The Steady Rise of Kesterite Solar CellsClick to copy article linkArticle link copied!
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Could slow and steady win the race to sustainable energy generation? While perovskite solar cells are currently at the forefront of photovoltaic research activity, we argue in this Viewpoint that devices based on the mineral kesterite could be the dark horse of next-generation solar energy conversion. The most urgent research challenges for this technology are outlined.
Thin-film photovoltaic (PV) technologies offer an economically promising and flexible means to harness solar energy. (1) Compared to silicon, the use of materials that absorb sunlight strongly allows for less material to be used, lowering cost and opening up more possibilities for integrating the solar cell modules with buildings. Only a small number of commercial thin-film technologies have achieved power conversion efficiencies (PCEs) greater than 20%: CdTe and Cu(In,Ga)(S,Se)2 (CIGS) have champion device PCEs of 21%. (2) The toxicity of cadmium and competition in the supply of indium are limiting factors for the large-scale utilization of these technologies.
In the emerging thin-film PV market, hybrid halide perovskites such as methylammonium lead iodide (CH3NH3PbI3) have received an immense amount of research interest, with over 3000 publications in relation to solar cells since 2009. The efficiency of laboratory-scale halide perovskite devices has risen sharply, especially over the course of the first 500 publications focused on the material (see Figure 1). The current record for a perovskite solar cell is 22.1%. (2) Progress in PCE with respect to the number of publications on this material is beginning to plateau. Furthermore, stability issues and concerns over the lead content of halide perovskites remain as challenges for the commercialization of this technology. (3)
Figure 1
Figure 1. Comparison of the cumulative research output for Cu2ZnSn(S,Se)4 (kesterite) and CH3NH3PbI3 (halide perovskite) related photovoltaic technologies against year and the record light-to-electricity conversion efficiency. The publication data has been generated from Web of Science (February 15, 2017) for kesterite (search terms: “kesterite OR stannite” AND “solar cell”) and halide perovskite (search terms: “halide perovskite OR iodide perovskite OR hybrid perovskite” AND “solar cell”) solar cells.
Solar cells based on the kesterite mineral structure, including Cu2ZnSnS4 (CZTS), Cu2ZnSnSe4 (CZTSe), and their alloys Cu2ZnSn(SxSe1–x)4 (CZTSSe), stand out from other thin-film PV candidates for being composed of earth-abundant and nontoxic elements. Since the first kesterite solar cell was fabricated in 1997, the PCE of the champion device has risen from 0.66% (4) to the current certified record of 12.6% set in 2013, (5) with a 13.8% small-area device reported in late 2016. (6) These efficiencies fall far below the 28% predicted for this technology from the Shockley–Queisser limit. However, kesterites have achieved relatively little attention, with fewer than 1000 publications to date related to photovoltaic applications. A directed focus in research effort may see this sustainable PV technology achieve PCEs approaching those of hybrid perovskites or commercial thin-film PV technologies, perhaps as well with much improved operational stability.
There are numerous synthetic routes to kesterite thin films, including both vacuum-based deposition and nonvacuum-based solution processing. Nonvacuum approaches are desirable for scalability and feasible industrial production, including electrodeposition, (7-10) nanocrystal dispersion, (11-13) hydrazine-based deposition, (14) and other pure-solution approaches. (15-17) The current champion CZTSSe solar cell was fabricated using the hydrazine-based solution method developed at the IBM T. J. Watson Research Center. (5) Most record devices since 1997 have been produced by solution-based film deposition approaches, (18) alluding to the potential for large-scale fabrication, which is required to support a terawatt PV industry.
The relatively low efficiency of kesterite-based solar cells is attributed to a large deficit in the open-circuit voltage (VOC) relative to the band gap of the absorber layer. This is universal for high-performance kesterite devices, with the deficit being even larger for the pure sulfide material. While there is a consensus that the VOC deficit is the key limiting factor for devices, the origin remains very much an open question. (19) There are a number of hypotheses to account for the VOC deficit, which can be separated into three categories:
(i) | A non-Ohmic back electrical contact, usually Mo/CZTS, which could result in a high recombination velocity; | ||||
(ii) | A poorly optimized interface between CZTS and the CdS buffer layer, which could also result in rapid electron–hole recombination; | ||||
(iii) | Large amounts of defects and disorder in the bulk of the absorber layer, limiting minority charge carrier lifetimes and enhancing recombination processes. |
In the best devices, it has been shown that the back contact with Mo is Ohmic in nature. (20) The borrowed device architecture from CIGS solar cells involves the use of a CdS buffer layer, and analysis of the conduction band offset between CdS and CZTSSe also does not imply any major limitation on device performance. (21) This then leaves us with consideration of defects and disorder in the bulk of the absorber layer.
Advances in defect engineering, through modification of synthesis, deposition, and/or annealing procedures, enable a reduction in the impact of extended defects such as grain boundaries, passivation of surfaces, and production of pinhole-free thin films. However, point defects such as site vacancies and antisites will remain present even for carefully processed samples. (22) Devices fabricated from high-quality single crystals have demonstrated PCEs of 10%, with a VOC deficit similar to that of thin-film solar cells. (23)
Kesterites are an example of a multinary semiconductor, with a zinc-blende-related structure (space group type I4̅) and the general chemical formula of Cu2-MII-MIV-X4 (X = O, S, Se, Te). A consequence of the many components of the material is an increase in the number of possible lattice defects, with particular concern for cation disorder. (24) In CZTS, disorder among Cu and Zn metals would seem particularly likely because of the chemical similarity of the two species, which are neighbors in the periodic table. Indeed, first-principles calculations predict that the neutral defect pair [CuZn– + ZnCu+] has a low formation energy, (25) implying a high equilibrium concentration, and there is a large body of experimental evidence for Cu/Zn disorder from neutron diffraction, synchrotron X-ray diffraction, and near-resonant Raman studies. (22) It is now universally accepted that Cu/Zn disorder will be present to a high degree even in high-quality thin films. However, the impact of this type of disorder on device performance is the subject of ongoing debate in the community. (19, 26)
The open-circuit voltage of a PV device is limited by the band gap of the absorber material, but defects can modify the underlying electronic band structure (Figure 2). Lattice disorder can result in localized states near the top of the valence band or at the bottom of the conduction band. At sufficiently high concentrations, these interact to form an impurity band. As the density of defects increases, the tail states become more prevalent and penetrate further into the band gap. Localized states and associated band tailing in the electronic structure lead to band gap narrowing. (22, 27) As such, peaks in the photoluminescence spectra are red-shifted to energies below those of the optical band gap obtained from internal quantum efficiency measurements (which reflects only extended electronic states). (28) Band tailing in CZTSSe devices is roughly twice as severe as that in CIGS devices, and it has been suggested that Cu/Zn disorder in CZTSSe is one contributing factor. In comparison, cation disorder in CIGS could be expected to be less prevalent because Cu is less chemically similar to Ga/In than Cu is to Zn, increasing the energy cost associated with making substitutions among any of these two species with Cu compared to that of Zn with Cu. Furthermore, it has been observed that band tailing is less severe in devices made from Ag2ZnSnSe4, (29) where cation disorder could be expected to be suppressed because of the greater ionic radii mismatch between Ag+ and Zn2+ than Cu+ and Zn2+.
Figure 2
Figure 2. Acceptor defect-induced energy level within the band gap of a semiconductor, broadening to an impurity band with increased defect density until at sufficiently high defect concentrations the band merges with the valence band maximum, resulting in a reduced band gap. Such tail states would limit the open-circuit voltage accessible in a photovoltaic device and could explain the voltage deficit observed in kesterite-based solar cells. Also listed are the range of possible acceptor and donor defects in Cu2ZnSnS4; a large number of possible charge-neutral defect clusters can also be formed.
To our knowledge no direct connection between bulk disorder in the kesterite absorber layer and photovoltaic performance has yet been made. Postannealing treatments can be used to reduce the prevalence of Cu/Zn antisites. (30) Devices of varying degrees of Cu/Zn disorder in the absorber layer produced in this way demonstrated that the VOC changes by the same amount as the optical band gap. The postannealing treatment therefore had no impact on VOC deficit. (19) The lack of direct connection between Cu/Zn antisite concentration and photovoltaic performance could be because, despite reduction in Cu/Zn disorder, the antisite concentration is still high from the perspective of device performance. Alternatively, it could be because there are other defects that are more important for controlling device properties at the current performance level. It is worth noting that the primary factors limiting performance could differ considerably for devices produced using different absorber fabrication procedures or for different performance levels for samples produced using the same approach.
Beyond Cu/Zn antisite defects, another explanation for the VOC deficit is the presence of defects with levels deep in the band gap that could be acting as centers for nonradiative recombination. In particular, defects involving Sn result in deeper charge transition states owing the higher charge and larger radius of Sn relative to Cu and Zn. (31) Deep-level defects in CZTS have been predicted to have a formation energy that is higher than those of the shallow defects; (16) therefore, it could be expected that they will be less prevalent. However, the presence of “killer centers” (32) even in low concentrations could be limiting device performance. It is possible that the formation energy of such centers may be reduced by the specific environmental conditions during synthesis and chemical potentials of the constituent elements.
To summarize, some key challenges and opportunities for kesterite solar cells include the following:
1. | Missing activation step. For CdTe, a postdeposition chemical treatment of CdCl2 or similar is required to “activate” the absorber layer (33) (e.g., passivate grain boundaries, enlarge grain size), while for CIGS a Cu-rich processing stage is needed during the 3-stage evaporation process. (34) Identifying a similar activation process for kesterites may enable a step-change in PV action. | ||||
2. | Accurate material and device measurements. The wide variety of demonstrated growth processes and conditions makes comparison of reported physical properties challenging. Accurate and consistent measurements concerning carrier generation, transport, recombination, and collection for different absorbers would provide valuable insights to overcome device bottlenecks. | ||||
3. | Quantifying defects and disorder. A number of different descriptors are being used in the field to quantify disorder, including Raman spectra, optical spectra, and neutron diffraction. However, the site disorder averaged over a macroscopic sample does not provide insights into the microscopic cation distribution that will interact with photogenerated electrons and holes. More accurate local structure techniques and materials simulations could provide valuable insights. | ||||
4. | Alternative device architectures. Relatively little effort has been spent on looking at alternatives to the standard Mo/CZTS/CdS device configuration. Beyond simple component replacement (e.g., Mo for W or ZnS for CdS), other device configurations such as p–i–n could result in enhanced photovoltaic performance. |
Two large research consortia have recently been funded to address some of these questions. One is PVTEAM, led by L. M. Peter at the University of Bath (U.K.) that links with the SPECIFIC Innovation and Knowledge Centre for scale-up, and STARCELL, led by E. Saucedo at IREC (Spain) that combines 13 partners across Europe, Japan, and the United States. At a time when further breakthroughs in perovskite solar cells will become increasingly difficult to achieve, a focused effort on kesterites and related materials could have major impacts. While this technology has been relatively slow to emerge, the combination of earth-abundant and nontoxic elements in a chemically stable compound is the solution that sustainable thin-film photovoltaics requires.
References
This article references 34 other publications.
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- 3Park, N.-G.; Grätzel, M.; Miyasaka, T.; Zhu, K.; Emery, K. Towards stable and commercially available perovskite solar cells Nat. Energy 2016, 1, 16152 DOI: 10.1038/nenergy.2016.152Google Scholar3https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC2sXhtVersL0%253D&md5=0c0a441bdcb4d193c128dc29b186fa9dTowards stable and commercially available perovskite solar cellsPark, Nam-Gyu; Gratzel, Michael; Miyasaka, Tsutomu; Zhu, Kai; Emery, KeithNature Energy (2016), 1 (11), 16152CODEN: NEANFD; ISSN:2058-7546. (Nature Publishing Group)Solar cells employing a halide perovskite with an org. cation now show power conversion efficiency of up to 22%. However, these cells are facing issues towards commercialization, such as the need to achieve long-term stability and the development of a manufg. method for the reproducible fabrication of high-performance devices. Here, we propose a strategy to obtain stable and com. viable perovskite solar cells. A reproducible manufg. method is suggested, as well as routes to manage grain boundaries and interfacial charge transport. Electroluminescence is regarded as a metric to gauge theor. efficiency. We highlight how optimizing the design of device architectures is important not only for achieving high efficiency but also for hysteresis-free and stable performance. We argue that reliable device characterization is needed to ensure the advance of this technol. towards practical applications. We believe that perovskite-based devices can be competitive with silicon solar modules, and discuss issues related to the safe management of toxic material.
- 4Katagiri, H.; Sasaguchi, N.; Hando, S.; Hoshino, S.; Ohashi, J.; Yokota, T. Preparation and evaluation of Cu2ZnSnS4 thin films by sulfurization of E-B evaporated precursors Sol. Energy Mater. Sol. Cells 1997, 49, 407 DOI: 10.1016/S0927-0248(97)00119-0Google Scholar4https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADyaK2sXmvFKmtrg%253D&md5=63787577bd92271d132d5f9e713b85d8Preparation and evaluation of Cu2ZnSnS4 thin films by sulfurization of E-B evaporated precursorsKatagiri, Hironori; Sasaguchi, Nobuyuki; Hando, Shima; Hoshino, Suguru; Ohashi, Jiro; Yokota, TakaharuSolar Energy Materials and Solar Cells (1997), 49 (1-4), 407-414CODEN: SEMCEQ; ISSN:0927-0248. (Elsevier)By sulfurization of E-B evapd. precursors, CZTS (Cu2ZnSnS4) films could be prepd. successfully. This semiconductor does not consist of any rare-metal such as In. The x-ray diffraction pattern of CZTS thin films showed that these films had a stannite structure. This study estd. the optical band gap energy as 1.45 eV. The optical absorption coeff. was in the order of 104 cm-1. The resistivity was in the the order of 104 Ω-cm and the conduction type was p-type. Fabricated solar cells, Al/ZnO/CdS/CZTS/Mo/Soda Lime Glass, showed an open-circuit voltage up to 400 mV.
- 5Wang, W.; Winkler, M. T.; Gunawan, O.; Gokmen, T.; Todorov, T. K.; Zhu, Y.; Mitzi, D. B. Device Characteristics of CZTSSe Thin-Film Solar Cells with 12.6% Efficiency Adv. Energy Mater. 2014, 4, 1301465 DOI: 10.1002/aenm.201301465Google Scholar5https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC2cXnvVShtb8%253D&md5=87a6fd2fd03588d5b9d82fbfe79c2d1aDevice Characteristics of CZTSSe Thin-Film Solar Cells with 12.6% EfficiencyWang, Wei; Winkler, Mark T.; Gunawan, Oki; Gokmen, Tayfun; Todorov, Teodor K.; Zhu, Yu; Mitzi, David B.Advanced Energy Materials (2014), 4 (7), 1301465/1-1301465/5CODEN: ADEMBC; ISSN:1614-6840. (Wiley-Blackwell)The device characteristics of copper zinc tin sulfoselenide thin-film solar cells with 12.6% efficiency are studied. The current-voltage characteristics, quantum efficiency, capacitance, and electron-beam-induced current (EBIC) measurements are discussed in detail.
- 6
Reported at PVSEC-36 by a research team led at DGIST in South Korea. A 0.181 cm2 solar cell was certified at 13.80% by KIER.
There is no corresponding record for this reference. - 7Jiang, F.; Ikeda, S.; Harada, T.; Matsumura, M. Pure Sulfide Cu2ZnSnS4 Thin Film Solar Cells Fabricated by Preheating an Electrodeposited Metallic Stack Adv. Energy Mater. 2014, 4, 1301381 DOI: 10.1002/aenm.201301381Google Scholar7https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC2cXnvVShsL8%253D&md5=d016de5552e67c0df43c4ae63afecad6Pure Sulfide Cu2ZnSnS4 Thin Film Solar Cells Fabricated by Preheating an Electrodeposited Metallic StackJiang, Feng; Ikeda, Shigeru; Harada, Takashi; Matsumura, MichioAdvanced Energy Materials (2014), 4 (7), 1301381/1-1301381/4CODEN: ADEMBC; ISSN:1614-6840. (Wiley-Blackwell)This article describes the pure sulfide Cu2ZnSnS4 thin film solar cells fabricated by preheating electrodeposited metallic stack. A champion solar cell with sunlight conversion efficiency of 8% (active area, without an MgF2 anti-reflectance layer) is described. The dependence of sunlight conversion efficiency and other device parameters on preheating duration is also discussed on the basis of surface microstructures and bulk compns.
- 8Guo, L.; Zhu, Y.; Gunawan, O.; Gokmen, T.; Deline, V. R.; Ahmed, S.; Romankiw, L. T.; Deligianni, H. Electrodeposited Cu2ZnSnSe4 Thin Film Solar Cell with 7% Power Conversion Efficiency Prog. Photovoltaics 2014, 22, 58 DOI: 10.1002/pip.2332Google Scholar8https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC3sXhvFejtbnN&md5=d0ea406ecf5da9185c1cfa0a23d27d16Electrodeposited Cu2ZnSnSe4 thin film solar cell with 7% power conversion efficiencyGuo, Lian; Zhu, Yu; Gunawan, Oki; Gokmen, Tayfun; Deline, Vaughn R.; Ahmed, Shafaat; Romankiw, Lubomyr T.; Deligianni, HarikliaProgress in Photovoltaics (2014), 22 (1), 58-68CODEN: PPHOED; ISSN:1062-7995. (John Wiley & Sons Ltd.)High performance Cu2ZnSnSe4 (CZTSe) photovoltaic materials were synthesized by electrodeposition of metal stack precursors followed by selenization. A champion solar cell with 7.0% efficiency is demonstrated. This is the highest efficiency among all of the CZTSe solar cells prepd. from electrodeposited metallic precursors reported to-date. Device parameters are discussed from the perspective of material microstructure and compn. in order to improve performance. In addn., a high performance electrodeposited CZTS (S only) solar cell was demonstrated and its device characteristics were compared against the CZTSe (Se only) cell. Using secondary ion mass spectrometry for the anal. of the chem. compn. of the absorber layer, a higher concn. of oxygen in the electrodeposited absorber is thought to be the root cause of the lower performance of the electrodeposited CZTS or CZTSe solar cells with respect to a solar cell fabricated by evapn. The grain boundary areas of Sn-rich compn. are thought to be responsible for the lower shunt resistance commonly obsd. in CZTSe devices. We measured the longest minority carrier lifetime of 18 ns among all reported kesterite devices. This work builds a good baseline for obtaining higher efficiency earth-abundant solar cells, while it highlights electrodeposition as a low cost and feasible method for earth-abundant thin film solar cell fabrication.
- 9Yao, L.; Ao, J.; Jeng, M.-J.; Bi, J.; Gao, S.; He, Q.; Zhou, Z.; Sun, G.; Sun, Y.; Chang, L.-B. CZTSe solar cells prepared by electrodeposition of Cu/Sn/Zn stack layer followed by selenization at low Se pressure Nanoscale Res. Lett. 2014, 9, 678 DOI: 10.1186/1556-276X-9-678Google Scholar9https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A280%3ADC%252BC2MvntVCqtQ%253D%253D&md5=0b1948a4ad2a854b72218c4e75c9d9dbCZTSe solar cells prepared by electrodeposition of Cu/Sn/Zn stack layer followed by selenization at low Se pressureYao Liyong; Ao Jianping; Bi Jinlian; Gao Shoushuai; He Qing; Zhou Zhiqiang; Sun Guozhong; Sun Yun; Jeng Ming-Jer; Chang Liann-Be; Chen Jian-WunNanoscale research letters (2014), 9 (1), 678 ISSN:1931-7573.Cu2ZnSnSe4 (CZTSe) thin films are prepared by the electrodeposition of stack copper/tin/zinc (Cu/Sn/Zn) precursors, followed by selenization with a tin source at a substrate temperature of 530°C. Three selenization processes were performed herein to study the effects of the source of tin on the quality of CZTSe thin films that are formed at low Se pressure. Much elemental Sn is lost from CZTSe thin films during selenization without a source of tin. The loss of Sn from CZTSe thin films in selenization was suppressed herein using a tin source at 400°C (A2) or 530°C (A3). A copper-poor and zinc-rich CZTSe absorber layer with Cu/Sn, Zn/Sn, Cu/(Zn + Sn), and Zn/(Cu + Zn + Sn) with metallic element ratios of 1.86, 1.24, 0.83, and 0.3, respectively, was obtained in a selenization with a tin source at 530°C. The crystallized CZTSe thin film exhibited an increasingly (112)-preferred orientation at higher tin selenide (SnSe x ) partial pressure. The lack of any obvious Mo-Se phase-related diffraction peaks in the X-ray diffraction (XRD) diffraction patterns may have arisen from the low Se pressure in the selenization processes. The scanning electron microscope (SEM) images reveal a compact surface morphology and a moderate grain size. CZTSe solar cells with an efficiency of 4.81% were produced by the low-cost fabrication process that is elucidated herein.
- 10Scragg, J. J.; Dale, P. J.; Peter, L. M. Synthesis and Characterization of Cu2ZnSnS4 Absorber Layers by an Electrodeposition-Annealing Route Thin Solid Films 2009, 517, 2481 DOI: 10.1016/j.tsf.2008.11.022Google ScholarThere is no corresponding record for this reference.
- 11Larramona, G.; Bourdais, S.; Jacob, A.; Chone, C.; Muto, T.; Cuccaro, Y.; Delatouche, B.; Moisan, C.; Pere, D.; Dennler, G. 8.6% Efficient CZTSSe Solar Cells Sprayed from Water-Ethanol CZTS Colloidal Solutions J. Phys. Chem. Lett. 2014, 5, 3763 DOI: 10.1021/jz501864aGoogle Scholar11https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC2cXhslCjtL3I&md5=6d76aae599772f24f1e404e9050eee678.6% Efficient CZTSSe solar cells sprayed from water-ethanol CZTS colloidal solutionsLarramona, Gerardo; Bourdais, Stephane; Jacob, Alain; Chone, Christophe; Muto, Takuma; Cuccaro, Yan; Delatouche, Bruno; Moisan, Camille; Pere, Daniel; Dennler, GillesJournal of Physical Chemistry Letters (2014), 5 (21), 3763-3767CODEN: JPCLCD; ISSN:1948-7185. (American Chemical Society)Copper zinc tin sulfide-selenide, Cu2ZnSn(S1-xSex)4 (CZTSSe), thin film photovoltaic devices were fabricated using a fast and environmentally friendly prepn. method, consisting of the following steps: An instantaneous synthesis of a Cu-Zn-Sn-S (no Se) colloid, a nonpyrolytic spray of a dispersion of this colloid in a water-ethanol mixt., and a sequential annealing first in a N2 atmosphere and second in a Se atm. The achievement of cell efficiencies up to 8.6% under AM1.5G (cell area 0.25 cm2) and without antireflecting coating indicates that this method can compete with other vacuum-based or more complex wet deposition methods.
- 12Guo, Q.; Ford, G. M.; Yang, W.-C.; Walker, B. C.; Stach, E. A.; Hillhouse, H. W.; Agrawal, R. Fabrication of 7.2% Efficient CZTSSe Solar Cells Using CZTS Nanocrystals J. Am. Chem. Soc. 2010, 132, 17384 DOI: 10.1021/ja108427bGoogle Scholar12https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC3cXhsVGksbzP&md5=3851f2e1db6e9f152fbcc8bbea3bb1a7Fabrication of 7.2% efficient CZTSSe solar cells using CZTS nanocrystalsGuo, Qi-Jie; Ford, Grayson M.; Yang, Wei-Chang; Walker, Bryce C.; Stach, Eric A.; Hillhouse, Hugh W.; Agrawal, RakeshJournal of the American Chemical Society (2010), 132 (49), 17384-17386CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)Earth abundant copper-zinc-tin-chalcogenide (CZTSSe) is an important class of material for the development of low cost and sustainable thin film solar cells. The fabrication of CZTSSe solar cells by selenization of CZTS nanocrystals is presented. By tuning the compn. of the CZTS nanocrystals and developing a robust film coating method, a total area efficiency as high as 7.2% under AM 1.5 illumination and light soaking has been achieved.
- 13Zhou, H.; Hsu, W.-C.; Duan, H.-S.; Bob, B.; Yang, W.; Song, T.-B.; Hsu, C.-J.; Yang, Y. CZTS nanocrystals: a promising approach for next generation thin film photovoltaics Energy Environ. Sci. 2013, 6, 2822 DOI: 10.1039/c3ee41627eGoogle Scholar13https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC3sXhsV2it7nO&md5=1fdd619011b07047656a9462d54f7bc6CZTS nanocrystals: a promising approach for next generation thin film photovoltaicsZhou, Huanping; Hsu, Wan-Ching; Duan, Hsin-Sheng; Bob, Brion; Yang, Wenbing; Song, Tze-Bin; Hsu, Chia-Jung; Yang, YangEnergy & Environmental Science (2013), 6 (10), 2822-2838CODEN: EESNBY; ISSN:1754-5706. (Royal Society of Chemistry)A review. Cu2ZnSn(S,Se)4 (CZTSSe) has received considerable attention as a material capable of driving the development of low-cost and high performance photovoltaics. Its high absorption coeff., optimal band gap, and non-toxic, naturally abundant elemental constituents give it a no. of advantages over most thin film absorber materials. In this manuscript, we discuss the current status of CZTSSe photovoltaics, and provide a comprehensive review of Cu2ZnSnS4 (CZTS) and Cu2ZnSnSe4 (CZTSe) nanocrystal (NCs)-based fabrication methods and solar cell characteristics. The focus will be on the relevant synthetic chem., film deposition, and the prodn. of high efficiency photovoltaic devices. Various colloidal synthesis routes are currently used to form the highest quality CZTSSe film from the nanocrystals with controllable phase, size, shape, compn., and surface ligands. A variety of recipes are summarized for producing nanocrystal inks that are appropriate for forming CZTSSe absorber materials with a wide range of controllable optoelectronic properties. Deposition and post-processing, such as annealing and selenization treatments, play an important role in defining the phase and structure of the resulting material. Various film treatment strategies are outlined here, and their resulting material quality, device performance, and dominant photovoltaic loss mechanisms are discussed. Suggestions regarding needed improvements and future research directions are provided based on the current field of available literature.
- 14Bag, S.; Gunawan, O.; Gokmen, T.; Zhu, Y.; Todorov, T. K.; Mitzi, D. B. Low band gap liquid-processed CZTSe solar cell with 10.1% efficiency Energy Environ. Sci. 2012, 5, 7060 DOI: 10.1039/c2ee00056cGoogle Scholar14https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC38XmsVWqtLY%253D&md5=90eb5694c92ba6c302ea2865edc22e27Low band gap liquid-processed CZTSe solar cell with 10.1% efficiencyBag, Santanu; Gunawan, Oki; Gokmen, Tayfun; Zhu, Yu; Todorov, Teodor K.; Mitzi, David B.Energy & Environmental Science (2012), 5 (5), 7060-7065CODEN: EESNBY; ISSN:1754-5706. (Royal Society of Chemistry)A low band gap liq.-processed Cu2ZnSn(Se1-xSx)4 (CZTSSe) kesterite solar cell with x ≈ 0.03 is prepd. from earth abundant metals, yielding 10.1% power conversion efficiency. This champion cell shows a band gap of 1.04 eV, higher minority-carrier lifetime, lower series resistance and lower Voc deficit compared to our previously reported higher band gap (Eg = 1.15 eV; x ≈ 0.4) cell with similar record efficiency. The ability to vary the CZTSSe band gap using sulfur content (i.e., varying x) facilitates the examn. of factors limiting performance in the current generation of CZTSSe devices, as part of the thrust to achieve operational parity with CdTe and Cu(In,Ga)(S,Se)2 (CIGSSe) analogs.
- 15Xin, H.; Katahara, J. K.; Braly, I. L.; Hillhouse, H. W. 8% Efficient Cu2ZnSn(S,Se)4 Solar Cells from Redox Equilibrated Simple Precursors in DMSO Adv. Energy Mater. 2014, 4, 1301823 DOI: 10.1002/aenm.201301823Google ScholarThere is no corresponding record for this reference.
- 16Haass, S. G.; Diethelm, M.; Werner, M.; Bissig, B.; Romanyuk, Y. E.; Tiwari, A. N. 11.2% Efficient Solution Processed Kesterite Solar Cell with a Low Voltage Deficit Adv. Energy Mater. 2015, 5, 1500712 DOI: 10.1002/aenm.201500712Google ScholarThere is no corresponding record for this reference.
- 17Su, Z.; Sun, K.; Han, Z.; Cui, H.; Liu, F.; Lai, Y.; Li, J.; Hao, X.; Liu, Y.; Green, M. A. Fabrication of Cu2ZnSnS4 solar cells with 5.1% efficiency via thermal decomposition and reaction using a non-toxic sol–gel route J. Mater. Chem. A 2014, 2, 500 DOI: 10.1039/C3TA13533KGoogle Scholar17https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC3sXhvVOgtb%252FM&md5=871592091807cc9d093933b86f788d80Fabrication of Cu2ZnSnS4 solar cells with 5.1% efficiency via thermal decomposition and reaction using a non-toxic sol-gel routeSu, Zhenghua; Sun, Kaiwen; Han, Zili; Cui, Hongtao; Liu, Fangyang; Lai, Yanqing; Li, Jie; Hao, Xiaojing; Liu, Yexiang; Green, Martin A.Journal of Materials Chemistry A: Materials for Energy and Sustainability (2014), 2 (2), 500-509CODEN: JMCAET; ISSN:2050-7496. (Royal Society of Chemistry)Earth-abundant Cu2ZnSnS4 (CZTS) has been confirmed as a promising semiconductor material for thin film solar cells. To meet the requirements of high-efficiency and low-cost for photovoltaic technologies, a modified thermal decompn. sol-gel method with low-cost and low-toxicity for CZTS thin film prepn. is presented, and the detailed formation mechanism of the thin film is investigated to obtain an optimized process. By introducing non-aq. thiourea-metal-oxygen sol-gel processing, as well as applying extrinsic dopants and chem. etching, high-quality and phase-controlled CZTS thin films with homogeneous elemental distribution and a low impurity content have been synthesized. Based on the modified sol-gel method, solar cells with a structure of Ni:Al/ZAO/i-ZnO/CdS/CZTS/Mo/glass have been fabricated, and a power conversion efficiency of 5.10% is obtained, indicating its potential for high-throughput and high power conversion efficiency photovoltaic devices.
- 18Liu, X.; Feng, Y.; Cui, H.; Liu, F.; Hao, X.; Conibeer, G.; Mitzi, D. B.; Green, M. The current status and future prospects of kesterite solar cells: a brief review Prog. Photovoltaics 2016, 24, 879 DOI: 10.1002/pip.2741Google ScholarThere is no corresponding record for this reference.
- 19Bourdais, S.; Choné, C.; Delatouche, B.; Jacob, A.; Larramona, G.; Moisan, C.; Lafond, A.; Donatini, F.; Rey, G.; Siebentritt, S. Is the Cu/Zn disorder the main culprit for the voltage deficit in kesterite solar cells? Adv. Energy Mater. 2016, 6, 1502276 DOI: 10.1002/aenm.201502276Google ScholarThere is no corresponding record for this reference.
- 20Gunawan, O.; Gokmen, T.; Mitzi, D. B. Suns-VOC characteristics of high performance kesterite solar cells J. Appl. Phys. 2014, 116, 084504 DOI: 10.1063/1.4893315Google Scholar20https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC2cXhsVWksbfP&md5=4fb57ed5313a7854bbd4195d3956c9adSuns-VOC characteristics of high performance kesterite solar cellsGunawan, Oki; Gokmen, Tayfun; Mitzi, David B.Journal of Applied Physics (Melville, NY, United States) (2014), 116 (8), 084504/1-084504/9CODEN: JAPIAU; ISSN:0021-8979. (American Institute of Physics)Low open circuit voltage (VOC) was recognized as the no. one problem in the current generation of Cu2ZnSn(Se,S)4 (CZTSSe) solar cells. The authors report high light intensity and low temp. Suns-VOC measurement in high performance CZTSSe devices. The Suns-VOC curves exhibit bending at high light intensity, which points to several prospective VOC limiting mechanisms that could impact the VOC, even at 1 sun for lower performing samples. These VOC limiting mechanisms include low bulk cond. (because of low hole d. or low mobility), bulk or interface defects, including tail states, and a nonohmic back contact for low carrier d. CZTSSe. The nonohmic back contact problem can be detected by Suns-VOC measurements with different monochromatic illuminations. These limiting factors may also contribute to an artificially lower JSC-VOC diode ideality factor. (c) 2014 American Institute of Physics.
- 21Gershon, T.; Gokmen, T.; Gunawan, O.; Haight, R.; Guha, S.; Shin, B. Understanding the relationship between Cu2ZnSn(S,Se)4 material properties and device performance MRS Commun. 2014, 4, 159 DOI: 10.1557/mrc.2014.34Google Scholar21https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC2cXitFOku73I&md5=ab98ce64853d9e3bb531b60ff55c2e51Understanding the relationship between Cu2ZnSn(S,Se)4 material properties and device performanceGershon, Talia; Gokmen, Tayfun; Gunawan, Oki; Haight, Richard; Guha, Supratik; Shin, ByunghaMRS Communications (2014), 4 (4), 159-170CODEN: MCROF8; ISSN:2159-6867. (Cambridge University Press)Cu2ZnSn(S,Se)4 (CZTSSe) photovoltaics (PV) have long been considered promising candidates for large-scale PV deployment due to the availability of constituent elements and steady improvements in device efficiency over time. The key limitation to high efficiency in this technol. remains a deficit in the open-circuit voltage with respect to the band gap. The past decade has seen significant progress toward understanding how the various material properties such as bulk and surface compn., point defects (intrinsic and extrinsic), and grain boundaries all impact the optoelectronic properties of CZTSSe materials, and consequently device performance. This paper aims to summarize what is known about the CZTSSe bulk and surfaces, and how these material properties may be related to the Voc deficit.
- 22Shin, D.; Saparov, B.; Mitzi, D. B. Defect Engineering in Multinary Earth-Abundant Chalcogenide Photovoltaic Materials Adv. Energy Mater. 2017, 1602366 DOI: 10.1002/aenm.201602366Google ScholarThere is no corresponding record for this reference.
- 23Lloyd, M. A.; Bishop, D.; Gunawan, O.; Mccandless, B. Fabrication and performance limitations in single crystal Cu2ZnSnSe4 Solar Cells 2016 IEEE 43rd Photovoltaic Specialists Conference (PVSC) 2016, 3636 DOI: 10.1109/PVSC.2016.7750352Google ScholarThere is no corresponding record for this reference.
- 24Baranowski, L. L.; Zawadzki, P.; Lany, S.; Toberer, E. S.; Zakutayev, A. A review of defects and disorder in multinary tetrahedrally bonded semiconductors Semicond. Sci. Technol. 2016, 31, 123004 DOI: 10.1088/0268-1242/31/12/123004Google Scholar24https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC2sXhs1yju7o%253D&md5=7d307fab9979316d2675e0421896547dA review of defects and disorder in multinary tetrahedrally bonded semiconductorsBaranowski, Lauryn L.; Zawadzki, Pawel; Lany, Stephan; Toberer, Eric S.; Zakutayev, AndriySemiconductor Science and Technology (2016), 31 (12), 123004/1-123004/16CODEN: SSTEET; ISSN:0268-1242. (IOP Publishing Ltd.)Defects are crit. to understanding the electronic properties of semiconducting compds., for applications such as light-emitting diodes, transistors, photovoltaics, and thermoelecs. In this review, we describe our work investigating defects in tetrahedrally bonded, multinary semiconductors, and discuss the place of our research within the context of publications by other groups. We applied exptl. and theory techniques to understand point defects, structural disorder, and extended antisite defects in one semiconductor of interest for photovoltaic applications, Cu2SnS3. We contrast our findings on Cu2SnS3 with other chem. related Cu-Sn-S compds., as well as structurally related compds. such as Cu2ZnSnS4 and Cu(In,Ga)Se2. We find that evaluation of point defects alone is not sufficient to understand defect behavior in multinary tetrahedrally bonded semiconductors. In the case of Cu2SnS3 and Cu2ZnSnS4, structural disorder and entropy-driven cation clustering can result in nanoscale compositional inhomogeneities which detrimentally impact the electronic transport. Therefore, it is not sufficient to assess only the point defect behavior of new multinary tetrahedrally bonded compds.; effects such as structural disorder and extended antisite defects must also be considered. Overall, this review provides a framework for evaluating tetrahedrally bonded semiconducting compds. with respect to their defect behavior for photovoltaic and other applications, and suggests new materials that may not be as prone to such imperfections.
- 25Chen, S. Y.; Yang, J.-H. H.; Gong, X. G.; Walsh, A.; Wei, S.-H. H. Intrinsic point defects and complexes in the quaternary kesterite semiconductor Cu2ZnSnS4 Phys. Rev. B: Condens. Matter Mater. Phys. 2010, 81, 245204 DOI: 10.1103/PhysRevB.81.245204Google Scholar25https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC3cXosVyisL8%253D&md5=5911ab778637c784c36483ea18663cf7Intrinsic point defects and complexes in the quaternary kesterite semiconductor Cu2ZnSnS4Chen, Shiyou; Yang, Ji-Hui; Gong, X. G.; Walsh, Aron; Wei, Su-HuaiPhysical Review B: Condensed Matter and Materials Physics (2010), 81 (24), 245204/1-245204/10CODEN: PRBMDO; ISSN:1098-0121. (American Physical Society)Current knowledge of the intrinsic defect properties of Cu2ZnSnS4 (CZTS) is limited, which is hindering further improvement of the performance of CZTS-based solar cells. Here, the authors have performed 1st-principles calcns. for intrinsic defects and defect complexes in CZTS, from which the authors have the following observations. (i) It is important to control the elemental chem. potentials during crystal growth to avoid the formation of secondary phases such as ZnS, CuS, and Cu2SnS3. (ii) The intrinsic p-type cond. is attributed to the CuZn antisite which has a lower formation energy and relatively deeper acceptor level compared to the Cu vacancy. (iii) The low formation energy of many of the acceptor defects will lead to the intrinsic p-type character, i.e., n-type doping is very difficult in this system. (iv) The role of elec. neutral defect complexes is predicted to be important, because they have remarkably low formation energies and electronically passivate deep levels in the band gap. For example, [CuZn-+ZnCu+], [VCu-+ZnCu+], and [ZnSn2-+2ZnCu+] may form easily in nonstoichiometric samples. The band alignment between Cu2ZnSnS4, CuInSe2 and the solar-cell window layer CdS also was calcd., revealing that a type-II band alignment exists for the CdS/Cu2ZnSnS4 heterojunction. The fundamental differences between CZTS and CuInSe2 for use in thin-film photovoltaics are discussed. The results are expected to be relevant to other I2-II-IV-VI4 semiconductors.
- 26Scragg, J. J. S.; Larsen, J. K.; Kumar, M.; Persson, C.; Sendler, J.; Siebentritt, S.; Platzer Björkman, C. Cu-Zn Disorder and Band Gap Fluctuations in Cu2ZnSn(S,Se)4: Theoretical and Experimental Investigations Phys. Status Solidi B 2016, 253, 247 DOI: 10.1002/pssb.201552530Google Scholar26https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC2MXhsFGhtb7M&md5=f0d49480d885afc209ee9a887a71c6d8Cu-Zn disorder and band gap fluctuations in Cu2ZnSn(S,Se)4: Theoretical and experimental investigationsScragg, Jonathan J. S.; Larsen, Jes K.; Kumar, Mukesh; Persson, Clas; Sendler, Jan; Siebentritt, Susanne; Platzer Bjoerkman, CharlottePhysica Status Solidi B: Basic Solid State Physics (2016), 253 (2), 247-254CODEN: PSSBBD; ISSN:0370-1972. (Wiley-VCH Verlag GmbH & Co. KGaA)Cu2ZnSn(S,Se)4 (CZTS(e)) solar cells suffer from low-open-circuit voltages that have been blamed on the existence of band gap fluctuations, with different possible origins. In this paper, we show from both theor. and exptl. standpoints that disorder of Cu and Zn atoms is in all probability the primary cause of these fluctuations. First, quantification of Cu-Zn disorder in CZTS thin films is presented. The results indicate that disorder is prevalent in the majority of practical samples used for solar cells. Then, ab initio calcns. for different arrangements and densities of disorder-induced [CuZn + ZnCu] defect pairs are presented and it is shown that spatial variations in band gap of the order of 200 meV can easily be caused by Cu-Zn disorder, which would cause large voltage losses in solar cells. Expts. using Raman spectroscopy and room temp. photoluminescence combined with in situ heat-treatments show that a shift in the energy of the dominant band-to-band recombination pathway correlates perfectly to the order-disorder transition, which clearly implicates Cu-Zn disorder as the cause of band gap fluctuations in CZTS. Our results suggest that elimination or passivation of Cu-Zn disorder could be very important for future improvements in the efficiency of CZTS(e)-based solar cells.
- 27Van Mieghem, P. Theory of band tails in heavily doped semiconductors Rev. Mod. Phys. 1992, 64, 755 DOI: 10.1103/RevModPhys.64.755Google Scholar27https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADyaK38XltlOrs7k%253D&md5=081f7cbec8f0ab9a405317875982d1f3Theory of band tails in heavily doped semiconductorsVan Mieghem, PietReviews of Modern Physics (1992), 64 (3), 755-93CODEN: RMPHAT; ISSN:0034-6861.The different classes of theories that describe the band tailing of the d. of states in heavily doped semiconductors are reviewed with many refs.
- 28Gokmen, T.; Gunawan, O.; Todorov, T. K.; Mitzi, D. B. Band tailing and efficiency limitation in kesterite solar cells Appl. Phys. Lett. 2013, 103, 103506 DOI: 10.1063/1.4820250Google Scholar28https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC3sXhtl2rur%252FL&md5=72e4de46c98db2b4f58d458d991419baBand tailing and efficiency limitation in kesterite solar cellsGokmen, Tayfun; Gunawan, Oki; Todorov, Teodor K.; Mitzi, David B.Applied Physics Letters (2013), 103 (10), 103506/1-103506/5CODEN: APPLAB; ISSN:0003-6951. (American Institute of Physics)A fundamental performance bottleneck for hydrazine processed kesterite Cu2ZnSn(S,Se)4 (CZTSSe) solar cells with efficiencies reaching >11% can be the formation of band-edge tail states, which quantum efficiency and photoluminescence data indicate is roughly twice as severe as in higher-performing Cu(In,Ga)(S,Se)2 devices. Low temp. time-resolved photoluminescence data suggest that the enhanced tailing arises primarily from electrostatic potential fluctuations induced by strong compensation and facilitated by a lower CZTSSe dielec. const. The implications of the band tails for the voltage deficit in these devices are discussed. (c) 2013 American Institute of Physics.
- 29Gershon, T.; Sardashti, K.; Gunawan, O.; Mankad, R.; Singh, S.; Lee, Y. S.; Ott, J. A.; Kummel, A.; Haight, R. Photovoltaic Device with over 5% Efficiency Based on an n-Type Ag2ZnSnSe4 Absorber Adv. Energy Mater. 2016, 6, 1601182 DOI: 10.1002/aenm.201601182Google ScholarThere is no corresponding record for this reference.
- 30Scragg, J. J. S.; Choubrac, L.; Lafond, A.; Ericson, T.; Platzer-Björkman, C. A low-temperature order-disorder transition in Cu2ZnSnS4 thin films Appl. Phys. Lett. 2014, 104, 041911 DOI: 10.1063/1.4863685Google Scholar30https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC2cXjtlyqtLs%253D&md5=0444978629d2ef4d755f129b93938731A low-temperature order-disorder transition in Cu2ZnSnS4 thin filmsScragg, Jonathan J. S.; Choubrac, Leo; Lafond, Alain; Ericson, Tove; Platzer-Bjoerkman, CharlotteApplied Physics Letters (2014), 104 (4), 041911/1-041911/4CODEN: APPLAB; ISSN:0003-6951. (American Institute of Physics)Cu2ZnSnS4 (CZTS) is an interesting material for sustainable photovoltaics, but efficiencies are limited by the low open-circuit voltage. A possible cause of this is disorder among the Cu and Zn cations, a phenomenon which is difficult to detect by std. techniques. We show that this issue can be overcome using near-resonant Raman scattering, which lets us est. a crit. temp. of 533 ± 10 K for the transition between ordered and disordered CZTS. These findings have deep significance for the synthesis of high-quality material, and pave the way for quant. investigation of the impact of disorder on the performance of CZTS-based solar cells. (c) 2014 American Institute of Physics.
- 31Chen, S.; Walsh, A.; Gong, X.-G.; Wei, S.-H. Classification of lattice defects in the kesterite Cu2ZnSnS4 and Cu2ZnSnSe4 earth-abundant solar cell absorbers Adv. Mater. 2013, 25, 1522 DOI: 10.1002/adma.201203146Google Scholar31https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC3sXit1Kms7c%253D&md5=8dc20228591c6104fbfcadfe1bbc72edClassification of Lattice Defects in the Kesterite Cu2ZnSnS4 and Cu2ZnSnSe4 Earth-Abundant Solar Cell AbsorbersChen, Shiyou; Walsh, Aron; Gong, Xin-Gao; Wei, Su-HuaiAdvanced Materials (Weinheim, Germany) (2013), 25 (11), 1522-1539CODEN: ADVMEW; ISSN:0935-9648. (Wiley-VCH Verlag GmbH & Co. KGaA)A review of theor. research on defect formation and ionization in kesterite materials, based on new systematic calcns., and compared with the better studied chalcopyrite materials CuGaSe2 and CuInSe2. The kesterite-structured semiconductors Cu2ZnSnS4 and Cu2ZnSnSe4 are drawing considerable attention recently as the active layers in earth-abundant low-cost thin-film solar cells. The addnl. no. of elements in these quaternary compds., relative to binary and ternary semiconductors, results in increased flexibility in the material properties. Conversely, a large variety of intrinsic lattice defects can also be formed, which have important influence on their optical and elec. properties, and hence their photovoltaic performance. Exptl. identification of these defects is currently limited due to poor sample quality. Four features are revealed and highlighted: (i) the strong phase-competition between the kesterites and the coexisting secondary compds.; (ii) the intrinsic p-type cond. detd. by the high population of acceptor CuZn anti-sites and Cu vacancies, and their dependence on the Cu/(Zn+Sn) and Zn/Sn ratio; (iii) the role of charge-compensated defect clusters such as [2CuZn+SnZn], [VCu+ZnCu] and [ZnSn+2ZnCu] and their contribution to nonstoichiometry; (iv) the electron-trapping effect of the abundant [2CuZn+SnZn] clusters, esp. in Cu2ZnSnS4. The calcd. properties explain the exptl. observation that Cu poor and Zn rich conditions (Cu/(Zn+Sn) ≈ 0.8 and Zn/Sn ≈ 1.2) result in the highest solar cell efficiency, as well as suggesting an efficiency limitation in Cu2ZnSn(S,Se)4 cells when the S compn. is high.
- 32Stoneham, A. M. Non-radiative transitions in semiconductors Rep. Prog. Phys. 1981, 44, 1251 DOI: 10.1088/0034-4885/44/12/001Google Scholar32https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADyaL38XitVyrt7Y%253D&md5=7b69cd5af0b6bda2ba714e41b36bc383Nonradiative transitions in semiconductorsStoneham, A. M.Reports on Progress in Physics (1981), 44 (12), 1251-95CODEN: RPPHAG; ISSN:0034-4885.A review with many refs.
- 33Major, J. D.; Treharne, R. E.; Phillips, L. J.; Durose, K. A low-cost non-toxic post-growth activation step for CdTe solar cells Nature 2014, 511, 334 DOI: 10.1038/nature13435Google Scholar33https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC2cXhtFOjtrrK&md5=9db3cf862c3a25c97e608a7d2d9a362cA low-cost non-toxic post-growth activation step for CdTe solar cellsMajor, J. D.; Treharne, R. E.; Phillips, L. J.; Durose, K.Nature (London, United Kingdom) (2014), 511 (7509), 334-337CODEN: NATUAS; ISSN:0028-0836. (Nature Publishing Group)Cadmium telluride, CdTe, is now firmly established as the basis for the market-leading thin-film solar-cell technol. With lab. efficiencies approaching 20 per cent, the research and development targets for CdTe are to reduce the cost of power generation further to less than half a US dollar per W (ref. 2) and to minimize the environmental impact. A central part of the manufg. process involves doping the polycryst. thin-film CdTe with CdCl2. This acts to form the photovoltaic junction at the CdTe/CdS interface and to passivate the grain boundaries, making it essential in achieving high device efficiencies. However, although such doping has been almost ubiquitous since the development of this processing route over 25 years ago, CdCl2 has two severe disadvantages; it is both expensive (about 30 cents per g) and a water-sol. source of toxic cadmium ions, presenting a risk to both operators and the environment during manuf. Here we demonstrate that solar cells prepd. using MgCl2, which is non-toxic and costs less than a cent per g, have efficiencies (around 13%) identical to those of a CdCl2-processed control group. They have similar hole densities in the active layer (9 × 1014 cm-3) and comparable impurity profiles for Cl and O, these elements being important p-type dopants for CdTe thin films. Contrary to expectation, CdCl2-processed and MgCl2-processed solar cells contain similar concns. of Mg; this is because of Mg out-diffusion from the soda-lime glass substrates and is not disadvantageous to device performance. However, treatment with other low-cost chlorides such as NaCl, KCl and MnCl2 leads to the introduction of elec. active impurities that do compromise device performance. Our results demonstrate that CdCl2 may simply be replaced directly with MgCl2 in the existing fabrication process, thus both minimizing the environmental risk and reducing the cost of CdTe solar-cell prodn.
- 34Gabor, A. M.; Tuttle, J. R.; Albin, D. S.; Contreras, M. A.; Noufi, R.; Hermann, A. M. High-efficiency CuInxGa1-xSe2 Solar Cells Made From (Inx,Ga1-x)2Se3 Precursor Films Appl. Phys. Lett. 1994, 65, 198 DOI: 10.1063/1.112670Google Scholar34https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADyaK2cXkvVOmsbk%253D&md5=353036a4d86cf581c24371c8593d1ec6High-efficiency CuInxGa1-xSe2 solar cells made from (Inx,Ga1-x)2Se3 precursor filmsGabor, Andrew M.; Tuttle, John R.; Albin, David S.; Contreras, Miguel A.; Noufi, Rommel; Hermann, Allen M.Applied Physics Letters (1994), 65 (2), 198-200CODEN: APPLAB; ISSN:0003-6951.In, Ga, and Se were co-evapd. to form precursor films of (Inx,Ga1-x)2Se3. The precursors were then converted to CuInxGa1-xSe2 by exposure to a flux of Cu and Se. The final films were smooth, with tightly packed grains, and had a graded Ga content as a function of film depth. Photovoltaic devices made from these films showed good tolerance in device efficiency to variations in film compn. A device made from these films resulted in the highest total-area efficiency measured for any non-single-crystal, thin-film solar cell, at 15.9%.
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Figure 1
Figure 1. Comparison of the cumulative research output for Cu2ZnSn(S,Se)4 (kesterite) and CH3NH3PbI3 (halide perovskite) related photovoltaic technologies against year and the record light-to-electricity conversion efficiency. The publication data has been generated from Web of Science (February 15, 2017) for kesterite (search terms: “kesterite OR stannite” AND “solar cell”) and halide perovskite (search terms: “halide perovskite OR iodide perovskite OR hybrid perovskite” AND “solar cell”) solar cells.
Figure 2
Figure 2. Acceptor defect-induced energy level within the band gap of a semiconductor, broadening to an impurity band with increased defect density until at sufficiently high defect concentrations the band merges with the valence band maximum, resulting in a reduced band gap. Such tail states would limit the open-circuit voltage accessible in a photovoltaic device and could explain the voltage deficit observed in kesterite-based solar cells. Also listed are the range of possible acceptor and donor defects in Cu2ZnSnS4; a large number of possible charge-neutral defect clusters can also be formed.
References
This article references 34 other publications.
- 1Polman, A.; Knight, M.; Garnett, E. C.; Ehrler, B.; Sinke, W. C. Photovoltaic materials – present efficiencies and future challenges Science 2016, 352, aad4424 DOI: 10.1126/science.aad4424There is no corresponding record for this reference.
- 2Green, M. A.; Emery, K.; Hishikawa, Y.; Warta, W.; Dunlop, E. D. Solar Cell Efficiency Tables (Version 49) Prog. Photovoltaics 2016, 24, 3 DOI: 10.1002/pip.2728There is no corresponding record for this reference.
- 3Park, N.-G.; Grätzel, M.; Miyasaka, T.; Zhu, K.; Emery, K. Towards stable and commercially available perovskite solar cells Nat. Energy 2016, 1, 16152 DOI: 10.1038/nenergy.2016.1523https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC2sXhtVersL0%253D&md5=0c0a441bdcb4d193c128dc29b186fa9dTowards stable and commercially available perovskite solar cellsPark, Nam-Gyu; Gratzel, Michael; Miyasaka, Tsutomu; Zhu, Kai; Emery, KeithNature Energy (2016), 1 (11), 16152CODEN: NEANFD; ISSN:2058-7546. (Nature Publishing Group)Solar cells employing a halide perovskite with an org. cation now show power conversion efficiency of up to 22%. However, these cells are facing issues towards commercialization, such as the need to achieve long-term stability and the development of a manufg. method for the reproducible fabrication of high-performance devices. Here, we propose a strategy to obtain stable and com. viable perovskite solar cells. A reproducible manufg. method is suggested, as well as routes to manage grain boundaries and interfacial charge transport. Electroluminescence is regarded as a metric to gauge theor. efficiency. We highlight how optimizing the design of device architectures is important not only for achieving high efficiency but also for hysteresis-free and stable performance. We argue that reliable device characterization is needed to ensure the advance of this technol. towards practical applications. We believe that perovskite-based devices can be competitive with silicon solar modules, and discuss issues related to the safe management of toxic material.
- 4Katagiri, H.; Sasaguchi, N.; Hando, S.; Hoshino, S.; Ohashi, J.; Yokota, T. Preparation and evaluation of Cu2ZnSnS4 thin films by sulfurization of E-B evaporated precursors Sol. Energy Mater. Sol. Cells 1997, 49, 407 DOI: 10.1016/S0927-0248(97)00119-04https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADyaK2sXmvFKmtrg%253D&md5=63787577bd92271d132d5f9e713b85d8Preparation and evaluation of Cu2ZnSnS4 thin films by sulfurization of E-B evaporated precursorsKatagiri, Hironori; Sasaguchi, Nobuyuki; Hando, Shima; Hoshino, Suguru; Ohashi, Jiro; Yokota, TakaharuSolar Energy Materials and Solar Cells (1997), 49 (1-4), 407-414CODEN: SEMCEQ; ISSN:0927-0248. (Elsevier)By sulfurization of E-B evapd. precursors, CZTS (Cu2ZnSnS4) films could be prepd. successfully. This semiconductor does not consist of any rare-metal such as In. The x-ray diffraction pattern of CZTS thin films showed that these films had a stannite structure. This study estd. the optical band gap energy as 1.45 eV. The optical absorption coeff. was in the order of 104 cm-1. The resistivity was in the the order of 104 Ω-cm and the conduction type was p-type. Fabricated solar cells, Al/ZnO/CdS/CZTS/Mo/Soda Lime Glass, showed an open-circuit voltage up to 400 mV.
- 5Wang, W.; Winkler, M. T.; Gunawan, O.; Gokmen, T.; Todorov, T. K.; Zhu, Y.; Mitzi, D. B. Device Characteristics of CZTSSe Thin-Film Solar Cells with 12.6% Efficiency Adv. Energy Mater. 2014, 4, 1301465 DOI: 10.1002/aenm.2013014655https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC2cXnvVShtb8%253D&md5=87a6fd2fd03588d5b9d82fbfe79c2d1aDevice Characteristics of CZTSSe Thin-Film Solar Cells with 12.6% EfficiencyWang, Wei; Winkler, Mark T.; Gunawan, Oki; Gokmen, Tayfun; Todorov, Teodor K.; Zhu, Yu; Mitzi, David B.Advanced Energy Materials (2014), 4 (7), 1301465/1-1301465/5CODEN: ADEMBC; ISSN:1614-6840. (Wiley-Blackwell)The device characteristics of copper zinc tin sulfoselenide thin-film solar cells with 12.6% efficiency are studied. The current-voltage characteristics, quantum efficiency, capacitance, and electron-beam-induced current (EBIC) measurements are discussed in detail.
- 6
Reported at PVSEC-36 by a research team led at DGIST in South Korea. A 0.181 cm2 solar cell was certified at 13.80% by KIER.
There is no corresponding record for this reference. - 7Jiang, F.; Ikeda, S.; Harada, T.; Matsumura, M. Pure Sulfide Cu2ZnSnS4 Thin Film Solar Cells Fabricated by Preheating an Electrodeposited Metallic Stack Adv. Energy Mater. 2014, 4, 1301381 DOI: 10.1002/aenm.2013013817https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC2cXnvVShsL8%253D&md5=d016de5552e67c0df43c4ae63afecad6Pure Sulfide Cu2ZnSnS4 Thin Film Solar Cells Fabricated by Preheating an Electrodeposited Metallic StackJiang, Feng; Ikeda, Shigeru; Harada, Takashi; Matsumura, MichioAdvanced Energy Materials (2014), 4 (7), 1301381/1-1301381/4CODEN: ADEMBC; ISSN:1614-6840. (Wiley-Blackwell)This article describes the pure sulfide Cu2ZnSnS4 thin film solar cells fabricated by preheating electrodeposited metallic stack. A champion solar cell with sunlight conversion efficiency of 8% (active area, without an MgF2 anti-reflectance layer) is described. The dependence of sunlight conversion efficiency and other device parameters on preheating duration is also discussed on the basis of surface microstructures and bulk compns.
- 8Guo, L.; Zhu, Y.; Gunawan, O.; Gokmen, T.; Deline, V. R.; Ahmed, S.; Romankiw, L. T.; Deligianni, H. Electrodeposited Cu2ZnSnSe4 Thin Film Solar Cell with 7% Power Conversion Efficiency Prog. Photovoltaics 2014, 22, 58 DOI: 10.1002/pip.23328https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC3sXhvFejtbnN&md5=d0ea406ecf5da9185c1cfa0a23d27d16Electrodeposited Cu2ZnSnSe4 thin film solar cell with 7% power conversion efficiencyGuo, Lian; Zhu, Yu; Gunawan, Oki; Gokmen, Tayfun; Deline, Vaughn R.; Ahmed, Shafaat; Romankiw, Lubomyr T.; Deligianni, HarikliaProgress in Photovoltaics (2014), 22 (1), 58-68CODEN: PPHOED; ISSN:1062-7995. (John Wiley & Sons Ltd.)High performance Cu2ZnSnSe4 (CZTSe) photovoltaic materials were synthesized by electrodeposition of metal stack precursors followed by selenization. A champion solar cell with 7.0% efficiency is demonstrated. This is the highest efficiency among all of the CZTSe solar cells prepd. from electrodeposited metallic precursors reported to-date. Device parameters are discussed from the perspective of material microstructure and compn. in order to improve performance. In addn., a high performance electrodeposited CZTS (S only) solar cell was demonstrated and its device characteristics were compared against the CZTSe (Se only) cell. Using secondary ion mass spectrometry for the anal. of the chem. compn. of the absorber layer, a higher concn. of oxygen in the electrodeposited absorber is thought to be the root cause of the lower performance of the electrodeposited CZTS or CZTSe solar cells with respect to a solar cell fabricated by evapn. The grain boundary areas of Sn-rich compn. are thought to be responsible for the lower shunt resistance commonly obsd. in CZTSe devices. We measured the longest minority carrier lifetime of 18 ns among all reported kesterite devices. This work builds a good baseline for obtaining higher efficiency earth-abundant solar cells, while it highlights electrodeposition as a low cost and feasible method for earth-abundant thin film solar cell fabrication.
- 9Yao, L.; Ao, J.; Jeng, M.-J.; Bi, J.; Gao, S.; He, Q.; Zhou, Z.; Sun, G.; Sun, Y.; Chang, L.-B. CZTSe solar cells prepared by electrodeposition of Cu/Sn/Zn stack layer followed by selenization at low Se pressure Nanoscale Res. Lett. 2014, 9, 678 DOI: 10.1186/1556-276X-9-6789https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A280%3ADC%252BC2MvntVCqtQ%253D%253D&md5=0b1948a4ad2a854b72218c4e75c9d9dbCZTSe solar cells prepared by electrodeposition of Cu/Sn/Zn stack layer followed by selenization at low Se pressureYao Liyong; Ao Jianping; Bi Jinlian; Gao Shoushuai; He Qing; Zhou Zhiqiang; Sun Guozhong; Sun Yun; Jeng Ming-Jer; Chang Liann-Be; Chen Jian-WunNanoscale research letters (2014), 9 (1), 678 ISSN:1931-7573.Cu2ZnSnSe4 (CZTSe) thin films are prepared by the electrodeposition of stack copper/tin/zinc (Cu/Sn/Zn) precursors, followed by selenization with a tin source at a substrate temperature of 530°C. Three selenization processes were performed herein to study the effects of the source of tin on the quality of CZTSe thin films that are formed at low Se pressure. Much elemental Sn is lost from CZTSe thin films during selenization without a source of tin. The loss of Sn from CZTSe thin films in selenization was suppressed herein using a tin source at 400°C (A2) or 530°C (A3). A copper-poor and zinc-rich CZTSe absorber layer with Cu/Sn, Zn/Sn, Cu/(Zn + Sn), and Zn/(Cu + Zn + Sn) with metallic element ratios of 1.86, 1.24, 0.83, and 0.3, respectively, was obtained in a selenization with a tin source at 530°C. The crystallized CZTSe thin film exhibited an increasingly (112)-preferred orientation at higher tin selenide (SnSe x ) partial pressure. The lack of any obvious Mo-Se phase-related diffraction peaks in the X-ray diffraction (XRD) diffraction patterns may have arisen from the low Se pressure in the selenization processes. The scanning electron microscope (SEM) images reveal a compact surface morphology and a moderate grain size. CZTSe solar cells with an efficiency of 4.81% were produced by the low-cost fabrication process that is elucidated herein.
- 10Scragg, J. J.; Dale, P. J.; Peter, L. M. Synthesis and Characterization of Cu2ZnSnS4 Absorber Layers by an Electrodeposition-Annealing Route Thin Solid Films 2009, 517, 2481 DOI: 10.1016/j.tsf.2008.11.022There is no corresponding record for this reference.
- 11Larramona, G.; Bourdais, S.; Jacob, A.; Chone, C.; Muto, T.; Cuccaro, Y.; Delatouche, B.; Moisan, C.; Pere, D.; Dennler, G. 8.6% Efficient CZTSSe Solar Cells Sprayed from Water-Ethanol CZTS Colloidal Solutions J. Phys. Chem. Lett. 2014, 5, 3763 DOI: 10.1021/jz501864a11https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC2cXhslCjtL3I&md5=6d76aae599772f24f1e404e9050eee678.6% Efficient CZTSSe solar cells sprayed from water-ethanol CZTS colloidal solutionsLarramona, Gerardo; Bourdais, Stephane; Jacob, Alain; Chone, Christophe; Muto, Takuma; Cuccaro, Yan; Delatouche, Bruno; Moisan, Camille; Pere, Daniel; Dennler, GillesJournal of Physical Chemistry Letters (2014), 5 (21), 3763-3767CODEN: JPCLCD; ISSN:1948-7185. (American Chemical Society)Copper zinc tin sulfide-selenide, Cu2ZnSn(S1-xSex)4 (CZTSSe), thin film photovoltaic devices were fabricated using a fast and environmentally friendly prepn. method, consisting of the following steps: An instantaneous synthesis of a Cu-Zn-Sn-S (no Se) colloid, a nonpyrolytic spray of a dispersion of this colloid in a water-ethanol mixt., and a sequential annealing first in a N2 atmosphere and second in a Se atm. The achievement of cell efficiencies up to 8.6% under AM1.5G (cell area 0.25 cm2) and without antireflecting coating indicates that this method can compete with other vacuum-based or more complex wet deposition methods.
- 12Guo, Q.; Ford, G. M.; Yang, W.-C.; Walker, B. C.; Stach, E. A.; Hillhouse, H. W.; Agrawal, R. Fabrication of 7.2% Efficient CZTSSe Solar Cells Using CZTS Nanocrystals J. Am. Chem. Soc. 2010, 132, 17384 DOI: 10.1021/ja108427b12https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC3cXhsVGksbzP&md5=3851f2e1db6e9f152fbcc8bbea3bb1a7Fabrication of 7.2% efficient CZTSSe solar cells using CZTS nanocrystalsGuo, Qi-Jie; Ford, Grayson M.; Yang, Wei-Chang; Walker, Bryce C.; Stach, Eric A.; Hillhouse, Hugh W.; Agrawal, RakeshJournal of the American Chemical Society (2010), 132 (49), 17384-17386CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)Earth abundant copper-zinc-tin-chalcogenide (CZTSSe) is an important class of material for the development of low cost and sustainable thin film solar cells. The fabrication of CZTSSe solar cells by selenization of CZTS nanocrystals is presented. By tuning the compn. of the CZTS nanocrystals and developing a robust film coating method, a total area efficiency as high as 7.2% under AM 1.5 illumination and light soaking has been achieved.
- 13Zhou, H.; Hsu, W.-C.; Duan, H.-S.; Bob, B.; Yang, W.; Song, T.-B.; Hsu, C.-J.; Yang, Y. CZTS nanocrystals: a promising approach for next generation thin film photovoltaics Energy Environ. Sci. 2013, 6, 2822 DOI: 10.1039/c3ee41627e13https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC3sXhsV2it7nO&md5=1fdd619011b07047656a9462d54f7bc6CZTS nanocrystals: a promising approach for next generation thin film photovoltaicsZhou, Huanping; Hsu, Wan-Ching; Duan, Hsin-Sheng; Bob, Brion; Yang, Wenbing; Song, Tze-Bin; Hsu, Chia-Jung; Yang, YangEnergy & Environmental Science (2013), 6 (10), 2822-2838CODEN: EESNBY; ISSN:1754-5706. (Royal Society of Chemistry)A review. Cu2ZnSn(S,Se)4 (CZTSSe) has received considerable attention as a material capable of driving the development of low-cost and high performance photovoltaics. Its high absorption coeff., optimal band gap, and non-toxic, naturally abundant elemental constituents give it a no. of advantages over most thin film absorber materials. In this manuscript, we discuss the current status of CZTSSe photovoltaics, and provide a comprehensive review of Cu2ZnSnS4 (CZTS) and Cu2ZnSnSe4 (CZTSe) nanocrystal (NCs)-based fabrication methods and solar cell characteristics. The focus will be on the relevant synthetic chem., film deposition, and the prodn. of high efficiency photovoltaic devices. Various colloidal synthesis routes are currently used to form the highest quality CZTSSe film from the nanocrystals with controllable phase, size, shape, compn., and surface ligands. A variety of recipes are summarized for producing nanocrystal inks that are appropriate for forming CZTSSe absorber materials with a wide range of controllable optoelectronic properties. Deposition and post-processing, such as annealing and selenization treatments, play an important role in defining the phase and structure of the resulting material. Various film treatment strategies are outlined here, and their resulting material quality, device performance, and dominant photovoltaic loss mechanisms are discussed. Suggestions regarding needed improvements and future research directions are provided based on the current field of available literature.
- 14Bag, S.; Gunawan, O.; Gokmen, T.; Zhu, Y.; Todorov, T. K.; Mitzi, D. B. Low band gap liquid-processed CZTSe solar cell with 10.1% efficiency Energy Environ. Sci. 2012, 5, 7060 DOI: 10.1039/c2ee00056c14https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC38XmsVWqtLY%253D&md5=90eb5694c92ba6c302ea2865edc22e27Low band gap liquid-processed CZTSe solar cell with 10.1% efficiencyBag, Santanu; Gunawan, Oki; Gokmen, Tayfun; Zhu, Yu; Todorov, Teodor K.; Mitzi, David B.Energy & Environmental Science (2012), 5 (5), 7060-7065CODEN: EESNBY; ISSN:1754-5706. (Royal Society of Chemistry)A low band gap liq.-processed Cu2ZnSn(Se1-xSx)4 (CZTSSe) kesterite solar cell with x ≈ 0.03 is prepd. from earth abundant metals, yielding 10.1% power conversion efficiency. This champion cell shows a band gap of 1.04 eV, higher minority-carrier lifetime, lower series resistance and lower Voc deficit compared to our previously reported higher band gap (Eg = 1.15 eV; x ≈ 0.4) cell with similar record efficiency. The ability to vary the CZTSSe band gap using sulfur content (i.e., varying x) facilitates the examn. of factors limiting performance in the current generation of CZTSSe devices, as part of the thrust to achieve operational parity with CdTe and Cu(In,Ga)(S,Se)2 (CIGSSe) analogs.
- 15Xin, H.; Katahara, J. K.; Braly, I. L.; Hillhouse, H. W. 8% Efficient Cu2ZnSn(S,Se)4 Solar Cells from Redox Equilibrated Simple Precursors in DMSO Adv. Energy Mater. 2014, 4, 1301823 DOI: 10.1002/aenm.201301823There is no corresponding record for this reference.
- 16Haass, S. G.; Diethelm, M.; Werner, M.; Bissig, B.; Romanyuk, Y. E.; Tiwari, A. N. 11.2% Efficient Solution Processed Kesterite Solar Cell with a Low Voltage Deficit Adv. Energy Mater. 2015, 5, 1500712 DOI: 10.1002/aenm.201500712There is no corresponding record for this reference.
- 17Su, Z.; Sun, K.; Han, Z.; Cui, H.; Liu, F.; Lai, Y.; Li, J.; Hao, X.; Liu, Y.; Green, M. A. Fabrication of Cu2ZnSnS4 solar cells with 5.1% efficiency via thermal decomposition and reaction using a non-toxic sol–gel route J. Mater. Chem. A 2014, 2, 500 DOI: 10.1039/C3TA13533K17https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC3sXhvVOgtb%252FM&md5=871592091807cc9d093933b86f788d80Fabrication of Cu2ZnSnS4 solar cells with 5.1% efficiency via thermal decomposition and reaction using a non-toxic sol-gel routeSu, Zhenghua; Sun, Kaiwen; Han, Zili; Cui, Hongtao; Liu, Fangyang; Lai, Yanqing; Li, Jie; Hao, Xiaojing; Liu, Yexiang; Green, Martin A.Journal of Materials Chemistry A: Materials for Energy and Sustainability (2014), 2 (2), 500-509CODEN: JMCAET; ISSN:2050-7496. (Royal Society of Chemistry)Earth-abundant Cu2ZnSnS4 (CZTS) has been confirmed as a promising semiconductor material for thin film solar cells. To meet the requirements of high-efficiency and low-cost for photovoltaic technologies, a modified thermal decompn. sol-gel method with low-cost and low-toxicity for CZTS thin film prepn. is presented, and the detailed formation mechanism of the thin film is investigated to obtain an optimized process. By introducing non-aq. thiourea-metal-oxygen sol-gel processing, as well as applying extrinsic dopants and chem. etching, high-quality and phase-controlled CZTS thin films with homogeneous elemental distribution and a low impurity content have been synthesized. Based on the modified sol-gel method, solar cells with a structure of Ni:Al/ZAO/i-ZnO/CdS/CZTS/Mo/glass have been fabricated, and a power conversion efficiency of 5.10% is obtained, indicating its potential for high-throughput and high power conversion efficiency photovoltaic devices.
- 18Liu, X.; Feng, Y.; Cui, H.; Liu, F.; Hao, X.; Conibeer, G.; Mitzi, D. B.; Green, M. The current status and future prospects of kesterite solar cells: a brief review Prog. Photovoltaics 2016, 24, 879 DOI: 10.1002/pip.2741There is no corresponding record for this reference.
- 19Bourdais, S.; Choné, C.; Delatouche, B.; Jacob, A.; Larramona, G.; Moisan, C.; Lafond, A.; Donatini, F.; Rey, G.; Siebentritt, S. Is the Cu/Zn disorder the main culprit for the voltage deficit in kesterite solar cells? Adv. Energy Mater. 2016, 6, 1502276 DOI: 10.1002/aenm.201502276There is no corresponding record for this reference.
- 20Gunawan, O.; Gokmen, T.; Mitzi, D. B. Suns-VOC characteristics of high performance kesterite solar cells J. Appl. Phys. 2014, 116, 084504 DOI: 10.1063/1.489331520https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC2cXhsVWksbfP&md5=4fb57ed5313a7854bbd4195d3956c9adSuns-VOC characteristics of high performance kesterite solar cellsGunawan, Oki; Gokmen, Tayfun; Mitzi, David B.Journal of Applied Physics (Melville, NY, United States) (2014), 116 (8), 084504/1-084504/9CODEN: JAPIAU; ISSN:0021-8979. (American Institute of Physics)Low open circuit voltage (VOC) was recognized as the no. one problem in the current generation of Cu2ZnSn(Se,S)4 (CZTSSe) solar cells. The authors report high light intensity and low temp. Suns-VOC measurement in high performance CZTSSe devices. The Suns-VOC curves exhibit bending at high light intensity, which points to several prospective VOC limiting mechanisms that could impact the VOC, even at 1 sun for lower performing samples. These VOC limiting mechanisms include low bulk cond. (because of low hole d. or low mobility), bulk or interface defects, including tail states, and a nonohmic back contact for low carrier d. CZTSSe. The nonohmic back contact problem can be detected by Suns-VOC measurements with different monochromatic illuminations. These limiting factors may also contribute to an artificially lower JSC-VOC diode ideality factor. (c) 2014 American Institute of Physics.
- 21Gershon, T.; Gokmen, T.; Gunawan, O.; Haight, R.; Guha, S.; Shin, B. Understanding the relationship between Cu2ZnSn(S,Se)4 material properties and device performance MRS Commun. 2014, 4, 159 DOI: 10.1557/mrc.2014.3421https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC2cXitFOku73I&md5=ab98ce64853d9e3bb531b60ff55c2e51Understanding the relationship between Cu2ZnSn(S,Se)4 material properties and device performanceGershon, Talia; Gokmen, Tayfun; Gunawan, Oki; Haight, Richard; Guha, Supratik; Shin, ByunghaMRS Communications (2014), 4 (4), 159-170CODEN: MCROF8; ISSN:2159-6867. (Cambridge University Press)Cu2ZnSn(S,Se)4 (CZTSSe) photovoltaics (PV) have long been considered promising candidates for large-scale PV deployment due to the availability of constituent elements and steady improvements in device efficiency over time. The key limitation to high efficiency in this technol. remains a deficit in the open-circuit voltage with respect to the band gap. The past decade has seen significant progress toward understanding how the various material properties such as bulk and surface compn., point defects (intrinsic and extrinsic), and grain boundaries all impact the optoelectronic properties of CZTSSe materials, and consequently device performance. This paper aims to summarize what is known about the CZTSSe bulk and surfaces, and how these material properties may be related to the Voc deficit.
- 22Shin, D.; Saparov, B.; Mitzi, D. B. Defect Engineering in Multinary Earth-Abundant Chalcogenide Photovoltaic Materials Adv. Energy Mater. 2017, 1602366 DOI: 10.1002/aenm.201602366There is no corresponding record for this reference.
- 23Lloyd, M. A.; Bishop, D.; Gunawan, O.; Mccandless, B. Fabrication and performance limitations in single crystal Cu2ZnSnSe4 Solar Cells 2016 IEEE 43rd Photovoltaic Specialists Conference (PVSC) 2016, 3636 DOI: 10.1109/PVSC.2016.7750352There is no corresponding record for this reference.
- 24Baranowski, L. L.; Zawadzki, P.; Lany, S.; Toberer, E. S.; Zakutayev, A. A review of defects and disorder in multinary tetrahedrally bonded semiconductors Semicond. Sci. Technol. 2016, 31, 123004 DOI: 10.1088/0268-1242/31/12/12300424https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC2sXhs1yju7o%253D&md5=7d307fab9979316d2675e0421896547dA review of defects and disorder in multinary tetrahedrally bonded semiconductorsBaranowski, Lauryn L.; Zawadzki, Pawel; Lany, Stephan; Toberer, Eric S.; Zakutayev, AndriySemiconductor Science and Technology (2016), 31 (12), 123004/1-123004/16CODEN: SSTEET; ISSN:0268-1242. (IOP Publishing Ltd.)Defects are crit. to understanding the electronic properties of semiconducting compds., for applications such as light-emitting diodes, transistors, photovoltaics, and thermoelecs. In this review, we describe our work investigating defects in tetrahedrally bonded, multinary semiconductors, and discuss the place of our research within the context of publications by other groups. We applied exptl. and theory techniques to understand point defects, structural disorder, and extended antisite defects in one semiconductor of interest for photovoltaic applications, Cu2SnS3. We contrast our findings on Cu2SnS3 with other chem. related Cu-Sn-S compds., as well as structurally related compds. such as Cu2ZnSnS4 and Cu(In,Ga)Se2. We find that evaluation of point defects alone is not sufficient to understand defect behavior in multinary tetrahedrally bonded semiconductors. In the case of Cu2SnS3 and Cu2ZnSnS4, structural disorder and entropy-driven cation clustering can result in nanoscale compositional inhomogeneities which detrimentally impact the electronic transport. Therefore, it is not sufficient to assess only the point defect behavior of new multinary tetrahedrally bonded compds.; effects such as structural disorder and extended antisite defects must also be considered. Overall, this review provides a framework for evaluating tetrahedrally bonded semiconducting compds. with respect to their defect behavior for photovoltaic and other applications, and suggests new materials that may not be as prone to such imperfections.
- 25Chen, S. Y.; Yang, J.-H. H.; Gong, X. G.; Walsh, A.; Wei, S.-H. H. Intrinsic point defects and complexes in the quaternary kesterite semiconductor Cu2ZnSnS4 Phys. Rev. B: Condens. Matter Mater. Phys. 2010, 81, 245204 DOI: 10.1103/PhysRevB.81.24520425https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC3cXosVyisL8%253D&md5=5911ab778637c784c36483ea18663cf7Intrinsic point defects and complexes in the quaternary kesterite semiconductor Cu2ZnSnS4Chen, Shiyou; Yang, Ji-Hui; Gong, X. G.; Walsh, Aron; Wei, Su-HuaiPhysical Review B: Condensed Matter and Materials Physics (2010), 81 (24), 245204/1-245204/10CODEN: PRBMDO; ISSN:1098-0121. (American Physical Society)Current knowledge of the intrinsic defect properties of Cu2ZnSnS4 (CZTS) is limited, which is hindering further improvement of the performance of CZTS-based solar cells. Here, the authors have performed 1st-principles calcns. for intrinsic defects and defect complexes in CZTS, from which the authors have the following observations. (i) It is important to control the elemental chem. potentials during crystal growth to avoid the formation of secondary phases such as ZnS, CuS, and Cu2SnS3. (ii) The intrinsic p-type cond. is attributed to the CuZn antisite which has a lower formation energy and relatively deeper acceptor level compared to the Cu vacancy. (iii) The low formation energy of many of the acceptor defects will lead to the intrinsic p-type character, i.e., n-type doping is very difficult in this system. (iv) The role of elec. neutral defect complexes is predicted to be important, because they have remarkably low formation energies and electronically passivate deep levels in the band gap. For example, [CuZn-+ZnCu+], [VCu-+ZnCu+], and [ZnSn2-+2ZnCu+] may form easily in nonstoichiometric samples. The band alignment between Cu2ZnSnS4, CuInSe2 and the solar-cell window layer CdS also was calcd., revealing that a type-II band alignment exists for the CdS/Cu2ZnSnS4 heterojunction. The fundamental differences between CZTS and CuInSe2 for use in thin-film photovoltaics are discussed. The results are expected to be relevant to other I2-II-IV-VI4 semiconductors.
- 26Scragg, J. J. S.; Larsen, J. K.; Kumar, M.; Persson, C.; Sendler, J.; Siebentritt, S.; Platzer Björkman, C. Cu-Zn Disorder and Band Gap Fluctuations in Cu2ZnSn(S,Se)4: Theoretical and Experimental Investigations Phys. Status Solidi B 2016, 253, 247 DOI: 10.1002/pssb.20155253026https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC2MXhsFGhtb7M&md5=f0d49480d885afc209ee9a887a71c6d8Cu-Zn disorder and band gap fluctuations in Cu2ZnSn(S,Se)4: Theoretical and experimental investigationsScragg, Jonathan J. S.; Larsen, Jes K.; Kumar, Mukesh; Persson, Clas; Sendler, Jan; Siebentritt, Susanne; Platzer Bjoerkman, CharlottePhysica Status Solidi B: Basic Solid State Physics (2016), 253 (2), 247-254CODEN: PSSBBD; ISSN:0370-1972. (Wiley-VCH Verlag GmbH & Co. KGaA)Cu2ZnSn(S,Se)4 (CZTS(e)) solar cells suffer from low-open-circuit voltages that have been blamed on the existence of band gap fluctuations, with different possible origins. In this paper, we show from both theor. and exptl. standpoints that disorder of Cu and Zn atoms is in all probability the primary cause of these fluctuations. First, quantification of Cu-Zn disorder in CZTS thin films is presented. The results indicate that disorder is prevalent in the majority of practical samples used for solar cells. Then, ab initio calcns. for different arrangements and densities of disorder-induced [CuZn + ZnCu] defect pairs are presented and it is shown that spatial variations in band gap of the order of 200 meV can easily be caused by Cu-Zn disorder, which would cause large voltage losses in solar cells. Expts. using Raman spectroscopy and room temp. photoluminescence combined with in situ heat-treatments show that a shift in the energy of the dominant band-to-band recombination pathway correlates perfectly to the order-disorder transition, which clearly implicates Cu-Zn disorder as the cause of band gap fluctuations in CZTS. Our results suggest that elimination or passivation of Cu-Zn disorder could be very important for future improvements in the efficiency of CZTS(e)-based solar cells.
- 27Van Mieghem, P. Theory of band tails in heavily doped semiconductors Rev. Mod. Phys. 1992, 64, 755 DOI: 10.1103/RevModPhys.64.75527https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADyaK38XltlOrs7k%253D&md5=081f7cbec8f0ab9a405317875982d1f3Theory of band tails in heavily doped semiconductorsVan Mieghem, PietReviews of Modern Physics (1992), 64 (3), 755-93CODEN: RMPHAT; ISSN:0034-6861.The different classes of theories that describe the band tailing of the d. of states in heavily doped semiconductors are reviewed with many refs.
- 28Gokmen, T.; Gunawan, O.; Todorov, T. K.; Mitzi, D. B. Band tailing and efficiency limitation in kesterite solar cells Appl. Phys. Lett. 2013, 103, 103506 DOI: 10.1063/1.482025028https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC3sXhtl2rur%252FL&md5=72e4de46c98db2b4f58d458d991419baBand tailing and efficiency limitation in kesterite solar cellsGokmen, Tayfun; Gunawan, Oki; Todorov, Teodor K.; Mitzi, David B.Applied Physics Letters (2013), 103 (10), 103506/1-103506/5CODEN: APPLAB; ISSN:0003-6951. (American Institute of Physics)A fundamental performance bottleneck for hydrazine processed kesterite Cu2ZnSn(S,Se)4 (CZTSSe) solar cells with efficiencies reaching >11% can be the formation of band-edge tail states, which quantum efficiency and photoluminescence data indicate is roughly twice as severe as in higher-performing Cu(In,Ga)(S,Se)2 devices. Low temp. time-resolved photoluminescence data suggest that the enhanced tailing arises primarily from electrostatic potential fluctuations induced by strong compensation and facilitated by a lower CZTSSe dielec. const. The implications of the band tails for the voltage deficit in these devices are discussed. (c) 2013 American Institute of Physics.
- 29Gershon, T.; Sardashti, K.; Gunawan, O.; Mankad, R.; Singh, S.; Lee, Y. S.; Ott, J. A.; Kummel, A.; Haight, R. Photovoltaic Device with over 5% Efficiency Based on an n-Type Ag2ZnSnSe4 Absorber Adv. Energy Mater. 2016, 6, 1601182 DOI: 10.1002/aenm.201601182There is no corresponding record for this reference.
- 30Scragg, J. J. S.; Choubrac, L.; Lafond, A.; Ericson, T.; Platzer-Björkman, C. A low-temperature order-disorder transition in Cu2ZnSnS4 thin films Appl. Phys. Lett. 2014, 104, 041911 DOI: 10.1063/1.486368530https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC2cXjtlyqtLs%253D&md5=0444978629d2ef4d755f129b93938731A low-temperature order-disorder transition in Cu2ZnSnS4 thin filmsScragg, Jonathan J. S.; Choubrac, Leo; Lafond, Alain; Ericson, Tove; Platzer-Bjoerkman, CharlotteApplied Physics Letters (2014), 104 (4), 041911/1-041911/4CODEN: APPLAB; ISSN:0003-6951. (American Institute of Physics)Cu2ZnSnS4 (CZTS) is an interesting material for sustainable photovoltaics, but efficiencies are limited by the low open-circuit voltage. A possible cause of this is disorder among the Cu and Zn cations, a phenomenon which is difficult to detect by std. techniques. We show that this issue can be overcome using near-resonant Raman scattering, which lets us est. a crit. temp. of 533 ± 10 K for the transition between ordered and disordered CZTS. These findings have deep significance for the synthesis of high-quality material, and pave the way for quant. investigation of the impact of disorder on the performance of CZTS-based solar cells. (c) 2014 American Institute of Physics.
- 31Chen, S.; Walsh, A.; Gong, X.-G.; Wei, S.-H. Classification of lattice defects in the kesterite Cu2ZnSnS4 and Cu2ZnSnSe4 earth-abundant solar cell absorbers Adv. Mater. 2013, 25, 1522 DOI: 10.1002/adma.20120314631https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC3sXit1Kms7c%253D&md5=8dc20228591c6104fbfcadfe1bbc72edClassification of Lattice Defects in the Kesterite Cu2ZnSnS4 and Cu2ZnSnSe4 Earth-Abundant Solar Cell AbsorbersChen, Shiyou; Walsh, Aron; Gong, Xin-Gao; Wei, Su-HuaiAdvanced Materials (Weinheim, Germany) (2013), 25 (11), 1522-1539CODEN: ADVMEW; ISSN:0935-9648. (Wiley-VCH Verlag GmbH & Co. KGaA)A review of theor. research on defect formation and ionization in kesterite materials, based on new systematic calcns., and compared with the better studied chalcopyrite materials CuGaSe2 and CuInSe2. The kesterite-structured semiconductors Cu2ZnSnS4 and Cu2ZnSnSe4 are drawing considerable attention recently as the active layers in earth-abundant low-cost thin-film solar cells. The addnl. no. of elements in these quaternary compds., relative to binary and ternary semiconductors, results in increased flexibility in the material properties. Conversely, a large variety of intrinsic lattice defects can also be formed, which have important influence on their optical and elec. properties, and hence their photovoltaic performance. Exptl. identification of these defects is currently limited due to poor sample quality. Four features are revealed and highlighted: (i) the strong phase-competition between the kesterites and the coexisting secondary compds.; (ii) the intrinsic p-type cond. detd. by the high population of acceptor CuZn anti-sites and Cu vacancies, and their dependence on the Cu/(Zn+Sn) and Zn/Sn ratio; (iii) the role of charge-compensated defect clusters such as [2CuZn+SnZn], [VCu+ZnCu] and [ZnSn+2ZnCu] and their contribution to nonstoichiometry; (iv) the electron-trapping effect of the abundant [2CuZn+SnZn] clusters, esp. in Cu2ZnSnS4. The calcd. properties explain the exptl. observation that Cu poor and Zn rich conditions (Cu/(Zn+Sn) ≈ 0.8 and Zn/Sn ≈ 1.2) result in the highest solar cell efficiency, as well as suggesting an efficiency limitation in Cu2ZnSn(S,Se)4 cells when the S compn. is high.
- 32Stoneham, A. M. Non-radiative transitions in semiconductors Rep. Prog. Phys. 1981, 44, 1251 DOI: 10.1088/0034-4885/44/12/00132https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADyaL38XitVyrt7Y%253D&md5=7b69cd5af0b6bda2ba714e41b36bc383Nonradiative transitions in semiconductorsStoneham, A. M.Reports on Progress in Physics (1981), 44 (12), 1251-95CODEN: RPPHAG; ISSN:0034-4885.A review with many refs.
- 33Major, J. D.; Treharne, R. E.; Phillips, L. J.; Durose, K. A low-cost non-toxic post-growth activation step for CdTe solar cells Nature 2014, 511, 334 DOI: 10.1038/nature1343533https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC2cXhtFOjtrrK&md5=9db3cf862c3a25c97e608a7d2d9a362cA low-cost non-toxic post-growth activation step for CdTe solar cellsMajor, J. D.; Treharne, R. E.; Phillips, L. J.; Durose, K.Nature (London, United Kingdom) (2014), 511 (7509), 334-337CODEN: NATUAS; ISSN:0028-0836. (Nature Publishing Group)Cadmium telluride, CdTe, is now firmly established as the basis for the market-leading thin-film solar-cell technol. With lab. efficiencies approaching 20 per cent, the research and development targets for CdTe are to reduce the cost of power generation further to less than half a US dollar per W (ref. 2) and to minimize the environmental impact. A central part of the manufg. process involves doping the polycryst. thin-film CdTe with CdCl2. This acts to form the photovoltaic junction at the CdTe/CdS interface and to passivate the grain boundaries, making it essential in achieving high device efficiencies. However, although such doping has been almost ubiquitous since the development of this processing route over 25 years ago, CdCl2 has two severe disadvantages; it is both expensive (about 30 cents per g) and a water-sol. source of toxic cadmium ions, presenting a risk to both operators and the environment during manuf. Here we demonstrate that solar cells prepd. using MgCl2, which is non-toxic and costs less than a cent per g, have efficiencies (around 13%) identical to those of a CdCl2-processed control group. They have similar hole densities in the active layer (9 × 1014 cm-3) and comparable impurity profiles for Cl and O, these elements being important p-type dopants for CdTe thin films. Contrary to expectation, CdCl2-processed and MgCl2-processed solar cells contain similar concns. of Mg; this is because of Mg out-diffusion from the soda-lime glass substrates and is not disadvantageous to device performance. However, treatment with other low-cost chlorides such as NaCl, KCl and MnCl2 leads to the introduction of elec. active impurities that do compromise device performance. Our results demonstrate that CdCl2 may simply be replaced directly with MgCl2 in the existing fabrication process, thus both minimizing the environmental risk and reducing the cost of CdTe solar-cell prodn.
- 34Gabor, A. M.; Tuttle, J. R.; Albin, D. S.; Contreras, M. A.; Noufi, R.; Hermann, A. M. High-efficiency CuInxGa1-xSe2 Solar Cells Made From (Inx,Ga1-x)2Se3 Precursor Films Appl. Phys. Lett. 1994, 65, 198 DOI: 10.1063/1.11267034https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADyaK2cXkvVOmsbk%253D&md5=353036a4d86cf581c24371c8593d1ec6High-efficiency CuInxGa1-xSe2 solar cells made from (Inx,Ga1-x)2Se3 precursor filmsGabor, Andrew M.; Tuttle, John R.; Albin, David S.; Contreras, Miguel A.; Noufi, Rommel; Hermann, Allen M.Applied Physics Letters (1994), 65 (2), 198-200CODEN: APPLAB; ISSN:0003-6951.In, Ga, and Se were co-evapd. to form precursor films of (Inx,Ga1-x)2Se3. The precursors were then converted to CuInxGa1-xSe2 by exposure to a flux of Cu and Se. The final films were smooth, with tightly packed grains, and had a graded Ga content as a function of film depth. Photovoltaic devices made from these films showed good tolerance in device efficiency to variations in film compn. A device made from these films resulted in the highest total-area efficiency measured for any non-single-crystal, thin-film solar cell, at 15.9%.