Symposium ES20—Thin-Film Chalcogenide Semiconductor Photovoltaics

Thin-film Ag-CIGS (or ACIGS) solar cells and modules produced by MiaSolé Hi-Tech yield high efficiencies rivaling polycrystalline silicon-based modules but are based on a high-throughput, PVD-based deposition process on flexible stainless steel substrates. The process is unique among thin-film solar manufacturers in that a complete device stack is produced without a vacuum break in less than one hour. Using this manufacturing system, a high level of process control and stability is possible via real-time control of the deposition process.

To continue performance improvements that close the gap between manufacturing material and champion small devices, a better understanding of the fundamental device properties is required. This requires development of a device model that accounts for electrical performance characteristics observed under light and voltage stress over time for comparison to real-world solar cell performance.

Our recent effort to generate a fundamental device model began with employing a wide variety of characterization results from our own measurements, those of our research partners, and features of many CIGS materials that have previously been excluded in prior device models. Additionally, device parameters must be based on actual device measurements and constituents, as heterojunction ACIGS solar cell devices are complex and errors compound quickly. The overall objective was to keep the model as simple as possible and look for generally good agreement among a variety of simulated measurements.

Most of the effort has been devoted to understanding the defect structure of the ACIGS absorber layer. Conventional capacitance-voltage profiling is supplanted in our measurements by high-speed C-V profiling (HSCV) that avoids many complications of transient charge effects in highly defective materials such as ACIGS. An important consequence of these measurements is the discovery that MiaSolé absorber layers (as well as other CIGS devices) can be defined by an acceptor-rich layer (ARL) adjacent to the CdS buffer layer. This ARL has been shown to have profound consequences for device electrical performance, as it can degrade fill factor and reduce efficiency.

The physical basis for the ARL is a matter of some debate. A variation in static shallow acceptor concentration is one possible source, but it predicts neither device metastability nor measured high concentrations of DLTS-visible defects. Charge accumulation at high concentrations of deep acceptors would be possible, but accounts for neither metastability nor carrier capture kinetics observed in DLTS. Instead we hypothesize that V(Se)-V(Cu) divacancy defects are responsible for charge accumulation in the ARL, and their simulated response to light- and voltage-stressing is consistent with our measurements. This hypothesis is qualitatively consistent with positron annihilation spectroscopy (PAS) measurements of our absorber. Predicted shallow acceptor and donor levels of the divacancy also provide an explanation for the long-observed high dopant compensation in CIGS, and at the concentrations predicted by our model, it is possible that the divacancies alone can be used to explain most of the charge and doping profile of the absorber.

Emphasis of the divacancy defect in our device model can guide MiaSolé Hi-Tech to new directions for device improvement that leverage our ability to directly manipulate absorber stoichiometry in our unique thin-film solar manufacturing system.

In this work, we present a suite of characterization tools developed to accurately and systematically analyze losses in non-Si photovoltaic solar cell technologies, in particular thin-film chalcogenide devices. This characterization platform will enable easy access to the implied open-circuit voltage (iV_oc) and fill factor (iFF) of devices, to their pseudo fill factor (pFF) and thus series resistance (R_s) at maximum power point, and to their complete-stack contact resistance (R_c). Together with calculated detailed-balance performance limits and traditional J-V measurements, these metrics will enable a precise accounting of recombination and resistive losses, from fundamental efficiency limits to experimental device performance. The characterization platform includes systematic determination of the absorber optical parameters (n & k), optical modeling of the device, calculation of the iV_oc from measurement of the external radiative efficiency (ERE) of the absorber, J_sc-V_oc measurements, and measurement on the contact stack resistance using through-the-absorber transfer line method (TLM).
Cd(Se)Te solar cells, our test system, provide a prime example of the potential impact of the techniques we propose to develop and implement: although they exhibit strong photoluminescence response, record poly-Cd(Se)Te cells have bandgap-voltage deficits (W_oc) of approximately 550 mV, as compared with below 400 mV for all other mature PV technologies. Similarly, these record Cd(Se)Te devices have FFs below 80%, when other mature cells are near or above 85%. Frustratingly, a systematic identification of the origin of these sub-par performances – for example recombination or resistive losses – has been lacking, thus slowing down the development of these technologies. Similarly, it is often asserted that Cd(Se)Te cells need a better back (hole) contact. Although we believe this is true, it is currently unknown how high the V_oc and FF could be for a given cell if it had a perfect back contact.
Using the proposed set of characterization techniques, users will be able to identify the sources of inefficiencies in thin-film chalcogenide solar cells, from bulk material quality and surface passivation to contact selectivity and resistivity. Rapid absorber and contact improvement will thus be possible.

Several characterization techniques are usually performed on a localized region of any material because of convenience in a routine basis. However, lateral non-uniformities in chemical composition, structural defects and even the presence of voids can be found in a polycrystalline solar cell, i.e. CI(G)S, influencing the local carrier properties of the material. This implies that some of the material properties extracted from local characterization methods may not be representative of the full device performance complicating the correlation between measurements. For example, time-resolved photoluminescence (TRPL) is typically measured locally, and the effective lifetime values determined (including the influence of front/back recombination, etc.) are closely related to the open circuit voltage which is a macroscopic parameter that might be affected by any carrier lifetime non-uniformity. Therefore, information about the influence of spatial inhomogeneities on the carrier transport is important to assess whether it is enough to use local parameters to correlate different characterization results —and to predict the performance of the solar cell—, or to what extent it is required to consider non-uniform parameters that could be more representative of the whole device.

In this contribution, we use transient PL measurements spatially resolved in the micrometer-scale range to access the carrier properties of different absorbers. The TRPL setup is coupled into a microscope enabling to operate in confocal or widefield illumination mode. Widefield mode is used to map the samples in a broader range while confocal mode attempts to resolve the influence of the grains inside the structure with the highest optical resolution of the system (< 1 µm). Besides all the mechanisms involved in the determination of the effective carrier lifetime such as surface recombination, charge separation, etc., the use of confocal mode requires the consideration of lateral diffusion of carriers as well as the proper evaluation of the sample performance due to possible high injection effects. Accordingly, the interpretation of the measurements in both modes is aided by 2D simulations, which are also compared to the macroscopic I-V parameters of the devices. Specifically, 2D device simulations allow us to quantify the lateral diffusion of carriers as well as to include other key parameters involved in TRPL measurements, i.e. surface recombination or carrier mobility. All simulations use as input measured absorption coefficient as a function of the bandgap grading for improved modelling. Finally, results from other spatially resolved techniques will be shown to correlate device performance with TRPL mapping and support the main findings.

Numerical simulation of solar cell device physics for predicting the performance, design and optimization of solar cells is a well explored active area of research. There are many tools and software packages like AFORS-HET, SCAPS-1D, AMPS-1D, PC1D, Silvaco TCAD, Sentaurus TCAD, etc., which are available freely or commercially, to perform numerical simulation of solar cells. The fundamental physics equations solved are continuity equations for charge carriers, namely electrons and holes, and the Poisson equation for electrostatic potential. In the continuity equation for electrons and holes the generation and recombination term is modeled through radiative processes, the SRH process, the Auger process etc. For the SRH generation recombination process an effective lifetime is assumed in the modeling. Combining this model with the boundary conditions, light and temperature simulation conditions, one can calculate I-V curves, efficiencies, fill factors etc., for the solar cell.
As the solar cell ages performance is affected. This can be microscopically explained through the transport of different defects present in the solar cell. The transport of defects can also cause metastability in solar cells. In our previous work we explained the metastable behavior of CdTe solar cells by studying the Cu related defect transport along with the carrier transport.
In this work, we present a novel Unified Solver for studying carrier and defect transport on an equal footing. The generation recombination term in the continuity equation for defects corresponds to the formation and transformation of defects. This formation and transformation of defects along with generation and recombination process for charge carriers is represented as a defect chemical reaction. Hence, we call our model as reaction-drift-diffusion modeling of solar cell. The drift-diffusion equations for defects require the diffusion constants and activation energies of the defect to be known and the defect chemical reaction require reaction rate constants to be known. These parameters are calculated using Density Functional Theory (DFT).
Since the main goal of our research work is to study short time metastability and long-time reliability concerns of cadmium telluride (CdTe) photovoltaics, special attention has been placed in the design of the solver to be able to produce results ranging from ns to hours/days/years. The solver gives us possibilities to explicitly account for all transient effects with free carriers (simulation of time resolved photoluminescence) and defects (simulation of performance instabilities, IV hysteresis etc). Various generation recombination processes can be represented as additional defect chemical reactions. Moreover, the Unified Solver supports accurate treatment of interfaces and grain boundaries that are crucial for the explanation of the operation of CdTe and other chalcogenide PV technologies.
The Unified Solver is benchmarked against Silvaco simulations of a homojunction and heterojunction solar cell. Excellent agreement is observed between the Unified Solver and Silvaco results for the key solar-cell parameters (short-circuit current and open-circuit voltage). Next, the Unified Solver is employed in constant temperature 2D simulation of chlorine diffusion annealing in a cadmium telluride (CdTe) system under insulating boundary conditions (isolated system). Chlorine is introduced in the system as a neutral interstitial at a half corner of the (1um×1.2um) structure. The concentration of chlorine interstitial is 1e16 cm^-3 and the system is kept at a temperature of 750K. The sample is annealed for 240s using the test case of chlorine defect reactions. The time evolution of chlorine substitutional defect (Cl_A⁺) is presented. Emulation of process temperature profiles is also presented in the talk.

For solar technology to compete with traditional energy sources, a continued decrease in photovoltaic (PV) energy generation cost is needed. Next-generation PV technology can achieve this by utilizing earth-abundant materials and low-cost processing techniques. Chalcogenide compounds are of particular interest in this area due to their prolific use in energy applications and amenability to high-throughput, low-cost processing techniques. Chalcogenides semiconductors have shown particular success in PV applications with materials such as Cu(In,Ga)(S,Se)₂ (CIGS), Cu₂ZnSn(S,Se)₄ (CZTS), and CdTe. However, developing new materials and processing techniques require extensive research efforts to achieve the required high-performance goals. Complex device processing and non-ideal optoelectronic properties associated with early stage materials often prolong material development.

In this work, our approach to developing next-generation semiconductors for PV is shown. We utilize low-cost, scalable solution-based processing techniques which are amenable to high-throughput optimization and a variety of chalcogenide precursors. Furthermore, we utilize advanced optoelectronic and structural characterization to guide the material development process; this provides rapid feedback for accelerated material development through accurate screening of early-stage materials for relevant optoelectronic properties and optimal synthesis parameters. In particular, we focus on the extraction of optoelectronic properties relevant for device performance – without the need for device fabrication – through optical techniques. Previous work using these techniques for developing CZTS, CIGS, and perovskites will be shown, with application to new materials currently in development. Ultimately, the successful development of new semiconductor materials requires a cross-disciplinary approach linking fundamental material properties and processing to the device-relevant optoelectronic properties.

Sb₂(S,Se)₃, is becoming a relevant thin film chalcogenide semiconductor with different technological applications such as: superconductivity, electronic components, electrode for sodium-ion batteries, photodetectors and as emerging photovoltaic absorber. In particular, and for this last application, the material has shown remarkable improvements in the last few years, demonstrating solar cells in superstrate configuration with power conversion efficiencies reaching 7.6%. In fact, and similarly to CdTe, most of the devices reported in the literature so far have been prepared using this configuration. This has opened interesting perspectives for their use in solar energy conversion applications, also taking into account the 1D crystalline organization of the material, with in principle benign grain boundaries and anisotropic conduction properties. Additionally Sb₂(S,Se)₃ has shown a high flexibility degree in terms of substrate type, due to the relatively low synthesis temperatures required for optimal high quality polycrystalline growth (300-400 C), allowing deposition onto polymeric, steel, ceramic and TCO/glass substrates. This versatility makes this compound very promising for ubiquitous applications such as building integrated photovoltaics (BIPV) (flexible, bifacial, and semi-transparent), wearables, or autonomous IOT applications among others.
In this work we present a systematic optimization study of the synthesis of Sb₂Se₃ thin films using substrate configuration solar cells, by a sequential process based on reactive annealing under Se atmosphere of thermally evaporated Sb layer precursors. The study is centered in the analysis of Sb precursor thickness and reactive thermal annealing conditions (annealing temperature, time, and pressure) on the compositional, structural and morphological properties of the layers. We observe the formation of continuous layers with large and homogeneous crystals, reporting for first time a weak photoluminescence signal close to 1.3eV in agreement with the band gap value obtained by IQE, and a systematic vibrational characterization under resonant and non- resonant Raman conditions that allows report 15 peaks of the 30 expected.
After a first optimization on Mo coated soda lime glass substrates, we report a promising power conversion efficiency of 5.3% in substrate configuration with a V_OC of 403 mV (the highest value reported for this configuration to the best of our knowledge), close to the 7.6% certified world record in superstrate one.
Additionally the study is complemented with a wide characterization of the fundamental properties of Sb₂Se₃ layers and devices using morphological and physic-chemical characterization (Photoluminescence, SEM, XRF, XRD), and with a complete analysis of the impact of the absorber stoichiometry under different regimes (Se-rich, Se-poor, Sb-rich and Sb-poor conditions). All this will be correlated with the optoelectronic characterization (JV, IQE, CV) of the solar cells. Finally, the main challenges to develop Sb₂Se₃ type solar cells in substrate configuration will be reviewed in the frame of the obtained results.

The non-cubic antimony chalcogenides, i.e., Sb₂Se₃, formed by quasi-one-dimensional ribbons can enhance light absorption and carrier transport by tuning the ribbon direction using the close-space sublimation (CSS) deposition. The power conversion efficiency (PCE) ~7% is found to be associated with the ribbon direction, which was investigated with theoretical calculation and experimental measurement in the Sb₂Se₃ films and devices. The substrate temperature and film thickness are critical for the fine-tuning of ribbon orientations during the CSS deposition. Our results show that [211]-preferred orientation leads to the minimum series resistance and highest light absorbance in the device. The device reliability measurement and in-depth elemental profiling analysis suggest that the interdiffusion between window layer and absorber layer dominate the degradation mechanism. This observation demonstrates that Sb₂Se₃-like quasi-one-dimensional materials with van der Waals boundaries can achieve scalable production at low cost and hold great potential for next-generation solar cell.

Tandem cells are considered as the next generation solar cell to overpass the intrinsic efficiency limitations of single junction solar cells, fixed by the Schockley Queisser limit below 33% and more probably below 30%. Silicon solar cells are already approaching the theoretical limit of about 29%, and represent 95 % of the present PV Market. Tandem solar cells on silicon would make possible to search for 43% theoretical limit, making practical module efficiencies of more than 30%. Combinations with hybrid perovskite and III-V top cells are presently focusing a lot of attention, but an alternative is to develop top cells based on chalcogenide materials, which technologies are well proved on the market (CdTe and CIGS). In the presentation we will report on researches carried out on silicon/CIGS solar cells. Based on the experience on single junction CIGS low band solar cells (1.15 eV) at about 20% efficiency, wide gap CIGS solar cells (1,6-1,8 aV) are grown on single cristal silicon substrates by coevaporation, with lattice parameters matching with silicon, thanks to compositional adjustments (Ga to In, S to Se). In order to give more flexibility in interface electrical properties engineering, the silicon substrate is functionnalized by a thin III-V buffer layer, based on GaInAlAsP lattice matched alloys¹. Epitaxial growth of CIGS has been successfully demonstrated and first devices have been elaborated to address the formation of high efficiency top cells alone, using Si/III-V as a selective back contact. Further structural, chemical and luminescence characterization will be reported.
Ref :
¹ D. Lincot et al., Proceedings of EUPVSEC 2018

A large gap still remains between the achieved conversion efficiencies and the Shockley-Queisser limit for Cu(In,Ga)Se₂ (CIGS) based solar cells. Some of the current limitations such as parasitic absorption losses are known and solutions are being developed. For some other limitations the origin is not well understood yet, but is required for knowledge based improvements. Thorough materials and device characterization of highly efficient solar cells can help to understand the origins of the remaining losses.
We used advanced materials and device characterization including high resolution transmission electron microscopy, time-of-flight secondary ion mass spectrometry, and time resolved photoluminescence (TRPL) combined with multidimensional device modelling to confine the origins of the remaining losses. We will summarize our recent findings on optical, compositional, structural and electronic properties in multi-stage co-evaporated CIGS layers and at interfaces to adjacent layers. In particular, we will describe how surface recombination velocities, minority carrier lifetime and mobility can be extracted from TRPL measurements and what we can learn from these findings. Further, we will present the presence of undesired compositional non-uniformities & voids and discuss their formation mechanism and role on device performance. Combined with recent findings obtained on narrow band gap CIS absorbers our results indicate possible inherent limitations originating from current fabrication methods and device architecture, which leads to new strategies how the actual CIGS solar cell efficiency can be pushed closer to the Shockley-Queisser value.

Recently, 22.8% efficient Cu(In,Ga)Se₂ (CIGS) thin film solar cell has been reported, demonstrating its great potential as a competitor to silicon solar cells. Adopting solution method to prepare highly efficient CIGS light absorbing materials is crucial for reducing the cost of CIGS solar cell fabrication and achieving large-scale production. We have previously reported 10.3% copper indium selenide (CIS) solar cell with the absorber fabricated from single DMF molecular precursor solution, demonstrating the great potential of DMF as relatively environmental benign solvent for solution processed thin film solar cells. The metallic element ratio (Cu/In or Cu/(In+Ga)) is the key factor to affect the quality of absorber materials. Here, we have systematically investigated the effect of the Cu/In ratio (0.85 to 1.2, from Cu-poor to Cu-rich) in the precursor solution on the CIS device performance. We found that solar cell efficiency increases with the Cu/In ratio from 0.85 to 1.05 and then decreases from 1.05 to 1.2. The best device was achieved from a ratio of 1.05 with a peak power conversion efficiency (PCE) of 12.20%, a short circuit current density (J_sc) of 36.12 mA/cm², an open circuit voltage (V_oc) of 0.499 V, and a fill factor (FF) of 67.57%. Further, by gallium alloying, CIGS solar cell with a PCE of 13.6% has been fabricated with a V_oc of 0.600 V, J_sc of 34.78 mA/cm² and FF of 65.17% under similar Cu-rich (Cu/(In+Ga)=1.05, Ga/(In+Ga)=0.2) condition. Our results for the first time demonstrate highly efficient CIS/CIGS solar cells can be achieved from absorbers grown under Cu-rich conditions. Characterization of the absorber materials and solar cell devices are undergoing to understand the mechanism behind the phenomenon observed.

Cadmium telluride (CdTe) thin film solar cells are renowned photovoltaic materials for its high absorption coefficient, suitable band gap and low manufacturing cost. The fabrication of highly efficient devices includes reduced intrinsic defects and improved interfaces at front and back contacts. The back-contact processing includes the formation or deposition of tellurium (Te) layer to make an ohmic contact with a metal. Here, we report wet-chemical treatments of CdTe using various iodine compounds and sodium tetrafluoroborate (NaBF₄). The iodine compounds tested are elemental iodine (I₂), ammonium iodide (NH₄I), mixture of iodine and ammonium iodide (I^-/I₃^-) and formamidine iodide (FAI). The treated surfaces were studied using Raman spectroscopy, X-ray diffraction (XRD), scanning electron microscopy (SEM), and energy dispersive X-ray spectroscopy (EDS). The treatment with iodine compounds produced Te rich layer on the back surface based on Raman, XRD patterns and changed surface morphology. The fabrication of the devices after these treatments, except I₂-propanol, improved the open-circuit voltage (V_OC) and fill factor (FF) of the devices. The I₂-propanol etching produced tellurium iodide (TeI) in β-phase on the surface which reduced device performance. The photoconversion efficiency of CdTe devices after treatment is up to 14.0% (V_OC 841 mV and FF 78%) while untreated devices have an efficiency of about 12.7% (V_OC 814 mV and FF 73%). In case of BF₄^- treated samples, it did not produce Te rich layer, but it seems like BF₄^- ions are passivating CdTe surface with enhanced V_OC and FF compared to standard devices.

CdTe solar technology is producing electricity at costs less than conventional fuels in many regions. Yet despite its commercial maturity, there is still headroom to increase performance significantly by addressing fundamental material challenges including compensation, hole density, carrier lifetime, and interfaces. This presentation will describe ongoing work to understand and overcome these challenges.

Last advances on kesterite (Cu₂ZnSn(S,Se)₄) corroborate the importance of doping and alloying strategies, not only for tuning the properties of this family of materials, but also for achieving higher efficiencies and broadening their possible range of application. Alkaline (Li, Na, K) and Ge doping; as well as Ag, Cd and Ge alloying are demonstrating a high potential by enhancing different aspects of these materials. In particular, the partial or total substitution of Sn by Ge in kesterites is an emerging approach aiming to improve their efficiency and broaden their band gap up to 2.2 eV with direct application in advanced solar cell tandem concepts, or semi-transparent photovoltaics for building integrated photovoltaics (BIPV) among others. Many studies have proved that the use of Ge as doping or alloying element influences drastically the overall parameters of Sn-based kesterite devices improving carrier lifetime, boosting the V_OC, and increasing the conversion efficiency in solar cell devices. The highest efficiency reported until now for pure Cu₂ZnGeSe₄ (CZGSe) compound is a promising 7.6% achieved with an annealing under H₂Se environment and employing very similar process parameters to those typically used for the pure Sn alloy. This work aims to study and optimize the main parameters affecting the synthesis of CZGSe comparing their main characteristics with the pure Sn compound.
For this purpose, Cu₂ZnGeSe₄ thin films were synthesized onto Glass/Mo substrates by a sequential process, based on the sputtering of Cu, Zn and Ge metallic layers, followed by a reactive annealing under a Se+Ge atmosphere inside a graphite box. Parameters like metallic stack order; annealing temperature (450-550 C), pressure (1-1000 mbar), routine (one or two-step annealing) and time; as well as the composition and of the use of chemical etchings were investigated. Additionally, a combinatorial sample with compositional gradients was prepared covering the full Cu/Zn, Cu/Ge and Zn/Ge range of interest, in order to correlate composition with the formation of secondary phases and defects. All the samples were fully characterized using a complete set of techniques (XRF, Raman spectroscopy, XRD, SEM and EDX) and by preparing and measuring solar cell devices.
After trying several stack orders, Cu/Zn/Ge is selected as the most promising one, leading to the formation of less secondary phases. The optimization of the annealing parameters confirms that a two stage process is the best suited for high quality absorbers. This includes a stage at relatively low temperature (330 C) for the synthesis of the CZGSe compound, and a second step at relatively high temperature (480 C) for its crystallization, revealing that lower annealing temperatures than in the case of the pure Sn compound are required. The compositionally graded sample shows that best efficiencies are obtained for absorbers with Cu/Ge ratio around 0.67 and Zn/Ge around 1.10, suggesting that Cu and Zn-poorer conditions than for pure Sn kesterite are required. Using this compositionally graded sample, we will present a deep analysis of the correlation of the different optoelectronic parameters with composition, including the analysis of secondary phase and possible point defects formation. With a first optimization which includes an etching of the absorbers in KCN, a champion cell with an efficiency of 6.5% (V_OC = 556 mV, FF = 59.6%, J_SC = 19.6 mA/cm²) is reported. Additional experiments introducing different sources of Ge during the annealing process (GeSe₂, GeSe, and pure Germanium) in order to change the partial pressure inside the graphite box will also be presented.

Multinary semiconducting compounds of the I-III-VI₂ group (I – Cu, Ag; III – Al, Ga, In; VI – S, Se, Te) with crystal structure of chalcopyrite CuFeS₂ are at the focus of current research as absorbing materials which are used in solar cells. By this reason studying the Cu-Fe-S system and determination the composition ranges of the existence of ternary compounds in this system can help to develop the methods of the improvement of the properties of the above-mentioned compounds of the I-III-VI₂ group for solar cells. The goal of the present paper is to prepare thin films of haycockite Cu₄Fe₅S₈ and to study their optical properties.

Thin films on glass substrates were deposited by flash evaporation technique from previously prepared ingots of haycockite Cu₄Fe₅S₈. Morphology of thin films and their chemical composition were investigated using the Hitachi TM3000 Tabletop Microscope equipped by Energy Dispersive Spectrometer (EDS). The magnification was varied from ×50 to ×10000. The X-ray studies were carried out using monochromatic Cu K_α-radiation (1.5406 Å, step size 0.01° or 0.04°, counting time 10 s). The Rietveld analysis of the X-ray powder diffraction data was done using the FullProf software. X-ray photoelectron spectroscopy (XPS) measurements were performed with PHI 5000 VersaProbe II XPS system using a monochromatic Al K_α source (15 kV). The pass energy was 117.4 eV for survey scans and 29.35 eV for high-resolution scans. The pressure in the analysis chamber was maintained below 5×10^-8 Pa for data acquisition. The data was processed using Multipak software, version 9.6.0. The XPS data was internally referenced to the C-C C1s peak (BE for C-C of 284.78 eV). The optical transmission was measured using Cary 500 spectrophotometer in the wave lengths interval from 400 nm to 2800 nm. Studies of the effects of illumination on the surface voltage were taken using surface photovoltage (SPV) technique in the energy range of 0.5 – 5.0 eV.

It was found that thin films have the chemical composition with the atomic content of Cu, Fe, and S of 25.21, 27.77, and 47.02 at. % with the atomic ratios of Cu/Fe and S/ (Cu + Fe) equaling to 0.91 for the atomic ration Cu/ Fe and 0.90 for the atomic ratio S/ (Cu + Fe) while these theoretical values for haycockite Cu₄Fe₅S₈ are equal to 0.80 and 0.89 respectively. Thin films of chalcopyrite consist of separate grains with the approximately equal areas of about (1000 – 2000) μm². It may be proposed that this structure appeared during cooling thin films because it completely covers the surface of thin films. The small inclusion of the second phase with the chemical composition close to talnakhite Cu₉Fe₈S₁₆ was detected. The most common occurrence of the inclusion of the second phase along the borders of the grain shows that they are may be also responsible for the cracking of thin films. Energy band gap was estimated using both transmission and SPV methods. Usage of thin films of haycockite Cu₄Fe₅S₈ as absorber in solar cells is discussed.

Acknowledgment. B. Korzun would like to thank PSC-CUNY for financial support of the studies under project TRADA-49-552.

Kesterite Cu₂ZnSn(S,Se)₄ (CZTSSe) semiconductors, composed of non-toxic and earth-abundant elements, have great potential as low cost and mass production photovoltaic absorber materials. However, the best CZTSSe solar cell, obtained from hydrazine based ink by IBM in 2014, only has an efficiency of 12.6%[1], much lower than similar structured Cu(In,Ga)Se₂ (CIGS) solar cell (22.6%)[2]. The development of kesterite Cu₂ZnSn(S,Se)₄ thin-film solar cells is currently hindered by the large open-circuit voltage deficit (V_oc-def). The possible reasons for the large V_oc-def include large tail states (electrostatic potential fluctuation and bandgap fluctuation), Cu-Zn antisite order-disorder, formation of deep defects and interface recombination, which is dominate still under debating. Here, we have explored different strategies to understand the V_oc deficit in DMSO solution processed CZTSSe solar cells. First, we found that the V_oc-def as well as efficiency of DMSO solution processed CZTSSe solar cells is strongly affected by the precursor tin oxidation state (Sn²⁺/Sn⁴⁺) in the solution. The power conversion efficiency achieved from Sn⁴⁺ solution was 13.2% with a V_oc-def of only 0.570 V, much lower than the current world record CZTSSe device (0.617 V), whereas the efficiency obtained from Sn²⁺ solution was only 7% with V_oc-def higher than 0.600 V. Investigation by solution chemistry and film morphology characterization reveals that the huge difference comes from different complexation of the Sn with ligand (thiourea) and solvent (DMSO) which lead to different reaction pathways from solution to solid state film and thus dramatic absorber quality and device performance. Second, we have used Ag or Ge alloying to further investigate the their effects on the V_oc-def of Sn⁴⁺ solution processed CZTSSe solar cells. Characterization of CZTSSe absorber materials including XRD, Raman, SEM, EDX, TEM, and PL and device performance show positive effect of the alloying on V_oc-def issue. Our results shed new lights on how to suppress CZTSSe solar cell V_oc-def and improve their efficiency to higher level.

Crystalline antimony-based nitrides have long posed fabrication challenges due to the high nitrogen chemical potential required for formation and the tendency of antimony to react with oxygen and moisture to form amorphous oxynitrides. To date, Zn₂SbN₃ is the only reported crystalline antimony nitride in which Sb functions as a cation with a positive oxidation state. This material warrants further investigation not only due to its unique nature, but also due to its advantageous optoelectronic properties, which may be tunable like those of other ternary nitrides. Theoretical calculations predict this material to have a direct band gap (1.7eV), low effective mass for electrons (0.15-0.19m_e) and moderate effective masses for holes (2.4 m_e), which suggests that Zn₂SbN₃ has promise as a photoactive absorber. [1] Experimental measurements show room-temperature photoluminescence activity and band edge alignment appropriate for the hydrogen evolution reaction.

In this work we report on the detailed investigation of the optoelectronic properties of thin-film Zn₂SbN₃ and their correlation to the films’ composition. Films were grown through combinatorial co-sputtering and characterized by spatially-resolved measurements. Experimental results show that the wurtzite-derived crystal structure is stable over a large range of cation composition (Zn/(Zn+Sb) = 60-80%) as determined by X-Ray Fluorescence. 4-point-probe measurements reveal 200-400 kOhm resistance, tunable with composition. UV-visible spectroscopy shows an absorption onset around 1.7 eV. This will be further investigated through photoluminescence and Hall effect measurements. Growth parameter space will be mapped by varying factors such as growth rate, substrate temperature, and the use of a N₂-cracker to enhance the nitrogen chemical potential. A fuller understanding of the relationship between fabrication conditions and film properties will help evaluate the attainability of this new ternary nitride’s predicted utility as a photovoltaic material.

[1] Arca et al., J. Am. Chem. Soc., 2018, 140, 4293

Currently CZTS sulfide absorbers for solar cells with the highest efficiency have all been produced by vacuum techniques which thus seem to be superior to solution processing for CZTS cells. Sputtering and pulsed laser deposition are typical vacuum techniques, which are employed for solar cell absorbers of complicated stoichiometry. Both techniques are well-known for stoichiometric transfer of chemical compounds such as metal oxides and nitrides from a target to a substrate. The two techniques are non-thermal since the emitted atoms from the target are quite energetic and may have an energy distribution with a tail up to more than 10 eV. This non-thermal arrival energy is also known to be beneficial for growth of layers with high crystallinity.
In the photovoltaic literature there seems be a limited number of examples of a comparison between the absorbers and cells produced by either sputtering deposition (SD) or pulsed laser deposition (PLD) except for an inconclusive study without production of solar cells by Sun et al. [1].
We have recently produced solar cells by co-sputtering deposition and PLD with a record efficiency of 6.6 % for SD, while that by PLD reached 5.2 % [2]. While the efficiency was comparable, there were several processing differences, in particular in the thermal annealing steps. It is not clear how different the ideal thermal profiles for the two techniques are under similar deposition conditions, i.e., same type of Mo-coated substrate and precursor films of the same thickness and composition.
Furthermore, for single-target SD the composition of the films can be very difficult to tune because of preferential sputtering from the growing film induced by the arrival of fast atoms or ions from the target. In contrast, for standard PLD (with a single target) the composition can be varied by adjusting the laser fluence, since the transfer of copper atoms from target to film is increasing with fluence (and with 0.8 J/cm2 as the optimum fluence) [2,3].
We will discuss a few other PLD experiments: The CZTS-films can be doped with additional elements, e.g. by Ag using doped targets without any additional complications in the PLD process. Also the production of oxide precursor films is comparatively easy with an oxide target for PLD, while it is not yet clear how successful a detour via metal oxides to sulfides would be for co-SD.
Detailed results for the comparison between films made by SD and PLD as well as examples PLD-films of other compositions will be shown in this contribution.
[1] L. Sun et al., Journal of Crystal Growth 361 (2012) 147–151
[2] A. Cazzaniga, A. Crovetto et al., Solar Energy Materials & Solar Cells 166 (2017) 91–99
[3] J. Schou et al., Applied Physics A) 124 (2018) 78

Various chalcogenides (Cu(In, Ga)(S, Se)₂, CdTe, Cu₂ZnSn(S, Se)₄) have been adopted as successful absorber materials for application in thin-film photovoltaic (PV) and photoelectrochemical (PEC) devices. However, despite being commercialized (e.g. Cu(In,Ga)(S,Se)₂, CdTe) or showing performance advances (e.g., Cu₂ZnSn(S,Se)₄), several concerns and material limitations remain regarding scarcity/toxicity (e.g., In and Ga/ Cd) and Cu-Zn anti-site disordering in earth-abundant Cu₂ZnSn(S,Se)₄(CZTSSe) chalcogenides.Cu₂BaSn(S, Se)₄(CBTSSe) has recently gained substantial attention as an alternative semiconductor material due to desirable properties for solar energy conversion applications, and reduced tendency for antisite disorder relative to Cu₂ZnSn(S, Se)₄. In this study, as an alternative to more expensive vacuum-based film-deposition processes, we report a low-toxicity solution-based process for the fabrication of high-quality CBTSSe absorber layers with µm-scale film thickness and grain size. The facile process involves spin-coating an environmentally benign solution of highly soluble, inexpensive and commercially available precursors, Ba(NO₃)₂, Cu(CO₂CH₃)₂and SnI₂ in DMSO, followed by sequential sulfurization/selenization annealing. A high-temperature pre-baking step under sulfur vapor is needed for each film layer to avoid forming the impurity phase, Ba(SO₄) when starting from the soluble Ba(NO₃) reagent. Our reproducible approach forms a dense, 1-µm-thick, single phase CBTSSe absorber layer with large grains (0.9-4.5 µm) and a tunable band gap (e.g., 1.68 eV under the typical processing conditions employed). Additionally, we demonstrate the first prototype solution-deposited CBTSSe PEC device, exhibiting a photocurrent of ~10mA/cm²at 0 V_RHE((increasing to ∼12 mA/cm²during the stability test), comparable with analogous devices based on vacuum-processed CBTSSe films, as well as stable hydrogen evolution for more than 10 hours. These results demonstrate the prospects for low-cost solution-processing of high-quality CBTSSe film and absorbers for thin film PV and PEC cells.

Cd chalcogenide films are direct band gap semiconductors that can harvest photon energy in a wide energy range from 2.42 eV (CdS) to 1.74 eV (CdSe) by compositional tuning or chemical gradation. Growth of such films using chemical bath deposition is advantageous due to low production costs and the ability to easily control bath composition and resulting film microstructure. We have recently demonstrated monocrystalline CdS and CdSe deposited on GaAs substrates by chemical bath deposited.^{1, 2} In this work we present the chemical, structural and photovoltaic properties of Cd(S,Se) solid solution thin films.³ We present evidence for the formation of chemically graded films which can be expected since thiourea (S precursor) decomposes faster than sodium selenosulfate (Se precursor) and as a result, S anions are first to react with the substrate surface. The physical and chemical properties of the films were characterized using x-ray powder diffraction, scanning electron microscopy, analytical transmission electron microscopy and energy dispersive spectroscopy mapping. Finally, current-voltage measurements of a GaIn-eutectic/GaAs/Cd(S,Se)/In device were conducted at room temperature under 1 sun. We demonstrate increasing photo-response with S/Se ratio in CdS_xSe_1-x based photovoltaic cells. This work presents a comprehensive and applicable study on chemical bath deposited Cd chalcogenide thin films.

1. O. Friedman, A. Upcher, T. Templeman, V. Ezersky and Y. Golan, Journal of Materials Chemistry C, 2017, 5, 1660-1667.
2. O. Friedman, D. Korn, V. Ezersky and Y. Golan, CrystEngComm, 2017, 19, 5381-5389.
3. O. Friedman, O. Moschovitz and Y. Golan, CrystEngComm, 2018, 20, 5735-5743.

Optically transparent p-type conductors enable development of transparent electronics, and opportunities to develop high-efficiency bifacial photovoltaic devices. Sulfide materials offer an interesting alternative to oxides for the photovoltaic applications due to better hole transport properties. We report on the structural, optical, and electronic properties of earth-abundant p-type transparent conducting barium copper sulfide (BCS) thin films fabricated using solution-processing. The BCS thin films were characterized using scanning electron microscopy (SEM), X-ray diffraction (XRD), UV–Vis–NIR spectroscopy, and spectroscopic ellipsometry (SE). A BCS film of thickness 100 nm can transmit > 70% of the visible light with band gap ∼ 2.4 eV. Based on SEM images, initial BCS films look compact with a grain size of ∼15 nm. Our studies show that these films are conductive with a carrier concentration of ∼10¹⁹ cm^-3, making them promising hole transport materials for photovoltaic applications. We will discuss cadmium telluride (CdTe) devices fabricated using BCS layer as an interfacial layer between CdTe and a standard Cu/Au. Additionally, we will extend our study to fabricate bifacial CdTe devices employing the transparent BCS buffer layer and a conducting oxide as the back contact.

Polycrystalline II-VI and silicon are the most mature material systems for photovoltaic applications, both produced and used at the GW-scale by the photovoltaic industry. Their combination in tandem devices – with a silicon bottom cell and a wide-bandgap (1.70-1.75 eV) II-VI top cell – is, thus, a promising pathway. However, such a development has so far been stymied by the absence of a suitable wide-bandgap polycrystalline II-VI absorber, with high enough material quality to ensure a performant top cell. In the particular case of the wide-bandgap alloy Cd_1-xZn_xTe, this is largely due Zn out-diffusion during the CdCl₂ process – required to “activate” the material by passivating internal defects and, thus, to achieve high-efficiency polycrystalline CdTe and CdSeTe devices – leading to a final film with a bandgap closer to 1.5-eV CdTe than 1.7-eV Cd_1-xZn_xTe. Reducing the intensity of the CdCl₂ treatment while still achieving activation of the material is believed to be a key step to enable high material quality 1.7-eV II-VI cells.
In the case of non-Zn-alloyed materials, CdSe_xTe_1-x films exhibit substantially stronger luminescence than their CdTe counterparts before the CdCl₂ passivation treatment step, indicating a higher as-deposited material quality and, correspondingly, a longer minority carrier lifetime after CdCl₂ treatment. Following these results, we theorize that incorporation of Se into Cd_1-xZn_xTe would lead to higher material quality of as-deposited wide bandgap films. We expect that such films would require a lower intensity CdCl₂ treatment to activate, leading to limited Zn out-diffusion. Hence, we deposited the quaternary alloy Cd_1-xZn_xSe_yTe_1-y using a modified close space sublimation (CSS) process from sources containing CdSe_yTe_1-y and Zn. The deposited films were characterized by Transmission and Scanning Electron Microscopy, Energy Dispersive X-Ray Spectroscopy, and X-ray Diffraction in order to assess their material properties and composition as a function of deposition parameters. Optoelectronic properties were assessed with Transmittance, Photoluminescence, and Time-Resolved Photoluminescence measurements. Preliminary device results will be reported.

High bandgap kesterite Cu₂ZnSnS₄ (CZTS) has attracted world-wide attention in recent years owing to the earth-abundance and non-toxicity of its constituents and the realization of high and stable performance [1]. The large deployment of photovoltaic (PV) in the future would demand stable, abundant and non-toxic materials similar to Si, in either flexible/rigid single junction thin film solar cells or tandem cells with Si bottom cell. In this regards, CZTS is more advantageous over other single junction cells and their associated top cell for tandem cells. For its industrial-scale deployment, obtaining high efficiency CZTS is the first prerequisite. Until recently, however, state-of-the-art CZTS devices were limited under 10%, being far below the Schockley-Queisser (SQ) limit of about 33% efficiency under terrestrial conditions [2]. Major limitations lie in the large open circuit voltage deficit (Voc-deficit) and a comparably low fill factor (FF). The loss mechanisms for the Voc and FF have been extensively investigated and the hetero-junction interface quality is one the most proposed factors that contribute to the performance limit.
One of the critical elements of forming a desirable interface structure in hetero-junction PV technology is the junction-formation step [3]. At present, the preferred approach in CZTS community is to deposit a thin CdS layer by chemical bath deposition (CBD) method, where a suitable amount of ammonium hydroxide (ammonia) is applied as the complexing agent. The function of the ammonia has been extensively studied in CIGS (1) clean the absorber surface before the deposition, (2) control the cation slow release, (3) supress the formation of detrimental impurity like Zn(OH)2 at the interface and (4) minimize the metastability behaviour of the device. In our successive ionic layer adsorption and reaction (SILAR) process for alternative ZnCdS buffer deposition, however, no ammonia has been applied. In order to further optimize the interface quality, we explored adding ammonia in the cation solution to realize its similar function in CdS deposition process.
In summary, the CZTS/ZnCdS interface quality is optimized by applying ammonia as complexing agent in the precursor solution during the SILAR process for ZnCdS deposition. Excess oxide and hydroxide impurities formed at the interface are limited to a low level, which reduces the related electronic defects in the depletion region and alleviates the metastability behavior caused by the Zn(OH)₂. The application of ammonia also enables the control of reaction mechanism, improving the epitaxial growth, thereby reducing the interface recombination. Finally, efficiency of up to 10% is achieved by modifying the interface structure thanks to the chemistry method.

Wide bandgap Cd_1-xZn_xTe (CZT) is of interest for a top cell in tandem devices because it has a tunable bandgap and is expected to leverage the low cost CdTe fabrication process. However, the vapor pressure of ZnCl₂ is orders of magnitude higher than that of CdCl₂ at Cl activation temperatures, which results in Zn loss during processing, thereby reducing the bandgap. In the CZT films, the Zn can be replaced with the Cd of the CdCl₂ treatment or simply leave Zn vacancies in the film. Either of these effects are detrimental to device performance and make it challenging to fabricate higher bandgap absorbers based on CdTe. Here, we investigate CZT composition for films activated in an enclosed box with varying CdCl₂ and ZnCl₂ vapor pressures. We will determine how controlling the vapor pressure of the two Cl sources affects the evolution of the CZT film. These results will allow us to determine if Zn loss can be avoided.

Polycrystalline Cu(In, Ga)Se₂ (CIGSe) quaternary semiconducting material is a mesmerizing absorber layer to fabricate the high-efficiency thin film photovoltaic device. Among the different evaporation growth processes of CIGSe thin film, the deposition of copper, Indium, Gallium, and selenium stacking layer structure is one of the versatile methods due to its simplicity, good stoichiometry, and resulting throughput efficiency in photovoltaic devices. In the experiment, the metallic elements copper (Cu), indium (In), and gallium (Ga) were deposited in specific orders at room temperature and then the whole structure was selenized at (450-550) °C with different time of selenization (30-60 min). Selenization effect on structural, morphological, optical, compositional and electrical properties of prepared films was studied. Results show that the structural, morphological and optical properties of CIGSe thin films strongly depend on the growth mechanism, annealing temperature, time of selenization and chemical composition. X-ray diffraction (XRD) pattern showed the chalcopyrite structure of CIGSe with (112) preferred plane. The surface morphology and chemical composition of selenized thin films were investigated by scanning electron microscope (SEM) equipped with energy-dispersive X-ray spectroscopy (EDS. The grains were transmuted from featureless melted to well faceted bladed tetragonal grains of CIGSe by SEM images. Raman spectroscopy exhibited A₁ mode peak along with B₂/E mode peak. The root mean square (RMS) roughness of the films was studied by atomic force microscopy (AFM) and was ranged between 60 to 90 nm. The depth profile study showed that the distribution of elements depends upon the elemental order in the stacking structure. Optical testing was conducted by UV spectrophotometry and observed the absorption coefficient in the order of 10⁵ cm^-1. The carrier concentration was varying from 10¹⁶ to 10 ¹⁷ cm^-3 by hall effect Van der Pauw method. The results showed that 450 °C and 500 °C of selenization temperature was not sufficient for the inter-diffusion of all elements forming a good CIGSe absorber layer. Instead, the film selenized at 550 °C for 60 min showed a better result which could be a good absorber layer for highly efficient CIGSe based thin film solar cells

Copper gallium diselenide (CuGaSe₂) and copper aluminum diselenide (CuAlSe₂) belong to the
I-III-VI₂ group of compounds and are extensively studied as absorbing materials in solar cells and as active elements in optical filters. To optimize their properties, it is necessary to control their surface properties. The goal of the present paper is to investigate the effects of etching on the surface of the CuGaSe₂ – CuAlSe₂ single crystals and compare the results of chemical etching and argon ion beam sputtering.

Single crystals of seven alloys of the (CuGaSe₂)_1-x (CuAlSe₂)_x system, with x = 0.20, 0.40, 0.50, 0.60, 0.70, 0.80, and 0.90, were grown by chemical vapor transport (CVT) in an evacuated quartz tube using iodine as a transporting agent. Typical dimensions of the grown plate-like single crystals were about 10 mm × 5 mm × 0.5 mm with a well-developed surface (112).

X-ray photoelectron spectroscopy (XPS) measurements were performed with PHI 5000 VersaProbe II XPS system using a monochromatic Al K_α source (15 kV). The pass energy was 117.4 eV for survey scans and 29.35 eV for high-resolution scans. The pressure in the analysis chamber was maintained below 5×10^-8 Pa for data acquisition. The data was processed using Multipak software, version 9.6.0. The XPS data was internally referenced to the C-C C1s peak (BE for C-C of 284.78 eV). Raman measurements in backscattering configuration were performed at room temperature using a WITec alpha 300R confocal Raman imaging system with an excitation Nd:YAG laser at a wavelength of 532 nm and a 20x objective lens.

The untreated, chemically etched, and Ar⁺-sputtered surfaces of single crystals were investigated, and the effects of chemical etching and then Ar⁺ sputtering on the same surface of a crystal were determined. The spectra of the untreated samples showed a typical oxidized surface, strong O1s and C1s signals. The Se3d spectra revealed an additional component (Se⁴⁺) due to the existence of SeO₂ at the surface of the untreated samples. An additional signal corresponding to I3d_5/2 at 619.1 eV was detected, showing the presence of single crystals of CuI on the surface of the untreated samples. Chemical etching for 30 min removed iodide compounds. After Ar⁺ sputtering for 30 s, the oxygen and carbon contamination at the surface was removed. After the treatments, no detectable lattice damage was observed. The change of parameters of Ar⁺ sputtering from 4 mm², 2 kV, 1 μA to 5 kV, 25 mA, resulted in the appearance of an Al sub-peak corresponding to the neutral state of Al.

The position and shape of the Se3d peak on the composition of the solid solutions were investigated and explained by the binding behavior of the Se atoms in the (CuGaSe₂)_1-x (CuAlSe₂)_x system. There was a correlation of the Raman active modes with the change in chemical composition of this system.

Recently, different works related to the improvement of SnS-based thin film for solar cells applications have been reported. These solar cells (SCs) are built with complex configurations and fabricated using expensive techniques to obtain high short circuit current densities, J_sc, above of 20 mA/cm², and power conversion efficiencies of 4.36%. However, the SCs often have low values of open circuit voltage, V_oc, below of 400 mV, which limit their application to industrial scale. In order to incresase the V_oc, some researchers have developed either transition metals-doped SnS films or the synthesis of novel semiconductor ternary materials based in Sn and S such as Sn-Sb-S (TAS): SnSb₂S₄, SnSb₄S₇, and Sn₂Sb₂S₅. Conventionally, ternary compounds are synthetized by deposition of individual intercalated layers of SnS and Sb₂S₃, followed by annealing at high temperatures.
In this work, SnS and Sb₂S₃ thin films has been obtained by in-situ chemical bath deposition. The effect of the sulfur load, time deposition and pH bath solution has been studied. Antimony and tin chloride were used as metal ion sources, thioacetamide as sulfur ion source, and tartaric as the complexing agent. Chemical bath deposition was carried out at 80°C varying the concentration of sulfur and time of deposition while the pH of the solution was kept between 8 and 9. Then, the resulting samples were heated in nitrogen at temperatures from 300 to 500 °C to make them crystalline and form the ternary compounds. The results showed that the film thickness depends of the sulfur load, deposition time, pH and the temperature during the heating. The thickness of the as-deposited film without excess of sulfur was of 117 nm for a time deposition of 2 h. An increase on the sulfur load produced thicker films with thickness above of 250 nm. The SEM analysis of the as-deposited films showed a morphology of elongated grains of 100 nm, which increase their size after heating. The optical band gap of the as-deposited films was of 1.6 eV, which decreased to 1.46 and 1.24 eV after heating the films at 300 and 500°C, respectively. All the TAS films exhibited optical absorption coefficients higher than 10⁴ cm^-1. Moreover, the photo-conductivity of the as-deposited films was of 10^-9 Ω^-1 cm^-1 while those heated at 300 and 500 °C showed photo-conductivities of 10^-8 to 10^-6 Ω^-1 cm^-1. Finally, the evaluation of these ternary absorbers into the FTO/CdS/TAS/C structure gave a V_oc of 462 mV and a J_sc of 2.37 mA/cm² under white-LED light. These preliminary results demonstrated that TAS absorbers could be good candidates to obtain higher V_oc values than those obtained with the SnS-based SCs.

Low temperature-processed Cu(In,Ga)Se₂ (CIGS) thin film absorber is important for demonstrating a functional CIGS solar cell on plastic substrates. However, control of the band profile mainly dominated by the Ga concentration profile in the CIGS film has been challenging due to slow atomic diffusion at low temperatures. Here, a systematic engineering of Ga profile in CIGS films is reported by employing a thin Ag precursor layer prior to CIGS co-evaporation. By increasing the Ag precursor layer thickness, Ga gradient was mitigated along with improved CIGS grain size, which enhanced the overall solar cell performance. Formation of liquid-phase Ag-Se could provide mobile channels for Ga diffusion along the grain boundaries and expedite CIGS recrystallization process at such a low temperature.

The recent efficiency advances in solar cells based on Cu₂ZnSnS₄ (CZTS) and related high bandgap alloys make them promising compounds for application in tandem cells, in particular when monolithically integrated with a Si-based bottom cell, with the prospect of an all earth abundant, stable, efficient and industrially feasible tandem solar cell. However, in fabrication of monolithic CZTS/Si tandem structures, two major problems must be overcome: 1) a proper interlayer between top and bottom cell has to be developed, since the tunnel junctions typically used in III-V tandem cells are not trivially available, and 2) unlike in their Perovskite/Si tandem counterpart, the processing of the CZTS top cell involves at least one high temperature step (>500 C) which must not damage the bottom Si cell structures nor contaminate them with elements such as Cu or S diffusing from CZTS.
In this contribution, we approach these problems by using a thermally resilient n-type Si Tunnel Oxide Passivated Contact (TOPCon) bottom structure with a thin (<25 nm) TiN layer at the interface between the top and bottom cells, serving both as diffusion barrier and recombination layer. Using Microwave Detected Photoconductance Decay (µ-PCD) and Secondary Ion Mass Spectrometry (SIMS), we show that a regime for CZTS production exists where contamination of the bottom Si cell is kept at low levels, with post-process carrier lifetimes exceeding 1.5 ms, corresponding to an implied Voc (iVoc) of above 700mV. Based on these findings, we develop and characterize a series of monolithic CZTS/Si tandem devices with different interface configurations and barrier layer thicknesses. We discuss the optical impact of the different tandem interfaces used on the overall tandem performance. Moreover, we compare the photovoltaic properties of the CZTS films used as the top cells with our single junction CZTS baseline, based on co-sputtering. Although a natural trade-off exists which narrows down the processing regimes necessary to simultaneously achieve efficient top and bottom cells, our results do not reveal any a priori fundamental limitations to achieving efficient monolithically integrated CZTS/Si tandem solar cells.

For all thin film tandem configuration, perovskite and Cu(In,Ga)Se₂ (CIGS) solar cells represent ideal candidates for top and bottom devices due to their suitable energy band gaps. In this contribution we will show how CuInSe₂ (CIS) based absorbers can be pushed from 15% to beyond 19% without changing the optical bandgap of 1.0 eV. To do so we suppress back contact recombination by adding a graded composition to the back interface, increase carrier lifetime by additional post deposition treatment and increase absorber quality by shifting to a more stoichiometric composition between Cu and group III elements. Combined with semitransparent perovskite top cells we demonstrate efficiencies above 24% in four-terminal tandem configuration on rigid substrate. Further, the possibility to develop highly efficient perovskite and CIGS solar cells on flexible substrates lays the foundations to lightweight flexible tandem devices by high-throughput roll-to-roll manufacturing. We present a multistage vacuum-solution deposition method for perovskite absorber to tailor PbI₂ growth morphology. With this approach, we demonstrate a steady-state efficiency of 14.0% for NIR-transparent perovskite solar cells (average transmittance of ~74% between 800 and 1000nm), on a flexible front sheet which is used for flexible CIGS module encapsulation. Flexible perovskite/CIGS tandem with 19.6% efficiency is obtained in four-terminal configuration. Our work represents a step forward towards highly efficient low band gap CIS solar cells and provides insights into growth of layers and interfaces for high efficiency flexible perovskite solar cells. Furthermore, latest results on both four-terminal as well as monolithic two-terminal thin film tandem devices will be discussed.

CuGa₃Se₅ thin film is polycrystalline wide band gap material related to chalcopyrites. It is a promising absorber candidate for top cell application in both for tandem photovoltaic (PV) solar cells and photoelectrochemical (PEC) water splitting devices. Zn_1-xMg_xO is an attractive n-type contact layer for CuGa₃Se₅ and other PV absorber technologies due to its tunable conduction band offset as a function of Mg concentration. In this study, Zn_1-xMg_xO is being investigated as the buffer/contact layer for CuGa₃Se₅ based PV and PEC devices.

CuGa₃Se₅ thin films with Cu/Ga = 0.36 composition are deposited by single-stage co-evaporation, and Zn_1-xMg_xO films are deposited using combinatorial RF sputtering. The electrical, optical and structural properties of the Zn_1-xMg_xO films as function of Mg compositions are characterized. A baseline device is fabricated in a similar fashion to CIGS devices, with CdS window layer and i-ZnO buffer layer. Zn_1-xMg_xO is studied as replacement for both CdS and i-ZnO, and is grown on as-deposited, water-treated, and Cd²⁺ solution treated CuGa₃Se₅ absorber thin films. X-ray photoelectron spectroscopy (XPS) is performed on the treated films to study the surface defect chemistry.

The resulting CuGa₃Se₅/Zn_1-xMg_xO device JV and EQE performance is observed to be dependent on the temperature and the duration of the treatments. Replacing CdS/i-ZnO with Zn_1-xMg_xO improves the carrier collection in the short wavelengths due to the higher bandgap of Zn_1-xMg_xO. The measured device performances are directly related to the Zn/Mg ratio in the contact layer, and supports the significance of conduction band optimization. Open circuit voltage up to 920 mV is measured, which is important for PEC water splitting and tandem PV application.

A characterization study using atom probe tomography to understand crystal growth and other micro-structural aspects on molybdenum di-sulfide (MoS₂) thin films ~100nm is presented in here. The film was deposit over silicon di-oxide (SiO₂) coupons using radio frequency at 275W at high vacuum~10-9 from commercial MoS₂ 99.9% purity target. From scanning electron microscopy, it was possible to observe vertically crystal growth with high-porosity and three-dimensional reconstruction from spatial distribution from atom probe tomography indicates low impurities and no grain boundaries within molybdenum disulfide cystallites on film matrix. As complementary, electrical transport -3V to 3V at room temperature and X-ray pole figures were also measured on the films to understand crystal grain orientation.

Polycrystalline thin-film CdTe device efficiency of over 19% has been demonstrated at Colorado State University using a repeatable and scalable sublimation process. Record efficiency for research scale CdTe photovoltaics has been reported to be over 22% and highest reported module efficiency is over 18% by First Solar Inc. Use of Se alloying with CdTe to form a lower band-gap CdSe_xTe_1-x (CST) alloy has led to higher devices efficiencies with better current collection. Recent materials characterization has revealed that Se may be responsible for more than just reduction of band-gap. With CdCl₂ treatment Cl is known to very effectively passivate defects at CdTe grain boundaries. Recent materials and electrical characterization have shown that Se has a very similar behavior to Cl at the grain boundaries of polycrystalline CdTe. These include Se decorating the grain boundaries like Cl in the while cross-section cathodoluminescence maps show substantially higher signal intensity in regions with Se. Double heterostructures fabricated using CdSeTe with Al₂O₃ as a passivating oxide have been measured to have several hundred nanosecond bulk recombination lifetime. These films were also measured to have surface recombination velocity of under 100 cm/s. These results among others emphasize that Se also helps in passivation of grain boundaries. High absorber quality may help in mitigating voltage deficit that has traditionally been the largest limiting factor in CdTe photovoltaics in achieving efficiency closer to Shockley-Queisser limit. In addition to graded absorber devices and double heterostructures, replacing the traditional Cu doping with more advanced group V doping is an important research effort that maybe critical for future improvements in device efficiency. Nitrogen, Phosphorous, Arsenic and Antimony doping has been investigated by several research groups. Advanced co-sublimation hardware developed at Colorado State University a unique capability of depositing films for group V doping under over pressure of Cd. Such deposition under Cd overpressure causes formation of V_Te that is known to be favorable for incorporation of group V dopant and activation. Utilizing the high absorber and interface quality material and higher doping, efforts are in progress to fabricate polycrystalline devices with open-circuit voltage over 1V and device efficiency over20%.

We report on simultaneous incorporation of sodium and potassium in the absorber layer of all-sputtered Ag-CIGS solar cells fabricated in a MiaSolé roll-coater multi-chamber manufacturing tool. The MiaSolé process begins with a stainless-steel substrate, which is unwound into the roll-coater where the following layers are consecutively deposited by sputtering: diffusion barrier layer, alkali-doped Mo back electrode, absorber layer, CdS buffer layer, and TCO window layer. Addition of potassium to the absorber has been reported to lead to higher cell efficiency, and in the majority of cases potassium is introduced via a post-deposition treatment (typically as KF, after CIGS deposition). In the MiaSolé process, both sodium and potassium are introduced from the molybdenum back electrode, which is sputter-deposited from Mo rotary targets, doped with sodium and potassium molybdates, respectively. Changing the sputter deposition power and varying the depth of the doped layer inside the Mo electrode allows us to vary the amount of alkali metals delivered to the absorber.
Addition of the K-doped back electrode to the Na-doped Ag-CIGS baseline process leads to a higher solar cell efficiency (up to 0.5%abs), mostly driven by higher FF and Voc as a result of reduced interfacial recombination and reduced voltage dependent collection losses. Auger electron spectroscopy (AES) and secondary-ion-mass spectroscopy (SIMS) results indicate that potassium primarily accumulates near the main junction and leads to reduced Cu/(Ga+In) in that region, similarly to observations reported in the literature. However, in contrast to numerous reports, we do not observe any evidence for the formation of secondary phases, such as e.g. K-In-Se, according to x-ray photoelectron spectroscopy (XPS) measurements.
In the MiaSolé inline sputtering process, the last stage of the absorber growth converts the absorber film to a Cu-poor composition via sputtering from targets with reduced Cu content. We demonstrate that lower surface Cu content is required for high efficiency in the presence of potassium. Furthermore, the degree of potassium accumulation at the junction appears to increase linearly with decreasing Cu/(Ga+In) ratio in that region and correlates with increased cell efficiency.
Flexible modules made from the optimized baseline (Na-only) and K-doped (Na+K) cells yield comparable light-soaking (24 hours under 1 sun) and accelerated damp-heat (85^oC/85%RH at 1000hrs) performance suggesting no adverse impact of introduction of K on module reliability.

In recent years, the quality of cadmium telluride based solar cells has increased with efficiencies exceeding 22%. This high efficiency can be attributed to the improved bulk quality of the CdTe bulk. As with silicon-based solar cells, an improved bulk quality warrants the development of a passivating and carrier-selective contact structure that is capable of extracting holes and repelling electrons or vice-versa. Such a structure has already been well-developed for silicon based solar cells through the implementation of an amorphous silicon (a-Si:H) based contact structure and is the basis of the highest efficiency silicon solar cell to date. In the CdTe world, a similar contact based on p-type a-Si:H on monocrystalline CdTe produced an open-circuit voltage of greater than 1 V indicating the potential of a-Si:H(p) to act as a hole-selective contact. Ultimately, the fill factor of this high V_oc device was fairly low at 65.5% – one of the major requirements for a passivating and carrier-selective contact – indicating that the contact stack may not be majority-carrier conductive. Overall, the has been a marked shift to developing polycrystalline CdTe films as the absorber layer in devices. In this work we aim to answer whether a-Si:H(p) (in conjuction with other thin film materials) can play the role of a hole-selective contact to polycrystalline CdTe absorber layers. To accomplish this task we vary the properties of the contact stack and find that the photoluminescence of the device dramatically increases after the addition of Al₂O₃ and a-Si:H(p) layers indicating an excellent passivation of the surface. Additionally, we find a V_oc of 779 mV indicating the hole selectivity of the layer, but the fill factor of these devices is relatively low with a value of 65.0% indicating that the a-Si:H(p) contact stack is not currently majority-carrier conductive. By manipulating the interface and bulk properties of our a-Si:H(p)based stack we show that a-Si:H(p) can indeed act as a carrier-selective contact to solar cells based on CdTe and its alloys.

The optoelectronic properties of CdIn₂S₄ spinel single crystal have been widely investigated until the end of the 1980s. Interestingly, these studies report the presence of defect levels which location in the band gap strongly depends on cationic In-Cd exchanges in the tetrahedral sites. Since a compound derived from this phase is suspected to be formed at the interface between KF-treated Cu(In,Ga)Se₂ and (CBD)CdS, re-investigating this material as thin film makes sense and is the purpose of the present contribution.
Thin films of Cd_1-xIn_2+2x/3S₄ were grown through the co-evaporation of indium, sulfur and CdS. The fluxes of indium and CdS were adjusted so that films of the following compositions could be achieved x=0 (CdIn₂S₄), x=0.6 or x=1 (In₂S₃). Those 325±5 nm-thick layers were deposited on glass substrates heated at 400°C. The structural properties of these thin films were investigated by X-Ray diffraction; they all crystallize in the spinel structure, but the cationic disorder could hardly be determined. All of these films were also characterized optically; the evolution of their absorption coefficient with incident energy is strongly dependent on x. Their electrical characteristics were investigated through Hall-effect measurements, temperature dependence of conductivity, and photoconductivity. In addition, the formation energies of point defects were theoretically evaluated by DFT. These studies all converge toward the conclusion that the optoelectronic properties of Cd_1-xIn_2+2x/3S₄ strongly depend on x (which is likely to imply cationic disorder), but most probably also on another parameter difficult to control, that is sulfur deficiency. To explore this latter parameter, all kinds of films (x=0, 0.6 or 1) were annealed for 72h at 300°C in an evacuated sealed tube with or without additional sulfur pellets. Thereby, the parameter δ (denoting the amount of sulfur vacancies) could be qualitatively investigated. The films obtained, Cd_1-xIn_2+2x/3S_4-δ, were also structurally, optically and electrically characterized. The conclusion of these investigations is that there exists a complex correlation between Cd/In ratio and the amount of S vacancies, strongly influencing defects level distribution within the bang gap of these thiospinel.
The structural and optoelectronic characterizations of these thin films lead us to conclude that the electronic nature and distribution of the defect levels is dependent on both x and δ. This makes the electronic properties of these materials diversely wavelength dependent. Such a conclusion is likely to provide new bricks to the understanding of CIGSe/CdS hetero-junction photosensitivity.

The most commonly applied buffer layer in photovoltaic devices employing Cu(In,Ga)Se₂ (CIGS) as active absorbing material, especially in highly efficient devices, is CdS. The parasitic optical absorption of CdS (E_g = 2.4 eV) results in a reduced photocurrent density. A simple reduction of the CdS layer thickness, however, leads to a reduced device performance due to a loss in open-circuit voltage (V_OC) attributed mainly to insufficient coverage on the rough surfaces and possible sputter damage of the CIGS absorber surface during the deposition of the window layer. In this contribution an alternative window layer is presented replacing the sputtered ZnO. Highly resistive and transparent Zn_xTi_yO is deposited by atomic layer deposition (ALD) onto the substrate/Mo/CIGS/CdS stack. The finished devices are then characterized by current-voltage and external quantum efficiency (EQE) measurements. ALD is chosen for this purpose since for thin and resistive oxide layers a conformal coverage of the rough surface and a precise thickness control are paramount. Further, ALD allows for a compositional grading of Zn_xTi_yO at a nanometer scale. It is found that with ALD-Zn_xTi_yO instead of sputtered ZnO the CdS thickness can be reduced without adversely affecting the Voc (~0.7 V) and therefore a gain in current density (~0.8 mA cm^-2) from a reduced parasitic absorption of both CdS and ZnO is achieved. Temperature dependent I-V measurements and its correlation with the FF support the benefit of a compositional grading of Zn_xTi_yO.

A series of efficiency improvements of Cu(In,Ga)Se₂ (CIGSe) thin-film solar cells have been achieved over the last 7 years by the development of alkali-fluoride post-deposition treatments (PDT) [1]. These processes modify the optoelectronic properties of CIGSe providing an improved surface for subsequent buffer growth by chemical bath deposition (CBD) [2,3] and favoring a reduction of the buffer thickness, as compared with non-PDT interfaces [4].
We present a Kelvin probe force microscopy (KPFM) study of the influence of different alkali-fluoride PDTs on the CIGSe absorber surface properties and the interface formation to the CBD buffer layers. The surface potential variation at more than 200 grain boundaries was measured, allowing for a meaningful statistical analysis. A strong difference for different alkali elements (K, Rb, and Cs) is found, which shows a clear correlation to the open-circuit voltage of reference solar cell devices. For the KF-PDT samples strong variations in the potential variation at the grain boundaries are observed, exhibiting large hole and electron barriers, whereas for RbF and CsF we only observe small electron barriers and neutral grain boundaries, suggesting an effective passivation effect.
For the interface formation between co-evaporated CIGSe subjected to a RbF-PDT with the solution-grown buffer layers CdS and Zn(O,S) we studied the evolution of the work function with increasing buffer layer thickness. Information about effective charge carrier separation at the forming interface was obtained from positive surface photovoltage (SPV) measurements. Surprisingly, for the Zn(O,S) buffer layer a negative SPV was found, which could be explained with the help of simulations by the presence of acceptor-type defects at the surface of Zn(O,S).
Our KPFM results provide valuable insights for the further improvements of CIGSe solar cell efficiencies.
[1] P. Jackson et al., Phys. Status Solidi RRL 9, 28 (2015); P. Jackson et al., Phys. Status Solidi RRL 10, 583 (2016).
[2] A. Laemmle et al. Phys. Status Solidi RRL 7, 631 (2013).
[3] F. Pianezzi et al., Phys. Chem. Chem. Phys. 16, 8843 (2014).
[4] A. Chirila et al., Nature Materials 12, 1107 (2013).

Currently, the Cu(In,Ga)Se₂ (CIGSe) solar-cell devices exhibiting highest conversion efficiencies of close to 23% have been fabricated using polycrystalline absorber layers which contained alkali metals already during growth or into which alkali metals were introduced via a postdeposition treatment (see, e.g., Ref. 1). The beneficial effects of alkali metals on the materials and device properties of CIGSe solar cells are diverse, and it is convenient to divide them into those affecting the bulk properties (such as the net doping density of the CIGSe layer), as well as into those acting at the interfaces to the n-type buffer layer / back contact and at line and planar defects. In various experiments (see, e.g., Refs. 2 and 3), the presence of alkali metals at (random) grain boundaries (GBs) in CIGSe thin films was confirmed. This presence often leads to the premature assumption that alkali metals are responsible for a passivating effect at GBs.
The present work will discuss systematically the impact of various alkali metals, i.e., Na, K, and Rb, on the electrical properties of GBs, i.e., on the barrier heights and on the recombination velocities. Concerning the barrier heights, it can be understood by using the equations provided by Seto [4] that introducing Na into polycrystalline CIGSe increases the barrier heights at the GBs via increased net doping density. Indeed, this effect was verified by means of scanning probe microscopy analysis. The presence of K and Rb can not be expected to have an impact on the barrier height.
As for the recombination velocity, the values extracted from electron-beam-induced current and cathodoluminescence measurements on devices with Na-free, Na-containing, and Na/Rb-containing CIGSe absorbers exhibited for most of the GBs the same order of magnitude, 10³ cm/s, regardless of whether the conversion efficiency of the solar cell was about 10%, 15% or about 22%. Thus, there is not any indication for a substantial passivation effect of any alkali metal at GBs in polycrystalline CIGSe thin films.

[1] P. Jackson, R. Wuerz, D. Hariskos, E. Lotter, W. Witte, M. Powalla, Effects of heavy alkali elements in Cu(In,Ga)Se2 solar cells with efficiencies up to 22.6%, Phys. Stat. Solidi (RRL) 10 (2016) 583-586.
[2] O. Cojocaru-Mirédin, T. Schwarz, D. Abou-Ras, Assessment of elemental distributions at line and planar defects in Cu(In,Ga)Se₂ thin films by atom probe tomography, Scripta Mater. 148 (2017) 106-114.
[3] P. Schöppe, S. Schönherr, R. Wuerz, W. Wisniewski, G. Martínez-Criado, M. Ritzer, K. Ritter, C. Ronning, C.S. Schnohr, Rubidium segregation at random grain boundaries in Cu(In,Ga)Se₂ absorbers, Nano Energy 42 (2017) 307-313.
[4] J.Y.W. Seto, The electrical properties of polycrystalline silicon films, J. Appl. Phys. 46 (1975) 5247.

Polycrystalline Cu(In,Ga)Se₂ (CIGS) appears nowadays as one of the most promising candidates for the fabrication of high efficiency thin film solar cells. However, the current record efficiency (22,9 % [1]) is still lower than the one of the single crystalline GaAs thin film solar cells (28 % [1]). The discussion about whether this lower performance is attributed to the nature of the surface or grain boundaries (GBs) electronic properties is still ongoing [2].

In this work, we investigated the surface and grain boundary properties of CuInSe₂ (CISe) with scanning tunneling microscopy (STM) and spectroscopy carried out under ultra-high vacuum conditions. This technique enabled us to measure the local density of states (LDOS) with nanometer precision. In particular, we studied the effect of potassium cyanide (KCN) etching, the influence of heating in vacuum and pre-electrolyte treatment with Cd²⁺ and NH₄OH. Our STM results are supplemented with X-ray photoelectron spectroscopy (XPS) and photoluminescence (PL) measurements. Scanning tunneling spectroscopy (STS) maps measured on KCN etched CISe (KCISe) samples are mostly in agreement with reference [3]. We measured signatures of a defective surface with a high density of surface states, which can be improved, to a certain extend via an UHV annealing to 280°C [4]. However, in CISe solar cells the heterojunction is usually made with CdS and consequently we studied the absorbers after a Cd-pre-electrolyte treatment (Cd²⁺CISe). This enabled us to expose the surface to a high amount of Cd²⁺ ions without actually growing a CdS layer, which would render an investigation of the CISe surface with STM impossible.

Interestingly, the surface electronic properties are changing substantially after the Cd²⁺ treatment, and we observed (a) less inhomogenties and (b) a reduction of the surface defect density, which manifested itself by a shift in the Fermi-level at the surface. Compared to the KCISe samples we do not observe distinct features at the grain boundaries anymore.

XPS measurements gave important insights into the changes of the surface composition after the various treatments. We observed an Indium depletion on the (Cd²⁺CISe) compared to the KCISe samples, which suggests that the Cd ions are substituting Indium at the surface. We compared the surface compositions to the compositions after mild sputtering with Argon ions. We found that in both cases, the surface is Cu-depleted (in agreement with most reports in literature) and most of the Cd was confined to the first few nanometers.

The PL measurements on both types of samples (KCIS vs Cd²⁺CIS) showed no considerable change on the bulk electronic properties. A detailed analysis of these observations will be presented and discussed.

Reference
[1] M. A. Green et al., Prog. Photovolt: Res. Appl. 26, 2018, 427
[2] S. Siebentritt, Sol. Energy Mater Sol., 95, 2011, 1471
[3] H. Mönig et al., Phys. Rev. Lett., 105, 2010, 116802-1
[4] H. Mönig et al., Adv. Mater. Interfaces, 1, 2014, 130004

High-efficiency Cu(In,Ga)Se₂ (CIGS) thin-film solar cells are achieved by depositing CIGS absorber layers using a three-stage co-evaporation process. During the second stage of this process, Cu-Se deposition on a (In,Ga)₂Se₃ precursor during heating leads to the formation of a CIGS layer that passes from a Cu-poor to a Cu-rich composition. It has been reported that the interdiffusion of Cu and In/Ga between CIGS and Cu-Se layers is essential for the grain growth of CIGS during this Cu-poor to Cu-rich transition^1-5. However, little has been known about how the atomic structures evolve during this process.

In the present study, we use the CuInSe₂ (CIS) system, as it is simpler than CIGS for an initial study. Cu-Se was deposited on top of CIS, and the grain growth was directly monitored during in-situ heating in a scanning electron transmission microscope (STEM). The compositional changes were revealed simultaneously with electron energy-loss spectroscopy (EELS). The corresponding time-series STEM images and elemental maps will be presented in as videos in the conference. The results show that planar defects play an important role in grain growth. The grains with high-density planar defects tend to be consumed by the grains without planar defects, and this does not occur when there is no extra Cu-Se layer deposited on CIS. Simultaneously acquired STEM and EELS spectrum images reveal that Cu-rich grain boundaries (GBs) between highly defected and non-defected CIS grains migrate towards the highly defected CIS grains during heating. Further EELS elemental mapping reveals Cu enrichment and In depletion at most GBs within regions having widths of more than 10 nm. Atomically resolved STEM images show a gradual phase change from a chalcopyrite CIS structure at CIS grain interiors towards anantifluorite Cu_2-xSe structure at GBs, which is also evidenced by atomic-resolution EELS. In this gradual phase change, the Se sublattice stays the same, while In and Cu interdiffuse gradually without any dislocations between the Cu_2-xSe and CIS phases. Moreover, In atoms are found at interstitial sites in Cu_2-xSe near GBs. Density functional theory (DFT) calculations show that the diffusion barrier of In atoms from interstitial to interstitial sites in Cu_2-xSe is very low. These results not only show that Cu diffusion at GBs assists the recrystallization of CIS, but also reveal the mechanism beneath it, which will be discussed at the conference.

Acknowledgment: Research funded in part by the Helmholtz Virtual Institute “Microstructure Control for Thin- Film Solar Cells”, VH-VI–520 project. CL acknowledges the support from Max Planck Society.

References:

1. R. Klenk et al., A model for the successful growth of polycrystalline films of CuInSe₂ by multisource physical vacuum evaporation, Adv. Mater. 5, 114-119 (1993)
2. N. Barreau et al., Recrystallization of CIGSe layers grown by three-step processes: a model based on grain boundary migration, Acta Mater. 58, 5572-5577 (2010)
3. R. Mainz et al., Sudden stress relaxation in compound semiconductor thin films triggered by secondary phase segregation, Phys. Rev. B 92, 155310 (2015)
4. H. Rodriguez-Alvarez et al., Recrystallization of Cu(In,Ga)Se₂ thin films studied by X-ray diffraction, Acta Mater. 61, 4347-4353 (2013)
5. H. Stange et al., Diffusion-induced grain boundary migration as mechanism for grain growth and defect annihilation in chalcopyrite thin films, Acta Mater. 111, 377–384 (2016)

The nanoscale optoelectronic properties of materials can be especially important for polycrystalline photovoltaics including many sensor and solar cell designs. For thin film solar cells such as CdTe, the open-circuit voltage and short circuit current are especially critical performance indicators, often varying between and even within individual grains. A new method for directly mapping the open-circuit voltage leverages photo-conducting AFM, along with an additional proportional-integral-derivative feedback loop configured to maintain open circuit conditions while scanning. Alternating with short circuit current mapping efficiently provides complementary insight into the highly microstructurally sensitive local and ensemble photovoltaic performance. Furthermore, direct open circuit voltage mapping is compatible with tomographic AFM, which additionally leverages nanoscale serial sectioning. The 2-Dimensional and 3-Dimensional results for CdTe solar cells during in situ illumination reveal local to mesoscale contributions to PV performance based on order of magnitude variations in photovoltaic properties with distinct grains, at grain boundaries, and for sub-granular planar defects.

The widely reported reaction route for kesterite Cu₂ZnSnSe₄ (CZTSe) formation involves a reaction route containing binary selenides, i.e. Cu₂Se + SnSe + ZnSe + 1/2Se₂ -----> Cu₂ZnSnSe₄. The high volatility of SnSe_y phase at commonly used annealing temperatures (500 – 550 °C) makes this reaction pathway less favorable in terms of achieving a resilient control of the composition and quality of the grown material. In the present work, we report a strategy to control the pathway of kesterite formation to prevent the formation and subsequent losses of binary Sn-chalcogenides. In our view, any kesterite formation reaction which starts from metallic precursors consists of two parts, i.e. metal alloying regime (T < T_Se-onset) and chalcogen reaction regime (T > T_Se-onset), where the temperature of selenization onset (T_Se-onset) can vary between different processes. We have investigated the time and temperature dependent formation of metal alloys and chalcogenide phases in the fabrication procedure, which starts from sputtered precursors and leads to cell efficiencies of > 11%. The phase formation and the evolution of the elemental composition were tracked by means of in-situ energy-dispersive X-ray diffraction (EDXRD). After interrupting the process at various temperatures and durations, depth dependent Raman measurements were performed. The results indicate that the formation and preservation of Cu-Sn alloys in combination with the suppressed formation of Cu-Zn phases before selenization onset plays a key role in avoiding the generation of volatile Sn-chalcogenides. This strategy strongly hinders the SnSe_y formation, therefore helps in avoiding any Sn loss and allows for maintaining constant Sn amount in the layer throughout the annealing process. We further demonstrate, that by adding additional Sn into the reaction chamber it is possible to change the composition of the absorber during the annealing process, which happens due to the formation of SnSe_y vapor, which can react with the sample. The results show that this fabrication procedure is capable of growing CZTSe in the Cu-rich regime and turning the overall composition towards the Cu-poor regime during the selenization process, similar to the common phase transition in CIGSe processing. This adds another degree of freedom to the further optimization of CZTSe solar cells. We will show suggestions for the most promising reaction pathway in the phase diagram and discuss the implication for future process development. First results from the modified fabrication series following these suggestions will be shown, indicating the impact of the nominal amount of Cu-excess during the selenization step on the obtained solar cell results.

In recent years, point defect engineering has proven beneficial for many types of thin-film solar cells. Particularly, defect control has been adopted for kesterite absorbers resulting in an increase in open-circuit voltage (V_OC). Due to its quaternary nature with three cations of similar coordination environment, the material is very sensitive to synthesis conditions, and the energy barrier to disorder is low. The most common point defects present in Cu₂ZnSnS₄ (CZTS) are Cu_Zn and V_Cu, which also account for the p-type doping of the material. However, the number of possible defects and defect complexes is substantial, and most are a cause for undesired recombination inside the absorber layer. One commonly observed phenomenon is Cu/Zn-disorder, which originates from the similar ionic radii of Cu and Zn, can result in band tailing and a decrease in V_OC.
By replacing Cu with an element of a larger ionic radius, such as Ag, the degree of Cu/Zn-disorder is decreased because Ag is less likely to exchange positions with Zn. In other words, the formation energy of Ag_Zn antisite defects is high and thus renders the incident unlikely. Until now, several new cations have been studied for different deposition techniques, however, only partly for the solution processing route we apply here.
We fabricate CZTS through spin-coating a precursor ink consisting of DMSO, thiourea, and chloride salts in consecutive layers until the desired thickness is achieved. A power conversion efficiency of 4% has been achieved for an alkali-free absorber, which is on the order of the non-doped record cell by Tiwari et al., [1]. The V_OC was 580 mV, which lags behind the record pure-sulfide CZTS device of 683 mV, [2], and suggests that there is room for improvement.
In this work, we screen the effect of incorporating various types of new cations into the kesterite, including Ag and Ba by direct addition into the precursor ink. A comparison will be given in terms of the structure, morphology, and opto-electronic properties.

[1] D. Tiwari et al., Chem. Mater. 2016, 28, 14, 4991-4997.
[2] C. Yan et al., ACS Energy Lett. 2017, 2, 4, 930-936.

The tunability of the band-gap in kesterite absorbers by choosing different S/Se ratios offers the possibility to create a band-gap gradient throughout a single absorber layer of Cu₂ZnSn(S_xSe_1-x)₄. This type of band-gap gradient is deeply investigated for CIGS technologies where it is commonly used for a significant enhancement of the device performance, by allowing for combination of optimized optical properties and reduction of charge carrier recombination. In this work, we present a novel method for introducing a band-gap grading via sulfurization of pure selenide Cu₂ZnSnSe₄ thin films by using hydrogen sulfide (H₂S). The impact of various process parameters of H₂S assisted sulfurization such as pressure, temperature, and duration on the properties of the resulting Cu₂ZnSn(S_xSe_1-x)₄ absorbers are discussed. In particular, the elemental S/Se gradient throughout the sample after the process, the total content of sulphur as well as its impact on absorber band-gap and quality are investigated. Raman depth profiling confirms the formation of a grading with a relatively high sulphur concentration at the front surface which decreases towards the back side of the absorber, resulting in a front graded Cu₂ZnSn(S_xSe_1-x)₄ with a surface band-gap of E_g ^front ≈ 1.10 eV and a difference between front and back side of ΔE_g > 100 meV. The results indicate that selenium depletion from the existing material and S-Se exchange are taking place simultaneously during the process, affecting also material properties other than the grading profile. Inaccurate selection of process parameters outside a stable range can lead to decomposition of the Cu₂ZnSnSe₄ layer and the formation of ZnS secondary phase at the absorber surface. Finally, the effects and advantages of the presented H₂S assisted process are further discussed, including the strong kinetic nature of this process which appears to allow for a fast incorporation of a sulphur gradient. Results on solar cell devices will be presented.

For thin film solar cells to compete with the current market dominate silicon ones, reducing production cost is the key. Large area solution processing (to replace vacuum based deposition) to reduce fabrication cost and using earth abundant elements to lower materials cost are both effective ways to achieve this goal. Here, we report over 13% efficient copper indium gallium sulfoselenide (CIGSSe) and copper zinc tin sulfoselenide (CZTSSe) solar cells respectively fabricated from DMF and DMSO molecular precursor inks. We have first used DMF as single solvent to prepare precursor solution and fabricated 10.3% efficient copper indium sulfoselenide (CISSe) solar cells without any etching.¹ We have now systematically investigated the effect of the Cu/In ratio in the precursor solution on the CISSe device performance. We found that solar cell performance increases from Cu-poor (0.85) to slightly Cu-rich and then decreases for further Cu-rich condition with a peak power conversion efficiency (PCE) of 12.20% achieved from the Cu/In ratio of 1.05. Further, by gallium alloying, CIGSSe solar cell with a PCE of 13.6% has been fabricated under similar Cu-rich (Cu/(In+Ga)=1.05, Ga/(In+Ga)=0.2) condition. Our results for the first time demonstrate highly efficient CIS/CIGS solar cells can be achieved from absorbers grown under Cu-rich condition. Characterization of the absorber materials and solar cell devices are investigated to understand the mechanism behind the phenomenon observed. The main issue for CZTS solar cell further development is the large open-circuit voltage deficit (V_oc-def) which could be due to large tail states, Cu-Zn antisite order-disorder, formation of deep defects and interface recombination. DMSO precursor solution has been demonstrated to be a promising approach for fabricating high performance CZTSSe solar cells.^2,3 Here, we have explored different approaches to understand the V_oc-def of DMSO based CZTSSe solar cells. First, we found device V_oc (as well as efficiency) is strongly affected by the precursor tin oxidation state (Sn²⁺/Sn⁴⁺) in the precursor solution. A PCE of 13.2% has been achieved from Sn⁴⁺ solution with a V_oc-def of only 0.570 V, much lower than the current world record CZTSSe device (0.617 V), whereas the efficiency obtained from Sn²⁺ solution was only 7% with V_oc-def higher than 0.600 V. The reason for the huge difference has been investigation by solution chemistry, film morphology, material property and device characterizations. Second, we have found that Ag or Ge alloying have positive effect on the V_oc-def of DMSO solution processed CZTSSe solar cells. Our results shed new lights on suppressing the V_oc-def and improving the efficiency of CZTSSe solar cells.

References:
1. J. Jiang; S. Yu; Y. Gong; W. Yan; R. Zhang; S. Liu; W. Huang; and H. Xin; Solar RRL, 2018, 1800044.
2. H. Xin, J. K. Katahara, I. L. Braly, H. W. Hillhouse, Adv. Energy Mater. 2014, 4. 201301823
3. H. Xin, S. M. Vorpahl, A. D. Collord, I. L. Braly, A. R. Uhl, B. W. Krueger, D. S. Ginger, H. W. Hillhouse, Phys. Chem. Chem. Phys. 2015, 17, 23859.

Kesterite Cu₂ZnSnSe₄ (CZTSe) based materials belong to I₂-II-IV-VI₄ quaternary compound semiconductor and the composition control are relatively complicated. However, the composition control is very important to obtain high efficiency solar cells. Actually, a specific composition of Cu-poor and Zn-rich composition have been used for high efficiency solar cells, while the off-stoichiometric composition results in formation of many defects. In order to overcome the problem, the several methods are introduced to relieve the off-stoichiometric effect, we demonstrated high conversion efficiency by Ge and Na incorporation [1, 2]. Especially, 12.3% conversion efficiency was obtained using Ge incorporated Gu₂Zn(Sn,Ge)Se₄ alloy [2].
As mentioned above, the composition control is important however, they are mainly on cation elements, and selenium control has not been so seriously considered compared to these composition controls. One of the reasons is that the selenium control has a small effect on the composition, however selenium related defects are considered to play an important role on electrical properties. In this study, we investigate the impact on the selenium control at growth and annealing, and demonstrate high efficiency of CZTSe solar cells through the selenium control.
To obtain high quality CZTSe thin films, many researchers use two step growth method; first one is precursor deposition or low temperature compound growth, and second one is annealing. In this study, CZTSe thin films were grown by a coevaporation method at 340°C using molecular beam epitaxy equipment, and the selenium supply was controlled with the selenium cell temperature. And the annealing was also conducted controlling selenium heating temperature by a quartz tube furnace with three heating zones. Although closed type annealing, at which sample is sealed in a box, is widely used, however, we used an “open type” annealing system, which has greater flexibility to control the annealing conditions, environmental species and its amount with independence from the treatment temperature, treatment pressure, carrier gas flow, and so on.
We investigated the effects of selenium supply both at the growth and annealing, and found that the efficiency gradually decreased for the higher selenium supply at the growth, on the contrary, the efficiency drastically decreased for the lower selenium supply. It was also found that the dependence of selenium supply at the annealing treatment was not so strong compared to that at the growth. The optimal efficiency was obtained with controlling annealing selenium and fixed growth selenium, and conversion efficiency (9.4% -> 10.2% -> 11.6%) improved mainly with Voc (0.401 V -> 0.406 V -> 0.419 V) and FF (0.592 -> 0.647 -> 0.687) in spite of almost constant Jsc (.39.6 mA/cm² -> 38.7 mA/cm² -> 40.2 mA/cm²). The carrier concentration was almost identical with the conditions and the improvement is considered to be due to reduction of selenium related deep defects. The conversion efficiency of best performing cell 12.0% with Voc=0.418 V, Jsc= 42.1 mA/cm², and FF=0.683.

References
[1] S. Kim, K.M. Kim, H. Tampo, H. Shibata, and S. Niki, Appl. Phys. Express 9 102301 (2016).
[2] H. Tampo, K.M. Kim, S. Kim, H. Shibata, and S. Niki, J. Appl. Phys, 122 (2017) 023106.

Record efficiencies for Cu(In,Ga)Se₂ (CIGSe) thin-film solar cells have been reached using KF [1], RbF [2], and CsF [3,4] post-deposition treatments (PDT). The underlying mechanisms for the observed performance boost have, however, not yet been completely understood. In this contribution, we present the results of hard and soft x-ray photoelectron spectroscopy studies which non-destructively probe the depth-dependent chemical and electronic properties of low-temperature processed CIGSe absorbers as a function of alkali PDT deposition rate. We find a link between the degree of Cu depletion at the absorber surface and increasing alkali content, in agreement with Ref. [5]. Depth-dependent valence band photoemission profiles indicate a shift of the valence band maximum (VBM) at the CIGSe surface away from the Fermi level with higher alkali content.
It has been reported [6] that alkali elements penetrate into the absorber, especially at grain boundaries as a result of PDT. To investigate potential effects at the back contact, an absorber subjected to a combined NaF/RbF PDT was lifted off the Mo substrate; both sides were studied using hard x-ray photoemission. We detect significant quantities of Rb at the back surface of the absorber, and observe that Rb content is anticorrelated with Na on both front and back CIGSe surfaces. The Cu and Ga depletion resulting from NaF/RbF PDT is seen only at the front side of the CIGSe surface, not at the backside. Furthermore, the spectral shape of the Se and In derived shallow core level lines differs between front and back surfaces, indicating a different chemical structure. Se was found on the Mo substrate after liftoff as a result of MoSe₂ formation at the CIGSe/Mo interface.
In our presentation, we will ultimately aim to link our findings to the reported efficiency increase of respective CIGSe solar cells.

[1] Chirila et al., Nature Materials, 12, 1107-1111 (2013)
[2] Jackson et al., Phys. Status Solidi RRL 10, No 8, 583–586 (2016)
[3] Solar frontier press release (http://www.solar-frontier.com/eng/news/2017/1220_press.html, 2018-01-12)
[4] PVSEC 27, session 2ThO4.3, talk by Jyh-Lih Wu (Solar Frontier)
[5] Avancini et al., Chem. Mater. 29 (22), 9695–9704 (2017)
[6] Schöppe et al., Nano Energy, Vol 42, 307-313 (2017)

On the way to achieving high efficiencies for CuIn_0.7Ga_0.3Se₂ (CIGS) thin film solar cells, alkali doping has become a necessary step. However, it is expected that a migration of the various alkali atoms over the lifetime of the cell will lead to efficiency losses [1].
Logically, ensuring the reliability of a solar cell material is a paramount step towards the commercialization of a product. Indeed, it is necessary for companies to able to guarantee the efficiencies of the “as-sold” panels over periods of time that expand well over 20 or even 25 years.
Experiments studying the reliability of the CIGS solar cells and their different components have already been performed in the past [1-4], nevertheless, the exact effects of alkali, and more generally, the precise reasons for, and effect of, the degradation, are still widely unknown.
In order to evaluate the viability of some of the most common techniques used to produce highly efficient thin-film solar cells, ultra-thin coevaporated CIGS solar cell absorber material is produced and doped with various alkali atoms (Na, K, ...).
The absorber material is deposited in ultra-thin (400-500nm) layers on soda-lime glass (SLG) equipped with a Si(O,N) diffusion barrier. By reducing the amount of bulk material, the aim is to reduce the concentration of defects in the material and limit their effects as much as possible. The barrier prevents the diffusion of alkali atoms from the SLG into the CIGS and allows for a better control of the concentration of alkali atoms in the layer. Additionally, it allows for a better isolation of the effects of the different alkalis by preventing an uncontrolled mix of alkali from being present in the absorber material.
All alkalis are deposited in their fluoride form (NaF, KF, etc.) either by evaporation, prior to CIGS deposition, or by a post deposition treatment.
We submitted the produced cells to highly degrading conditions in a damp heat environment. This aggressive setting is expected to simulate standard outdoor condition equivalent to 20 years in an experimentally reasonable timespan of a 1000 hours.
We monitored the effects of the degradation by regular current-voltage (IV) measurements all along the 1000h experiment. These measurements showed a clear, and alkali concentration dependent, degradation of the solar cells as time increases.
Using Atom Probe Tomography (APT) on degraded as well as on undegraded (reference) cells, it was possible to show that in the absence of a potential, the alkali atom migration is minimal, and that K can be mostly found in the grain boundary region of the absorber material. Given the lack of the expected alkali migration, the main reason for the degradation of the cells seems to be the presence of water that seeps into the grain boundaries of the solar cell material, which could also be observed using APT.
The possibility to reverse that degradation mechanisms involving water inclusion is currently under investigation.


References:

[1] Theelen et al., Prog. Photovol, 23 (2015), p. 537-545.
[2] Theelen et al., Proc. 42^nd IEEE PVSC, p. 1-6.
[3] Daume et al., Proc. 28^th EUPVSEC, p. 2192-2198.
[4] M. Theelen, Degradation of CIGS Solar Cells, PhD Thesis, 2015, available: http://www.cigsdegradation.com/mobile/index.html

Cu(In,Ga)Se₂-based (CIGS) solar cells with an ultra-thin absorber layer (<500 nm) allow manufacturing cost reduction but exhibit low efficiencies due to poor light absorption and back contact recombination. To achieve high-efficiency ultra-thin CIGS solar cells, it is necessary to develop reflective back contacts that enhance CIGS absorption.

In this work we present the first ultra-thin CIGS solar cells prepared directly on top of an ohmic and reflective back contact. The conventional Mo back contact is replaced by a flat multi-layer stack including a silver mirror and a top ITO layer. The thermal stability as well as the high reflectivity and conductivity of these reflectors have been confirmed by TEM/EDX analysis, optical and electrical measurements.

We demonstrate the direct fabrication of complete ultra-thin solar cells (500-nm-thick CIGS) on top of reflective back contacts with a 13.4%-efficiency. CIGS thin films were co-evaporated in a three-stage process. Decreasing CIGS deposition temperature from 550°C to 500°C resulted in increased cell efficiencies both for reflective and Mo back contacts. Using GD-OES and TEM/EDX investigations, we show that the lower temperature limits the formation of a detrimental gallium oxide layer at CIGS/ITO interface, and leads to a higher V_OC thanks to the steeper GGI grading in ultra-thin CIGS layers. Furthermore, CIGS solar cells prepared at 500°C on a back-reflector result in a J_SC gain of 2.3 mA/cm² as compared to a Mo back contact (from 26.2 to 28.5 mA/cm²), with increased EQE in the infrared region due to enhanced back reflection. We then demonstrate that combining an additional 3-nm-thick Al₂O₃ layer on top of back reflectors with a NaF precursor layer can also limit Ga₂O₃ formation at CIGS/ITO interface and contribute to CIGS rear passivation. Finally, further improvements are expected with an additional ARC and a patterning of the reflective back contact.

Numerical electromagnetic simulations of complete solar cells have been performed for different CIGS thicknesses and various mirror materials. Simulated CIGS absorptions show good agreement with our experimental EQEs, and confirm that parasitic absorption in Mo can be avoided by using a reflective back contact. We also demonstrate the possibility to reach a short-circuit current of J_SC = 36.3 mA/cm² for a 150-nm-thick CIGS absorber with a silver nanostructured mirror¹. Preliminary results of nanostructured back contacts fabricated by nanoimprint lithography will be also presented.

[1] J. Goffard et al., ‘Light Trapping in Ultrathin CIGS Solar Cells with Nanostructured Back Mirrors’, IEEE Journal of Photovoltaics, vol. 7, no. 5, pp. 1433–1441, (2017)

Secondary phases in Cu(In,Ga)Se₂ based thin film solar cells were found at the backside of the absorber layer in contact to the back contact. Such buried secondary phases are difficult to detect, but might influence the solar cell performance. Therefore, it is desirable to develop a methodology to localize and investigate them in detail. The CIGS layer was fabricated in a co-evaporation process under special conditions. Localization and further investigations can be done by removing the absorber from the back contact or by cleaving the sample and investigating the cross section. The secondary phases can be observed using an electron microscope. Energy dispersive X-ray spectroscopy (EDS) measurements reveal a varying composition of the absorber elements. However, the forced detachment of the back contact from the absorber destroys the solar cell stack and might influence the appearance of the secondary phase itself. Furthermore, investigations of the solar cell stack as a whole are no longer possible. To overcome these problems, we electrically contacted a complete solar cell inside an electron microscope and used electron-beam-induced current (EBIC) to locate buried secondary phases. In EBIC images, the secondary phases show a reduction of the current density depending on the size and the morphology of the secondary phase. Hence, the reduction of the electron-beam-induced current allows the localization of secondary phases in a reliable way without destroying the solar cell. For verification and further investigations, we prepared thin cross section lamellas via focused ion beam at the localized buried phases. Several thin lamellas of the complete solar cell stack, including the underlying substrate, were investigated via EDS with a high spatial resolution of tens of nanometers revealing compositional variations.

The incorporation of Ag in Cu(In,Ga)Se₂ (CIGSe) is understood to induce certain improvements in the opto-electronic properties of the absorber, which is reflected in enhanced conversion efficiencies [1]. Ag atoms substitute for Cu forming a pentenary (Ag,Cu)(In,Ga)Se₂ (ACIGSe) alloy with a wider band gap compared to CIGSe absorber. The Ag alloying also allows to reduce the deposition temperatures due to the lower melting point of Ag, which as a result decreases the density of lattice defects in the absorber. Synthesizing such alloy on high strain K-rich substrates along with KF post deposition treatment displays higher device performance compared to similarly processed CIGSe devices without Ag [2]. The diffusion of alkali metals from the substrate in ACIGSe compared to CIGSe as well as the distribution of Ag in the absorbers are matters that can elucidate differences in the opto-electronic properties and therefore requires further attention.

Amongst various analytical techniques, atom probe tomography (APT) is an extremely promising tool to characterize the chemical distribution in three dimensions with sub-nm resolution [3]. APT is used in the study here as a main tool to examine local compositions within ACIGSe and the distribution of alkali metals within absorbers grown on different substrates; soda lime glass (Na-rich) and high strain point glass (K-rich), respectively. Na and K were found to segregate to different lattice defects in the absorber structures. Correlative measurements using transmission Kikuchi diffraction and APT were therefore carried out to investigate the crystallographic and chemical nature of grain boundaries and twin boundaries detected within the ACIGSe structures. Although ACIGSe is a promising system for thin film solar cell technology, more research is certainly required to demonstrate if the beneficial effects observed for Ag additions are worth to pay for, considering its higher cost compared to Cu.


[1] J. H. Boyle, B. E. McCandless, W. N. Shafarman, and R. W. Birkmire, “Structural and optical properties of (Ag,Cu)(In,Ga)Se₂ polycrystalline thin film alloys,” J. Appl. Phys., vol. 115, no. 22, 2014.
[2] M. Edoff et al., “High V_oc in (Cu,Ag)(In,Ga)Se₂ Solar Cells,” IEEE J. Photovoltaics, vol. 7, no. 6, pp. 1789–1794, 2017.
[3] E. A. Marquis, M. Bachhav, Y. Chen, Y. Dong, L. M. Gordon, and A. McFarland, “On the current role of atom probe tomography in materials characterization and materials science,” Curr. Opin. Solid State Mater. Sci., vol. 17, no. 5, pp. 217–223, 2013.

CuIn_xGa_1-xSe₂ (CIGS) is a promising absorber material for high-efficiency, low-cost thin-film photovoltaics, with a significant increase in efficiency in the last 3 years. To further improve the device performance, a better understanding of the absorber layer defects is required. One puzzling behavior is the effect of light soaking (LS). CIGS solar cells exhibit metastable performance under extended duration illumination. This has been proposed to be related to amphoteric behavior of (V_se+V_Cu) divacancies; under light illumination, the equilibrium distribution of the donor and acceptor configuration of these divacancies changes. However, this hasn’t been shown experimentally [1]. Here, we report on metastable effects using Raman analysis during light soaking. Commercial Ag-alloyed Cu(In,Ga)Se₂ (ACIGS) sputter-deposited samples coated with CdS, were obtained from MiaSolé HiTech. The samples were fabricated in a production-scale roll-coater on flexible stainless-steel substrates. Na and K alkali metals were incorporated from the Mo back electrode. Current-voltage analysis of the samples under the light soaking, dark heat, and combinations of both were measured at MiaSolé. Raman spectra were measured every 1-2 minutes over a total time of 200 minutes using continuous 532 nm laser illumination. The sample temperature was maintained at 12°C by a Peltier device mounted under the sample. The excitation depth at this wavelength is ~ 100nm. The laser power was set at the lowest level at which reliable data could be obtained to prevent peak shifts due to local heating. Higher laser powers produced different but variable results. The results showed time-dependent variation in the A1 (CIGS) mode at 176 cm^-1_. In addition, a broad shoulder related to the ordered defect compounds such as Cu(In,Ga)₃Se₅ was detected at ~155 cm^-1. After 60 min illumination, the intensity of the CIGS A1 mode dropped while the ordered defect structure peak grew. For relatively stable samples, the A1/ODC stabilized at higher values after 200 min exposure; however, unstable samples stabilized at lower ratios. This suggests re-ordering of Cu-Se bonds under illumination. We are planning to study these effects as a function of temperature to establish the kinetic rate parameters of the changes.

[1] S. Lany, A. Zunger, “Light- and bias-induced metastabilities in Cu(In,Ga)Se₂ based solar cells caused by the (V_Se-V_Cu) vacancy complex”, J. Appl. Phys. 100 (2006) 113725.

Zn-based materials such as ZnO, ZnS, or Zn(O,S) are being widely used for optoelectronic devices, thin film transistors, and photovoltaic devices due to their wide bandgap, chemical stability, inexpensiveness, and nontoxicity. Zn(O_1-x,S_x) films especially show very interesting changes in electronic properties when the sulfur-to-oxygen composition ratio (x) is varied. With increasing x between 0 < x < 0.5, the conduction band minimum (CBM) hardly changes and the valence band maximum (VBM) moves up, while, for x larger than 0.5, it is the CBM that moves up and the VBM remains nearly unchanged, which yields the bandgap bowing. A deposition method is an important issue to consider in the fabrication of the Zn(O,S) thin films with high quality, composition controllability, and industrial applicability. Among various methods of Zn(O,S) deposition, reactive sputtering is an attractive choice because it is manufacturing-friendly. However, a highly energetic plasma during the sputtering is likely to make undesired effects especially when forming very thin films.
In this study, the Zn(O,S) thin films were prepared using a single ZnS target and O₂/Ar mixture gas. Despite careful control of the sulfur-to-oxygen composition ratio and its gradient through the vertical direction, we found that the sulfur-to-oxygen composition ratio changed along the depth direction; the bottom layers became O-rich. Additionally, Zn(OH)₂ secondary phase was observed at the bottom side of the Zn(O,S) thin films, which induced much lower VBM. The properties of the fabricated thin films and solar devices were characterized by a transmission electron microscopy, an energy dispersive spectroscopy, an X-ray photoelectron spectroscopy, and a current-voltage measurement. In this work, we present several strategies to mitigate the unwanted compositional variations and the formation of secondary phases to achieve high quality Zn(O,S) thin films with reactive sputtering. The details of these strategies will be presented.

Kesterite Cu2ZnSnS4 (CZTS) photovoltaics have been comprehensively investigated in the past decades but are still hampered by a relatively large open circuit voltage (Voc) deficit, which is due to the non-radiative recombination within the CZTS bulk and hetero-interface region. Heterojunction interface engineering is of great importance to tackling the interface recombination problem by optimizing the band alignment between the absorber and the buffer and interface passivation. In this work, we reported a detailed characterization of the microstructure at the interface of the CZTS device with three different buffer materials using high-resolution TEM technique. It has been revealed that an ultra-thin ZnS or Zn (S, O) nanolayer can be formed between the CZTS and the buffer for the subsequent growth of ZnSnO and ZnCdS layer, while such feature has not been observed in the CdS buffer sample. Such ultra-thin ZnS or Zn (S, O) is found grown epitaxially at the CZTS surface with high crystallinity as the lattice mismatch between ZnS and the kesterite CZTS is negligibly small. The presence of this nano-structure between the buffer and CZTS absorber may benefit the device performance by reducing interface crystalline defects and thereby decrease the interface recombination while acting as a tunneling layer for current transport.

Crystal structure of (In,Ga)₂Se₃ precursor material was studied by X-ray absorption fine structure (XAFS) in order to prepare high efficiency Cu(In,Ga)Se₂ (CIGS) solar cells. On the basis of the results, preparation process of high quality CIGS thin films is discussed.
The CIGS films are mainly prepared by a 3-stage process. In the 3-stage process, the initial stage consists of deposition of an (In,Ga)₂Se₃ precursor layer. In the second stage, the precursor layer is exposed to Cu and Se fluxes to form Cu-rich CIGS. In the third stage, In, Ga, and Se are added to the Cu-rich CIGS layer in order to obtain a slightly (In,Ga)-rich CIGS film. Then, the property for the first layer of (In,Ga)₂Se₃ can affect the quality and crystal orientation of CIGS, which is finally obtained in 3-stage process. The crystal structure of In₂Se₃ is hexagonal layered defect wurtzite type structure. The crystal structure of Ga₂Se₃ is monoclinic defect sphalerite type structure. Therefore, the crystal structure of (In,Ga)₂Se₃ is predicted to be affected by a Ga content. In this work, we synthesized (In,Ga)₂Se₃, and clarified their crystal structures.
(In,Ga)₂Se₃ was prepared by a mechanochemical method. (In_1-xGa_x)₂Se₃ was characterized by X-ray diffraction (XRD) and XAFS. The XRD patterns of (In_1-xGa_x)₂Se₃ were gradually changed with Ga content. The Rietveld analysis for the XRD patterns clarified the crystal structure of (In_1-xGa_x)₂Se₃. (In_1-xGa_x)₂Se₃ with 0 ≤ x ≤ 0.5 had a layered defect wurtzite type structure. (In_0.4Ga_0.6)₂Se₃ and (In_0.3Ga_0.7)₂Se₃ had an ideal defect wurtzite type structure. The materials with much higher Ga content had mixed phase of defect wurtzite type structure and defect sphalerite type structure. When Ga content was higher than 50 mol%, lattice parameters drastically approached those of ideal defect wurtzite type structure.
Se K-edge X-ray absorption near edge structure (XANES) spectra of (In_1-xGa_x)₂Se₃ also changed with depending on Ga content. The XANES spectra of (In_1-xGa_x)₂Se₃ were fitted by three gaussian functions (peak 1, peak 2 and peak 3) and an arctangent function. The energy position of peak 1 was linearly shifted to higher energy with increasing Ga content. The area of peak1 and peak 2 changed at x = 0.5. The area of peak 1 increased and that of peak 2 decreased. Peak 1 is reported to be attributed to In(s, p) or Ga(s, p)-Se(p) anti-bonding orbital. The anti-bonding orbital was main component of CBM. Therefore, these results indicate that CBM of (In_1-xGa_x)₂Se₃ varied due to the improvement of transition probability to In(s, p) or Ga(s, p)-Se(p) anti-bonding orbital. The improvement of transition probability also showed change of the crystal structure of (In_1-xGa_x)₂Se₃. Since an electron was easy to flow into the anti-bonding orbital due to the change of the crystal structure, the bond length varied and lattice parameter was changed.

Barium disilicide (BaSi₂) shows great promise as a new material for thin-film solar cells. It has a suitable bandgap of 1.3 eV, large optical absorption coefficients exceeding those of CIGS,^[1] and a large minority-carrier diffusion length of about 10 μm.^[2] We achieved a conversion efficiency of 9.9% in p-BaSi₂/n-Si heterojunction solar cells.^[3] We next target BaSi₂ pn homojunction solar cells. To this end, high-quality BaSi₂ light absorbers with low defect density (N_T) are inevitable. In this work, we investigated defect levels and their values of N_T in the BaSi₂ layers by deep level transient spectroscopy (DLTS), and succeeded in reducing defects in BaSi₂ films.
In the first experiment, we investigated the defect levels in undoped BaSi₂ absorber layers by DLTS. After thermal cleaning (TC) under UHV at 800°C, we grew a 500-nm-thick undoped BaSi₂ layer by MBE using an optimized Ba/Si ratio on n-Si (111) (ρ = 1–4 Ωcm) to form a heterojunction diode. The electron concentration (n) of the BaSi₂ was below 10¹⁶ cm^-3. The n of the Si substrate was 2 × 10¹⁵ cm ^-3 at RT. Hence, the depletion region stretches in both sides. However, the defects on the Si substrate decrease to a value below the detection limit^[4]. For this reason, the detected peaks indicate defects in BaSi₂. An upward facing peak caused by minority-carrier (hole) trap level (H1) was observed at approximately 126 K. The hole trap level was calculated to be approximately 0.27 eV from the valence band maximum, and N_T was 1×10¹³ cm^-3. This value is approximately 100 times smaller than that reported ^[5]. As the DLTS signal near 125 K decreased as |V_R| decreased and hence it approached the BaSi₂/Si interface. This result shows detected defects existed in BaSi₂, and their density decreased around the interface. When we changed the number of injected carriers by changing the pulse width t_pw, the DLTS signal increased sharply as the t_pw increased from 0.1 to 10 ms, indicating the presence of point defects. First-principles calculation has revealed that Si vacancy (V_Si) is most likely to occur.^[6] One of the calculated defect levels due to V_Si is in good agreement with H1. In order to understand why the density of V_Si decreased in BaSi₂ close to the heterointerface, RBS measurement was carried out. The depth profiles of Ba to Si atomic ratios showed that BaSi₂ was in excess of Si around the interface. This result means that Si atoms diffuse into the BaSi₂ layer from the Si substrate during MBE growth and fill the Si vacancies. From this result, it is expected that promoting Si diffusion from the substrate is effective for reducing the density of V_Si.
In the second experiment, in order to promote the diffusion of Si atoms from the Si substrate, we raised the substrate temperature (T_Sub.) during MBE growth from 580 to 650°C. In this experiment, we varied the Ba to Si deposition rate ratio (R_Ba/R_Si) from 0.4 to 4.7. The photoresponsivity changed drastically depending on R_Ba/R_Si and reached a maximum of around 2.7 A/W at 1.55 eV under the bias voltage of –1 V applied between the top and bottom electrodes when the R_Ba/R_Si was around 1.2. This value is more than three times larger than that reported ^[7], suggesting that the density of V_Si was significantly suppressed.

[1] K. Toh et al., Jpn. J. Appl. Phys. 50 (2011) 068001.
[2] M. Baba et al., J. Cryst. Growth 348 (2012) 75.
[3] S. Yachi et al., Appl. Phys. Lett. 109 (2016) 072103.
[4] Y. Yamashita et al., Jpn J. Appl. Phys. 57 (2018) 025501.
[5] H. Takeuchi et al., Jpn. J. Appl. Phys. 54 (2015) 07JE01.
[6] M. Kumar et al., J. Mater. Chem. A 5 (2017) 25293.
[7] R. Takabe et al., J. Appl. Phys. 123 (2018) 045703.

Cu₂(In,Ga)Se₄ (CIGSe) solar cells have recently set a new record efficiency of 22.9%, thus demonstrating a continuous improvement of this technology. Currently, the standard thickness for the CIGSe absorbers is 2-3 µm. Reduction in material consumption will help to further reduce the cost. When approaching the material reduction by reducing the absorber thickness to below 700 nm leads to increase in production quantity but, on the other hand, it considerably reduces light absorption in absorber layer causing in strongly enhanced recombination at the back contact. Here, we will focus on the optical aspect and investigate a novel means to generate broadband plasmonic generation in ultrathin CIGSe layers for enhanced absorption in solar cells.
We report a novel approach to generate plasmons in CIGSe thin films in the wide spectral range. In this work, a new methodology to generate plasmons in the CIGSe thin film using dual-ion beam sputtering (DIBS) system is reported. Sputtering of material during growth by utilizing secondary DC-ion source present in the DIBS system leads to the generation of different metallic and metal oxide nanoclusters of its constituent elements. Moreover, the formation of nanoclusters and generation of plasmons are verified by electron energy loss spectra obtained by UPS analysis, FESEM, and spectroscopic ellipsometry. Additionally, calculation of valence bulk, valence surface, and particle plasmon resonance energies are performed, and indexing of each plasmon peaks with corresponding plasmon energy peak of the different nanoclusters are done. This approach offers a convenient method to produce plasmon resonance across the broad spectral range spanning 2.1-10.74 eV. Further, use of this dual ion beam sputter instigated plasmon enhanced absorbers can be a novel mean to enhance the performance of photovoltaic devices.

Thin film solar cells based on the absorber material Cu(In,Ga)Se₂ have achieved conversion efficiencies well over 22 %. The recent push in record efficiencies was mainly realized by applying an alkali fluoride post deposition treatment (PDT) to the absorber. Best results were obtained for heavy alkali elements like Rb and Cs. In order to study the beneficial effect of the heavy alkali elements we investigated highly efficient Cu(In,Ga)Se₂ solar cells, whose absorbers were subjected to either a RbF-PDT or a CsF-PDT. Thin cross sectional lamellas were cut out of the entire devices and analyzed via a combination of synchrotron based spatially resolved X-ray fluorescence analysis and different electron microscopy techniques. It is evident that Rb accumulates at random high-angle grain boundaries, at the p-n junction, and at the interface between the absorber and MoSe₂ layer. The agglomeration of Rb at the grain boundaries is accompanied by a reduced Cu concentration and slightly increased In and Se concentrations. Furthermore, variations in the local composition of the absorber at the p-n junction indicate the formation of a secondary phase, which shows a laterally inhomogeneous distribution. In case of the CsF-PDT, we obtained similar results. Hence, the improved solar cell performance due to heavy alkali fluoride PDT can be expected to originate from a favorable modification of the p-n junction, the random high-angle grain boundaries or the back contact interface or from a combination of these effects.

Reducing the thickness of Cu(In,Ga)Se₂ (CIGS) absorber layers has potential to decrease its cost significantly, but has drawbacks like incomplete absorption and increased back-contact recombination, both resulting in power conversion efficiency losses. One solution is to implement a rear surface passivation layer, which has potential to reduce rear surface recombination velocity and increase rear internal reflection [1]. Alumina (Al₂O₃) is such an ideal passivation layer, but unfortunately also acts as an electron and diffusion barrier layer, and thus prevents current flow and sodium (Na) diffusion. As is discussed below, our novel approach to generate point contact openings in this passivation layer overcomes both problems at once. The proposed method is to use sodium fluoride (NaF) on top of the Al₂O₃ passivation layer, which will generate contact openings during selenization. This applied approach is industrially viable, as compared to proven methods – e.g. using nanoparticles, e-beam or nano-imprint lithography – which are too expensive, time-consuming or not applicable for larger areas. D. Ledinek et al. previously proposed using a very thin layer of Al₂O₃ rear surface passivation in combination with NaF deposition to enhance the electrical characteristics of CIGS solar cells. In their study this surface passivation is claimed to allow tunneling [2]. In our study, we prove that point contacts have been generated in this Al₂O₃ surface passivation layer. Atomic layer deposition (ALD) was used to deposit very thin Al₂O₃ layers on molybdenum (Mo) rear contact, and NaF was deposited on these layers by spin coating. Thereafter CIGS layers (±500 nm thick) were grown by single stage co-evaporation at 550°C, followed by standard solar cell process described in [3]. To detect the contact openings in the passivation layer, scanning electron microscopy (SEM) combined with energy dispersive X-ray (EDX) spectroscopy was applied. Glass/Mo/Al₂O₃/NaF characterization samples were used for this analysis, where the co-evaporation process was mimicked by using a selenization step. After the selenization, point openings in the thin Al₂O₃ passivation layers were determined by SEM, and supported by EDX measurement. To investigate the impact of Al₂O₃ thickness on passivation of the CIGS rear surface, time resolved photo-luminescence (TR-PL) measurements on finished absorber layers (capped with CdS) were used. According to these TR-PL results, a 6 nm thick passivation layer gave the highest free charge carrier lifetime and PL response in all sets of samples. This was confirmed in full solar cell devices, where a significant increase in power conversion efficiency, and gain in open-circuit voltage, current density and fill-factor values was measured for the 6 nm Al₂O₃ rear passivated CIGS cells, as compared to unpassivated reference cells. Hence, it can be concluded that by using a simple, cost-effective and fast way, i.e. ALD for Al₂O₃ and spin coating for NaF, we succeeded to make point contact openings to passivate the back surface of ultra-thin CIGS solar cells. The ongoing work focuses on different aspects: (i) At present the point openings are still submicron size, now we try to make them nano-size. (ii) We also concentrate on controlling the density of the contact openings, e.g. by changing the temperature of ALD process or the molarity of NaF solution. (iii) This approach also enables us to investigate different metal oxides as passivation layers, such as HfO₂ and TiO_2, in combination with the same contact opening approach. (iv) And finally, we are investigating this rear surface passivation approach for other absorber layer deposition/growth techniques, e.g. a 2-step sputtering + selenization method.

[1] Poncelet, O. et al. DOI: 10.1016/j.solener.2017.03.001
[2] Ledinek, D. et al. DOI: 10.1016/j.solmat.2018.07.017
[3] Garud, S. et al. DOI: 10.1002/pssa.201700826.

The highest conversion efficiencies for Cu(In,Ga)Se₂ (CIGSe) thin-film solar cells are currently close to 23%. In spite of extraordinary photovoltaic performance, the open-circuit voltage (V_oc) and the short-circuit current density (j_sc) do not reach levels of the theoretical, radiative Shockley-Queisser limit. The present work reports about potential limitations caused by enhanced nonradiative recombination at grain boundaries (GBs) and by electrostatic potential fluctuations. A CIGSe solar cell with a conversion efficiency of almost 21% (without anti-reflection coating) and a stacking sequence ZnO:Al/(Zn,Mg)O/CdS/CIGSe/Mo/glass, of which the coevaporated CIGSe absorber underwent a RbF postdeposition treatment (PDT), was characterized. In order to get an insight into the microstructural, compositional, electrical and optoelectronic properties, electron backscatter diffraction, energy-dispersive X-ray spectrometry (EDX), electron-beam-induced current (EBIC), and cathodoluminescence (CL) measurements were performed on a cross-sectional specimen in a scanning electron microscope. In addition, quantitative photoluminescence (PL) hyperspectral imaging and capacitance-voltage (C-V) analyses were carried out as well as two-dimensional device simulations, in order to corroborate the microscopy results.

From EBIC results, the width of the space-charge region (w_SCR) and the minority-carrier diffusion length in the quasi-neutral region (L_D) can be correlated to results obtained by absolute PL and C-V measurements. EDX elemental distribution analysis did not show any substantial, spatial variations in composition, except for the Ga/In gradient perpendicular to the substrate, which can be also correlated to the detected spectral shift in the CL peak in the same perpendicular direction. However, the impact of electrostatic potential, lifetime, as well as band-gap fluctuations present in neighboring regions in the absorber layer on the device performance of CIGSe solar cells can be neglected. This result was confirmed by two-dimensional device simulations.

In contrast, recombination velocities at GBs were determined to be on the order of 10³ cm/s from CL profiles extracted across GBs, which is the same order of magnitude that was determined also for GBs in various Na-containing CIGSe thin films without any PDT. Two-dimensional device simulations show that indeed, such recombination velocities could be one origin for the limited V_oc and j_sc in CIGSe solar cells.
V_oc and j_sc in CIGSe solar cells.

Understanding the relationship of photoexcited carrier lifetimes, mobilities, and recombination mechanisms to structural properties and processing of photovoltaic (PV) absorber materials is critical to the design of efficient solar cells. Carrier dynamics in PV absorbers have conventionally been characterized by time-resolved photoluminescence (TRPL), but TRPL may not be suitable or straightforward for all absorbers. Alternative non-contact methods can enable measurement of ultrafast carrier dynamics for a wider range of materials. Here we combine the complementary analysis methods of time-resolved terahertz spectroscopy (TRTS) and near-infrared transient reflectance (NIR-TR) spectroscopy along with TRPL to assess photoexcited carrier dynamics in high-quality kesterite Cu₂ZnSnSe₄ (CZTSe) crystals with different cation compositions. The single-crystalline nature of the sample eliminates complications arising from grain boundaries, secondary phases and interfaces associated with thin film growth. An example copper-poor, zinc-rich (Cu/(Zn+Sn) = 0.87 and Zn/Sn = 1.22) single-crystal-based PV device exhibited J_sc = 28.2 mA/cm² and efficiency of 6.2%, with V_oc of 400 mV consistent with the quasi-Fermi level splitting determined using absolute photoluminescence. NIR-TR showed picosecond-scale cooling and relaxation of carriers into a distribution of band tail states while TRTS revealed a characteristic time scale of 200 – 260 ps for recombination. Hall effect and TRTS measurements revealed electron and hole mobilities in the range of 50 – 100 cm²/Vs and a majority carrier density of 9x10¹⁶ cm^-3. These dynamics result in a characteristic minority carrier diffusion length of less than 200 nm, leading to incomplete carrier collection, as confirmed by a strongly decreasing external quantum efficiency at long wavelengths for a device prepared on the crystal.

Carrier dynamics and device performance strongly depended on cation composition. Near-stoichiometric compositions (Cu/(Zn+Sn) > 0.92 and Zn/Sn < 1.12) had much shorter carrier lifetimes of less than 20 ps, which lead to J_sc = 16.2 mA/cm², V_oc = 220 mV, and efficiency = 1.7%. Low carrier lifetime, which may be due to the high density of cluster defects related to Cu_Sn/Sn_Zn antisites, limits J_sc and V_oc, as well as efficiency. To reduce the defect concentration, Ag was used as a substitute for Cu since the covalent radius of Ag is approximately 15% larger than that of Cu and Zn. With Ag/(Ag+Cu) = 0.29, lifetime increased by more than an order of magnitude to 9 ns, with associated increase in diffusion length up to ~1 µm. Improved performance with Ag alloying had previously been observed for polycrystalline thin films, with improvements partially ascribed to larger grains and changes to the chemistry of the grain boundaries. However, the dramatically increased lifetime that we observe with Ag alloying in single crystals indicates that reductions in bulk defect densities are likely the dominant factor. Incorporating Ag into CZTSSe devices may lead to increased efficiencies. Our approach combining ultrafast spectroscopy and device measurements can lead to more detailed understanding of performance-limiting photophysical processes and can subsequently accelerate the development of more efficient PVs.

The photoexcited carrier dynamics in optoelectronic devices such as photovoltaic and thermoelectric devices is a major research topic over the past decades. Chalcogenide-based Cu(In,Ga)Se₂ (CIGS) thin film has attracted considerable attention as a material for photovoltaic devices with high radiation tolerance in space environment. However, very little empirical work had been done until recently, related with the studies on radiation damage and photoexcited carrier dynamics with radiation exposure in CIGS thin film.
In this paper, we carried out the irradiation of H⁺ ion beams with injection energy of 200 keV into CIGS thin film by varying the concentration of H⁺ ion beams dose from 10¹⁴/cm² to 10¹⁶/cm². To investigate the optical and electrical property, the carrier concentration and hall mobility were evaluated as a function of H⁺ ion beams dose. To study photoexcited carrier dynamics after ion beams-bombardment, optical pump-THz probe (OPTP) spectroscopy were measured, which is sensitive to behavior of free carriers within ultrashort time scales of ~ ps. Contrary to pure CIGS thin film, the carrier lifetimes of ion beams-bombarded CIGS thin film were drastically decreased, which is directly related to the surface defect states induced by radiation damages. Moreover, carrier lifetimes varied with the concentration of H⁺ ion dose due to different settlements of ion beams along the depth direction into the CIGS film.
Based on carrier-recombination process, we introduce an interpretation of the photocarrier dynamics in terms of surface defect states existed in the ion beams-bombarded CIGS thin film as a function of H⁺ ion dose.

Over the last decades, the significant increase in short circuit current density has been considered as one of the most important factors that led to the current record efficiency CdTe solar cells. The improvement of short circuit current density is mainly achieved by incorporating Se into CdTe. The impact of Se incorporation is commonly attributed to the optimal band alignment introduced by the formation of CdSe_1-xTe_x alloy. However, other possible mechanisms that may lead to the enhanced device performance, including the changes in the carrier density profiling, trap state distributions in bulk and at interfaces, and barrier heights are yet to be systematically investigated.
In this work, a series of electrical characterization measurements are carried out to explore the effects of incorporating a CdSe interlayer in CdTe solar cells. Temperature dependent current density-voltage (J-V-T) measurements are performed to extract the Schottky back barrier heights for devices with different CdS and CdSe thicknesses. Temperature dependent capacitance–voltage (C-V-T) measurements are used to compare the charge carrier density distributions through the absorber layers. In addition, thermal admittance and impedance spectroscopy measurements are carried out to explore the defects in bulk and at interfaces, conductivity, and carrier mobilities. Combining these advanced electrical characterization techniques allows us to probe the origins of the enhanced performance in CdSe_1-xTe_x solar cells. Our study would supply a better understanding of Se incorporation in CdTe absorber layers, thus providing guidelines to further improvement of CdTe solar cells.

In many photovoltaic materials the impact of disorder and band tails is discussed controversially and it is unclear whether extended band states or localized tail states dominate charge carrier dynamics.
Here, we integrate the effect of band tails on charge carrier distribution, transport and recombination into one narrative and deduce the consequences for solar cells.
To this end, we probe photoexcited charge carriers in a kesterite-semiconductor as a model system over their full life span from femtoseconds to microseconds by time-resolved terahertz spectroscopy and photoluminescence spectroscopy.
We find that occupation of the band tails is strongly temperature-dependent. Above a characteristic temperature charge carrier are activated out of Urbach band tails. Then charge carriers occupy dominantly shallow Gaussian band tails in the probed kesterite thin film. At room temperature we directly observe the thermalisation of charge carriers into these tail states within the first 2 ps. The occupation of these tail states reduces the quasi-Fermi level splitting and the open circuit voltage of a potential solar cell by ca. 80 meV.
The mobility of the charge carriers decreases to 144 cm²/Vs while they thermalize into the tail states. This mobility value is at least one order of magnitude lower than in a theoretical tail-free kesterite-semiconductor. Further, the mobility at terahertz frequencies indicates that charge carriers localize in tail states with a typical spatial extend of 11 nm.
The recombination kinetics of photoexcited charge carriers can be described within the Shockley-Read-Hall model of defect assisted recombination. Unambiguous signs of an influence of band tails are not measured at room temperature, but secondary effects from the affected carrier distribution and transport are discussed. The non-radiative recombination is found to be the major bottleneck for a potential solar cell and reduces the open circuit voltage of a potential solar cell by 270 meV.
However, below the characteristic temperature of 200 K charge carrier relax by multiple-trapping into the deep Urbach tail and lead to a strongly injection-dependent charge carrier distribution, transport and recombination as previously observed for amorphous semiconductors.

The absence of toxic and scarce elements makes kesterite (Cu₂ZnSnS₄) a promising inorganic thin film photovoltaic technology, but its performance advancement is hindered by a substantial voltage loss. This is commonly expressed as the open circuit voltage deficit (V_oc-deficit = E_g – V_oc) which requires bandgap determination. The band tails that characterize kesterite absorption spectrum make bandgap evaluation inaccurate and dependent on the method, thus Voc-deficit interpretation is unreliable for proper performance comparison. It was demonstrated by Yao et al. (Phys. Rev. Appl., 4, 1 (2015)) that from the reciprocity principle and the external quantum efficiency it is possible to measure voltage losses avoiding the bandgap estimation.

Here we present the analysis of the radiative open circuit voltage limit Voc,rad and the non-radiative voltage losses (ΔV_oc,nrad = V_oc,rad - V_oc) based on the external quantum efficiency (EQE) and electroluminescence (EL) measurements of a series of kesterite devices. As a result of Sn content variation from 27 at% to 32.2 at% the V_oc ranges from 423 to 465 mV, while the bandgap and Voc,rad increase of about 100 mV, thus ΔV_oc,nrad increments of about 60 mV. It is observed that for the device with the lowest Sn concentration of 27 at%, Voc and V_oc,rad were the lowest of the series whereas electroluminescence quantum yield (QE_LED) was the highest (5.2×10^-5 %). This indicates that the lowest photovoltage does not always correlate with a high non-radiative recombination in kesterite solar cells. This is also supported by temperature dependent current-density (JV-T) and capacitance-frequency (Cf-T) measurements confirming the Sn variation has no appreciable influence on the defect distribution.

It is concluded that QE_LED obtained by means of EL and EQE measurements is a suitable indicator of non-radiative losses for compositionally complex absorbers like kesterite.

Cadmium telluride (CdTe) photovoltaic device efficiencies show significant improvement with the incorporation of a cadmium selenium telluride (CdSeTe) alloy layer deposited between the magnesium zinc oxide (MgZnO) emitter and CdTe absorber layers. CdSeTe/CdTe devices are fabricated by close-space sublimation with a total absorber thickness of 1.5 µm. At this thickness, both the CdSeTe and CdTe layers are fully depleted. The incorporation of the CdSeTe layer is confirmed by microscopy measurements and shows minimal grading of Se into the CdTe layer. Loss analysis calculated from optical measurements shows that the CdSeTe layer acts as the primary absorber in the CdSeTe/CdTe structure, absorbing at least 90% of the photons. Current density-voltage measurements show an increase in current density of up to 3 mA/cm² with the addition of a CdSeTe layer that is due to a band gap shift from approximately 1.5 to 1.43 eV for CdTe and CdSeTe/CdTe absorbers respectively, where band gaps are determined by quantum efficiency and spectral photoluminescence measurements. Improvement in CdSeTe/CdTe device quality compared to CdTe is reflected in spectral photoluminescence measurements which show peak intensities many times greater when Se is incorporated into the absorber. The voltage deficit, calculated as the difference between the optimal absorber band gap and the measured open circuit voltage, shows improvement with the incorporation of the CdSeTe layer. This is corroborated by improved electroluminescence intensity, which is proportional to voltage deficit. The incorporation of CdSeTe as part of the absorber layer in CdTe device structures improves device current density and voltage deficit and has therefore increased device efficiencies by 2% absolute for CdSeTe/CdTe absorbers with a total thickness less than two microns.

Cu(In, Ga)Se₂ (CIGS) solar cells have been studied for space applications for a number of years due to their radiation tolerance and low cost. Most early work has studied CIGS on soda lime glass, which is too bulky for space applications, and would not allow for the complex folding arrays that have been proposed for future deep space missions. Here, commercially available flexible CIGS solar cells are investigated under conditions consistent with those around the outer planets. Performance under these low temperature low intensity (LILT) conditions is critical to study, as these conditions can reveal issues not present under normal operation [1]. Current density-voltage (J-V) and external quantum efficiency (EQE) measurements under different temperature, illumination, and bias conditions are performed to investigate the environmental effects of these deep space conditions. Of particular interest at low temperature is the behavior of the well-known V_Cu-V_Se metastable defect [2], and its potential role in device performance under LILT conditions. Here, evidence of a photo-activated barrier is observed, which is attributed to the same defect center, although its effect on power generation at LILT conditions appears to be minimal. Additionally, proton irradiation with high fluence levels is investigated to emulate certain conditions near Jupiter. Although degradation is observed, self-healing behavior under annealing conditions is also evident.

[1] D. A. Scheiman and D. B. Snyder, "Low intensity low temperature (LILT) measurements of state-of-the-art triple junction solar cells for space missions," in 2008 33rd IEEE Photovoltaic Specialists Conference, 2008, pp. 1-6.
[2] S. Lany and A. Zunger, "Light- and bias-induced metastabilities in Cu(In,Ga)Se₂ based solar cells caused by the (V_Se-V_Cu) vacancy complex," Journal of Applied Physics, vol. 100, no. 11, p. 113725, 2006/12/01 2006.

Real time spectroscopic ellipsometry (RTSE) measurements have been performed on copper indium diselenide (CuInSe₂) narrow bandgap solar cell materials. In these studies, thin film CuInSe₂ absorber structures have been deposited in one-stage and two-stage thermal co-evaporation processes on smooth c-Si wafer substrates held at temperatures in the range of 300-570 °C. Analysis of the RTSE data for one-stage CuInSe₂ yields the structural evolution and the complex dielectric functions (ε₁, ε₂) of the individual CuInSe₂ layers, the latter relevant for the elevated temperatures of deposition. In this analysis, information on the structural evolution, i.e. the surface roughness and underlying bulk layer thicknesses versus time, is obtained with monolayer-level sensitivity during thin film nucleation and coalescence. Complex dielectric functions (ε₁, ε₂) of deposited films as well as their bandgap energies and broadening parameters are obtained at different measurement temperatures applying the combination of RTSE and in situ SE measurements during sample cooling. All such deduced information on the structural and optical properties provides insights into CuInSe₂ film crystallinity and grain growth processes. In the two stage processes, CuInSe₂ is obtained by depositing either In₂Se₃ or Cu_2-xSe first, followed by exposure to either Cu+Se or In+Se evaporant, respectively. Real time SE has been applied to both stages of each CuInSe₂ deposition in order to compare the structural and optical property evolution characteristic of the two different routes for CuInSe₂ fabrication. The goal of this work is CuInSe₂ of optimum stoichiometry and grain structure obtained under RTSE monitoring and control for use as a bottom cell absorber material in tandem solar cell applications. Results correlating structural evolution and ultimate device performance will be reported.

Recently, Cu₂ZnSn(S,Se)₄ (CZTSSe) solar cell has attracted many attentions due to several advantages of CZTS(e) such as earth-abundant, cheap, nontoxic, high absorption coefficient (~10^-4 cm^-1) and tunable bandgap (1~1.5eV). [1-4] So far, the highest efficiency of CZTS(e) solar cell is 12.6% achieved by IBM team but its power conversion efficiency (PCE) still cannot compete with the commercial thin-film solar cells, e.g., CIGS and CdTe. There are numerous issues need to be solved to enhance the PCE (e.g., back contact losses, interfacial losses, deep defects). Especially, back contact losses cause degradation of CZTS(e) solar cell due to interface defects between CZTS(e) and MoS(e)₂ produced by sulfo-selenized Mo-coated soda lime glasses (SLG).
In this work, the PCE of CZTSSe solar cells were greatly improved by a simple but effective method, wherein a CdS nano-layer was deposited between the Mo substrate and the metallic precursors prior to CZTSSe sulfo-selenization, producing p+-CTS(e) and ZnS(e), which helps reduce interface recombination, on the bottom of the CZTSSe absorber layer. The mainly improvement results from increasing fill factor (FF) and short circuit current due to less recombination at back interfaces. Moreover, a 9.61% PCE (~10.6% in the cell effective area) CZTSSe cell with improved FF from 44% to 64% has been attained.
The morphology, elemental composition, and distribution of the absorber layers are being examined by X-ray diffraction (XRD), X-ray fluorescence spectrometry (XRF), scanning electron microscopy (SEM), Transmission electron microscope (TEM), and Raman spectroscopy.

References
[1] V. Tunuguntla, W.C. Chen, P.H. Shih, I. Shown, Y.R. Lin, C.H. Lee, J.S. Hwang, L.C. Chen and K.H. Chen, J. Mater. Chem. A, 2015,3, 15324-15330
[2] Y.R. Lin, V. Tunuguntla, S.Y. Wei, W.C. Chen, D. Wong, C.H. Lai, L.K. Liu, L.C. Chen and K.H. Chen, Nano Energy, 2015, 16, 438
[3] W.C. Chen, C.Y. Chen, V. Tunuguntla, S.H. Lu, C. Su, C.H. Lee, K.H. Chen and L.C. Chen, Nano Energy, 2016, 30, 762-770
[4] C.Y. Chen, B. S. Aprillia, W.C. Chen, Y.C. Teng, C.Y. Chiu, R.S. Chen, J.S. Hwang, K.H. Chen, and L. C. Chen, Nano Energy, 2018, 51, 597-603

Alkali metal (Na, K, Rb, and Cs) incorporation or post-deposition treatment (PDT) is known to be effective in improving the conversion efficiency of Cu(In,Ga)Se₂ (CIGSe) and Cu(InGa)(SSe)₂ thin-film solar cells. Among them, Cs has been known to boost V_OC and J_SC in CIGSe, and the world-record cell is also based on this technique. A new world-record CIGSSe efficiency of 22.9% has been achieved in the research center of Solar Frontier K.K.by CsF-PDT. However, CsF-PDT effects have been rarely studied, and therefore, it is worth to investigate the Cs incorporation.
This study reveals CsF-post-deposition treatment (PDT) effects on a Cu(InGa)Se₂ (CIGSe) cell in comparison with KF-PDT effects. After CIGSe growth, we performed three different alkali-PDTs (KF, CsF, and combined KF + CsF) on CIGSe thin films, and examined the material and cell properties. Both KF- and CsF-PDT CIGSe cells showed similar cell efficiency improvements (~17.5%) compared to the untreated CIGSe cell (~16%). However, the reasons for the improvement differed. The CIGSe cell with KF-PDT exhibited further reduction in the recombination at the CdS/buffer interface compared to the CsF-PDT CIGSe cell. On the other hand, the CIGSe cell with CsF-PDT exhibited further reduction in the recombination in bulk compared to the KF-PDT CIGSe cell. In addition, the CIGSe cell with combined alkali-PDT exhibited a largely enhanced conversion efficiency (18.4%) due to cooperative alkali incorporation effects. This device exhibited reduced recombination at both the CdS/CIGSe interface in the space charge region (SCR) and bulk compared to all other cells. Finally, we achieved a highly efficient combined alkali-PDT CIGSe device (20.15%) by applying a MgF₂ anti-reflective coating layer.

The biggest difficulty for improving the power conversion efficiency of Cu₂ZnSn(S, Se)₄ (CZTSSe)/CdS/ZnO thin film solar cells is improving the open circuit voltage (V_OC). Probably leading causes of the V_OC deficit in CZTSSe devices have been identified as bulk recombination, band tails, and the intertwined effects of CZTSSe/CdS band offset, interface defects, and interface recombination. In this work, we replaced the CdS buffer layer by Mg-doped ZnO (Mg_.26Zn_.74O) and measure the interface of n-type Mg_.26Zn_.74O / p-type CZTSSe heterojunction grown by dual ion beam sputtering. The valence band offset at n-type Mg_.26Zn_.74O (MZ1) /p-CZTSSe heterojunction interface can be determined by the core-level photoemission-based method. Here, the calculation of was done by two methods: (1) considering different core level peaks of Se 3d and Mg 2p for CZTSSe and MZ1, respectively, and (2) considering same core-level peaks of Zn 3d for CZTSSe and MZ1. The values of valence band onset (VBOn), as obtained by linear extrapolation method for MZ1 and CZTSSe films are 2.97 and 0.54 eV, respectively. Figure. 1 (a) shows the core-level peaks of the MZ1/CZTSSe heterojunction. The core-level spectra of Zn 3d recorded on the MZ1/CZTSSe heterojunction is dominated by one symmetric peak, implying a uniform bonding state. Figure 1(b) shows transmission spectra using ellipsometry. Figure 1(c) shows the energy band diagram calculated using Ultra violet photoelectron spectroscopy (UPS).

Thin film CIGS solar cells have reached an efficiency of almost 23%. However, when going to modules the efficiency is max 19% but often less [1]. One reason for this large gap is that the process steps that are required to make the individual solar cells so efficient are not easily upscalable to module production. To resolve this, we are aiming to make the CIGS solar cells structure simpler and more cost effective. We will do this by decreasing the thickness to ~ 500 nm and using an ungraded absorber layer grown in a single stage co-evaporation step. This makes the process simpler, fast and requires less material. However, since the thin layers are rather different in their depth profile and grain size compared to the thicker 3-stage co-evaporated layers, re-investigation of some commonly used process steps like alkali addition is required. In this contribution we investigate the effect of KF post deposition treatment (KF-PDT) on our thin ungraded absorber layers.
To investigate the effect of KF-PDT, we use a Mo/Si(O,N)/glass substrate. The Si(O,N) layer functions as an alkali diffusion barrier and in this way the amount of alkali are controlled by adding them separately. The Na was added by evaporating NaF before CIGS growth and K was added by spincoating a KF solution onto the absorber layer and do a post anneal in a nitrogen atmosphere. While Se is commonly added during PDT to avoid Se losses, we found that this is not required for our thin absorber layers. The molarity of the KF solution, temperature and method of the anneal were varied and optimized further.
Absorber layers with and without KF-PDT were characterized with room temperature photoluminescence (RT-PL), time resolved photoluminescence (TR-PL), GDEOS, GI-XRD and SEM. Except for the alkali, the KF treatment had no effect on the depth profile. After KF treatment, the Na was virtually absent, while there was no K in the absorber layer before KF treatment. This has been shown earlier for 3-stage samples [2]. RT-PL however, did reveal significant changes. The intensity was up to 2 orders higher after KF treatment. This increase was used to predict an increase in Voc, which corresponded very well with the measured Voc increase. The spectra of KF treated absorbers were blue-shifted compared to the untreated absorber layers. That the emission can blue-shift is possible since the PL spectra of the single stage absorber layers is slightly below the band gap and coming from tail states. The measured blue-shift after KF treatment is then attributed to a reduction of tail states.
Finished solar cells were characterized with IV and CV measurements. An increase in Voc, FF and net acceptor concentration for the KF treated absorber layers were measured. However, when the initial doping was much higher, we found a decrease in net acceptor concentration after KF treatment. The Voc and FF still improved though. This shows that the net doping is determined by the amount of K and excludes Voc improvement due to increased net acceptor concentration only. It has been proposed as well that the KF treatment improves the pn junction quality [3]. To look more in detail at the pn junction, a few cells were illuminated with blue light. Here we found a clear distinction between KF treated and untreated cells. Without KF treatment the IV curve showed a kink at negative bias, while after KF treatment the kink was completely removed. This is a direct signature of an improved pn junction. At last, we determined J0s from the measured dark curves. Here we found an approximately one order decrease of the J0 for the KF treated cells. The overall efficiency has increased more than 2%, reaching the 10% for our 500 nm single stage processed solar cells. This is without any passivation or back reflector, showing the potential of the proposed process.

[1] Green et. al. DOI: 10.1002/pip.3040
[2] Pianezzi et. al. DOI 10.1002/pssr.201307238
[3] Chirila et. al. DOI: 10.1038/NMAT3789

Polycrystalline CdTe-based solar cells have reached efficiencies of over 22% in the recent years. Although current density values have been improved by incorporation of Se and elimination of CdS as a window layer, open circuit voltage (V_oc) is still well below its theoretical maximum. The road towards high V_ocand 25%-efficient devices requires a combination of low interface recombination velocity, higher lifetime, and higher carrier concentration in the CdTe absorber. It was recently demonstrated that the impact of the interface recombination on the performance of the device is more pronounced as carrier concentration and lifetime increase. Therefore, precise engineering of the conduction band alignment with CdTe absorber is needed at the front interface to reduce recombination. In this work we perform a combinatorial study of Mg_1-xZn_xO (MZO) buffer layer prepared by co-sputtering of Zn and Mg in oxygen-containing atmosphere. This allows to probe band gap variation of about 0.4 eV across a 2-inch substrate. In addition, the effects of MZO deposition temperatures on the device performance are studied. Introduction of MZO into the device architecture also allows for higher CdCl₂ treatment temperatures to be used to obtain larger CdTe grains compared to devices with CdS window layers, where delamination may occur at higher CdCl₂ treatment temperatures. Although larger grains and fewer grain boundaries may be favorable for carrier lifetime, this also requires modification of our standard Cu activation process that has been used with devices employing CdS window layer. Therefore, in this work we also study the effect of CdCl₂ treatment temperature on Cu activation using rapid thermal processing (RTP). Kelvin probe is used to determine the work function variation of MZO across the substrate for better understanding of band alignment with CdTe absorber. Device performance is evaluated using J-V characteristics.

Cu₂ZnSnS₄ (CZTS), a quaternary semiconductor, is an earth abundant and sustainable semiconductor material that is used as an absorber layer for thin film solar cells. In the present work, CZTS thin films were deposited onto soda lime glass (SLG) substrates by RF magnetron sputtering using the following stacking sequence CuS/ ZnS/ SnS at a substrate temperature of 300^oC. The samples were then sulphurised at 400⁰C in H₂S atmosphere for 60 min. A comparison of as-deposited and sulphurised CZTS thin films for their composition are discussed in this paper. The composition of the CZTS thin film samples were studied using X-ray photoelectron spectroscopy, which revealed the presence of Cu, Zn, Sn, S in required ratio. The presence of O and C was also detected due to atmospheric contamination. It is evident from the atomic % calculation that the stoichiometry has improved for the sulphurised samples. The formation of stoichiometric CZTS is an important factor to use it as an absorber layer in a thin film solar cell. The Ultra-violet photoelectron spectroscopy was used to study the ionization energy, electron affinity, electronic band gap of the material and the data were in relation to reported results. Thus, from the XPS results it is obvious that the sulphurised CZTS thin films can be used as absorber layer in a thin film solar cell.

We performed density functional theory calculations using CASTEP algorithm to understand the electronic strucure of MoS₂ layers over Silicon Oxides. Our findings, indicate a vertical alignment of MoS₂ layers as most geometrical stable and perturbation of density of states for d-orbital from semiconducting into a semi-metallic was found and it could be correlated to linear behavior of I-V experimental curves as measured by four-point probe on MoS₂/SiO₂ thin films (~100nm).

In the past decades many materials capable of photovoltaic solar energy conversion have been developed. Several of those materials are already commercialized, including monocrystalline and polycrystalline silicon, as well as several thin-film materials such as cadmium telluride (CdTe) and copper indium gallium arsenide (CIGS). The efficiency of these materials is still increasing and in some cases approaching the Shockley-Queisser limit. However, real-life outdoor illumination and climate conditions often lead to performance significantly below the performance under standard test conditions (AM1.5G, 1 kW/m², 25^oC). Many environmental factors such as varying solar spectrum, light intensity, and temperature influence the performance as is shown in previous work.^[1] Diffuseness of light is a parameter that is thought to have an influence on the performance of solar modules as well. In this work we present real-life performance data of several silicon and thin-film modules with data recorded from our solar field in Amsterdam, NL. The diffuseness is measured with a novel measurement tool that uses intrinsic shading for providing an effective and inexpensive way to measure how diffuse the sunlight is. We compare the data to predicted performance from simple diode model in order to determine the physical nature of performance losses and gains. We further use our modelling tool to predict real-life efficiencies of many photovoltaic materials and predict the important optimizing parameters in order to increase realistic performance in countries with climates similar to the Netherlands.

[1] Futscher, M. H., & Ehrler, B. (2017). Modeling the Performance Limitations and Prospects of Perovskite/Si Tandem Solar Cells under Realistic Operating Conditions. ACS Energy Letters, 2(9), 2089-2095.

Thin film solar cells based on the absorber material Cu(In,Ga)Se₂ have achieved conversion efficiencies well over 22 %. The recent push in record efficiencies was mainly realized by applying an alkali fluoride post deposition treatment (PDT) to the absorber. Best results were obtained for heavy alkali elements like Rb and Cs. In order to study the beneficial effect of the heavy alkali elements we investigated highly efficient Cu(In,Ga)Se₂ solar cells, whose absorbers were subjected to either a RbF-PDT or a CsF-PDT. Thin cross sectional lamellas were cut out of the entire devices and analyzed via a combination of synchrotron based spatially resolved X-ray fluorescence analysis and different electron microscopy techniques. It is evident that Rb accumulates at random high-angle grain boundaries, at the p-n junction, and at the interface between the absorber and MoSe₂ layer. The agglomeration of Rb at the grain boundaries is accompanied by a reduced Cu concentration and slightly increased In and Se concentrations. Furthermore, variations in the local composition of the absorber at the p-n junction indicate the formation of a secondary phase, which shows a laterally inhomogeneous distribution. In case of the CsF-PDT, we obtained similar results. Hence, the improved solar cell performance due to heavy alkali fluoride PDT can be expected to originate from a favorable modification of the p-n junction, the random high-angle grain boundaries or the back contact interface or from a combination of these effects.

The highest conversion efficiencies for Cu(In,Ga)Se₂ (CIGSe) thin-film solar cells are currently close to 23%. In spite of extraordinary photovoltaic performance, the open-circuit voltage (V_oc) and the short-circuit current density (j_sc) do not reach levels of the theoretical, radiative Shockley-Queisser limit. The present work reports about potential limitations caused by enhanced nonradiative recombination at grain boundaries (GBs) and by electrostatic potential fluctuations. A CIGSe solar cell with a conversion efficiency of almost 21% (without anti-reflection coating) and a stacking sequence ZnO:Al/(Zn,Mg)O/CdS/CIGSe/Mo/glass, of which the coevaporated CIGSe absorber underwent a RbF postdeposition treatment (PDT), was characterized. In order to get an insight into the microstructural, compositional, electrical and optoelectronic properties, electron backscatter diffraction, energy-dispersive X-ray spectrometry (EDX), electron-beam-induced current (EBIC), and cathodoluminescence (CL) measurements were performed on a cross-sectional specimen in a scanning electron microscope. In addition, quantitative photoluminescence (PL) hyperspectral imaging and capacitance-voltage (C-V) analyses were carried out as well as two-dimensional device simulations, in order to corroborate the microscopy results.

From EBIC results, the width of the space-charge region (w_SCR) and the minority-carrier diffusion length in the quasi-neutral region (L_D) can be correlated to results obtained by absolute PL and C-V measurements. EDX elemental distribution analysis did not show any substantial, spatial variations in composition, except for the Ga/In gradient perpendicular to the substrate, which can be also correlated to the detected spectral shift in the CL peak in the same perpendicular direction. However, the impact of electrostatic potential, lifetime, as well as band-gap fluctuations present in neighboring regions in the absorber layer on the device performance of CIGSe solar cells can be neglected. This result was confirmed by two-dimensional device simulations.

In contrast, recombination velocities at GBs were determined to be on the order of 10³ cm/s from CL profiles extracted across GBs, which is the same order of magnitude that was determined also for GBs in various Na-containing CIGSe thin films without any PDT. Two-dimensional device simulations show that indeed, such recombination velocities could be one origin for the limited V_oc and j_sc in CIGSe solar cells.
V_oc and j_sc in CIGSe solar cells.

Understanding the relationship of photoexcited carrier lifetimes, mobilities, and recombination mechanisms to structural properties and processing of photovoltaic (PV) absorber materials is critical to the design of efficient solar cells. Carrier dynamics in PV absorbers have conventionally been characterized by time-resolved photoluminescence (TRPL), but TRPL may not be suitable or straightforward for all absorbers. Alternative non-contact methods can enable measurement of ultrafast carrier dynamics for a wider range of materials. Here we combine the complementary analysis methods of time-resolved terahertz spectroscopy (TRTS) and near-infrared transient reflectance (NIR-TR) spectroscopy along with TRPL to assess photoexcited carrier dynamics in high-quality kesterite Cu₂ZnSnSe₄ (CZTSe) crystals with different cation compositions. The single-crystalline nature of the sample eliminates complications arising from grain boundaries, secondary phases and interfaces associated with thin film growth. An example copper-poor, zinc-rich (Cu/(Zn+Sn) = 0.87 and Zn/Sn = 1.22) single-crystal-based PV device exhibited J_sc = 28.2 mA/cm² and efficiency of 6.2%, with V_oc of 400 mV consistent with the quasi-Fermi level splitting determined using absolute photoluminescence. NIR-TR showed picosecond-scale cooling and relaxation of carriers into a distribution of band tail states while TRTS revealed a characteristic time scale of 200 – 260 ps for recombination. Hall effect and TRTS measurements revealed electron and hole mobilities in the range of 50 – 100 cm²/Vs and a majority carrier density of 9x10¹⁶ cm^-3. These dynamics result in a characteristic minority carrier diffusion length of less than 200 nm, leading to incomplete carrier collection, as confirmed by a strongly decreasing external quantum efficiency at long wavelengths for a device prepared on the crystal.

Carrier dynamics and device performance strongly depended on cation composition. Near-stoichiometric compositions (Cu/(Zn+Sn) > 0.92 and Zn/Sn < 1.12) had much shorter carrier lifetimes of less than 20 ps, which lead to J_sc = 16.2 mA/cm², V_oc = 220 mV, and efficiency = 1.7%. Low carrier lifetime, which may be due to the high density of cluster defects related to Cu_Sn/Sn_Zn antisites, limits J_sc and V_oc, as well as efficiency. To reduce the defect concentration, Ag was used as a substitute for Cu since the covalent radius of Ag is approximately 15% larger than that of Cu and Zn. With Ag/(Ag+Cu) = 0.29, lifetime increased by more than an order of magnitude to 9 ns, with associated increase in diffusion length up to ~1 µm. Improved performance with Ag alloying had previously been observed for polycrystalline thin films, with improvements partially ascribed to larger grains and changes to the chemistry of the grain boundaries. However, the dramatically increased lifetime that we observe with Ag alloying in single crystals indicates that reductions in bulk defect densities are likely the dominant factor. Incorporating Ag into CZTSSe devices may lead to increased efficiencies. Our approach combining ultrafast spectroscopy and device measurements can lead to more detailed understanding of performance-limiting photophysical processes and can subsequently accelerate the development of more efficient PVs.

The photoexcited carrier dynamics in optoelectronic devices such as photovoltaic and thermoelectric devices is a major research topic over the past decades. Chalcogenide-based Cu(In,Ga)Se₂ (CIGS) thin film has attracted considerable attention as a material for photovoltaic devices with high radiation tolerance in space environment. However, very little empirical work had been done until recently, related with the studies on radiation damage and photoexcited carrier dynamics with radiation exposure in CIGS thin film.
In this paper, we carried out the irradiation of H⁺ ion beams with injection energy of 200 keV into CIGS thin film by varying the concentration of H⁺ ion beams dose from 10¹⁴/cm² to 10¹⁶/cm². To investigate the optical and electrical property, the carrier concentration and hall mobility were evaluated as a function of H⁺ ion beams dose. To study photoexcited carrier dynamics after ion beams-bombardment, optical pump-THz probe (OPTP) spectroscopy were measured, which is sensitive to behavior of free carriers within ultrashort time scales of ~ ps. Contrary to pure CIGS thin film, the carrier lifetimes of ion beams-bombarded CIGS thin film were drastically decreased, which is directly related to the surface defect states induced by radiation damages. Moreover, carrier lifetimes varied with the concentration of H⁺ ion dose due to different settlements of ion beams along the depth direction into the CIGS film.
Based on carrier-recombination process, we introduce an interpretation of the photocarrier dynamics in terms of surface defect states existed in the ion beams-bombarded CIGS thin film as a function of H⁺ ion dose.

Over the last decades, the significant increase in short circuit current density has been considered as one of the most important factors that led to the current record efficiency CdTe solar cells. The improvement of short circuit current density is mainly achieved by incorporating Se into CdTe. The impact of Se incorporation is commonly attributed to the optimal band alignment introduced by the formation of CdSe_1-xTe_x alloy. However, other possible mechanisms that may lead to the enhanced device performance, including the changes in the carrier density profiling, trap state distributions in bulk and at interfaces, and barrier heights are yet to be systematically investigated.
In this work, a series of electrical characterization measurements are carried out to explore the effects of incorporating a CdSe interlayer in CdTe solar cells. Temperature dependent current density-voltage (J-V-T) measurements are performed to extract the Schottky back barrier heights for devices with different CdS and CdSe thicknesses. Temperature dependent capacitance–voltage (C-V-T) measurements are used to compare the charge carrier density distributions through the absorber layers. In addition, thermal admittance and impedance spectroscopy measurements are carried out to explore the defects in bulk and at interfaces, conductivity, and carrier mobilities. Combining these advanced electrical characterization techniques allows us to probe the origins of the enhanced performance in CdSe_1-xTe_x solar cells. Our study would supply a better understanding of Se incorporation in CdTe absorber layers, thus providing guidelines to further improvement of CdTe solar cells.

In many photovoltaic materials the impact of disorder and band tails is discussed controversially and it is unclear whether extended band states or localized tail states dominate charge carrier dynamics.
Here, we integrate the effect of band tails on charge carrier distribution, transport and recombination into one narrative and deduce the consequences for solar cells.
To this end, we probe photoexcited charge carriers in a kesterite-semiconductor as a model system over their full life span from femtoseconds to microseconds by time-resolved terahertz spectroscopy and photoluminescence spectroscopy.
We find that occupation of the band tails is strongly temperature-dependent. Above a characteristic temperature charge carrier are activated out of Urbach band tails. Then charge carriers occupy dominantly shallow Gaussian band tails in the probed kesterite thin film. At room temperature we directly observe the thermalisation of charge carriers into these tail states within the first 2 ps. The occupation of these tail states reduces the quasi-Fermi level splitting and the open circuit voltage of a potential solar cell by ca. 80 meV.
The mobility of the charge carriers decreases to 144 cm²/Vs while they thermalize into the tail states. This mobility value is at least one order of magnitude lower than in a theoretical tail-free kesterite-semiconductor. Further, the mobility at terahertz frequencies indicates that charge carriers localize in tail states with a typical spatial extend of 11 nm.
The recombination kinetics of photoexcited charge carriers can be described within the Shockley-Read-Hall model of defect assisted recombination. Unambiguous signs of an influence of band tails are not measured at room temperature, but secondary effects from the affected carrier distribution and transport are discussed. The non-radiative recombination is found to be the major bottleneck for a potential solar cell and reduces the open circuit voltage of a potential solar cell by 270 meV.
However, below the characteristic temperature of 200 K charge carrier relax by multiple-trapping into the deep Urbach tail and lead to a strongly injection-dependent charge carrier distribution, transport and recombination as previously observed for amorphous semiconductors.

The absence of toxic and scarce elements makes kesterite (Cu₂ZnSnS₄) a promising inorganic thin film photovoltaic technology, but its performance advancement is hindered by a substantial voltage loss. This is commonly expressed as the open circuit voltage deficit (V_oc-deficit = E_g – V_oc) which requires bandgap determination. The band tails that characterize kesterite absorption spectrum make bandgap evaluation inaccurate and dependent on the method, thus Voc-deficit interpretation is unreliable for proper performance comparison. It was demonstrated by Yao et al. (Phys. Rev. Appl., 4, 1 (2015)) that from the reciprocity principle and the external quantum efficiency it is possible to measure voltage losses avoiding the bandgap estimation.

Here we present the analysis of the radiative open circuit voltage limit Voc,rad and the non-radiative voltage losses (ΔV_oc,nrad = V_oc,rad - V_oc) based on the external quantum efficiency (EQE) and electroluminescence (EL) measurements of a series of kesterite devices. As a result of Sn content variation from 27 at% to 32.2 at% the V_oc ranges from 423 to 465 mV, while the bandgap and Voc,rad increase of about 100 mV, thus ΔV_oc,nrad increments of about 60 mV. It is observed that for the device with the lowest Sn concentration of 27 at%, Voc and V_oc,rad were the lowest of the series whereas electroluminescence quantum yield (QE_LED) was the highest (5.2×10^-5 %). This indicates that the lowest photovoltage does not always correlate with a high non-radiative recombination in kesterite solar cells. This is also supported by temperature dependent current-density (JV-T) and capacitance-frequency (Cf-T) measurements confirming the Sn variation has no appreciable influence on the defect distribution.

It is concluded that QE_LED obtained by means of EL and EQE measurements is a suitable indicator of non-radiative losses for compositionally complex absorbers like kesterite.

Cadmium telluride (CdTe) photovoltaic device efficiencies show significant improvement with the incorporation of a cadmium selenium telluride (CdSeTe) alloy layer deposited between the magnesium zinc oxide (MgZnO) emitter and CdTe absorber layers. CdSeTe/CdTe devices are fabricated by close-space sublimation with a total absorber thickness of 1.5 µm. At this thickness, both the CdSeTe and CdTe layers are fully depleted. The incorporation of the CdSeTe layer is confirmed by microscopy measurements and shows minimal grading of Se into the CdTe layer. Loss analysis calculated from optical measurements shows that the CdSeTe layer acts as the primary absorber in the CdSeTe/CdTe structure, absorbing at least 90% of the photons. Current density-voltage measurements show an increase in current density of up to 3 mA/cm² with the addition of a CdSeTe layer that is due to a band gap shift from approximately 1.5 to 1.43 eV for CdTe and CdSeTe/CdTe absorbers respectively, where band gaps are determined by quantum efficiency and spectral photoluminescence measurements. Improvement in CdSeTe/CdTe device quality compared to CdTe is reflected in spectral photoluminescence measurements which show peak intensities many times greater when Se is incorporated into the absorber. The voltage deficit, calculated as the difference between the optimal absorber band gap and the measured open circuit voltage, shows improvement with the incorporation of the CdSeTe layer. This is corroborated by improved electroluminescence intensity, which is proportional to voltage deficit. The incorporation of CdSeTe as part of the absorber layer in CdTe device structures improves device current density and voltage deficit and has therefore increased device efficiencies by 2% absolute for CdSeTe/CdTe absorbers with a total thickness less than two microns.

Cu(In, Ga)Se₂ (CIGS) solar cells have been studied for space applications for a number of years due to their radiation tolerance and low cost. Most early work has studied CIGS on soda lime glass, which is too bulky for space applications, and would not allow for the complex folding arrays that have been proposed for future deep space missions. Here, commercially available flexible CIGS solar cells are investigated under conditions consistent with those around the outer planets. Performance under these low temperature low intensity (LILT) conditions is critical to study, as these conditions can reveal issues not present under normal operation [1]. Current density-voltage (J-V) and external quantum efficiency (EQE) measurements under different temperature, illumination, and bias conditions are performed to investigate the environmental effects of these deep space conditions. Of particular interest at low temperature is the behavior of the well-known V_Cu-V_Se metastable defect [2], and its potential role in device performance under LILT conditions. Here, evidence of a photo-activated barrier is observed, which is attributed to the same defect center, although its effect on power generation at LILT conditions appears to be minimal. Additionally, proton irradiation with high fluence levels is investigated to emulate certain conditions near Jupiter. Although degradation is observed, self-healing behavior under annealing conditions is also evident.

[1] D. A. Scheiman and D. B. Snyder, "Low intensity low temperature (LILT) measurements of state-of-the-art triple junction solar cells for space missions," in 2008 33rd IEEE Photovoltaic Specialists Conference, 2008, pp. 1-6.
[2] S. Lany and A. Zunger, "Light- and bias-induced metastabilities in Cu(In,Ga)Se₂ based solar cells caused by the (V_Se-V_Cu) vacancy complex," Journal of Applied Physics, vol. 100, no. 11, p. 113725, 2006/12/01 2006.

Real time spectroscopic ellipsometry (RTSE) measurements have been performed on copper indium diselenide (CuInSe₂) narrow bandgap solar cell materials. In these studies, thin film CuInSe₂ absorber structures have been deposited in one-stage and two-stage thermal co-evaporation processes on smooth c-Si wafer substrates held at temperatures in the range of 300-570 °C. Analysis of the RTSE data for one-stage CuInSe₂ yields the structural evolution and the complex dielectric functions (ε₁, ε₂) of the individual CuInSe₂ layers, the latter relevant for the elevated temperatures of deposition. In this analysis, information on the structural evolution, i.e. the surface roughness and underlying bulk layer thicknesses versus time, is obtained with monolayer-level sensitivity during thin film nucleation and coalescence. Complex dielectric functions (ε₁, ε₂) of deposited films as well as their bandgap energies and broadening parameters are obtained at different measurement temperatures applying the combination of RTSE and in situ SE measurements during sample cooling. All such deduced information on the structural and optical properties provides insights into CuInSe₂ film crystallinity and grain growth processes. In the two stage processes, CuInSe₂ is obtained by depositing either In₂Se₃ or Cu_2-xSe first, followed by exposure to either Cu+Se or In+Se evaporant, respectively. Real time SE has been applied to both stages of each CuInSe₂ deposition in order to compare the structural and optical property evolution characteristic of the two different routes for CuInSe₂ fabrication. The goal of this work is CuInSe₂ of optimum stoichiometry and grain structure obtained under RTSE monitoring and control for use as a bottom cell absorber material in tandem solar cell applications. Results correlating structural evolution and ultimate device performance will be reported.

Recently, Cu₂ZnSn(S,Se)₄ (CZTSSe) solar cell has attracted many attentions due to several advantages of CZTS(e) such as earth-abundant, cheap, nontoxic, high absorption coefficient (~10^-4 cm^-1) and tunable bandgap (1~1.5eV). [1-4] So far, the highest efficiency of CZTS(e) solar cell is 12.6% achieved by IBM team but its power conversion efficiency (PCE) still cannot compete with the commercial thin-film solar cells, e.g., CIGS and CdTe. There are numerous issues need to be solved to enhance the PCE (e.g., back contact losses, interfacial losses, deep defects). Especially, back contact losses cause degradation of CZTS(e) solar cell due to interface defects between CZTS(e) and MoS(e)₂ produced by sulfo-selenized Mo-coated soda lime glasses (SLG).
In this work, the PCE of CZTSSe solar cells were greatly improved by a simple but effective method, wherein a CdS nano-layer was deposited between the Mo substrate and the metallic precursors prior to CZTSSe sulfo-selenization, producing p+-CTS(e) and ZnS(e), which helps reduce interface recombination, on the bottom of the CZTSSe absorber layer. The mainly improvement results from increasing fill factor (FF) and short circuit current due to less recombination at back interfaces. Moreover, a 9.61% PCE (~10.6% in the cell effective area) CZTSSe cell with improved FF from 44% to 64% has been attained.
The morphology, elemental composition, and distribution of the absorber layers are being examined by X-ray diffraction (XRD), X-ray fluorescence spectrometry (XRF), scanning electron microscopy (SEM), Transmission electron microscope (TEM), and Raman spectroscopy.

References
[1] V. Tunuguntla, W.C. Chen, P.H. Shih, I. Shown, Y.R. Lin, C.H. Lee, J.S. Hwang, L.C. Chen and K.H. Chen, J. Mater. Chem. A, 2015,3, 15324-15330
[2] Y.R. Lin, V. Tunuguntla, S.Y. Wei, W.C. Chen, D. Wong, C.H. Lai, L.K. Liu, L.C. Chen and K.H. Chen, Nano Energy, 2015, 16, 438
[3] W.C. Chen, C.Y. Chen, V. Tunuguntla, S.H. Lu, C. Su, C.H. Lee, K.H. Chen and L.C. Chen, Nano Energy, 2016, 30, 762-770
[4] C.Y. Chen, B. S. Aprillia, W.C. Chen, Y.C. Teng, C.Y. Chiu, R.S. Chen, J.S. Hwang, K.H. Chen, and L. C. Chen, Nano Energy, 2018, 51, 597-603

Alkali metal (Na, K, Rb, and Cs) incorporation or post-deposition treatment (PDT) is known to be effective in improving the conversion efficiency of Cu(In,Ga)Se₂ (CIGSe) and Cu(InGa)(SSe)₂ thin-film solar cells. Among them, Cs has been known to boost V_OC and J_SC in CIGSe, and the world-record cell is also based on this technique. A new world-record CIGSSe efficiency of 22.9% has been achieved in the research center of Solar Frontier K.K.by CsF-PDT. However, CsF-PDT effects have been rarely studied, and therefore, it is worth to investigate the Cs incorporation.
This study reveals CsF-post-deposition treatment (PDT) effects on a Cu(InGa)Se₂ (CIGSe) cell in comparison with KF-PDT effects. After CIGSe growth, we performed three different alkali-PDTs (KF, CsF, and combined KF + CsF) on CIGSe thin films, and examined the material and cell properties. Both KF- and CsF-PDT CIGSe cells showed similar cell efficiency improvements (~17.5%) compared to the untreated CIGSe cell (~16%). However, the reasons for the improvement differed. The CIGSe cell with KF-PDT exhibited further reduction in the recombination at the CdS/buffer interface compared to the CsF-PDT CIGSe cell. On the other hand, the CIGSe cell with CsF-PDT exhibited further reduction in the recombination in bulk compared to the KF-PDT CIGSe cell. In addition, the CIGSe cell with combined alkali-PDT exhibited a largely enhanced conversion efficiency (18.4%) due to cooperative alkali incorporation effects. This device exhibited reduced recombination at both the CdS/CIGSe interface in the space charge region (SCR) and bulk compared to all other cells. Finally, we achieved a highly efficient combined alkali-PDT CIGSe device (20.15%) by applying a MgF₂ anti-reflective coating layer.

Reducing the thickness of Cu(In,Ga)Se₂ (CIGS) absorber layers has potential to decrease its cost significantly, but has drawbacks like incomplete absorption and increased back-contact recombination, both resulting in power conversion efficiency losses. One solution is to implement a rear surface passivation layer, which has potential to reduce rear surface recombination velocity and increase rear internal reflection [1]. Alumina (Al₂O₃) is such an ideal passivation layer, but unfortunately also acts as an electron and diffusion barrier layer, and thus prevents current flow and sodium (Na) diffusion. As is discussed below, our novel approach to generate point contact openings in this passivation layer overcomes both problems at once. The proposed method is to use sodium fluoride (NaF) on top of the Al₂O₃ passivation layer, which will generate contact openings during selenization. This applied approach is industrially viable, as compared to proven methods – e.g. using nanoparticles, e-beam or nano-imprint lithography – which are too expensive, time-consuming or not applicable for larger areas. D. Ledinek et al. previously proposed using a very thin layer of Al₂O₃ rear surface passivation in combination with NaF deposition to enhance the electrical characteristics of CIGS solar cells. In their study this surface passivation is claimed to allow tunneling [2]. In our study, we prove that point contacts have been generated in this Al₂O₃ surface passivation layer. Atomic layer deposition (ALD) was used to deposit very thin Al₂O₃ layers on molybdenum (Mo) rear contact, and NaF was deposited on these layers by spin coating. Thereafter CIGS layers (±500 nm thick) were grown by single stage co-evaporation at 550°C, followed by standard solar cell process described in [3]. To detect the contact openings in the passivation layer, scanning electron microscopy (SEM) combined with energy dispersive X-ray (EDX) spectroscopy was applied. Glass/Mo/Al₂O₃/NaF characterization samples were used for this analysis, where the co-evaporation process was mimicked by using a selenization step. After the selenization, point openings in the thin Al₂O₃ passivation layers were determined by SEM, and supported by EDX measurement. To investigate the impact of Al₂O₃ thickness on passivation of the CIGS rear surface, time resolved photo-luminescence (TR-PL) measurements on finished absorber layers (capped with CdS) were used. According to these TR-PL results, a 6 nm thick passivation layer gave the highest free charge carrier lifetime and PL response in all sets of samples. This was confirmed in full solar cell devices, where a significant increase in power conversion efficiency, and gain in open-circuit voltage, current density and fill-factor values was measured for the 6 nm Al₂O₃ rear passivated CIGS cells, as compared to unpassivated reference cells. Hence, it can be concluded that by using a simple, cost-effective and fast way, i.e. ALD for Al₂O₃ and spin coating for NaF, we succeeded to make point contact openings to passivate the back surface of ultra-thin CIGS solar cells. The ongoing work focuses on different aspects: (i) At present the point openings are still submicron size, now we try to make them nano-size. (ii) We also concentrate on controlling the density of the contact openings, e.g. by changing the temperature of ALD process or the molarity of NaF solution. (iii) This approach also enables us to investigate different metal oxides as passivation layers, such as HfO₂ and TiO_2, in combination with the same contact opening approach. (iv) And finally, we are investigating this rear surface passivation approach for other absorber layer deposition/growth techniques, e.g. a 2-step sputtering + selenization method.

[1] Poncelet, O. et al. DOI: 10.1016/j.solener.2017.03.001
[2] Ledinek, D. et al. DOI: 10.1016/j.solmat.2018.07.017
[3] Garud, S. et al. DOI: 10.1002/pssa.201700826.

Thin film CIGS solar cells have reached an efficiency of almost 23%. However, when going to modules the efficiency is max 19% but often less [1]. One reason for this large gap is that the process steps that are required to make the individual solar cells so efficient are not easily upscalable to module production. To resolve this, we are aiming to make the CIGS solar cells structure simpler and more cost effective. We will do this by decreasing the thickness to ~ 500 nm and using an ungraded absorber layer grown in a single stage co-evaporation step. This makes the process simpler, fast and requires less material. However, since the thin layers are rather different in their depth profile and grain size compared to the thicker 3-stage co-evaporated layers, re-investigation of some commonly used process steps like alkali addition is required. In this contribution we investigate the effect of KF post deposition treatment (KF-PDT) on our thin ungraded absorber layers.
To investigate the effect of KF-PDT, we use a Mo/Si(O,N)/glass substrate. The Si(O,N) layer functions as an alkali diffusion barrier and in this way the amount of alkali are controlled by adding them separately. The Na was added by evaporating NaF before CIGS growth and K was added by spincoating a KF solution onto the absorber layer and do a post anneal in a nitrogen atmosphere. While Se is commonly added during PDT to avoid Se losses, we found that this is not required for our thin absorber layers. The molarity of the KF solution, temperature and method of the anneal were varied and optimized further.
Absorber layers with and without KF-PDT were characterized with room temperature photoluminescence (RT-PL), time resolved photoluminescence (TR-PL), GDEOS, GI-XRD and SEM. Except for the alkali, the KF treatment had no effect on the depth profile. After KF treatment, the Na was virtually absent, while there was no K in the absorber layer before KF treatment. This has been shown earlier for 3-stage samples [2]. RT-PL however, did reveal significant changes. The intensity was up to 2 orders higher after KF treatment. This increase was used to predict an increase in Voc, which corresponded very well with the measured Voc increase. The spectra of KF treated absorbers were blue-shifted compared to the untreated absorber layers. That the emission can blue-shift is possible since the PL spectra of the single stage absorber layers is slightly below the band gap and coming from tail states. The measured blue-shift after KF treatment is then attributed to a reduction of tail states.
Finished solar cells were characterized with IV and CV measurements. An increase in Voc, FF and net acceptor concentration for the KF treated absorber layers were measured. However, when the initial doping was much higher, we found a decrease in net acceptor concentration after KF treatment. The Voc and FF still improved though. This shows that the net doping is determined by the amount of K and excludes Voc improvement due to increased net acceptor concentration only. It has been proposed as well that the KF treatment improves the pn junction quality [3]. To look more in detail at the pn junction, a few cells were illuminated with blue light. Here we found a clear distinction between KF treated and untreated cells. Without KF treatment the IV curve showed a kink at negative bias, while after KF treatment the kink was completely removed. This is a direct signature of an improved pn junction. At last, we determined J0s from the measured dark curves. Here we found an approximately one order decrease of the J0 for the KF treated cells. The overall efficiency has increased more than 2%, reaching the 10% for our 500 nm single stage processed solar cells. This is without any passivation or back reflector, showing the potential of the proposed process.

[1] Green et. al. DOI: 10.1002/pip.3040
[2] Pianezzi et. al. DOI 10.1002/pssr.201307238
[3] Chirila et. al. DOI: 10.1038/NMAT3789

Polycrystalline CdTe-based solar cells have reached efficiencies of over 22% in the recent years. Although current density values have been improved by incorporation of Se and elimination of CdS as a window layer, open circuit voltage (V_oc) is still well below its theoretical maximum. The road towards high V_ocand 25%-efficient devices requires a combination of low interface recombination velocity, higher lifetime, and higher carrier concentration in the CdTe absorber. It was recently demonstrated that the impact of the interface recombination on the performance of the device is more pronounced as carrier concentration and lifetime increase. Therefore, precise engineering of the conduction band alignment with CdTe absorber is needed at the front interface to reduce recombination. In this work we perform a combinatorial study of Mg_1-xZn_xO (MZO) buffer layer prepared by co-sputtering of Zn and Mg in oxygen-containing atmosphere. This allows to probe band gap variation of about 0.4 eV across a 2-inch substrate. In addition, the effects of MZO deposition temperatures on the device performance are studied. Introduction of MZO into the device architecture also allows for higher CdCl₂ treatment temperatures to be used to obtain larger CdTe grains compared to devices with CdS window layers, where delamination may occur at higher CdCl₂ treatment temperatures. Although larger grains and fewer grain boundaries may be favorable for carrier lifetime, this also requires modification of our standard Cu activation process that has been used with devices employing CdS window layer. Therefore, in this work we also study the effect of CdCl₂ treatment temperature on Cu activation using rapid thermal processing (RTP). Kelvin probe is used to determine the work function variation of MZO across the substrate for better understanding of band alignment with CdTe absorber. Device performance is evaluated using J-V characteristics.

Cu₂ZnSnS₄ (CZTS), a quaternary semiconductor, is an earth abundant and sustainable semiconductor material that is used as an absorber layer for thin film solar cells. In the present work, CZTS thin films were deposited onto soda lime glass (SLG) substrates by RF magnetron sputtering using the following stacking sequence CuS/ ZnS/ SnS at a substrate temperature of 300^oC. The samples were then sulphurised at 400⁰C in H₂S atmosphere for 60 min. A comparison of as-deposited and sulphurised CZTS thin films for their composition are discussed in this paper. The composition of the CZTS thin film samples were studied using X-ray photoelectron spectroscopy, which revealed the presence of Cu, Zn, Sn, S in required ratio. The presence of O and C was also detected due to atmospheric contamination. It is evident from the atomic % calculation that the stoichiometry has improved for the sulphurised samples. The formation of stoichiometric CZTS is an important factor to use it as an absorber layer in a thin film solar cell. The Ultra-violet photoelectron spectroscopy was used to study the ionization energy, electron affinity, electronic band gap of the material and the data were in relation to reported results. Thus, from the XPS results it is obvious that the sulphurised CZTS thin films can be used as absorber layer in a thin film solar cell.

We performed density functional theory calculations using CASTEP algorithm to understand the electronic strucure of MoS₂ layers over Silicon Oxides. Our findings, indicate a vertical alignment of MoS₂ layers as most geometrical stable and perturbation of density of states for d-orbital from semiconducting into a semi-metallic was found and it could be correlated to linear behavior of I-V experimental curves as measured by four-point probe on MoS₂/SiO₂ thin films (~100nm).

In the past decades many materials capable of photovoltaic solar energy conversion have been developed. Several of those materials are already commercialized, including monocrystalline and polycrystalline silicon, as well as several thin-film materials such as cadmium telluride (CdTe) and copper indium gallium arsenide (CIGS). The efficiency of these materials is still increasing and in some cases approaching the Shockley-Queisser limit. However, real-life outdoor illumination and climate conditions often lead to performance significantly below the performance under standard test conditions (AM1.5G, 1 kW/m², 25^oC). Many environmental factors such as varying solar spectrum, light intensity, and temperature influence the performance as is shown in previous work.^[1] Diffuseness of light is a parameter that is thought to have an influence on the performance of solar modules as well. In this work we present real-life performance data of several silicon and thin-film modules with data recorded from our solar field in Amsterdam, NL. The diffuseness is measured with a novel measurement tool that uses intrinsic shading for providing an effective and inexpensive way to measure how diffuse the sunlight is. We compare the data to predicted performance from simple diode model in order to determine the physical nature of performance losses and gains. We further use our modelling tool to predict real-life efficiencies of many photovoltaic materials and predict the important optimizing parameters in order to increase realistic performance in countries with climates similar to the Netherlands.

[1] Futscher, M. H., & Ehrler, B. (2017). Modeling the Performance Limitations and Prospects of Perovskite/Si Tandem Solar Cells under Realistic Operating Conditions. ACS Energy Letters, 2(9), 2089-2095.

The biggest difficulty for improving the power conversion efficiency of Cu₂ZnSn(S, Se)₄ (CZTSSe)/CdS/ZnO thin film solar cells is improving the open circuit voltage (V_OC). Probably leading causes of the V_OC deficit in CZTSSe devices have been identified as bulk recombination, band tails, and the intertwined effects of CZTSSe/CdS band offset, interface defects, and interface recombination. In this work, we replaced the CdS buffer layer by Mg-doped ZnO (Mg_.26Zn_.74O) and measure the interface of n-type Mg_.26Zn_.74O / p-type CZTSSe heterojunction grown by dual ion beam sputtering. The valence band offset at n-type Mg_.26Zn_.74O (MZ1) /p-CZTSSe heterojunction interface can be determined by the core-level photoemission-based method. Here, the calculation of was done by two methods: (1) considering different core level peaks of Se 3d and Mg 2p for CZTSSe and MZ1, respectively, and (2) considering same core-level peaks of Zn 3d for CZTSSe and MZ1. The values of valence band onset (VBOn), as obtained by linear extrapolation method for MZ1 and CZTSSe films are 2.97 and 0.54 eV, respectively. Figure. 1 (a) shows the core-level peaks of the MZ1/CZTSSe heterojunction. The core-level spectra of Zn 3d recorded on the MZ1/CZTSSe heterojunction is dominated by one symmetric peak, implying a uniform bonding state. Figure 1(b) shows transmission spectra using ellipsometry. Figure 1(c) shows the energy band diagram calculated using Ultra violet photoelectron spectroscopy (UPS).

In this work, we investigate the effects of heat, light and voltage bias on the metastable behavior of CIGS solar cells through accelerated stress tests (AST) on the order of days (50 - 100 hours) in accordance to the thin-film PV stabilization tests in IEC 61215-2 standards. A comprehensive analysis of devices with different alkali treatments using current-voltage (J-V), capacitance-voltage (C-V), quantum efficiency (QE), time-resolved photoluminescence (TRPL), and time-of-flight secondary ion mass spectroscopy (ToF-SIMS) to propose a preliminary mechanism for changes in device parameters with AST. Additionally, in-situ J-V characterization has been used to determine activation energies of change in V_oc to further understand the effect of Na incorporation on the underlying mechanisms.
Three sample types were prepared at ZSW for accelerated stress testing: Type A with baseline Na, Type B with intended reduced Na with the introduction of a sputtered AlO_x-AlN barrier to reduce Na diffusion from the substrate and Type C with baseline Na plus a RbF post-deposition treatment. The CIGSe layer was deposited by a coevaporation process and the CdS buffer layer was deposited by chemical bath deposition (CBD) according to standard procedures. The buffer-window stack for Types A, B and C samples was CBD CdS followed by sputtered undoped ZnO and Al-doped ZnO layers. The cells were completed with Ni/Al/Ni grid contacts and the cell separation was done by mechanical scribing. Three primary stress factors considered in this study are voltage (low and high internal field), illumination (light and dark), and temperature (room temperature to 85 C). Under illumination and open-circuit (OC) conditions, the photogenerated charge carriers must recombine entirely internally, resulting in a decrease in built-in field. However, under short-circuit (SC) condition, the photogenerated current is able to freely flow and the internal field is equivalent to the equilibrium field. For all stress tests, in-situ JV measurements were taken every 30 min at AST conditions in addition to before and after room temperature JV, CV and QE data collection. A select set of samples have also been characterized by TRPL and ToF-SIMS to understand the evolution of minority carrier lifetime and atomic concentration with stress.
An increase in V_oc upon light soaking under open-circuit stress condition suggests an increase in net doping density resulting in a space-charge region of reduced width. Capacitance-voltage data indicates an increase in shallow acceptor concentration and redistribution of shallow donors in the depletion region. Short-circuit stress results in decrease in V_oc along with a decrease in shallow donor density for Type A and Type B devices with activation energies of 1.46 eV and 0.78 eV, respectively. In contrast, V_oc increase is observed for RbF-treated devices under both open-circuit and short-circuit stress conditions. Our data indicates that alkali treatments may result in an increase in activation energy of the underlying mechanisms of change in V_oc. ToF-SIMS measurements indicate higher O levels after stress in CIGS samples prepared with Na barrier, and higher levels of Na for devices without barrier. At temperatures > 65 C, higher concentration of a deep acceptor defect 0.55 eV above valence band is observed. Experimental light- and heat-soaking results supported by compositional analyses and SCAPS-1d simulations will be discussed and potential mechanisms of performance change under stress will be proposed.

Part of this work was performed under the auspices of the U.S. DOE by LLNL under Contract DE-AC52-07NA27344.

Transparent back contact (TBC) CIGS solar cell is promising for high-efficiency solution to semi-transparent, bifacial, and tandem applications. Transparent conducting oxide (TCO) such as ITO, AZO, FTO has been explored as an alternative back contact for the TBC CIGS solar cell, but its use has not been successful enough to achieve high efficiency comparable to Mo back contact. The main hurdle is non-ohmic junction at TCO/CIGS derived from the low work function of TCO and the n-type semiconducting Ga-O phase formed at the interface. Despite the inherent problems, several literatures recently reported that the interface could be switched to electrically ohmic but without proper explanation.
In this work, we developed methods to control the electrical properties of ITO/CIGS interface in a way that carrier transport is easier. The temperature for depositing CIGS was reduced from 550 ^oC to 450 ^oC to avoid the detrimental Ga-O reaction at ITO/CIGS interface. Then the structural and electrical properties at the interface were investigated for thin or thick ITO (200nm, 600nm), ITO/Mo, and SiO_x/ITO, with particular focus on Na doping effect, which provided insights on its electrical nature, GaO_x properties and their correlation with Na engagements. Here, by controlling the Na doping scheme, we show that the hole transporting property at indium-tin oxide (ITO)/CIGS back contact can be substantially improved, regardless of the GaO_x formation. Na incorporation from the glass substrate during GaO_x forming phase created defective states at the interface which allowed efficient hole extraction from CIGS, while post Na treatment after GaO_x formation did not play such a role. Furthermore, we discovered that almost GaO_x–free interface could be made by reducing the underlying ITO film thickness, which revealed that ITO/CIGS junction is inherently Schottky. In the GaO_x-free condition, post Na treatment could eliminate the Schottky barrier and create ohmic junction due to generation of conducting paths at the interface, which is supported by our photoluminescence analysis. On the other hand, another solution, applicable to the 550 ^oC CIGS process, was developed to make ITO/CIGS interface more ohmic. It aims at suppressing the GaO_x formation or change its electrical properties by way of doping the external elements. It turned out to be effective in controlling the interfacial properties, reducing the gap in efficiency with Mo/CIGS cells.
On the basis of the understanding as elaborated above, we tried to optimize the fabrication process and achieved ITO/CIGS solar cell efficiency exceeding 16%, and furthermore to develop monolithic-integrated modules for the semi-transparent applications. For the latter work, laser scribing technology that is more available for high speed and precise patterning with controlled damage has been applied to electrically isolate the back contact (P1 scribing) and the top electrode (P3 scribing), and to provide the path connecting both electrodes (P2 scribing). In addition, laser technology has also been developed to remove the aperture area without shunt loss for light transmission. With respect to transparent PV application, it takes more advantage over the strategy of thinning the light absorber (here CIGS) in that it can decouple the light transmission from the light power conversion. For the first time, we demonstrated mini-module of ITO/CIGS solar cell without electrical loss due to laser damages, and then transparent mini-module with 30% aperture area removed. The results mean that the laser scribing can be successfully utilized without the electrical shunting loss to fabricate the TBC CIGS monolithic-integrated module.

A conversion efficiency up to 20% have been recently achieved on the thin film cells using epitaxial Cu(In, Ga)Se₂ (epi.-CIGSe) absorbers [1]. This high performance, large domain size and surface smoothness make the epi.-CIGSe one of the excellent platforms to investigate electronic structure and effect of post deposition treatment (PDT) on it. In this study, we have studied surface potential, surface photovoltage over epi-CIGSe, band alignment at CdS/epi.-CIGSe interface and effect of KF-PDT on them by Kelvin probe force microscopy, and photoemission and inverse photoemission spectroscopy.
Epi.-CIGSe absorbers composed of CuIn_0.3Ga_0.7Se₂ bottom- and CuIn_0.6Ga_0.4Se₂ top-layer grown on GaAs (001) substrate by molecular beam epitaxy were used as specimens. The cells using CdS buffer and Al-doped ZnO window with adoption of KF-PDT and light soaking showed a conversion efficiency of 20%. Details of preparation conditions were shown in the previous paper [1]. Surface photovoltage were determined from changes in surface potential induced by illumination of 685 nm light with an intensity about 10 mW/cm². CdS buffer was deposited onto the epi.-CIGSe by in-situ evaporation. All of the measurements were performed at a pressure of low 10^-8 Pa scale.
AFM measurements reveal that surfaces of the epi.-CIGSe consist of wide and atomically flat terraces and ridges with a height below few tens nm. The latter is parallel to anti-phase domain boundaries along [110] direction of the chalcopyrite phase. It is noteworthy that any edges of crystalline facets are hardly observed. Surface potential of the polycrystalline CIGSe films consists of two components and the width of each component is about 120 mV. These features are attributed to variations in crystalline planes and in surface terminations over poly-CIGSe. KF-PDT selectively suppressed the higher voltage-side component that should be attributed to anion-termination dominant facets. For the epi.-CIGSe without KF-PDT, widths of each component were less than 60 mV. KF-PDT on the epi.-CIGSe converts the double component feature to a single component one, and it drastically reduces the dispersion width down to 30 mV. These results reveal that the variation in orientations of crystalline planes and that in the surface terminations are dominant origins of the potential fluctuation over the CIGSe films. The junction between CdS and KF-treated epi.-CIGSe shows surface photovoltage above 550 mV. It exceeds an energy separation between conduction band minimum of the CIGSe and of the CdS. It means that conduction band alignment at this junction is so-called type-I, favorable for reducing interfacial recombination. These results reveal that epi.-CIGSe intrinsically has the tunable electronic structure for high performance, and that uniform feature of its surface is beneficial to reduce the open circuit voltage loss.
[1] J. Nishinaga, T. Nagai, T. Sugaya, H. Shibata and S. Niki, Appl. Phys. Express 11 (8), 082302 (2018).

The efficiency of Cu(In,Ga)(S,Se)₂-based (CIGSSe) thin-film solar cells has significantly increased over the last few years, both on the lab- as well as the module-scale. In order to achieve even higher efficiencies, larger open circuit voltages are desired, which can, in principle, be realized by increasing the absorber band gap, e.g., by increasing the Ga/(Ga+In) (GGI) and/or S/(S+Se) (SSSe) ratio of the absorber. Traditionally, the AVANCIS GmbH absorber exhibits a high SSSe and no Ga at the surface. Recently, the GGI at the absorber surface has also been increased. Consequently, it becomes an area of interest whether and how the band alignment of the buffer/absorber interface is modified by this approach. Hence, we have conducted a detailed study of the chemical and electronic structure of this interface.
In this contribution, we present results for the heterojunction between a sputtered Zn(O,S) buffer and the CIGSSe absorber using a unique combination of spectroscopies, namely x-ray and UV photoelectron spectroscopy (XPS and UPS), x-ray emission spectroscopy (XES), and inverse photoemission spectroscopy (IPES). We find a significant increase of the absorber band gap at the surface for increasing GGI and SSSe ratios. Furthermore, our results paint a detailed picture of the electronic structure at the Zn(O,S)/CIGSSe interface, which will be discussed in view of the observed efficiency improvements and previously published results obtained for similar heterojunctions with different preparation processes and parameters.

Recently solar cells based on non-toxic and Earth-abundant materials, for example heterojunction solar cells based on kesterite crystal structure, are required in order to renewably produce energy on the multi-terawatt scale. However, cell performances based on kesterite are low compared with those of CIGS and CdTe; hence we need to improve cell performances based on kesterite. Here, it is well-known that band alignment of the p/n heterointerface plays an important role for the high performance heterojunction solar cells. In this study, we have studied band alignment of CdS/Cu₂Zn(Sn_1-xGe_x)Se₄(CZTGSe) with Ge composition x from 0 to1 by in situ X-ray photoelectron spectroscopy, ultraviolet photo-, and inverse photoemission spectroscopies (XPS, UPS, and IPES).
The p-type light absorbers composed of CZTGSe thin films are deposited by the co-evaporation technique on Mo coated soda lime glass substrate. After KCN treatment and thermal annealing, these samples were treated by the aqueous NH₃ solution in order to remove the absorbed carbon and oxygen from CZTGSe surface in Ar circulated glovebox. To evaluate the band alignment of CdS/CZTGSe heterointerface, n-typed CdS buffer was deposited onto the CZTGSe by thermal evaporation technique. He(I_a) and Al(K_a) lines were used as the light source in the UPS measurements. All of the measurements (XPS, UPS, IPES) and CdS depositions were performed in ultra-high vacuum condition (pressure < low 10^-8 Pa) without air exposure.
The UPS and IPES reveal the Fermi-levels are located near the center of the bandgap of CZTGSe with x from 0 to 1. Although this hole concentration is much smaller than that (~10¹⁶ cm^-3) in the CZTGSe bulk, laser-irradiated XPS revealed that the origin of the hole deficiency near the surface is due to defects distributed closer to CZTGSe surface than in the bulk, rather than Fermi-level pinning at the CZTGSe surface. Moreover, we determined the conduction and valence band offset (CBO and VBO) at the CdS/CZTGSe heterointerface relative to Ge composition x by using XPS, UPS, and IPES measurements by stepwise CdS deposition (e.g. CdS film thickness = 0, 3, 6, 30 nm). The CBOs decreased from +0.36 to -0.01 with increasing of x. A positive CBO indicates that the position of the conduction band minimum (CBM) of the CdS is higher than that of CZTGSe, which is so-called “spike” conduction band alignment whereas the band alignment is a flat in x = 1. In contrast, the VBOs remain constant and are around 1.1 eV. We revealed these CBO and VBO relative to Ge composition can be explained by the theoretical calculation. In the case of high performance CIGS heterojunction solar cells, the CBO at the CdS/CIGS heterointerface have CBOs from 0 to ~0.30 eV. Concerning it, we conclude CBOs at the CdS/CZTGSe heterointerface with x from 0 to 1 are significant adequate value for the heterojunction solar cells in a view point of electronic structure at the interface. Despite of it, cell performances of our CdS/CZTGSe heterojunction solar cells drastically decreased with increasing of x over 0.2. We will discuss the relationship between band alignment of CdS/ CZTGSe and solar cell performances.

Among the alternatives for CdS buffer layers in Cu(In,Ga)Se₂ (CIGSe)-based thin-film solar cells, In_xS_y is a very promising candidate, leading to cell efficiencies very close to, or sometimes even above, CIGSe devices with a chemical bath-deposited (CBD) CdS buffer layer. Additionally, increasing the Ga/(Ga+In) (GGI) ratio allows increasing the bulk band gap of the absorber, which in turn is expected to lead to higher open-circuit voltages. To optimize the design for both, GGI ratio and alternative buffer layer, an important prerequisite is a detailed knowledge of the electronic and chemical structure of CIGSe absorber surfaces with varying GGI ratios, their interfaces with CBD-In_xS_y buffer layers, and the chemical and electronic properties of the final (i.e., full-thickness) buffer layer. In this contribution, laboratory-based photoelectron spectroscopies (XPS, UPS, IPES), x-ray excited Auger electron spectroscopy (XAES), as well as synchrotron-based soft x-ray emission spectroscopy (XES) are used to acquire this information. With these complementary spectroscopic methods, important insights into, e.g., elementary ratios at the surface of the investigated absorbers, intermixing processes at the interface, buffer layer growth properties, and characteristic species in the buffer layer, are gained. The results of these measurements allow us to discuss the chemical and electronic structure of the CBD-In_xS_y/CIGSe interface for different GGI’s.

An assessment of the intrinsic electronic properties of chalcopyrite surfaces is crucial to understand the characteristics of the window/absorber interface and related post-deposition treatments in chalcopyrite solar cells. However, a basic understanding of the involved processes is complicated by surface oxidation, granular inhomogeneities and grain boundaries. To gain access to the fundamental electronic properties of chalcopyrite surfaces, we focus on epitaxially grown CuInSe₂ (epi-CISe), which forms the energetically most favorable {112} facets upon slight Cu depletion.² Scanning tunneling spectroscopy (STS) is a valuable tool in assessing whether chalcopyrite surfaces are electronically defective or passivated.³ Here, we carry out STS measurements on epi-CISe surfaces along with supplementary surface-sensitive techniques such as X-ray, ultraviolet and inverse photoelectron spectroscopies (XPS/UPS/IPES). In this way, the local defect density is correlated with the energy level alignment and dipole effects at the surface upon various annealing and wet-chemical treatments relevant for device fabrication.
We compare the initial (as-grown) condition of epi-CISe samples (without any air exposure) to the states after oxidation in air, etching in NH_3- or KCN, as well as subsequent annealing in ultra-high vacuum (UHV). In the as-grown condition, the sample surface is found to be intrinsically passivated with a low defect level density. Based on this, a consistent determination of the surface band gap by STS and UPS/IPES reveals a value of 1.65 eV in accordance with density functional theory calculations reported earlier.⁴ Furthermore, our measurements provide evidence for the absence of any local or global surface dipole(s). These findings are in line with previous theoretical models predicting that the intrinsic dipole(s) of the polar facets in CISe are compensated by shallow surface defects.^3-5However, our STS data do not show any spectral signatures of the corresponding defect levels, possibly due to their shallow nature. In contrast, upon oxidation in air, an overall high surface defect level density in conjunction with a considerable net-surface dipole associated with a potential drop of ~ 0.7 eV at the surface is observed. This surface dipole is accompanied by an upward band bending that shifts the surface Fermi level position from mid-gap (for the as-grown state) to p-type conditions. The high defect level density together with the dipole layer and the upward band bending persist even after removal of the surface oxides by wet-chemical NH_3-- or KCN etching. Yet, a subsequent annealing in UHV restores the initial sample condition, as concluded from the absence of surface dipoles and passivation of surface defect levels. Our results shed new light on previous reports on lateral inhomogeneities on polycrystalline chalcopyrite surfaces^3,6, emphasizing the relevance of oxidation-induced dipole effects on chalcopyrite surfaces for the window/absorber interface design in chalcopyrite solar cells.

[1] Solar Frontier. Solar Frontier Achieves World Record Thin-Film Solar Cell Efficiency of 22.9%. Press Release (2017).
[2] Siebentritt, S. et al., Appl. Phys. Lett. 88, 2005–2007 (2006).
[3] Bröker, S. et al., ACS Appl. Mater. Interf. 7, 13062–13072 (2015).
[4] Hinuma, Y. et al., Phys. Rev. B 86, 1–7 (2012).
[5] Jaffe, J. E. et al., Phys. Rev. B 64, 241304 (2001).
[6] Mönig, H. et al., Adv. Mater. Interf. 1, 1–7 (2014).

CIGSe solar cells are known to exhibit strong metastable behaviour and, in consequence, lot of experiments do not follow textbook interpretations. To name only two most basic measurements - light current voltage characteristics, and hence the parameters which determine the efficiency, as well as free carrier concentrations revealed by capacitance profiling, depend very strongly on the direction of the voltage sweep and/or the history of the sample. Therefore, sooner or later, everyone dealing with electro-optical characterization of CIGSe solar cells experiences a kind of frustration, and metastable behaviour is often seen as an obstacle to understanding and progress.
In this presentation we will show that, if analyzed with care, metastabilities can also serve as a valuable tool for investigation of CIGSe-based devices. We will begin with a proposition of new classification of metastabilities. Then, we will critically review the theoretical models proposed to explain some of the observed phenomena with a special emphasis on their importance for the interpretation of experimental results. Finally, we will present how the analysis of metastable behaviour can shed some light on the role of sodium, potassium, and the deposition of different buffer layers.

A world record for Cu(In,Ga)(S,Se)₂ (CIGSSe) cells efficiency of 22.9% has been achieved recently by Solar Frontier K.K. using Cesium treatment (Cs treatment). A common explanation for the high efficiency of the cells after alkali metal treatment is that processing passivates defects at the p-type CIGS absorber surface or grain boundaries. It is also known that the high performances of CIGSSe treated with alkali metal are linked to an increase of the open circuit voltage (V_OC) [1]. However, the underlying physics of this effect has yet to be fully understood. To analyse the recombination mechanisms, we investigated the open current voltage at different excitation wavelengths and intensities of CIGSSe solar cells treated with potassium treatment (K-treatment). Based on the dependence of the CuInSe absorption coefficient with wavelength, low energy radiation will penetrate deeper in the alloy than high energy one. Thus, the variations in wavelength and intensity open the possibility to study the recombination at different depths in the cells. Looking at V_OC evolution, we can deduce the recombination rate of the CIGSSe as a function of the depth. The results of such experiment show that the untreated samples V_OC dependence with excitation intensity also presents a dependence with the excitation wavelength, while the K treated samples does not have such a behaviour. This evidence a defect level in the absorber of the untreated sample which obstruct the collection of the carriers in the bulk. Using SCAPS-1D device modelling software, with a double grading structure, we investigated the recombination rate in the solar cell as a function of the depth. We evidence two different regions of recombination, in the bulk and in the depletion region. We show that the amount of recombination in each of the region depends both on the excitation wavelength as well as on the excitation intensity. The simulation was also done for different defect density, which shows that for a low defect density, recombination occurs only in the bulk, while for high defect density, recombination also occurs in the depletion layer. This creates a loss for the carrier collection, reducing the solar cell efficiency. The increase of the V_OC in K-treated sample is then well explained by suppressing defect levels in the absorber layer.

[1] K. L. Chopra et al., Prog. Photovolt. 12, 69-92 (2004).

Copper indium gallium disulfide Cu(In,Ga)S₂ or CIGS is a promising absorber material for thin film solar cells due to its direct band gap energy that can be tuned from 1.5 eV (pure CIS) to 2.4 eV (pure CGS). It is suitable to make single- and multi-junction solar cells. In fact, it could be used as top cell in a tandem configuration with Si or CIGSe as bottom cell.
However, Cu(In,Ga)S₂-based solar cells still suffer from a low efficiency compared to their counterpart Cu(In,Ga)Se₂-based solar cells, that have reached a record conversion efficiency of 22.9%. This is mainly due to a low open circuit voltage.
Recently, a new fundamental breakthrough came when Hiroi et al. reached the world record efficiency of 15.5% with pure sulfide CIGS solar cell^[1] and open circuit voltage up to 973 mV, using processing temperatures above those used traditionally (>550°C).
Nevertheless, there is a lack of knowledge regarding the recombination processes and intrinsic defects for this chalcopyrite material. In fact, a deeper understanding is needed to better control the electronic properties, therefore to further improve the performance of devices based on this absorber material.

In this work, we present a detailed photoluminescence (PL) investigation of polycrystalline CIS thin film and we compare the results to those reported in literature.
The sulfide CIS absorbers were deposited on molybdenum coated soda lime glass in a PVD system under copper excess (Cu-rich) in a single stage coevaporation using a deposition temperature of 650 °C. The secondary phase CuS was removed by KCN etching, reaching a Cu over In ratio of 1.
The low temperature PL measurements were carried out with a 663 nm wavelength laser as excitation source using a liquid Helium flow cryostat. Moreover, temperature dependent admittance measurement (ADM) were carried out on both finished devices and aluminium coated absorbers to form a Schottky contact.
The PL spectra recorded at 10 K are dominated by excitonic luminescence together with defect related peaks at 1.39 eV, 1.35 eV, 1.31 eV and 1.27 eV.
The excitation dependent measuremets over seven order of magnitude reveal that the 1.39 eV is a donor-acceptor transition (DA) with an energy shift of the peak position of 3 meV/decade. The same shift has been found for the other transitions at lower energy.
The three peaks at 1.35 eV, 1.31 eV and 1.27 eV well match the Poisson distribution as phonon replica of the main peak (1.39 eV) with a Huang-Rhys factor of 0.45 and a phonon energy of about 40 meV. We therefore attribute this series of peaks to a DA transition with its phonon-replica.
In order to get the energy position of the defects involved in the DA transition, temperature dependent PL has been performed. We found that at 90 K the luminescence clearly becomes a free to bound transition (FB), with the shallower defect ionized. From its peak position, we extracted a defect energy of 140 meV. This value is in good agreement with the activation energy of the main step of the capacitance measurements on both devices and Schottky contact.

We do show that the 140 meV defect is a bulk defect and not an interface defect as reported in some previous works. In addition, we attribute the 1.35 eV peak to the first phonon replica of the main DA transition, together with second and third one clearly visible. This is in contradiction with previous reported studies that generally explain the 1.35 eV transition as DA with a deeper donor involved, because the further phonon replica could not be detected.

[1] H. Hiroi, Y. Iwata, K. Horiguchi, and H. Sugimoto, IEEE J. of Photovoltaics, 6, 309 (2016).

The V_OC of kesterite solar cells is limited by high levels of non-radiative recombination, which reduces the quasi-Fermi level splitting (QFLS) in a semiconductor. The QFLS is the maximum achievable V_OC that a semiconductor can generate in a solar cell and it can be determined from the external photoluminescence quantum yield (EQE_PL). We evaluate the V_OC deficit by comparing the calculated QFLS to their theoretical maximum V_OC (in the absence of non-radiative recombination). The use of quantitative PL for calculating the V_OC deficit enables the possibility to use it in bare absorbers, powders or single crystals. This allows for differentiation of non-radiative recombination losses introduced by other layers of a solar cells stack or by the presence of grain boundaries.
We use hyperspectral quantitative PL imaging to characterize several kesterite type materials in the form of thin films, powders and single crystals. One of the recent strategies tested to overcome the V_OC deficit limitation of kesterites has been the partial replacement of Cu, Zn and Sn cations by other elements. We also analyzed the effect in the QFLS of the incorporation of several elements in kesterite-based photovoltaic materials, including Li and Ge in high efficiency solar cells and Ag in CZTSe powders. We observe that the EQE_PL increases by the incorporation of Ag and Li in CZTSSe. Based on our results we propose some recommendations to further reduce non-radiative recombination and the V_OC deficit in kesterite solar cells by cationic substitution.

Recent studies have shown that correct conduction band alignment at the emitter/absorber interface in photovoltaic devices leads to reduced interface recombination and higher device performance. Mg_xZn_1-xO (MZO) is an interesting choice for the emitter because (1) it is a wide bandgap material that will not absorb photon and (2) the conduction band can be controlled by changing the Mg concentration. Consequently, MZO is being incorporated into CdTe devices. Unfortunately, few groups have demonstrated the improved device performance when using MZO alone. This suggests that the band alignment is not correct or new interface defects are being introduced. Here we investigate the carrier lifetime of sputtered CdTe devices using steady-state and time resolved photoluminescence spectroscopy (TRPL). Our preliminary results show that carrier lifetime increases as the bandgap of the MZO increases from 3.4 eV to 3.7 eV. We attribute the reduction of interfacial recombination to improved band bending in the CdTe as a function of better band alignment between MZO and CdTe. We will extend our study to determine if the conduction band alignment alone is responsible for reduced recombination by varying the MZO dopant density. We will show that TRPL can be a quick tool to determine if appropriate band alignment and bending have been achieved.

Point defects play an important role in determining a wide variety of materials properties and detailed analyses of their role often necessitates the evaluation of their formation energies and transition levels. Generally, formation of point defects involves displacing few atoms from their ideal positions meaning that defect formation is very localized process that involves very few degrees of freedom of a system. In this talk, I will illustrate how machine learning techniques can be used to predict formation energies and transition levels of native defects in III-V and II-VI semiconductors with a quantifiable uncertainty using a set of simple descriptors, like branch point energy and dangling bond energies. These descriptors can capture local neighborhood information as well as structural information and can be evaluated easily from ab initio simulations. Our analysis opens avenues for predicting the stability of defects in complex materials and alloys by training a coarse-grained model which can significantly simplify experimental design and material synthesis.

This work was performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory under Contract DE-AC52-07NA27344.

Impurities and defects in the band gap are critically important in photovoltaic (PV) materials, including thin film chalcogenides, as recombination centers, dopants, or passivants. Predicting formation energies and charge transition levels for defects in these PV materials is thus paramount; density functional theory (DFT) calculations have been widely applied for such studies. However, the requirement of large supercells and inclusion of charged states make these computations very expensive, and trends and knowledge from previous calculations are not exploited in subsequent ones. In this work, we use DFT to generate a substantial computational dataset of the formation energies and transition levels of vacancy, interstitial and substitutional point defects in Cd-based pure and alloyed chalcogenide systems (CdX, where X is one or a combination of S, Se, Te). The hybrid HSE06 functional is used for these calculations to correct for the band gap and band edge miscalculation that is characteristic of the standard PBE functional for this class of materials. We then apply machine learning techniques to extract crucial design rules and predictive models from the data, via the intermediate process of numerically representing any defect in terms of easily calculated structural and electronic properties of defect atoms, their known compounds, and using information from cheaper, lower-fidelity calculations. Using correlation analysis and regression techniques, we obtain accurate predictive models for formation energy and every relevant transition level that require only a handful of descriptors as input. These models are used to make predictions for defect energy levels in thousands of unexplored cases, such as Cd(Se,Te) which is of current interest, thus expanding our knowledge to wide areas of the chalcogenide chemical space. The machine learning strategy used here can lead to a framework for accelerated prediction of defect properties and allow efficient materials design for performance improvements in Cd-chalcogenides as well as other PV materials.

Cadmium telluride (CdTe) solar cells have been researched for decades and through this time very few key technological step changes have been critical to its success: I. the superstrate process that forms a buried passivated junction, II. the annealing in a chlorine containing atmosphere, III. a stable copper back-contact, and IV. the introduction of selenium to grade the absorber structure. First Solar’s thin-film CdTe solar cells have been certified at 22% cell efficiency. The presentation will review our understanding of the performance limiting mechanisms and opportunities to further improve efficiency. Desirable improvements often conflict with established processes and material choices and this places significant constraints on the development of new technology components. One of these improvements, is to replace the p-type dopant in the absorber layer with a dopant from the 5^th column of the periodic table. Over the last two years, First Solar has made steady progress on realizing a Group-V doping in polycrystalline CdTe and the first milestone was the consistent demonstration of greater than 10¹⁶ cm^-3 hole concentration with a flat doping profile and narrow depletion width. It has been challenging to produce the desired improved Voc with an alternative dopant and we will review evidence that may point to a direction on how to overcome the limitations.
First Solar’s leading thin-film manufacturing capabilities are in process of ramping the new utility-power optimized Series 6 module. Series 6 modules will have power ratings up to 440 Watts with an optimized design to lower installation and other balance of system cost. It will preserve First Solar existing CdTe module power density advantage superior temperature coefficient, spectra and shading response.

As CIGS PV technology has matured to production on an industrial scale, its long-term stability becomes increasingly important: The electric yield and thus the revenue of a PV system depend on both the initial conversion efficiency as well as its development over time. Increasing this long-term stability by understanding and lowering the degradation of CIGS technology is therefore a key strategy for its market success. Furthermore, increasing the long-term stability of individual solar cells allows to lowering the demands and thus the cost of barrier materials necessary for flexible CIGS modules. Literature has revealed that large differences in the stability of CIGS modules in the field have been observed. Therefore it is important to identify, predict and prevent degradation mechanisms.
An overview of degradation mechanisms occurring in CIGS solar cells and modules (observed in the laboratory and field experiments as obtained from literature) will be presented.

A first reason for the long term performance loss of CIGS solar cells and module is water ingress. Tests have shown that humidity can cause the increase of sheet resistance of the ZnO:Al front contact, while it can also deteriorate the interconnection conductivity, especially in the P2. Both effects can lead to an increased series resistance. Methods to mitigate the degradation of the ZnO:Al front contact on cell level are thicker layers, higher deposition temperatures, post deposition annealing, higher carrier concentrations and smoother underlayer substrates. The use of alternative TCOs like ITO and IO:H can also improve reliability. Moreover, the presence of humidity combined with a bias voltage can lead sodium migration, which can cause severe shunting. Strong effect of the sodium content on this degradation mechanism has been observed. Fortunately, most sources reveal that exposure to only elevated temperatures and illumination do not harm, or even improve solar cell performance.

Another degradation mechanism occurring in the field is power loss due to partial shading of CIGS modules. If one or several cells in a module are covered by e.g. bird dropping, humans, animals or trees, a reversal of the power flow can occur in the shaded area. In a module cells are connected in series, due to this series connection a current is forced through the shaded cells, resulting in a reversed power flow over the shaded cells, often referred to as negative or reversed bias. It was observed that very short shading events of seconds could lead to degradation of the modules.
Modules degraded due to partial shade show very typical symptoms, which are: Reduction of FF due to reduced I_MPP/I_sc ratio; Small dark spots in electroluminescence due to localized shunts corresponding with worm-like defects, visible by naked eye; Non-permanent effects were also observed: the electrical performance can either reduce and increase over time after exposure to reverse bias. This effect seems strongly dependent on the module type.
The reverse characteristics can be modified by a change in buffer layer composition and thickness, although the relation with degradation has up till now not been made.

Another degradation mechanisms that mostly has been observed in the field is Potential-Induced Degradation (PID). This is caused by the exposure of modules in the (negative) end of a string to voltages as high as 1000 V. PID has been identified on CIGS modules by a combination of electroluminescence and IV measurements. PID is also a common defect for crystalline silicon modules, where it is associated with sodium migration, mostly from the front glass. For CIGS, the sodium could originate from the substrate glass.

Cu(In,Ga)(S,Se)₂ (CIGSSe) based thin-film solar cells have recently achieved the outstanding performance of 22.9% efficiency at the laboratory scale. In contrast, the best performance of CIGSSe commercial modules does not exceed 17.5%. Such a difference is partially inherent to the actual architecture of the modules (P1, P2, P3 patternings), leading to a decreased active area, an increased series resistance as well as optical losses due to the decreased sheet resistance of the transparent conductive oxide (e.g. ZnO:Al). To overcome these limitations, we propose a modified architecture of modules where the cells are interconnected via metallic grids. This monolithic approach consists in using Ni/Al/Ni grids to electrically connect the rear contact of the cell N to the top window layer of the cell N+1. Independently of absorber band gap energy, this architecture allows (i) increasing the width of the cells, thereby decreasing the surface ratio of inactive area and (ii) decreasing the thickness of the ZnO:Al layer without resistive losses, thereby minimize the optical losses.
In the present study, standard cells of 0.5cm² and alternative modules composed of 3 cells connected in series are investigated. The most impressive result concerns the FF achieved with the alternative module, which reaches the value of 81 %, yielding an active area efficiency of 17.2%. The V_OC of the alternative module is 2.05 V (i.e. 684 mV per cell), which is close but still lower than the V_OC of the standard small-area cell (702 mV). In order to further explore the potential origins of such lowered V_OC/cell (mini-module) relative to that of small-area cells, additional characterizations (SEM, Raman, XPS, XRD) were performed and led to the following hypotheses. As all of the layers composing the devices (i.e. small cell and mini-module) were grown simultaneously, the parameters potentially explaining the observed lowered V_OC/cell of the mini-module are (i) the influence of Mo patterning, (ii) the influence of Mo/MoSe₂/grid contacts, (iii) the influence of the modified properties of the Cu(In,Ga)Se₂ grown on soda-lime glass (within the etched Mo zones). Each of these hypotheses will be detailed and discussed during the presentation of this work.
The photovoltaic performance demonstrated thanks to this alternative architecture opens new routes to drastically decrease the gap observed between small-area-cells and industrial modules.

Reducing the gap between module and cell efficiency is one of the most important topics for CIGS module manufacturing. Efficiency records on cell level are > 20%, where most commercial modules are in the range between 14 and 18% efficiency. In all types of CIGS modules that are based on an insulating substrate (glass, polyimide etc.) a monolithic interconnection is used to separate and series connect cells. These cell separations and interconnections of cells out of a full area CIGS substrate are made using scribes or ’patterning steps’, that selectively remove material by means of either a mechanical needle or a laser. Typical cell sizes are around 5 mm due to relatively high sheet resistance of the transparent conductive oxide (TCO). The area that is needed for the monolithic interconnection does not contribute to the output of the module and is referred to as the dead zone. The size of the dead zone is typically in the range of ~350 um for commercial modules. The ratio between this dead zone and the small cell size is one of the factors that play a large role in the cell to module losses.
The selective removal steps or scribes that are present in the monolithic interconnection can be made using laser scribing (P1, P2) or mechanical patterning (P2, P3). Laser scribing has the advantage of precision and lesser peripheral damage as compared to mechanical scribe. Industrially also laser scribing is preferred due to wear of the needle in mechanical patterning. Higher yield and smaller dead area in the interconnection make that successful laser scribing is important for improvement of the module efficiency.

In the monolithic interconnection the P1 scribe separates the metal back contact and is typically done with a laser. The P2 creates the connection between the bottom and top contact of neighboring cells and both laser as well as mechanical scribes are used. The P3 that isolates the top contact (TCO) of the cells is typically made using a mechanical scribe. Laser scribing of the P3 however is challenging because CIGS is very thermally sensitive. It has been shown that laser patterning of CIGS induces formation of conductive species. These conductive species create shunts in the scribe that are detrimental to the efficiency [1].

In this work we show the use of Raman spectroscopy for identification of shunt-free P3 scribe using a pulsed laser. Ablation of the TCO is the minimum requirement to form the P3, but removal of CIGS is an added advantage since CIGS has lateral conductivity. Melting of CIGS which leads to formation of conductive species needs to be avoided for good isolation in the P3.

Material removal by a typical laser scribing process involves rapid heating, melting and evaporation material. The thermal sensitivity of the CIGS can results in molten scribe edges, redeposition of conductive material and compositional changes within the layer next to the scribe. Different laser parameters were systematically studied for pulsed infrared laser scribing of CIGS including pulse duration, pulse overlap and laser fluency. Raman spectroscopy in conjunction with SEM, EDX and lock-in thermography was used to identify compositional changes in the area of the scribes.

In our research we have shown that Raman spectroscopy is a very suitable technique to monitor melting and shunt-free scribing in CIGS. We will explain our approach of using Raman line-scans over the laser scribe and the use of a contour plot. This approach allows for clear differentiation between shunted versus shunt-free scribes looking at the broadening of the main 177 cm-1 CIGS peak and the appearance of a CIS peak at 290 cm-1. Finally we show that shunt-free laser scribing of a P3 in CIGS is possible using a pulsed infrared laser.

[1] C. Schultz, G.A.F. Basulto, S. Ring, C. Wolf, R. Schlatmann, and B. Stegemann, IEEE Journal of Photovoltaics, 2017, 7(5), 1442–1449

Solar cells based on Cu(In,Ga)Se2 (CIGS) present efficiency of up to 22.9%1 and is highest among other thin film photovoltaics, but considering Shockley-Queisser limit (33.7%) there is still a big room for improvement. Impressively, over the years, its maximum efficiency is achieved in its polycrystalline form, even with its small grain sizes of 2 μm. Previous studies have shown an important role of all grain boundaries (GBs) 2–4 towards its output efficiency. Here using electron beam induced current (EBIC) technique we found that most of the GBs are indeed beneficial for the device, however a small percentage (15%) of GBs was found detrimental. Hence, in this work we have isolated these detrimental GBs and perform correlative EBIC-EBSD and atom probe tomography (APT) on them to identify its type, structure and composition as shown in Figure 1. These measurements are also performed on beneficial and neutral boundaries. Results show that detrimental GBs are mainly enriched in Cu and O and depleted in Ga. Whereas characteristics of a beneficial GB are Cu depletion, no O and no change in Ga. With sufficient statistics, we have obtained characteristics of ‘detrimental’ GBs5, which could be avoided in future by variation in deposition parameters and post deposition treatments.
References:
1 https://www.pv-magazine.com/2017/12/20/solar-frontier-hits-new-thin-film-cell-efficiency-record/
2 C. Persson and A. Zunger, Phys. Rev. Lett. 91, 266401 (2003).
3 O. Cojocaru-Mirédin, T. Schwarz, and D. Abou-Ras, Scripta Materialia (2017).
4 M. Raghuwanshi, E. Cadel, P. Pareige, S. Duguay, F. Couzinie-Devy, L. Arzel, and N. Barreau, Applied Physics Letters 105, 13902 (2014).

Graded bandgap concepts have been shown to be a disruptive issue in order to achieve high efficiency devices in thin film chalcogenide photovoltaics. In the case of kesterite (Cu₂ZnSn(S_xSe_(1-x))₄ or CZTSSe, in short) based solar cells, two main strategies are being explored in order to achieve bandgap grading: cationic substitution (Cu by Ag, Zn by Cd, Sn by Ge and Si) and anionic (S-Se) compositional engineering. Both the cationic and anionic substitutions are complementary as they primarily affect either the conduction (cation) or valence (anion) bands. The specific control of the anionic composition along the volume of kesterite absorbers has been found challenging due to the similar formation energy of selenide and sulfide compounds that favors S-Se solid solutions over phase segregation. This work presents the development of a novel and non-toxic reactive annealing process that allows the synthesis of CZTSSe solar cells with S-Se sharp graded compositional profiles and sheds light onto the dynamics of anionic bandgap grading in kesterite solar cells. This by employing different S sources (pure S > CH₄N₂S > SeS₂ > SnS, ordered by expected reactivity), trying to control perfectly the S and Se content of the final CZTSSe layers and to form an S-enriched top surface. After an optimization of the annealing parameters, we demonstrate the feasibility of a graded composition CZTSSe phase with high S content at the top by introducing pure S during the cooling down process of the CZTSe kesterite. Multi-wavelength depth-resolved Raman spectroscopy measurements allowed to characterize the complex graded structure and estimate the composition of the CZTSSe layers. A bandgap > 1.20 eV within the depletion zone and < 1.03 eV towards the back contact interface is obtained this way. These results are correlated with in-depth Auger Electron Spectroscopy (AES), Grazing Incidence X-Ray Diffraction (GIXRD) and X-ray fluorescence (XRF), confirming the possibility of achieving a complex graded anionic compositional, and therefore, graded bandgap profiles. The fabricated solar cell devices show conversion efficiencies > 9 %, with a Voc deficit of 420 mV, which currently represent a promising result for a non-optimized graded absorber.
In summary, preliminary results in which the samples are first selenized and then, in-situ superficially sulfurized, show the possibility of creating an inverted grading profile with higher bandgap towards the surface of the absorber by using pure elemental S source. We will show that obtaining S-Se graded bandgap profiles (both at the front and back interfaces) is only possible when the S is introduced in an out-of-equilibrium state. Strategies for achieving more complex graded bandgap profiles will be discussed including effects on device performance using theoretical analysis by SCAPS modeling.

Up to date, kesterite has drawn a significant attention as an absorber material for future photovoltaic applications. Recent research aims at understanding the nature of this quaternary material and intends to improve the device efficiency further. In this work, we performed a compositional study on CZTSe kesterites grown by a two-step process. Films are processed by sputtering of precursors containing elemental Zn and alloyed CuSn layers, and an annealing procedure at 530 °C under selenium atmosphere, additionally placing elemental Sn within the susceptor. We distinguish two mechanisms for composition control and demonstrate their tuning for optimization of the absorber fabrication process.
A direct composition control can be achieved in the classical manner by varying the sputtering duration for each layer of the stack. Another indirect control parameter is the tuning of the selenium amount: This is found to impact the final composition of the absorber by i) stabilizing the formation reaction of CZTSe, ii) hindering possible SnSe loss from the CZTSe absorber at high temperatures, and iii) reacting with the elemental Sn in the selenization chamber to SnSe_y assisting for the incorporation of additional Sn into the film via SnSe_y vapour. Each of these three mechanisms appears to be influenced by the amount of Se available in the chamber.
Elemental analysis of the grown absorbers shows a variation of the Cu/Zn and Zn/Sn ratios in the range of ≈0.9-1.6 and ≈1.1-2.1 respectively. Solar cell results demonstrate that the studied system is highly tolerable for the compositional variations, showing device efficiencies between 9 and 11% even at significantly off-stoichiometric compositions. By fine-tuning the discussed parameters, improved solar cells were obtained reaching a V_oc of 476 mV for pure selenide kesterite and device efficiencies around 12%.

II-IV-V₂ semiconductors such as ZnSnN₂ and ZnSiP₂ offer the promise of optoelectronic properties on par with the III-Vs combined with the tunability of complex multinary materials like CIGS, due to their ternary composition and the ability to control cation order parameter. For example, ZnSnN₂ has been predicted to exhibit band gap tunability from 1-1.5 eV based on variation in cation disorder, which arises mainly from local violations of the octet rule, with band gap narrowing occurring when N-centered motifs deviate from their ideal 2Zn + 2Sn coordination. Other II-IV-V₂ materials such as ZnSiP₂, ZnGeP₂, and ZnGeN₂ exhibit similar phenomena. The nitride materials of this family exhibit defect tolerant behavior and are photoactive as polycrystalline thin films, while the phosphides are lattice-matched to Si, and can be grown epitaxially for tandem photovoltaic applications. We have grown thin films of several nitride and phosphide members of this family and have observed strong photoluminescence behavior, promising energy conversion properties in a photoelectrochemical configuration, and tunable carrier concentration. This talk will focus on experimental methods for synthesis and characterization of these materials (including combinatorial methods), evidence of their potential for photovoltaic applications, and calculations that provide a physical understanding of disorder and its impact on relevant properties.

Copper Indium Gallium Diselenide (CIGS) photovoltaic devices usually exhibit variations in device performance when exposed to heat, voltage bias, and/or light. The degree of such variations and their reversibility depends on fabrication processes and impurity concentrations. Typically, light soaking at elevated temperatures increases the apparent doping concentration and open circuit voltage while heating in the dark tends to reverse those gains. Although the underlying mechanism is not clear, this metastable behavior is often attributed to transitions of the copper-selenium (V_Se-V_Cu) divacancy defect between donor and acceptor states. A better understanding of metastability could improve the utility of test standards and suggest pathways to increase power conversion efficiency.

In this work, physical models of metastability are quantitatively evaluated by comparison to current density-voltage (JV) data collected in-situ during light soak at various temperatures and applied bias. Our time-dependent JV data at different stress conditions provide a unique platform for testing mechanistic hypotheses. Additional insight is gained by comparing CIGS devices with either CdS or Zn(O,S) buffer layers. Metastability mechanisms based on defect kinetics associated with lattice relaxations are coupled with semiconductor device simulations to predict how JV, quantum efficiency (QE), and capacitance voltage (CV) characteristics vary with time under specific stress conditions. The evaluated defect kinetics models include the (V_Se-V_Cu) divacancy complex with the previously calculated activation energies as well as more generic models of charge-induced shallow or deep defects. Ab-initio calculations are also conducted to elucidate possible chemical species and lattice relaxation parameters.

Our results suggest that the (V_Se-V_Cu) divacancy model alone does not explain much of the observed metastability data due to the low transition energy barrier from the donor to acceptor state. Calculations with the generic defect models suggest that the data can be described by the formation of shallow acceptors upon electron capture with an activation energy close to 1 eV. In some cases, the formation of deep recombination centers with similar activation energy must also be evoked to account for losses in open circuit voltage during light soaking at higher temperatures. In general, these types of mechanisms are associated with phenomena such as persistent photoconductivity and self-compensation when a system is sufficiently flexible to readjust in response to a shift away from equilibrium. In the case of metastability, it appears that electron injection in CIGS causes the system to respond by creating defects that reduce the electron concentration. This work quantifies the rates and magnitudes of the response.

Part of this work was conducted under the auspices of the U.S. DOE by LLNL under Contract DE-AC52-07NA27344.

Both group-V element doping and Cd-rich composition are important techniques for overcoming the open-circuit voltage deficiency in CdTe photovoltaic device by improvements of high p-type doping and long minority carrier lifetime. However, group-V doping simultaneously involves the formation of AX compensating donor like defect which results in low doping activation. We present a comprehensive experimental approach to reveal equilibrium As doping limit of a few times 10¹⁷ cm^-3 in Cd-rich CdTe by very gentle cooling process. As-doped Cd-cirh CdTe single crystal with high hole concentration > 10¹⁷ cm^-3 and long bulk lifetime > 20 ns can be obtained. We demonstrate CdTe photovoltaic device with high open-circuit voltage of 900 mV by As-doped Cd-rich bulk single crystal as absorbing layer. These results indicate that group-V doping and Cd-rich composition enable the improvement of open-circuit voltage deficiency in CdTe photovoltaic device.

Cu(In,Ga)Se₂ (CIGS)-based solar cell have yielded the highest light-to-electrical energy conversion efficiency of all thin-film solar cells. To achieve a high conversion efficiency, incorporation of alkali metal (AM) impurities in CIGS has become critical. In addition, light induced and voltage-bias induced metastabilities could further enhance the performance of p-type CIGS by increasing hole concentration. However, the relationship between AM impurities and positive light/voltage-bias induced metastabilities is not well understood and remains largely unexplored. It has been experimentally observed that a large amount of AM impurities segregate at grain boundaries and the absorber/buffer interface, but they can diffuse into the grain depending on the growth conditions or over long aging times. Therefore, analysis of AM impurities both in the grain and at the grain boundaries are important to elucidate their role on metastabilities.
In this work, we performed first principles calculations with a hybrid functional to explore effects of AM impurities on electronic properties of CuInSe₂(CIS), particularly on light-induced metastabilities. First, we calculated binding interaction between AM and divacancy complexes (V_Se–V_Cu), which has been speculated to exhibit metastable properties in CIGS. We found that formation of (V_Se–AM_Cu) complexes, in which AM is located at the V_Cu site, is energetically favorable. The binding energy of the (V_Se–AM_Cu) complexes with respect to the (V_Se–V_Cu) and AM_Cu increases in order of increasing atomic size of AM (Na < K < Rb). Similar to the (V_Se–V_Cu) complex, the (V_Se–AM_Cu) complexes also have different donor and acceptor configurations, and conversion between these two stable configurations can occur by overcoming energy barriers accompanied by accepting electrons/releasing holes. Our calculations predict that the required energy for the forward reaction (donor to acceptor conversion) is 0.16, 0.42, and 0.60 eV eV for the (V_Se–Na_Cu), (V_Se–K_Cu), and (V_Se–Rb_Cu), respectively, which is larger than 0.05 eV for the (V_Se–V_Cu). This suggests that formation of the metastable acceptor configuration would be largely suppressed for the (V_Se–AM_Cu) complexes with heavier AM.
Second, we examined geometries, energetics, and electronic structures of AM_Cu and its associated defect complexes at grain boundaries. Among many possible grain boundaries, we constructed (112)Σ3 twin boundaries (TB), which are commonly observed in CIGS. Three different (112)Σ3 twin boundaries were found with different Se local coordination environment at the boundary; for the most stable TB, all Se atoms at the TB are regularly coordinated with 2 Cu and 2 In, while for the least stable TB, Se atoms at the TB are coordinated with either 3 Cu and 1 In or 1 Cu and 3 In. Our calculations demonstrate that the formation energy of AM_Cu decreases at all three TB considered in this work with respect to that in the bulk, while the amount of decrease is larger for the TB containing more irregularly coordinated Se atoms at the boundary. Also, we found that heavier AM has stronger tendency (Na < K < Rb) to segregate at the grain boundaries, which agree well with the experimental observations demonstrating that K or Rb locates at the grain boundaries by kicking out Na at the grain boundaries. Furthermore, AM_Cu interaction with other point defects and defect complexes will be discussed to understand more comprehensive effects of AM impurities at the grain boundaries.

Light-induced enhancement of the performance as a result of an increase of the open-circuit voltage and fill factor is widely observed in Cu(In,Ga)Se₂ (CIGS) solar cells with Zn-compound buffer layers [1,2]. However, the reason for the improved efficiency is still under discussion. The present study was conducted in order to investigate the microscopic effects of light-soaking (LS) in Cu(In,Ga)Se2 solar cells with solution-grown CdS and Zn(O,S) buffer layers. Electron-beam induced current (EBIC) measurements were carried out before and after the LS under AM 1.5G illumination, as well as after keeping samples in the dark. In addition, cathodoluminescence and energy-dispersive X-ray spectrometry analyses were performed in order to gain insight into possible effects on the local radiative recombination and the elemental distribution. By evaluating EBIC profiles extracted perpendicular to the p-n junction, values for the widths of the space-charge regions, which are (roughly) proportional to N_A^-0.5 [3], and for the diffusion lengths of minority charge carriers were determined [4]. It is revealed that for solar cells with both, CdS/CIGS and Zn(O,S)/CIGS heterojunctions, the diffusion length increases significantly already after 30 min LS treatment. The doping density slightly increases during the LS process. In addition, the influence of electron-beam irradiation and heat-light soaking was investigated.

1. T. Kobayashi, H. Yamaguchi, T. Nakada, “Effects of combined heat and light-soaking on device performance of Cu(In,Ga)Se2 solar cells with ZnS(O,OH) buffer layer”, Prog. Photovolt.: Res. Appl. 22, pp. 115-121 (2014).
2. W. Witte, D. Hariskos, M. Powalla, “Comparison of charge distributions in CIGS thin-film solar cells with ZnS/(Zn,Mg)O and CdS/i-ZnO buffers”, Thin Solid Films 519, pp.7549-7551 (2011).
3. R. Scheer and H.-W. Schock, “Chalcogenide Photovoltaics: Physics, Technologies, and Thin Film Devices” (Wiley-VCH, Weinheim, 2011).
4. C. Donolato, “Evaluation of diffusion lengths and surface recombination velocities from electron beam induced current scans”, Appl. Phys. Lett. 43(1), pp. 120-122 (1989).