CZTSe solar cells prepared by electrodeposition of Cu/Sn/Zn stack layer followed by selenization at low Se pressure
© Yao et al.; licensee Springer. 2014
Received: 27 June 2014
Accepted: 3 December 2014
Published: 15 December 2014
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.
Thin film materials such as copper indium gallium diselenide (CIGS)  and cadmium telluride (CdTe)  are attracting much attention owing to their potential applications in the harvesting of solar energy. However, restrictions on the usage of the heavy metal Cd and limited supplies of In and Te are projected to limit the fabrication of existing chalcogen-based technologies to <100 GWp annually [3, 4]. Low-cost earth-abundant copper-zinc-tin-chalcogenide kesterites, Cu2ZnSnS(Se)4, and Cu2ZnSn(S,Se)4 have been studied as potential alternatives to CIGS or CdTe [5–8]. Recently, a liquid-based copper zinc tin sulfur-selenium (CZTSSe) solar cell with an efficiency of 12.6% was developed , and a 14-cm2 sub cell with an efficiency of over 10.8% has been fabricated . This rapid increase in the efficiency of CZTSSe solar cells signifies their huge potential in the future [5, 9–11]. Many methods, including both vacuum and non-vacuum methods, such as sputtering deposition , co-evaporation [6, 7], solution ink printing [5, 9, 11], and electrodeposition [13, 14], have been used to prepare Cu2ZnSnS(e)4 layers. Among these methods, electrodeposition has the following advantages: (a) it does not require a vacuum, (b) it is low-cost, (c) it provides thin films that are uniform over a large area, and (d) it enables the accurate control of morphology and composition [15, 16]. Unlike the co-electrodeposition of copper-zinc-tin, the electrodeposition of stacked metal layers allows precise control of the quantity deposited and is effective under a large range of deposition conditions of temperature, pH, and concentration of the main salt or addition agent. The morphology of the stacked metal layers can be easily controlled. Post-annealing improves the uniformity of the distribution of elements and promotes the conversion of elemental phases to alloyed phases, which have a significant effect on the formation of CZTSSe thin films .
As is well known, a two-step process is widely used to synthesize high-quality CZTS(Se) thin films . In this process, the precursor layer is prepared by vacuum or non-vacuum methods and then thermal annealing is performed at high sulfur or selenium pressure [7, 13]. However, selenization at high S(Se) pressure is likely to form a thick molybdenum disulfide/diselenide (MoS(Se)2) layer [5, 7, 13, 14]. An excessively thick MoS(Se)2 layer degrades the performance of the device [18, 19]. Therefore, treatment under low Se pressure has been suggested to prevent the formation of such a thick molybdenum diselenide (MoSe2) layer. However, the potential loss of the Sn metal from Cu2ZnSnSe4 (CZTSe) thin films at high temperature raises the additional difficulty of controlling the composition and phase of the films. Adding tin selenide (SnSe x ) gas during annealing can inhibit the decomposition of CZTSe thin films . This work studies the use of a low Se pressure  and additional SnSe x vapor in the preparation of high-quality CZTSe thin films.
The composition of the CZTSe thin film was obtained using a Magix(PW2403 (PANalytical LTD., the Netherlands)) X-ray fluorescent spectrometer (XRF) with an Rh-anode, which was calibrated by inductively coupled plasma spectroscopy (ICP). The structures of the selenized samples were elucidated using a Philips X-pert Pro diffractometer (PANalytical Ltd., the Netherlands) with Cu radiation and a Renishaw inVia Raman spectroscope (Renishaw Ltd., UK). Surface and cross-sectional observations were made using a scanning electron microscope (SEM, JEOL JSM-6700 (JEOL Ltd., Akishima-shi, Japan)). The depth profiles of the elements were obtained by secondary ion mass spectroscopy (SIMS, IMS-4 F, CAMECA, Nancy, France). Current–voltage (J-V) measurements of CZTSe solar cells were made under illumination by a standard AM1.5 spectrum of 100 mW/cm2 at room temperature using a constant-light solar simulator, which was calibrated using a standard monocrystalline Si solar cell.
Results and discussion
The composition of the CZTSe thin films prepared by various selenization conditions of A1, A2, and A3
Cu/(Zn + Sn)
The intensities of the XRD (112) and (204) peaks of CZTSe thin films prepared by various selenization conditions
Preparing high-quality CZTSe thin films under low selenium pressure is difficult owing to severe Sn loss. The presence of SnSe x is critical to the formation of stoichiometric CZTSe thin films, especially when selenization is performed at high temperature and low Se pressure. A higher SnSe x partial pressure yields better crystallinity of CZTSe with a preferred (112) orientation. A very thin MoSe2 layer may be present at the interface between Mo and the CZTSe layer following selenization at a low selenium pressure and a substrate temperature of 530°C. CZTSe solar cells with an efficiency of 4.81% are formed by the low-cost electrodeposition of a Cu/Sn/Zn stack layer followed by selenization at a low Se pressure.
LY is a Ph.D student in the Institute of Photoelectronic Thin Film Devices and Technology, Nankai University, China.
JA is a professor in the Institute of Photoelectronic Thin Film Devices and Technology, Nankai University, China.
M-JJ is a professor in the Department of Electronic Engineering in Chang Gung University, Taiwan.
JB is a Ph.D student in the Institute of Photoelectronic Thin Film Devices and Technology, Nankai University, China.
SG is a senior engineer in the Institute of Photoelectronic Thin Film Devices and Technology, Nankai University, China.
QH is a senior engineer in Institute of Photoelectronic Thin Film Devices and Technology, Nankai University, China.
ZZ is a senior engineer in the Institute of Photoelectronic Thin Film Devices and Technology, Nankai University, China.
GS is a senior engineer in the Institute of Photoelectronic Thin Film Devices and Technology, Nankai University, China.
YS is a professor in Institute of Photoelectronic Thin Film Devices and Technology, Nankai University, China.
L-BC is a professor in the Department of Electronic Engineering in Chang Gung University, Taiwan.
J-WC is a master student in the Department of Electronic Engineering in Chang Gung University, Taiwan.
The authors would like to thank the National Science Council of the Republic of China, Taiwan, for financially supporting this research under Contract No. NSC101-2221-E-182-068.
- Jackson P, Hariskos D, Wuerz R, Wischmann W, Powalla M: Compositional investigation of potassium doped Cu(In, Ga)Se2 solar cells with efficiencies up to 20.8%. Physica Status Solidi Rapid Res Lett 2014, 8: 219–222.View ArticleGoogle Scholar
- Mathew X, Thompson GW, Singh VP, McClure JC, Velumani S, Mathews NR, Sebastian PJ: Development of CdTe thin films on flexible substrates—a review. Solar Energy Mater Solar Cells 2003, 76: 293–303.View ArticleGoogle Scholar
- Andersson BA: Materials availability for large-scale thin-film photovoltaics. Prog Photovoltaics Res Appl 2000, 8: 61–76.View ArticleGoogle Scholar
- Wadia C, Alivisatos AP, Kammen DM: Materials availability expands the opportunity for large-scale photovoltaics deployment. Environ Sci Technol 2009, 43: 2072–2077.View ArticleGoogle Scholar
- Wang W, Winkler MT, Gunawan O, Gokmen T, Todorov TK, Zhu Y, Mitzi DB: Device characteristics of CZTSSe thin-film solar cells with 12.6% efficiency. Adv Energy Mater 2014, 4.Google Scholar
- Repins I, Beall C, Vora N, DeHart C, Kuciauskas D, Dippo P, To B, Mann J, Hsu WC, Goodrich A, Noufi R: Co-evaporated Cu2ZnSnSe4 films and devices. Solar Energy Mater Solar Cells 2012, 101: 154–159.View ArticleGoogle Scholar
- Shin B, Gunawan O, Zhu Y, Bojarczuk NA, Chey SJ, Guha S: Thin film solar cell with 8.4% power conversion efficiency using an earth-abundant Cu2ZnSnS4 absorber. Prog Photovoltaics Res Appl 2013, 21: 72–76.View ArticleGoogle Scholar
- Hiroi H, Sakai N, Kato T, Sugimoto H: High Voltage Cu2ZnSnS4 Submodules by Hybrid Buffer Layer. In Proceedings of the IEEE Photovoltaic Specialists Conference 39th: 16–21 Jun. Tampa, FL; 2013:16–21.Google Scholar
- Todorov TK, Reuter KB, Mitzi DB: High-efficiency solar cell with earth-abundant liquid-processed absorber. Adv Mater 2010, 22: E156-E159.View ArticleGoogle Scholar
- Todorov TK, Tang J, Bag S, Gunawan O, Gokmen T, Zhu Y, Mitzi DB: Beyond 11% efficiency: characteristics of state‒of‒the‒art Cu2ZnSn(S, Se)4 solar cells. Adv Energy Mater 2013, 3: 34–38.View ArticleGoogle Scholar
- Winkler MT, Wang W, Gunawan O, Hovel HJ, Todorov TK, Mitzi DB: Optical designs that improve the efficiency of Cu2ZnSn(S, Se)4 solar cells. Energy Environ Sci 2014, 7: 1029–1036.View ArticleGoogle Scholar
- Katagiri H, Jimbo K, Maw WS, Oishi K, Yamazaki M, Araki H, Takeuchi A: Development of CZTS-based thin film solar cells. Thin Solid Films 2009, 517: 2455–2460.View ArticleGoogle Scholar
- Jiang 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.View ArticleGoogle Scholar
- Jeon JO, Lee KD, Seul Oh L, Seo SW, Lee DK, Kim H, Jeong J, Ko MJ, Kim B, Son HJ, Kim JY: Highly efficient copper–zinc–tin–selenide (CZTSe) solar cells by electrodeposition. Chem Sus Chem 2014, 7: 1073–1077.View ArticleGoogle Scholar
- Deligianni H, Ahmed S, Romankiw LT: The next frontier: electrodeposition for solar cell fabrication. Interface-Electrochemical Soc 2011, 20: 47.Google Scholar
- Dudchak IV, Piskach LV: Phase equilibria in the Cu2SnSe3–SnSe2–ZnSe system. J Alloys Compd 2003, 351: 145–150.View ArticleGoogle Scholar
- Redinger A, Berg DM, Dale PJ, Valle N, Siebentritt S: Route Toward High-Efficiency Single-Phase Cu ZnSn (S, Se) Thin-Film Solar Cells: Model Experiments and Literature Review. In Photovoltaic Specialists Conference (PVSC), 2011 37th IEEE. 1st edition. Seattle, WA; 2011:200–206.Google Scholar
- Scragg JJ, Wätjen JT, Edoff M, Ericson T, Kubart T, Platzer BC: A detrimental reaction at the molybdenum back contact in Cu2ZnSn(S, Se)4 thin-film solar cells. J Am Chem Soc 2012, 134: 19330–19333.View ArticleGoogle Scholar
- Shin B, Bojarczuk NA, Guha S: On the kinetics of MoSe2 interfacial layer formation in chalcogen-based thin film solar cells with a molybdenum back contact. Appl Phys Lett 2013, 102: 091907.View ArticleGoogle Scholar
- Scragg JJ, Ericson T, Kubart T, Edoff M, Platzer BC: Chemical insights into the instability of Cu2ZnSnS4 films during annealing. Chem Mater 2011, 23: 4625–4633.View ArticleGoogle Scholar
- Yaws CL: Handbook of Vapor Pressure, Volume 4. In Inorganic Compounds and Elements. Houston, Texas: Gulf Publishing Company; 1995.Google Scholar
- Guo L, Zhu Y, Gunawan O, Gokmen T, Deline VR, Ahmed S, Romankiw LT, Deligianni H: Electrodeposited Cu2ZnSnSe4 thin film solar cell with 7% power conversion efficiency. Prog Photovoltaics Res Appl 2014, 22: 58–68.View ArticleGoogle Scholar
- Chakrabarti DJ, Laughlin DE: The Cu - Se (copper-selenium) system. J Phase Equilibria 1981, 2: 305–315.Google Scholar
- Chang C: Processing and Characterization of Copper Indium Selenide for Photovoltaic Applications. In PhD Thesis. University of Florida, Physics, CondensedMatter; 1999.Google Scholar
- Binnewies M, Milke E: Thermochemical Data of Elements and Compounds. New York: Wiley-VCH; 2002.View ArticleGoogle Scholar
- Sharma RC, Chang YA: The Se - Sn (selenium-tin) system. J Phase Equilibria 1986, 7: 68–72.Google Scholar
- Redinger A, Berg DM, Dale PJ, Siebentritt S: The consequences of kesterite equilibria for efficient solar cells. J Am Chem Soc 2011, 133: 3320–3323.View ArticleGoogle Scholar
- Altosaar M, Raudoja J, Timmo K, Danilson M, Grossberg M, Krustok J, Mellikov E: Cu2Zn1–xCdxSn(Se1–ySy)4 solid solutions as absorber materials for solar cells. Physica Status Solidi (a) 2008, 205: 167–170.View ArticleGoogle Scholar
- Chao Z: Research on Selenization and Sulfization of Electrodeposited Cu-In-Ga Metallic Precursor. PhD thesis. Nankai University, Electronic Science and Technology; 2013.Google Scholar
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