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.
KeywordsCu2ZnSnSe4 (CZTSe) Electrodeposition Cu/Sn/Zn precursors Selenization Solar cells
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.
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