Recent advancements in thin film Cu₂ZnSn(S_xSe_1-x)₄ (CZTSSe) solar cells have proven the ability for power conversion efficiencies >11%, indicating the potential of this low cost, earth abundant material system as a viable alternative to copper indium gallium diselenide (CIGSe) and cadmium telluride (CdTe) absorbers¹. The initial appeal of CZTSSe as a viable p-type absorber comes from its ideal band gap of 1.0 to 1.5 eV, depending upon the sulfur/selenium content in the device. While band gap tuning has chiefly been explored through control of the S/(S+Se) ratio in CZTSSe, recent research has shown the additional ability for band gap tuning through partial substitution of Sn for Ge into the crystal lattice, forming Cu₂Zn(Sn_yGe_1-y)(S_xSe_1-x)₄ (CZTGeSSe)². This presentation demonstrates material characterization as well as increased device performance associated with Ge-incorporation into the CZTGeSSe crystal lattice.
Material characterization of sintered CZTGeSSe absorbers has revealed a shift in the x-ray diffraction peaks associated with the decreasing lattice constants upon Ge incorporation. Similarly, Raman spectroscopy reveals a shift in the main A/A1 Raman mode of CZTSe through Sn substitution by Ge. CZTSSe and CZTGeSSe phase purity and binary/ternary identification was carried out through depth profiling Raman spectroscopy utilizing lasers of various emission energies. This analysis has demonstrated heterogeneous phase formation on the absorber surface following selenization for both material systems suggesting further room for improvement.
Current optimization of Ge incorporation into CZTGeSSe has achieved an increase in device performance to 9.4% total-area power conversion efficiency for 30% Ge/(Sn+Ge), over 8.4% for CZTSSe. To understand the device improvements associated with Ge-incorporation, electrical and optical characterization was carried out using IV, EQE, and TRPL measurements on devices with various Ge/(Sn+Ge) ratios. Electrical characterization has shown a linear increase in Voc associated with Ge-incorporation, ranging from .41 V for CZTSSe to .50 V for CZTGeSSe with 50% Ge substitution. Similarly, EQE measurements have shown a linear increase in band-gap associated with Ge incorporation ranging from 1.10 eV for CZTSSe to 1.27 eV for CZTGeSSe (50% Ge). Furthermore, time-resolved photoluminescence measurements on CZTSSe and CZTGeSSe (30% Ge) show an increase in the minority carrier lifetime associated with Ge incorporation. The effect of an increased minority carrier lifetime can similarly be observed in voltage-biased EQE measurements, which reveal a decrease in the voltage dependent carrier collection in Ge-containing devices.
[1] D. A. R. Barkhouse et, al., Prog. Photovolt: Res. Appl., 20, 2012, pp. 6
[2] G. M. Ford et al., Chem. Mater., 23, 2011, pp. 2626

We report on the electrical, optical and physical properties of Cu2ZnSnSe4 (CZTSe) solar cells consisting of an absorber layer fabricated by selenization of sputtered Cu, Zn, Sn multilayers. The Cu, Zn, Sn multilayers are sequentially DC-sputtered from metal targets onto Mo on soda lime glass substrates. The metal layers are then selenized for 15 to 30 minutes at temperatures between 450°C and 520°C in a continuous flow of pure H2Se gas. Under AM1.5G illumination, the best 1x1 cm2 CZTSe solar cell shows an efficiency of 6.35 % with a maximum short circuit current density of 31.3 mA/cm2, an open circuit voltage of 390 mV and a fill factor of 52%. Whereas the bulk absorber layer quality seems to be good, with a best measured minority carrier lifetime of 9.7 ns, the limiting factors for cell efficiency seem to be the rather high series resistance, low shunt resistance and interface related recombination.
On a set of 11 solar cells with efficiencies varying between 0.5 % and 6.35 %, it is noted that the doping density as measured by Drive Level Capacitance Profiling is exponentially correlated to the Zn/Sn ratio of the CZTSe crystals as measured by Energy Dispersive X-ray Spectroscopy [1]. Also the defect peak as measured by admittance spectroscopy on the same set of solar cells increases exponentially as the Zn/Sn ratio increases. Zn and/or Sn are therefore either directly involved or are at least influencing this rather deep defect level, which seems to be also responsible for doping in the CZTSe absorber layers. It will also be shown that the open circuit voltage of the solar cells, the minority carrier lifetime and the photoluminescence peak position of the absorber layer correlate with the height of the defect peak as measured by admittance spectroscopy.
[1] G. Brammertz, Y. Ren, M. Buffière, S. Mertens, J. Hendrickx, H. Marko, A. E. Zaghi, N. Lenaers, C. Köble, J. Vleugels, M. Meuris, J. Poortmans, Thin Solid Films, DOI: 10.1016/j.tsf.2012.10.037.
Acknowledgments
We would like to acknowledge Tom De Geyter, Greetje Godiers and Guido Huyberechts from Flamac in Gent for sputtering of the metal layers. AGC is acknowledged for providing substrates. The Flemish ‘Strategisch Initiatief Materialen&’ (SIM) SoPPoM program is acknowledged for their collaboration. Hamamatsu Photonics is acknowledged for providing the C12132 near infrared compact fluorescence lifetime measurement system.

The current knowledge about the correlation between variations from stoichiometry and electrical properties of CZTS based solar cells is rather limited. In this work we analyse the charge carrier transport properties using EBIC measurements of solar cell crossections based on CZTS with different compositions. All samples are produced in a single step co-evaporation process. From EBIC measurements we estimate the depletion width in the absorber as well as the diffusion length. Together with the doping density of the buffer layer it is therefore possible to estimate the effective doping density of the CZTS absorber.
In order to analyse the dependence of the charge carrier transport properties on the exact composition of the CZTS phase, which can be significantly different from the total sample composition due to the presence of secondary phases, we performed a complete phase analysis using X-ray absorption spectroscopy. By a combination of X-ray absorption spectroscopy at different k-edges of the cations and anions secondary phases can be detected and precisely quantified. Together with the total sample composition we are able to determine the exact composition of the CZTS phase.
For a stoichiometric composition as well as for strongly Zn-rich and Cu-poor conditions we find a small charge carrier collection width which is on the order of 40 nm towards the buffer- absorber interface. For slightly Zn-rich and Cu-poor samples we find a larger charge carrier collection width on the order of 500 nm. These results are compared with measurements of the I-V-characteristics, the external quantum efficiency and the capacitance at room temperature which allows us to develop a consistent picture of the device properties.
Due to the formation of ZnS as a secondary phase we find Cu-rich conditions of the CZTS phase also in samples which are Cu-poor and Zn-rich. This might give rise to the observed similar charge carrier transport properties of stoichiometric and strongly Cu-poor and Zn-rich samples.

Kesterite Cu2ZnSn(Se,S)4 (CZTSSe) is an emerging thin film solar cell technology unrestrained by material availability issues suffered by other leading technologies such as CdTe and CuInGaSe (CIGS). We recently demonstrated solution-processed CZTSSe cells with world record efficiency of 11.1%. We review the current status of material and device physics understanding from a series of characterization and comparison studies against CIGS to identify key performance bottlenecks in CZTSSe such as high Voc deficit and low fill factor issues. Recent admittance spectroscopy study also reveals distinct and fundamental features in CZTSSe (vs. CIGS) such as low dielectric constant and dominant deep acceptor level responsible for diverging series resistance (and fill factor collapse) at low temperature. These findings help to identify key areas of improvements to realize high performance and large scale CZTSSe technology in the near future.

Cu2ZnSn(S,Se)4 (CZTS) is a very promising material system for next generation thin film solar cell. It exhibits optical and electronic properties comparable to those of the Cu(In,Ga)(S,Se) 2 (CIGS) and CdTe material systems while consisting entirely of nontoxic constituents that avoid the scarcity and cost issues associated with indium and tellurium. The materials advantages of CZTS also come at a cost: the complex phase of this quandary material impedes challenges for processing to achieve a single phase kesterite while involving volatile components (SnS, Zn). Developing a simple and reliable fabrication route for producing single phase kesterite material is essential for an in-depth exploration of its intrinsic materials properties and ultimately the identification of current barrier and feasible avenues toward further improvement.
Here we develop a simple and reliable solution approach to process high quality CZTS films using fully dissolved hydrazine-based CZTS precursor complexes in which each of the elemental constituents are mixed on the molecular level. It is the first time by introducing an additive that all constituents of CZTS, especially Zn-contained compound, are incorporated to form a homogenous precursor solution. Solute chemistry and dissolution mechanism is investigated by identifying the molecular species dissolved via X-ray crystallography and solution Raman. The concept to introduce additives into hydrazine solution, could also be used for solution processing other interested materials beyond the realm dissolvable by hydrazine and excess chalcogen developed from Mitzi. Hydrazine solution and the additive introduced here has the advantages of achieving a chemically cleaner film compared with other solution based deposition approaches. This molecular solution process enables a precise stoichiometry control by individually varying each component solution to achieve the resulting single phase of CZTS. The conversion efficiency beyond 8% is demonstrated from this solution approach at its early stage.The low temperature formation together with moleculary homogeneity demonstrates a competitive process vs vacuum-based approaches not only in terms of cost-competitive, but also providing the option to overcome those challenges for a better material properties control toward an improved efficiency. Advantages demonstrated from this molecular solution process imply the potential for future scale-up production compatible with roll-to-roll or other liquid coating system.

Chalcopyrite materials and devices have been characterized by a wide range of nanochemical and nanostructural techniques and detailed current/voltage and capacitance methods. The results described here include tunneling microscopy and spectroscopy, photoluminescence, transmission electron microscopy, temperature and frequency-dependent capacitance with and without light bias, temperature-dependent current/voltage and others as time permits. The results suggest that the devices are strongly influenced by band tails and that the band tail structure of AgInSe2 is very different from that of CuInSe2. While the implication could be that AIS would yield superior devices, there is evidence that in that material there are additional deep defects that may limit performance. At the same time there is considerable evidence for a defect 800 meV above the valence band edge that influences the maximum Voc that can be obtained. The conclusion of these studies is that a combination of this defect and the band tails ultimately determines device performance. Fortunately both of these can be studied by non-destructive methods.

Chalcopyrite-based solar cells have reached efficiencies exceeding 20% in recent years. These improvements have been achieved by a Cu-poor growth of the CuInxGa(1-x)Se2 (CIGSe) absorber, resulting in a CIGSe surface with a distinct Cu-deficiency, in contrast to the more stoichiometric bulk composition. This growth mode reduces recombination at the interface between the chalcopyrite absorber and the CdS buffer.
However, CIGSe absorbers grown in Cu excess exhibit beneficial bulk characteristics such as larger grains, a lower defect density, higher mobilities, and a lower bulk recombination. Cu-rich prepared CIGSe absorbers are therefore expected to have the potential to achieve a higher solar cell performance. An understanding of the interplay between surface and bulk properties and how to tune them is of critical importance for Cu-rich prepared CIGSe devices if they are to outperform their Cu-poor grown counterparts.
A series of Cu-poor and Cu-rich grown CuInSe2 (CISe) absorbers were fabricated to study the effects of various surface treatments on the stoichiometry and electronic properties of the surface compared to the bulk. In an effort to produce a Cu-poor surface while maintaining a Cu-rich bulk, the Cu-rich grown CISe absorber surface was KCN-etched to remove Cu2-xSe surface species, followed by the deposition of an additional InxSey layer and a subsequent annealing step (“In-Se treatment”). The absorber after each surface modification step was studied and compared to a Cu-poor grown CISe absorber, fabricated following a three-stage In-Se, Cu-Se, In-Se growth process.
The samples were investigated by photoelectron spectroscopy using UV (UPS) and x-ray (XPS) excitation, as well as by soft x-ray emission spectroscopy (XES). By combining these techniques we achieve a non-destructive “depth-profile” of the chemical and electronic structure from the surface into the bulk. The In-Se-treated sample shows a strong reduction of the surface Cu signal compared to that of both the original Cu-rich and Cu-poor grown surfaces. This observation is also supported by the more bulk-sensitive XES measurements. The “depth-resolved” position of the valence band maximum (VBM) with respect to the Fermi level (EF) was determined using UPS and XPS. For all samples, we find that the VBM shifts away from the Fermi level when approaching the surface.
Our results will be complemented by surface-sensitive inverse photoemission measurements (IPES, to probe the conduction band minimum) and bulk-sensitive UV-Vis optical reflection measurements (to probe the optical bulk band gap). This will yield a complete picture of the chemical and electronic structure of these differently prepared CISe thin-film solar cell absorbers.

A way to lower the manufacturing cost of Cu(In,Ga)Se2 (CIGS) thin film solar cells is to use flexible polymer substrates instead of glass substrates. Because such substrates require a low temperature during absorber deposition, the efficiency of the cells remains slightly lower (18.7% [1]) compared to CIGS on glass substrates (20.3% [2]). Partial compensation of this efficiency loss might be accomplished by Sb doping [3] of the absorber, which is reported to have a positive effect on the morphology of this layer. In this work the defect structure of Sb doped CIGS solar cells is investigated using optical and electrical spectroscopic techniques. Experiments were performed on cells deposited on soda lime glass substrate, adding a thin Sb layer (8, 12 nm) onto the Mo back contact prior to the CIGS absorber deposition. The results are compared with those for cells without Sb doping using the same process.
Fourier-Transform near infrared photocurrent measurements in the 10-300K range demonstrate that the band gap of Sb-doped samples is larger than for undoped samples. Photoluminescence spectra in the 5-100K region provide information on shallow-level defects. Deep-Level Transient Spectroscopy spectra of Sb-doped cells exhibit two features not encountered for non-doped cells: 1) a peak at lower temperature than the N1 signal and 2) incomplete charge carrier freeze-out down to 8 K. While the first result appears to be the fingerprint of an extra non-Ohmic contact in the solar cell structure [4], the second suggests the introduction of a very shallow acceptor by Sb doping. As a salient feature one can accurately monitor the partial hole freeze-out in the 40-60 K range and determine the signature of the intrinsic defects that provide the p-type conductivity of the CIGS absorber using Admittance Spectroscopy.
[1] A. Chirila, S. Buecheler, F. Pianezzi, P. Bloesch, C. Gretener, A. R. Uhl, C. Fella, L. Kranz, J. Perrenoud, S. Seyrling, R. Verma, S. Nishiwaki, Y. E. Romanyuk, G. Bilger, and A. N. Tiwari, Nature Mater. 10, 857-861 (2011)
[2] P. Jackson, D. Hariskos, E. Lotter, S. Paetel, R. Wuerz, R. Menner, W. Wischmann, and M. Powalla, Progr. Photovolt. 19, 894-897 (2011)
[3] T. Nakada, Y. Honishi, Y. Yatsushiro, 37th IEEE Photovoltaic Specialists Conference (2011)
[4] J. Lauwaert, S. Khelifi, K. Decock, M. Burgelman, and H. Vrielinck, J. Appl. Phys. 109, art. no. 063721 (2011)

Thin films of Tin(IV) Sulfide (SnS₂) and alloy films of CuZnS, CuSnS_x and Cu₂ZnSnS₄ were created by Atomic Layer Deposition (ALD) in a hot wall reactor on slides of quartz glass and on a nanoporous matrix of Titanium Dioxide (TiO₂). A separate feedthrough injector was used for each precursor, and the metal sources were Tin(IV) Acetate, Zn(TMHD)₂, and Cu(TMHD)₂, while the sulfur source was H2S generated in situ via a reaction between Aluminum Sulfide (Al₂S₃) and water. For the films of SnS₂, energy-dispersive x-ray spectroscopy (EDX) and extended x-ray absorption fine structure (EXAFS) data show that the films are a sulfur-rich phase of SnS_x with x~2. For the alloy films, nanometer thick depositions of tin sulfide, zinc sulfide, and copper sulfide were stacked in alternating layers to observe the formation of alloy phases on these scales. In the films of CZTS, the ratios of elements was controlled by varying the number of deposition cycles for the component compounds, i.e. using a sequence of ALD reactions to deposit 5 cycles of copper sulfide, then 5 of zinc sulfide, then 5 of tin sulfide, or alternatively 10 cycles of copper sulfide, then 6 of zinc sulfide, then 5 of tin sulfide to control the Zn/Sn and Cu/(Zn+Sn) ratios of the resulting films.

Native point defects such as vacancies and self-interstitials exist ubiquitously in compound semiconductors. They can significantly affect thermal and electrical properties of the materials, and become critical for improving the performance and thermal management of thin-film devices including photovoltaics and thermoelectrics. In this work we seek to develop a universal and correlative understanding of the thermal and electrical effects of native defects.
Experimentally, point defects can be introduced into thin film compound semiconductors using high-energy particle irradiation (3 MeV alpha particles). Point defects with controllable concentration are generated uniformly throughout the film. Theoretically, the electrical effect of native defects can be described by the amphoteric defect model, while their effects on thermal transport can be modeled by considering additional acoustic phonon scattering at these point defects.
In this talk we will present a case study on Cadmium Oxide (CdO) and a series of CdO-related alloys. CdO is a unique transparent conductive oxide with exceptionally high mobility (>100 cm²/Vs) with electron concentration exceeding 10²¹ cm^-3. Hence it has an excellent electrical conductivity and transmission window in the range from 400 nm to >1500 nm, making this material an ideal transparent conductor for photovoltaics with low band gap absorbers. We introduce point defects into these thin films to alter their electrical transport which is measured by Hall effect, and their thermal conductivity determined by the three-omega method. We will discuss the results of this study in the framework of our universal understanding of native defects on the thermal and electrical properties of compound semiconductors.

Solution-based deposition of Cu2ZnSn(S,Se)4 (CZTSSe) thin films for photovoltaics is regarded as a low-cost, high-throughput alternative to the vacuum-based approach. Successful fabrication with such methodology relies on the solid-state chemical transformation of soluble precursors followed by subsequent annealing and grain growth. Herein we present a comprehensive study in which we monitor the formation of Cu2ZnSnS4 (CZTS) and Cu2ZnSnSe4 (CZTSe) thin films from a series of precursors via X-ray diffraction and Raman spectroscopy. The precursors we use in this study include (A) a molecular precursor fabricated by Hugh W. Hillhouse et al., (B) mixed nanocrystals capped with metal chalcogenide complexes reported by our group, (C) a hydrazinium-based particulate precursor developed by David B. Mizti et al., and (D) our recently-synthesized CZTS (or CZTSe) nanocrystals capped with ammonium sulfide ligands. Amongst these precursors, precursor D is adopted as the baseline to observe the sintering of CZTS and CZTSe nanocrystals, while precursors A, B and C differ in chemical compositions as well as sizes of dispersion. Our experimental results indicate that the molecular precursors react to form CZTS at lower temperature, but subsequent sintering occurs at higher temperature compared to nanocrystalline precursors. These phenomena can be explained by the lattice dynamics arguments widely used in solid-state chemistry. We hope our research will contribute to the rational design of liquid-phase precursors for solution-based process of CZTSSe films.

Thin film solar cells with p-type absorber layers composed of earth abundant copper zinc tin sulfoselenide (Cu2ZnSn(Sy,Se1-y)4, or CZTSSe) offer significant promise as a viable, low-cost photovoltaic technology. Characterized by an ideal band gap in the range 1.0 - 1.5 eV and a high absorption coefficient, solution-processed CZTSSe solar cells have achieved power conversion efficiencies greater than 11% [1]. Although this technology has developed rapidly in recent years, the realization of the full potential of CZTSSe solar cells requires better understanding of the fundamental physics involved in device fabrication. In particular, the formation of large grained CZTSSe absorber layers from a nanocrystal film represents an important step in the device fabrication process that is currently poorly understood.
In this presentation, we report the observation of two distinct size-correlated particle composition domains in a solution-based synthesis of copper zinc tin sulfide (CZTS) nanocrystals that has led to device efficiencies in excess of 7% [2]. Devices fabricated using particles from each domain individually exhibit inferior performance compared to devices based on the mixture of particles reported in [2]. In order to understand the effect of inter-particle heterogeneity on absorber layer formation, we investigate the role of particles from each domain in the sintering of nanocrystal films in the presence of selenium vapor (selenization). To observe differences in the sintering mechanism for various particle size and composition profiles, energy-dispersive x-ray diffraction (EDXRD) and x-ray fluorescence (XRF) spectra were collected during the selenization process. We find that the selenization of small (i.e. < 5 nm), Cu- and Sn-rich nanocrystals leads to the formation of copper selenide intermediate phases and subsequently to a fast and direct formation of CZTSe. In contrast, the selenization of larger (> 15 nm), Zn-rich nanocrystals shows a rather slow transition from the sulfide to the selenide phase. Ultimately, studying the effect of nanoparticle size and composition on absorber layer sintering is leading to better understanding of the physics involved in the growth of high quality, thin film absorbers.
[1] Todorov, T. K., Tang, J., Bag, S., Gunawan, O., Gokmen, T., Zhu, Y. and Mitzi, D. B., “Beyond 11% Efficiency: Characteristics of State-of-the-Art Cu2ZnSn(S,Se)4 Solar Cells”. Adv. Energy Mater., 2012, doi: 10.1002/aenm.201200348.
[2] Guo, Q., Ford, G.M., Yang, W.-C., Walker, B.C., Stach, E.A., Hillhouse, H.W., and Agrawal, R., "Fabrication of 7.2% Efficient CZTSSe Solar Cells Using CZTS Nanocrystals". J. Am. Chem. Soc., 2010, 132 (33), pp 17384-17386. doi: 10.1021/ja108427b.

Cu2ZnSnS4 (CZTS) is a promising indium-free absorber for thin layer photovoltaic devices. It contains only abundant and non-toxic materials. Electrodeposition is an interesting low cost, low temperature process. In this work, absorbers are made by one step electrodeposition of copper, tin and zinc and a pre-annealing of the precursor. An annealing in selenium atsmosphere is necessary to obtain the CZTSe phase. The best electrodeposition cell exhibits a 7.1 % conversion efficiency made by stack [1] and a 3.4 % conversion energy by one step electrodeposition [2].
The electrodeposition is carried out in 500 mL three-electrode cells. The solutions contain copper, tin and zinc sulphates (10 mM each), two complexing agents: C4H6O6 (50 mM) and Na3C6H7O6 2H2O (100 mM) and K2SO4 (100 mM) as supporting electrolyte. The pH is adjusted at 4.75 with H2SO4. Voltammetry and chronoamperomety are carried out with a rotating disc electrode and a quartz crystal microbalance is used for in situ growth rate studies.
Citrate allows the solubilisation of Sn(II) salts and brings the deposition potentials of the 3 metals closer. Thanks to studies at different potentials, the Cu-Sn-Zn co-deposition is achieved at -1.25 V/NHE. The deposition is fast: 1 µm in seven minute deposition. The precursor composition is zinc rich and copper poor (it has been measured by inductively coupled plasma). Due to the highly negative potential, dendritic morphology is observed. X ray diffraction shows a Cu-Zn phase with metallic Sn.
With the dendritic morphology, the precursor layer is porous. A pre-annealing is necessary to increase the density of the layer. SEM observations and X-ray diffraction have been performed to optimize the deposition and pre-annealing treatment.
After annealing in selenium atmosphere, Raman characterization shows the presence of the CZTSe phase. With lasers at different wavelengths, the presence of binary and ternary compounds is also evidenced. The degradation of CZTSe during selenization has been shown by Raman characterization too. For long annealing, CZTSe grains get smaller and CuxSe appears.
[1] S. Ahmed, K. B. Reuter, O. Gunawan, L. Guo, L. T. Romankiw, et H. Deligianni, laquo; A High Efficiency Electrodeposited Cu2ZnSnS4 Solar Cell raquo;, Advanced Energy Materials.
[2] A. Ennaoui, M. Lux-Steiner, A. Weber, D. Abou-Ras, I. Kötschau, H.-W. Schock, R. Schurr, A. Hölzing, S. Jost, R. Hock, T. Vob, J. Schulze, et A. Kirbs, laquo; Cu2ZnSnS4 thin film solar cells from electroplated precursors: Novel low-cost perspective raquo;, Thin Solid Films, vol. 517, no. 7, p. 2511-2514, févr. 2009.

Cu2ZnSnS4 [CZTS] and Cu2ZnSnSe4 [CZTSe] are considered as potential alternatives of Cu(In, Ga)Se2 and CdTe absorbers in thin film solar cells, because of their non-toxic, earth-abundant features and tunable band-gap energy to the optimum for a single junction cell. So far, most of the solar cells based on these absorbers have been fabricated using liquid precursor method or sulfurization or selenization methods of metal precursor. Some surface-treatments such as chemical etching have been adopted prior to and after the junction formation. Large differences of chemical activity of the constituents and possibilities of formation of secondary phases of this multinary compound, however, suggest the variation of surface electronic structure as well as band alignment at buffer/absorber interface, which are crucial factors to cell performances. In this study, effect of chemical treatments and deposition methods of CdS buffer on electronic structure of CZTS, CZTSe surface and CdS/CZTS interface have been studied by means of photoemission and inverse photoemission spectroscopy.
CZTS and CZTSe films were respectively prepared by sulfurization of metal precursor deposited by sputtering and evaporation under Cu-poor process conditions. Chemical treatments of the absorber surfaces were carried out in a grove-box with oxygen and nitrogen background level below 20 ppb, connected to the analysis-system. CdS buffers with various thicknesses were deposited on CZTS layer by chemical bath deposition CBD and evaporation. CBD-CdS/CZTS samples were transported using vacuum-package. Surfaces of CZTS and CZTSe etched with aqueous solution of KCN showed Cu-depleted and oxidized feature with thickness of few nm and Zn- and Sn-rich composition. In addition to selective etching of Cu, elements with higher oxygen-affinity were selectively pulled out by KCN treatment. The KCN-treatment resulted in a surface of CZTS and CZTSe with wide band gap above 1.85 and 1.75 eV, respectively. Interfaces between these surfaces and CBD-CdS showed negative conduction band offset CBO. On the other hand, etching using H2O-only or non-aqueous solution of HCl for 1 min resulted in development of bulk-like electronic structure; conduction band minimum CBM and valence band maximum VBM the treated CZTS and CZTSe were +0.9, -0.6 and +0.6, -0.5 eV, respectively. Dependence of IPES spectra of CBD-CdS/H2O- or HCl-treated CZTS on thickness of the CBD-CdS showed a suppression of negative feature of CBO, accompanied with interface-induced downward band bending by about 0.1 eV. On the other hand, negative CBO about -0.3 eV was observed in in-situ IPES for co-evaporated-CdS/CZTS interfaces. Properties of the cells using the junction above, up to present, indicated relaxation of the negative CBO should be favorable to better performances.

Quaternary semiconductors, Cu2ZnSnS4 and Cu2ZnSnSe4 which contain only earth-abundant elements, have been considered as the alternative absorber layers to Cu(In,Ga)Se2 (CIGS) for thin film solar cells although CIGS-based solar cells have achieved efficiencies over 20 %. Cu2ZnSn(SxSe(1-x))4 (CZTSSe)-based solar cells have reached efficiencies over 11 % using hydrazine-based solution process. Hydrazine can dissolve chalcogenides by breaking them down to molecular dimension, allowing the preparation of precursor inks which are compatible with different common liquid coating techniques. In this work we report an air-stable route for preparation of CZTSSe thin film absorbers by a solution process based on the binary and ternary chalcogenide nanocrystal precursors dispersed in organic solvents [X. Lin et. al; RSC Adv., 2012, 2, 9798-9800]. Our CZTSSe absorber layers were obtained by spin coating of the ink precursors followed by annealing under Ar/Se atmosphere at temperature up to 580°C. We have investigated the influence of the ink precursors&’ ratio as well as annealing conditions on the reduction or elimination of detrimental secondary phases. Our strategy is the formation of CZTSSe from the reaction of copper, zinc, and tin chalcogenide precursors with additional Se during annealing. The objective is to eliminate the intermediate ZnS phase which is mostly present after annealing. The morphology and phase formation were investigated with scanning electron microscopy and grazing incident X-ray diffraction. Raman spectroscopy was utilized to better identify secondary phases such as ZnS and copper tin sulfide. Solar cells were completed by chemical bath deposited CdS buffer layer followed by sputtered i-ZnO/ZnO: Al bi-layers and evaporated Ni/Al grids.

Recently the material group Kesterite Cu2ZnSn(S,Se)4 (CZTSSe) has emerged as a absorber for thin film solar cells. Basically, this material is derived from CuInS2 by replacing In(III) with Zn(II) and Sn(IV). CZTSSe contains only abundant and non-toxic elements and has direct band gap energy in the range of 1.0-1.5 eV with a high absorption coefficient above 10^4 cm-1, that fulfils the requirements for thin film solar cells. Solar cells with CZTSSe absorber layers have already reached an energy conversion efficiency of 11.1 % [T. K. Todorov et. al; Adv. Energy Mater. (2012)].
In this work SnS, ZnS and Cu3SnS4 (CTS) nano-particles (NPs) are used as precursors to form CZTSSe. All our binary and ternary NPs are prepared via a chemical route that allowed the creation of monodispersed NPs using organic ligands [X. Lin et. Al; RSC Adv., 2012,2, 9798-9800]. The precursors are then deposited by either spin coating or dip coating on ZnO nanorod (NR) arrays, which are electrochemically grown in an aqueous solution. In a previous work we observed the formation of CZTSSe by the reaction between CTS and the ZnO-NR array. The latter vanished during the annealing process [Jaison Kavalakkatt et al. Thin Solid Films, in press]. However our objective is the use ZnO NRs as a nano-structured substrate while keeping the advantage of high surface/volume ratio. Therefore, in order to prevent from interdiffusion and reaction processes at the absorber/substrate interface, a MoO3 barrier layer was first deposited on ZnO NRs prior to the deposition of the CZTSSe precursors. The stability of the barrier layer during the annealing in reactive atmospheres is investigated by means of photoelectron spectroscopy. Further investigations of the structural properties are performed using of X-ray diffraction (XRD) and Raman spectroscopy. The formation of CZTSSe on the structured substrates is also analysed by XRD and Raman, to explore the binary and ternary phases such as ZnS, ZnSe, CuxS and CuxSnSy. Further systematic study is carried out using energy dispersive X-ray spectroscopy (EDX) mapping in a scanning electron microscope. The spatial resolution of the element concentration allows a quantitative analysis of the CZTS layers. Losses of organic ligands during the annealing process can also be determined. The fabrication of the ZnO-NRs/MoO3/CZTSSe structure as a new concept of solar cells will be discussed.

Cu2ZnSn(S1-xSex)4 (CZTSSe) compounds are very promising candidates to replace Cu(In,Ga)Se2 in thin film solar cell applications thanks to their optical properties - bandgap range from 1.0 to 1.5 eV and absorption coefficient higher than 104 cm-1. Moreover the relative abundance of each element and their non-toxicity make them suitable for cost effective photovoltaic applications. We are focusing on synthesizing CZTSSe layers using a low cost-effective method and this allowed us to fabricate solar cells exhibiting power conversion efficiencies up to 6%.
The proposed work is to establish and explain new approaches to improve the CZTSSe physical properties and performances of photovoltaic devices. In this context two different studies were developed.
(i) We investigated the influence of Na on the growth of CZTSSe thin films. First, we examined the influence of Na incorporated in Mo-coated Eagle2000 (0.1 wt% Na) - borosilicate (4 wt% Na) and soda-lime glass (12 wt% Na) during selenization of the vacuum deposited precursors. Second, we repeated the experiment with Na-doped Molybdenum (5 wt% Na) on the same substrates to increase the Na content in the CZTSSe absorber. The Na-content in the CZTSSe coming from both the substrate and the Mo were quantified by second ion mass spectroscopy (SIMS) analysis. Different characterization techniques (including photoluminescence) indicated that we are able to grow a better quality material when increasing Na-content in both the substrate and Mo. The material characterization results were compared to the device performances.
(ii) The formation of stable and low resistance back contacts, as well as better back reflectors, for CZTSSe thin films are critical challenges associated with strong effect on the performances of Mo/CZTSSe/CdS/ZnO:Al solar cells. For this purpose we replaced the Mo-back contact by various metals with higher working function (W, Ni, Au, Pd, Pt), and compared the measured device performances with numerical simulations.

Tin monosulfide (SnS) as a photovoltaic absorber has high potential solar energy conversion efficiency of above 20% due to its bandgap (1.1 eV indirect and 1.4 eV direct) and its high absorption coefficient (>104 cm-1) at energies above the direct absorption edge. The material is also attractive because its constituent elements are environmentally benign, earth-abundant and inexpensive. Despite such promising properties, SnS solar cells have achieved only 1.3% efficiency to date. This presentation will cover our efforts in producing high quality SnS absorbers and incorporating them into complete solar cell devices. We have developed several approaches for producing SnS polycrystalline thin films on various substrates. Through both solution and physical vapor deposition (PVD) methods, we demonstrate control over SnS thin film growth and morphology. Solution methods include chemical bath deposition and pyrolysis of molecular precursor inks, whereas PVD methods include sputtering and evaporation. Depending on the deposition conditions, we can produce either highly nanostructured films or smooth pinhole-free thin films.
The best results were obtained using a vapor transport deposition (VTD) approach that we developed to be compatible with the high-rate VTD systems used in CdS/CdTe solar cell manufacturing. This is possible because under the proper conditions we can deposit SnS thin films from a SnS powder source in a fashion similar to the VTD process used for CdTe and CdS. TEM images and X-ray diffraction patterns indicate that SnS films grown using our VTD process are orthorhombic and highly crystalline. Optical and electrical measurements of SnS devices will also be presented. UV-vis and ellipsometry measurements indicate the expected bandgap. VTD-deposited films exhibit both photoconductivity and relatively high resistivities of ~50 kOhms/square. The latter is important because low resistivity has been one of the reasons prior attempts to make SnS solar cells yielded low efficiencies. We will present the performance of our VTD-SnS solar cell devices and discuss what challenges remain to improve efficiency.

Recombination loss due to electrically active defects in the absorber material is one of the most important limiting factors of photovoltaic device performance. Understanding the defects in kesterite Cu2ZnSn(S,Se)4(CZTSSe) is critical for the continued development of solar cells based on this material, but challenging due to the complex nature of this polycrystalline multinary material. Here we present a comparative study on CZTSSe alloys with three different bandgaps by introducing different fractions of sulfur during the annealing process. Through the use of admittance spectroscopy, drive level capacitance profiling and capacitance-voltage profiling, we observed the influences of sulfur content in CZTSSe on defect activation energy, defect density, and the corresponding effects on devie performance. The high sulfur content device showed a higher defect energy level (0.183 eV) as well as a high bulk defect density (8.2*10^15 cm-3), which led to strong recombination losses as indicated by low external quantum efficiency values and a large open-circuit voltage deficit. In contrast, the low sulfur content device showed the best efficiency (7.4%) and high external quantum efficiency as the result of a relatively low defect activation energy (0.134 eV) and a low bulk defect density (8*10^14 cm-3). In all, enlarging the bandgap of the absorber by an increase of the sulfur content leads to significantly less desirable defect behavior, which is accompanied by an overall reduction in device efficiency.

In thin film solar cells based on calcopyrite semiconductors as light absorbers (CuIn_xGa_1-xSe₂ or CIGSe), interface losses are a concern for manufacturers. The fabrication process includes co-evaporation or sequential deposition of the CIGSe elements on the back contact, a metallic (molybdenum) layer, leading to the welcome formation of an interface layer of MoSe₂ [1]. The paramenters controlling the formation and morphology of this layer, which enhances the collection of holes, are far from being elucidated. In particular, no informations are available at the atomic scale on the thermodynamics and the kinetics of selenium adsorbed on Mo surfaces.
We present here results of first principles calculations of Se adsorbed on the Mo(110) surface at various coverages, using plane waves pseudopotential density functional theory calculations. We have addressed the issue of the accuracy of surface energies and adsorption energies by comparing some well established semi-local functionals and a recently proposed one (PBEq2d [2]), supposed to improve the description of quasi-two-dimensional systems. We compare our results to the known features of sulphur adsorption on the same surface and predict reconstructions that are expected to be observed experimentally.
Finally, we will discuss about Se mobility on the surface and possible implications for surface reactions that could take place in particular with Na, whose presence during the deposition
stages is considered necessary.
[1] R. Caballero et al., Appl. Phys. Lett. 96, 092104 (2010).
[2] L. Chiodo et al., Phys. Rev. Lett. 108, 126402 (2012).

Large-area Cu₂ZnSnS₄ (CZTS) thin films were deposited by low-cost spray pyrolysis on Mo-coated soda-lime glass (SLG) substrates at varied substrate temperatures of 573-673°K. Heterojunction was formed by deposition of n-CdS window layer via chemical bath method followed by sputtered i-ZnO buffer layer on top of sprayed p-CZTS absorber. CZTS film deposition parameters and post deposition processing conditions were optimized to obtain best photovoltaic performance. Structural, morphological, and compositional characterization of spray deposited CZTS absorber layers were carried out by x-ray diffraction (XRD), Raman spectroscopy, scanning electron microscopy (SEM), energy dispersive x-ray analysis (EDX), atomic force microscopy (AFM), and x-ray photoelectron spectroscopy (XPS). Optical and electrical properties were measured by UV-Vis spectroscopy, van der Pauw, and Hall-effect measurements. The fabricated solar cell of an active area ~1.2 cm² exhibited a conversion efficiency of ~0.62% under AM1.5 simulated solar radiation. Our results show that large area CZTS solar cells and monolithically integrated arrays can be fabricated by low-cost spray pyrolysis technique for high throughput roll-to-roll commercial production. Details of CZTS film deposition, heterojunction fabrication, and characterization results will be presented.

In the last years the interest in SnS as absorber material for thin film solar cells has increased remarkably. Despite the favorable optical properties of SnS only low efficiencies were reported until now. The highest conversion efficiencies were obtained using spray pyrolysis and chemical plating for the SnS deposition and CdS as buffer layer. In this work superstrate CdS/SnS solar cells have been prepared by thermal evaporation of SnS with conversion efficiencies of up to 0.7 %, which are remarkable results for evaporated SnS solar cells. The determination of the band alignment at the CdS/SnS interface with in-situ X-ray photoemission spectroscopy revealed an initial growth of SnS2 and conduction band offset of -0.6 eV between CdS and SnS2 and 0.2 eV between SnS2 and SnS. This non-ideal band alignment limits the open circuit voltage of the device. Therefore the use of alternative buffer layers has to be considered. Cd1-xZnxS, ZnO, InGaxSyOz and InSxOy have already been investigated, but could not keep up with CdS so far. In this work besides CdS, In2S3 and CdOxSy were chosen. The CdS and CdOxSy were prepared by RF sputtering under argon or argon-oxygen flow respectively, while the In2S3 layers have been grown from thermal evaporation of the compound material onto Al:ZnO coated glass. Different parameters for the buffer layer deposition have been varied and investigated with respect to the performance of the solar cell including the thickness of the CdS layer, the stoichiometry of the CdOxSy layer and the substrate temperature during the In2S3 deposition. After the SnS deposition the solar cell structures were completed by the deposition of a gold back contact. The solar cells were characterized with IV- and quantum efficiency measurements. It was found that the buffer layers reduce effectively the leakage current at pinholes in the SnS layer. In case of the CdS buffer material an improvement of the characteristic values was observed if a post-annealing of the solar cell was carried out. As a result conversion efficiencies of up to 0.7 % with short circuit currents of over 8 mA/cm2 were obtained. In addition also the band alignments of the interfaces In2S3/SnS and CdOxSy/SnS were determined with in-situ X-ray photoemission spectroscopy. Possible reasons for the differences and similarities in performances of the solar cells with different buffer layers will be discussed.

The quaternary chalcogenide semiconductors Cu₂-B^II-C^IV-X₄ (B^II-Zn, Cd, Hg; C^IV-Si, Ge, Sn; X-S, Se, Te) have drawn wide interest for their nonlinear optical properties and potential application as thin film solar cell absorbers [1], photocatalysts for solar water splitting [2] and high-temperature thermoelectric materials [3].
Cu₂ZnSiSe₄ belong to the adamantine family of quaternary chalcogenides crystallizing in the wurtzstannite structure [8]. In this compound each selenium atom has four nearest neighbor cations (two copper, one zinc, one silicon) at the corners of the surrounding tetrahedron. Recent ab-initio calculations show, that the lowest energy structure of Cu₂ZnSiSe₄ is the wurtzkesterite type structure [9] in contrast to wurtzstannite type, usually obtained in experiments. To clarify this issue a structural study on single crystals is of great importance. Here we employed a chemical vapor transport method to grow single crystals of Cu₂ZnSiSe₄. The structural characterization of the single crystals was carried out by X-ray diffraction at two different temperatures - room temperature and 150K, using an Xcalibur E diffractometer supplied with an EOS CCD space detector and a monochromatic source of MoK_α radiation (graphite monochromator). The structure was refined by the full matrix least squares method on F² with anisotropic displacement parameters using the program SHELXL 97. The XRD data analysis shows, that Cu₂ZnSiSe₄ single crystals adopt the orthorhombic wurtzstannite type structure (space group Pmn21). The values of the lattice parameters were determined as a =7.8208(2) Å, b = 6.73380(10) and c = 6.45290(10) Å (at room temperature). Detailed results for the single crystal X-ray diffraction study of Cu₂ZnSiSe₄ single crystals at different temperatures will be presented.
Acknowledgments: Financial supports from IRSES PVICOKEST 269167, BMBF MDA11\002 and STCU # 5402 projects are acknowledged. One of us (SL) would like to thank Humboldt foundation for support.
References:
[1] H. Katagiri, K. Jimbo, S. Yamada, T. Kamimura, W.S. Maw, T. Fukano, T. Ito, T. Motohiro, Appl. Phys. Expr., 1 (2008), p. 041201.
[2] D. Yokoyama, T. Minegishi, K. Jimbo, T. Hisatomi, G. Ma, M. Katayama, J. Kubota, H. Katagiri, K. Domen, Appl. Phys. Expr., 3 (2010), p. 101202.
[3]M.L. Liu, F.Q. Huang, L.D. Chen, I.W. Chen, Appl. Phys. Lett., 94 (2009), p. 202103.
[4] W. Schafer, R. Nitsche, Mater. Res. Bull. 12 (1974), p. 645.
[5] S. Chen, A. Walsh, Y. Luo, J.H. Yang, X.G. Gong, S.H. Wei, Phys. Rev. B, 82 (2010), p. 195203

Cu₂ZnGeSe₄ is one of the quaternary semiconductors Cu₂-B^II-C^IV-X₄ (B^II-Zn, Cd, Hg; C^IV-Si, Ge, Sn; X-S, Se, Te) belonging to the adamantine compound family, which are considered as very interesting, due to their possible applications in optoelectrics and non-linear optics [1, 2]. It has been suggested that Cu₂ZnGeSe₄ presents low and high temperature structural modifications [3, 4]. The low temperature phase of Cu₂ZnGeSe₄ shows the tetragonal stannite type structure (space group I-42m) [3], whereas the crystal structure of the high temperature phase has not as yet been reported. In contrast to these findings recent first principal calculation predicts the kesterite type phase (space group I-4) to be the ground state structure for this material [5]. Thus it is of great interest to study the effect of the growth method on the structural properties of Cu₂ZnGeSe₄. In this report, Cu₂ZnGeSe₄ samples were grown by three different methods: chemical vapour transport, using iodine as transport agent, a modified Bridgman method and solid state reaction of the elements. An electron microprobe system equipped with wavelength dispersive X-ray analysis (WDX) was used to determine the chemical composition of the obtained samples. The structural analysis was performed by powder X-ray diffraction at room temperature. The data were collected using a PANalytical X&’pertPro MPD diffractometer equipped with CuK_α radiation (lambda;=1.54056 Å) in a focusing Bragg-Brentano geometry, and a subsequent Rietveld analysis of the diffraction data using the FullProf suite software [6] was applied. A comparative study of the crystal structure and the phase content of the Cu₂ZnGeSe₄ samples grown by the three different methods will be presented.
Acknowledgments: Financial supports from IRSES PVICOKEST 269167, BMBF MDA11\002 and STCU # 5402 projects are acknowledged. One of us (SL) would like to thank Humboldt foundation for support.
References:
[1] N. Nakayama, K. Ito, Appl. Surf. Sci. 92 (1996) 171.
[2] H. Matsushita, T. Maeda, A. Katsui, T. Takizawa, J. Cryst. Growth 208 (2000) 416.
[3] O.V. Parasyuk, L.D. Gulay,Ya.E. Romanyuk, L.V. Piskach, J. Alloys Comp. 329 (2001) 202-207;
[4] Ya.E. Romanyuk, O.V. Parasyk, J. Alloys Comp. 348 (2003) 195-202.
[5] S. Chen, A. Walsh, Y. Luo, J.H. Yang, X.G. Gong, S.H. Wei, Phys. Rev. B, 82 (2010), p. 195203 (8pp.)
[6] Juan Rodriguez-Carvajal and Thierry Roisnel, www.ill.eu/sites/fullprof

Electrodeposition is one of the promising non-vacuum methods to fabricate thin-film compound semiconductor materials because of its simplicity of apparatus, efficient material utilization, possible formation of a compact film required for solar cell applications, and scalability and manufacturability into a large scale. In the present study, we investigated the fabrication of kesterites Cu2ZnSnS4 (CZTS) and Cu2ZnSnSe4 (CZTSe) photoabsorbers using two kinds of electrodeposition routes: (1) sequential electrodeposition of Cu/Sn/Zn layers with subsequent sulfurization and (2) sequential electrodeposition of Cu-Zn-Sn-Se/Cu-Sn-Se bilayer from electrolytes containing corresponding metal and selenium ions followed by annealing under argon (Ar) or selenium (Se) atmospheres. In the former procedure, due to the formation of island-shaped Sn deposits, resulting CZTS films tended to form inhomogeneous compositions with rough surface morphologies. In order to improve qualities of the CZTS film, we tried to optimize the deposition condition and addition of a pre-annealing before sulfurization. As a result, one of the CZTS sample achieved improvement of compactness having relatively flat morphology. The current best efficiency achieved on thus-obtained CZTS-based cell has reached 5.6%. In the latter approach, post annealing of the as-deposited bilayer under Ar induced significant losses of Sn and Se components. However, these losses could be suppressed by introduction of Se vapor during annealing. As a result, solar cell based on thus-obtained CZTSe film exhibited appreciable device properties: the highest efficiency of 1.1% was obtained based on the CZTSe film prepared by annealing the bilayer at 575 oC for 5 min. Further work to improve the device properties are now in progress.

A crucial threshold in the usage of the benign class of ZnX systems (X = O, S, Se, or Te) in photocatalysis and photovoltaics is our ability to engineer (i.e., reduce) their band gap to desired values. In the present contribution, we show using first-principles density functional theory (DFT) and hybrid DFT calculations that a nanolayer of ZnX when coherently placed on a substrate of ZnY (Y = O, S, Se, Te; with X and Y being mutually exclusive), would significantly enhance the range of absorption or emission energies. This is an example of heterostructures providing a natural way of manipulating materials and their properties through epitaxial strain, and, in turn, providing a powerful opportunity to control the band gap as well as the relative position of band edges.
Our present contribution on ZnX/ZnY nano-hetero-architectures builds on past work in which the possibility of band gap reduction in bulk single-crystal ZnX systems through imposition of uniaxial strains was unequivocally shown using conventional DFT, hybrid DFT and many-body perturbation theory [1, 2]. Here, we extend that work to study the effect of built-in strains due to constraints of coherency at interfaces between dissimilar systems. We considered (0001), (10-10), and (11-20) interfacial planes for heterostrucutre formation, with thickness of each layer ranging from 2-3 nm. Such architectures lead to both in-plane biaxial strains and quantum confinement. Coherency of the overlayer with the substrate is modeled by appropriately constraining the in-plane lattice parameters of the heterostructure. The highest band gap change is observed when any of the ZnX systems is under a biaxial tensile strain in the (10-10) plane. The largest variation in the band edges is always confined to the unstrained material. It was also found that strained material displays a diminished quantum-confinement tendency compared to the unstrained material. In general this finding opens the possibility to tune the band gap through proper choice of materials making up the nano-heterostructure. Among the various combinations of systems considered, a ZnSe nanolayer on a ZnTe substrate is particularly promising for solar cell application. ZnSe shows 50% reduction in the band gap from its equilibrium value of 2.7 eV, and the continuous variation of the band gap in the ZnTe side of the heterostructure is an added advantage. [1] Yadav et. al, Phys. Rev. B, 81, 144120 (2010)]; [2] Yadav et. al., Appl. Phys. Lett., 100, 241903 (2012).

Cu(In,Ga)Se₂ (CIGS) are among the most promising materials for thin-film solar cells. Recently, a substitution of In and Ga in CIGS by other elements has become an important issue because In and Ga are expensive rare metals. Therefore, Cu₂ZnSnS₄ (CZTS) is anticipated as an In-free absorber material of thin-film solar cells. IBM group fabricated Cu₂ZnSn(S,Se)₄ (CZTSSe) solar cells with an efficiency of 11.1% by the hybrid coating process [1].
We prepared Cu deficient Cu_2(1-x)ZnSnSe₄ and characterized their crystal structures by XRD and XAFS [2]. Then, we characterized their optical properties. The band gaps of the CZTSSe solid solutions determined from diffuse reflectance spectra of the powders and transmittance spectra of the films. The band gap (E_g) of the Cu₂ZnSn(S_xSe_1-x)₄ solid solution linearly increases from 1.05 eV for CZTSe (x=0.0) to 1.51 eV for CZTS (x=1.0) [3]. In our previous study, We fabricated high-density CIGSe films by a "printing and high-pressure sintering" (PHS) process and obtained a CIGSe solar cell with 3.2% efficiency [4]. In this study, we fabricated CZTSSe films by PHS process [5] and CZTSSe solar cells with the devise structure of Ag/ ITO/i-ZnO/CdS/CZTSSe/Mo/soda-lime glass.
Elemental powders such as Cu, Zn, Sn, S, and Se were weighted to give a molar ratio of Cu:Zn:Sn:S:Se=2:1:1:4x:4(1-x). Cu₂ZnSn(S_xSe_1-x)₄powders were synthesized by heating at 550^oC for 5 h in an N₂ gas atmosphere. Particulate precursor ink was prepared by mixing the obtained CZTSSe powder with an organic solvent. The precursor CZTSSe layer was deposited on a Mo coated soda-lime glass substrate by a screen-printing technique. The organic solvent was removed from the screen-printed film by heating in an N₂ gas atmosphere. The porous precursor layer was sintered into dense polycrystalline CZTSSe films by high-pressure sintering and post-annealing in an N₂ gas atmosphere.
Then, we fabricated the CZTSSe solar cells with a devise structure of Ag/ITO/i-ZnO/CdS/CZTSSe/Mo/soda-lime glass. A CdS buffer layer was formed by a conventional chemical bath deposition (CBD). The i-ZnO, ITO layers and Ag grid were deposited by a RF-sputtering technique. The I-V characteristics of the CZTSSe solar cells are estimated under standard conditions.
[1] T. K. Todorov et al., Adv. Energy Mater., DOI:10.1002/aenm.201200348.
[2] F. Gao, S. Yamazoe, T. Maeda, and T. Wada, Jpn. J. Appl. Phys. 51, 10NC28 (2012).
[3] F. Gao, S. Yamazoe, T. Maeda, K. Nakanishi, and T. Wada, Jpn. J. Appl. Phys. 51, 10NC29 (2012).
[4] T. Wada, J. Kubo, S. Yamazoe, A. Yamada, and M. Konagai, Proc. 25th EU-PVSEC/5th WCPEC, pp. 3465-3467 (2010).
[5] T. Wada, F. Gao, T. Maeda, and S. Yamazoe, Proc. 26th EU-PVSEC, pp. 2452-2455 (2011).

In order to search for new absorber materials for inorganic thin film photovoltaics that combine the avoidance of complex phase synthesis and the advantage of using non-poisonous and abundant elements, Bi₂S₃ films were grown on various substrates and examined for their suitability for solar cell application. Recent publications reported Bi₂S₃ to show an n-type conductivity, a wide range of conductivity of 10^-4-10² (Omega;cm)^-1 and a band gap down to 1.3 eV, which suggests its use for photovoltaic devices.
In the work presented, Bi₂S₃ thin films were deposited by thermal evaporation in an ultra-high vacuum system with a base pressure below 10^-7 mbar on both glass substrates and commercial TCOs at substrate temperatures of 80-400°C.
The influences of substrate material, deposition temperature and sulfur vapor pressure were investigated. In addition to structural, optical and transport measurements, detailed analyses were conducted using photoluminescence and photoelectron spectroscopy.
Thermally evaporated thin films showed a polycrystalline structure and band gaps of 1.3 -1.4 eV for elevated substrate temperatures (> 200°C); by Hall measurements, an n-type conductivity with a charge carrier density of asymp; 10¹⁵ cm^-3 and a mobility of 1-10 cm²V^-1s^-1 was confirmed. Photoluminescence spectroscopy revealed a clear increase of material quality with respect to its opto-electronic properties under a rise of substrate temperatures up to 400°C; furthermore a notable sensitivity to sulfur vapor pressures with a narrow optimum region was found by photoluminescence.
The first analyses of Bi₂S₃ by means of hard X-Ray photoelectron spectroscopy (HAXPES) were conducted at the PETRA III synchrotron. Clear effects of substrate temperature and vapor pressure on the electronic states could be observed and connected to respective trends investigated by photoluminescence spectroscopy. Moreover, HAXPES was used for electronic analysis of various Bi₂S₃/TCO interfaces.

Copper oxides are unusual when compared with other metal oxide semiconductors in that they demonstrate p-type behaviour. Cuprous oxide (Cu2O) has been studied extensively as a photovoltaic material, predominantly as a potential photo-absorber despite having a non-ideal band gap of ~2.1eV. Cupric oxide (CuO) has a band gap which is more useful and is combined with good electrical conductivity. Despite this there have been relatively few studies conducted on this material. Cupric oxide has potential not just as a photo-absorber but also as a base material for a range of p-type conductors, both transparent and non-transparent. If it can be successfully degenerately doped, it could be a useful p-type conductor in its own right. Combined with other metal oxides to increase the band gap, a range of p-type transparent conductive oxides (TCOs) and transparent semiconductors can be synthesised. This study provides an electrical, optical and structural analysis of intrinsic cupric oxide thin films with the aim of establishing a starting point for the investigation of these interesting materials. Cupric oxide thin films have been deposited on soda-lime glass substrates from a pre-formed stoichiometric ceramic target by sputtering using a radio frequency power supply. It has been found that changing the substrate temperature and oxygen partial pressure during deposition has a dramatic effect on both film conductivity and band gap. By carefully controlling the deposition parameters it is possible to produce films with electrical and optical properties to suit a range of applications including contact materials for thin film photovoltaic cells and LEDs.

Cuprous sulfide (Cu₂S) is a non-toxic and low-cost p-type thin film semiconductor and therefore reconsidered [1] as an alternative for CdTe or CuIn_xGa_1-x(S,Se)₂ in thin film solar cells. We have studied Cu₂S absorber layers prepared by physical vapor deposition (PVD) with thicknesses of approximately 500 nm and varying deposition parameters by calibrated spectral photoluminescence (PL) and by confocal PL with lateral resolution Δx asymp; 0.9 µm.
Calibrated photoluminescence experiments as a function of temperature T and excitation fluxes were performed to obtain the absolute PL-yield and to calculate the splitting of the quasi-Fermi levels (QFL) mu; at an excitation flux equivalent to the AM1.5 spectrum and the absorption coefficient α, both in the temperature range 20 K le; T le; 340 K. The PL-spectra reveal two peaks at E_low = 1.17 eV and E_high = 1.3 eV, whereas the low energy peak is only detectable at temperatures T < 200 K. The samples show an impressive QFL-splitting of µ > 700 meV associated with a pseudo band gap of E_g = 1.25 eV. The high energy peak shows an unexpected temperature behavior, namely an increase with rising temperature at variance with the behavior of QFL-splitting that decreases from extrapolated T = 0 K value of µ = 1.3 eV with rising T. The PL-yield versus temperature will be discussed in terms of radiative transitions in electronic 3- and 4-level systems.
Our observations indicate that, contrary to common believe, it is not the PL-yield, but rather the QFL-splitting that is the comprehensive indicator of the quality of the excited state in an illuminated semiconductor. A further examination of the lateral variation of opto-electronic properties by confocal PL and the surface contour shows no detectable correlation between Cu₂S grains/grain boundaries and the PL-yield or QFL-splitting. Additionally, the QFL-splitting as well as the pseudo band gap show a homogeneity over tens of µm.
[1] L.L. Kazmerski, F.R. White, G.K. Morgan, Appl. Phys. Lett. 29, 268 (1976)

Cost effective solar cells with high efficiency based on abundant non-toxic materials is the long term target of present research and development. Multijunction material saving thin film solar cells including nanostructure or nanoparticles is considered as an important option for future solar cell technologies. Several materials which have promising photovoltaic properties have been identified, e.g. copper(II) oxide (CuO) and copper(I) oxid (Cu2O) should fulfill the before mentioned requirements with suitable band gap energies of about 1.4 eV and 2 eV.
In this study commercially available CuO nanoparticles are investigated and modified in order to improve their electronic properties for use as active absorber material in solar cells. According to TEM results the particles are agglomerated with an average particle size of 30 nm. The absorption characteristics were determined by photothermal deflection spectroscopy (PDS). Electronic properties and microstructure were investigated by Photoluminescence (PL) and Raman spectroscopy as well as TEM.
Microscopic laser annealing experiments to improve the material quality were performed in air and N2 atmosphere. Laser annealing in air leads to an improvement of the material as can be deduced from the increase of the room temperature PL signal at 1.3 eV which is attributed to band-band recombination. The phase transition temperature from CuO to Cu2O could not be reached for the high oxygen partial pressure. Therefore laser annealing was done in N2. An improvement of the CuO was also observed while annealing in N2 atmosphere at low laser power. Further increasing of the laser power induces the phase transformation to Cu2O as the characteristic Cu2O Raman modes have been observed. PL at room temperature exhibits strong band edge luminescence as well as defect luminescence probably caused by copper and oxygen vacancies. We estimated the particles temperature within the laser focus by Raman spectroscopy via stokes-antistokes ratio as well as by the temperature induced peak shift of the characteristic modes. In order to monitor the influence of the thermal treatment on the absorption characteristics by PDS, samples were annealed in air as well as N2 atmosphere in an oven. PDS reveals an absorption edge at about 1.5 eV and a decreasing subgap absorption with increasing annealing temperature due to reduction of defects. PDS measurements of the samples annealed in N2 show the phase transformation at a temperature of 750 C indicated by a shift of the absorption band to about 2 eV. This is confirmed by Raman spectra and the band edge PL of Cu2O which increases by several orders of magnitude at annealing up to 900 C. Due to the strong PL Signal obtained from CuO and Cu2O we conclude that both materials are particularly suitable for photovoltaic applications.

The Cu2ZnSnS4 (CZTS) nanowrie array, which was a candidate for light absorbance layer of environmental friendly photovoltaic materials, have been successfully formed in an anodic aluminum oxide (AAO) template by two-step electroplating method. At first, we formed AAO template with 70 nm pore on Mo electrode by anodization of sputtered Al film. Then, CuSn and CuZn were alternately electroplated into AAO pores as a precursor of CZTS. The sulfurization of CuSn/CuZn nanowire was carried out by annealing at 650 oC in CS2 gas ambient.
We carried out characterizations such as morphology and crystalline structure and light absorption property of the CZTS nanowire arrays. The typical size of nanowire was about 70 nm in diameter and 1000nm in height. We observed enhancement of light absorption in the CZTS nanowire arrays compared with flat film sample. The results of absorbance for different size of CZTS nanowire arrays are also going to be presented on site, and the nanowire size dependence of light absorbance will be discussed.

Secondary phases are very often detrimental to the solar cell performance. In the quaternary CZTSe a large amount of binaries and ternaries can occur during growth therefore it is important detect and characterize them. We have investigated solar cell absorbers of different solar cell with conversion efficiencies ranging from 2 % to 6.3 %.
The absorber layers have been fabricated in a sequential process where in a first step Cu, Zn, Sn and Se are coevaporated at 320°C followed by a high temperature treatment in a Se and SnSe atmosphere. The films are analysed via depth resolved Raman spectroscopy, room temperature depth resolved Photoluminescence (RT-PL) and secondary ion mass spectroscopy (SIMS). In order to correlate the RT-PL and the Raman measurements with the sample composition the SIMS results have been calibrated to the composition given by energy dispersive X-ray spectroscopy (EDX) performed at 20 keV using a Monte Carlo simulation of the EDX detection profile.
A wide range of RT-PL transitions is seen however five peak energies namely 0.85 eV, 0.9 eV, 0.96 eV, 1.02 eV and 1.24 eV are sufficient to fit all spectra. The intensities and the ratios of these transitions dramatically change within the depth of the absorber and between different absorbers. The RT-PL peak at 1.24 eV is higher than the band gap of CZTSe thus related to a higher band gap material. According to the phase diagram and the composition ZnSe is the most probable secondary phase. This transition has been correlated to ZnSe before*. The presence of ZnSe in the sample is confirmed by 457.9 nm excitation wavelength Raman spectroscopy. The assignment of this 1.24 eV RT-PL transition to defect luminescence in ZnSe is strengthened by the correlation of the PL intensity to the ZnSe content derived from both the composition and the Raman spectroscopy. The peak at 0.85 eV correlates with the literature band gap of Cu2SnSe3. It appears at the surface of solar cell absorbers that show a low VOC and is therefore attributed to a Cu-Sn-Se phase with a lower band gap than the CZTSe. The peaks at 0.90, 0.96 and 1.02 eV are preliminarily attributed to kesterite and stannite phases with an additional strong defect luminescence. This attribution is supported by the comparison with the band gap obtained from quantum efficiency measurements, which indicate the band gap of the main transport channel, by known differences in band gap between kesterite and stannite and by a detailed analysis of the Raman spectra, which indicates the presence of stannite and kesterite phases.
*Redinger, et al. Photovoltaics, IEEE Journal of 1(2) 200-206.

SnSe thin films of 50 and 200 nm were produced by thermal evaporation of SnSe precipitate. The films obtained are amorphous. A post-deposition heating for 30 min in nitrogen (10 Torr) at 300 °C, makes the films crystalline. The films have an optical band gap of 1.5 eV, p-type conductivity and photoconductivity σ of 0.26 (Omega; cm)^-1. According to calculations assuming one electron-hole pair per photon absorbed, solar cells using a 200 nm SnSe thin film as absorber can generate a current density (J_L) of 28 mA/cm² (AM1.5G). In the present study, as the cell structure of TCO/CdS/SnSe was found to be unstable, we have used thin films of antimony chalcogenides (200 nm) as an additional absorber with SnSe in solar cells. In TCO/CdS/Sb₂S₃/SnSe/C cell heated in a nitrogen atmosphere at 300 °C for 30 min, V_oc of 607 mV, J_sc of 3.9 mA/cm², eta; of 0.6 %, and FF of 0.33 are obtained. When the SnSe thickness is 50 nm, J_sc is only 2 mA/cm². This establishes the role of SnSe as an optical absorber contributing to the photo-carrier generation. When an Sb₂(S,Se)₃ solid solution replaces Sb₂S₃ in the solar cell structure, J_sc is significantly increased to 10.25 mA/cm² due to a reduction in band gap from 1.8 to 1.5 eV of this absorber, but due to the same reason, the V_oc drops to 275 mV. The overall cell efficiency is higher, 0.88 %, and the fill factor is practically unchanged, 0.31. When a CdSe thin film is added at the CdS/Sb₂(S/Se)₃ interface, the fill factor and efficiency are improved. Having processed the cell structures at 300 °C, these cell parameters are retained over a long period of observation and measurement.

Pyrite (FeS₂) is a non-toxic, earth abundant chalcogenide with desirable characteristics for application as a thin film photovoltaic absorber including a modest band gap of 0.95 eV and a large optical absorption coefficient (>10⁵ cm^-1). Conventional synthesis approaches employ thermal sulfurization of iron-based films or precursors. These routes inherently produce substoichiometric contaminate phases (Fe_1-xS) that once formed, are difficult to completely remove. Thermodynamics suggests that hematite (α-Fe₂O₃) may be directly transformed to FeS₂ in the presence of sufficiently high sulfur activity. In this work, we demonstrate pyrite synthesis using a H₂S plasma to sulfurize hematite nanorods. Controlled delivery of sulfur via plasma exposure time resulted in a systematic increase in film stoichiometry from 0.0 - 2.0 S:Fe atomic ratio as measured by calibrated energy dispersive X-ray spectroscopy. The application of plasma dramatically enhanced both the rate of sulfurization and the quality of the resulting material. Complete conversion from Fe₂O₃ to FeS₂ was achieved at a moderate temperature of 400 °C and a chalcogen partial pressure <6 × 10^-5 atm. Raman and X-ray photoelectron spectroscopy were used to track sulfur incorporation and supported a direct solid-state transformation to FeS₂. The presence of FeS phases, which are readily detected during thermal processing, are absent in the plasma-assisted process. The apparent direct optical band gap of the film systematically decreased from 2.2 to ~1.2 eV with increasing sulfur content, and the stoichiometric films displayed high absorption coefficients (~10⁵ cm^-1) above the band gap. Preliminary photoelectrical characterization showed room temperature conductivity of the FeS₂ layers was on the order of 10^-4 S cm^-1 and approximately doubled under calibrated solar illumination.

Cu₂ZnSnSe₄, or ‘CZTS&’, is being widely developed as an earth-abundant solar absorber. As a photoabsorber layer, several processing challenges present themselves to achieve the desired kesterite phase while maintaining the control over composition to tailor properties. Among these challenges include phase segregation, grain boundary issues, and control of the interface with other device layers. Furthermore, the composition of the film is employed to engineer the band gap and achieve the highest efficiency devices. Scanning transmission X-ray microscopy (STXM) offers the ability to map the chemical and electronic structure of materials on length scales less than 50 nm by collecting near edge X-ray absorption fine structure (NEXAFS) spectra at each image pixel. The NEXAFS is a lifetime and experimentally broadened projection of the local-unoccupied density of states, which can be used to interpret the element-specific electronic and physical structure of a material. We report on preliminary studies CZTS layers and devices examining the Cu, Zn, and Se L_2,3 edges across several different points of the device stack. We probe the L levels to provide more information regarding the local electronic and atomic structure near grain boundary regions and possible defect structures throughout the layer. The results are described in context of the processing conditions in effort to achieve higher device efficiencies.

Iron (II) disulfide is a semiconductor that has considerable promise as the absorbing component of solar conversion modules. Pyrite is an attractive photovoltaic material (0.95 eV) because of the high abundance, low cost, and environmental compatibility of its constituent elements. With a large absorption coefficient in the visible region (6.5x10⁵ cm^-1), very thin films may be used reducing materials cost. In this work, we explore low temperature hydrothermal synthesis conditions for the growth of polycrystalline pyrite thin films.
Our strategy employed gold on silicon as the substrate for film growth. In principle, the strong affinity of sulfur or sulfide for gold would promote strong adhesion between pyrite and the substrate. To prepare the wafers, gold (50 nm) is deposited onto a wetting layer of chromium (5 nm) by thermal evaporation onto single crystal silicon (100). The resulting polycrystalline gold surface has a dominant {111} texture. Synthesis of iron (II) disulfide follows the methods of Wu et al and Wadia et al, which were designed for nanoparticle synthesis. In the first method, the substrate is placed in a Teflon lined autoclave and immersed in an aqueous solution of FeSO₄, Na₂S₂O₃, and sulfur. The Parr reactor is sealed and held at temperature (150-230 ^oC) for 16 hours. The effects of temperature and initial pH on the final phase composition and film morphology were investigated. In the second method, the substrate is placed in a Teflon lined autoclave and immersed in an aqueous solution of FeCl₃#9679;6H₂O and (C₂H₅O)₂P(S)SNH₄ with and without the surfactant (C₁₆H₃₃)N(CH₃)₃Br, CTAB. The Parr reactor is sealed and held at 230 ^oC for 4 hours. The effects of reactant concentration and presence of surfactant were investigated.
Iron (II) disulfide films on gold from the first method range from primarily marcasite at low temperatures to a dominant pyrite phase at high temperatures. Films from the second method yield a dominant pyrite phase with the surfactant affecting particle morphology. Scanning electron microscopy (SEM) imaging shows that the polycrystalline films are continuous, uniform over a majority of the 1” wafer, and composed of densely packed micron sized particles with well-formed facets. Film thickness depends on preparation and was found to lie within 1-10 microns. Powder x-ray diffraction was measured by grazing angle and Bragg-Brentano configurations showing evidence of preferred orientation for marcasite and pyrite with the first and second methods, respectively. While the pyrite phase was randomly oriented within films from the first method, marcasite had a dominant (101) or (020) texture. Films from the second method are predominately pyrite phase and showed the surfactant affecting surface roughness, defect concentration and amount present of the (100) facet. A detailed summary of the SEM, XRD, EBSD, and optical spectra results will be presented.

We report synthesis of colloidal metallic nanoparticles using liquid-phase pulsed laser ablation, and electrophoretic deposition of the nanoparticles for fabrication of Cu₂ZnSnSe₄ (CZTS) thin film solar cells. First, colloidal metallic nanoparticles of Cu-Zn-Sn (CZT) alloys are produced by pulsed laser ablation of a bulk alloy target in common organic solvents. The nanoparticles are found to be electrostatically stabilized against agglomeration without addition of stabilizing ligands. The colloid&’s electric charging mechanism and modulation of nanoparticle surface charge are systematically investigated. The nanoparticles are also examined to identify their phases and composition. Precursor CZT thin films are fabricated by electrophoretic deposition (EPD) in the as-made colloids without transferring to another solvent and without using binders. Under an external electric field applied in the solvent, the deposition is facilitated by the eletrokinetic mobility acquired by the electrically charged nanoparticles. Compact, crack-free, and high purity CZT films are obtained after optimization of the deposition process. Particular advantages of the nanoparticle EPD approach include high deposition rates in the micron/min range and compliant coating on flexible substrates or complex surfaces. Finally, after selenization and coating of CdS window layer and top conducting layers, CZTS solar cells on flexible metal substrates are fabricated with an energy conversion efficiency up to 4.77%. We will also report high resolution transmission electron microscopy (HRTEM) characterizations of the CZTS thin film extended defects such as phase segregation and grain boundaries (GBs) which are the predominant defects affecting the carrier transportation and recombination. With this work and our previous work on CIGS solar cells fabricated in the same approach, we will demonstrate a new route for non-vacuum fabrication of chalcopyrite thin film solar cells. This approach is fast and clean, requires no hazardous chemicals, produces minimal waste, and is implementable to large-scale flexible substrates.

Vapor phase chalcogenisation of Cu-Sn-Zn metallic precursors is a potentially low cost and scalable method of thin film Cu2ZnSn(S,Se)4 (CZTSSe) fabrication. However different absorber properties were achieved using different stacking orders of metals.[1-3] None of the published studies explained why some metal stacking orders are successful whilst others are not. Previously we investigated the influence of selenium partial pressure inside the reaction chamber on the reaction pathway.[4] It was shown that preferably all elements must be present in the same place in the same time to avoid the formation of secondary phases. Secondary phases are known to be detrimental to device performance and thus a synthesis route which minimized secondary phase formation or which made them accessible for easy removal by etching processes is highly desirable.[5] We will show that different precursor stacking orders lead to different spatial distributions of secondary phases in the annealed absorber.
Our hypothesis is that for any metal precursor stack where Sn or Zn are not in direct contact with sufficient Cu to form a stable alloy phase (e.g. Cu6Sn5 or Cu5Zn8) they will tend to diffuse to find the Cu that they need. This alloying is slow at room temperature, but accelerated during the heating ramp of the annealing step, before the chalcogen has time to react with the precursors. As a consequence two dimensional or three dimensional metallic structures emerge before the chalcogenisation step and this leads to different formation routes during the chalcogenisation step. Using mass transport controlled electrodeposition and soft annealing we synthesize CuSnZn alloys. Depending on the way the layers are deposited two dimensional and three dimensional structures can be formed. We present chemical and structural experimental evidence on the sub micron scale for the two dimensional and three dimensional precursor structures, as well as the consequence of this during the chalcogenization step for the absorber layers. For exactly the same annealing conditions it will result in different absorber homogeneity. These new insights can be used to explain why some of the synthesis routines described in literature yield much better efficiencies than others.
[1] Araki et al., Thin Solid Films, 517 (2008) 1457-1460,
[2] Yoo et al., Thin Solid Films, 518 (2010) 6567-6572.
[3] Ahmed et al., Advanced Energy Materials, 2 (2012) 253,
[4] Arasimowicz et al., E-MRS Spring Meeting 2012,
[5] Wätjen et al., Applied Physics Letters, 100 (2012) 173510

To be capable of harvesting solar energy on a scale large enough to meet global energy demands, photovoltaic materials must be abundant and cost-effective. In_xGa_1-xN is an example of a tunable band gap material that is valuable for solar cell use, but In incorporation limits scalability, and growth of In_xGa_1-xN with large x has been hindered by phase separation. Instead, we propose replacing the group III elements with a combination of group II (Zn) and group IV (Sn, Ge) elements, which are abundant and low-cost. Calculations predict that ZnSn_xGe_1-xN₂ will have a band gap this is tunable over a range of about 1.4 eV to 2.9 eV, which still covers a large portion of the solar spectrum. Another advantage of ZnSn_xGe_1-xN₂ is the smaller lattice mismatch between ZnSnN₂ and ZnGeN₂, suggesting that phase separation will not be as energetically favorable.
Thin films of ZnSn_xGe_1-xN₂ were deposited on c-sapphire using reactive RF magnetron sputtering with varying x measured by energy dispersive spectroscopy. First, films with x = 1 are characterized because there are no reports of ZnSnN₂ synthesis in the literature, and the expected band gap of about 1.4 eV makes this material important for photovoltaics. X-ray powder diffraction measurements of ZnSnN₂ reveal peaks that match with a wurtzite-derived Pna2₁ orthorhombic unit cell, as has been shown for ZnGeN₂. The lattice parameter of our ZnSnN₂ is derived from 2theta; X-ray diffraction measurements and from selected area diffraction in a transmission electron microscope, and matches well with the expected values.
On c-sapphire, ZnSn_xGe_1-xN₂ films are predominantly oriented with <001> normal to the interface. X-ray diffractograms reveal an increasing (002) peak position in 2theta; as x decreases, indicating an absence of any observable phase separation between ZnSnN₂ and ZnGeN₂. The optical band gap is estimated from absorption obtained using spectroscopic ellipsometry, and increases from ~2.2 eV to ~3.1 eV as x decreases. Films with x = 1 have a larger band gap than expected, attributed to the Burstein-Moss effect, which is further evidenced by a large donor carrier concentration from Hall measurements.
We have synthesized thin films of ZnSn_xGe_1-xN₂ with a wide range of x values and no observable phase separation, demonstrating a tunable band gap. We believe that ZnSn_xGe_1-xN₂ is a promising low-cost and earth-abundant alternative to In_xGa_1-xN for photovoltaics.

Solution processing of semiconductor devices is a promising route to manufacture low-cost photovoltaics. Electrochemical deposition in particular is used in a number of hybrid and solid state devices. In the case of fully electrochemically processed devices, one layer is electrodeposited on a conducting substrate and subsequent layers are deposited on top of the pre-existing stack. For example, in ZnO/Cu₂O devices, ZnO is electrodeposited on ITO, and Cu₂O is grown on the ITO/ZnO layers. Thus, the ZnO film properties (such as carrier concentration, film density, and orientation) can dictate the Cu₂O growth and have a heavy influence on the device properties. In this investigation, we vary the ZnO deposition temperature and are able to increase the open circuit potential (V_OC) of ZnO/Cu₂O devices by > 80 %. For any solar cell, V_OC is heavily influenced by interface states, shunting pathways, as well as the film carrier density which defines the depletion region. We use impedance and admittance spectroscopy to examine how each of these properties change with the ZnO deposition temperature and correlate their behavior with the increase in V_OC.

Fully electrochemically deposited (ED) ZnO/Cu₂O solar cells are inhibited by a collection length (< 1mu;m) which is much less than the depletion width (~3 mu;m) of the device. This discrepancy in length scales makes the simultaneous achievement of efficient charge collection and high open-circuit voltage difficult to obtain. Using a nascent deposition method, Atmospheric Atomic Layer Deposition (AALD), we show that Cu₂O can be grown with a carrier concentration approximately 2 orders of magnitude higher than the ED Cu₂O film. Using both of these low temperature, scalable techniques we are therefore able to fabricate an ED-ZnO/ED-Cu₂O/AALD-Cu₂O⁺ device with a reduced depletion width that is commensurate with the collection length of the Cu₂O. Modeling shows that the device design builds in a high, uniform electric field which increases the charge collection from both incident light and light reflected off the back electrode. Using this structure, we achieve a 28 % improvement in device efficiency compared to a fully-ED cell and obtain the highest reported short circuit current density (6.32 mA cm^-2) for this system grown under atmospheric conditions.

The Zn(1-x)Mn(x)S thin films could be considered as potential window layer material in heterojunction solar cells. One of the most important problems of hetero-structures formation is lattice mismatch between layers. In order to avoid large lattice mismatch the solid solutions of the II-VI compounds can be used. Introduction of the impurities allows controlled variation of the lattice parameter, and hence provide low lattice mismatch at layer interfaces.
In present work we have studied influence of the Mn concentration on structural properties of the ZnS films. The Zn(1-x)Mn(x)S fims were deposited from the powder which contains 30% of the Mn on glass substrate.The evaporation temperature was 1473 K and substrate temperature changed from 423 to 723 K. The chemical composition of the samples was studied by the X-ray spectral microanalysis, phase analysis and investigation of substructural parameters (scattering domain size, concentration of the dislocation and microstresses) were performed by the X-ray diffraction.
It was determined that thin films contain two phase, namely cubic Zn(1-x)Mn(x)S and small amount MnS phase. The Zn(1-x)Mn(x)S phase has high-oriented [111] texture. The Mn concentration is monotonically decrease from 5.8 to 1.7 atm % with substrate temperature increasing from 373 K to 723 K. Whereas (Zn+Mn)/S ratio is increasing from 0.22 from 1.57 atm% within 373-623 K temperature range, then decrease to about 1.08 atm% at 723 K substrate temperature.
The XRD study shows that lattice parameter of the samples is changes from the 0.56322 to 0.56522, level of microstresses from 0.393 to 3.88 E-3 , scattering domain size from 38.18 to 208.66 nm and concentration of the dislocation from 4.97 to 8.53 E+14 lin/m^2. As a result the optimal growth conditions of the Zn(1-x)Mn(x)S with controllable Mn concentration was determined.

The SnS compound semiconductors thin films is promising material for the heterojunction solar cells due to optimal band gap energy (1.4 eV) and high light adsorption coefficient (105 cm-1). Also SnS composed of abundant nontoxic element, which makes the use of this material more beneficial than CdTe and CIGS.
We report results of structural study of the SnS films obtained by the close spaced vacuum sublimation. The samples were deposited on the cleaned glass substrate from the pre-annealed SnS2 powder.
The substrate and evaporator temperatures were varied from 573 to 673 K and from 773 to 973 K. Some samples were annealed after the deposition during 30 min also.
The structural study of the SnS films was performed by the X-ray diffractometr in the range of diffraction Bragg angles from 20 to 80 using a Ni-filtered Cu radiation source. The phase analysis was carried out by the comparison of interplane distance and peak intensity values with the JCPDS reference data. The texture of the thin films was estimated with the help of Harris method.
It was determined that obtained thin films consist of two phase with different concentration, namely SnS and SnS2 which have orthorhombic and hexagonal structure. The XRD study revealed high-oriented [111] texture of the films. Also analysis of the XRD data of the samples deposited under different growth conditions shows that concentration of SnS2 phase is increased with substrate temperature decreasing. Thereby the optimal growth and annealing conditions of single-phase SnS films were determined in present study.

Pyrite, FeS₂, is a non-toxic, earth abundant semiconductor that offers several potential advantages as a photovoltaic material, including low cost, large absorption coefficients and band gaps that are suitable for the harnessing of solar energy. Conventional thin film deposition techniques require the use of a post-deposition annealing step in elemental sulfur in order to achieve stoichiometric material. This extra step requires precise control over sulfur exposure, time, and temperature, and it would be desirable to eliminate this cumbersome process. In this talk we introduce pulsed plasma-enhanced chemical vapor deposition (PECVD) as an alternative technique for thin film pyrite synthesis. In pulsed PECVD a mixture of iron pentacarbonyl (IPC, Fe(CO)₅) diluted in H₂S is delivered continuously to the reactor while the plasma is pulsed using square wave modulation at low frequency (~1 Hz). The concept is that IPC absorbs during the plasma off step, and that it is fully sulfurized in situ during the plasma on step. The process offers digital control over thickness with control on the order of ~1 Å/pulse. In this work we describe the dependence of pyrite deposition rate and material quality as a function of relevant variables such as H₂S:IPC ratio, pulse sequence, plasma power, and substrate temperature. Films are characterized using a suite of analytical techniques including Raman, XRD, FESEM, and UV-Vis-NIR spectrophotometry, and we report on the process-structure-property relationships in this system.

Famatinite is a relative new class of sustainable absorber material, utilizing cost effective and abundant elements. Band gap engineered, modified Famatinite thin films and nanocrystals were synthesized using novel solution based methods. A testing analog of copper antimony sulfide film-electrolyte interface was created in order to evaluate photoelectrochemical performance of thin film of absorber materials. Many different redox couples were selected for this purpose, based on their relative band offset with copper antimony sulfide. A detailed investigation has been carried out by changing stoichiometry of films and corresponding surface and optical characterization results has been evaluated. A summary of favorable processing parameters of films showing enhanced photoelectrochemical response is presented.

Cu(In,Ga)Se2 (CIGS) solar cells exhibit high conversion efficiencies of over 20% and have become a viable option for large-scale power generation. CIGS thin films are grown via chalcogenization of sputtered metal precursors for large-area application and commercialization. However, In and Ga, which are used in the preparation of the absorber, are very rare and expensive elements.
Recently, an alternative ternary sulfide, Cu2SnS3 (CTS), has been considered as a promising candidate for photovoltaic applications. CTS has a high absorption coefficient of 10^4 cm^-1, and its bandgap energy is reported in the range of 0.9-1.8 eV. Small-area solar cells with efficiencies of less than 3% have been fabricated. The use of nontoxic and abundant elements is the main reason for the interest in this material as an alternative to thin film solar cell technologies based on CIGS. Furthermore, CTS thin films also can be grown via the chalcogenization process. Therefore, it should be possible for CTS thin films to be utilized for large-area applications. In addition, it should be easy to control the composition of CTS compared with CIGS or Cu2ZnSn(S,Se)4 (CZTSSe) because CTS consists of only three elements.
However, only a few reports on the preparation of CTS thin films have been reported. Without understanding the fundamental physical properties of CTS, it is impossible to make a breakthrough similar to what was achieved with CIGS solar cells. In this presentation, the growth of CTS thin films by sulfurization of sputtered Cu-Sn metal precursors will be described. CTS thin films were grown at around 420C by simple sulfurization process and had approximately 2-mu;m-diameter grains. The result is the first step toward realizing CTS related solar cells.

Cu(In,Ga)(S,Se)2 (CIGSSe), Cu2ZnSn(S,Se)4 (CZTSSe), and SnS have a high absorption coefficient and a suitable direct bandgap energy for solar cells. These solar cells are able to be fabricated by chalcogenization. Chalcogenization is commercially the most desirable process because it can be used to prepare large-area films economically with a simple dry process. However, CIGSSe has some problems such as the high cost of In and Ga and the toxicity of Se. One problem with CZTSSe is that it is not easy to grow CZTSSe without any secondary phases, because it is a quaternary compound semiconductor. On the other hand, SnS is composed of elements that are economical, safe for both the environment and the human body, and abundant in nature. Furthermore, SnS is easy to control the composition because it is a binary compound. SnS solar cells have been fabricated by sulfurization using the S element.
The mechanisms involved in the fabrication of CIGSSe and CZTSSe by chalcogenization have been clarified. In contrast, the growth mechanism of SnS fabricated by sulfurization has not been clarified yet, because SnS thin films are poor quality. In this presentation, SnS thin films will be grown to control the carrier density for preparing high quality SnS solar cells.
Sn layer was prepared by RF sputtering method on a soda-lime glass substrate. It was sulfurized using S vapor in a quartz tube reactor. The sulfurization temperature and time were 150-540C and 40-90 min, respectively.
The carrier density of SnS thin films was controlled by the atomic ratio of S/Sn. This result may indicate that tin vacancy (VSn) acts as a shallow acceptor and is mainly responsible for the p-type conductivity of SnS. The carrier density of SnS thin films grown by controlling the molar fraction of S vapor was changed from 1015 cm-3 to 1018 cm-3.

Wet chemistry and its possibility to use roll to roll deposition is a promising approach for cost reduction in thin film photovoltaics. It relies on dispersions or inks containing molecular or particulate precursors for, e.g., CIGS or CZTS thin films. In the case of particle-based kesterite precursors, different possibilities exist to form nanoparticle thin films that can be transformed into dense CZTS. Here, we present synthesis routes based on colloidal chemistry that result in quaternary kesterite particles, CZTS or CZTSe, ternary compounds, such as Cu2SnS(e)3, Cu2ZnS(e)2 or ZnSnS(e)x) or binary compounds (CuxS(e), ZnS(e), SnS(e)x). Importantly, we show that all nanoparticle precursors can be stabilized by carbon free, inorganic ligands. This enables various combination of precursors to be explored, and thus optimize the formation of dense CZTS(e) thin films with little, if any, carbon contamination

The kesterites Cu₂ZnSn(S,Se)₄, which are composed of earth abundant elements and with a high absorption coefficient, have been shown as an attractive candidate for thin film photovoltaic solar cells. The highest efficiency reported to date, 11.1 %, made by using hydrazine solutions [1], is still far away from the 20.3% -efficiency achieved for Cu(In,Ga)Se₂ solar cells [2]. One of the main difficulties found for these compounds is the existence of a narrow region of single phase kesterite, which seems to be crucial for obtaining efficient devices.
In the present work, Cu₂ZnSnS₄ (CZTS) thin films have been grown onto Mo-coated glass and soda-lime glass by flash evaporation at low substrate temperature (100° C). Afterwards, a thermal treatment was carried out in Ar atmosphere [3]. Bulk compounds were previously synthesized in order to be used as evaporation sources. The thin films grown by using the CZTS bulk compound and ZnS, SnS and CuS binary compounds have been compared. Different chemical reactions were investigated to obtain the desired kesterite composition for solar cells. A preferential re-evaporation of Zn has been observed during the flash evaporation process independent of the evaporation source used. The compositional, structural, morphological and optical properties of the thin films have been investigated before and after the thermal treatment. An enhanced cristallinity, a band gap energy of around 1.5 eV and large-grain structures were obtained after the annealing treatment. The formations of a MoS_x layer and voids sometimes at the Mo/CZTS interface have been observed by scanning electron microscopy. The effect of the thermal treatment on the formation of the back interface will be discussed.
[1] T.K. Todorov et al, Adv. Energy Matter 2012, doi: 10.1002/aenm.201200348.
[2] P. Jackson et al, Prog. Photovolt: Res. Appl. 19 (2011) 894.
[3] R. Caballero et al, Thin Solid Films, doi: 10.1016/j.tsf.2012.10.028

Cu2ZnSnS4 (CZTS) is a promising semiconductor to be used as the absorber layer in thin film solar cells due to its main properties: band gap close to 1.5 eV and absorption coefficient higher than 10^4 cm-1. The grown films tend to be p-type, and the fact that the compound involves only abundant elements with low toxicity levels, favors its use as a realistic alternative to Cu(In,Ga)Se2.
In this work we studied CZTS thin films in order to evaluate the influence of the growth parameters on their optical properties. The films were prepared through sulphurization of metallic precursor stacks. The number of Zn/Sn/Cu periods of the metallic precursors was changed (1, 2, 4) and the sulphurization was performed either in a graphite box or under sulphur flux. The investigation of optical properties was done through photoluminescence measurements in the temperature range 7-300 K and under different excitations powers.
Higher luminescence intensity was obtained for the sulfurization in S flux than in a graphite box, as shown from the significant increase of the signal-to-noise ratio. Additionally, for both methods of sulphurization, the increase of the number of the periods of the metallic precursors resulted in an increase of the intensity. These results were correlated to the evolution of the average grain size observed from cross-section micrographs obtained in scanning electron microscopy studies of all samples. The fabrication of solar cells revealed that the highest efficiency was obtained for the CZTS film formed through sulphurization in S flux of four periods of metallic precursors. The optical properties of this film, which showed a composition close to stoichiometry, as obtained from energy dispersive X-ray spectroscopy, were investigated more deeply. The dependences of the emission on temperature and excitation power allowed the discussion of the nature of the radiative recombination as well as the non-radiative channels responsible for the thermal quenching of the luminescence. The observed behavior cannot be explained by donor-acceptor pair transitions. Nevertheless, the results including the asymmetric shape the emission can be interpreted considering the existence of potential fluctuations.
The results of this study are relevant in the optimization process of the growth of CZTS thin films.

Several families of soluble precursors for preparation of Cu2ZnSn(S,Se)4 (CZTS) have been explored in fulfilling low cost and large scale production of photovoltaic during last decades. Hydrazinium chalcogenidometallates, organometallic compounds or nanocrystals have been investigated and some of which even demonstrated higher efficiency than that prepared from vacuum deposition method. The sacrificial groups or ligands exited in these precursors help bind to metals or chalcogen ions to provide solubility in desired solvents, but elimination of these species during thermal decomposition results in substantial volume contraction or undesirable impurities. Here, we demonstrated a novel pathway from soluble precursors to CZTS for thin film optoelectronics application. These soluble precursors are composed of colloidal nanocrystals and metal chalcogenide complexes (MCCs) as the surface ligands. Zn/Sn chalcogenide complex is presented as an instance of MCCs, in which Zn/Sn ligand both provide colloidal stability and are essential components for CZTS. Cu2-xS or Cu2-xSe nanocrystals can be capped by Zn/Sn complex to form CZTS conveniently, which avoid the complexity of mixing Cu2-xSe and ZnS nanocrystals with Sn2S64- or Sn2Se64- ligands. The ratio of Cu, Zn and Sn in CZTS phase, which is very critical for the photovoltaic performance, can be simply controlled by the ratio of Cu2-xS nanocrystals and Zn/Sn complex that ratio of Zn/Sn is judiciously selected and fixed. X-ray diffraction and Raman spectroscopy are employed to determine the formation of well-crystallized quaternary phase during annealing, when the inorganic ligands are reacting with NC cores. Typical p-type transport of CZTS is observed by field-effect transistor measurement, that the drain current is increased by applying negative gate voltage. CZTS films processed in this method feature larger grain size and higher phase purity than ones from traditional precursors, which have confirmed the prospects for economical and practical applications.

Copper sulfide (Cu2-xS) films of nanometer thickness have been grown by atomic layer deposition (ALD) and their structural, optoelectronic, and surface properties investigated as a function of time and storage environment. As-prepared and prior to exposure to room ambient, low conductivity films are obtained. Exposure to air results in a rapid rise in conductivity due to heavy p-type doping combined with only a slight reduction in mobility. Storage in a < 0.1 ppm oxygen and water atmosphere dramatically slows but does not halt the rise in conductivity with time. The evolving electrical properties are correlated with a change in both crystalline phase, optical properties, and surface chemistry. Metal oxide overlayers grown by atomic layer deposition are shown to dramatically slow the changes, even under ambient conditions. These results inform the search for a highly earth-abundant and non-toxic solar absorber with optimal bandgap.

While many materials have been synthesized by ALD, the technologically important metal sulfides are underexplored, and homogeneous quaternary metal sulfides are absent from the literature. We present ALD process to synthesize Cu2ZnSnS4 (CZTS), a potentially low cost semiconductor being explored for photovoltaic applications. Two approaches to ALD thin films are taken: one in which a trilayer stack of binary metal sulfides (i.e., Cu2S, SnS2 and ZnS) is deposited and mixed by thermal annealing, and one in which supercycle strategy is utilized. Both routes rely on the solid state diffusion of chalcogenides for mixing. Challenges to ALD growth include nucleation, ion-exchange between the film and the volatile chemical precursors, and phase-stability of binary SnS2. The thin films were made with no sulfurization step. Photoelectrochemical measurements under simulated AM1.5 illumination using Eu3+ as an electron acceptor demonstrate that the films are photoactive.

Cu₂ZnSn(S,Se)₄ (CZTSSe) is a material of high interest in the field of solar cell applications. Besides the thin film technology, where the absorber layer is a polycrystalline thin film of the semiconductor, crystalsol applies a technology where micrometer scale (50-100µm) single crystals (“monograins”) are fixed in a polymer matrix to form the core of a flexible solar cell.
CZTS and CZTSe crystallize in the tetragonal kesterite type structure [1, 2], which is characterized by alternating cation layers of CuSn, CuZn, CuSn, and CuZn at z=0, 1/2, 1/2, and 3/4 in the direction of the crystallographic c-axis. A differentiation between the isoelectronic cations Cu⁺ and Zn²⁺ is not possible using lab X-ray diffraction methods due to their nearly similar scattering power. But their neutron scattering lengths are different, thus neutron diffraction opens the possibility to answer the question of isoelectronic cation distribution [3].
Using CZTSSe monograin powder in a neutron diffraction experiment, the unique opportunity to investigate an absorber material directly used in a solar cell is given. Here we present the results of a structural investigation of CZTSSe monograin powder based on neutron diffraction experiments. We focused on intrinsic point defects, which have an important impact on the monograin device efficiency.
The samples were prepared from binary compounds in the liquid phase of flux material in evacuated quartz ampoules. To determine the chemical composition of the powder samples, wavelength dispersive X-ray measurements (WDX) have been performed using a JEOL-JXA 8200 electron microprobe analysis system (EMPA). Calibrated elemental standards were used to determine the chemical composition of the samples as precisely as possible. The observed backscattered electron micrographs, recorded during EMPA analysis, gave reliable information about the chemical homogeneity and the phases present in the prepared material. Neutron powder diffraction data were recorded at the high resolution powder diffractometer at the Berlin Research Reactor BERII. The neutron data treatment was performed by full pattern Rietveld refinement using the kesterite structure as starting model in the analysis. This procedure resulted in the determination of lattice parameters and cation site occupancies. In order to determine the cation distribution in the crystal structure of the CZTSSe monograins, which is the basis for the evaluation of the intrinsic point defects, the method of the average neutron scattering length was applied [4]. The presentation is rounded by a discussion of the evaluated intrinsic point defects concentrations.
[1] S. Schorr et al., Europ. J. Mineral. 19 (2007) 65
[2] S. Schorr, Sol. En. Mat. Sol. Cells 95 (2011) 1482
[3] S. Schorr et al., Ad. En. Mat. 13 (2011) 737
[4] S. Schorr et al. in: Advanced characterization techniques for thin film solar cells, ed. by D. Abou-Ras, T. Kirchartz, U. Rau, Wiley, 2011

Thin film solar cells are developed by thermal evaporation of Sb₂S₃ on chemically deposited CdS(80-120 nm) thin films prepared on transparent conductive oxide (SnO₂:F) coated glass. Colloidal graphite paint is applied on Sb₂S₃, and the entire cell structure is heated at 290 °C in nitrogen for 30 min. Subsequently colloidal silver print is painted on the graphite electrode and dried at 75 °C for 30 min to complete the cell fabrication process. These cells show V_oc 670 mV, J_sc 4.3 mA/cm², and conversion efficiency 1.0% for Sb₂S₃ film thickness 115 nm. At higher or lower film thickness, the cell parameters are inferior. The optical absorption coefficient > 10⁵ cm^-1 at photon energy 2-3 eV of a 115 nm film with direct gap 1.52 eV (forbidden transitions) suggests light-generated current density of 16.2 mA/cm² (AM1.5G solar radiation) for this absorber thickness. The shortfall in J_sc is ascribed to collection loss of photo-generated carriers due to small crystallite grain diameters, 20 nm, persisting even upon heating the film. Various methodologies are being implemented to overcome this: heating the film in sulfur-rich atmosphere; use of an appropriate flux to aid grain-growth in the film; replacing the back contact of graphite paint with evaporated carbon films, or use of alternate back contacts to aid efficient collection. The use of Sb₂(S/Se)₃ solid solution thin film as absorber is yet another basic approach. These methodologies have direct relevance to the development of antimony chalcogenide absorber films for a scalable PV technology, in view of low cost, abundance and low toxicity of antimony.

CuSrN is one of the few thermodynamically stable Cu-N-based compounds and crystallizes in the NiBaN-type structure. This crystal structure is composed of linear chains of Cu-N, kinked at every 3rd N atom, with N atoms being tetrahedrally coordinated to Sr atoms. Such rather exotic structure results in peculiar electrical and optical properties for CuSrN. We present a theoretical investigation of these properties by means of advanced ab initio calculations, showing the photovoltaic relevance of such material as a thin-film Earth-abundant absorbing layer. Our GW-based calculation shows that CuSrN has a direct band gap of 1.0 eV while its absorption onset lies at 1.2 eV. Defect calculation reveals a potential moderately p-type doping due to the low formation energy of Cu vacancies. Moreover, stability with respect to air or water was also investigated and subsequently, the critical limitations of CuSrN were addressed. Overall, this study uncovers the possibility of a new paradigm for thin-film solar cells based on Cu-N materials as the absorbing layer.

It has previously been shown that germanium alloying of Cu2ZnSn1-xGex(S,Se)4 (CZTGeSSe) absorber layers can lead to significant efficiency improvements in kesterite-based thin film solar cells. With a germanium content of 30% device conversion efficiencies up to 9.4 % have been achieved, using a low-cost nanocrystal-based precursor-selenization process. The reason for the efficiency improvement is not fully clear at present. Since replacing Sn with Ge increases the band gap of kesterite semiconductors, analogies with the effects of Ga-alloying in Cu(In,Ga)Se2 chalcopyite solar cells comes to mind. In this study the defect physics of Ge-containing CZTGeSSe vs. pure Cu2ZnSn(S,Se)4 (CZTSSe) thin film absorber layers is investigated by photoluminescence spectroscopy. From low-temperature excitation-dependent photoluminescence measurements we find evidence for compensation for all of the investigated samples, with the observation of strong blue shifts of about 14 meV/decade excitation intensity. This low temperature emission is interpreted as a quasi-donor-acceptor pair (QDAP) transition in the presence of strong potential fluctuations. The main difference between a pure CZTSSe and a Ge-containing CZTGeSSe sample is a blue shift of the transition energy by about 80 meV for the latter sample. This blue shift is consistent with a approximately 80 meV larger band gap estimated for the Ge-containing sample from external quantum efficiency measurements on accompanying devices made from the same absorber material. With increasing measurement temperature maximum of the photoluminescence transition energy red shifts and exhibits a strong quenching of the photoluminescence yield, with activation energies of about 15 and 90 meV for both the pure CZTSSe and the Ge-containing samples. We attribute the smaller activation energy to the thermal release of carriers trapped in fluctuating bands, whereas the larger activation is assigned to a shallow level involved in QDAP. The observation of near-band gap luminescence at room temperature for both CZTSSe and CZTGeSSe indicates a relatively high electronic quality of the materials.

Thin film solar cells based on kesterite semiconductors Cu2ZnSnS4 (CZTS) and Cu2ZnSnSe4 (CZTSe) are suitable materials for low cost solar cells consisting of earth abundant or readily available elements. Non-vacuum based deposition methods are especially interesting due to their inherent potential for low manufacturing costs and high throughput processing. Up to now the highest reported efficiency of > 11 % has been achived for a non-vacuum deposited Cu2ZnSn(S,Se)4 absorber using hydrazine as solvent.
We investigate the effect of different annealing conditions on the properties of CZTS absorbers, focusing on the compositional changes on the CZTS surface. The control of the absorber surface is important for achieving optimum p-n junction and maximum current collection in kesterite solar cells. It is known, however, that the CZTS surface is not stable and tends to decompose at elevated temperatures, resulting in deteriorated material and often Sn-poor surface composition.
Precursors are prepared by non-vacuum deposition of solutions containing metal salts and thiourea. The amorphous precursor layer is converted into crystalline CZTS material during annealing in chalcogen atmosphere, which requires the careful adjustment of substrate temperature, annealing duration and chalcogen partial pressure.
In-situ XRD is used to understand the formation mechanism of CZTS under different sulfur or SnS containing atmospheres during annealing.
Combined visible and UV Raman spectroscopy together with surface sensitive XPS measurements show very distinct surface composition and elemental gradients depending on the chosen annealing conditions.Sufficient chalcogen supply only in the precursor does not prevent surface decomposition leading to the presence of ZnS and binary Cu and Sn sulfides. XPS shows pronounced surface gradients for Cu and Zn distribution for differently treated precursors. Na diffusion from the soda lime glass substrate is only observed for high substrate temperatures Tsub > 550 °C. Additional sulfur containing atmosphere during the heat treatment is needed to improve the CZTS surface. UV Raman showed that a very pronounced occurrence of ZnS phase can be suppresed if an excess of sulfur is supplied.

Iron pyrite is an earth abundant semiconductor with material characteristics that make it attractive for photovoltaics. A band gap of 0.95 eV and high optical absorption (α > 10^5 cm^-1 for hnu; > 1.3 eV ) could allow iron pyrite to compete with silicon photovoltaics on efficiency while utilizing less material.[1] However, production of reliable device-quality material and high efficiency devices has been challenging.[2,3] Sulphur annealing of metallic iron precursor films is a well-studied method for producing iron pyrite thin films. Phase transformation introduces stress into the iron pyrite that can cause delamination if annealing conditions are not managed properly or adhesion layers are not employed.
Voided iron precursors provide a means to enable unconstrained volume expansion during phase transformation and eliminated stress in the iron pyrite films. We present the use of Glancing Angle Deposition (GLAD) to engineer the void fraction of metallic iron precursor films and achieve films without cracking or buckling. Void fraction can be used to move iron pyrite films between planar and columnar morphologies. Within a narrow window of void fraction planar films consisting of large crystallite >100 nm in size (measured by XRD and SEM) can be produced. Large crystallites may be important for reducing grain-boundary recombination of photo-carriers. X-ray diffraction studies demonstrate that the films have good crystalline phase purity, without any indication of marcasite. Crystalline interfaces between crystallites can also be seen in via high-res TEM, although surface oxidation leads to an amorphous layer on the crystallite surface. Hall measurements were used to determine the resistivity (2.2 - 7.6 Omega; cm) carrier concentration (10^18 - 10^19 cm^-3) and mobility (~0.1 cm^-2 V^-1 s^-1) consistent with other films in the literature. Optical Tauc plots indicate a 0.89 eV bandgap. A photocarrier lifetime of 27 ps was observed by ultrafast optical pump (800 nm) and THz probe measurements. Although this lifetime is insufficient for photovoltaic applications [3], the control offered by void fraction engineering provides additional opportunities to optimize the sulphur annealing process to create high-quality iron pyrite.
[1] C. Wadia, a P. Alivisatos, D.M. Kammen, Environmental Science & Technology 43 (2009) 2072-2077.
[2] A. Ennaoui, S. Fiechter, C. Pettenkofer, N. Alonsovante, K. Buker, M. Bronold, C. Hopfner, H. Tributsch, Solar Energy Materials and Solar Cells 29 (1993) 289-370.
[3] P. Altermatt, Solar Energy Materials and Solar Cells 71 (2002) 181-195.

Crystalline kesterite Cu₂ZnSnS₄ (CZTS) is an attractive candidate for the absorber material in low-cost and high-volume photovoltaic modules due to the abundance and low-cost of its constituent elements and its near-optimal direct band gap. Development of CZTS solar cells has progressed rapidly in recent years, in spite of challenges presented by a complex intrinsic point defect chemistry and growth kinetics, which lead to a small processing window for producing high-quality CZTS films. In this study we report on room temperature synthesis of thin films along the Cu₂SnS₃ - CZTS - ZnS tie line in the quasi-ternary CuS - ZnS - SnS phase system. This approach has yielded a range of semiconducting alloys (hereafter a-CZTS) with highly tunable, intrinsic control over the optical band gap and carrier concentration. By increasing the Zn content of the films [according to (Cu₂SnS₃)_1-x(ZnS)_x] the optical band gap can be continuously varied from 1.2 eV to at least 2.8 eV while maintained a high (> 10⁴ cm^-1 at E_G) optical absorption coefficient. Small variations in the Cu:Sn ratio, relative to the nominally stoichiometric 2:1 ratio in crystalline CZTS, provides control of sheet resistance over three orders of magnitude. Increasing the Zn content in the films also increases the resistivity, giving an overall range for a-CZTS alloys from ~ 1 ohm/sq to > 10⁸ ohm/sq. Preliminary EXAFS measurements indicate that the atomic structure in a-CZTS films maintains the tetrahedral coordination of crystalline CZTS. On the other hand, optical absorption measurements indicate a high degree of structural disorder, and x-ray diffraction measurements are consistent with the existence of nanometer-scale crystalline grains. The structural disorder on the cation sublattice that results from low-temperature synthesis, constrained by tetrahedral coordination of both anions and cations, leads to the observed high degree of tunability of the opto-electronic properties. All semiconductor materials that have been successfully developed for PV applications thus far, including Si, a-Si:H, CIGS, CdTe, CZTS and III-V alloys, are characterized by tetrahedral coordination. In the case of amorphous semiconductors, it is believed that tetrahedral coordination plays a critical role in unpinning the Fermi level to allow effective control over doping levels.[Street, Hydrogenated Amorphous Silicon, Springer, ch. 4] In contrast, most common amorphous chalcogenide semiconductors are not tetrahedrally coordinated, which results in mid-gap Fermi-level pinning and limited utility in technological applications such as PV. Initial electrical, optical and structural characterization of a-CZTS films will be presented, as well as preliminary results from photovoltaic solar cells made with a-CZTS absorber layers. Potential applications of these materials in tandem or multijunction solar cells that might be enabled by the wide tunability of band gap and carrier concentration will also be discussed.

Terawatt-scale energy demands motivate the investigation of new visible-range, direct bandgap semiconductor materials that are earth abundant and low-cost, for photovoltaic and photoelectrochemistry applications. In this context, the goal of this work is to develop and evaluate a new class of Zn-based ternary-alloy and quaternary-alloy semiconductor materials, Zn-IV-N₂ (IV = Sn, Ge, Si). First principles electronic structure calculations using hybrid density functional theory indicate that these nitrides have similar properties to their III-N counterparts, like InN and InGaN, with tunable band gaps spanning the spectrum from infrared to ultraviolet wavelengths and favorable band alignments for photoelectrochemistry. Theory has guided the experimental parameter space of the Zn-IV-N₂ alloys and experimental techniques like X-ray spectroscopy and spectroscopic ellipsometry (SE) have been used to directly probe the optoelectronic structure. With calculated and measured band gaps between 1.8 eV to 3eV, Zn-IV-N₂ alloys are of great interest for band gap engineered light absorber materials, especially since the optical gap in ZnSn_xGe_yN₂ can be tuned from 1.8 to 2.5eV, band gap values that lie between ZnSnN₂ and ZnGeN₂ gaps.
The focus of this presentation will be theory, optoelectronics and X-ray spectroscopy of ZnSnN₂, ZnGeN₂ and ZnSn_xGe_yN₂ alloys. X-ray absorption and emission spectroscopy (XAS/XES) in conjunction with resonant inelastic X-ray scattering (RIXS) have been used to study the N, Sn and Ge partial density of states. Experimentally determined partial density of states, using XAS (N K-edge, Sn M-edge, and Ge L-edge) and N XES, is compared with ab-initio calculations of the Zn-IV-N₂ alloys and overall a good agreement is found. In N K-edge XAS, comparison of the bulk sensitive total fluorescence yield with the surface sensitive total electron yield indicates conduction band filling on the surface. X-ray absorption near-edge spectroscopy (XANES) and extended X-ray fine structure (EXAFS) of Ge (K-edge) and Sn (L_II-edge) of the ZnSn_xGe_yN₂ alloys were also used to determine the chemical state and local structure of absorbing atoms. Optoelectronic measurements from SE and photoluminescence will be discussed in context of defect states in the ZnSnN₂ alloys.

Thin-film Cu₂ZnSn(S,Se) ₄ (CZTSSe) is a promising absorber material for low-cost, scalable photovoltaic applications. There has been substantial progress recently in the performance of CZTSSe devices, in spite of a complex intrinsic point defect chemistry, and poorly understood kinetic processes during film growth and processing. Recent first-principles calculations predict that chalcogen vacancies form deep levels in CTZSSe materials that could negatively impact critical opto-electronic properties, including minority-carrier lifetime and electron and hole mobilities. As a consequence, CZTSSe materials synthesized under low or high vacuum conditions, or via liquid-phase precursors, are often subjected to a post-deposition annealing process under relatively high chalcogen partial pressures. In order to understand the kinetics that control chalcogen vacancy annihilation during post-deposition, high-chalcogen annealing processes, a systematic investigation of chalcogen diffusion within the CZTSSe system was undertaken. Thin-film CZTSe samples were prepared via co-evaporation on Mo-coated soda-lime glass. Prior to diffusion measurements, samples were mechanically polished to less than 20-nm rms roughness to minimize roughness-related artifacts and improve depth resolution in subsequent secondary ion mass spectrometry (SIMS) measurements. Diffusion anneals were carried out in a multi-zone tube furnace with variable S and Se partial pressure, and Ar as an inert carrier gas. Following pre-anneals under Se-ambient conditions to equilibrate the point-defect concentrations, diffusion anneals were performed by introducing S vapour into the furnace, while maintaining a constant overall chalcogen pressure. The activation energy for S diffusion in CZTSe was extracted from SIMS profiles acquired from samples annealed at different temperatures. Fundamental connections between the diffusion measurements and the overall defect chemistry within the CZTSSe system will be discussed.

Thin film compound semiconductor solar cells have been traditionally based on chalcogenide absorber materials, such as the famous defect-tolerant Cu(In,Ga)Se2 (CIGS) with chalcopyrite structure. More recently, the quest for new thin film absorbers led researchers to structurally equivalent II-IV-V2 pnictides in a hope to mimic all beneficial defect properties of CIGS structure.
Here we present a new paradigm in alternative thin film absorbers - Earth-abundant ternary copper nitrides. These Cu-M-N materials feature chemical rather than structural similarity to CIGS. Namely, good mixing of energy-matched Cu d and N p states in the anti-bonding states at the valence band maximum of Cu-M-N is expected to result in defect tolerance in the same way as Cu d and Se p states lead to defect tolerance in CIGS.
The parent of all ternary Cu-M-N materials is binary Cu3N. The advantages of this compound for thin film solar cell absorber applications are absorption onset which is close to optimal (1.6 eV) and fast growth of uniaxial textured thin films grown by reactive sputtering at ambient temperature. These high quality Cu3N layers have moderate concentration of electric charge carriers suitable for solar cell absorber applications. The main disadvantage is a low 0.9 eV indirect forbidden band gap.
One way to address the non-optimal absorber properties of binary Cu3N is to consider ternary copper nitrides. Presently, there are very few such materials documented in crystallographic databases. In this project we considered two documented and several new ternary copper nitrides.
One documented ternary copper nitride CuTaN2 is synthesized using an ion exchange reaction andshows a 1.5 eV absorption onset in diffuse reflectance. Theoretical calculations indicate that this onset corresponds to an indirect band gap in this material and suggest a related previously unknown CuNbN2 compound would have better optical properties.
Another ternary copper nitride CuSrN has been synthesized in the past using sodium flux crystal growth method. This material has an optical absorption onset of 1.2 eV and a moderate concentration of copper vacancies, according to first-principles theoretical calculations. One potential disadvantage of this material for practical applications is its high sensitivity to ambient atmosphere (H2O and O2).
In searching for Cu-M-N photovoltaic absorbers, theoretical calculations identified several other ternary copper nitrides with properties promising for photovoltaic absorber applications. In particular, 6 new thermodynamically stable ternary copper nitrides and 7 Cu-M-N materials with a suitable combination of electronic and optical band gaps have been identified by density functional theory calculations. Experimental synthesis of these materials is in progress.
This research is supported by the U.S. Department of Energy, office of Energy Efficiency and Renewable Energy, as a part of a Next Generation PV II project within the SunShot initiative.

Improvements in efficiency, cost, and scalability of thin film photovoltaic technologies are needed to make solar electricity competitive with conventional energy sources. To support these goals, it is desirable to diversify the materials base for inorganic thin-film PV, and to identify new technologies based on inexpensive and abundant materials that can achieve high efficiencies. One possible approach to this task is to use iterative coupling of predictive materials theory and high throughput experiments in a sustainable framework for rapid development of novel thin film solar cells.
In this poster we will present the overall rapid development approach as well as provide more details on experimental and theoretical research methods. As a part of the approach, predictive first principles theory can be used to identify and evaluate promising materials based on their band-structure, optical, electrical and interface properties. Suitable candidates are then synthesized and optimized using high-throughput combinatorial deposition, spatially resolved characterization and automated analysis setup. Finally, photovoltaic device prototypes are fabricated where both predictive theory and high-throughput deposition techniques are employed to optimize the junction partners and contacts. The overall goal of this approach is to provide a proof of concept for novel inorganic PV technologies with the potential for a game-changing impact in the PV sector.
Predictive theoretical first principles calculations provide accurate estimates of properties related to band structure (optical absorption spectra, effective masses), defects (dopants, carrier concentration and recombination centers) and interfaces (band offsets, interface states). The accuracy necessary for making predictions is achieved by using state of the art GW methods for band structure calculations, corrected first principles thermodynamics methods for defect calculations, and computationally inexpensive empirical pseudopotential methods for interface calculations.
High-throughput experiments are preformed in three steps that include combinatorial synthesis, mapping characterization and automated analysis. The thin films are prepared using co-sputtering from angled sources as a function of chemical composition and temperature that spatially vary across the substrate. The resulting samples are characterized for composition, structure, and optical, electrical and surface properties. Automated analysis is performed using a custom developed software package that correlates the measured properties with each other and with the synthesis conditions. These three steps are applicable to both materials optimization and device prototyping.
The project “Rapid Development of Earth-abundant Thin Film Solar Cells” is supported as a part of the SunShot initiative by the U. S. Department of Energy, Office of Energy Efficiency and Renewable Energy under Contract No. DE-AC36-08GO28308 to NREL

Zinc tin nitride (ZnSnN2) is a novel earth-abundant semiconductor. It is a member of the II-IV-nitrides, that are analgous to the well-characterized III-nitrides currently employed in light-emitting diodes and sensors. Hybrid DFT simulations1 of electronic and crystal structure have motivated fabrication of ZnSnN2, as its calculated direct band gap of 1.4eV lies in a permissible regime for solar energy conversion. Like the III-Nitrides, that impart alloying to tune the band gap energy, tunable alloys of Zn-IV-Nitrides have been demonstrated.2 Thus, ZnSnN2, its alloys, and the Zn-IV-Nitrides, are being developed for their potential applications in photovoltaics, light-emitting diodes, and sensors.
Zinc tin nitride thin films are fabricated by RF reactive sputtering on sapphire and GaN templates on sapphire.1 By x-ray diffraction studies, polycrystalline thin films have grain sizes ranging from 5-15nm. Sample resistivities are in the 10-2 Ohm-cm range, exhibiting high n-type doping. Hybrid density functional theory simulations have indicated a potentially low electron effective mass 2 which, combined with a high carrier concentration, may lead to a Burstein-Moss effect, where the Fermi level sits above the conduction band minimum. However, ZnSnN2 exhibits semiconducting properties, as shown by temperature dependent resistivity measurements, x-ray photoelectron spectra of the valence band region, and photoresponse behavior.
Photoelectron spectra measurements of treated and untreated surfaces have been obtained and subsequent workfunctions have been determined. A corresponding range of solid-state and liquid contacts for Schottky and photoelectrochemical cells have been attempted. Reducing ZnSnN2 carrier concentration might allow better junction rectification. Ohmic junctions formed from solid-state and liquid contacts have shown that samples are photoconductive. Presence of a photoresponse provides a positive outlook for development of ZnSnN2 for applications in solar cells or photon sensors.

The introduction of extrinsic impurities into a semiconductor can improve device performance by forming electrically benign defects or defect complexes that reduce the concentration of electrically active defects. For example, in Cu(InGa)Se2 (CIGS) it is widely accepted that the inclusion of sodium atoms forms neutral sodium on copper site (NaCu) defects which improve photovoltaic device performance by limiting the formation of indium on copper site (InCu) recombination centers [1-3]. In addition, the formation energies of the intrinsic defect complexes that are responsible for the resilient self-compensation phenomenon, such as [CuZn+ZnCu] and [VCu+ZnCu] in Cu2ZnSnSe4 (CZTS), also depend on semiconductor composition [4] and the concentration and type of impurities. The solution space of potentially beneficial impurities and impurity combinations for CZTS is vastly unexplored, and experimental exploration of these defect chemistries has received little attention. This is primarily because experiments investigating composition dependence of optoelectronic properties typically require substantial processing time for each composition and are susceptible to batch-to-batch variability due to the large number of variables involved.
To address this challenge, we present here the first report of an automated high throughput combinatorial synthesis strategy being applied to the CZTS material system. We first discuss the design, construction, calibration, and utilization of a custom spray coater instrument capable of reproducibly generating CZTS films with continuously varying composition and with impurity mole fractions spanning five orders of magnitude. We then highlight results obtained with this instrument that investigate the role of Group IIA elements and other impurities on the optoelectronic properties of CZTS films and devices. Data is reported for films made from nancrystal inks and from true solutions with an emphasis on photoluminescence mapping.
1. Wei, S. H.; Zhang, S. B.; Zunger, A., Effects of Na on the electrical and structural properties of CuInSe2. J. Appl. Phys. 1999, 85, (10), 7214-7218.
2. Schuler, S.; Siebentritt, S.; Nishiwaki, S.; Rega, N.; Beckmann, J.; Brehme, S.; Lux-Steiner, M. C., Self-compensation of intrinsic defects in the ternary semiconductor CuGaSe2. Physical Review B 2004, 69, (4), 9.
3. Li, Z. B.; Wang, X.; Yao, K. L., Electronic structures and optical properties of Cu1-xNaxInSe2 by first-principle calculations. Solid State Communications 2010, 150, (33-34), 1514-1517.
4. Chen, S. Y.; Yang, J. H.; Gong, X. G.; Walsh, A.; Wei, S. H., Intrinsic point defects and complexes in the quaternary kesterite semiconductor Cu2ZnSnS4. Physical Review B 2010, 81, (24), 245204-245214.

Iron pyrite (FeS₂) is an earth-abundant non-toxic semiconductor that has generated renewed interest due to its promising properties for solar-energy conversion (band gap of 0.95 eV and high absorption coefficient α ~ 10⁵ cm^-1). Despite its attractive properties, the best conversion efficiency demonstrated for any solar conversion devices based on single crystal pyrite has remained very low (< 3%, as photoelectrochemical liquid junction solar cells) and no example of solar conversion efficiencies in nanostructure pyrite has been demonstrated despite intense recent synthetic efforts. In order to understand whether this originates from an intrinsic defect problem of pyrite materials, we will report our electrochemical and photoelectrochemical studies on pyrite single crystals grown by chemical vapor transport together with the transport studies on pyrite nanostructures. We analyze the J-V characteristics and impedance spectroscopy of pyrite single crystals and discuss how their observed “metal-like” behavior could be passivated by post-growth treatments. On the other hand, we report the synthesis and complete structural characterization of phase pure single crystalline pyrite nanorods and nanoribbons grown via a vapor phase synthesis. Using them as a material platform, the semiconductor transport properties can be characterized in detail after growth and after post-growth treatment. Finally, we can provide a comprehensive explanation into how the observed properties for pyrite single crystals and nanostructures are explained by the defects in pyrite materials, and how does such understanding can be used to improve pyrite material for solar energy conversion.

Copper (I) oxide is a wide-gap material that recently attracted renewed interest as an earth abundant photovoltaic absorber material. Present conversion efficiencies of Cu₂O cells are below 4%, but we suggest that the efficiency could be much increased by significantly reducing the very high absorption threshold of Cu₂O around 2.5 eV that originates from the dipole-forbidden transition at the band gap energy at 2.1 eV. A further improvement of the efficiency should be achievable by increasing the p-type doping which stays below 10¹⁵cm^-3 in pure Cu₂O. In order to manipulate both the optical and electrical properties, we employ theory to predict the range of properties accessible by alloying both divalent cations (M = Mg, Zn, Cd) and isovalent chalcogens (X = S, Se). In the experimental part of this project we then aim to synthesize the most promising alloy compositions.
The theoretical predictions are based on supercell defect calculations, GW band-structure calculations, and thermodynamic modeling, taking into account the effects of defect pairing and changes of the band edge positions with composition. Considering a composition window for the Cu_2-2xM_xO_1-yX_y alloys of 0 le; (x,y) le; 0.2, which might be accessible in non-equilibrium thin-film growth, we predict a wide range of possible band gaps from 1.7 to 2.6 eV, and net doping concentrations between p=10¹⁹ cm^-3 and n=10¹⁷cm^-3. These results indicate that optimized PV materials should be accessible in the suggested alloy system. Initial Pulsed Laser Deposition of Zn-doped Cu₂O films shows that alloys in the targeted composition range are achievable.
This work is supported as part of the SunShot initiative by the U.S. Department of Energy, Office of Energy Efficiency and Renewable Energy under Contract No. DE-AC36-08GO28308 to NREL.

The increasing cost and the limited supply of elements like In and Te have led to a widespread interest in developing novel earth-abundant thin-film photovoltaic materials for future large-scale PV deployment. In addition, the goal of cost-competitiveness called out by the SunShot initiative requires high cell efficiencies exceeding those of current thin-film technologies. A material of high current interest is Cu2ZnSn(S,Se)4 (CZTS) from which devices with remarkable conversion efficiencies above 10% have been achieved. Nevertheless, the chemical complexity resulting from the combination of four difference valences (I2-II-IV-VI4) leads to challenges in the control of composition and electronic properties.
In an effort to accelerate the development of novel PV materials through a combination of materials theory and high-throughput synthesis and characterization, we are exploring the potential of In-free ternary copper chalcogenides. Two interesting families of ternary copper sulfide materials have the third element that belongs to group IVB and groups VB of the periodic table. The prototypical examples of such materials are Cu2SnS3 and CuSbS2. According to previous experimental studies, Cu2SnS3 has tetrahedrally bonded crystal structure and ~1 eV band gap. Previous studies indicate that CuSbS2 has layered crystal structure and ~1.8 eV band gap.
As part of the theoretical evaluation of this set of materials, we predict band-structures and optical properties of the Cu2SnS3 and CuSbS2 and the compounds formed by isovalent alloying on the Sn and S sites. We demonstrate that the optimal band gap of 1.3 eV for a single junction cell and high optical absorption of ~10⁴ cm^-1 can be achieved in this class of materials. We additionally perform defect studies to elucidate the doping trends.
The project “Rapid Development of Earth-abundant Thin Film Solar Cells” is supported as a part of the SunShot initiative by the U. S. Department of Energy, Office of Energy Efficiency and Renewable Energy under Contract No. DE-AC36-08GO28308 to NREL.

The minority carrier lifetime is considered one of the most critical and variable parameters in photovoltaic materials. A significant challenge in evaluating unconventional semiconductor materials for photovoltaic applications is accurately measuring the bulk minority carrier lifetime due to the difficultly of separating surface from bulk recombination. We are developing a new two-photon lifetime tomography technique to separate bulk minority carrier lifetime from surface effects. This new technique is being developed through a SunShot BRIDGE grant and will enable rapid screening of photovoltaic materials and optimization of processing conditions by avoiding the need to construct full devices. This technique will also enable the generation of three-dimensional minority carrier lifetime and charge collection efficiency maps that will be useful in identifying efficiency bottlenecks for both new and conventional (e.g. CdTe and CIGS) thin film PV materials.
The presented two-photon absorption measurements use photons with energies below the band gap to excite carriers in the semiconductor. Because it relies on a nonlinear process, excitation only occurs within a diffraction-limited focal volume in the sample. By adjusting the focal depth from the surface into the bulk of the film it is possible to move this excitation volume into the bulk of the film and directly excite carriers within the film. Using two-photon absorption as the excitation source thus allows the independent examination of the front surface, bulk, and buried interfaces of thin films.

Semiconductor nanowire arrays hold the potential for inexpensive, highly efficient solar cells because they can be processed in solution, they exhibit light trapping for enhanced light absorption, and their radial p-n junctions can accommodate materials with short minority carrier diffusion lengths. Our previous work developed a cation-exchange process in solution to prepare CdS@Cu2S heteroepitaxial core-shell nanowires for inexpensive solar cells, whose performance exceeded bulk Cu2S-CdS solar cells in terms of open circuit voltage (0.61V) and fill factor (80.8%). This was achieved because the interface between the CdS core and the Cu2S shell is heteroepitaxial with a low defect density, which leads to nearly perfect charge separation and low minority carrier recombination. However, for these CdS@Cu2S solar cells, the efficiency was limited because the 20-nm Cu2S shells produced by this process cannot effectively absorb all incident photons. Furthermore, the use of Cu2S for the outer shell material raises the requirements for encapsulation of a final module because of concerns about the stability of copper sulfide.
To address these drawbacks, a solution-based process has been developed using cation exchange to create Cu2S@CdS core-shell heteroepitaxial nanowires on Cu2S arrays that are grown under mild conditions. This core-shell geometry is confirmed by high-resolution transmission electron microscopy (HRTEM) and elemental mapping using energy dispersive spectroscopy (EDS). This Cu2S@CdS geometry should yield improved absorption of light and power conversion efficiency in solar cells. This chemistry is promising for the creation of solution-processable solar cells with high efficiency made from Cu2S@CdS heteroepitaxial core-shell nanowire arrays.

Cu₂ZnSn(S,Se)₄--based materials (CZTSSe) have recently been attracting much attention as next-generation solar cell materials because of the successful realization of CuInGaSe₂(CIGS) solar cells and III-element-free chalcogenide semiconductors. CZTSSe-based materials are a type of I₂-II-IV-V₄ quaternary compound semiconductor and the composition control is important to obtain high quality and high conversion efficiency of CZTSSe-based solar cells. In this study, we demonstrate a method of the composition control of Cu₂ZnSnSe₄-based materials (CZTSe) by coevaporation method. CZTSe films were deposited by the thermal evaporation of elemental Cu, Zn, Sn, and Se all together, a type of coevaporation method. Growth temperature was fixed at 370°C.
We investigated a tendency of the composition of CZTSe films using a triangular plot of the composition of CZTSe films, and the triangular apexes consist of CuSe, ZnSe, and SnSe. Note that Se content is fixed at 50% in the triangular plot, which was confirmed from experimental results. From the triangular plot, all the samples were found to be closely distributed on the line connected by ZnSe and Cu₂ZnSnSe₄. In this study, CZTSe composition was controlled by adjusting the Cu, Zn, and Sn beam fluxes, which were individually changed (a much higher Se flux was always used). That is, the composition ratios were significantly shifted from the supply ratios, and the compositions were automatically fixed on the line. All the CZTSe films in this study showed a single-phase nature, by results of XRD and Raman measurements. Therefore, it is concluded that single-phase CZTSe can exist widely on the composition line between ZnSe and Cu₂ZnSnSe₄. The single-phase region was much wider than the reported phase diagram; however, the origin of this was not clear yet.
The composition on the line can be expressed as Cu_2xZnSn_xSe_(1+3x), where x denotes the partition ratio of ZnSe and Cu₂ZnSnSe₄. CZTSe was grown on the specific line, so we can express the composition by one parameter by projecting the actual composition to the specific line. The parameter is the thermo equilibrium line projected parameter (TP) defined as,
TP=([Sn]/[Zn]+[Cu]/2[Zn])/2,
where [Sn], [Zn], and [Cu] are the atomic concentrations of Sn, Zn, and Cu, respectively. The first term indicates the projection to the line connecting ZnSe and Cu₂ZnSnSe₄ from the actual composition point started from Cu vertex in the triangular plot, and the second term is the projection to the line from the actual composition point started from Sn vertex. TP is their mean. TP corresponds to x if the composition of CZTSe can be expressed as Cu_2xZnSn_xSe_(1+3x). In this study, single phase CZTSe films were widely obtained from TP=1.0 to 0.4.

Solar cells based on Cu(In,Ga)Se2 typically show transient behavior of the open-circuit voltage under illumination. Recent theoretical [1] and experimental [2] results pointed out that this metastability may be related to a (VSe-VCu) defect complex, which changes its character from donor-like to acceptor-like upon capture of two electrons. It is predicted (and experimentally verified) that the acceptor concentration increases under illumination. If this is the case, however, it is easy to explain a variation of Voc: either increasing for bulk recombination or decreasing for interface recombination. According to the defect-complex model the transient behavior should depend on the Se and Cu content of the sample. In order to verify this assumption, a series of Mo/Cu(In,Ga)Se2/CdS/ZnO solar cells were investigated, with absorber layers prepared by means of a three-stage co-evaporation process with varying Se flow. However, also cells with different Ga content have been investigated. We find that both the Se and the Ga content in the samples have an influence on the transient behavior. Most surprisingly, samples without metastable Voc where found. In this presentation, the results of Voc(t) experiments are related to other investigations such as photoluminescence decay and positron lifetime spectroscopy.
[1] S. Lany & A. Zunger, J. Appl. Phys. 100 (2006) 113725.
[2] M. Igalson et al., Thin Sol. Films 515 (2007) 6142.

Although polycrystalline CuIn_1-xGa_xSe₂ (CIGSe) based thin film solar cells show efficiencies above 20% the epitaxial counterpart shows efficiencies below 10%. This work investigates epitaxial CIGSe based solar cells to identify the differences between polycrystalline and single crystalline devices. The pure materials CuInSe₂ (CISe) and CuGaSe₂ (CGSe) were grown on highly doped GaAs substrates by metal organic vapor phase epitaxy (MOVPE). The solar cells were finished using the commonly used CdS buffer layer, prepared by a chemical bath deposition followed by a sputtered ZnO window layer. However, CISe and CGSe did not show reasonable efficiencies. The main problem for the CISe cells is that our MOVPE process yields n-type CISe when growing Cu-poor films. On the other hand, CGSe samples showed cracks because of a combination of tensile lattice mismatch with the substrate and a significant difference of the coefficient of thermal expansion of CGSe and GaAs.
To allow a lattice matched growth and thus a reduction of strain effects and to ensure a p-type character of the absorber layer, the growth process was optimized to fabricate CIGSe samples. These devices show efficiencies up to 6.7%. All samples were analyzed with temperature dependent IV measurements showing the feature of a second diode, reducing the fill factor, and suffer from a high series resistance. The extrapolated open circuit voltage indicates a dominant recombination process at the interface. The temperature dependence of the diode factor indicates that the devices are limited by tunnelling processes.
The device simulation program SCAPS was used to clarify the origin of the second diode. The simulation can reproduce the shape of the IV curves indicating that the interface to the GaAs is crucial. The occurrence of the second diode depends further on the combination of the conduction band offset and on the doping density. Capacitance measurements indicate high doping levels, which are consistent with a tunnelling assisted recombination.
Thus, the devices suffer from a secondary diode at the CIGSe/GaAs interface, from a high series resistance, and from tunnelling assisted interface recombination. The improved understanding of the short comings of epitaxial devices allows further improvements. The lower efficiency of epitaxial devices is by no means an indication of a beneficial effect of grain boundaries.

Current compound semiconductor thin film solar cells are empirically developed over decades. Processes and device structures have been identified enabling high conversion efficiencies and low cost production of modules based on Cu(In,Ga)Se2 and CdTe absorbers. The limited abundance of In and the toxicity of CdTe are, however, demanding other semiconductors to be used as absorber materials. More recently, Kesterite compound solar cells have reached conversion efficiencies of 10%. Despite increasing worldwide research activities, conversion efficiencies of other polycrystalline inorganic compound semiconductor materials remain below 5%. The possible reasons for low efficiency with new absorber materials are manifold: (i) materials may have low minority carrier lifetime due to recombination at impurities, point or extended defects like grain boundaries and interfaces; (ii) transport of charge carriers may be very slow due to polaronic transport, also resulting in low diffusion lengths; (iii) poor nucleation behaviour may lead to pinholes which produce shunts; (iv) inappropriate deposition techniques may be the origin of insufficient nucleation and/or of defect formation in the absorber and/or interfaces; (v) inappropriate device structures and/or contact materials may severely limit the open circuit voltage. In efficient solar cells all these issues must be solved. Due to the huge amount of different materials, processes, device structures, extensive research activities are required. Even with large scale research projects, only little progress can be achieved within the typical project duration and breakthroughs are often only achieved by chance. For an efficient use of resources, it is therefore imperative to develop reliable criteria and experimental strategies for selecting the most promising materials at an early stage of investigation. This presentation provides an overview of the general insights obtained within the PINET (p-i-n network) project, which is funded by the German Ministry for Education and Research. Within this project, different semiconductors like Kesterites, SnS, Cu2S, Bi2S3, and Cu2O are investigated with respect to their growth, optoelectronic bulk and interface properties in the different institutions. The studies are complemented by preparation of thin solar cells, p-type transparent conducting oxides as contact materials, and DFT calculations.

The electronic properties of interfaces in semiconductor devices are crucially dependent on the
detailled atomic structure of the contact plane.
Most studies on solar cell interfaces are carried out on technologically prepared interfaces. In this study we start from idealized single crystalline interfaces prepared by MBE, MOMBE, ALD etc. under very well defined UHV conditions and investigated in situ by UPS, XPS, LEED, STM and XPEEM.
In particular we report on our attempts to model the junction in chalcopyrite thin films by well defined interfaces to clarify the influence of grain boundaries, lateral inhomogenities and chemical variations across and aside the contact plane. Chalcopyrites of the Type CuInX2 (X=S,Se) were grown by MBE as single crystalline samples in various orientations and were studied by surface analytical tools like XPS, UPS, LEED, STM and XPEEM in situ. Especially the application of synchrotron radiation in photo emission experiments is an extremely powerful tool to gain insight into the morphology and structure of hetero contacts. In a single deposition experiment it is possible to determine the band alignment, band bending, chemical reacted interfaces and their crystalline structure with high accuracy. By following the development of the contact phase to ZnO, ZnSe, ZnS step by step in an UHV environment, all properties of the interface are determined on an atomic scale with high resolution. Beside the formation of an ordered vacancy compound of the absorber the existence of various interfacial layers are detected and their influence on the parameters of a cell is discussed.
For CuInSe2 the formation of Cu poor interface layers is observed by SRXPS by the formation of the interface to ZnSe buffer layers. The development of Cu-poor surface phases was discussed by Zunger et al. and is here detected unambiguously.
To determine the band alignment valence band spectra have to be recorded to obtain the valence band onset. Here we will show that the right value can only be obtained by using synchrotron radiation as the correct position of the valence band in k-space has to be determined at the Γ-point.

Reactive annealing of metallic Cu-In-Ga precursors in selenium and/or sulfur containing atmospheres is of high interest for the commercial manufacture of Cu(In,Ga)(Se,S)2 absorber layers, having already achieved power conversion efficiencies of more than 17 % for sub-modules [1]. To date, this process has been predominantly conducted using soda-lime glass (SLG) substrates, which limit the reaction temperature to 600C. However, photovoltaic specialty glasses are under development that permit processing temperatures up to 650C. Recent results for Cu(InGa)Se2 co-evaporation at temperatures between 600C and 650C have demonstrated more uniform Ga profiles and increased device efficiency at wider band gaps [2, 3]. In the present work, we examine whether higher processing temperatures enabled by alkali containing specialty glass substrates are also beneficial for the reactive annealing process.
The reactive annealing of Cu(InGa)(SeS)2 exhibits a number of commonly observed issues at the Cu(InGa)(SeS)2/Mo interface, including poor adhesion, Ga accumulation, and void formation [4]. While these issues can be controlled with varying degrees of success at lower temperatures on SLG, increased processing temperature offers an additional pathway to address these issues. Cu-In-Ga metallic precursors are prepared by sputtering multiple layers of Cu0.77Ga0.23 alloy and elemental In onto Mo-coated specialty glass substrates. To form Cu(InGa)(SeS)2, the precursors are then reacted using a three-step process [4] which is similar to that used in manufacturing by Solar Frontier [1]. The process consists of a low temperature (400C) selenization step using H2Se gas, followed by a high temperature (600C to 650C) crystallization/reaction step in inert Ar atmosphere, and a subsequent sulfization step using H2S gas. In this study, we show that the high temperature reaction leads to a significant increase in grain size by a factor of 2 to 3 in lateral extension of the grain. Furthermore, a decrease in void coverage at the Cu(InGa)(SeS)2/Mo interface from 35 % to 10 % is observed. In addition, we discuss how the high reaction temperature influences the Ga and S profiles as well as the alkali content in the film. The effect of observed changes in film composition and morphology are compared to photovoltaic device characteristics.
References:
[1] H. Sugimoto et al., Photovoltaic Specialists Conference (PVSC), 2011 37th IEEE, pp.003420-003423, 19-24 June 2011
[2] M.A. Contreras et al., Photovoltaic Specialists Conference (PVSC), 2011 37th IEEE, pp.000026-000031, 19-24 June 2011
[3] J. Haarstrich et al., Solar EnergyMaterials&SolarCells 95 (2011) 1028-1030
[4] K. Kim et al., Journal of Applied Physics 111, 083710 (2012)

Conventionally, either elemental co-evaporation or selenization of CuGaIn precursors was used to produce the reliable Cu(InGa)Se2 (CIGS) thin film photovoltaic absorbers. In particular, the rapid thermal processing (RTP) of Se-coated Cu-Ga-In metal precursors was successfully scaled-up by Avancis company, reporting record efficiencies of 15.5% at 30x 30 cm2 and 13.9% at 65x165 cm2 for fully integrated modules. However, there are many parameters requiring optimization in order to achieve a high-quality CIGS solar cell absorber by RTP of Mo/CuGa/In/Se precursors, including the Cu-Ga-In metal stacking order, Se layer thickness, temperature ramp rate, annealing temperature, time, and partial pressure of Se.
In this contribution, stacked layer of glass/Mo/CuGa/In/Se precursors were prepared on Mo-coated low-alkali glass with a thickness of 1.8 mm by sequential sputtering of CuGa (25-28 Ga at.%) alloy and pure In targets, and then thermal evaporation of Se with no substrate heating. Detailed thermal behavior and phase evolution of precursors during rapid thermal processing with different ramp rate in the range of 0.5 and 4 K/s are discussed on the basis of results of X-ray diffraction and scanning electron microscope analyses.
The thermal behavior of Se layer was investigated, mainly focusing on the effect of Se phase on adhesion strength of Mo/CIGS interface. In the precursor structure of Mo/CuGa/In/Se, it is expected that the crystallization of Se layer may produce the considerable mechanical stress onto metal precursor and subsequently result in the poor adhesion of CIGS layer to Mo, evidenced by the delamination of CIGS layer during the post-processing. The pre-annealing step at 250 deg.C with different ramp rate (4, 0.5 and 0.1 K/s) and time (6, 5 and 0 min) was adopted to ensure the complete melting of crystalline Se and minimization of Se layer loss. The results showed that higher ramp rate for pre-annealing was recommended to improve the incorporation of Ga into CIGS chalcopyrite structure while maintaining the reliable adhesion strength of CIGS to Mo layer.
To investigate the phase evolution of Mo/CuGaIn/Se precursors with temperature, in-situ high-temperature X-ray diffraction technique was used. The results showed that as-deposited amorphous Se layer would be crystallized at around 120 deg.C, and then melt at 220 deg.C. The formation of CIGS phase was initiated at around 150 deg.C and saturated at 350-400 deg.C.

NaF precursor layers are used to provide sodium in photovoltaic devices based on Cu(In,Ga)Se2 (CIGS) in cases where in-diffusion of Na from the substrate has been inhibited by a barrier or where Na-free substrates are used. NaF can be evaporated from a resistively heated source in vacuum using a quartz crystal monitor for thickness control. A NaF layer thickness of a few tenths of nanometers deposited onto the Mo back contact is enough to yield solar cell efficiency results similar to those of devices where Na has been provided by in-diffusion from a soda-lime glass substrate. If the NaF thickness is exaggerated, the CIGS is found to peel off during chemical bath deposition of CdS. In this study NaF precursor layers have been deposited onto Mo-coated polished sintered alumina substrates. As reference material Mo-coated soda lime glass substrates have been used. After initial characterization of the NaF layers, the samples have been loaded into the CIGS co-evaporation vacuum system. Further analysis was carried out after interrupted CIGS co-evaporation runs. For every run a new set of samples was loaded. Samples were unloaded after vacuum exposure and heat-up, after pre-heating in Se-containing atmosphere, after starting the CIGS deposition, and two more interrupts during the CIGS process. Finally, samples from the full CIGS process were analyzed. The influence of substrate temperatures on the status of the NaF layer during was investigated using 540 °C and 620 °C, but the preheating and first stage of the CIGS process was set to 450 °C in both cases. To avoid substrate bending of the soda lime glass the higher temperature was only used for the alumina substrate. Two different NaF thicknesses were investigated. The resulting films were analyzed using XPS and SIMS as well as by microscopy methods. As deposited NaF films are found to be fully covering or almost fully covering the Mo back contact layer. During pre-heating, the NaF is stable and decomposition is not observed. During the CIGS evaporation, decomposition of NaF leads to incorporation of Na and minor amounts of F in the CIGS layer but substantial amounts of both Na and F in the Mo layer. Results indicate that the NaF layer decomposes gradually and that remaining NaF may be found at the Mo/CIGS interface during the co-evaporation. After the complete CIGS process no sign of NaF is found at theMo/CIGS interface for the investigated samples.

Increasing the band gap energy of the absorber to the optimal value (~1.45 eV) while realizing the efficiency enhancement would be a key breakthrough in CIGS thin film technology. Unfortunately, current CIGS cells show a maximum efficiency at much lower band gap energy (~1.2 eV). Higher efficiency directly translates into reduced cost of module manufacturing, and further reduces the balance of the system cost. Recent results at NREL show that cell efficiency can be maintained as the Ga content is increased if the temperature is increased and the Se partial pressure modified. In this study in-situ high-temperature XRD was used to determine the reaction pathways with higher gallium content at higher temperature and Se pressure when simulating the NREL 3-stage process.
Bilayer precursors of (In,Ga)2Se3/CuSex were deposited by molecular beam deposition on sodium free substrates for Ga composition in the range 0.25 < XGa < 0.75. High resolution room temperature XRD patterns showed that with increase in gallium content the crystal structure of (In, Ga)2Se3 changed from hexagonal to zincblende. Reaction pathways were followed using in-situ high temperature XRD with Se overpressure and no major differences in the reaction pathway were observed with change in Ga composition. The reaction pathway is summarized as CuSe=CuSe2=CuSe + (In,Ga)2Snot;e3=CIGS with CIGS forming using liquid assisted growth. The reaction was complete at <500 oC as evidenced by MoSe2 formation. Increasing the temperature beyond 500 oC caused peeling of the film in some cases due to the unfavorable MoSe2 orientation (002). The reaction kinetics for CIGS formation was obtained from isothermal studies and rate parameters were estimated from Avrami and parabolic models.

Critical issues in the reaction of metal precursors in Se- and/or S- containing atmospheres to form CIGS (when S included, CIGSS) are void formation and poor adhesion at the CIGSS/Mo interface. These can result in poorer device performances yield compared to co-evaporated CIGS. These voids are considered to result from the agglomeration of slow reacting intermetallic phases during selenization [1, 2]. In this work, we examine effects of Sb, Bi, or Te interlayers materials inserted between the metal precursor and Mo to control void formation by modifying surface/interface energies and potentially widen the process window by improving adhesion. The group V elements, Sb and Bi, have been reported to enhance recrystallization of CIGSS via a surfactant effect and Te is known to induce wetting of metallic species on Mo [3-5].
Ten nm-thick interlayers of Sb, Bi, or Te were deposited onto Mo prior to sputter deposition of the Cu-Ga-In precursors. The precursors were then converted to CIGSS absorbers by the three-step H₂Se/Ar/H₂S reaction [1] and solar cells were fabricated. Characterization of the precursor and reacted films included the size and density of the void formation, film structure and morphology, adhesion, and device performance. The surface morphologies and microstructures of the precursors were changed by the interlayers and, in particular, the Te interlayer induced nearly complete intermixing between Cu-Ga intermetallics and In. None of the interlayers eliminated the void formation at the CIGSS/Mo interface after reaction though void size was reduced. Compared to a CIGSS cell with no interlayer (control), CIGSS cells with the Sb or Bi interlayer exhibited poor device performances due to reduced V_OC. In contrast, the Te interlayer did not affect device performance and appeared to improve the compositional tolerance in terms of Cu/(Ga+In). The influences of the interlayers to CIGSS film adhesion to Mo will also be investigated and discussed in the final paper.
References
[1] K. Kim et al., J. Appl. Phys. 111, 083710 (2012).
[2] G. M. Hanket et al., J. Appl. Phys. 102, 074922 (2007).
[3] M. Yuan et al., Thin Solid Films 519, 852 (2010).
[4] T. Nakada et al., Conference records of the 37th IEEE PVSC, 160 (2011).
[5] B. M. Basol, et al., Conference records of 22nd IEEE PVSC, 1179 (1991).

A possible strategy to increase throughput in manufacturing of Cu(In,Ga)Se₂ (CIGS) solar modules is to reduce the thickness of the absorber by reducing the deposition time. This approach would also lead to a decrease in material consumption. Recently, we reported an efficiency of 18.7% on a polyimide substrate with CIGS deposited by a low-temperature multistage co-evaporation process. When reducing the thickness, scaling down the developed deposition process proportionally to the standard deposition time leads to lower cell performance. Therefore, the growth process for thinner CIGS absorbers has to be adapted in order to maintain an efficiency comparable to that of CIGS with standard thickness.
In this study, different growth processes are implemented for the deposition of CIGS layers with thickness in the range of 0.7-1.3 µm on flexible polyimide films. Maximal Cu content, final Cu content and Ga/In ratio are varied in order to actively modify the compositional grading. X-ray diffraction (XRD), scanning electron microscopy (SEM), x-ray fluorescence (XRF), and secondary ion mass spectrometry (SIMS) are used to characterize the microstructure, the morphology and the compositional properties of evaporated CIGS layers in order to understand their influence on device performance. Current-voltage, external quantum efficiency and capacitance measurements are used to characterize the optoelectronic properties of the solar cell devices. Results are compared to those obtained on record efficiency devices.
Absorber films with grains as large as the film thickness and smooth interfaces have been directly obtained by modifying the multistage co-evaporation profile. For an absorber thickness of 0.8 µm, comparable Voc and FF as for reference high efficiency devices have been obtained, leading to conversion efficiencies higher than 15%. Main loss compared to thicker samples is due to a lower current density and can be explained by insufficient light absorption and non-optimal Ga-grading compared to reference devices. Suggestions for further improvements will be given, based on a detailed analysis of the different compositional profiles. The influence of Ga-dip position and relative front and back grading on carrier separation and collection will be discussed.

Cu(InGa)Se2 (CIGSe)-based thin film solar cells have the potential to reach very high power-conversion efficiencies above 20 % at low production costs and energy consumption. However, owing to its four constituents and extrinsic impurities such as Na, CIGSe is a very complex material. Furthermore, CIGSe thin films used for photovoltaics are usually polycrystalline. In the case of other semiconductors, such as Si and GaAs, internal interfaces, i.e., grain boundaries (GB), are known to lead to an enhanced recombination reducing the charge carrier mobility. In the case of CIGSe, there is still a lack of knowledge about the role of grain boundaries.
In the present study, we investigate the electrostatic potential at a single Σ9 GB in CuGaSe2 (CGSe), which serves as a model system. A CGSe bicrystal was grown on top of a GaAs bicrystal exhibiting a single Σ9 GB by use of metal organic vapor phase epitaxy. We apply in-line electron holography in transmission electron microscopy, density functional theory (DFT) calculations, and multislice simulations on this CGSe Σ9 GB in order to study the electrostatic potential on a length scale of a few tens of nanometers. Our measurements show that the electrostatic potential is reduced by about 0.8 V in a region of 1.3 nm around the GB. In comparison with previously analyzed GBs, we find the magnitude of the reduction in the electrostatic potential to decrease with increasing GB symmetry, i.e., from random GBs towards highly symmetric Σ3 GBs.
Although compositional changes have been found at GBs in previous studies, our work indicates that the reduction measured in the electrostatic potential can be conclusively explained by the reduced atomic density at the GB core. This is supported by the valence-charge density distribution calculated by DFT, which shows negligible difference between atoms near the GB cores and atoms within the grain interiors. We obtain a quantitative agreement of the measured and the simulated electrostatic potential profile, if we account for the experimental limitations of electron holography as applied. Our DFT calculations suggest that our results are transferable to the case of Σ9 GBs in CuInSe2.

Cu(In,Ga)Se2 (CIGS) thin-films solar cells possess a high efficiency, despite the polycrystalline structure of the absorber layer. One of the major factors controlling the cell efficiency is the diffusion of impurities from the soda-lime glass substrate into the absorber layer and to the CdS/CIGS p-n junction. However, the interaction between the defects and the impurities at the internal interfaces is not completely understood. This is due to a lack of information on the local chemical changes across the internal interfaces at the nanoscale.
In this work, the internal interfaces (p-n junction and CIGS grain boundaries) of a CIGS thin-film solar cell were explored at atomic scale by means of atom probe tomography (APT). The experimental findings of this work show that the CIGS surface (first 2-3 atomic monolayers) is Cu-depleted, Ga-depleted and Cd-enriched. This observation is a strong indication for the existence of an inverted p-n junction within the first layers of the CIGS absorber layer. These recent findings support our previous results obtained on the CdS/CuInSe2 p-n junction for a CuInSe2 thin-film solar cell [1].
Furthermore, the CdS/CIGS p-n junction is decorated by mainly O, but also slightly by Na. High impurity content was also detected inside the CdS buffer layer. We may assume that Na and O are segregated at the CIGS surface during the CIGS deposition at ~ 600°C and successively diluted in the CBD solution to be finally redeposited with the CdS layer. One might speculate that the Na and O impurities in the CdS layer could change the electrical properties of the CdS layer, as it is the case for CIGS and CIS, and thereby improve the device performance.
Na (K) and O impurities were found to decorate not only the CdS/CIGS interface but also the CIGS grain boundaries [2]. Furthermore, atom probe tomography technique showed the Cd diffusion into the CIGS layer along the grain boundaries (more than 50 nm length). The high diffusivity of Cd along CIGS GBs is an indication for a high concentration of Cu vacancies at the GBs.
The present results are compared with the existing “electronic” grain boundary models. The aim is to understand the correlation between impurities and point defects at the internal interfaces of CIGS and possible consequences for the cell efficiency.
References :
[1] O. Cojocaru-Mirédin et al., Appl Phys. Lett. 98 (2011) 103504.
[2] O. Cojocaru-Mirédin et al., J. of Photovoltaics. 1 (2011) 207-212.

One of the critical issues of Cu(In,Ga)Se2 (CIGS) thin film devices is the grain boundary (GB) electrical properties and its role in the PV performance. Many characterization and theoretical studies propose inactive recombination of carriers at the GBs, which is considered a key point for the polycrystalline material to reach a high efficiency of Eff>20%. In addition to this benign characteristic, device modeling shows that a carrier inversion at the GBs can even enhance the conversion efficiency by facilitating a carrier collection channel along the GBs if this enhancement can suppress the drawback of electrostatic potential fluctuation. Indeed, potential measurements using scanning Kelvin probe force microscopy (SKPFM) indicated a positive potential and charged GBs, which is likely caused by Na segregation to the GBs.
In this contribution, we report a direct measurement of electrical conduction channels through GBs of CIGSe and Cu2ZnSnSe4 (CZTSe) films, using scanning spreading resistance microscopy (SSRM). SSRM is a modified version of conductive atomic force microscopy (C-AFM), with a wide range of current sensor (20 pA-200 mu;A) and a logarithm current-voltage conversion. A critical point of SSRM leading to the success in Si microdevice characterization is that the wide current sensor range allows the AFM-probe to be indented hard into the sample material, so that the probe-sample contact resistance, which dominates the overall resistance being measured, is minimized. We applied this technique to CIGSe and CZTSe, and found a higher conduction channel along the GBs of the thin films than in the grain interior. For CIGSe, the resistivity among the grains varies in ~50-500 Omega;cm. The resistivity at the GBs is smaller than in the grain interior by an amount of ~1/10-1/2 of the grain resistivity. The channel width is ~30-100 nm, consistent with the tip apex size (d~70 nm), indicating that the channel width is <100 nm and the resolution is limited by the probe size. This conduction channel along the GB was mapped only when the probe was indented hard with a force larger than ~500 nN. This force on the tip caused significant damage to the film&’s surface layer after scanning, so a large amount of strained and dangling bonds are expected at the tip-sample contact, which minimizes the contact resistance. We verified that the measurement is not an artifact of probe-sample contact area by (1) scanning multiple times in the same area with changes of surface morphology or surface damages but with the consistent conduction channels, and (2) measuring consistently higher conductance and SKPFM potential on the same GBs. The SSRM on CZTSe shows similar results to CIGSe, illustrating similarity of the two thin film materials in their electrical properties of GBs as well as other defects. Resistivity of CZTSe is about one order of magnitude smaller than CIGSe. Discussions of the material and device physics behind these measurement results will be presented.

A key feature of polycrystalline materials is the existence of extended defects such as stacking faults, twins, dislocations, and grain boundaries. These extended defects are considered detrimental to solar cell performance because they create deep levels that act as harmful non-radiative recombination centers. Using a combination of advanced electron microscopy and first-principles calculations, we have studied the atomic structure and electronic properties of extended defects in polycrystalline CdTe thin films. In this presentation I will review our results that explain in detail why the extended defects are detrimental to solar cell performance and how and how the detrimental effects may be passivated.

The University of Illinois at Chicago (UIC) in collaboration with EPIR Technologies, Inc. has studied polycrystalline CdS/CdTe solar device interfaces using a recently acquired spherical aberration corrected cold field emission scanning transmission electron microscope (STEM). Devices were fabricated in the standard superstrate structure with CdS deposited by chemical bath deposition and CdTe by close space sublimation. Through the implementation and optimization of a high resistivity buffer layer in conjunction with thinning CdS, UIC and EPIR have produced a device with an NREL verified efficiency of 15.3% on commercially available TCO coated glass. This study focuses on examining these devices in more detail using STEM for atomic-level characterization.
Small cross sections of the samples are removed using a focused ion beam without damaging device interfaces. This is followed by standard TEM sample thinning techniques. High angle annular dark field (HAADF) images in conjunction with energy dispersive x-ray spectroscopy (EDS) of these cross sections provide a view of interface differences between devices on a micrometer scale. Grain size is found to vary from less than 0.5 micrometers near the interface to greater 1 micrometer at a depth of 2 micrometers. This indicates that surface analysis of grains may not provide a good indication of grain size near the junction where generation of carriers is largest. Twin boundary structures within the bulk of the CdTe grains are clearly identified on an atomic scale by HAADF images. Electron energy loss spectroscopy (EELS) scans across these twin boundaries have indicated large stoichiometric shifts between twins in samples that have not been treated with the standard CdCl2 annealing process. These shifts in stoichiometry are not observed in samples that have been treated with CdCl2. The reasoning of implication of this result is still being explored.

Electroluminescence (EL) is the complementary physical action to the normal operating mode of a solar cell or module. Therefore, spatially and/or spectrally resolved EL is an attractive and meaningful tool for the characterization of these devices. In recent years, EL was widely used to gain quantitative information, e.g., on resistive losses within a solar module or on the fundamental properties of light absorption and emission in a photovoltaic material. Thus, the method is useful in a wide range of length scales from the square meter size of a solar module down to the microscopic aspect of radiative transitions in a photovoltaic material. The contribution will discuss the fundamental aspects of photo- and electroluminescence of Cu(In,Ga)Se2 at various temperatures and how EL can be used to characterize the material under the normal operation conditions of solar cells and modules. It will be investigated under which circumstances basic principles like the reciprocity between EL emission and photovoltaic quantum efficiency or the linear superposition of EL and photoluminescence are strictly valid or have to be modified. An especial focus will be given to the influence of metastabilities on the interpretation of spectrally and spatially resolved EL. Finally, several methods are discussed that allow for a quantitative interpretation of spatially resolved EL images.

Although everyone agrees that carrier lifetime is important to understand thin-film PV limitations, the interpretation of time-resolved photoluminescence (TRPL) measurements is complicated and the available published analysis is very limited. Therefore, it is necessary to continually re-examine the meaning of these measurements.
We use numerical modeling with Sentaurus Device software as a window inside the TRPL measurement. We look at the carriers&’ spatial distribution after a short light pulse and how it changes at different moments during the decay. The change of carrier distribution alters the electric field, concentration gradients and recombination rates. As a result, the process dominating the decay usually changes with time. In addition to electron and hole recombination, the decays are affected by carrier dynamics including carrier diffusion and drift. Although the minority -carrier lifetime is often used as a measure for material quality, not only the electron, but also hole recombination affects the TRPL decay and the device performance. The hole recombination is especially important in the part of absorber adjacent to the heterointerface. We perform the TRPL analysis for variation of electronic parameters such as doping, defect density and capture cross section. In each case we determine which mechanism dominates the TRPL decays, and connect it with the device performance.
Achieving 20% and higher efficiency in thin-film CdTe should be practically achievable, but it will require a higher degree of understanding the recombination mechanisms, and separating the various types of recombination involved. In order to do this, one needs confidence in the measurements output. While trying to interpret TRPL decays, these simulations give insight into carrier dynamics in CdTe solar cells and help understand their device physics. This abstract is subject to government rights.

Chalcogenide photovoltaic materials are attractive options for thin film solar cells due to their effective optical absorption and inexpensive fabrication processes. However, the power conversion efficiency of commercial cadmium telluride (CdTe) solar modules is asymp;13 %, well below the theoretical maximum value of asymp;30 %. The underlying physical mechanisms for the low efficiency are presently not well understood. Here, we investigate local photovoltaic properties with a focus on the difference between the grain bulk (< a few µm in size) and the grain boundary in CdTe absorber. Local current-voltage measurements were performed using nano-contacts in conjunction with local carrier (electron-hole pairs) generation using electron beams. Electron beam induced current was used to measure local efficiency with a spatial resolution as high as asymp;20 nm both on the top surface and in cross-section of the device. Furthermore, we have developed a novel approach for high-resolution and high-throughput photocurrent imaging using a thin film (<50 nm) of colloidal quantum dots assembled on the CdTe absorber. Under low energy (<6 keV) electron beam irradiation, the electron energy is fully absorbed by the quantum dot layer, exciting cathodoluminescence with wavelengths in the visible range. The emission from the quantum dots serves as a local photon source in the near-field, which can be spatially addressable by positioning the electron beam at the location of interest. The results show that, in a well-optimized material, a large fraction of grain boundaries displays higher photocurrent as compared to grain bulk effectively serving as a three-dimensional distributed photocurrent collector.

Polycrystalline thin-film solar cells based on the compound semiconductors CuInSe2 (CIS) and Cu(In,Ga)Se2 (CIGS) as absorber materials are important for photovoltaic applications because of their high energy conversion efficiency, long-term stable performance, and low-cost production. One of the major factors controlling the cell efficiency is the diffusion of impurities from the soda-lime glass substrate into the absorber layer. The determination of the grain boundaries composition at the nanoscale of the CI(G)S absorber layer was possible thanks to the atom probe tomography (APT). By using this technique, the redistribution of the impurities within the CI(G)S grain boundaries was directly observed.
This work gives an insight into the three-dimensional elemental distribution in some CI(G)S absorber layers using pulsed-laser APT. A novel approach of preparing site-specific atom-probe specimens using combined focused-ion beam, electron backscatter diffraction, and transmission electron microscopy is presented in this work. This method allows selected internal interfaces with a known location in CIGS thin-films to be studied by APT. Furthermore, we focus also here on the concentration and distribution of impurities, namely Na, K, and O atoms, out-diffusing from the soda lime glass substrate into the absorber.
Based on these experimental results, we conclude that impurities, as Na and O, contribute to the electrical properties of the extended crystal defects present at the grain boundaries. Indeed, in this work we propose that the passivation of these defects by Na and O play an important role in enhancing the efficiency of CI(G)S solar cells.

Thin film solar cells consist of several layers, like in the typical ZnO/CdS/CIGS/Mo architecture. Each deposition process influences the previous layers. Hence, the final device is the most appropriate for assessing the effect of defects in the absorber, e.g. using electrical characterization with capacitance spectroscopy. Such characterization techniques measure the response of the whole structure after applying a time dependent potential over it. This can be measured statically (current-voltage), dynamically (capacitance-voltage), and via admittance spectroscopy (AS) or Deep-Level Transient Spectroscopy (DLTS) recording a current or capacitance transient after a pulse. The first main challenge is to explain the observed signals. Afterwards one may use these explanations to improve the solar cell efficiency.
Although recently in CIGS technology efficiencies have been achieved beyond 20% on glass substrates [1] and exceeding 17% on a flexible polymer substrates [2], a lot of questions remain unanswered regarding the fundamental signals experimentalists observe for them. Among those are the frequently observed anomalies in fundamental parameters as open circuit voltage, short circuit current and fill factor, as listed in reference [3].
In this work we focus on the properties of the DLTS signals observed for CIGS solar cells, as typically measured on cells produced by EMPA (ETH Zürich, Switzerland). At low temperature a signal labeled N1 is usually observed, whose appearance seems independent of the manufacturing technology. A second peak appears at higher temperatures and gives rise to slow capacitance transients near room temperature. Following ref. 3, we label this signal N2. We demonstrate that both N1 and N2 exhibit the typical features of a non-ohmic contact [4]. Based on these findings we introduce the idea of modeling all interfaces in a structure as rectifying barriers, connected in series with one another. We show that such an electric circuit is able to mimic the typical DLTS and AS spectroscopy of a CIGS solar cell. The CIGS DLTS spectra are furthermore compared with those of other thin film solar cells, e.g. CdTe/CdS and P3HT-PCBM organic solar cells and with other layered electronic devices (Al-Ge-Au structures). It is remarkable that such structures all exhibit signals with similar properties as the fundamental signals N1 and N2, confirming their origin as additional barriers. Although DLTS and AS spectroscopy were originally invented to measure charging and decharging of defects, the main features observed in thin film solar cells all appear to be related with non-Ohmic contacts in the structure.
[1] P. Jackson et al., Progr. Photovolt. 19, 894-897 (2011)
[2] A. Chirila et al., Nature Mater. 10, 857-861 (2011)
[3] R. Scheer and H.W. Schock, Chalcogenide Photovoltaics, Physics, Technology and Thin Film Devices, Wiley (2011)
[4] J. Lauwaert et al., J. Appl. Phys. 109, art. no. 063721 (2011)

A novel phosphonic acid-based synthesis for tunable and monodisperse Cu_xIn_yS₂ (CIS) quantum dots (QDs) is presented. These QDs are band gap-tunable from 1.5 to 2.1 eV, with high monodispersity and colloidal stability. Interestingly, a method for the introduction of localized surface plasmon resonances (LSPRs) in the near-infrared through changes in the reaction parameters is discussed. This discovery has spurred interest in the field of plasmonic CIS incorporation into QDPVs, in particular as thin windows in n-i-p junctions and as sensitizers in quantum dot sensitized architectures. A new approach concerning ligand functionality is being taken which eliminates the need for ligand exchange procedures, which generally result in significant cracking and deformations of thin films and/or non-air stable QDs. Specifically, phenylphosphonic acid has emerged as a promising option for natively-ligated QD thin films, due to its short length and significant surface stabilization. Hall voltage measurements are performed on defined thin films of QDs, deposited efficiently by a commercial airbrush. The acquisition of experimental data such as dopant density and carrier mobility will lead to better informed device design and execution.

In general, the intention of most chalcopyrite thin film solar cell companies to produce modules without the chemical bath deposited (CBD) CdS buffer layers has not yet been solved to full satisfaction. The CBD CdS buffer layer is still the state of the art in highest efficiency solar cells and serves as reference to which all alternatives have to compare. The main reason to this is that the CdS very reliably offers a stable, high quality junction independent of specific absorber surface conditions. Furthermore, the impact of CdS on the electronic properties of the photovoltaic device can be explained theoretically to a great extent. In contrast to this, devices with Cd-free buffer layers are much more dependent on specific absorber properties such as its surface composition and many show pronounced metastability effects such as the light soak effect. It is fair to say that the role that alternative buffer layers play in the junction formation is by far less understood. In our contribution we will exemplarily address the junction between industrial Cu(In,Ga)(S,Se)2 absorbers and In2S3 buffer layers. For the buffer deposition, compound In2S3 is sublimated in vacuum. This junction suits well as a model because it can be built at low substrate temperature, showing poor photovoltaic performance, and can then be enhanced in a controlled manner to match the performance of CdS references by a thermal post-deposition treatment. Thermal compound evaporation offers a simple and pure deposition process without chemical precursors (and no potentially metastable oxides/hydroxides) involved. The thermal evaporation of In2S3 is attractive for replacing the state-of-the-art CdS both due to the high efficiencies (>15%) achieved and the in-line friendly and simple process involved. The performance of devices with evaporated In2S3 layers depends crucially on an annealing at temperatures around 200°C, which leads to a strongly reduced recombination and consequently enhanced Voc and fill factor. It is well established among the scientific community that the thermal treatment triggers interdiffusion phenomena across the interface, namely a pronounced Cu diffusion out of the absorber and into the buffer layer. The question remains in which way the Cu diffusion is related to the optimized electronic properties of the junction. In this contribution, we characterize the optimized, annealed In2S3/Cu(In,Ga)(S,Se)2 interface and compare it to the non-optimized In2S3/Cu(In,Ga)(S,Se) and the CdS/Cu(In,Ga)(S,Se)2 interface to come up with a model how after annealing and diffusion the optimized interface effectively suppresses recombination. The applied analysis methods include detailed, temperature-dependent current-voltage analysis, transmission electron micrographs with energy-dispersive x-ray fluorescence imaging and X-ray photoelectron spectroscopy measurements.

One approach to raise the efficiency of Cu(In,Ga)Se₂ (CIGS) solar cells is the application of an alternative buffer layer with a higher bandgap energy like Zn(O,S) instead of the common CdS. With sputtering of Zn(O,S) the S/(S+O) ratio, which determines the bandgap energy of Zn(O,S) and band offsets to CIGS, can be precisely controlled, whereas for Zn(O,S) buffers from chemical bath deposition (CBD) this is more complex. In addition, the dry sputtering process is well suited to roll-to-roll applications with a flexible substrate material.
This contribution compares electrical and optical properties, chemical composition as well as band alignment of CIGS cells with sputtered Zn(O,S) to those of cells with CBD-Zn(O,S) buffers and discusses the relationship to the solar cell parameters.
Different Zn(O,S) buffers and standard CBD-CdS films were prepared on CIGS deposited with an in-line multi-stage co-evaporation process on Mo-coated glass. The efficiency level of our reference cells with the standard CdS/i-ZnO/ZnO:Al sequence is eta; = 16 - 18 %. The CBD-Zn(O,S) buffers were deposited from a zinc sulphate, ammonia, and thiourea solution or by RF-sputtering from a ceramic ZnS target with different O₂ contents in the sputter gas. We replaced the common i-ZnO layer used for cells with CdS buffers with RF-sputtered (Zn,Mg)O for our Zn(O,S)-buffered cells.
The chemical composition of different Zn(O,S) buffer layers was measured by means of secondary ion and quantified sputtered neutral mass spectrometry (SIMS and SNMS) depth profiles. Valence band offsets between CIGS and buffer layers were determined with X-ray photoelectron spectroscopy (XPS).
Cells with sputtered Zn(O,S) exhibit efficiencies up to 13 % and the open-circuit voltage V_OC and the short-circuit current strongly depend on the S/(S+O) ratio. For S-rich compositions the photocurrent is blocked and for S-poor films we observe low V_OC values as a result of the different band offsets between CIGS and Zn(O,S) and also the formation of sulfates. For CBD-Zn(O,S) the valence band offsets to CIGS derived from XPS scatter widely around 1.5 eV whereas for CdS the values are constant at 1.0 eV. One reason beside variations in the S/(S+O) ratio can be the higher amounts of hydroxide in CBD-Zn(O,S) layers.
With SNMS and SIMS depth profiles we could show that the chemical composition of the CBD-Zn(O,S) layers is homogeneous whereas sputtered Zn(O,S) exhibit different gradients in the S/(S+O) ratio, hydroxide, and sulfate distribution depending on the substrate temperature and amount of O₂ in the sputter gas.
With a post-annealing step we can often increase the conversion efficiency of devices with CBD-Zn(O,S). This is not the case for our CIGS cells with sputtered Zn(O,S) and the reference cells with CdS buffer. An explanation could be the higher hydroxide amounts in CBD-Zn(O,S) or the Zn diffusion into CIGS as a result of the higher deposition temperatures during the sputter process.

Roll-to-roll deposition techniques for the fabrication of chalcopyrite solar cells are of mayor interest and are a promising alternative to state of the art vacuum processes. However, for roll-to-roll processes the preparation of precursor materials like nanoparticle inks is a crucial point. In this work a study on the preparation technique of copper-Indium intermetallic nanoparticles was conducted. The preparation of the nanoparticles is based on chemical reduction of copper and indium cations with sodium borohydride. Different parameters are discussed regarding their influence on (1) Cu/In ratio within the synthesised nanoparticles, (2) size and shape of the nanoparticles, (3) yield of the synthesis. Results show a strong dependency of the Cu/In ratio of the nanoparticles and the yield of the synthesis on the synthesis parameters. The influence of different parameters like (a) the Cu2+/In3+ cation ratio within the precursor solution, (b) the ratio of metal cations to BH4- anions and (c) the dropping rate of the copper-indium precursor solution are discussed. The Cu/In ratio within the nanoparticles can mainly be controlled by the Cu2+/In3+ cation ratio and the dropping rate of copper-indium precursor solution. The yield of the synthesis shows saturation behaviour depending on the ratio of metal cations to BH4- anions. Shape and size of the nanoparticles are independent of the varied parameters.

Cu(In,Ga)Se₂ devices with the superstrate configuration (glass/window/buffer/Cu(In,Ga)Se₂/contact) are interesting for several reasons. The superstrate configuration opens the possibility to deposit the window layer at elevated temperature to improve its optoelectronic properties. When using this device structure it is furthermore possible to texture the window layer for light trapping and engineer the reflectance of the back contact. At the same time this approach eliminates the need for a transparent back cover, which potentially leads to a cost reduction.
A critical issue for the superstrate device structure is to control the junction formation when Cu(In,Ga)Se₂ is deposited onto the buffer layer. Since Cu(In,Ga)Se₂ is typically deposited at elevated temperature, inter-diffusion and reactions at the interface play an important role. In the present study Cu(In,Ga)Se₂ is deposited by co-evaporation onto a ZnO buffer layer under various conditions. The chemical interactions are investigated by X-ray photoelectron spectroscopy (XPS) depth profiling. Substrates for Cu(In,Ga)Se₂ deposition are prepared by room temperature sputtering of ZnO onto indium tin oxide (ITO) on glass. For depth profile studies a 100 nm thick Cu(In,Ga)Se₂ film is deposited onto ZnO by co-evaporation at 350 ^oC. The effect of an additional in-situ annealing step at 550 ^oC is investigated.
Based on the depth profile of Auger and photoelectron peaks of gallium it is concluded that Cu(In,Ga)Se₂ and ZnO reacts during the 550 ^oC in-situ annealing. The reaction leads to formation of an approximately 10 nm thick Ga₂O₃ layer at the interface. Formation of Ga₂O₃ is prevented when the annealing step at 550 ^oC is avoided. Presence of Ga₂O₃ is furthermore investigated by transmission electron microscopy (TEM) studies and by peeling of the samples at the Cu(In,Ga)Se₂ /Ga₂O₃/ZnO interface. The peeled surface is investigated with XPS and glancing incidence x-ray diffraction (GIXRD).
Detrimental effects of the Ga₂O₃ interlayer are discussed. A large spike is expected in the conduction band alignment at the Ga₂O₃/Cu(In,Ga)Se₂ interface. The Ga₂O₃ layer therefore acts as a barrier for photocurrent, severely limiting the performance superstrate devices with a ZnO buffer layer. Suggestions are made to avoid the formation of this unfavorable interlayer and hence to improve the device performance. This includes modifications of the process to reduce the thermal load during deposition and improvement of the ZnO buffer layer.

Incorporation of small amounts of Na has been shown to have a beneficial effect on the conversion efficiencies of Cu(In,Ga)Se2 (CIGS) solar cells [1]. A sufficient Na content can be obtained, for instance, by growing the absorber layer on top of a soda-lime glass substrate, resulting in Na diffusion from the substrate into the absorber layer. However, the mechanisms behind the improved efficiency are still not properly understood. In this work, we have addressed the questions involving the role of sodium in CuInSe2 (CIS) using first-principles calculations. We have evaluated the feasibility of different sodium-related defects in the CIS lattice and studied its migration properties to find out where sodium ends up after entering the CIS layer. We have also studied how its presence affects the electronic properties of the material.
[1] U. Rau and H.W. Schock. Appl. Phys. A 69, 131 (1999)

As researchers seek alternatives to CdS as the window layer for copper indium gallium diselenide and related chalcogenide solar cells, it is important to identify methods to passivate the copper indium gallium diselenide surface adjoining the window layer. We have investigated chemical passivation of CuInSe2 (CIS) thin films by trioctylphosphine sulfide (TOP:S), a molecule that has been used for passivation of II-VI semiconductors and which also provides excellent passivation of GaAs. For passivated CuInSe2, we find a 4.76X enhancement in photoluminescence intensity relative to unpassivated films. The CuInSe2 thin films were prepared on Mo-coated sodalime glass. A 400 nm thick Mo electrode was sputtered first, followed by a 1300 nm thick Cu/In alloy layer formed by co-sputtering. Then the thin films were furnace annealed in a selenization step at 550 oC. For passivation, the CuInSe2 thin film was immersed in TOP:S solution in a glove box for 48 hours under room temperature, followed by a 10 min toluene rinse to remove the excess TOP:S. From X-ray photoemission spectroscopy, a binding energy centered at 162 eV was observed which corresponded to the S 2p peak. Additional shoulders at higher energy side of In 3d3/2 (452 eV) and 3d5/2 (445 eV) were resolved which indicated the formation of In2S3. Quantitatively, it indicated 17% In on the surface of the thin film bonded to S. There were no change in Cu 2p1/2 and 2p3/2 peaks (952 and 932 eV, respectively), which meant there was no bonding formed between Cu and S. Besides, P 2p3/2 peak (133 eV) was observed, together with obvious broadening in C 1s peak (285 eV), which suggested that TOP:S molecules still resided on the sample surface. This layer is ultrathin, because there is no topographic difference between passivated and unpassivated CuInSe2 films from SEM measurements. When increasing the reaction temperature from room temperature to 80 oC, 29.2% further improvement in PL intensity (6.15X in total compared to unpassivated CuInSe2 films) was observed.

High-efficiency Cu(In,Ga)Se₂ (CIGS) thin-film solar cells are commonly prepared by the evaporation process in conjunction with the selenization using H₂Se gas. These are also the deficiencies in CIGS fabrication since evaporation method is unsuitable for large-area production and H₂Se is highly toxic. CIGS solar cells prepared by sputtering in conjunction with non-toxic selenization process hence attract considerable research interests. This work prepares the CIGS absorption layer by depositing an InGa₂Se₃ (IGS) layer on Mo/soda lime glass substrate prior to the deposition of CIGS layer via magnetron sputtering at various working pressures (1sim;30 mtorr). The stacking layers were then annealed in Se vapor ambient at 560°C to form the CIGS absorption layers. Afterward, the CIGS thin-film solar cell samples with the structure of Mo/CIGS/CdS/i-ZnO/IZO/Al were prepared and their performance was evaluated.
X-ray diffraction analysis revealed the IGS insertion effectively promotes the grain growth of CIGS layers and the crystallinity improves with the increase of working pressure of IGS deposition. Moreover, the addition of IGS layer might remedy the In and Se loss encountered in sputtering deposition of CIGS layer. It was found that the CIGS layer prepared without the IGS insertion exhibits the stoichiometric ratio of Cu:(In,Ga):Se asymp; 1:0.84:1.53 while that of the CIGS layer prepared by the stacking layer method is Cu:(In,Ga):Se asymp; 0.91:1:1.82. Notably, the remedy of In and Se loss simultaneously alleviated the excessive Cu in CIGS layer and suppressed the undesired Cu₂Se phase.
UV-Vis spectroscopy indicated the bandgap of CIGS decreases from 1.35 to 1.12 eV when the working pressure of IGS deposition increases from 1 to 30 mtorr. The CIGS containing the 30-mtorr IGS layer exhibited the best transport properties with p-type carrier concentration = 7.68×10¹⁹ cm^minus;3 and mobility = 3.37 cm²V^minus;1sec^minus;1 as revealed by the Hall measurement. Such a CIGS layer subjected to the 560°C post annealing in Se ambient was then transferred to the solar cell preparation and open-circuit voltage (V_oc) = 0.48 V, short-circuit current density (J_sc) = 0.32 mA/cm², fill factor (FF) = 53.9% and conversion efficiency (eta;) = 8.3% were achieved under the AM1.5 illumination condition. The IGS insertion effectively improved the device performance since the CIGS device containing CIGS layer prepared without IGS insertion exhibited V_oc = 0.37 V, J_sc = 0.23 mA/cm², FF = 41.36% and eta; = 3.48%. Composition analysis revealed the Ga/(In+Ga) ratio increase from 0.175 to 0.4 from the surface to the bottom of CIGS layer prepared with the IGS insertion. In addition to the improvement on crystallinity and transport properties, the IGS insertion might also result in a gradient Ga distribution in CIGS layer and thus improve the CIGS thin-film solar cell performance.

Non-vacuum printing techniques compatible with roll-to-roll manufacturing technologies promise large-area and low-cost flexible thin film solar cell production. Here, Cu(In,Ga)(S,Se)₂ (CIGSSe) solar cells were prepared on flexible stainless steel substrates from nanoparticulate Cu(In,Ga)S₂ precursor films. The absorber layer was obtained using a simple non-vacuum ink-printing method and a subsequent selenization process without the need of hazardous H₂Se or any additional reducing step. Short selenization duration of 3-5 minutes on a rapid thermal annealing system with elemental selenium vapour ensures high throughput for a cost-efficient production. A Ti diffusion barrier layer was sputtered onto the substrate to avoid diffusion of detrimental elements from the substrate. The incorporation of Na in CIGSSe was provided by sputtering a Na-doped Mo layer onto the Ti diffusion barrier layer before the deposition of the Mo back contact. Complete solar cells were prepared by conventional methods. The contribution will discuss the processes and their influences on layer morphology, composition and cell performance comparing rigid glass substrates with flexible stainless steel. With a similar approach we could already demonstrate power conversion efficiencies > 8% on rigid glass [1], here we report on comparable efficiencies up to 6% on stainless steel. [1] I. Klugius, R. Miller, A. Quintilla, T. M. Friedlmeier, D. Blázquez-Sánchez, E. Ahlswede, M. Powalla, Physica Status Solidi - Rapid Research Letters6, 297 (2012).

Long term stability is crucial to any product as the underlying technology moves from lab scale manufacturing to industrial production, like Cu(In,Ga)Se₂ (CIGSe) solar cells that move beyond production volumes of 1 GW_P/year now. Sodium is important to build highly efficient CIGSe solar cells, however little is known of its effect on the long term stability. The latter is mainly jeopardized by humidity, thus damp heat (85 % relative humidity at 85°C) is widely used as an accelerated aging procedure.
In this contribution we study the degradation process of CIGSe solar cells fabricated on flexible polyimide substrates with respect to the sodium content of the absorber layer. Unencapsulated cells were exposed to damp heat and investigated via current-voltage (IV), capacitance-voltage (CV), external quantum efficiency (EQE) and electroluminescence (EL) measurements to reveal the electrical and optical properties as well the lateral homogeneity of the cells. Secondary-ion-mass-spectrometry (SIMS) measurements were performed in order to determine the elemental distribution of the chalcopyrite constituents, as well as sodium, fluorine and oxygen in the cell.
We distinguished two stages of aging by cyclic damp heat treatment and electric measurements. First, the efficiency improves slightly, and second the cells degrade continuously. Thereby the proportion of improvement to degradation decreases for higher sodium contents. We found the overall degradation to proceed faster for cells with higher sodium content. Since the initial efficiency increases with sodium, it was possible to find an optimum sodium content for both, good initial efficiency and damp heat stability.
By combining the results of CV, EL, EQE and SIMS measurements, we conclude that the corrosion of the molybdenum back contact and its interface to the CIGSe absorber is the main contribution to the degradation of the solar cell. A decrease of the sodium aggregation at the back contact interface after damp heat treatment was shown by SIMS. We observed a decreased capacitance for samples with higher sodium contents that we attribute to local corrosion of the molybdenum contact. The EL measurements confirm the local corrosion of the solar cell as well as the overall dependence of the degradation rate on the initial sodium content of the CIGSe cell.

Chalcopyrite (CuInSe2) based solar cells have achieved lab efficiencies of over 20% [1], and due to the high absorption coefficient of this material, only thin films are required (< 2 µm). Electrodeposition offers a low cost and resource efficient method of synthesis. Yet, films produced by depositing Cu, In and Se in a single step are inhomogeneous through their depth and of nanocrystalline structure. In order to produce device quality material where the elements are distributed homogeneously through the depth and with crystal sizes of hundreds of nanometres, thermal treatment is essential. Conventional furnace techniques are in the time scales of tens of minutes. In this work it is shown that by using a single, one second laser pulse the material&’s crystallinity can be dramatically increased and absorber layers produced with potential for use in solar cell fabrication. Such a rapid annealing process would significantly reduce manufacturing time and thus be an attractive industrial process.
Using a 1064 nm Nd:YAG laser, the full, 1µm depth of the absorber layer is annealed without wasteful heat loss to the surroundings. XPS shows atomic rearrangement of the films, towards bulk stoichiometry, to be possible in this timescale. Our previous work has shown that low laser fluences (50 Wcm-2) can be used to produce films of crystallinity comparable to furnace annealed samples, but with timescales of up to a minute and probable loss of volatile Se [2]. In a single, one second laser pulse, both the surface and the bulk crystallinity of the films is improved as evidenced by Raman spectroscopy and XRD, where, for example, 1080 Wcm-2 reduces the FWHM by 72%, close to that realised in a 30 minute furnace method. SEM images show this annealing to be solid state, i.e. no melting occurred, and EDX proves stoichiometry is maintained.
Annealing is achieved without the requirement of a selenium atmosphere, only a thin (250 nm) layer of selenium evaporated onto the sample surface before laser treatment. This avoids using toxic H2Se or solid Se to maintain a saturated atmosphere, increasing costs, forming unwanted secondary phases and causing selenisation of the Mo back contact. Photoelectrochemical measurements made on the absorber layers demonstrate their p-type conductivity and a superior photocurrent response is observed for the Se capped precursor layers than layers without the Se cap.
Photoluminescence (PL) measurements were performed at room temperature showing that laser annealed absorber layers have an increase of radiative recombinations (RR) compared to the precursor layers and a shift of the maximum of the PL spectra. RR are proportional to the quasi-Fermi level splitting and thus to the maximum achievable Voc of the final solar cell.
[1] Jackson, P.; et al. Prog. Photovolt: Res. Appl. 2011; 19; 894-897
[2] Bhatia, A; Proc. SPIE 2012; 8473

CIGS solar cells have excellent radiation tolerance, which means their electrical properties do not degrade by 1MeV electrons. Conversely, cell performance is impaired with exposure to 10MeV proton irradiation, similar to other solar cell types. The radiation damage to the cells caused by proton irradiation gradually recovers when the irradiated cells are kept even at room temperature and the recovery rate is temperature-dependent. The radiation defect in CIGS solar cells, which decrease the cell performance, was reported as an In antisite defect. However, it remains unclear whether the other types of defects, namely Cu, Ga and Se Frenkel-pair in CIGS, which are simultaneously generated by radiation, impair cell performance or not. Therefore, we investigated these defects in CIGS solar cells induced by low energy electrons, enabling the type of radiation defect in the solar cells to be selected.
Electrons with energy of less than 500keV triggered copper- and gallium-related defects. The cells were irradiated with 250keV electrons at below 200 K because the radiation defects could be recovered with a thermal annealing effect.
The electron-induced defects were characterized by electroluminescence (EL), which is a powerful tool to analyze radiation defects in semiconductors. The EL image of CIGS solar cells before electron irradiation at 120K described small grains. The figure is thought to be the grain of the CIGS. After 250 keV electron irradiation of the CIGS cell, the cell illuminated uniformity compared to before the electron irradiation and the observed grains were unclear. In addition, the EL intensity rose with increasing electron fluence, meaning the change in EL efficiency may be attributable to the increased likelihood of non-irradiative recombination in intrinsic defects due to electron-induced defects. Since the phenomenon of light soaking is reported for CIGS solar cells, the 250 keV electron radiation effects for CIGS solar cells might be equivalent to the light soaking effect.
Radiation defects, which impair the cell performance in CIGS solar cells, lead to a decline in EL intensity. Conversely, 250 keV electrons increase the EL intensity of CIGS solar cells. The electrical property of defects irradiated with electrons does not correlate with that exposed to radiation defects. Therefore, it may be suspected that the copper related defects in CIGS degrade the CIGS solar cells.

Surface plasmon resonance (SPR) is the promising phenomenon to increase light absorption and reduce reflectance by trapping light in thin film solar cells. Metal nanoparticle like gold (Au) and silver (Ag) has revealed prominent localized surface plasmon resonance (LSPR) at UV, visible and near infrared (NIR) wavelengths. In this study, we investigate the SPR with Au nanoparticles onto the Cu(In,Ga)Se2 (CIGS) absorption layer. CIGS absorber layers have been fabricated by using two stages sputtering-selenization systems. In the first stage, CIG precursor layers were fabricated by using two targets of In and CuGa and onto the Mo coated polyimide (PI) substrate by using DC sputtering. The CIG precursors were converted into CIGS absorption thin film by selenization process at 400 oC. After the formation of CIGS thin film, Au nanoparticles were deposited for increasing time from 10 to 120 seconds by using sputter coater at 10-2 mbar pressure with 2mA current. The X-ray diffraction (XRD) patterns confirm the formation of Au:CIGS nanocomposite structure with prominent peak shift of CIGS reflections and presence of Au phase. The morphologies and atomic (at.%) composition uniformity onto the surface and along depth were extensively analysed with field effect scanning electron microscope (FESEM) and energy dispersive spectroscopy (EDS) respectively. The optical properties as transmittance, reflectance and absorbance of Au:CIGS layers were found to expand in the infrared region for by controlling the size of Au particles. Further, PL spectrums also confirms the surface plasmon resonance (SPR) with wide bad gap of Au:CIGS layers as the defect density decreases due to the deposition of Au nanoparticles onto the CIGS layer. Such SPR effect in Au:CIGS absorption layer will be a key parameter to further improve performance of the solar cell.

In the Cu(In,Ga)Se2 (CIGSe) based solar cells technology, the indium world supplies might become an issue if CIGS thin-film solar cells are produced in very large volumes. A thinner CIGSe layer (down to 0.2 µm compared to 2 µm actually), with minor loss in performance, could reduce the direct materials usage and thereby the material costs. In previous works we showed that reducing the CIGSe thickness by etching down to 500 nm mostly leads to Jsc losses due to the reduction of photons absorption, mainly in the large wavelength range. However for a direct deposition of CIGSe absorber, the question is if once one considers much thinner absorber layers, the same growth mechanisms as for thick films can be transposed or not. We have investigated different deposition processes to grow ultrathin Cu(In,Ga)Se2 films by coevaporation. The target thickness is around 200 nm. The main challenges are to grow homogeneous large grain material, with a controlled composition, and to avoid back contact recombination. Three strategies have been tested: adapting the 3-stage process with reduced material flows, evaporating all the metals in one stage, innovating with a new process tuned to allow the deposition of ultrathin CIGS films. The resulting films have been mainly investigated by X-ray Diffraction (XRD), Raman spectroscopy, Scanning Electron Microscopy (SEM), and Glow Discharge Optical Emission Spectroscopy (GDOES). Standard solar cell devices were completed and optoelectronic measurements were performed. Finally, the effect of some simple light management techniques to decrease the parasitic absorption in the cell such as replacing the CdS/i-ZnO buffer layers by ZnS/ZnMgO buffer layers or the Mo back contact by a higher reflective metal, such as gold has been studied.

Laboratory scale thin film solar cells based on co-evaporated Cu(In,Ga)Se2 (CIGSe) absorber and chemical bath deposited (CBD) CdS buffer have reached conversion efficiency of 20 %. Contradictory conclusions concerning the optimal absorber Cu content for reaching such very high efficiency have been reported in the literature. In the present study, we have investigated the photovoltaic parameters of cells prepared from 3-stage grown absorbers with Cu content varying from y = [Cu]/([In]+[Ga]) = 0.80 up to y ~ 0.95 by increasing the duration of the 3rd stage; with our standard CBD recipe, resulting in 50 nm-thick CdS, no significant difference could be observed from one sample to another. In contradiction, for thinner CdS buffers (down to 20 nm-thick), the absorber Cu-content appears crucial for the device performance (mainly impacting the Voc). As a result, we observe that absorbers with the highest Cu content tolerate much thinner buffers without Voc and FF losses; we could therefore improve the cell efficiency thanks to an improved Jsc (gain in the short wavelength region without long wavelength collection loss). In order to explore the origins of such Cu-content influence, I(V,T) measurements have been performed on this set of samples; the results clearly show specific voltage distribution relative to CdS thickness and CIGSe Cu-content. From these conclusions, one can suggest that the decrease of CdS thickness without Voc and FF losses appears possible if the absorber Cu-content is as close as possible to that of stoichiometry.

The decrease of absorber thickness in co-evaporated Cu(In,Ga)Se2(CIGSe)-based solar cell is important for both material consumption and production cycle time issues. Since the short circuit current (Jsc) progressively decreases with the absorber thinning due to the reduction of photons absorption, optical management seems to be essential to maintain high efficiencies in submicron absorber CIGSe solar cells.
The present study proposes alternative approaches to improve conventional back contact by using diffuse reflectance.
In this study, we propose Mo/metal/ZnO multilayer as a back contact. On the one hand side, surface roughness of ZnO layer are controlled by wet chemical etching. This rugosity induces the light scattering and have significant impact on the back contact reflectance allowing Jsc improvement. On the other hand, both the Voc and FF of the cells are expected to be hindered due to the perturbation of the standard Mo/Mose2/CIGSe interface. When ZnO roughness, multilayer stack properties, and absorber growth conditions are optimized, Jsc improvement can be achieved without Voc and FF losses.

In this work we report the preparation of photovoltaic structures using AgSb(SxSe1-x)2 as absorber layer and CdS as window layer. n-CdS thin films were deposited on FTO coated glass substrates from a chemical bath containing CdCl2, TEA, NH4OH and TU at 70°C for 20 min, an annealing at 400 °C was applied to these films to increase conductivity. Then p-AgSb(SxSe1-x)2 thin films were formed on the FTO/CdS by heating multilayers of Sb2S3/Ag/Se at 200 °C for 1 h followed by an increase in temperature to 375 °C for another 15 min, all in vacuum. For this, first Sb2S3 thin films of thickness 600 nm were grown on FTO/CdS from a solution containing SbCl3 and Na2S2O3 followed by Ag thin film of thickness 105 nm by thermal evaporation. These multilayers of FTO/CdS/Sb2S3/Ag were dipped in an acidic solution of Na2SeSO3 (pH 4.5), at room temperature to deposit selenium thin films. The Se deposition time in absorber layer AgSb(SxSe1-x)2 was 6 h. Structure, morphology, optical and electrical properties for the p-AgSb(SxSe1-x)2 were measured. X-ray Photoelectron Spectroscopy and conductive AFM studies of AgSb(SxSe1-x)2 thin films will also be presented. The PV structures showed conversion efficiencies greater than 2 % under illumination using a 150 W solar simulator and an AM1.5 filter.

In CIGS-based chalcogenide absorbers device efficiencies of 20% have been achieved. There has been a substantial interest in tandem cell structures in past decade to achieve a discrete jump in device efficiencies for thin film photovoltaics. Top cell absorbers with band-gap of 1.6 eV - 1.7 eV are the optimum choice for the wide band-gap top cell in the tandem structure. CGS, CIGSS, CIAS and similar materials have been investigated. However, for all these materials the material quality tends to degrade for the required band-gap range which poses a significant problem of lattice mismatch (interface recombination) and high defect density in the absorber. Thus limiting the open circuit voltage and thus efficiency of these devices. Large grain size is required to reduce grain boundary density in order to achieve low grain boundary recombination losses. In this work we propose extending CIGS-based materials to include all the group three IIIB elements: In, Ga, and Al to form CIAGS. Films were deposited from elemental sources using a UHV co-evaporation system at a base pressure of 2E-7 torr. Band-gap measurements using transmission spectroscopy were done and a substantial increase in the band-gap was observed for moderate amounts of Al and Ga in CIAGS films compared to CIGS/CIAS. We have determined the variation of band-gap with composition of the film according to following equation : Eg = 1+ 0.67*x + 0.11*x(1-x) + 1.7*y - 0.6*y*(1-y) + 3.5*x*y , where x and y are the Ga and Al fraction in the film respectively. A composition of Al/III = 0.2 and Ga/III = 0.3 is required for a band-gap of 1.67 eV. We investigated the morphology of the films and about 400 nm average grain size was observed for band gap as large as 1.7 eV determined from scanning electron micrograph. The films showed strong (112) orientation as determined by x-ray diffraction.

Heterojunctions based on chalcopyrite Cu(In,Ga)Se2 absorbers and CdS buffer layers (typically prepared by chemical bath deposition) are the most efficient thin-film system for photovoltaics. Replacement of the buffer process and/or material is attractive with respect to cost, in-line manufacturing and efficiency. This replacement proved to be less straightforward than originally assumed, but long-term research has nevertheless produced feasible options.
The range of possible applications of chalcopyrite-based devices can be extended by preparing on light-weight flexible substrates. It is believed that polyimide foils (PI) are a more universal substrate option than metal foils. They are, however, also the more challenging option because they require the absorber to be prepared at significantly lower substrate temperatures (low-T absorbers).
In this contribution we will report on our initial results, combining absorbers grown at low temperatures on PI with In2S3 buffer layers prepared by thermal evaporation or Spray-Ion Layer Gas Reaction (ILGAR). For the latter we have investigated two different precursors, namely Indiumacetylacetonate and Indiumchloride in ethanol. Using these approaches, we were able to achieve reasonable performance of a flexible Cd-free cell. However, we find that the process stability, reproducibility and device performance that had previously been achieved in conjunction with the standard absorbers have - to a certain degree - been lost and need to be reestablished. Low-T absorbers are characterized by inferior morphology with a higher density of grain boundaries and a larger internal surface area. Furthermore, the low preparation temperature causes kinetic limitations in phase formation which may result in segregations at internal and external surfaces.
We have characterized surface and bulk properties of absorbers and buffers as well as current transport in the devices. We compare these data for the three different In2S3 preparation methods introducing characteristic differences (0-12% Cl content, preparation temperature). We will show that the specific properties of low-T absorbers amplify effects which are already known to play an important role in junction formation in general:
- diffusion of constituting elements and contaminations across the heterointerface,
- modification of absorber properties by the buffer layer process environment, and
- the need for wet chemical surface conditioning.
So far we have achieved a flexible Cd-free cell with 11.5% efficiency without anti-reflective coating (Reference cell with CdS was 14.2%). Further progress will depend on tighter control of the buffer layer process conditions. It may also be necessary to narrow the gap between the properties of low-T and standard absorbers.

CuInSe2 is an important semiconductor material for thin film solar cell applications because of its high absorption coefficient, thus layers of around 2 mu;m thickness are required to absorb most of the usable solar radiation. There are several deposition techniques such as physical vapor deposition, electrodeposition, sputtering, etc. that can be used to prepare CuInSe2-based thin films. However, electrodeposition (ED) presents some advantages compared to other techniques because deposition of CuInSe2-based thin films can be performed at room temperature, atmospheric pressure and over large areas. Moreover it allows good control of film thickness and is self purifying; so that low purity precursor materials can be used. So, in this work, we report the electrodeposition of CuInSe2 thin films from a buffered bath performed on rigid and flexible substrates; soda-lime glass with a sputtered Mo layer and polyimide/Mo. In order to find out the deposition potential values to grow the CuInSe2, cyclic voltammetry studies were performed in a scanning range from 0.6 to -0.9 V (vs. Ag/AgCl). These studies were done using a buffered Cu-In-Se electrolyte purged with N2 at a 25 °C. During the whole scanning process the N2 gas flow was removed from the electrolyte but kept on top of the surface into the electrochemical cell. With the information obtained from these studies, CuInSe2 thin films were prepared at a deposition potential of -0.5 V (vs. Ag/AgCl) until a thickness of ~ 2 mu;m was reached. The electrodeposited layers were characterized by X-ray diffraction (XRD) to study their structural properties, Scanning Electron Microscope (SEM) for morphology details and Energy Dispersion Spectroscopy (EDS) for chemical composition. Results have shown that good quality polycrystalline films with a chemical composition very close to the stoichiometry of CuInSe2 can be obtained by ED. XRD results showed that as-ED CuInSe2 thin films have broad peaks, an indication of low crystallinity and small particle size. SEM images revealed a very compact and uniform morphology. In order to increase the grain size, it is necessary to perform a thermal treatment at high temperatures, so that CuInSe2 thin films deposited on soda-lime glass substrates/Mo were selenized at 540 °C and for the polyimide/Mo at 360 °C during 30 min, respectively. The thermal treatment was performed in a sealed tubular furnace containing a mixture of H2:N2 (10%:90%) and elemental shots of selenium as our Se source instead of the H2Se/Ar gas mixture used in a previous work [1]. Once the CuInSe2 films were selenized and characterized, they were used to prepare photovoltaic devices, the preliminary results will be shown here.
References
[1] M.E. Calixto, K.D. Dobson, B.E. McCandless BE, and R.W. Birkmire, J. of The Electrochemical Society, vol. 153 (6) G521-G528 (2006).

The CBD-Zn(S,O) remains one of the most studied Cd-free buffer layer for replacing CBD-CdS in Cu(In,Ga)Se2 based solar cells and has already demonstrated its potential to lead to high efficiencies solar cells and modules. The CBD-Zn(S,O) films have often been synthesized in basic medium using a zinc salt, with thiourea as sulfur precursor and ammonia as complexing agent. However, a key issue to implement a CBD-Zn(S,O) process in a CIGS production line is both the deposition time and the homogeneity of the deposition, which depend on the growth mechanism of the buffer layer. Several studies during the past years have been showing that it is possible to vary the deposition rate, the morphology and the composition of the buffer layer if a second ligand is introduced in the deposition bath.
The present contribution objective is to compare and determine the mechanisms involved during the deposition of CBD-Zn(S,O) using different additives presented in the literature. In that respect, the effects of various additives, complexing or non-complexing agents, such as H2O2, ethanolamine, and H2O2-trisodium citrate, on the structural, chemical, morphological and opto-electrical properties of Zn(S,O) thin films and on the final solar cell properties were investigated. A theoretical approach, based on thermodynamic and solubility calculations, has provided insights on the solution chemistry and on the effect of these additives on the growth of the CBD-Zn(S,O). In-situ growth deposition studies as a function of deposition parameters were then carried out using quartz crystal microbalance measurements (QCM) in order to deduce the impact of each additive on the growth mechanism of the buffer layer. The composition of the Zn(S,O) films deposited was determined by X-Ray Photoelectron Spectroscopy. The morphology and the structure of the films were analyzed and compared using Scanning Electron Microscopy, X-Ray Diffraction and Raman Spectroscopy. As the use of additive is relevant only if the solar cells present good properties, the final step of the study was to analyze the impact of the different Zn(S,O) formulations on CIGSe/Zn(S,O)/ZnMgO/ZnO:Al solar cell parameters.
Based on these results, a model for the role of each additive on the growth mechanism of alternative Zn(S,O) buffer layer is proposed. This allowed us to introduce a new approach for a better control of the CBD-Zn(S,O) reaction kinetics and of the homogeneity, leading to high efficiency solar cells, better than CdS references and with deposition times lower than 10 min.

Thin film solar cells with Cu(In,Ga)Se2 (CIGSe) absorber prepared by co-evaporation reach high efficiencies even on flexible polymer substrates [1,2]. The use of flexible polyimide (PI) substrates offers the advantage of roll-to-roll fabrication and monolithic device interconnection, which are both of importance regarding manufacturing cost. Due to their light weight and high radiation tolerance CIGSe thin film devices are attractive for space applications. Commercially available PI foils, however, only tolerate lower temperatures, which affects the dynamics of phase formation during the absorber growth process and also hinders the interdiffusion of In and Ga [3]. This leads to a more pronounced Ga double gradient when relying on a standard multi-stage CIGSe co-evaporation process [1,4].
To study the interdiffusion of In and Ga during low temperature CIGSe growth on Mo coated PI foil and glass substrates, we performed several co-evaporation processes with varied In, Ga and Cu flux profiles of nominally identical integral element composition. Glow discharge optical emission spectroscopy (GDOES) was performed to measure the depth dependent composition. Solar cell devices were manufactured and the devices were analyzed by IV, external quantum efficiency (EQE) and scanning electron microscopy (SEM). To investigate the spatial distribution of charge carrier collection, measurements by electron beam induced current (EBIC) are in progress.
In agreement to [1], the GDOES results show that the evaporation flux profiles can effectively adjust the In and Ga gradient in the low temperature growth process. The resulting Ga gradient affects the IV parameters, the EQE, the band-gap and the morphology of the absorber. Cell efficiencies between 11 - 14% and 8-12% were achieved on glass and PI foil, respectively. Changing the Ga gradient did not improve the overall cell performance. The co-evaporation of Ga within the Cu stage, however, leads to a higher Voc and band-gap compared to our standard low-temperature process; this can be attributed, by GDOES, to a higher [Ga]/([Ga]+[In]) ratio close to the hetero-junction. A low [Ga]/([Ga]+[In]) ratio close to the back contact or a shift of the notch of the Ga gradient in the bulk of the absorber results in a poor current collection in the near infrared of the EQE.
[1] A. Chiril#259;, Nature Materials 10, 857-861 (2011)
[2] R. Caballero et al., Prog. Photovolt: Res. Appl. 19, 547-551 (2011)
[3] D. Greiner et al., laquo; In situ x-ray analysis of a low-temperature Cu(In,Ga)Se2 co-evaporation process on flexible polyimide foilraquo;, 22th PVSEC Hangzhou, China (2012)
[4] S. M. Schleussner et al., Prog. Photovolt: Res. Appl. 20 (2012)

Even though Cu(In,Ga)Se2 (CIGS) solar cells have already been commercialized, their degradation mechanisms, such as radiation, humidity, and high temperature, have not yet been comprehensively clarified. One of the reasons for this is the difficulty of direct observation around the CdS/CIGS heterointerface. In fact, several interfaces are not uniform because of the grain boundary and thickness undulation of the CIGS layer.
Electrochemical impedance spectroscopy (EIS) is a nondestructive method of investigating electrochemical devices, using transfer functions such as impedance and admittance of the equivalent circuits of these devices. EIS measures the current response to various frequencies of voltages. In fact, EIS is applicable to various inhomogeneous phenomena such as corrosion, electroplating, and the electrode of battery. One of the advantages of EIS is that it can measure which capacitance and resistance is changed by existence of defects, grains, and diffusion of atoms. Therefore, the use of impedance spectroscopy (IS), which is applied to EIS for semiconductor devices, is appropriate for investigating these interface properties.
The constant phase element (CPE) is one of the most important equivalent circuit parameters. It indicates the non-ideal quality of a capacitor, which could make it vulnerable to interface inhomogeneity. The CPE is able to reflect defects and degradation properties around the CdS/CIGS interface. In this presentation, the defects and degradation properties of CIGS solar cells around the CdS/CIGS interface are investigated by IS. An equivalent circuit is proposed that reflects the complex structure of CIGS solar cells.
Impedance measurements were conducted at frequencies ranging from 10 Hz to 1 MHz. For all the measurements, a 10 mV AC signal and a DC bias less than VOC were applied to the test device. The measurements were conducted under dark and light (under AM1.5 light irradiation) conditions at room temperature.
IS reveals the electrical properties and defect physics around the pn junction of CIGS solar cells. In addition, the depletion layer width and uniformity, which are related CdS/CIGS degradation properties, can also be observed. This is the first step toward the practical application of IS as a new method for characterizing the heterogeneity of a pn interface.

Both on laboratory scale as well as in large area industrial production, chalcopyrite based solar cells have been processed for many years with a CdS buffer layer between absorber and transparent front contact deposited in a chemical bath process. However, CdS is toxic and therefore its usage increases the production and recycling costs. A substitution of CdS by In₂S₃ provides a Cd-free alternative, which can be integrated in a dry inline production. In order to understand this alternative heterojunction, we have investigated the stoichiometry and electronic properties of the nominal In₂S₃/Cu(In,Ga)(S,Se)₂ (CIGSSe) interface using photoelectron spectroscopy. Upon deposition of In₂S₃, no diffusion of absorber elements was observed, while our measurements indicate the formation of an indium-rich composition close to the In₆S₇-phase at the surface of the buffer layer. To account for the heat treatment as it occurs in the subsequent cell process, the samples were tempered up to 200 °C. This treatment leads to a distinct diffusion of copper and sodium from the absorber into the buffer layer resulting in a smaller band gap in the buffer layer.

Chalcopyrite-structure Cu(In,Ga)Se2 (CIGS) alloys are a suitable material for fabricating high efficiency, low-cost solar cells, and CIGS cells have been commercialized using the selenization technique recently. The Cu(In,Ga)(S,Se)2 (CIGSSe) alloys are also attracting attention as promising candidates for use as a light-absorbing medium of multiple-junction or tandem solar cells due to the wide variation in the bandgap energy (1.0-2.5 eV). Especially, the conduction and valence band levels can be controlled by changing the solid compositions of In/Ga and S/Se, respectively. However, growing single-phase CIGSSe alloys of high Cu(In,Ga)S2 mole fraction is difficult because of unwanted compositional separation due to the difference in the reaction rates of the two end-point compounds. In general, the selenization process is regarded as a slow reaction because it might be limited by reaction rate with Se atoms. On the other hand, the sulfurization process is a rapid reaction because it might be limited by diffusion speed of S atoms. Therefore, it is difficult to obtain CIGSSe film of uniform composition of S-Se by a sequential chalcogenization process such as selenization following sulfurization.
Here we propose the use of less hazardous, easy to handle, and reasonable cost metalorganic sources, diethylselenide [(C2H5)2Se: DESe] and ditertiarybutylsulfide [(t-C4H9)2S: DTBS], for obtaining Cu(In,Ga,Al)Se2 and CuInS2 films by the selenization or sulfurization method, respectively. Since a metalorganic source decomposes into atomic Se/S more easily than H2Se/H2S gas or Se/S vapor, the reaction rate is higher. From the technological viewpoint, CIGSSe thin films can be grown simultaneously by the selenization/sulfurization method using organometallic sources. However, reaction mechanisms of simultaneous chalcogenization for obtaining CIGSSe films have not been clarified. In this presentation, reaction mechanisms of CuIn(S,Se)2 thin film growth by the simultaneous chalcogenization will be investigated. The process was utilized to obtain homogeneous CIGSSe quaternary alloys of controlled amounts of Se and S organometal sources. Understanding of the simultaneous chalcogenization process may greatly assist the development of a high-performance and cost-effective commercial process.

Ga-doped ZnO (GZO) film is a promising transparent conductive oxide (TCO) material for use in electrodes of flat display panels and window layers of thin film solar cells. For ZnO-based TCO materials, the stability to damp-heat environment is a crucial research topic for practical applications. Recently, we have ensured the durability of GZO thin films to high humidity. Our group reported that, for the 100-nm-thick GZO (3 wt% Ga2O3) films, the humidity resistance property was greatly improved by indium co-doping (GZO:In, 0.25 wt% In2O3) together with the control of oxygen gas (O2) flow rates during film deposition. For GZO:In film grown at an O2 flow rate of 15 sccm, the relative change of electrical resistivity (ρ) was 12 % with ρ values of 4.1/4.6 mu;Omega;m before/after 500 h humidity test (60 °C and 95% relative humidity). The GZO film grown under the same condition had a relative change in ρ of 33 % with ρ values of 3.7/4.9 mu;Omega;m before/after the humidity test. Our research result showed indium co-doping can provide high humidity-resistant GZO:In films with low ρ. Herein, we will report the high-performance GZO:In films for the window layers in thin film solar cell devices.
We used ZnO sintered targets with contents of 3 wt% Ga2O3 and 0.25 wt% In2O3 to grow GZO:In films in ion plating with direct current arc-discharge system. GZO:In films with different thicknesses (0.1-1 mu;m) were deposited on glass substrates at 200°C under the O2 flow rate of 15 sccm. As the film thickness increased from 0.1 to 1 mu;m, the resistivity and sheet resistance decreased from 4.3 mu;Omega;m to 2.6 mu;Omega;m and from 42.7 Omega;/Sq. to 2.6 Omega;/Sq., respectively. Meanwhile, the mean optical transmittance (Tm) in the wavelength ranging from 0.4 to 1 mu;m decreased from above 86% to below 75%. For the sample with a thickness of ~0.5 mu;m had a low sheet resistance of 6 Omega;/Sq. and a Tm of 80%. The analysis of the results obtained by atomic force microscopy measurement revealed a quite flat surface with Root Mean Squared (RMS) roughness value of 0.7 nm, which will facilitate the surface roughness control of GZO:In films using wet-chemical etching processes. Compared to the commercial fluorine-doped tin oxide (FTO) coated glass, high-performance GZO:In films reported here showed the great potential to replace FTO coated glass for the window layer in thin film solar cell devices.
This work was supported by New Energy and Industrial Technology Development Organization (NEDO) under the National Project of Rare Metal Indium In Substitute Materials Development. Also, this work has been supported by Japan Society for the promotion of science JSPS (Kakenhi Grant Number 30320120), Basic Research A, the title is High performance ZnO-based hydrogen gas sensor.

Pulsed D.C Magnetron Sputtering (PdcMS) has been investigated for the first time to study the deposition of copper indium gallium diselenide (CIGS) thin films for photovoltaic applications. Pulsing the d.c. in the mid frequency region enhances the ion intensity and enables long term arc-free operation for high resistivity materials such as CIGS and has the potential to produce films with good crystalline properties, even at low substrate temperatures. However, the technique has not generally been applied to the absorber layers for photovoltaic applications. Growth of stoichiometric p-type CIGS with the desired electro-optical properties has always been a challenge, particularly over large areas, and has involved multiple steps and a dangerous selenization process to compensate for selenium vacancies. The films deposited by PdcMS had a nearly ideal composition (Cu0.75In0.88Ga0.12Se2) at temperatures ranging from no intentional heating to 400 oC. The films were found to be highly dense and pin-hole free. The stoichiometry was independent of heating during the deposition, but the grain size increased with substrate temperature, reaching about ~ 150 nm at 400 oC. Hot probe analysis showed that the layers were p-type. The physical, structural, electrical and optical properties of these films were analysed using SEM, EDX, XRD, four point probe, and UV-VIS-NIR spectroscopy. The material characteristics suggest that these films can be used for solar cell applications. This novel ion enhanced single step low temperature deposition technique may have a critical role in flexible and tandem solar cell applications compared to other conventional techniques which require high temperatures.

Chalcopyrite Cu(In,Ga)(S,Se)₂ (CIGSSe) film has a bandgap ranging from 1.05 to 2.40 eV by changing In/Ga and S/Se ratios. That is, it is possible to manufacture the next generation high-efficiency tandem cell with only CIGSSe-based cells. CIGSSe at regions of relatively low bandgap (< 1.2 eV) is studied comparatively positively. So far, Cu(In,Ga)Se₂-based small cells with the efficiencies of over 20% has been obtained by a three-stage process which is one of coevaporation process, and CIGSSe-based submodules with 17.8%using selenization and sulfurization process has been reported. On the other hands, there are few reports on the fabrication of CIGSSe-based solar cells of wide-bandgap regions. Therefore, growth mechanism and structural characterization for the wade bandgap CIGSSe film is not well analyzed. However, we believe that CIGSSe is candidate material for the top cell of a high-efficiency tandem cell.
In this study, CIGSSe films with wide-bandgap (>1.5 eV) with high Ga and/or S concentration were fabricated on Mo/soda lime glass substrates by a three-stage process. The thickness of the CIGSSe films was set to ~2 micron. A plasma-cracked cell was used as the S source. The S/(S + Se) and Ga/(In +Ga) ratios in the fabricated films were determined by electron probe micro analysis(EPMA). Also, in-plane distributions of elements on the film surface were obtained by EPMA. The depth profiles of elements in the fabricated films were measured through secondary ion mass spectrometry. The morphology of the films was investigated through scanning electron microscopy.
The fabricated solar cells had an Al-grid/n-ZnO/i-ZnO/CdS/CIGSSe/Mo/SLG structure with an area of approximately 0.5 cm² and its performance were measured.
A detailed analysis on compositional structure and cell performance will be addressed in this study.

In this study, we have challenged development of highly efficient flexible Cu(In, Ga) Se2 (CIGS) solar cells and submodules. Flexible thin-film solar cells based on CIGS absorber layers offer several advantages such as lightweight, application to curved surfaces, and space use. However, several issues must be solved to improve the efficiency of flexible CIGS solar cells. In this work, firstly, the photovoltaic performance of the laboratory-scale CIGS cells on rigid soda-lime glass (SLG) substrate was optimized. CIGS thin films of about 2 mu;m in thickness were grown by the three-stage process using a molecular beam epitaxy apparatus on the Mo-coated substrates. By optimizing the CIGS growth conditions such as substrate temperature, Se flux, and [Ga]/[In + Ga] composition ratio of CIGS films, a 19.8% efficiency CIGS solar cell with high open-circuit voltage (Voc: 0.761 V, Jsc: 33.3 mA/cm2, FF: 0.779, active area: 0.514 cm2, with antireflection coating) has been achieved to date. On the other hand, CIGS solar cells fabricated on alkali-free flexible substrates commonly require Na-doping into the CIGS absorber layers to enhance cell efficiency. The beneficial effects of Na on the electrical and structural properties of CIGS are widely known. Thus, Next, Na incorporation control technique, which the Na-compound deposited during CIGS growth, was developed. By applying the conditions of Na-doping suitable for three-stage process, the conversion efficiency of CIGS solar cells with alkali-free substrate comparable to that of CIGS cells with SLG substrate was achieved. By applying the high quality CIGS growth conditions and the Na-doping technique to flexible substrates such as a flexible Zirconia sheet or Ti-foil, flexible CIGS solar cells and submodules with high performance can be expected.

The synthesis of CIGS particle has been reported by so many researchers for CIGS solar cell. The synthesis method of CIGS powder was generally studied a liquid phase method such as solvothermal and colloidal process. These methods are easily controlled particle size, whereas the processes are complex and, also, mass production is difficult. Therefore, it is required that new method can produce a large scale by a lot and that the process is simple. Because of these reasons, mechanochemical method was selected among powder synthesis methods. But because CIGS powder can be easily reacted with organic solvents at room temperature, the CIGS phase can be changed easily with milling conditions, such as a kind of solvent, milling time and so on. But there are still insufficient reports for the synthesis and the phase change by mechanical milling. In this study, the phase of synthesized CIGS was observed with milling condition, and then, CIGS powder of single phase was synthesized by mechanochemical method. Morphologies of synthesized powder were observed with solvent and milling time. As a result, the milling condition was established without phase change of CIGS powder and the homogeneous phase of CIGS was synthesized. Crystallinity of synthesized powders was analyzed by XRD and their microstructure was observed by scanning electron microscope.

During the last decade, the colloidal synthesis of especially Cd - and Pb chalcogenide nanocrystals has been well-developed. Monodisperse sols of nanocrystals over a large size range have become available and are widely used as building blocks for electronic and optical devices. However, the issue of toxicity remains a major obstacle towards large-scale integration of semiconductor nanocrystals in applications. I-III-VI materials (Cu(In,Ga)(S,Se)₂ (CIGS) and related compounds) are interesting candidates towards greener chemistry and offer the possibility of band gap engineering both by using quantum confinement and altering material composition. Moreover, CIGS is a well-known absorber material for high efficiency thin-film photovoltaics, and the use of nanocrystals as precursor inks is an interesting route to decrease the production cost of CIGS solar cells.
Up to now, the synthesis of CIGS nanocrystals is mostly developed on an empirical basis. In this work, our aim is to understand the relation between reaction parameters (temperature, concentration, reaction time,..) on one hand and nanocrystal properties (size, shape, composition) on the other hand. We demonstrate that by altering reaction parameters in a rational way (1), CuInS₂ and CuGaS₂ nanocrystals with sizes from 5 to 20 nm can be obtained which vary in shape from quasi-spheres to flat hexagonal prisms. The ability to control the size and shape of CIGS nanocrystals is of particular importance for the deposition of crack-free thin films by, e.g., inkjet printing that can be incorporated in thin film solar cells.
(1) S. Abé, R. #268;apek, B. De Geyter, Z. Hens (2012), "Tuning the Postfocused Size of Colloidal Nanocrystals by the Reaction Rate: From Theory to Application", ACS Nano, 6, 1: 42-53.

Thin-film solar cells with Cu(In,Ga)Se2 absorber layers have reached record power conversion efficiencies of more than 20 % on glass substrates [1]. Only slightly lower efficiencies of up to 18.7 % have been shown when depositing such solar-cell stacks on flexible polyimide foils [2].
However, the specimen preparation for electron microscopy of these ZnO/CdS/Cu(In,Ga)Se2/Mo thin-film stacks on brittle substrates is very complicated. When applying mechanical and ion-beam polishing in conventional specimen preparation, delaminations of individual layers or generations of cracks within the thin-film stack are generally experienced. These damages deteriorate considerably the analysis of individual layers and their interfaces.
The goal of the present work is the study of microstructural, compositional, as well as electrical and optoelectronic properties of all layers and interfaces in complete solar-cell stacks on flexible polyimide foil substrates, if possible at identical positions. Preparation of cross-section specimens as well as their analyses is performed using a focused ion-beam machine. This approach allows for crystallographic contrast imaging of individual layers. Even the very thin MoSe2 layer between Cu(In,Ga)Se2 and Mo can be represented. Electron backscatter diffraction as well as by energy-dispersive X-ray spectrometry have been performed not only two- but also three-dimensionally [3]. The resulting microstructural and compositional properties can finally be related to electron-beam-induced current and cathodoluminescence analyses in order to reveal the structure-property relationships for Cu(In,Ga)Se2 solar cells on polyimide foils.
[1] P. Jackson et al, Prog. Photovolt.: Res. Appl. 19 (2011), p. 894.
[2] A. Chiril#259; et al, Nature Mater. 10 (2011), p. 857.
[3] D. Abou-Ras et al, Micron 43 (2012), p. 470.

The microstructural characterization of polycrystalline Cu(In,Ga)Se2 (CIGSe) thin films for solar cells has so far mainly been focused on point defects and grain boundaries. Systematical investigations on dislocations in these layers are still missing. These linear crystal defects can be considered to have substantial impact on device performance, also in view of that in CIGSe thin films, dislocation densities between 10^6-10^11 cm^-2 [1, 2] were reported.
In the present work, transmission electron microscopy (TEM) analyses were used to characterize individual dislocations in CIGSe thin films. The CIGSe thin film under investigation was deposited by multi-stage coevaporation processes with a [Cu]/([Ga]+[In]) ratio of about 0.81 and a [Ga]/([Ga]+[In]) ratio of about 0.28. TEM samples were prepared from completed ZnO/CdS/CIGSe/Mo/glass solar cell stacks. The electrostatic (scattering) potential, caused by atomic nuclei and electrons, was measured by means of inline electron holography at individual dislocations. We found decreased potential values with widths of 2-6 nm at dislocations. The depths of these potential wells vary depending on the orientation of the dislocations with respect to the electron beam. The possible origin of the potential wells, such as the displacement field around the dislocation core, local changes in composition, or charges at the dislocation core are discussed. We found that a Cu depletion in the vicinity of the dislocation core, as revealed by atom probe tomography, describes best the measured potential wells. Preliminary electron-beam-induced current measurements on a solar cell with an absorber layer of a high dislocation density of 10^10 cm^-2 do not exhibit any considerable reduction in short-circuit current in the CIGSe thin film. This may be related to Cu depletion leading to a lowered valence band maximum, which would implicate a hole barrier around the linear lattice defect.
[1] C.-M. Li et al., MRS Proceedings, 763, B4.2 (2003).
[2] J. Dietrich et al., IEEE Journal of Photovoltaics, 2 (3), 364-370 (2012).

In solar cell, the junction location directly relates to the interface recombination and the carrier transport over the junction. Consequently, the determination of the p-n junction location and its correlation with the metallurgical parameters plays a crucial role in Cu(In,Ga)Se2 (CIGS) solar cell design. However, in consistent results of metallurgical junction location have been reported either at the CIGS/CdS heterointerface (heterojunction) or on the CIGS side (buried homo-junction). The atomic force microscopy (AFM)-based electrical imaging techniques, scanning Kelvin probe force microscopy (SKPFM) and scanning capacitance microscopy/spectroscopy (SCM/SCS), directly measure electrostatic potential or carrier concentration of semiconductors in the nanometer resolutions. We have previously measured the potential across CIGS junction using SKPFM to determine the junction location, on a model device deposited on the GaAs(110) substrate. However, due to a large corrugation of the prepared cross-sectional surface, one cannot completely rule out experimental artifacts. By developing a polishing process using argon ion beam milling, we have achieved flat cross-sectional surfaces with a corrugation of ~20 nm across the device. This new surface preparation procedure has yield improvements in the SKPFM and SCM measurements. In our experiments, the cross-sections of the devices were polished in two steps, the first step with a 6kV argon beam for 3 hours to get a relative flat area, and the second step with a lower energy of ~4kV for 8 hours.
In the SKPFM measurement, one measures the changes of surface potential. In such a measurement, the location of the maximum electric field should correspond to the metallurgical junction. Our measurements show that the metallurgical junction is within several tens of nanometers from the CIGS/CdS interface. Because this distance is in the same order of the spatial resolution of the SKPFM and that the junction location from the surface potential data can deviate from its location in the bulk, it is important to use a complementary technique to determine the junction with a spatial resolution of <10nm. To achieve this, we have carried out SCS measurement. We measure the SCM dC/dV signal as a function of DC bias voltage (Vs) between the probe and sample. In p-side CIGS and n-side CdS/ZnO, the SCS spectra or dC/dV-Vs curves exhibit different line-shapes. The spectra were taken pixel by pixel across the junction with a step <10nm and the junction is within the transition region where the curves change from p-type to n-type characteristics. Detailed results and discussions will be presented.

Sodium plays a very important role in the development of CIGSeS chalcopyrite thin film solar cells. Sodium has a tendency to reduce detrimental point defects. It reduces compensating donors by substituting selenium vacancies Vse and therefore increases the p-type conductivity. Normally, the sodium precursor is deposited over the molybdenum back contact prior to the metallic precursor deposition in a two stage process. It could also be deposited over the metallic precursors. Since not much work has been carried out to investigate the effect of location of sodium precursor on the properties of the absorber films prepared by selenization of metallic precursors, a study was carried out in order to investigate this aspect. Initially, optimization of process parameters was carried out for the preparation of absorber films. Using the optimized parameters, absorber films were prepared with various amounts of NaF on top and below the CuInGa metallic precursors. The as-prepared absorbers were characterized using scanning electron microscopy (SEM), x-ray diffraction (XRD), and atomic force microscopy (AFM) to study the morphology and crystal structure. Increase in preferred orientation was observed for films with increase in the quantity of NaF. A significant difference in the crystallinity and morphology was observed for films with NaF in the front and at the back of precursor. The films with NaF in the front showed better crystallinity with compact and well-faceted grains compared to films with NaF at the back. Solar cell devices were also completed on the as-prepared absorber films. Devices prepared with absorber that had NaF in the front showed better open-circuit voltages compared to device that were prepared with NaF at the back. A detailed analysis is also being carried out to understand the effect of the NaF location on the thickness of MoSe2 layer formation and its effect on the device properties.

Cadmium sulfide films were deposited onto glass substrates by chemical bath deposition (CBD) from a bath containing cadmium acetate, ammonium acetate, thiourea, and ammonium hydroxide. The CdS thin films were annealed in argon (neutral atmosphere) or hydrogen (reducing atmosphere) for 1 h at various temperatures (300, 350, 400, 450 and 500 °C). The changes in optical and electrical properties of annealed treated CdS thin films were analyzed in detail. The results showed that the dielectric constants, refractive index, the band-gap and resistivity depend on the post-deposition annealing atmosphere and temperatures. Thus, it was found that these properties of the films, were found to be affected by various processes with opposite effects, some beneficial and others unfavorable. The energy gap and resistivity for different annealing atmospheres was seen to oscillate by thermal annealing. Recrystallization, oxidation, surface passivation, sublimation and materials evaporation were found the main factors of the heat-treatment process responsible for this oscillating behavior. Annealing over 400 °C was seen to degrade the optical and electrical properties of the film.

The future deployment of thin films photovoltaic technologies, especially CuIn_1-xGa_xSe₂ (CIGS) based ones, strongly depends on the development of non-destructive characterization methods, compatible with the accuracy, process rate and industrial environment conditions. In this frame, optical methods are very well suited and in particular using Raman spectroscopy, high levels of structural and chemical specificity can be obtained. This technique also requires no sample preparation or reagents, is contactless and non-destructive, being an ideal candidate for in-situ and in-line quality control. In this work, we present the application of pre-resonant Raman spectroscopy to the characterization of complete size modules (60 cm x 120 cm) using several excitation wavelengths (325 nm, 532 nm and 785 nm), where the absorbers were produced by the selenization of Cu/In/Ga electrodeposited multi-stacks onto Glass/Mo substrates. Subsequent CdS and i-ZnO/ZnO:Al layers were deposited by using chemical bath deposition and RF-sputtering to complete the modules. To demonstrate the viability of the proposed methodology, modules with different CIGS absorbers, CdS buffers and ZnO:Al windows layers were prepared and the optoelectronic parameters obtained under AM1.5 illumination conditions. Modules were also characterized using different excitation wavelengths by Raman spectroscopy, searching for pre-resonant conditions in each of the layers, and correlating the Raman characterization with the optoelectronic parameters. In particular, UV excitation wavelength has shown to be very useful for the characterization of ZnO:Al layers without interference of the others ones. A broad peak at approximately 505 cm^-1 has been associated to the presence of structural defects introduced by the doping, and their diminution correlated with the deterioration of the optoelectronic parameters of the modules. Analyzing the devices with 532 nm as excitation wavelength, a high specificity for the detection of CdS is obtained, correlating the intensity of the 300 cm^-1 characteristic peak with the properties of the buffer layer, without any interference of the absorber and windows layers. On the other hand, the use of 785 nm excitation wavelength allows us to selectively characterize the absorber in the presence of CdS and ZnO:Al, giving valuable information of the structural aspects of the space charge region. The Ga content and OVC formation at the absorber surface can be evaluated and correlated with the optoelectronic parameters. The study also shows, that spatial variations between the different regions of the modules, due to inhomogeneities in CIGS, CdS and ZnO:Al , can be effectively and simultaneously evaluated to feed in the fabrication process for possible deviations and quick in-line correction.

Cu(In,Ga)Se2 solar cells are usually processed in the substrate configuration and attempts to process them in the superstrate configuration so far has led to much lower power conversion efficiencies up to 12.8% [1]. On the other hand the superstrate configuration offers a number of advantages regarding industrial device fabrication as well as the option of direct application in tandem device structures. The drawback is the difficulty to design the heterointerface, which traditionally relies on buffer layers like CdS that are unstable at the high process temperatures mainly due to diffusion of the cations into the absorber during growth.
In this work superstrate Cu(In,Ga)Se2 solar cells are prepared by coevaporation at temperatures above 500°C directly on ZnO:Al/ZnO coated glass substrates without the presence of an additional buffer layer. Depending on the process conditions, the formation of a thin Ga2O3[2] or In2O3 layer has been observed at the interface. It was found that the In2O3 layer is highly defect rich and reduces the open circuit voltage dramatically, whereas the Ga2O3 layer acts as a diffusion barrier for Zn and reduces the recombination current in the cell. Using this approach, currently a max conversion efficiency of around 7% has been achieved. The solar cells show high current densities well above 30 mA/cm2 but relatively low fill factors and open circuit voltages. We attribute these losses to a photocurrent barrier induced by the high bandgap Ga2O3 in combination with a high defect density close to the interface, which is supported by admittance measurements on the same device structures. The devices are also found to exhibit strong metastabilities, such that light soaking and forward biasing the device leads to a large permanent increase in fill factor and open circuit voltage.
The device performance and metastability of these superstrate devices is modeled by numerical simulation, taking into account results from admittance spectroscopy and photoelectron spectroscopy of the interface properties. To overcome the efficiency limiting effects derived from this device model, different interface conditionings in combination with the application of a variety of buffer layers are carried out and will be presented.
[1] T. Nakada and T. Mise, “High-efficiency superstrate type CIGS thin film solar cells with graded bandgap absorber layers.” Proceedings 17th European Photovoltaic Solar Energy Conference,Munich, 2001.
[2] Terheggen, M.; Heinrich, H.; Kostorz, G.; Haug, F.J.; Zogg, H. and Tiwari, AN “Ga2O3 segregation in Cu(In,Ga)Se2/ZnO superstrate solar cells and its impact on their photovoltaic properties” Thin solid films, 2002, 403, 212-215

The chalcopyrite CuInSe₂ (CISe) semiconductor is an interesting ternary compound for thin film photovoltaic applications due to its physical properties such as bandgap value at ~1.03 eV, p-type conductivity and high light absorption coefficient (>10⁴ cm^-1). Various physical and chemical based techniques have been reported for the preparation of high-quality CISe films. Among them, electrodeposition (ED) is a very interesting alternative method because presents important advantages such as: low equipment cost because avoids both, high vacuum as well as high temperature requirements; exhibits also a high deposition rate and presents a relatively easy scalability to industrial level. In the literature, two basic methods of CISe ED have been reported: (i) co-deposition of the three elements in one or multi-steps ED, followed by a thermal annealing to enhance the crystalline quality of the as-grown layer; and (ii) a second approach was to deposit metallic precursors (Cu-In alloys or Cu/In multi-stacks) followed by reactive thermal treatment under Se atmosphere. The so called one-step ED process, has the drawback that usually the main chalcopyrite CISe phase is inevitably co-deposited together with several secondary phases (Cu-Au CISe phase, ordered vacancy compounds OVC&’s, elemental Se, Cu_xSe binaries, etc.), which extremely complicate the following thermal treatment and have shown to have a detrimental impact on the final solar cells optoelectronic parameters. In this communication, we report the preparation and characterization of ternary CISe, using a sequential ED of Cu and In metallic layers in separated baths onto Glass/Mo substrates, followed by a relatively low temperature thermal annealing treatment (450 ^oC) under Se containing atmosphere. The properties of the CISe absorber are strongly influenced by the morphological and structural characteristics of the metallic stacks precursors that depend on the ED operating conditions. In this case, the influence of pH and applied potential on the microstructure and morphology of the Cu-In precursors were studied. Moreover, we analyze the impact of the Cu/In atomic ratio on the electrical and optical properties of the CISe thin films, showing that the compositional issues play also a crucial role on the absorbers properties. Finally, we analyze the impact of the ED conditions on the final solar cells performance, preparing classical Glass/Mo/CISe/CdS/i-ZnO/ZnO:Al devices, and in a first optimization, we obtain a maximum 8.2% conversion efficiency solar cell. This conversion efficiency was obtained for a layer with a Cu/In ratio equal to 0.91 and annealed at low temperature (450 ^oC), being very interesting for the future development of low cost photovoltaic processes. We shall also discuss the importance that play both, compositional and ED parameters in the final solar cells characteristics.

CdTe-based thin film heterojunctions used for photovoltaic (PV) device fabrication continue to be a promising mean of direct solar energy-to-electricity conversion. One of the possibilities to explore the direct consequences of technology influence on the development of CdTe solar cells is a thorough study of low-T photoluminescence (PL). Defects and dopants are important to further enhancement of PV parameters and this technique can reveal the photo-active levels&’ properties and can be used to relate their influence to solar cell performance.
In this work, a comparative analysis of the localized levels has been investigated by PL in the 4.5-100K temperature range at different excitation intensities (0.01-30 mW, beam spot size of about 100 µm) of the Ar-Ion laser (514 nm). A measurement system consisting of a Czerny-Turner imaging spectrograph and a He flow cryostat equipped with spectrosil B windows was used. A spectral resolution of approx. 0.7 meV has been achieved.
The PL spectra recorded from the CdTe surface of ZnO/CdS/CdTe heterojunctions annealed in the presence of CdCl2 consist of several bands: (i) an impurity band, with peak intesity at 1.438 eV (4.7K), (ii) a band determined by D-A transitions is positioned at 1.565 eV, and (iii) a band determined by free excitons annihilation at 1.591 eV. The contour of the impurity band is determined by strong interactions with LO phonons. The increase of the PL excitation intensity causes a 12 meV shift to lower energy of the impurity band peak and determines bounded excitons annihilation to dominate the spectra in the excitonic region. Increasing the temperature (up to 77K) results in a redistribution of the probabilities of radiative transitions and at high excitation power, the appearance of a new band for energies less that 1.2 eV is observed.
The results obtained so far point at the need for additional PL investigations in the near infrared region (up to 1.3 µm). Thus, the PL experiments are being extended to near-IR region. In addition, the PL excitation wavelength is changed to ensure light absorption at different depths of the absorber. CdS/CdTe heterojunctions are investigated from the CdS/CdTe interface side as well.
PL studies of the as-deposited CdS/CdTe hetrojunctions as well as structures annealed in the presence of chlorides together with investigations of samples obtained by vapor transport with different Cd/Te ratio on glass substrates are underway and will provide new insights on the effect of intrinsic and extrinsic defects and complexes in CdTe.

Copper selenide (Cu_xSe) is a common secondary phase in copper indium gallium selenide (CIGS) and copper zinc tin selenide (CZTS) absorber layers. While a copper-rich absorber composition is claimed to enhance grain growth, it results in the formation of a Cu_xSe phase which can increase shunt conductance in the final solar cell. Selective etching of the secondary phase after grain growth solves this problem. However, the standard etchant, potassium cyanide (KCN) is toxic. In this study, the effect of several alternative etchants as well as a KCN reference solution on both copper-poor and copper-rich absorber layers has been investigated. Absorber layers were treated in alternative etchants containing different complexants to solubilize copper and different oxidizers to convert selenides to water-soluble selenates. After treatment periods of 30 s, 60 s, 120 s and 720 s, the solutions were characterized by ICP-AES to yield etching rates for copper, indium, gallium and selenium and the surface of the absorber layers was assessed by SEM-EDX and XPS. The oxidation step seems to be rate limiting: for an etch time of 720 s, both selenium and copper removal increase after a tenfold increase in oxidizer concentration. The cell performance of the etched absorbers was compared with that of KCN etched absorber layers.
The Flemish 'Strategisch Initiatief Materialen' (SIM) SoPPoM program is acknowledged for their support.

ZnO is an intrinsic, n-type semiconductor with a broad range of applications in optoelectronics and light-emitting diodes. As a semiconducting oxide material ZnO has low resistivity, high transmittance down to the UV spectral range, and good chemical stability. ZnO is thus a well-suited material to be used as transparent electrode for photovoltaic solar cells and electrodes. Although unintentionally doped ZnO is always n-type, due to the unavoidable presence of native defects that act as donors, a higher level of n-type doping can be attain by using group III metal elements such as Al, Ga, In, which substitute Zn. Substituting O by VII group elements, such as F or Cl, also results in an effective n-type doping [1].
CIGS solar cells use ZnO as window layer. Best results are obtained by sputtering a ZnO bilayer made of an unintentionally doped ZnO layer followed by a n-type ZnO:Ga layer. However, ZnO thin films can also be prepared by wet routes such electrodeposition (ED). ED presents some advantages compared to other techniques because deposition of ZnO thin films and other semiconductors can be performed at low temperature, atmospheric pressure and over large areas. Moreover it allows good control of film thickness through the control of deposited charge. Depending on the electrolyte, ZnO thin films can be deposited under several morphologies ranging from nanostructured and discontinuous films to extremely flat, compact and smooth films [2]. For photovoltaic window applications smooth films are desired because they exhibit higher transmittance.
We report on the synthesis and characterization of ZnO thin films prepared by ED in an organic electrolyte like DMSO for its use as optical window. Effective n-type doping of ZnO was achieved by varying the chlorine ion concentration in the electrolyte. The electrodeposited layers were characterized by XRD to study their structural properties, SEM for morphology details, Optical Spectroscopy to estimate the bandgap energy and Electrochemical impedance spectroscopy to calculate the doping donor concentration. Results have shown that good quality polycrystalline films can be ZnO obtained by ED. XRD results showed that as-deposited ZnO thin films exhibit narrow peaks, an indication of good crystallinity. SEM images revealed very compact and uniform layers, which is at the origin of the high transmittance. Further, when the chloride concentration in the bath increases an effective n-type doping of ZnO films takes place. n-type doping is evidenced by the drop of resistivity and the rise of donors concentration, obtained from Mott-Schottky measurements, as well as from the blue shift observed in the optical gap due to the Burstein-Moss effect. Preliminary results of photovoltaic devices prepared with ED-ZnO window layers will be shown.
References
[1] E. Chikoidze, M. Nolan, M. Modreanu, V. Sallet, P. Galtier; Thin Solid Films 516, 8146 (2008)
[2] H. Cui, M. Mollar, B. Marí; Optical Materials 33, 327 (2011)

The inability to harvest below bandgap light is a key factor limiting photovoltaic device performance. Two-photon upconversion provides a potential path to cost-effectively increase solar cell efficiency by allowing cells to use this low frequency light with minimal modification to the actual device structure. Recent encouraging reports of high upconverter efficiency make it now appropriate to take a critical look at the practical potential of upconverter enhanced solar cells and in this work we report on a systematic modeling study of realistic, finite-absorption bandwidth upconverters coupled with CdTe, CIGS, Silicon and Dye-Sensitized solar cells.
For the first time, upper bounds on efficiency improvements are reported as a function of the upconverter absorption bandwidth and spectral location. Surprisingly, due to the Fraunhofer absorption lines present in the AM1.5G solar spectrum, it is found that there are only two optimal absorption locations for upconverters when paired with low bandgap (<1.5 eV) solar cells.
Using our integrated simulation technique, which combines standard drift-diffusion device simulators with transfer matrix based light coupling calculations and a flexible phenomenological upconversion model, we identify architecture-specific challenges to upconverter/solar cell integration and provide detailed guidance to the upconversion research community on the general upconverter characteristics necessary for large efficiency improvements. Since upconverters are designed to be placed behind solar cells, the effect of rear illumination on solar cell quantum efficiency is investigated and found to be a critical factor limiting the applicably of upconversion to both CIGS and CdTe devices since carriers formed at the rear cannot diffuse to the pn junction.
We find that dye-sensitized solar cells are a very good candidate for enhancement with upconversion and we identify an upconverter that could improve the current world-record dye sensitized solar cell from 12 to over 15% power conversion efficiency. This is important because we also find that CIGS and CdTe solar cells are not good candidates for upconversion due to high back surface recombination losses and that while high efficiency crystalline silicon solar cells are able to effectively benefit from upconverted light, their low bandgap makes potential improvements relatively marginal.
Our work provides a simple way for solar cell researchers to estimate the benefits of upconverters on their devices and for upconversion researchers to identify the key remaining research challenges that need to be overcome for upconversion to play a role in future solar energy generation.

In single crystal CdTe, the electron drift mobility and Hall mobility range from 0.5 to 0.9×10³ cm²/Vs near room-temperature; typical sample thicknesses for the measurements are hundreds of mu;m. Very little is known about mobilities in thin-film CdTe that&’s used for solar cells, and device modelers have provisionally used the single-crystal values.
We report our measurements of the electron drift-mobility in thin-film CdTe solar cells. We used a photocapacitance technique to measure the electron drift mobility. The electron mobilities were too large to be resolved using the conventional photocarrier time-of-flight technique, which is limited to mobilities less than 10 cm²/Vs in these samples. The two techniques usually yield similar results for low mobility samples, but they do have different responses to trapping effects on the microsecond scale.
We have explored several research coupons with good properties under solar illumination (V_OC>0.7 V, FF>0.6). Some cells exhibit electron drift mobilities greater than 1.0×10² cm²/Vs, which could be consistent with single crystal band-mobility results. Some cells with comparably good solar cell parameters had electron mobilities less than 1.0×10² cm²/Vs. We have not yet identified the mechanism that lowers electron drift-mobilities in thin films below the range in single crystals. Based on temperature-dependence measurements, ionized impurity scattering has been proposed for single crystals with reduced mobilities.

The passivation of CdTe solar cells is necessary for the repeatable fabrication of high performing well-behaved devices. Traditionally, a process involving some form of CdCl2 along with high temperatures is employed for this purpose. In this series of experiments we investigate the effect of varying the traditional CdCl2 passivation of CdTe by adding other chlorides such as MgCl2 into the process. The effects of temperature set point, time, process gas, and pressure will also be investigated. Also of interest is the possibility of forming a highly doped field at the back of the device that would act as an electron reflector. An electron reflector would boost device performance by directing electrons back into the absorber layer and increasing the voltage while limiting recombination at the back of the device. By incorporating other chlorides, such as MgCl2, into the passivation process we attempt to build in such a field. The effects of varying the passivation process in this manner on device performance are characterized by current voltage and quantum efficiency measurements. Admittance spectroscopy and capacitance voltage measurements are used to study defect states, determine depletion width, and to investigate the presence of a highly doped field at the back of the device.

In2Se3 is a crucial p-type material for the formation of CIS-based or CIGS-based solar cells. In this study, we investigated systematically the influence of calcination temperature on the formation of In2Se3. XRD, Thermal Gravimetric Analysis (TGA), Raman spectroscopy, SEM, and TEM were used to characterize the phases and microstructure of In2Se3 powders. From the results of XRD, it was found that phase transformation of In2Se3 happened between 300 degree and 400 degree. When the temperature is raised to 400 degree and 500 degree, a pure In2Se3 phase is obtained. This result is also confirmed by TGA and Raman spectroscopy, and the temperature of phase transformation and vibration modes are demonstrated, respectively. By using Transmission Electron Microscopy with an attached energy-dispersive X-ray spectroscopy, the structure and composition of In2Se3 phase are also revealed. The ultimate goal of this study is to find the optimum conditions of precursor and process for objective compounds applied in the production of CIS-based or CIGS-based solar cells.

The technique of time-resolved-photoluminescence (TRPL) decay for material investigation has been applied since long. However, for non-ideal specimens/devices showing charge carrier separation, contact recombination, photon recycling, defect distributions and inhomogeneous excitation it is not a priori clear how to interpret the TRPL transients. In experiments, one can find multi-exponential decay, non-exponential decay and a variety of excitation dependencies. Using TCAD simulations, we investigate the influence of the different effects on the decay curve and on the derived TRPL lifetime. We first look at separate effects and next at effect combinations. It is found that photon recycling can well be neglected, curved TRPL transients may be an indication for charge separation, and a TRPL lifetime maximum at a certain excitation level may be the effect of Shockley-Read-Hall defect saturation and bimolecular radiative recombination. We give examples of CIGS layers and devices.

In spite of their high efficiency compared to other thin film PV technologies, the performance of CIGS solar cells should be further improved. Simultaneously, the cost can be lowered by reduced layer thickness of the active layer. Both aspects can be realized by using smart precision nanostructures dedicated to light management functionalities. The structures can be used to increase the incoupling efficiency of light into the absorber layer. Alternatively, such structures enable the further reduction in thickness of the absorber layer without loss in efficiency with significant cost saving as a result. For this, the depth profile of current collection in various thin film solar cell types are compared and we will discuss the preparation and application of various types of precision nanostructures in thin film photovoltaics. Three classes of precision nanomaterials will be discussed: metallic, (polymer containing) composite and metal oxide particles. For metallic nanoparticles, the distribution and the impact on optical characteristics on glass, transparent conductors and CIGS layers are presented.

Monodispersed colloidal semiconductor nanocrystals (NCs) are building blocks to harvest renewable energy for future demand. The extraordinary spectroscopic properties of NCs, caused by fine quantum space confinement regime, have made these materials promising candidates for thin-film optoelectronics, such as solar cells, light emitting devices, and photo sensors. In very recent years, there has been a considerable interest in the development of Field Effect Transistors (FETs) and solar cells having three dimensional NCs as active components. CdSe is a direct bandgap, II-VI compound semiconductor with the band gap of 1.74 eV as a bulk, however the band gap of a NC can be tuned by changing the size of the particle. CdSe NCs can be deposited on any surface by spin coating, or layer by layer dip coating. The optical properties of NCs have been investigated by many researchers; however the electronic properties of these NC thin films are not are not thoroughly explored due to their very poor conductivity caused by organic ligands. For optoelectronic applications the electronic coupling between NCs must be sufficient for efficient motion of charges from one NC to another. The electronic properties of colloidal NC solids can be enhanced through thermal annealing at moderate temperatures and chemical treatments.
Here we report the fabrication of uniform and conducting CdSe NC thin films on glass, Indium Tin Oxide (ITO) and on P-Si substrates by spin coat technique in a nitrogen filled glove box. The NCs synthesized in our lab show uniform size distribution, mono-dispersity and right stoichiometry. The treatment of CdSe NC thin films was studied using 1, 2-ethanedithiol (EDT) as a strong binding agent for improving the electrical and photo-response properties of CdSe NC-p-Si hetero junction solar cells. As spin coated NC films were compared with the films treated with EDT and annealed at different temperatures using Fourier transform infrared (FTIR) spectroscopy. FTIR spectra clearly indicate the complete removal of oleate organic ligands with EDT treatment. The Field Effect Scanning Electron Microscopy (FESEM) results clearly demonstrate highly uniform distribution of NCs on various substrates. Current-Voltage characteristics of devices made with these NCs shows photocurrent increased by few orders of magnitude. Our studies suggest that the combination of EDT treatment followed by shorter duration of CdSe NC film annealing enhance the electronic coupling in NC solids, leading to facile charge separation and transport. Our preliminary results from the fabrication of Copper Indium Gallium Selenide (CIGS) solar cell with CdSe NCs solid as a buffer layer will advance the applications of these materials in useful electronic-optoelectronic devices The current study suggests that NCs with small organic ligands are suited for solar energy conversion in solar cells. This work was supported by NSF-RISE and CREST (CNBMD).

Dating back to the very early days of research into CdTe as a photovoltaic material, the issue of sulfur diffusion from the CdS window layer into the CdTe absorber has been discussed [Vitrikhovskii, et al., Sov. Phys. Solid State, 1959]. Extensive research into the CdTeS pseudobinary phase diagram and the diffusivity of S in CdTe [Lane et al., J. Crystal Growth, 1999, Lane et al., Thin Solid Films, 2000] has indicated that concentrations of around 3 at% S can be expected in CdTe at a distance of 300 nm from the CdS/CdTe interface following CdCl2 activation of the cell. Additionally, the increased diffusivity at grain boundaries is predicted to cause significantly higher sulphur levels in the region of boundaries. Some TEM work on the diffusion of sulphur has been carried out [Terheggen et al., Thin Solid Films, 2003], finding sulphur at grain boundaries. The present study utilises STEM-EDS and STEM-EELS to investigate the issue of sulfur interdiffusion in CdTe as it occurs in real device structures rather than idealised systems. An investigation of CSS-grown CdTe and sputtered CdS has shown that sulfur interdiffusion is minimal, both in the bulk of the grains and at grain and twin boundaries. An optimised device (efficiency ~12%, 80 nm CdS), a thick CdS layer (300 nm, otherwise identical to optimised device) and a device with CdS sputtered at room temperature all show the same results; no measurable reduction in CdS thickness and no detectable alloying of sulphur into CdTe. Additionally, measurements are in progress of MOCVD-deposited CdTe and sputtered CdTe in order to determine how general the result is to the community as a whole. The consequences of this finding and the limits of the techniques, including the detection limits of EDS and EELS with reference to an ion-implantation standard, will be discussed.

A key advantage in the large scale production of thin film photovoltaics (PV) is that an inline process does not require the assembly of smaller cells into modules, as in the case of crystalline or polycrystalline silicon wafer based systems, but instead uses monolithically integrated cells. This well-known approach reduces cost and allows for continuous inline processes to be used, such as roll-to-roll production lines [1]. CdTe solar cells, deposited by atmospheric pressure metal organic chemical vapour deposition (AP-MOCVD), have achieved > 15 % [2] using evaporated gold back contacts. This back contacting process is convenient at the research scale but when moving to large scale, evaporated gold is relatively expensive compared to alternative contacting materials and is a fast diffuser in CdTe. Moving towards thin film PV modules means that alternative back contact needs to be assessed without resulting in excessive losses in device performance. Furthermore, the alternating scribing and deposition of different layers to form monolithic integration of cells, connected in series with each other, is still a challenge as electrical characteristics of thin film devices are influenced by the scribing parameters. The resolution and repeatability of the scribed lines, precision in substrate-pattern alignment, cell interconnects having low series resistance and high shunt resistance, are key to module performance [3]. It is therefore essential to optimise the laser parameters to create scribes with minimal HAZ (heat affected zone), smooth edges and no recast debris. This is further complicated as the area between the transparent conducting oxide (TCO) and back contact scribes (P1 and P3, respectively) is not active. With a sub-cell width of 10 mm or less to reduce the lateral conduction losses through the TCO and scribe lines in the order of several tens of microns in width, with the separation between P1 and P3 can be from tens to hundreds of microns. This non-active area must be minimised to increase the photocurrent generated by each sub-cell in the PV module. This paper investigates these challenges in terms of structural and electrical characteristics of thin film devices with respect to the scribing and contacting parameters used to form monolithic cell interconnections. A comparative study will be given between laser and mechanical scribing techniques with several metal back contacts for forming the monolithic interconnections of cells in CdTe thin film PV deposited by AP-MOCVD, over an area up to 37 cm2.
[1] A. Bosio, D. Menossi, S. Mazzamuto and N. Romeo, Thin Solid Films 519, 2011, 7522.
[2] A.J. Clayton, S.L. Rugen-Hankey, W.S.M. Brooks, G. Kartopu, V. Barrioz, D.A. Lamb, S.D.
Hodgson and S.J.C. Irvine, Proceedings of the PVSAT-8 Conference, April 2012, Northumbria
University, UK
[3] J. Perrenoud, B. Schaffner, S. Buecheler and A.N. Tiwari, Sol. Energ. Mat. Sol. C., 95, 2011, S8.

Cost/Wp is an important factor in developing the next generation of solar cells, but it must not be to the detriment of conversion efficiency. The goal for 3rd generation photovoltaic is to reach, and go beyond, the physical limits of single junction cells [1]. Dye sensitised solar cells (DSSC) have reached a conversion efficiency of 12.3 % over the wavelength range of 400 to 700 nm [2]. Thin film CdTe solar cells have recently been reported to achieve up to 17.3 % by First Solar [3]. In CdTe devices, the CdS window layer acts as a filter that absorbs light at short wavelengths. One approach to reduce these losses is to increase the band gap of the window layer, whilst maintaining its thickness to reduce shunting, by alloying it with Zn [4]. In doing this, a wide range of the solar spectrum is available for absorption between the CdZnS and CdTe band gaps. In order to further increase photon capture, two types of light harvesters can be used to absorb photons from different parts of the solar spectrum. In this context, the marriage of thin film and DSSC PV technologies may be able to offer efficiency enhancements whilst maintaining the benefits of each individual technology. To this effect, DSSC devices offer advantages in the nature of both the metal oxide photo-electrode and dye absorption bands which can be “tuned” to vary the optical performance of this part of the tandem device. Hence, the key challenge at this point is to optimise the tandem structure to achieve a net gain in conversion efficiency. Whilst DSSC/CIGS tandem cells have been reported in the literature [5], the origin of the optical losses has not been addressed. Using a 4-terminal DSSC/CdTe mechanically stacked tandem cell arrangement, the authors have studied the influence on the optical losses by varying parameters in the stacked tandem structure, using UV-VIS spectrometry, EQE and I-V measurements. This study will report on varying the glass thicknesses, the haze of TiO2 photo-electrodes and the absorbing dyes. Scattering losses at the glass/TCO/TiO2 interface can be reduced by up to 45 % in the DSSC top cell, via selection of the TiO2 photo-electrodes.
[1] W. Shockley and H.J. Queisser, J. Applied Phys., 32, 1961, 510.
[2] A. Yella, H.-W. Lee,.H.N. Tsao,.C. Yi, A.K. Chandiran, Md.K. Nazeeruddin, E.W.-G. Diau, C.-Y. Yeh, S.M. Zakeeruddin and M. Grätzel, Science, 334, 2011, 629.
[3] M. Kanellos, Greentech Solar, 26 July 2011 - accessed online (16/12/2011) http://www.greentechmedia.com/articles/read/first-solar-sets-efficiency-record-17.3-percent/.
[4] G. Kartopu, A.J. Clayton, W.S.M. Brooks, S.D. Hodgson, V. Barrioz,
A. Maertens, D.A. Lamb and S.J.C. Irvine, Prog. Photovolt: Res. Appl., 2012 (DOI: 10.1002/pip).
[5] P. Liska, K. R. Thampi, M. Grätzel, D. Bremaud, D. Rudmann, H. M. Ipadhyaya, A. N. Tiwari.
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Sputtered cadmium stannate (CTO, Cd₂SnO₄) thin films have shown great potential as transparent conducting oxides for thin film solar cells. Its high mobility provides superior optical and electrical performance compared to commercialized fluorine-doped tin oxide and tin-doped indium oxide. Substitution of a cadmium stannate/zinc tin oxide bilayer for the standard fluorine-doped tin oxide/tin oxide front contact enables larger short circuit current density, culminating in a world record CdTe solar cell in 2001. State of the art material is typically produced by a two-step process that involves room temperature sputtering of CTO followed by an annealing process where the sample is placed in direct contact with a cadmium sulfide film, often described as a proximity anneal. In this work we describe a process simplification in which high performance CTO is produced by sputtering a cadmium stannate/cadmium sulfide bilayer followed by annealing in an inert environment. Films were characterized by spectrophotometry, ellipsometry, Hall measurement, X-ray diffraction, electron microscopy, and energy dispersive spectroscopy. In this work we describe the optimization of both the standard proximity annealing process and the alternative bilayer approach. The latter process depends on the thickness of the CdS layer, and optimized films displayed resistivities less than 2×10^-4 Omega;-cm and mobilities approaching 60 cm²/V-s. The average transmission between 350 and 860 nm was greater than 90% for ~150 nm thick films. The elimination of the proximity annealing step should facilitate the continued development of this high performance TCO comprised of earth abundant elements.

Measurements of mobilities in polycrystalline chalcopyrite materials in general are strongly dominated by the influence of grain boundaries, such that reported values of mobilities show large variations and the estimation of intragrain mobilities remains a difficult task. On the other hand, previous studies of charge carrier dynamics by time-resolved photoluminescence have been limited by typical time-resolutions in the 50-100ps range. In the present study we employ femtosecond optical pump THz probe (OPTP) spectroscopy to access the microscopic mobilities and very fast charge carrier dynamics processes in polycrystalline CuInSe2 thin films.
Thin film samples of polycrystalline CuInSe2 were prepared by coevaporation using a standard 3-stage technique. The optical properties were investigated by OPTP spectroscopy. For stoichiometric CuInSe2, which was grown Cu-rich and subsequently KCN-etched, a Drude-like AC conductivity is observed. The variation of the pump-probe delay allows for probing the difference in hot and relaxed electron transport. At room temperature little difference is observed,with an estimated DC mobility of up to 1200cm2/Vs. However, at 4 K the hot electron mobility increases to 2400cm2/Vs at 4K while the relaxed electron mobility decreases to 800cm2/Vs, limited by charged impurity scattering.Lifetimes measured by time-resolved photoluminescence, as well as decay times measured by transient THz absorption indicate a non-radiative charge carrier lifetime smaller than 100ps for these stoichiometric CuInSe2 samples.
In contrast to these results we find that for Cu-poor grown CuInSe2 (as used in high-efficiency chalcopyrite thin film solar cells) the frequency dependence of the AC conductivity does not show Drude-like behavior, but rather indicates significant carrier localization. This behavior is most prominent at low temperatures and becomes more Drude-like at room temperature and at high excitation intensities, with much lower estimated DC mobilities < 100cm2/Vs. The photoconductivity for these films shows a very fast -ps decay followed by very long (ns) decay. These phenomena, which do not depend on whether the films are grown with or without the addition of Na, are explained by the trapping and localization of charge carriers in potential wells caused by the strong electronic compensation in these non-stoichiometric materials.

Zinc telluride (ZnTe) is an important back contact layer in thin film, cadmium telluride (CdTe)-based photovoltaics (PV) technology. It is believed that the electrical property of the ZnTe film affects the efficiency of a solar cell. In particular there is a great amount of interest in being able to control the carrier concentration of the ZnTe layer and thereby affect the position of the Fermi level across the device during the operation of the PV cell.
In this paper, we demonstrate deposition methods and conditions that allow the control of the electrical properties of nitrogen-doped ZnTe grown by RF magnetron sputtering. The electrical properties of the films, including carrier concentration and mobility were characterized using a van der Pauw Hall effect measurement method. We will demonstrate how the concentration of nitrogen in the plasma during the growth of the film impacts the conductivity of the ZnTe films. Films with hole concentrations in excess of 10^18 cm-3 and a high degree of crystallinity, as determined through X-ray diffraction measurements, were successfully grown. The films were characterized further with X-ray photoelectron spectroscopy (XPS) to analyze and understand the incorporation of nitrogen in the films. With the aid of an optical spectrophotometer, the bandgap of the films was measured and it was observed that the amount of nitrogen incorporated in the films resulted in the bandgap to change.

Recently, several techniques have been introduced to fabricate CuIn1-xGaxSe2 (CIGS) absorber layers such as co-evaporation, sputtering, spray pyrolysis, chemical method, etc. Especially, sputtering method is suitable for the good uniformity and large scale production of solar cells compared to any other manufacturing processes. However, two or three-stage sputtering process employing metal precursor deposition and post-selenization is not convenient to fabricate the CIGS absorber layer with a designed band gap. However, in order to design CIGS thin film solar cell showing a high efficiency the precise control of the band gap of the CIGS absorber layers is indispensible due to match solar spectrum that can be tailored by varying Ga content in the CIGS absorber layers. The band gap of the CIGS films varies from 1.0 to 1.6 eV with changing composition ratio Ga/(Ga+In) because CuInSe2 (CIS) and CuGaSe2 (CGS) are quite soluble solid solution.
In this study, we suggest a new technique to engineer the band gaps of the CIGS absorbers by one-step sputtering process utilizing CuInSe2 (CIS) and CuGaSe2 (CGS) ternary targets without selenization process. In the process we have fabricated the CIGS film libraries with different composition ratio Ga/(Ga+In)on a substrate by one batch of combinatorial sputtering method in which the CIS and CGS ternary targets were co-deposited under the optimized conditions.By using the technique, we could fabricated the CIGS film libraries with the composition ratio Ga/(Ga+In) of 0~1.0 on a substrate, of which the band gaps were successfully engineered from 0.98 ~1.60 eV.

Na-doped Mo (Mo:Na) has been of much interest as a Na source for CIGS solar cells, because its Na supply mechanism is similar to that of sodalime glass (SLG) and thus additional process is not necessary. For the low-temperature CIGS processes in which Na diffusion may be suppressed, the Mo:Na layer could also provide sufficient Na to CIGS films. However, since the Mo:Na layer should form part of back contact, it could interfere the role of Mo back contact in the formation of CIGS films, i.e. preferred orientation, Na content, the mechanical stability, etc. In order to maximize the advantages of Mo:Na layer with regard to high efficiency CIGS solar cells, we need to understand how various ways of introducing Mo:Na into back contact influence the performances of the CIGS solar cells. In this regard, Mo:Na layer was introduced onto the top and bottom of Mo back contact, respectively, and their thickness was varied from 10 nm to 50 nm for the top position and from 50 to 200 nm for the bottom position. CIGS films were deposited at 570 ^oC and 450 ^oC. Solar cell devices were completed by subsequently depositing CdS, i-ZnO, AZO, and finally Ni/Al electrode grid. The different positions of Mo:Na layer produced different trend of CIGS grain orientations; the top Mo:Na produced stronger (220)/(204) orientations, but the bottom one produced increasingly stronger (112) orientation with the Mo:Na thickness. This can be attributed to the competition between Na content and interface MoSe₂ structure, and the results were the same regardless of temperatures. Na doping in CIGS films were strongly dependent on the positions of Mo:Na as well as temperature. For high temperature process, the top Mo:Na layer was not effective in providing Na to CIGS films and adversely increased the series resistance. For low temperature process, the top Mo:Na layer was able to provide sufficient Na, similarly to the bottom Mo:Na, and the series resistance was not degraded. Rather thicker bottom Mo:Na increased the series resistances, degrading the solar cell efficiency. Together with bottom Mo:Na layer of optimal thickness, the top Mo:Na layer could be a good choice as Na source in the case of low-temperature CIGS process, but not good for high-temperature one.

Chalcopyrite Cu(In,Ga)Se₂ (CIGS) and its related multinary compounds have been attracting much attention for its highly efficient and cost-effective solar cell modules. Alkali metals, specifically sodium (Na), have been widely known to play important role to improve photovoltaic performance. Alkali doping by diffusion from soda lime glass (SLG) substrate is the one of the easiest way to incorporate alkali metals into CIGS absorber and high efficiencies of > 20% have been achieved with the convenient doping methods. Recently, advantages of transparent conductive oxides (TCOs) such as Al₂O₃, SnO₂, ZnO have been proposed, i. e., that enable bifacial or tandem structure with its transparent property and also light trapping with textured structure easily formed by optimizing deposition condition or chemical etching. But when TCOs are introduced as a back-contact on SLG substrates, the preferable diffusion of alkali metals are blocked by the oxide layers.
In this work we have found that introduction of thin Mo layer beneath TCO enhances diffusion of sodium through the TCO back contact into CIGS absorbers. CIGS absorbers were deposited on (1) Mo, (2) Mo/GZO and (3) Mo/GZO/Mo structure deposited on SLG substrates by three stage process. Na concentration of ~10¹⁹ atoms/cm³ was observed in CIGS absorbers grown on Mo/GZO/Mo hybrid back contacts by secondary ion mass spectroscopy (SIMS) whereas 2 orders smaller number of Na was found in CIGS layer grown on Mo/GZO substrate. Though the mechanisms have not been fully clarified yet, (1) formation of Na and Mo compounds such as Na₂MoO₄ and/or (2) changed crystal quality of GZO with existence of thin Mo layer underneath could enhance the diffusion of alkali metals.

We present the approach and new characterization technique of multi-dimensional admittance measurements. In standard admittance measurements, a semiconductor device is probed in the transverse dimension, between flat plate contacts. We extend such measurements to distributed, possibly non-uniform solar cells where one of the two contacts has very small (point-like) dimensions. As a result, both the real and displacement currents spread into lateral directions while flowing between the electrodes. Correspondingly, the probing electric field may result in contact voltages that are laterally not equipotential. The spatial voltage distribution will depend on the probing DC bias and AC frequency. The resulting measurement will give information about the system&’s lump parameters, such as open circuit voltage, sheet and shunt resistances, as well as the presence and location of shunts. Understanding of the measurement is developed through intuitive, analytic models. Numerical models, utilizing finite element circuits, are used to verify the analytic results, and also may be directly compared to or used to fit experimental data. While the presentation will focus on the physics and mathematical techniques, early experimental results will also be shown.
This work was performed under the auspice of the NSF award No. 1066749.

The CuIn1-xGaxSe2(CIGS) chalcopyrite thin films have been of increasing interest in efficient conversion of the light into electricity. They have a direct band gap, p-type conductivity, high absorption coefficient, high quantum efficiency, and great stability. They are also leading candidates for absorbers in high-efficiency heterojunction solar cells. In this study, we did fundamental research on the possibility of a photoconductor using CIGS chalcopyrite semiconductors for solid-state visible light image sensors. The typical structure of CIGS thin-film solar cells which has the highest efficiency composed of transparent conductive layer(Al-doped ZnO), resistive layer(undoped ZnO), n-type semiconductive layer(CdS), p-type semiconductive layer(CIGS), and metal electrode(Mo). However, toxic hazards in the usage of cadmium is undesirable. Moreover, the quantum efficiency of this structure in the short wavelength(< 520 nm) is deteriorated, because CdS(Eg=2.4 eV) thin film absorbs blue light. These are unfavorable for the solar cells and especially the visible light image sensors. For these reasons, a lot of effort has been dedicated to finding a Cd-free replacement material for the solar cells. On the other hand, the dark current of CIGS film itself is high for image sensors, because of the low resistivity of CIGS. To solve these problems, we applied a gallium oxide(Ga2O3) thin film as a n-type semiconductor layer. We supposed that the Ga2O3 thin film functions as a hole-blocking layer for CIGS films and suppresses an injection of holes from ITO electrode. The Ga2O3 thin films have n-type conductivity, a wide bandgap (4.7-4.9 eV), and high transmittance in the visible light. By widening the bandgap beyond that of CdS, a higher short wavelength quantum efficiency is expected in CIGS films. Furthermore, we compared undoped ZnO/CuGaSe2 structure with them. The ZnO thin films are also n-type semiconductor. Bandgap of them is 3.2-3.4 eV. We applied the CuGaSe2 thin films as a CIGS layer, because the energy bandgap of CuGaSe2 (1.64-1.67 eV) is suitable for the visible light(400-700 nm) image sensors. The electrical characteristics of heterojunctions composed of transparent conductive layer(indium-tin-oxide:ITO), n-type semiconductor layer(Ga2O3 or ZnO), p-type semiconductor layer(CuGaSe2) and metal electrode(Au) on glass substrates were examined to do basic study.
As a result, we could drastically reduced the dark current of CIGS by using Ga2O3. Then, the avalanche multiplication phenomenon was observed at an applied voltage of over 6 V.

Development of photovoltaic materials (PV) comprised of non-toxic, earth abundant elements are considered to be a major criterion to meet the ever increasing demand for energy [1]. Current PV technologies are mostly dominated by two most promising thin-films solar cells i.e., CdTe and Cu(In,Ga)Se2. However, due to price volatility issues (In, Ga), supply issues (In, Te), and environmental issues (Cd), these solar cell technologies are under question [2]. Recently, Cu2ZnSn(S,Se)4 shown promising properties as absorber material, however, there are still challenges with this system related to structural polymorphism, multi valence of Sn, and narrow stability range of multicomponent material [3]. Hence, the current research efforts are directed to search new inexpensive and earth abundant materials as thin-film solar cells.
Alternative ternary copper-sulfides system (CuxSbySz) such as CuSbS2, Cu3SbS3, and Cu3SbS4 has shown promising properties as potential low-cost sustainable absorber materials for thin-film solar cells in earlier studies [4]. However, thin-film based on these ternary copper sulfides system has not yet reach to device level and most of the research work are dedicate to improve the quality of the thin-films only. Hence, the fundamental physical properties of ternary CuxSbySz system are important to understand which, however, are not yet well discussed. Therefore, in present study, we employed first-principles calculation to analyzed structural, electronic and optical properties of CuSbS2, Cu3SbS3, and Cu3SbS4 compounds. We used the plane-wave projector augmented method and the Heyd-Scuseria-Ernzerhof (HSE06) screened hybrid functional as implanted in the VASP code.
We clarify that CuSbS2 and Cu3SbS3 are indirect band gap semiconductors while, Cu3SbS4 is a direct band gap material. The fundamental band gap energy is estimated to be Eg asymp; 1.73 eV for CuSbS2, Eg asymp; 2.01 eV for Cu3SbS3, and Eg asymp; 0.91 eV for Cu3SbS4. Calculated enthalpy of formation (ΔHf) suggests that these compounds are thermodynamically stable. Furthermore, investigation reveals that all these compounds have strong absorption spectra (>105 cm-1) in compare to other Cu-S based PV materials like CuInS2 and Cu2ZnSnS4.
References:
[1] C. Wadia, A. P. Alivisatos, and D. M. Kammen, Environ. Sci. Technol. 43, 2072 (2009).
[2] G. Phipps, C. Mikolajczak, and T. Guckes, Renewable Energy Focus 9, 58 2008; B. L Cohen, Geochim. Cosmochim. Acta 48, 203 (1984); P. Das, S. Samantaray, and G. R. Rout, Enviorn. Pollut. 98, 29 (1997); K. Zweibel, Science 328, 699 (2010).
[3] A. Redinger, D. M. Berg, P. J. Dale, and S. Siebentritt, J. Am. Chem. Soc. 133, 3320 (2011); T. Tanaka, A. Yoshida, D. Saiki, K. Saito, Q. Guo, M. Nishio, and T. Yamaguchi, Thin Solid Films 518, S29 (2010).
[4] Y. R. Lazcano, M. Nair, and O. Nair, J. Cryst. Growth 223, 399 (2001); Mod. Phys. Lett. 15, 667 (2001); A. Rabhi, M. Kanzari, and B. Rezib, Thin Solid Films 517, 2477 (2009).

The efficiency of thin-film solar cells based on the kesterite structured compound semiconductors Cu₂ZnSn(S,Se)₄ (CZTSSe) has already exceeded the 11 % limit [1] but is still far below the record efficiency of CdTe (17.3 %) [1] and Cu(In,Ga)Se₂ solar cells (20.3 %) [2].
Currently, the fabrication of high-performance CZTSSe solar cells having a Cu-poor and Zn-rich composition is impeded by the presence of secondary phases, which are formed as soon as growth conditions are outside the narrow homogeneity range of the CZTSSe phase. In general secondary phases (such as Zn(S,Se), Sn(S,Se), Cu_2-x(S,Se), Cu₂Sn(S,Se)₃) are detrimental to the solar cell efficiency.
The identification of these secondary phases with conventional methods is difficult or even impossible due to their similar crystal structure.
Therefore, Atom Probe Tomography (APT) was used in this work to investigate Cu₂ZnSnSe₄ (CZTSe) thin-film absorber layers prepared by co-evaporation on soda-lime glass substrate and post-deposition annealing.
We show the presence of a complex network of nanometer-sized CZTSe and ZnSe domains for the studied absorber layers. Some of the ZnSe domains are found to contain precipitates of Cu- and Sn-rich composition. Furthermore, segregation of sodium and sulfur/oxygen impurities was detected at the CZTSe/ZnSe interface. Both secondary phase formation and impurity distribution are discussed with respect to the growth conditions.
[1] M. A. Green, K. Emery, Y. Hishikawa, W. Warta, E. D. Dunlop, Prog. Photovolt: Res. Appl. 2012; 20 :606-614.
[2] P. Jackson, D. Hariskos, E. Lotter, S. Paetel, R. Wuerz, R. Menner, W. Wischmann, M. Powalla, Prog. Photovolt: Res. Appl. 2011; 19, 894.

High temperature multi-source co-evaporation has been the most successful way to create record efficiency Cu(InGa)Se2 devices, yet many groups have been unable to achieve record device efficiencies by transferring these methods to the Cu2ZnSnSe4 system. The difficulties stem from the dramatic difference in the thermochemical properties of the Cu-Zn-Sn-Se and Cu-In-Ga-Se systems. The primary issue confronting control of Cu2ZnSnSe4 film growth in vacuum is surface decomposition by the loss of volatile elements and binary compounds at the high temperatures necessary for Cu2ZnSnSe4 formation. The resulting films can thus exhibit secondary phases at the front and back interface as well as bulk inclusions, all of which are difficult to detect and can have strong effects on cell performance. A key advantage of co-evaporation at film formation temperatures is the potential to drive reaction chemistry towards desired properties by controlling the elemental incident fluxes throughout the deposition over a wide range of gas-phase compositions. Thus, despite the challenges in co-evaporation, with the proper fabrication procedure this equilibrium can be restored at the growth surface by managing substrate temperature and constituent effusion rates to mitigate decomposition and element loss.
In this paper, AES and XPS were used to investigate compositional gradients obtained at the front and rear interfaces of co-evaporated Cu2ZnSnSe4 films deposited on Mo/glass substrates at 500°C. Deposition with a uniform incident flux followed by shuttered vacuum cool-down yielded films with a ZnSe phase at the absorber/Mo interface and Cu-rich composition at the surface of the exposed film. Devices made with CdS/ZnO/ITO window layers on these absorbers never exceeded conversion efficiencies >1%. It was found that decomposition at the surface could be prevented by continuing effusion of Se and Sn during the cool-down of the substrate. Resulting films demonstrated more faceted grains and XPS showed more uniform depth profile near the surface consistent with less decomposition, as well as significantly improved device performance. Further improvements could be made by decreasing the substrate temperature to 300°C during the initial stages of deposition which helped reduce the ZnSe phase found at the Mo interface. This paper will present composition data on CZTSe films deposited under different deposition conditions where the substrate temperature, incident fluxes, and cool-down procedures were varied along with a thermochemical assessment of the predominant growth mechanisms. Solar cell current-voltage parameters will also be presented for representative cases.

We report the development of a general model of native point defect equilibrium for multinary semiconductors including the effects of vibrational entropy. We apply it to predict the concentrations of native defects in CZTS for typically used thin film processing conditions. The effects on the crystal&’s vibrational entropy accompanying the formation of each type of defect are different and result in significant changes in predicted concentrations for samples processed at typical temperatures. We also present results in terms of experimentally-accessible variables (i.e. the resulting composition rather than imposed chemical potentials), making a closer connection to experiment.
First-principles calculations of the vibrational density of states for a 64 atom supercell with and without each specific defect were carried out within the local density approximation using SIESTA. We first discuss the calculation of the vibrational entropy term and demonstrate that the ab-initio calculations predict temperature dependencies different than those from typically-used approximations. We then discuss the application of our framework to the defect equilibrium in CZTS during annealing in the presence of elemental sulfur in single-zone and two-zone processes. We focus on the resulting defect populations and predicted hole concentration after rapid quenching and slow cooling to simulate real sulfurization experiments.
As expected, two low-formation-enthalpy copper-related acceptors - the Cu vacancy and copper on zinc - dominate across the entire range of Cu compositions which makes manipulation of the hole concentration difficult. We establish that, despite its large formation enthalpy, when experimental diatomic sulfur vapor pressure curves are used to set the sulfur chemical potential, large concentrations of compensating sulfur vacancies are formed under typical single-zone annealing conditions.
We will discuss specific predictions of the model and demonstrate its applicability to other systems.

The kesterite semiconductor Cu2ZnSnS(e)4 is widely discussed as a suitable absorber layer to replace Cu(In,Ga)Se2 in thin film solar cells, if thin film photovoltaics are to be deployed on the terawatt scale. However, there are several challenges in growing thin films of kesterite due to the fact that there are three cations in the compound, as well as a fairly narrow compositional window to grow the semiconductor. This report is divided into two parts, the first part will discuss the growth of kesterite semiconductors suitable for use in photovoltaic devices, and the second part will discuss the ability of common X-ray diffraction and Raman scattering techniques to quantitatively discern the phase purity in the kesterite system.
The growth of Cu2ZnSn(S,Se)4 requires elevated temperatures regardless of the way that the elements are assembled on the substrate. At these elevated temperatures a careful choice of annealing environment is mandatory in order to maintain the integrity of the semiconductor. Particular attention should be paid to the partial pressure of chalcogen and tin chalcogenide within the annealing environment [1]. Simple thermodynamic calculations reveal that the relatively lower stability of Cu2ZnSn(S,Se)4 in comparison to CuInSe2 can be explained by the ease in which the oxidation state of tin may be lowered, and the relative volatility of its lower oxidation state chalcogenide as compared to the indium binary chalcogenides[2].
The partial pressure of the chalcogen is also important when considering the initial growth of the semiconductor from a metallic precursor. The concept of chalcogen partial pressure may be used to understand the kinetics of growth of the semiconductor and also be used to understand the formation reaction pathway of the kesterite. Another important variable is the microstructure of the metallic precursor which can be manipulated depending upon the way it is deposited and the thermal treatment it receives before it reacts with any chalcogen. These insights are important for developing methods to improve the absorber layer, and hence device performance.
Regarding secondary phase identification in thin films primarily consisting of Cu2ZnSnS4, we have carried out experiments to show that thin film X-ray diffraction and green excitation Raman scattering are insufficient to quantitatively detect the presence of Cu2SnS3 and ZnS secondary phases.
[1] A. Redinger, D. M. Berg, P. J. Dale, S. Siebentritt, J. Am. Chem. Soc. 2011,133, 3320
[2] J.J. Scragg, P.J. Dale, D. Colombara, L.M. Peter, Chem. Phys Chem. 2012, 13, 3035

Interest in Cu₂ZnSn(S,Se)₄ (CZTSSe) as a sustainable material for use in thin-film photovoltaics has been increasing rapidly in recent years. The prospect of producing CZTSSe devices without the requirement to employ costly vacuum based deposition methods has also attracted considerable attention. We recently reported the preparation of CZTSSe devices by a solution based synthetic route employing nanoparticle ink based precursor solutions coated onto molybdenum coated glass substrates and annealed at high temperature in the presence of elemental selenium (Y. Cao, et. al, J. Am. Chem. Soc.2012, 134, 15644). Finished devices were prepared by chemical batch deposition of CdS emitter followed by transparent front conductor via sputtering. Solar conversion efficiencies as high as 8.5% (1 sun, AM 1.5) have been obtained to date. We have also reported CZTSSe devices from simple solution based mixtures of molecular precursors. One of the intriguing features of devices prepared via both of these routes is the formation of a complex thin film microstructure during the selenization/annealing process. The resulting PV devices exhibit a large grain crystalline outer layer of CZTSSe which is separated from the Mo/MoSe₂ back contact by a carbon rich metal chalcogenide containing ‘small-grain&’ layer. In this paper we report on the detailed characterization of this microstructure and its impact on device processing and performance.

Two bulk defect levels of specific properties have been identified in polycrystalline and epitaxial thin film samples of two end-point compositions of Cu(In,Ga)Se2 chalcopyrites by using photocurrent spectroscopy - modulated photocurrent method and photoinduced current spectroscopy [1,2]. They act as electron traps and exhibit a wide spread of their two basic electronic parameters: attempt-to-escape frequency and activation energy which are correlated according to the Meyer-Neldel rule. In all sample categories (polycrystalline, epitaxial, CuGaSe2, CuInSe2, Cu-rich, Cu-poor) we observe a characteristic relation between decay rate of captured carriers and concentration of free holes: it increases with concentration of holes. This observation was made not only on different samples but also on one sample by changing measurement condition (under different secondary illumination fluxes). The highest activation energy and the lowest decay rate is observed in the n-type (in case of Cu-poor epitaxial CuInSe2) or highly compensated (in case of Cu-poor epitaxial CuGaSe2) samples. We conclude that two processes responsible for decay of captured charge, thermal emission of electrons and capture of holes proceed here with similar probabilities. Depending on hole concentration one or the other prevails in the experiment. The same levels have been detected also in some Schottky diodes and CIGS solar cells by capacitance spectroscopy. Since their specific characteristics are revealed in both thin films and in junctions we conclude that they originate from defect properties, and are not affected by the structure type or method used. Very low capture rate for holes, of the same order of magnitude as electron emission rate is a feature unusual for typical deep levels and might be an indication of a DX-type defect strongly correlated to the lattice. The InCu antisites and their complexes with copper vacancies are possible candidates since according to theoretical calculations [3] they constitute intrinsic DX centers. We conclude that although these defects are in abundance in solar cells, thanks to their low activity as recombination centers in their deep DX state they do not influence much the efficiency of baseline CIGS-based devices.
[1] A. Krysztopa, et. al. J. Appl. Phys. 110 (2011) 103711.
[2] A. Krysztopa, et al. J. Phys. D: Appl. Phys. 45 (2012) 335101.
[3] S. Lany, A. Zunger, Phys. Rev. Lett. 100 (2008) 016401.

Study of defects in Cu(In,Ga)Se₂(CIGS) is necessary for understanding its carrier recombination processes and enhancing its device performance. Since emission time of trapped carriers at deep defects is long, the deep defect is a convincing candidate for the carrier recombination center. In this study, transient photocapacitance spectroscopy (TPC) and photoluminescence using two kinds of wavelength for excitation (PL-TW) have been carried out to clarify the properties of the deep defects in CIGS. In the TPC measurements, we observed a difference in the capacitance transient measured in the dark and that measured under the sub-bandgap monochromatic light from a halogen lamp. The TPC spectra of CIGS solar cells exhibited a defect level with an optical transition energy of about 0.8 eV. The spectrum shape in the sub-bandgap region is independent of the Ga concentration. Therefore, the optical transition energy to the defect level is almost constant with about 0.8 eV from the valence band maximum[1,2]. The PL-TW involves two different pumping light sources, that is, 635 nm diode laser used as an above-gap excitation and 1550 nm diode laser whose energy corresponds to the defect level (0.8 eV) resulting in having a role of charge saturation at the defect level. For all specimens (regardless of Ga concentration), the photoluminescence peak intensity under the irradiation of two pumping light sources (635 nm and 1550 nm) was stronger than that measured with only the irradiation of 635 nm laser. The increase in the PL intensity under 1550 nm laser irradiation suggests a reduction in the non-radiative carrier recombination processes. The increase in occupation of trapped charges at the deep defects under the 1550 nm laser irradiation seems to cause a suppression of the non-radiative recombination process through the deep defects. This tendency was significantly enhanced with increasing the Ga content of the CIGS films. Thus, the reduction of deep defects is a crucial issue for improving the performance of the high-Ga-content CIGS solar cells.
[1] J.T.Heath et al., Appl.Phys.Lett. 80, 4540 (2002). [2] T.Sakurai et al., Thin Solid Films 517, 2403 (2009).

The defect content of a material is typically investigated computationally by determining the formation energies of relevant defects and defect complexes. The ones with low enough formation energies are then concluded to exist in the material in substantial quantities. Following a similar logic, the existence of several defect complexes in CuInSe2 (CIS) has been suggested in previous studies: In_{Cu}+2V_{Cu} [1], In_{Cu}+Cu_{In} [1], and V_{Cu}+V_{Se} [2]. While the significance of formation energies cannot be denied, they do not alone determine whether a specific complex would exist in the actual material. A defect complex consists of two or more individual point defects, some of which can migrate with ease, and some remain practically stuck where they are. Fast-diffusing defects meet other defects and, depending on whether they experience mutual attraction or repulsion, either cluster together or move away from each other. Diffusion kinetics is therefore equally important in predicting the defect microstructure of a material. By supplementing formation energy calculations with calculations of migration and binding energies, we have investigated clustering processes in CIS. We have re-evaluated the feasibility of previously suggested complexes and bring forth a new kind of complex that fulfills the requirements of low formation energy, kinetic formation, and stability.
[1] S.B. Zhang, S.-H. Wei, A. Zunger, and H. Katayama-Yoshida. Phys. Rev. B 57, 9642 (1998)
[2] S. Lany and A. Zunger. J. Appl. Phys. 100, 113725 (2006)

The thermodynamics and electronic properties of intrinsic point defects in CuInSe₂ and CuGaSe₂ were studied by calculations based on screened-exchange hybrid density functional theory. While self-doping and compensation in Cu(In,Ga)Se₂ has been understood to be due to copper vacancies and (In,Ga)_Cu antisites, our results suggest possible contributions also from the antisites Cu_In and Cu_Ga and from copper interstitials. Cu_In and Cu_Ga antisites exhibit two hole trap levels in the band gap, one of which can be attributed to the exprimentally observed N2 level and the other is probably related to deep levels as measured in photocapacitance spectroscopy. In contrast to the accepted view, complex formation of antisites with copper vacancies is found not to be decisive for explaining the favorable properties of CuInSe₂, since In_Cu is already shallow by itself. The results also raise doubts about the relevance of selenium vacancies and DX centers for experimentally observed metastabilities. Finally, guidance to the optimal preparation conditions with respect to the intrinsic point defect physics of CuInSe₂ and CuGaSe₂ for their application as solar cell absorbers is given in terms of the optimal chemical potentials.

CuInSe₂ (CIS) and Cu(In,Ga)Se₂ (CIGS) are successfully applied for thin-film photovoltaic devices. CIGS solar cells with high conversion efficiency have a typical device structure of TCO/ZnO/CdS/CIGS/Mo/soda-lime glass. The CdS layer is usually deposited by chemical bath deposition (CBD). At the CdS/CIGS interface, some of the Cd would be doped into the CIGS layer. In previous studies, it was demonstrated that a Cd-doped CIGS layer was formed at the CdS/CIGS interface during the CBD process[1-3]. Recently, we calculated the substitution energies of a Cd atom for a Cu or In atom in CIS[4]. We found that the substitution energy of a Cd atom for a Cu atom (Cd_Cu) in CIS is smaller than that for an In atom (Cd_In). The formation energy of the charge-neutral (Cd_Cu+V_Cu) pair is lowest; therefore, the (Cd_Cu+V_Cu) pair is easily formed during CBD of the CdS layer on the CIS layer, and a small amount of n-type Cd_Cu is also formed. Cd diffusion in the CIGS layer was examined by the radiotracer technique, and diffusion coefficient of Cd was determined by the Arrhenius equation D_Cd = 4.8 ×10^-4exp (-1.04 eV/k_BT ) cm²s^-1[5]. Most recently, we reported the migration mechanism of Cu and In in Cu-poor CIS[6]. Cu migration easily occurs in the CIS crystal.
In this study, we investigated the diffusion mechanism of Cd atom in CIS. We performed first-principles calculations within a density functional theory (program code: DMol³). To obtain the migration energy and transition state of Cd migration in CIS crystal, calculations using the linear (LST) and quadratic synchronous transit methods (QST) combined with the conjugate gradient (CG) method were performed. After that, the minimum energy pathway of Cd migration in CIS was obtained by the nudged elastic band (NEB) method. The migration energy of Cd in CIS was 0.99 eV. This agrees with the experimentally reported value of 1.04 eV[5]. The activation energy of Cd migration in CIS was smaller than that of Cu migration in CIS (1.06 eV)[6].
Some research groups have deposited a ZnS buffer layer instead of a CdS layer on CIGS films by CBD. Therefore, we investigated the diffusion of Zn atom in CIS. The migration energy of Zn in CIS was 1.30 eV, which is considerably larger that of Cd in CIS. Therefore, we consider that Zn migration occurs with more difficulty in the CIS crystal. This result is consistent with our previous experimental result[7].
[1] T. Wada et al., Proc. 2nd WCPEC, 1998, p. 403.
[2] K. Ramanathern et al., Proc. 2nd WCPEC, 1998, p. 477.
[3] T. Nakada et al., Appl. Phys. Lett. 74, 26 (1999).
[4] T. Maeda and T. Wada, Jpn. J. Appl. Phys., submitted.
[5] K. Hiepko et al., Appl. Phys. Lett. 99, 234101 (2011).
[6] S. Nakamura et al., Jpn. J. Appl. Phys., submitted.
[7] S. Nishiwaki et. al., Sol. Energy Mater. Sol. Cells 77 (2003) 359.

CdTe has many unique physical properties among the II-VI semiconductors. Due to its optimal band gap, high optical absorption, and easiness of synthesis, CdTe-based thin-film solar cell has become one of the leading candidates for low-cost, high-efficiency renewable photovoltaic technologies. However, although CdTe is the only II-VI compound that can be doped relatively easily either p or n type, the dopability of CdTe is relatively low, especially for p-type doping. The electron carrier life time and open circuit voltage are also low in polycrystalline CdTe solar cell. Therefore, to further improve the efficiencies and stability of CdTe-based solar cell, it is critical to understand how defects, grain boundaries, interfaces, and contact layers affect the solar cell performance. Using first-principles band structure methods we have studied the general chemical trends of defect formation in CdTe. We systematically calculated the formation energies and transition energy levels of intrinsic and extrinsic defects and defect complexes in CdTe and investigated the limiting factors for p-type and n-type doping in this material. Possible approaches to increase the doping limits are discussed. Our study also provides information about the possible deep trap states in CdTe bulk and grain boundaries. We suggest that Cd-poor growth conditions is beneficial in improving the hole carrier density but Cd-rich condition should provide sample with longer electron life time. Thus, the balance between these two conditions as well as using extrinsic doping could be the key in improving the performance of CdTe-based solar cells.

Despite being well established in production, issues remain for the contact or ‘back&’ side of CdTe-based PV devices. In order to produce devices with efficiencies above 10% the CdTe absorber layer must be treated with Cl, usually via a CdCl2 activation step. In addition to this, it is usually necessary to carry out a nitric-phosphoric acid (N-P) etch step on the CdTe back surface in order to achieve good electrical contact, and to remove the residues of the CdCl2 activation. A number of studies have examined the CdCl2 activation and N-P etch in isolation of one another [Ricco et al., J. Vac. Sci. Technol. A, 1984, Niles et al., Appl. Surf. Sci., 1998, Li et al., J. Vac Sci. Technol. A, 1999, Yan et al., Thin Solid Films, 2005], the present work brings together electrical (J-V-T evaluation of contact barrier height), surface chemistry (XPS) and microstructural information (TEM) for the first time. The TEM results in particular have proved particularly enlightening. The thickness of any Te-enriched layer on the back surface of the CdTe layer, previously reported [Yan et al., Thin Solid Films, 2005] as being in the 200 nm range, was found to be, at most, several nanometres. In addition, no evidence of the N-P etch preferentially attacking the grain boundaries was found for the etch times used to make high efficiency cells. The principle function of the N-P etch in fact seems to be the removal of residue from the CdCl2 activation step, it was however found that a 10 s etch was as effective in this as a 60 s etch. In both cases, some residue could still be found in the deeper recesses of the surface but the majority of the surface was free from residue. It is hoped that this increased knowledge of the processes applied to the back surface of the CdTe absorber will lead to processing improvements and hence higher efficiency cells.

The objective of the NREL 3-year CdTe Plan under the SunShot Program is to identify primary mechanisms that limit the open-circuit voltage and fill factor of polycrystalline CdTe PV devices, and develop alternative CdTe synthesis processes and/or device designs that avoid these limitations. Part of this project will rely on analysis of single-crystal and bi-crystal CdTe layers where point and extended defects can be introduced sequentially. These samples will allow studies to focus on likely sources of dominating recombination processes. The goals of the project include producing CdTe PV devices that demonstrates ge;20% conversion efficiency, while significantly improving our understanding of process and materials capable of attaining the SunShot cost goals of le;$0.50 per watt.
While several options are being investigated for the crystalline and pseudo-crystalline CdTe layers, one exciting pathway involves synthesis of CdTe layers onto crystalline Si by MBE heteroepitaxy at the University of Illinois at Chicago. Although CdTe/Si heteroepitaxy is relatively unfamiliar to researchers in the PV community, it has been used successfully for more than 20 years to produce high-quality CdTe surfaces needed for the production of large-area single-crystal HgCdTe IR detectors and focal-plane arrays. The process involves chemical and thermal preparation of Si(211) wafers, followed by deposition of As-passivation and ZnTe-accommodation layers. MBE-grown CdTe layers deposited on top of this “template” have been shown to demonstrate low etch-pit density (<106 cm-2) and high structural quality (FWHM=60 arcs). Initial studies of 10-µm-thick CdTe/Si samples at NREL have already established that recombination in these CdTe layers is distinct from either polycrystalline or crystalline materials, and that dislocation density determined by luminescence can yield complementary information relative to the historic etch-pit density measurements. This study will discuss these analyses, as well as report on other material properties relevant to the ultimate production of high-performance crystalline CdTe PV solar cells. This abstract is subject to government rights.

This paper addresses the role of CdTe defect chemistry on the open circuit voltage (Voc) of solar cells in the thermochemical processing regimes commonly encountered in present-generation CdTe devices. The highest Voc is ~900 mV for a bulk CdTe crystal with ITO which is only marginally higher than Voc = 865 mV obtained for polycrystalline CdTe films with CdS. Both fall ~300 mV short of the Voc expected for CdTe, having band gap Eg= 1.5 eV. The present 16-17% efficient superstrate CdTe cell uses a process based on high-temperature, T > 500°C, CdTe growth on CdS, coupled with optimized methods for incorporating oxygen, sulfur, copper, and chloride species in the CdTe film. Pushing cell conversion efficiencies beyond 20% will require increasing Voc beyond 1V, however the present pathway of processing optimization will likely yield Voc and efficiency converging on 900 mV and <20%, respectively. A new approach is required to overcome what appear to be limiting characteristics associated with CdTe microstructural and electronic properties and Cd-Te-O-S-Cu-Cl defect chemistry. This paper presents an interpretation of the 30+ years of CdTe cell development in a phenomenological context, separating thermodynamics from kinetic and dynamic effects of each processing step on the CdTe electronic properties believed to limit Voc, specifically carrier concentration, intra-grain trap density, charged structural defects, and passivation of grain surfaces and interfaces. Experimental results to support key concepts, using 14-16% efficient CdTe cells fabricated by the author, will be included. Concepts for new processing paths based on different processing dynamics and equilibria will be presented.

In the development of inexpensive and efficient solar cells, different architectures of thin film solar cell (TFSC) have been elaborated. The optical properties of CuInS2 (CIS), i.e. its direct gap (Eg = 1.5 eV) and its high absorption coefficient (α = 104 cm-1 at lambda; = 500 nm), make it a suitable candidate as absorber in solar cell devices.
A major challenge in the field of CIGS-type solar cells is the reduction of the indium content, a solution being the development of ultra thin (absorber thickness < 500nm) cells. Atomic Layer Deposition (ALD) is a method of choice to achieve the deposition of conformal films of controlled thickness. Hence, CIS films were deposited by ALD from CuCl, InCl3 and H2S at T = 350 - 450°C. Alternating Cu2S and In2S3 growth cycles did not lead to the deposition of films with the appropriate Cu/In atomic ratio. Instead, exchange reactions between adsorbed In2S3 and gaseous CuCl lead to In-free films, even when varying the injected quantity of InCl3, or lowering the deposition temperature.
A novel method for the deposition of CIS films that consists in a two-step process was developed. Here, In2S3 is grown on a Cu2S layer leading to the supercycle [CIS] = n1 x {Cu2S} + n2 x {In2S3}, with n1 = number of Cu2S cycles and n2 = number of In2S3 cycles. It allows the deposition of CIS films, as confirmed by spectrophotometry, X-ray fluorescence, EDX, MEB and grazing incidence X-ray diffraction studies. The Cu/In atomic ratio decreases with the n1/n2 ratio, and while some films are Cu-rich, films with a Cu/In ratio corresponding to a CuInS2 stoechiometry could be achieved for n1/n2 < 1/9. The characterization of films grown with n1/n2 > 1/9 has lead to an interesting growth mechanism study. Indeed, as observed by MEB-EDX, those films are heterogeneous and composed of two phases: one of {xCu = 74.0 %at, xIn = 0.1 %at; xS = 25.9 %at} atomic composition, and the other of {xCu = 28.5 %at, xIn = 23.6 %at; xS = 47.9 %at} atomic composition. The In-containing parts appear as “islands” surrounded by In-free parts; and their surfaces increase with the number of In2S3 cycles. However, photoluminescence studies revealed that only the “islands” have photoluminescence signals, at energies typical of both Cu2S and CuInS2. This is an interesting example of ALD of multinary compounds involving side-reactions that can occur and should be taken into account in every ALD process.
In a second stage, homogeneous CuInS2 films were used as absorber in CIGS-type solar cell. Complete devices (Mo/CIS/CdS/ZnO/ZnO:Al) were done and achieved moderate efficiency (eta; = 1,6 - 2,8%) due to low VOC values (VOC = 0,26 - 0,31 V) but relatively good EQE (up to 0,7 at lambda; = 550nm) and JSC values (JSC = 13,1 - 16,1 mA.cm-2), especially considering the low film thickness (<300 nm).
This study proves the feasibility of the synthesis of photoactive CIGS absorber by ALD, and gives a path toward the realization of ultra thin solar cells.

Iron pyrite (FeS₂) is an earth-abundant semiconductor with near-ideal properties as absorber layer in thin film solar cells, due to its high absorption coefficient, abundance and non-toxicity. However, all reported devices suffer from a low open-circuit voltage (V_OC), limiting their efficiency. To understand and overcome this limitation, we studied ultra-pure pyrite single crystals grown by a novel flux growth method, which display carrier mobilities up to 1300 cm²/Vs. They are compared to rigorously phase-pure thin films grown by a variety of techniques. We present temperature-dependent conductivity and Hall effect measurements and discuss electrical transport in these materials. While single crystals display n-type conductivity with high carrier mobility and low carrier concentration, thin films display p-type conductivity with low carrier mobility and high carrier concentration. We observe a strong surface effect and compensation of n-type single crystals into p-type conductors, which is explained with surface defects that induce acceptor states. We present a multilayer model that explains the measured resistivity and Hall coefficient data with a p-type surface inversion layer. Ongoing work on passivation of these surface defects will be presented as part of our efforts to produce efficient heterojunction solar cells with pyrite thin film absorber layers.

Thin film solar cells of Copper sulfide (Cu2S) have been investigated intensively in the past. Cells utilizing a Cu2S/CdS heterojunction reached efficiencies of up to 11% as well as open circuit voltages of ~600 mV. Despite their good performance research was ultimately abandoned due to long term stability issues which were related to the degradation of the Cu2S as well as the interdiffusion of copper ions into the CdS layer. Nevertheless copper sulfide is still a promising candidate for an alternative absorber material in thin film solar cells.
Cu2S layers have been deposited via thermal evaporation using various pre- and post-treatment parameters. The electrical and morphological properties have been investigated via in-situ XPS, SEM and XRD measurements.
A detailed optoelectronic characterization was carried out at the CvO University Oldenburg. It was found that deposition at room temperature followed by an additional annealing step dramatically increases the absorbers performance. Measurements of annealed Cu2S layers on glass without any surface passivation showed quasi-Fermi level-splittings of over 700 meV as well as an optical band gap of ~1.3 eV which indicates that Cu2S cells have not been brought to their full potential yet.
To better understand the processes in earlier device structures and discuss novel concepts for the application of Cu2S in thin film solar cells in-situ interface studies were carried out in the DAISY-MAT system (DArmstadt Integrated SYstem for MATerials research). Thin layers of Cu2S were deposited onto different substrates and characterized via XPS measurements, to determine the band alignments of Cu2S/CdS, ZnO/Cu2S as well as of Cu2S/Cu2O.
Additionally, solar cells with various device structures have been built. So far the cells suffer from shunting due to the inferior morphology of the annealed Cu2S films.

Tin sulfide SnS is currently reconsidered as an earth abundant photovoltaic (PV) absorber material. However, previously reported efficiencies of SnS cells were low and many of the basic PV relevant properties of SnS still remains obscure. Therefore, we present a coupled theoretical and experimental investigation of the optical and electrical properties of SnS thin films in order to assess the PV-potential of SnS absorbes. Our study includes GW-based band structure calculation, defect theory optical absorption spectroscopy and van der Pauw Hall effect measurements. We find that for typical thin-film thicknesses, the absorption edge of SnS lies 0.4 eV below our calculated band gap (Eg=1.07 eV),contrasting the common perception that SnS is a strongly absorbing direct gap material. The layered structure of SnS gives rise to anisotropic effective masses for both electrons and holes. Besides, native Sn vacancies have low formation energies under S-rich growth and explain the observed p-type doping. On the other hand, Sn-rich growth condition is detrimental to the PV capability of SnS as it favors the formation of deep levels, such as Sn antisites and S vacancies. This study highlights the value of theory to complement experiment and to fill critical knowledge gaps so to improve and optimize solar energy materials.

Thin-film solar cells based on a CdS/CdTe heterojunction are established candidates for cost-effective devices with high efficiency. For further improvement, efforts focus on the fact that absorption in the CdS layer reduces the flux of high-energy photons to the CdTe absorber. One solution is to increase the band gap of the material, e.g., by introducing O₂ into the Ar flow during RF sputtering of CdS. Such films have produced efficiencies of >15% in CdTe devices [1] and theoretically could reach efficiencies of >18.4%. In order to optimize this process and further improve device performance, a detailed understanding of the electronic and chemical structure at the surface, interface, and near-surface bulk is required. To study the electronic and chemical properties of such films, we employed surface-sensitive X-ray Photoelectron Spectroscopy (XPS) and near-surface bulk-sensitive Resonant Inelastic soft X-ray Scattering (RIXS), and investigated three samples with varying O₂/Ar flow rates near the empirically optimized ratio for solar cell performance (2.0 %, 2.3 %, and 2.5 % O₂/Ar). Using high-resolution XPS spectra of the S 2p region and a simultaneous fit of all spectra, we find at least three distinct sulfur species, including CdS and CdSO₄. We find the CdS intensity to decrease linearly as a function of O₂ partial pressure, while this trend is reversed for CdSO₄. The third species, attributed to other SO_x contributions, is most abundant in the 2.3 % O₂/Ar sample (i.e., at the O₂/Ar ratio employed in the best-performing devices produced by CSU). Near-surface bulk-sensitive X-ray Emission Spectroscopy (XES) at the Cd M_4,5 edge was used to study the local chemical environment of the Cd atoms in order to provide complementary information. The XES results again show an increase in CdSO₄ as a function of O₂ partial pressure, as well as spectral evidence for CdO. Furthermore, S L_2,3 RIXS was employed to derive the chemical environment of sulfur atoms in the bulk of the films, to complement the surface-sensitive XPS findings. In our presentation, we will focus on discussing the relative abundance of the different species at the surface and in the near-surface bulk, and discuss possible defects in the CdSO₄-matrix and their effect on the performance of CdTe-based thin-film solar cells.
1. X. Wu, Y. Yan, R. G. Dhere, Y. Zhang, J. Zhou, C. Perkins, and B. To, “Nanostructed CdS:O film: preparation, properties, and application,” physica status solidi (c)1, 1062 (2004).

CdS, often used as the window layer for CdTe solar cells, contributes very little to photocurrent collection in CdTe devices. Increasing this photocurrent, if possible, may lead to better efficiency. One way to attempt this is to increase the carrier lifetime in the CdS. Two processing steps that have been studied at Colorado State University (CSU) in hopes of increasing carrier lifetime in polycrystalline CdS are a CdCl2 treatment and plasma cleaning. This study looks at the effects of these treatments on CdS films using photoluminescence (PL) emission spectroscopy and time-resolved photoluminescence (TRPL). Measurements were taken on two sets of CdS films made by close-spaced sublimation (CSS) at CSU. One set of samples included a post-deposition CdCl2 treatment while the other set was made using plasma to clean the glass/TCO prior to CdS deposition. Room temperature PL emission spectroscopy was used to study band-to-band emission and defect properties. TRPL was applied to determine minority carrier lifetime. Results show that substrate preparation processes as well as CdS thicknesses affected the intensity of the PL signal. In particular, the post-deposition CdCl2 treatment increased PL by more than a factor of two. The plasma cleaning also increased the PL intensity, although less so than for the CdCl2 treatment. There was also no measurable variation in the band-to-band peak (of 2.45 eV) from sample to sample, even across sets. Furthermore, the 2.45 eV peak was the dominant one for both sets with a secondary peak at 2.15 eV for the CdCl2-treated and a very small peak at 1.6 eV for the plasma-cleaned samples that increased as CdS thickness decreased. There were no systematic differences in the quantitative intensities of these peaks. The TRPL measurements of the CdCl2-treated set showed a two-exponential decay with tau;1 = 0.5 ns and tau;2 = 2 ns for the as-deposited film and tau;2 increasing to 3 ns after the CdCl2 treatment. To the best of our knowledge, this is the first reported measurement of the minority carrier lifetime in polycrystalline CdS. We also observed correlations between longer lifetimes and stronger PL emission signals.

Thin film solar cells require a combination of a highly conductive transparent conducting oxide (TCO) and semi-insulating buffer or undoped oxide layer in order to reach high efficiency. Often, this bi-layer structure is composed of a heavily doped conducting layer (e.g. SnO₂:F or ZnO:Al) and a non-intentionally doped layer of the same host oxide (e.g. SnO₂ or ZnO). The target resistivity for the semi-insulating buffer is about 1 ohm-cm for CIGS and CdTe solar cells. Reducing the resistivity of the buffer layer hinders its ability to prevent shunt resistance losses, and increasing its resistivity results in excessively high series resistance in the cell. Maintaining a resistivity of 1 ohm-cm requires maintaining a very low carrier concentration in the buffer layer, which can be difficult if carriers are unintentionally incorporated during growth. SnO₂ and SnO₂:F are often produced by atmospheric pressure chemical vapor deposition (APCVD) using a chlorinated tin precursor. Chlorine is a strong donor in SnO₂, and the carrier concentration in the “undoped” or un-intentionally doped films grown from chlorinated precursors can be more than an order of magnitude higher than the level required to yield a resistivity of 1 ohm-cm. While chlorine-free tin precursors are available, they are not commercially appealing because of their toxicity, thermal reaction characteristics, and cost.
Recently, we have developed a method for suppressing the carrier resulting from unintentional chlorine or fluorine doping and for controlling the resistivity over several orders of magnitude by varying the deposition conditions. Zirconium can be added to high conductivity SnO₂:F to improve its optical properties. However, it can also be added in larger amounts to suppress carriers and increase resistivity in highly doped films. We can tune the resistivity of our buffer layer between 0.001 and 5 ohm-cm by varying the amount of Zr available in the CVD chamber. Here, we will present optical and electrical data from the buffer layers and device data from CdTe solar cells made using these buffers. We will also discuss the mechanism of carrier suppression and its impact on cell performance.

Atomic layer deposition (ALD) has gained a great interest in the recent years in the field of photovoltaic applications notably in cadmium-free materials for buffer layers in CIGS-based solar cells. In particular, indium sulfide (In2S3) and zinc sulfide (ZnS) based buffer layers deposited by this method have already shown efficiencies of respectively 16.4% and 18,5% . Those two materials have very different properties: β-In2S3 is a narrow bandgap semiconductor type (about 2 eV) while ZnS is a wide bandgap semiconductor type (3.6 eV). Thus, ZnS should be a better material to avoid proton losses due to light absorption out of the absorber. However In2S3 thin films exhibit a very low absorption with an indirect bandgap optical behaviour. The electronic properties of both materials are also very different in terms of the conduction band alignment at the buffer/CIGS interface. The interface with a coevaporated CuIn0.7Ga0.3Se2 with a bandgap of 1.15 eV forms a cliff in the case of ALD In2S3 as buffer layer, but a strong spike when ALD-ZnS is used.
In this study, mixed ZnInxSy films of different compositions have been synthesized by ALD from DEZ and H2S precursors at a deposition temperature between 180°C and 220°C. The film growth is monitored in-situ with the quartz crystal microbalance technique (QCM), which allows unique mechanistic insights. In-situ characterizations showed that surface exchange reactions and diffusion process occur during the deposition and influence the overall film composition. Atomic compositions, optical, electrical and structural properties of films have been determined. Solar cells have been prepared with these two layers. In parallel, benchmark studies were done with classical front end process (CdS by CBD/ZnOi/ZnO:Al by sputtering). The optoelectronic properties of the completed cells (I(V) curves and spectral response essentially) have been analyzed. The influence of the atomic composition of the layer, the deposition temperature and the type of window layer used on the cells performance were studied. The best cell obtained presents a conversion efficiency of 11.2 % (reference CdS: 12.3 %) for a buffer layer containing 28 % at of indium for a deposition temperature of 180°C. More importantly, this cell exhibits an open circuit voltage 15 mV higher than that of the reference. This study gives also new elements concerning the influence of a highly resistive ZnO layer in CIGS solar cells which appears to depend on the electronic properties of the buffer layer used. Further investigations and characterizations are necessary to evaluate the potential of this layer as buffer layer in CIGS based solar cells.

Chemical bath deposited (CBD)Zn(O,S) is among alternatives to (CBD)CdS buffer layer in Cu(In,Ga)Se2(CIGS)-based cells. Nevertheless, the performance reached with (CBD)Zn(O,S) appear varying from one sample to another and from one laboratory to another, which denotes that parameters of minority impact with (CBD)CdS have major influence on cells buffered with (CBD)Zn(O,S). Moreover, the literature reports, but not systematically, the necessity of substituting the standard resistive ZnO by (Zn,Mg)O and/or soaking the cells in UV-containing light in order to reach stable device operation conditions. Such differences in reported optimal device structures, performance and metastabilities drove us to investigate the impact of the three following parameters on the cells behaviour: (1) CIGS surface properties, (2) (CBD)Zn(O,S) layer thickness, (3) window characteristics. The first conclusion of this study is that all of these parameters are effectively observed influencing the electrical metastabilities of the devices. The second conclusion is that the achievement of stable devices is favoured by (1) using absorber which Cu content is close to the stoichiometry, (2) increasing the buffer thickness, (3) increasing the resistivity of r-ZnO. These observations suggest that the crucial parameter is the electron concentration in the close-to-interface CIGS layer which supports the model explaining fill factor metastabilities through the existence of a p+ layer at the CIGS near surface.

Contrary to CIGSe based solar cells with a CBD-CdS buffer layer, CBD-ZnS based solar cells require post-air-annealing and light-soaking procedures to achieve optimal conversion efficiencies. However, for industrial applications, it is better to eliminate these post-treatments. Recent works have identified ZnMgO as a partner for CBD-ZnS buffer layers to highly improve the efficiency of solar cells and strongly reduce their transient behavior. However the role of this layer within the device structure is still not well understood. The purpose of the present contribution is to focus on the impact of oxygen gas partial pressure during sputtering of i-ZnO and ZnMgO on the transient behavior of solar cells parameters when a CBD-ZnS buffer layer is used. Until now, it was considered that the CIGSe/buffer/window layer band live-up is the main feature to explain these behaviors. However, based on electrical characterization of cells, we have observed that the effect of light-soaking and heat-treatment is different on J-V characteristics depending on the quantity of oxygen present during the first deposition time of the i-ZnO or ZnMgO layers. In fact, we have noticed that when cells are prepared with standard i-ZnO, the efficiencies are very low and a pronounced transient behavior is observed. However, when the i-ZnO or ZnMgO is first formed by a few nanometers sputtered layer without any oxygen, depending on the thickness of this layer, the transient effects strongly decrease. It is then possible to reach efficiencies quite similar to the CdS reference cells, especially with ZnMgO, without any post-treatments. These results tend to provide evidence for the dominant influence of the interface between the Cd-free buffer layer and the window layer on solar cells parameters. And therefore, it encourages us to consider that an interface feature other than the band live-up is also important for the performance of the heterojunction. Based on chemical, structural and optoelectronic characterizations, we will try to present a new explanation of the performance limiting characteristics in Cd-free CIGSe cells. We will show that it is possible to strongly reduce the transient effect in Cd-free CIGSe based solar cells just by chemical interface engineering with simply reconsidering the i-ZnO or ZnMgO growth parameters.

Recently, Cu₂ZnSn(S,Se)₄ (CZTSSe) based solar cells have gained a lot of interest within the photovoltaic community. Being formed by earth abundant and/or low toxic elements, it is a potential future competitor of most mature CuIn_1-xGa_xSe₂ (CIGS) and CdTe technologies. Currently, the record efficiency is 11.1% for the CZTSSe phase, being still far of the champions efficiencies of CIGS (20.3%) and CdTe (17.3%). As in the case of CIGS technology, the electro-optical characteristic of the devices depends on the formation of a relatively thinner MoS₂/MoSe₂ layer (50-100 nm) at the interface region between the absorber and the back contact. Moreover, and because Kesterites are prepared under Cu-poor conditions avoiding the formation of Cu_xSe(S) as flux, to access a good crystallization high Se(S) vapor pressures and temperatures are required, compromising the integrity of the Mo back contact, by forming a very thicker MoSe(S)₂ layer. Also, the interface CZTSe(S)/MoSe(S)₂ is enormously affected forming typically large voids. The thickness of the formed interficial layer can be in principle adjusted along, with the use of a diffusion barrier layer, such as TiN. In this work the effect of a ZnO interficial nanolayer between the Mo and absorber ones during the selenization process is investigated. A thin (between 5 to 20 nm in thickness) ZnO layer was directly deposited onto Mo/glass, Mo/TiN/glass, TiN/Mo/glass and TiN/Mo/TiN/glass substrates. CZTSe absorber were prepared by selenization of Sn/Cu/Zn metallic multi stacks deposited by DC-magnetron sputtering, and thus solar cells using the typical CdS/i-ZnO/ZnO:Al structure were fabricated. Samples without the ZnO barrier layer was also used as reference in all cases. The Mo/CZTSe interfaces were characterized by using XRD, FIB-SEM, Raman and Auger spectroscopy. The analysis of the processed samples and cells reveals that the ZnO interficial layer does not act as a very effective barrier to avoid the formation of MoSe₂ like TiN, observing in all cases the formation of a relatively thick MoSe₂ layer. However, SEM cross section images showed that the use of a ZnO intermediate layer leds to a significantly improving at the interface. Our first characterization results suggest that this is related to the accumulation of Na at the interface when ZnO is present, and the subsequent formation of Na₂SeO₃, which helps to improve the morphology of the interface. This correlates with the existence of a significant improvement in the characteristics of the cells: the efficiency increased from 2.5% for the reference sample to 6% for the sample with a 10 nm thick ZnO layer. A discussion on the possible mechanisms involved in the improvement of the solar cells performance (including the analysis of the impact of the interfacial ZnO layer on the formation of secondary phases at the back region of the absorbers) will be reported, with a complete characterization of the CZTSe/MoSe₂ interface.

Reactive sputtering from metal targets in H2S containing plasma is an attractive, fast method for making CZTS films with tuneable composition. Deposition at lower substrate temperature gives films with a disordered cubic structure, while the kesterite phase can be obtained at higher temperature. Vacuum deposition at higher temperature is problematic for CZTS due to losses of Sn and S. Even if high deposition rate of Sn and supply of S can be employed during deposition, losses from the surface can be hard to prevent during cool-down. We therefore use a two-stage process with sputtering followed by rapid annealing at higher pressure. With this approach, devices with efficiencies up to 7.2% have been made, using a CdS/ZnO/ZnO:Al window.
In this work we compare reactive sputtering of CZTS to sulfurisation of metal precursors. We also report influence of precursor composition and sulphur supply during annealing on film and device properties. Finally, factors limiting performance of our champion devices are discussed.

Thermal stability is essential for the potential solar cell material Cu2ZnSnS4 (CZTS) in achieving a satisfactory photovoltaic device performance. Although the loss of Sn from CZTS has been reported, the basic decomposition mechanism of a CZTS system has not been well established yet, especially with regard to the role of active Cu1+ ions. This paper not only provides a deeper understanding of the change of Sn species, which includes an equimolar-isobaric vaporization mode transition and a solid-vapor phase transition in a self-generated atmosphere, but also reveals the oxidation state alternation (Cu¹⁺/Cu²⁺) and transfer mechanism of Cu species through carefully designed experiments and a reaction kinetic study. Cu ions are unexpectedly found to be active in affecting the degradation reaction by valance alternation and ion movement upon the application of a heat field to balance the derivation caused by a non-uniform temperature gradient. As a result, a Cu-Zn separation appears, with Cu accumulating near the hot area and Zn near the cold area. A decomposition reaction model of CZTS under a directional heat field is proposed to describe the elemental and electronic state change in atomic scale, and a perfect match is obtained between the model and the experimental results. This paper paves a way to solve the thermal stability issue of Cu₂ZnSnS₄.

The quaternary compound Cu2ZnSnS4 (CZTS) belongs to the family of I-II-IV-VI semiconductors. It has attracted a great interest due to its potential applications for sustainable thin-film solar cell devices. CZTS has large direct band gap (Eg ~ 1.5 eV), high absorption coefficients (>104 cm-1), intrinsic p-type conductivity and low thermal conductivity. Comparatively to the other important solar cell materials, the composition of naturally abundant and inexpensive elements such as Zn and Sn makes CZTS particularly attractive candidate for large-scale commercial application.
The vibrational properties of CZTS have been addressed in recent detailed theoretical investigations, and some infrared and Raman spectroscopy experiments on CZTS thin films. However, no detailed experimental data on the vibrational properties of CZTS single crystals has been reported yet.
The single crystals of CZTS with well-developed faces were grown by chemical vapor transport technique using iodine trichloride as a transport agent. A representative crystal is showing the as-grown basal (112) plane, comparatively-big-area (001), (110), (101) and (011) planes, as well as the small (100) plane. Energy dispersive X-ray analysis measurements showed some variations between the ratio of Cu and Zn among the examined samples. However, the average atomic ratio of Cu:Zn:Sn:S was found to be closed to 2:1:1:4.
Raman measurements were performed at room temperature utilizing the back-scattering configuration on a Renishaw inVia micro-Raman system. For the polarization measurement of the scattering light, a polarizer and a half-wave plate were used. The widely used Porto notation has been utilized in this study for the designation of the crystal and polarization directions. To determine the origin of the observed Raman modes, the polarization selection rules for backscattering configuration along [100], [001], [110] and [112] crystallographic directions of the crystals with kesterite (KS) and stannite (ST) structure have been used.
From a comprehensive analysis of the experimental spectra and comparison with the results of theoretical calculations, the positions and symmetry assignment of the observed Raman features are determined and identified. According to the selection rules, the values of A, B(TO), B(LO), E(TO), and E(LO) modes are found. The results reveal that the investigated CZTS single crystals have been crystallized in the KS structure. The obtained data may be used to clarify the existence of structural or phase inhomogeneities in CZTS absorber films of the solar cells.

Cu₂ZnSn(S,Se)₄ (CZTSSe) is an earth-abundant thin-film photovoltaic absorber with direct optical band gaps in the range 1.0 eV < E_g < 1.5 eV. Sulfur-rich compositions (E_g ~ 1.5 eV) are expected to provide a near-optimal match to the solar spectrum, leading in principle to optimized open-circuit voltage (V_OC) and device efficiency. In spite of this expectation, devices fabricated with Se-rich CZTSSe absorbers tend to perform better: champion device efficiencies for pure CZTS and Se-rich CZTSSe have been reported as 8.4% and 11.1%, respectively (1,2). Some of the performance deficit observed in CZTS devices might be attributable to poor conduction band alignment at the CdS/CZTS interface. According to recent first-principles calculations (3), the CZTS conduction band is pushed up relative to CZTSe, which could contribute to the creation of a substantial negative conduction-band (CB) offset between the CZTS and CdS layers at the interface that would effectively lower V_OC. Therefore, a key pathway to further improvement of CZTSSe device performance is to better understand band offsets at this critical interface. Another issue to relating to large-scale manufacturing and deployment of PV modules based on CZTSSe alloy absorbers is concerns over the toxicity of Cd-containing materials and manufacturing waste products. Both of these issues point toward the potential benefits of alternative buffer-layer materials in CZTSSe devices. A promising candidate is Zn(O,S), which is characterized by large optical bowing leading to broad tunability of both the valence- and conduction band edges. (4) Additionally, over some compositions Zn(O,S) has a larger band gap than CdS, which could increase short-circuit current (J_SC) in the device. In the present study, we report on band alignment at the prototypical ZnS/CZTS interface, as measured by x-ray and ultraviolet photoelectron spectroscopies (XPS/UPS). These measurements reveal a large positive CB offset between the absorber and the buffer layer that makes pure ZnS unsuitable for use in a CZTS PV device. Nevertheless, these results, in combination with separate XPS/UPS measurements on Zn(O,S) films, indicate that S-rich Zn(O,S) compositions could provide optimal CB alignment with S-rich CZTS material. Implications of ZnS/CZTS band alignment for grain-boundary passivation in Zn-rich CZTSSe absorber materials will also be discussed.
(1) B. Shin, O. Gunawan, Y. Zhu, N. A. Bojarczuk, S. J. Chey, and S. Guha, Prog. Photovolt.: Res. Appl., 2011.
(2) T.K. Todorov, J. Tang, S. Bag, O. Gunawan, T. Gokmen, Y. Zhu, and D.B. Mitzi, Adv. Energy Mater., 2012.
(3) S. Chen, A. Walsh, J.-H. Yang, X. G. Gong, L. Sun, P.-X. Yang, J.-H. Chu,and S.-H. Wei, Phys. Rev. B 83, 125201, 2011.
(4) C. Persson, C. Platzer-Björkman, J. Malmström, T. Törndahl, and M. Edoff, Phys. Rev. Lett. 97, 146403, 2006.

Cross sectional electron-beam induced current (EBIC) measurements show that charge carrier transport in CuInSe2/CdS/ZnO solar-cells is generation-dependent in some grains of the absorber layer. We have previously proposed a model based on the assumption of a thin p+ layer at the CuInSe2/CdS interface with a high density of deep acceptor-type defect states and a lowered valence band maximum with respect to the CuInSe2 layer to explain generation-dependent charge carrier transport [1]. A shallow donor-type defect state at the p+ layer/CdS interface of some grains of the absorber layer explains the observed grain specific inhomogeneities. Although this model is able to explain many of our experimental results, the material specific origin of the high acceptor density in the p+ layer has not been clarified yet.
For the present study, we performed cross sectional EBIC measurements on solar cells for which CdS was deposited directly after the absorber deposition with minimal exposure to air, and solar cells for which the absorber layers were intentionally exposed to air for one week before CdS deposition. X-ray photoelectron spectroscopy measurements on these absorber layer surfaces show different degrees of oxidation of the surfaces. The evaluation of a large number of cross-sectional EBIC profiles for the differently treated solar cells yields significant differences. Specifically, the number of grains where generation-dependent transport occurs is larger for solar cells with absorber layers exposed to oxygen before CdS deposition. These results indicate that the incorporation of oxygen is prone to play a crucial role for the effective doping density in the surface region of the absorber layer. A possible explanation is the passivation of donor type VSe or InCu point defects by oxygen as proposed in earlier studies [2]. We conclude that extrinsic doping cannot be neglected and can have a significant influence on the potential distribution within the device, charge carrier transport properties and in consequence the device efficiency.
[1] M. Nichterwitz et al, submitted
[1] D. Cahen et al., Appl. Phys. Lett. 54 (6), 558 (1999)

In view of the complexity of thin-film solar cells, which are comprised of a multitude of layers, interfaces, surfaces, elements, impurities, etc., it is crucial to characterize and understand the chemical and electronic structure of these components. This is particularly true for kesterite-based devices, because of the complexity of the Cu2ZnSn(S,Se)4 (CZTSSe) compound semiconductor absorber material alone. The already achieved efficiency of CZTSSe based solar cells - exceeding 11% [1] - is thus remarkable, since they are based on a heterostructure optimized for chalcopyrite absorbers over the past 30 years, without any kesterite-specific adaptations. Poorly designed interfaces can, however, lead to increased charge carrier recombination, limiting the device performance.
Using a combination of different advanced characterization tools, the “magic” of optimally designed chalcopyrite surfaces and interfaces was revealed over the past years. The optimized heterostructure was a key factor for chalcopyrite devices to yield the highest efficiencies among all thin-film solar cell technologies. To shed light on the similarities and differences, in this presentation our recent progress in the characterization of Cu2ZnSnS4 and Cu2ZnSnSe4 thin-film solar cell absorber surfaces and interfaces will be reviewed. It is demonstrated that the combination of different soft x-ray spectroscopies is an extraordinarily powerful method for illuminating the chemical and electronic material characteristics from many different perspectives, ultimately resulting in a comprehensive picture of these properties. The focus of the presentation will be on secondary impurity phases, electronic structure, native oxidation, surface composition, and band alignment with CdS, the commonly used buffer material. The emphasis will be on the comparison between different kesterite and chalcopyrite thin films and related device structures in order to unravel similarities and differences. This will finally answer the question whether kesterites and chalcopyrites are close cousins [2] - implying that kesterites can indeed deliver - or only distant relatives.
[1] T.K. Todorov, J. Tang, S. Bag, O. Gunawan, T. Gokmen, Y. Zhu, D.B. Mitzi, Adv. Energy Mater., DOI: 10.1002/aenm.201200348 (2012).
[2] I. Repins, N. Vora, C. Beall, S.-H. Wei, Y. Yan, M. Romero, G. Teeter, H. Du, B. To, M. Young, and R. Noufi, MRS Proceedings, 1324, mrss11-1324-d17-01 doi:10.1557/opl.2011.844 (2011).

The high efficiency of Cu(In,Ga)Se2 (CIGSe) based thin-film solar cell devices fabricated by NREL is, in part, due to an In-terminated surface that leads to a decrease in recombination losses, resulting in a higher fill factor and ultimately improving the device performance. One route to further increase device efficiency is the incorporation of additional Ga to shift the CIGSe bulk band gap towards the theoretical optimum value of 1.35 - 1.40 eV. Until recently, the performance of such wide-gap chalcopyrite absorbers was below that of their low-gap counterparts. Using a high-temperature CIGSe deposition path, however, the performance of wide-gap chalcopyrite absorbers was significantly improved.[1] A detailed understanding of the relationship of Ga content and the electronic and chemical surface and bulk properties of these wide-gap absorber materials is necessary to finally surpass the efficiency of their low-gap relatives.
It is hence the goal of our study to derive the electronic and chemical properties of In-terminated CIGSe absorbers, first for the baseline absorber, and then as a function of Ga content. For that purpose, we have investigated a respective sample set prepared at NREL by soft x-ray and electron spectroscopy [including x-ray (XPS) and ultraviolet (UPS) photoelectron spectroscopy and inverse photoemission (IPES) at UNLV, as well as by x-ray emission spectroscopy (XES) at the ALS]. XPS is conducted to investigate the surface composition of the samples, in particular focusing on the determination of the Ga/(In+Ga) ratio. This information is complemented by more bulk-sensitive chemical and electronic structure information derived by XES. Another emphasis of our study is put on determining the surface band-gap and position of the Fermi level (EF) within the band gap. This information is directly derived by combining surface-sensitive UPS (valence band) and IPES (conduction band) measurements. Furthermore, it is expected that understanding the interplay between In-termination and Ga content of the CIGSe thin films will help to pave the way towards an optimized bulk band gap energy, ultimately achieving high open-circuit voltages and improved CIGSe solar cell performance.

1 M.A. Contreras, L.M. Mansfield, B. Egaas, J. Li, M. Romero, R. Noufi, E. Rudiger-Voigt, and W. Mannstadt, Prog. Photovolt: Res. Appl. 20, 843 (2012).

MoO3 films with a high work function (5.5 eV), high transparency, and a wide bandgap (3.2-3.8 eV) are a potential candidate for the primary back contact of Cu(InGa)Se2 thin film solar cells. This may be advantageous to form ohmic contact in superstrate devices where the back contact will be deposited after the Cu(InGa)Se2 layer and MoSe2 layer doesn&’t form during Cu(InGa)Se2 deposition. In addition, the MoO3 may be used for a transparent back contact in tandem or bifacial cells. In this study, MoO3 films for use as a possible back contact for Cu(In,Ga)Se2 thin film solar cells were prepared by reactive RF sputtering under varying O2/(O2+Ar) ambient compositions. The effect of oxygen partial pressure and post processing on the structural, optical, and surface properties of the deposited films were investigated using x-ray diffraction, grazing incidence x-ray diffraction, UV/Vis/NIR spectrophotometry, and x-ray photoelectron spectroscopy. All of the as deposited films were amorphous, independent of applied oxygen partial pressure. Annealing resulted in crystallization of the films to the β-MoO3 and α-MoO3 phases at 300°C and 400°C, respectively. XPS results showed the Mo+6 oxidation state in all films studied with a small contribution of Mo+5 in samples prepared at lower O2/(O2+Ar) and/or low annealing temperatures. Increasing the oxygen partial pressure had no significant effect on optical transmittance of the films, and bandgaps in the range of 3.3-3.7 eV and 3.8-3.9 eV were obtained for the as deposited and annealed films, respectively. Cu(In,Ga)Se2 thin film solar cells prepared using an as deposited MoO3 back contact yielded an efficiency of ~14% with VOC = 647 (mV), JSC = 28.4 (mA), and FF. = 78.1%. The efficiency of cells with an annealed MoO3 back contact was limited to 7.4%, showing a blocking diode behavior in the forward J-V profile. This may be caused by the presence of a large barrier between the valence bands of the Cu(In,Ga)Se2 and MoO3, due to the higher bandgap of the annealed crystallized MoO3 films. SEM cross section studies showed uniform coverage of the as deposited MoO3 layer and formation of voids for the annealed MoO3 film. No noticeable structural changes of the Cu(In,Ga)Se2 absorber layer were observed for either cases.

CdTe photovoltaic (PV) devices fabricated in the nonstandard substrate configuration have attracted increasing interest recently because of their potential compatibility with flexible substrates, roll-to-roll processing, and building-integrated PV. While improvements to substrate-configured CdTe PV devices have been reported recently, these devices remain significantly less efficient than high performance superstrate PV devices. This limitation has been due, in part, to the difficulty of implementing a back contact that is both ohmic and stable through the high-temperature processing steps required for the CdTe film deposition and CdCl₂ heat treatment (HT). At NREL, we have fabricated substrate-configured CdTe PV devices with efficiencies of ~11% reproducibly, but these devices have typically exhibited significant rollover, leading to low fill factor (FF). In this study, we investigate Cu-containing and Cu-free back contact interface layers (BCILs) for substrate-configured CdTe PV devices. While several materials (e.g. ZnTe:Cu, Cu_xTe, Sb₂Te₃, and MoO_x) have been investigated as BCILs in substrate-configured CdTe PV devices in the past, and some have shown significant promise recently (MoO_x), others (e.g. MoSe₂, V₂O₅, WO₃, CuSe, and Cu₂Se) have been the subject of little or no discussion in the literature. Several BCIL materials will be studied using techniques including X-ray diffraction and secondary ion mass spectrometry (SIMS). SIMS will be used to study the incorporation of the BCIL constituents into the CdTe layer that results from diffusion during the high-temperature CdTe deposition and CdCl₂ HT. The accompanying PV devices will be studied using a variety of techniques, including current density-voltage (JV), temperature-dependent JV, capacitance-voltage, and admittance spectroscopy. The BCIL materials strongly influence the forward characteristics and FF of the PV devices. In addition, the effects of these BCIL materials differ depending on the CdTe film deposition temperature and method (close-spaced sublimation vs. thermal evaporation) and CdCl₂ HT parameters. While some of these BCIL materials have enabled no PV device improvement, others have contributed to FF values greater than 70% in PV devices with efficiencies of >10%, expanding the parameter space in which substrate-configured CdTe PV devices of reasonable efficiency can be produced.

Rapid advancement in CIGS device performance at MiaSolé has been accomplished by leveraging two powerful methodologies. First, the architecture of our continuous operation roll to cell process is uniquely suited to design of experiment (DOE) methods. Use of these highly structured experiments reduces ambiguities from noise factors and enables repeatable testing of interacting factors. In our deposition system multiple factors can be tested simultaneously such as chemical composition, deposition rate, film thickness, operating pressure, and growth temperature among others. In many cases, the interactions between different process parameters become the key to efficiency improvement. At MiaSolé the use of one of our production systems for development has additional advantages (i) large volume of repeatable results due to a stable and high output operation (ii) fast turn-around of experimental results (substrate to full stack tested cell within 60minutes) and (iii) reduction of further scale up issues when transitioning to full manufacturing of a new process.
The second methodology is to leverage our understanding of efficiency limitations through device characterization and modeling. In this case the goals are to identify specific mechanisms and portions of the film stack that yield to lower performance compared to a high efficiency CIGS device (such as a champion CIGS device). These comparisons are used to generate the best estimate on power loss contribution of each major mechanism and highlight the non optimum regions in the device. Inputs to device modeling come from simple IV tests to complex capacitance and optical measurements. Once we have strong agreement between the factors that drive performance, experiments are defined and then prioritized to test hypotheses.
We will discuss specific CIGS device improvement examples utilizing our methodologies as well as presenting development of our own analysis tools to extract critical parameters from large IV, CV, and QE datasets. We will also discuss methods of estimating physical parameters for CIGS devices that are challenging to measure directly.

Cu(In,Ga)Se2 (CIGS) is one of the most advanced absorber materials for thin film solar cells due to its direct bandgap, high absorption coefficient, and ability to yield good quality devices. CIGS-based solar cells have yielded the highest conversion efficiencies of all thin film solar cells, reaching up to about 20.3%. One of the techniques used to form CIGS layers is a two-stage approach, which involves deposition of a precursor layer on a substrate followed by a high temperature activation step that converts the precursor layer into solar cell grade CIGS. Although various techniques such as evaporation and sputtering have been employed to prepare precursor layers, electrodeposition is especially attractive due to its low cost, efficient utilization of raw materials and scalability to high-volume manufacturing. Having recognized the cost and manufacturability advantages of the two-stage techniques, SoloPower developed a fundamentally low-cost, high-rate, two-stage technique for CIGS formation based on electrodeposition. Conformal coverage, control of grain size, crystallinity (or lack thereof), and mixing (alloys) are easily afforded in electroplating. Several aqueous electroplating solutions both in acidic and alkaline regimes are formulated and optimized for achieving stable plating solutions for roll-to-roll electrodeposition that give adherent and high quality films with controllable molar compositions. An optimized rapid thermal annealing approach was developed to achieve device-quality CIGS absorber layers. In this paper, we discuss our advanced electrodeposition approaches for CIGS precursor formation as well as critical reaction techniques to form efficient absorber layers. We demonstrate the capability of our technology with cell efficiencies over 15.6 % on ~11.3 cm2 lab-size cells, and aperture efficiencies of 13.4 % for lightweight flexible modules from our roll-to-roll manufacturing line. We discuss these results in terms of current-voltage and quantum efficiency data.

The record efficiency of single cell thin-film Cu(In,Ga)Se₂ (CIGS)-based solar cells has recently surpassed 20 %. The best cells have been produced in laboratory-scale deposition systems with so-called 3-stage type of processes. In this contribution, we discuss synthesis of CIGS absorber layers for high efficiency devices under industrially more relevant conditions, with special focus on the effects of, and formation of, different Ga-gradings in the material. Based on device simulations performed in SCAPS, effects of front- and back-surface grading and the position of the so-called “notch” (Ga/(In+Ga) minimum) are discussed. With these results as starting point, the realization of different types of grading has been investigated under the constraints regarding evaporation rate profiles, deposition time, and substrate temperature that are put on industrially relevant processes. Film growth has been combined with electrical characterization and materials analysis, including glow discharge optical emission spectroscopy (GDOES) and scanning electron microscopy (SEM). It is shown that the formation of a desired type of grading is sensitive to both deposition time, substrate temperature, and the Cu/(In+Ga) ratio in the studied regime. Using an optimized CIGS deposition process, single cell efficiencies close to 20 % are demonstrated. This is achieved using an industrially relevant deposition process with a relative short deposition time (< 15 min.) and moderate substrate temperature (< 550 °C), and with an evaporation sequence which is different from a classical 3-stage process.

The development of low cost industrial processes is one of the key issues to help Cu(In,Ga)Se₂ (CIGS) based solar cells reach high competitiveness to silicon based solar cells. Such a low cost process can be achieved using a two-step technology based on the sequential electrodeposition of a metallic precursor followed by a rapid thermal processing (RTP). This kind of industrial process can reach very high efficiency devices only if an accurate optimization of critical process parameters has been already performed. Using previous studies done on this industrial process pilot line, five parameters have been selected because of their strong influence on material quality and solar cells efficiency: a factor related to electrodeposition of metallic copper (Cu), indium (In) and gallium (Ga) precursor, a factor linked to sodium deposition and three thermal treatment parameters (temperatures of several RTP furnace parts). We have studied the influence of each parameter (both linear and nonlinear behaviors) but also two interactions factors on device quality in order to improve our industrial process and to prevent any drift from the industrial process pilot line.
Cu, In and Ga have been electrodeposited on a Mo sputtered soda lime glass and annealed at low temperature (100-150°C) to get a stable phase before RTP process. Then a thin sodium fluoride film has been deposited by spray over the samples followed by the evaporation of selenium film using Vapor Transport Deposition technique. Finally, the whole stack has been annealed in a RTP furnace at atmospheric pressure. Samples have been characterized using standard methods to obtain optoelectronic properties using Intensity-Voltage, External Quantum Efficiency measurements, morphological properties using Secondary Electron Microscopy technique, chemical composition using both X-Rays Diffraction and Raman spectroscopy techniques and diffusion profiles using Glow Discharge Optical Emission Spectroscopy analysis. In addition, Raman scattering using different excitation wavelengths, Auger electron spectroscopy and photoluminescence measurements have been performed to help the non-destructive identification of the secondary phases for different depth regions in the layers. All these results have been correlated with process parameters.
Using statistical analyses, we have lit up some trends, especially between the sodium deposition factor and the incorporation of gallium through the absorber layer. Moreover, it seems that even if two interactions factors have a huge influence over solar cells quality, some compromises have to be done to reach very high efficiency modules.

The Spray-ILGAR (Ion Layer Gas Reaction) process is a technologically simple but chemically complex low cost and vacuum free technique that enables the deposition of thin semiconductor layers for numerous applications. It is a cyclic and sequential process in which a spraying (strictly speaking an aerosol assisted chemical vapor deposition) and a sulfurization step alternate to produce a thin film with process-dependent composition, band-gap and even morphology, ranging from closed and compact films to high quality nanodot films. The ILGAR process has been awarded the 2011 German High Tech Champion prize in the field of photovoltaics by the Fraunhofer Society.
In this current work we report on the use of the Spray-ILGAR process to deposit In₂S₃ buffer layers for chalcopyrite-based thin film photovoltaics. A novel route has been developed that allows the production of Cd-free Cu(In,Ga)(S,Se)₂ solar cells (0.5cm²) with certified record efficiencies of 16.1% on industrial absorbers from BOSCH Solar CISTech GmbH without any pre- or post treatment and with no anti-reflective coating and with comparable long term stability. The delicate chemistry of the process is discussed including the role of the indium precursor used (either InCl₃ or indium acetylacetonate) as well as the effect of the sulfurization step on the final performance of the devices. In the first case, the choice of the precursor alters fundamental parameters such as the crystallinity (investigated by scanning electron microscopy and x-ray diffraction), the bulk and surface composition of the nominal In₂S₃ layer (investigated by x-ray fluorescence and Rutherford backscattering, and x-ray photoelectron spectroscopy respectively) and even its band gap, with values ranging from 2.0 eV to 2.9 eV. The diffusion of sodium and copper into the buffer layer also depends on the indium precursor used. In the second case, the choice of the optimal H₂S flow rate and concentration during the sulfurization step helps to reduce and eliminate metastabilities that result in undesired light soaking effects. An optimal sulfurization of the precursor layers leading to stable devices is achieved when adding H₂S also during the spraying step.
The spray ILGAR technique has been scaled up and permits the in-line deposition of buffer layers, in contrast to the batch nature of the standard chemical bath deposition of CdS buffers. A prototype ILGAR coater for substrate sizes up to 30x30cm³ has been designed and constructed by SINGULUS Technologies AG and 30x30cm² modules have been produced with efficiencies of 13.7% (Cu(In,Ga)(S,Se)₂ from AVANCIS GmbH & Co. KG), comparable to the corresponding CdS references (13.6 % ±0.2).