Andriy Zakutayev, National Renewable Energy Laboratory
David O. Scanlon, University College London
Talia Gershon, IBM T.J. Watson Research Center
Symposium Support Aldrich Materials Science
IBM T.J. Watson Research Center
National Renewable Energy Laboratory
E3: PV Research Strategies
Tuesday PM, April 22, 2014
Westin, 3rd Floor, Franciscan I
2:30 AM - E3.01
Optimal Sunlight Harvesting in Photovoltaics and Photosynthesis
Marco Bernardi 1 Jeffrey C Grossman 2
1University of California, Berkeley Berkeley USA2Massachusetts Institute of Technology Cambridge USAShow Abstract
Materials employed to harvest sunlight are commonly recognized to be at a premium when their optical absorption peaks in the visible, extends to the infra-red, is panchromatic, and matched to the solar spectrum. By contrast, natural photosynthetic absorbers such as chlorophylls and carotenoids display absorption spectra with narrow peaks for yet unknown evolutionary reasons. Beyond such general observations, a rigorous treatment of sunlight harvesting optimization is still lacking.
In this talk, we provide a quantitative analysis of optimal solar energy harvesting in materials. We show a procedure to derive optimal absorption spectra for sunlight harvesting as a function of absorber thickness, and elucidate the concept of solar-matched absorption and its applicability limits. We demonstrate that the shape of the optimal absorption spectrum depends on the thickness and geometry of the absorber, and provide optimal absorption spectra for all thickness regimes. In addition, we define a procedure to rank photovoltaic materials for sunlight harvesting. We close our talk with a possible explanation of why absorption in plants photosynthetic pigments occurs in narrow energy windows.
2:45 AM - *E3.02
Rapid Development of Novel Photovoltaic Materials: The Computational Component
Stephan Lany 1
1Natl. Renewable Energy Lab. Golden USAShow Abstract
Novel, earth abundant materials are receiving increasing research interest as potential solution to the terawatt challenge. However, serious efforts to develop a photovoltaic technology based on alternative materials will require a solid proof of concept. The aim of the rapid development project is to accelerate the process from materials discovery, design, and device integration, by combining materials theory and combinatorial synthesis, characterization, and device fabrication. The role of theory is to (i) provide an initial assessment of basic photovoltaic properties of a candidate material, (ii) to explore novel materials structures and compositions, and (iii) to aid the device integration by addressing the issues related to the absorber/contact combination. This presentation will review recent computational studies in support of the rapid development, including Cu-Sn-S compounds, SnS-based alloys, and potential oxide photovoltaic materials systems.
3:15 AM - E3.03
Layered A2BBrsquo;O6 Perovskite Oxides with Low Band Gaps and Highly Dispersive Band Edges
Hungru Chen 1 Naoto Umezawa 1 2 3
1National institute for materials science, Japan Tsukuba, Ibaraki Japan2Japan Science and Technology Agency (JST) Saitama Japan3TU-NIMS Joint Research Center, Tianjin University Tianjin ChinaShow Abstract
The search for new materials for efficient solar energy conversion is one of the most active research areas globally. A material for such purpose should fulfill two basic requirements: it should be visible light sensitive and the carrier mobility should be high in the material. Many oxide materials with cations having a d0 or d10 electronic configuration can be rendered conductive with high carrier mobility but they do not absorb visible light due to large band gaps, e.g. transparent conducing oxides. In contrast, oxide materials with cations having partially filled d electrons can absorb visible light but the mobility of photo-generated carriers in these materials is rather low due to the localized character of d orbitals (flat bands). It is therefore desirable to combine the merits of the two categories of materials.
By hybrid density functional theory calculations, it is shown that materials with low band gaps and highly dispersive band edges can be realized in layered A2BB&’O6 perovskite oxides by utilizing the two dimensional character of the t2g conduction bands. La2MnTiO6, La2NiTiO6, Sr2FeNbO6 with alternating BO2 and B&’O2 layers along the  direction exhibit band gaps smaller than 2 eV. The valence band edge in each compound is composed of states from the element with partially filled d states, i.e. Mn2+, Ni2+ or Fe3+, whereas the conduction band edge is composed of highly dispersive unoccupied t2g states of the d0 element, i.e. Ti4+ or Nb5+. This means their band gap excitations are B → B&’ charge transfer transitions which are expected to result in spatially separated electron and hole pairs that migrate in separate layers. Furthermore, we predict that these layered A2BB&’O6 pervoskite can be synthesized under high pressure.
This work was partly supported by the Japan Science and Technology Agency (JST) Precursory Research for Embryonic Science and Technology (PRESTO) program.
3:30 AM - *E3.04
Accelerating Performance Improvements in Earth-Abundant Thin-Film Photovoltaic Devices
Riley E. Brandt 1 Rupak Chakraborty 1 Danny Chua 2 Katy Hartman 1 Rachel Heasley 2 Jasmin Hofstetter 1 Rafael Jaramillo 1 Jonathan Mailoa 1 Yun Seog Lee 1 Sang Woon Lee 2 Michael Lloyd 1 Jeremy Poindexter 1 Alex Polizzotti 1 Sin Cheng Siah 1 Vera Steinmann 1 Leizhi Sun 2 Roy G. Gordon 2 Tonio Buonassisi 1
1Massachusetts Institute of Technology Cambridge USA2Harvard University Cambridge USAShow Abstract
In light of the dramatic price reduction of crystalline silicon photovoltaic modules and the pressing need to address anthropogenic emissions, there is a renewed emphasis on accelerating efficiency increases in candidate Earth-abundant PV materials. These novel materials may serve as top-cell materials for silicon-based tandems, or as single-layer thin-film absorbers.
During the past three years, we have investigated electrochemically deposited cuprous oxide (Cu2O) and PVD/pulsed-CVD-deposited tin sulfide (SnS) for photovoltaic applications. With the benefit of hindsight, we highlight the critical investigations that identified and successfully engineered performance-limiting mechanisms in Cu2O and SnS-based devices, resulting in efficiency improvements from ~1.3% range to ~4.5% and beyond. The parallel evolution of these two material systems suggests certain elements of a common framework to improve the device efficiencies of novel PV materials.
Bulk engineering: We discuss efforts to improve bulk collection length, including approaches for bulk carrier-collection length measurements, effects of deposition conditions and post-growth annealing on collected current, bulk impurity assessment, and efforts to simulate point-defect evolution using kinetic models.
Surface engineering: We review the effect of interface recombination on Voc, the role and properties of native oxides, and grain-orientation specific properties in polycrystalline materials.
Device engineering: Often underappreciated in materials investigations, it is often necessary to resolve baseline device loss mechanisms before "unmasking" more detailed materials science. We review the effects of series and shunt resistance, optical interference, efforts to improve device reproducibility, and the role of advanced device characterization techniques coupled with simulation.
4:00 AM - E3.05
A New Allotrope of Silicon with a Quasidirect Band Gap for Solar Energy Conversion Technologies
Stevce Stefanoski 1 Duck Young Kim 1 Oleksandr Kurakevych 2 Timothy A Strobel 1
1Carnegie Institution of Washington Washington USA2IMPMC Universitamp;#233; P. amp; M. Curie Paris FranceShow Abstract
Silicon is the second most abundant element in the earth&’s crust, existing naturally within various oxygen-rich minerals. In addition to its low cost and abundance, silicon maintains a native oxide layer and is readily dopable by other elements. These advantages help silicon-based technologies to maintain a significant market share of photovoltaic devices, despite fundamental light absorption limitations arising from the indirect nature of the band gap. We report on the synthesis of a new low-density, orthorhombic allotrope of silicon, Si24. Si24 was formed by thermal “degassing” of sodium from a Na4Si24 precursor that was formed at high pressure. Si24 contains nano-porous channels along the crystallographic a-axis that are formed from six- and eight-membered sp3 silicon rings, which facilitates the sodium removal when the material is subjected to thermal degassing. Powder X-ray diffraction and energy dispersive spectroscopy analyses confirmed the high purity of the new phase. Si24 is dynamically stable at ambient pressure and is energetically more favorable than known metastable silicon allotropes. Using Density Functional Theory and quasiparticle (G0W0) calculations we estimate a direct band gap of 1.34 eV and an indirect band gap 1.30 eV. These calculations were corroborated by electrical resistivity and optical reflectivity measurements. This quasidirect band gap near 1.3 eV is close to the theoretically suggested optimal value for solar-energy conversion applications, namely the Shockley-Queisser limit. Si24 therefore shows potential for cost-effective and high-efficiency solar energy-conversion technologies.
E4: PV Interfaces
Tuesday PM, April 22, 2014
Westin, 3rd Floor, Franciscan I
4:30 AM - *E4.01
Heterojunction Interface Design for Earth Abundant Photovoltaic Devices
Harry A. Atwater 1
1California Institute of Technology Pasadena USAShow Abstract
Heterostructure interfaces can play a decisive role in the optoelectronic performance of earth abundant semiconductor absorbers in thin film photovoltaic applications. I will discuss examples from cuprous oxide, zinc phosphide photovoltaic, and Zn-IV nitride semiconductor heterostructures. We examine the dramatic dependence of open circuit voltage on interface stoichiometry at zinc oxide/cuprous oxide interfaces, and the effect of heterostructure design on current transport interfaces between zinc phosphide and ZnS, ZnSe, ZnO and CdS. Approaches to and prospects for low recombination activity interfaces and carrier selective contacts in earth abundant materials will be discussed.
5:00 AM - E4.02
Atomic Layer Deposited Ga2O3 Buffer Layers for Higher-Efficiency Cu2O Thin-Film Solar Cells
Yun Seog Lee 1 Danny Chua 2 Riley E Brandt 1 Sin Cheng Siah 1 Sang Woon Lee 2 Jonathan P Mailoa 1 Roy G Gordon 2 Tonio Buonassisi 1
1Massachusetts Institute of Technology Cambridge USA2Harvard University Cambridge USAShow Abstract
Thin-film solar cells using Earth-abundant materials are promising for renewable energy applications due to their low material cost and usage compatible with terawatt-level deployment, and inexpensive manufacturing potential. Cuprous oxide (Cu2O) is a p-type Earth-abundant semiconductor with over 20 % single-junction theoretical maximum power conversion efficiency. In particular, its large bandgap of 2 eV is suitable for a top-cell in tandem solar cells. However, due to non-ideal band alignments and interfacial defects, the power conversion efficiency of Cu2O-based heterojunction devices has remained low. The non-ideal heterojunction results in a significant energy loss by interface recombination, reducing the open circuit-voltage.
In this contribution, we increased the efficiency of Cu2O-based thin film solar cells to over 4 % by atomic layer deposition (ALD) of Ga2O3 buffer layers. We designed and synthesized ~10-nm-thick buffer layers that mitigated the interfacial recombination problem by improving band-alignments across the heterojunction. The buffer layers were deposited by ALD on electrochemically-deposited Cu2O layers prior to ALD of ZnO:Al. The buffer layer growth conditions were also engineered to minimize the density of Cu2+-related defects at the buffer layer/Cu2O interface. The band-alignment and the heterojunction quality were characterized by x-ray photoelectron spectroscopy and capacitance-frequency (C-f) measurements, respectively. The Ga2O3 buffer layer showed reduced conduction-band offset relative to the Cu2O layer, comparing with that of the amorphous zinc-tin-oxide buffer layer we used previously. We studied the device impacts of the buffer layer by measuring the J-V characteristics and quantum efficiency. We report an enhanced efficiency by increased open-circuit voltage exceeding 0.9 V
 Y. S. Lee et al., Energy Environ. Sci., 6, 2112-2118 (2013)
5:15 AM - E4.03
Improving SnS/Zn(O,S) Solar Cell Performance Through an Oxidized Passivation Layer at the SnS Surface
Helen Hejin Park 1 Rachel Heasley 1 Leizhi Sun 1 Danny Chua 1 Chuanxi Yang 1 Vera Steinmann 2 Katy Hartman 2 Rafael Jaramillo 2 Rupak Chakraborty 2 Riley E. Brandt 2 Tonio Buonassisi 2 Roy G. Gordon 1
1Harvard University Cambridge USA2Massachusetts Institute of Technology Cambridge USAShow Abstract
Thin film solar cells consisting of earth-abundant and non-toxic materials were made from atomic layer deposition (ALD) of Zn(O,S) as the n-type buffer layer, and pulsed chemical vapor deposition (pulsed-CVD) of SnS as the p-type absorber layer using an inexpensive Sn precursor. A thin oxidized passivation layer at the SnS/Zn(O,S) interface significantly improves the open-circuit voltage above 300 mV and fill factor above 50%, and record power conversion efficiencies over 3% were achieved. Solar cell performance is further optimized by adjusting the oxygen to sulfur ratio of Zn(O,S) [1,2,3] and by in situ nitrogen doping using ammonium hydroxide to serve as both oxygen and nitrogen sources.  H. H. Park, R. Heasley, and R. G. Gordon, Appl. Phys. Lett.102, 132110 (2013).  P. Sinsermsuksakul, K. Hartman, S. Kim, J. Heo, L. Sun, H. H. Park, R. Chakraborty, T. Buonassisi, and R. G. Gordon, Appl. Phys. Lett.102, 053901 (2013).  L. Sun, R. Haight, P. Sinsermsuksakul, H. H. Park, and R. G. Gordon, Appl. Phys. Lett.103, 181904 (2013).
5:30 AM - *E4.04
Design Principles of Advanced Thin Film Solar Cells Deduced from a Comparison of Established and Novel Absorber Materials
Wolfram Jaegermann 1
1TU Darmstadt Darmstadt GermanyShow Abstract
Thin film solar cells based on CdTe and CIGS have reached solar to power conversion efficiencies in the range of 20% after an extended development time. However, due to the criticality of some of the involved chemical elements there is a demand for alternative materials solution with similar conversion potentials. But so far most of the suggested alternatives are strongly limited in their performance. We will try with this contribution to deduce design principles for novel thin solar cells based on a systematic comparison of established and novel absorber materials.
Basis of this comparison are p-i-n structures of highly absorbing compound semiconductors which from a theoretical point of view are promising device structures for thin film solar cells. At first the device properties of CdTe and CIGS solar cells will be analysed with respect to the needs for obtaining high conversion efficiencies. As two probably most challenging duties we have identified the control of nucleation and growth also at low substrate temperatures as well as the engineering of interface properties. We will compare typical results which are considered to govern the success of CIGS and CdTe to the properties which have been obtained in the past for FeS2 and layered semiconductors like WS2. Recently we have also investigated novel absorber layers as e. g. SnS and previously studied layers of Cu2S. First results on the preparation and characterisation of thin films and simple devices of the materials will be presented and discussed.
It seems evident that in most cases the bulk properties of the deposited materials are already very promising when higher annealing temperatures can be applied avoiding a detrimental restructuring of the crystallites in the film forming pin-holes or grain boundary shunts which in principle could lead to reasonable conversion efficiencies. However, in most cases the contact materials are not appropriate and lead to only low built-in potentials due to the Fermi level pinning. For the electronic passivation of surface/interface states specifically engineered interfaces are needed as can be deduced from a comparion to successful devices.
E5: Poster Session: Earth-Abundant PV III
Tuesday PM, April 22, 2014
Marriott Marquis, Yerba Buena Level, Salons 8-9
9:00 AM - E5.02
Formation of Large-Grained Polycrystalline Cu2ZnSn(SxSe1-x)4 Thin Films through Annealing of Colloidal Nanocrystal Coatings
Boris D. Chernomordik 1 Amelie E. Beland 1 Donna D. Deng 1 Anne K. Hunter 1 Priyanka Ketkar 1 Eray S. Aydil 1
1University of Minnesota Minneapolis USAShow Abstract
A potentially low cost method for making Cu2ZnSnS4 (CZTS) or Cu2ZnSnSe4 (CZTSe) absorber layers for solar cells is annealing of films cast from colloidal dispersions of CZTS nanocrystals (NCs). Annealing such films in sulfur or selenium can transform them into polycrystalline thin films with micrometer size grains. Realization of inexpensive and high-efficiency CZTS and CZTSe solar cells via this approach requires a fundamental understanding of the effects of key annealing parameters on the evolution of film microstructure. To this end, we have studied the extent to which parameters such as S and Se vapor pressures, presence of alkali metal impurities, carbon content in the NC coating, annealing time, and annealing temperature affect the CZTS microstructure evolution. Films were cast from CZTS NC dispersions on substrates such as quartz, soda lime glass (SLG), Mo-coated quartz and Mo foil. NC films were than annealed in evacuated and sealed quartz ampoules containing known charges of solid sulfur and sometimes trace amounts of Na. The microstructure of annealed CZTS films is determined by three competing processes: densification by sintering of NCs, coarsening of NC grains by normal grain growth and surface energy driven abnormal grain growth. The NCs within the film grow and sinter, while abnormal grains nucleate on top of the nanocrystalline film and grow as large as 10 microns. Abnormal grain growth is driven by the high surface energy of the nanocrystals. The abnormal growth is enhanced at high temperatures, while the normal grain growth within the nanocrystalline film is more pronounced at high S vapor pressures and in the presence of alkali metals such as Na and K. Alkali metals exist as intentionally added impurities in SLG substrates and can diffuse into the CZTS film during annealing. Alternatively, controlled amounts of Na can be introduced into the annealing ampoule and eventually into the CZTS film via vapor phase transport. Either way, the presence of Na enhances grain growth. Larger grains are obtained on SLG than on quartz. The microstructure of the films annealed on SLG could be reproduced on quartz when Na is introduced into the CZTS films via the vapor phase. When the film is annealed in Se, its microstructure is different than that obtained when it is annealed in S. Annealing films in selenium can lead to the formation of a continuous layer of 2-5 µm size CZTSSe grains on quartz. The degree of Se incorporation depends on the annealing temperature and pressure. Annealing in Se also leads to the formation of a C and Se rich layer at the CZTS film-substrate interface. This segregation of C at the CZTSSe-substrate interface is commonly ascribed to the immediate formation of a capping/blocking layer of CZTSSe grains. However, we found that a continuous layer of CZTSSe grains is not necessary to observe C segregation to the film-substrate interface. In contrast, films annealed with S do not show such distinct C-rich layers.
9:00 AM - E5.06
Plasma-Assisted Molecular Beam Epitaxial Synthesis of Tailored Cu2O/ZnS/ZnO Heterojunction Interfaces
Yulia Tolstova 1 Samantha S. Wilson 1 Jeffrey P. Bosco 1 Harry A. Atwater 1
1California Institute of Technology Pasadena USAShow Abstract
Cu2O is an earth abundant semiconductor that has been identified as a promising photovoltaic material due to its high absorption and long minority carrier diffusion length. Cu2O is intrinsically p-type and thus requires a heterojunction partner to enable charge separation, which introduces the possibility of increased interface defects. Cu2O devices are highly susceptible to deleterious interface reactions which impede high cell performance. The highest reported photovoltaic efficiency for a Cu2O absorber device is 5.38%, whereas the detailed balance limit has been calculated to exceed 20%. One of the main reasons for low efficiency is instability of the Cu2O surface with respect to oxidation or reduction.
ZnS/Cu2O heterojunctions exhibit Type I band alignment and a clean stoichiometric interface can be achieved between Cu2O and ZnS by sputtering. However difficulties in doping ZnS as well as a large conduction band offset measured for ZnS/Cu2O band alignment preclude use of ZnS as an emitter, but make it a promising interfacial passivation layer for Cu2O/ZnO heterojunctions, which have been studied extensively. It has been shown that the technique used to deposit the buffer layer and heterojunction partner has a significant effect on device performance and cleanest interfaces produce highest efficiency devices. One way to achieve a cleaner heterojunction interface is to deposit the interface in situ by plasma-assisted Molecular Beam Epitaxy (PA-MBE), which allows precise interface control in an ultra-high vacuum (UHV) environment. In this work we deposit phase-pure Cu2O films on single-crystalline MgO substrates, followed by Zn-VI layers. The entire stack is deposited without breaking vacuum in order to achieve the cleanest possible interface. The heteroepitaxial orientation relationship is (110) Cu2O on (110) MgO and (110) Cu2O on (100) MgO, as determined by in situ Reflection High Energy Electron Diffraction (RHEED) and ex situ X-ray Diffraction (XRD). ZnO films exhibit (0001) texture. Phase purity of the interface is confirmed by X-ray Photoelectron Spectroscopy (XPS). The interface structure and defect landscape are further elucidated by cross-sectional Transmission Electron Microscopy (TEM).
9:00 AM - E5.07
Electrical and Optical Characterization of Phage-Templated Copper Sulfide
Mohammed Shahriar Zaman 1 Gabriel Bernard Grajeda 2 Elaine D Haberer 1 2
1University of California Riverside Riverside USA2University of California Riverside Riverside USAShow Abstract
Copper sulfide, an abundant, non-toxic p-type semiconductor material, has several stoichiometric and non-stoichiometric phases from Cu2S to CuS. The tunable electrical and optical properties of this material make it a potential candidate for photovoltaic and plasmonic device applications, yet some phases have been reported as unstable, particularly as nanoscale materials. Material stability is an important requirement for device reliability. In this work, we investigate the electrical and optical properties, as well as the stability of M13 viral-templated copper sulfide nanowires synthesized under room temperature and pressure. The filamentous geometry of the M13 virus, approximately 880 nm in length and 6 nm in diameter, is particularly well-suited for templated nanowire synthesis. Electrostatic interactions between the phage template and aqueous-based chemical precursors were utilized to synthesize nanocrystalline Cu1.8S along the length of the phage. Films of phage-templated Cu1.8S were prepared by drop-casting the synthesis product onto Au/Ti contact pads. The film thickness was highly non-uniform due to phage agglomeration and bundle formation during synthesis. The measured film thickness ranged from 120 nm to 5.8 mu;m with a median of 1.1 mu;m. In thinner regions of the film (<500 nm in thickness) the morphology became porous and phage bundles were easily identifiable. To study the electrical response and stability of the synthesized material, electrical resistance was measured for freshly made and 3 day aged samples under two different conditions: (1) ambient and (2) low oxygen and low humidity (< 0.1 ppm) conditions. Samples exposed to ambient conditions showed nearly a factor of 40 decrease in resistance while the resistance of samples stored under low oxygen, low humidity conditions only decreased by a factor of 13. As synthesized, the UV/Vis/IR absorption spectrum showed strong absorption at wavelengths below 800 nm due to band-to-band transition and a small, but broad peak in the infrared (IR) region attributable to localized surface plasmon resonance (LSPR) caused by free carriers. With prolonged exposure to ambient conditions, the height of the LSPR peak grew corresponding to an increase in free carrier concentration, which is consistent with the decrease in resistance observed with electrical measurements. XPS analysis revealed that after 3 days of exposure to ambient conditions the surface of the synthesized material developed a mixture of Cu deficient compositions and various oxides. Cu ions within these Cu deficient phases were converted to higher oxidation states, increasing the concentration of free carriers, specifically holes. These studies are a first step towards understanding the electrical characteristics and stability of phage-templated Cu1.8S material, both of which are critical to future device applications.
9:00 AM - E5.09
Metal Oxide Core-Shell Nanowires for Enhanced Photoelectrochemical Water Splitting
Hyun Joo Lee 1 Jung Eun Lee 1 Woo Ri Ko 1 Min Hyung Lee 1
1Kyung Hee University Yongin-si Republic of KoreaShow Abstract
The solar-assisted hydrogen evolution from the water splitting can provide susceptible and green source of energy. Here, we report metal oxide core-shell nanowires as photoanodes for a water-splitting photoelectrochemical (PEC) cell. The core-shell nanowires were synthesized by chemical and heat treatment on metal oxide (WOx) nanowires grown by chemical vapor deposition (CVD). The metal oxide core-shell nanowires showed enhanced PEC performance compared to original metal oxide nanowires due to increased light absorption in the visible. Also, the core-shell nanowires exhibit excellent chemical stability in a wide pH range and long-term stability against photocorrosion. Furthermore, plasmon-assisted photocurrent generation using the core-shell nanowires decorated with plasmonic nanoparticle will be discussed.
9:00 AM - E5.10
Sb2S3/SnSe Thin Film Solar Cells by Thermal Evaporation
Jose Escorcia-Garcia 1 Enue Barrios-Salgado 1 M. T. Santhamma Nair 1 P. Karunakaran Nair 1
1Universidad Nacional Autonoma de Mexico Temixco MexicoShow Abstract
Thin films of Sb2S3(50 nm) and SnSe (200 nm) produced by thermal evaporation are integrated into a solar cell structure SnO2:F/CdS(90 nm)/Sb2S3/SnSe/C-paint, which shows Voc of 550 mV, Jsc of 3.6 mA/cm2, FF of 0.38, and conversion efficiency of 0.75 %. Here, SnO2:F is a commercial transparent conductive oxide (TCO) coating (TEC-15, Pilkington). A CdS thin film of 90 nm in thickness is deposited on TCO substrates using chemical bath deposition at 80 °C during 1 h giving a specularly reflective film of optical band gap (Eg) 2.4 eV and photoconductivity σp, 0.005 (Omega; cm)-1. The Sb2S3 thin film is deposited by evaporating commercially available Sb2S3 powder (Sigma-Aldrich, 99%) at 10-5 Torr on to the glass/SnO2:F/CdS substrate at 400 °C. After such a substrate heating, the photoconductivity of CdS is found to increase to 0.1 (Omega; cm)-1. The Sb2S3 film produced by the thermal evaporation has an Eg of 1.8 eV and σp of 10-6 (Omega; cm)-1. Subsequently, SnSe thin film is deposited by evaporating SnSe precipitate obtained in the laboratory from chemical bath containing SnCl2.2H2O, triethanolamine, sodium selenosulfate (Na2SeSO3), sodium hydroxide (NaOH), and polyvinyl pyrollidone (PVP). The optical absorption coefficient of the SnSe thin film in the visible region is > 105 cm-1 and the Eg, 1.19 eV. Thermoelectric measurement on the SnSe film shows p-type conductivity with a thermoelectric power of 450 mu;V/K that suggests a carrier concentration of asymp;1016 cm-3. This film is photoconductive, with σp 10-3 (Omega; cm)-1. Optimization of the thickness as well as the thermal processing of the individual films is expected to lead to improved solar cell parameters.
9:00 AM - E5.11
Silver Antimony Sulfide-Selenide for Thin Film Solar Cells
Jesus Capistran 1 M. T. Santhamma Nair 1 P. Karunakaran Nair 1
1Universidad Nacional Autonoma de Mexico Temixco MexicoShow Abstract
Chemical deposition followed by thermal treatments to obtain thin films of crystalline silver antimony chalcogenides, AgSbS2 and AgSbS2-xSex, will be reported. Amorphous thin films are deposited on glass substrates kept for 1 h 15 min in a solution containing antimony trichloride (SbCl3), sodium thiosulfate (Na2S2O3) and silver nitrate (AgNO3) maintained at 40 °C. By heating these films in nitrogen at 180-320 °C, we have obtained thin films of crystalline cubic-AgSbS2. The thin film of AgSbS2, thus produced, has an optical band gap Eg = 1.89 eV and photoconductivity σph = 1.2x10-5 Omega; -1 cm-1. When the films are heated above 320 °C in a similar condition, decomposition occurs, and presence of elemental Ag and Sb is observed along with AgSbS2 in such films. The heat treatment of the amorphous film in presence of Se-vapor results in the formation of silver antimony sulfide selenide films, AgSbS2-xSex. XRD analysis confirms the formation of the solid solution AgSbS1.25Se0.75 or AgSbSe2 depending on the extent of Se-vapor available during the heat treatment. The crystalline grain diameter is 15 nm in both the films. These films show optical absorption coefficients α, 105 cm-1 in the visible region; Eg of 1.46 eV for AgSbS1.25Se0.75 and of 1.1 eV for AgSbSe2.The photoconductivity (p-type) σph is 2.7x10-2 Omega;-1 cm-1 of AgSbS1.25Se0.75 film. Thus the optical and electrical properties of the AgSbS1.25Se0.75 film are suitable for being a good absorber in solar cell. Presently, TCO(SnO2:F)/CdS/AgSbS2/C cells show open circuit voltage, Voc, 600 mV; short circuit current density, Jsc, 0.8 mA/cm2; and fill factor, FF, 0.53. Having heated the cell with the C-electrode at 240 °C in nitrogen atmosphere to promote crystallization of the absorber layer, the characteristics reported here are stable. We will include results on TCO(SnO2:F)/CdS/AgSbS1.25Se0.75/C cells as well, and illustrate a wide range of solar cell absorber materials for thin film and hybrid solar cell technology.
9:00 AM - E5.12
The Preparation and Characterization of FeS2 Pyrite Photovoltaic Ultrathin Films
Su-Ching Hsiao 1 Kuo-Wei Wu 1 Sheng-Hsin Huang 1 Shih-Hsiang Chiu 1 Lih-Hsin Chou 1
1National Tsing Hua University Hsinchu TaiwanShow Abstract
Semiconductor nanocrystals (NCs) are promising building blocks for next generation photovoltaic devices. FeS2 (pyrite phase) NCs have been considered as a superior semiconducting material for solar energy conversion and photo-electrochemical applications since it possesses an appropriate band gap of ~1 eV and a very high absorption coefficient (α >105 cm-1). These characteristics coupled with the low cost, environmental compatibility, and abundant elements in the crust make pyrite to be a potential candidate for solar cell absorption layer materials in the form of ultrathin films (< 100 nm). NCs coating technique, which requires a well-dispersed NC ink, is simple and low-cost compared with the traditional vacuum processes for depositing pyrite thin films. However, as the effect of surface energy of NCs is significantly higher than that of larger particles, NCs will tend to agglomerate and disable the formation of continuous, smooth ultrathin films.
In this report, well-dispersed, stable pyrite NC inks were successfully produced by beads-milling technique and smooth pyrite thin films less than 50 nm were produced by spin coating the dispersed pyrite NC inks on various substrates. The absorption coefficient of coated thin films increased as annealing temperature increased. Hall measurements show that these films were P-type. The room-temperature resistivity decreased and hall mobility increased after 500 oC annealing. These films can be used for not only traditional photovoltaic devices but also flexible devices.
The grazing incidence X-ray diffraction patterns (GIXRD) show the pyrite thin films changed to amorphous phase when annealing temperature was less than 350 oC. However, the thin films maintained pyrite phase with increased crystallinity when annealed above 400 oC. Meanwhile, the small amount of marcasite phase coexisted with the pyrite phase (major phase). These phenomena were observed for the first time. A model related to the phenomena will be proposed in this report. Other analyses were also utilized to study the pyrite thin films in this report. Fourier transform infrared (FTIR) was used to determine if there is any organic residue after annealing. The band gap of pyrite thin films was obtained by Tauc plot extracted from UV-Vis spectrum. The surface morphology and thickness of pyrite thin films were acquired from field emission scanning electron microscopy (FESEM).
9:00 AM - E5.13
Nanocrystals of Some Earth-Abundant Materials for Solar Cells
Karthik Ramasamy 1 3 Hunter Sims 2 3 Arunava Gupta 1 3
1The University of Alabama Tuscaloosa USA2The University of Alabama Tuscaloosa USA3The University of Alabama Tuscaloosa USAShow Abstract
Energy generation from non-fossil fuels has been rapidly accelerating to meet the growing global energy demand. Solar-based technologies are one of the major contributors towards meeting the energy needs. As compared to conventional silicon-based solar cells, thin film solar cell panels are lightweight and flexible and are thus preferable for a number of applications. In particular, thin films solar cells fabricated using CdTe and CuInGaSe2 (CIGS) are already widely used. Despite the success, these materials are composed of toxic and less-abundant elements such as tellurium, indium and gallium. An attractive alternative being extensively investigated is Cu2ZnSnS4 (CZTS), consisting of non-toxic and relatively abundant elements, with a large absorption coefficient (~ 1 x 104 cm-1) for solar radiation. In view of identifying other materials composed of sustainable and non-toxic elements for thin film solar cells, we have been investigating Cu2FeSnS4, CuCdxZn1-xSnS4 and Cu-Sb-S systems. We have developed solution state syntheses methods for phase, size, and shape-selective growth of Cu2FeSnS4, CuCdxZn1-xSnS4 and Cu-Sb-S nanocrystals. The optical and electrical properties of the nanocrystals have been studied. In particular, the copper-antimony-sulfide system consisting of four major phases, namely CuSbS2 (Chalcostibite), Cu12Sb4S13 (Tetrahedrite), Cu3SbS3 (Skinnerite) and Cu3SbS4 (Femitinite), have been investigated in detail. All four phases are p-type semiconductors and their energy band gap lie between 0.5 and 2 eV, with large absorption coefficient values over 105 cm-1. We have developed simple colloidal hot-injection methods for the phase-pure synthesis of nanocrystals of all four phases (CuSbS2, Cu12Sb4S13, Cu3SbS3, Cu3SbS4) and carried out detailed electronic structure calculations in order to access their suitability for solar energy applications. The details of the syntheses methods, structural and optical characterizations and band structure calculation will be presented.
9:00 AM - E5.14
Cu2ZnSnSe4 from Chemically Deposited Binary Films
Enue Barrios-Salgado 1 Jose Campos 1 M. T. Santhamma Nair 1 P. Karunakaran Nair 1
1Universidad Nacional Autonoma de Mexico Temixco MexicoShow Abstract
Thermal processing in presence of Se powder under nitrogen at 350 - 400 oC of chemically deposited thin film stack SnSe-ZnSe-Cu2-xSe precursor produces thin film of Cu2ZnSnSe4 (CZTSe). To develop Cu2ZnSnSe4, we used chemically treated glass substrates to deposit compact and specularly reflective SnSe thin films of 300 nm from a bath containing tin(II) chloride, triethanolamine, sodium hydroxide, sodium selenosulfate, and a small quantity of polyvinylpyrrolidone. The deposition bath was maintained at 26 oC. As deposited, the SnSe thin films have orthorhombic crystal structure, an optical band gap Eg of 0.95 - 1.18 eV, and a p-type electrical conductivity of 1 #8486;-1 cm-1. Thin films of ZnSe and Cu2-xSe are deposited on the SnSe films using chemical baths reported previously from our group [1, 2]. The films of CZTSe produced though heating the stack have tetragonal structure with lattice parameters of a = 5.68 Å and c = 11.92 Å and a crystallite diameter of 16 nm. The films are p-type, with electrical conductivity, 0.2 (Omega; cm)-1and Eg of 1 eV. As preliminary results, in a CdS/SnSe thin film solar cell structure, we have observed an open circuit voltage, Voc, of 225 mV; short circuit current density, Jsc, of 1.75 mA/cm2 under an illumination of 850 W/m2. The paper will deal with the morphology, composition, and other properties of the thin films of SnSe and CZTSe and results on solar cells using these two absorbers.
 V. M. García, M. T. S. Nair, and P. K. Nair, Semicond. Sci. Technol. 14 (1999) 366
 V. M. García, P. K. Nair, and M. T. S. Nair, J. Cryst. Growth, 203 (1999) 113
9:00 AM - E5.16
Vapor-Solid Interactions During the Formation of Cu2ZnSnS4 Thin Films
Melissa Johnson 1 Michael Manno 1 Xin Zhang 1 Chris Leighton 1 Eray S. Aydil 1
1University of Minnesota Minneapolis USAShow Abstract
The stability of Cu2ZnSnS4 (CZTS) thin films during synthesis and annealing at high temperatures (400-600 oC) is an important issue as it affects the chemical and phase composition of the film. For example, if the appropriate vapor phase species are not present in sufficient pressure, CZTS may decompose into binary sulfides and volatile SnS may leave the film, changing the film stoichiometry. The vapor phase composition over the CZTS film also affects the microstructure of the film, as we recently showed that impurities like Na and K may be volatilized from soda lime glass substrates. Widely used open-system annealing and sulfidation processes further complicate these effects as the vapor species and pressures change over time, thus a closed-system synthesis approach is well suited for studying vapor-film interactions. To this end, we synthesized CZTS films via ex situ isothermal sulfidation of co-sputtered Cu-Zn-Sn thin films on various substrates in a sealed quartz ampoule. The precursor film composition is controlled by varying the sputtering power of Cu, Cu-Zn, and Cu-Sn targets. The sulfidation ampoule with the precursor film is charged with varying amounts of S, Na and K, evacuated to a 10-6 Torr base pressure, sealed, and annealed at different temperatures and periods of time. The sulfidized films are characterized using a suite of methods including X-ray diffraction, confocal Raman spectroscopy and imaging, scanning electron microscopy, energy dispersive x-ray spectroscopy, and temperature dependent conductivity and Hall effect measurements. We explored a wide range of precursor metal compositions and found that the creation of SnS vapor was inevitable. While Sn loss through the formation of this phase has been well documented and is generally associated with CZTS decomposition, we found it to be essential during formation. During sulfidation, Cu2-xS, ZnS, and SnS form first. The volatility of SnS results in the loss of the vast majority of the Sn initially present in the film at temperatures as low as 300 oC. However, the SnS that escapes into the vapor phase is reincorporated into the film by reacting with the solid Cu2-xS remaining in the film to form the more stable Cu2SnS3 phase. Reincorporation of Sn is terminated when all of the Cu2-xS is consumed, and The Cu2SnS3 is later converted to CZTS through ZnS incorporation. Any excess SnS remaining in the vapor eventually condenses on the ampoule walls during cooling. This mechanism results in the self-regulation of Sn concentration within the film, such that the Cu/Sn ratio is always 2 for a very wide range of precursor film compositions. If the precursor film is Sn deficient, Cu2-xS remains in the final film, though the Sn deficiency can be corrected through the addition of elemental Sn in the sulfidation ampoule, creating additional SnS vapor. We find that these vapor-solid interactions are independent of substrate and whether or not Na and K are also present in the vapor phase.
9:00 AM - E5.18
Growth Mechanism of CZTSSe Thin Films from Binary and Ternary Chalcogenide Nanoparticles
Qijie Guo 1 Yanyan Cao 1 Jonathan V Caspar 1 Lynda K Johnson 1 Irina Malajovich 1 Dilip Natarajan 1 H. David Rosenfeld 1 Kaushik Roy Choudhury 1 Wei Wu 1
1DuPont Central Research and Development Wilmington USAShow Abstract
Interest in Cu2ZnSn(S,Se)4 (CZTSSe) as a sustainable material for use in thin-film photovoltaics (PV) has increased rapidly in recent years. In addition, low-cost solution-based CZTSSe thin-film deposition methods that offer attractive scalability and handling characteristics have also attracted considerable attention. Of particular interest is the two-step process via selenization of nanoparticle precursors, which has been demonstrated for the fabrication of highly efficient PV devices by various groups. One of the successful nanoparticle routes is based on a mixture of binary and ternary chalcogenide nanoparticles, where efficiencies of 8.8% without antireflection coating have been achieved. Previously, we have shown that following high-temperature selenization, the binary and ternary nanoparticle precursor film transforms into an intriguing bilayer structure consisting of a large-grain CZTSSe layer on top of a fine-grain carbon-rich layer. Detailed investigations have shown that the large-grain CZTSSe layer is the active layer in the resulting device and that its properties control device performance. Improved understanding and control of the bulk and interface properties of the large-grain CZTSSe layer could lead to further improvements in efficiency. In order to devise strategies to improve the material properties of the large-grain CZTSSe layer, a clear understanding of the mechanism of formation of the bilayer structure during the selenization process is needed. In this paper, we will present results from detailed mechanistic studies on the selenization of binary and ternary nanoparticles. Selenization runs were interrupted at different stages of the annealing process, and samples were examined in detail by a combination of microscopy and spectroscopy techniques. Based on the experimental results, a growth model for the formation of the bilayer CZTSSe thin film from the binary and ternary nanoparticle precursor layer is proposed.
9:00 AM - E5.19
Graded Index Optical Films for Antireflective Coupling, Transmission and Adiabatic Solar Energy Conversion and Storage
Juan J. Diaz Leon 1 2 R. Ernest Demaray 4 Roger W. Anderson 3 Nobuhiko P. Kobayashi 1 2
1University of California, Santa Cruz Santa Cruz USA2University of California, Santa Cruz Santa Cruz USA3University of California, Santa Cruz Santa Cruz USA4Antropy Inc. amp; Demaray LLC Portola Valley USAShow Abstract
The ability of absorbing an incident solar spectrum efficiently over a range of angles into a planar dielectric film for absorption, conversion or optical transmission requires improvement from antireflective (AR) films with discrete layers to graded index films with near “ideal” AR performance. Such graded films can also enable transmission of optical energy to couple devices with mismatching numerical aperture (NA), size or shape. High transparency allows in film waveguide transport after AR coupling. We present the simulation, deposition and characterization of vacuum thin films with the required range of refractive index and low optical absorption for the design of an optical coupler that will concentrate the sunlight into a thin film wave guide or an optical fiber with minimal loss, by virtue of the ability of varying the material deposited both laterally and axially. Modeling results show an optimum design for such graded index profiles using pulsed DC reactive magnetron sputtering with substrate bias for the deposition of the films. Optical ellipsometry, Scanning Electron Microscopy (SEM) and Energy-Dispersive x-ray Spectroscopy (EDS) were used for their characterization. These capabilities provide sub-wavelength antireflective coating to maximize the coupled light with in film waveguide transport including an adiabatic mode converter to couple the light to a subsequent waveguide or optical fiber for transmission, conversion or thermal storage and use at high concentration.
E1: Binary Sulfide PV Absorbers
David O. Scanlon
Tuesday AM, April 22, 2014
Westin, 3rd Floor, Franciscan I
9:30 AM - *E1.01
Material Design for Novel-Concept-Based Solar Cells --Sulfurization or Oxidization of ``Cheap'' Metals
Mutsumi Sugiyama 1
1Tokyo University of Science Chiba JapanShow Abstract
Even though extraordinary progress has been made in the field of Si and Cu(In,Ga)(S,Se)2 (CIGS) solar cells/modules over the years and their performance significantly improved, fabrication/material costs and safety still remain major concerns. In terms of material design for compound semiconductors, the most cost-effective, safe, and easy-handling anions are sulfur and oxygen. Nowadays, to obtain compound semiconductors by reacting inexpensive metal cations (precursors) with gas-phase anions has become a significant issue. For realizing this, sulfurization and oxidation are appropriate physical processes because they are based on simple thermal diffusion of anions into a metal precursor. In fact, sulfurization/selenization techniques are adopted for commercially manufacturing CIGS solar modules because of the cost effectiveness, scalability, and uniformity of such techniques.
Tin monosulfide (SnS) has a direct energy bandgap of 1.3 eV and a high optical absorption coefficient of 104 cmminus;1. Therefore, SnS is perceived to be a promising candidate as a cost-effective and earth-abundant inorganic material for use in the fabrication of next-generation solar cells. Although the theoretical conversion efficiency of SnS-based solar cells is high, the demonstrated efficiencies of such cells are still low. Some of the reasons for low efficiencies are considered to be the poor crystal quality of a SnS layer, and the mismatch in the band diagram at a pn-heterojunction. In fact, most researchers simply refer to the CIGS solar cells structure. The growth mechanism of SnS or the appropriate band offset of SnS-based solar cells has not yet been clarified because it shows metastable phase.
Wide-bandgap transparent conducting oxide (TCO) films exhibit only n-type conductivity. On the other hand, NiO shows only p-type conductivity, and is composed of inexpensive and less toxic elements. Recently, NiO has also been used in visibly transparent solar cells, which are attractive because their optical transparency permits greater flexibility in terms of installation locations. In addition, NiO films can be easily obtained just by the oxidation of Ni. We have proposed a “NiO-based invisible sensor” that receives electrical power from a “NiO-based invisible solar cell”. This sensor can be fixed on a wall or ceiling without a wired connection. Moreover, the user will not be aware of these invisible sensors and will not experience the stress associated with being monitored. Therefore, there is a great market opportunity for various environments, and the application possibilities for invisible devices based on NiO-based solar cells are limited only by imagination.
In this presentation, we will introduce some earth-abundant solar absorbers (SnS, Cu2SnS3, Cu2O, and related compounds) and TCOs (NiO, CuAlO2, and related compounds) for use in photovoltaics, with focus on materials research and device development, in terms of the most recent advances and experimental results.
10:00 AM - E1.02
Thin Film WSe2 for Use as a Photovoltaic Absorber Material
Qingleing Ma 1 Hrach Kyureghian 1 Joel D. Banninga 1 Ned Ianno 1
1University of Nebraska-Lincoln Lincoln USAShow Abstract