Promising Cd-free double buffer layer in CZTSSe thin film solar cells
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Published online 27 September 2020 | https://doi.org/10.1007/s40843-020-1419-8
Promising Cd-free double buffer layer in CZTSSe thin film solar cells 1
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Siyu Wang , Zhenwu Jiang , Zhan Shen , Yali Sun , Hongling Guo , Li Wu , Jianjun Zhang , 1 3 1* Jianping Ao , Hai Wang and Yi Zhang ABSTRACT Zn(O,S) film is widely used as a Cd-free buffer layer for kesterite thin film solar cells due to its low-cost and eco-friendly characteristics. However, the low carrier concentration and conductivity of Zn(O,S) will deteriorate the device performance. In this work, an additional buffer layer of In2S3 is introduced to modify the properties of the Zn(O,S) layer as well as the CZTSSe layer via a post-annealing treatment. The carrier concentrations of both the Zn(O,S) and CZTSSe layers are increased, which facilitates the carrier separation and increases the open circuit voltage (VOC). It is also found that ammonia etching treatment can remove the contamination and reduce the interface defects, and there is an increase of the surface roughness of the In2S3 layer, which works as an antireflection layer. Consequently, the efficiency of the CZTSSe solar cells is improved by 24% after the annealing and etching treatments. Simulation and experimental results show that a large band offset of the In2S3 layer and defect energy levels in the Zn(O,S) layer are the main properties limiting the fill factor and efficiency of these CZTSSe devices. This study affords a new perspective for the carrier concentration enhancement of the absorber and buffer layers by In-doping, and it also indicates that In2S3/Zn(O,S) is a promising Cd-free hybrid buffer layer for high-efficiency kesterite solar cells. Keywords: double buffer layer, CZTSSe, In-doping, band offset
INTRODUCTION In recent years, Cu2ZnSn(S,Se)4 (CZTSSe) has been regarded as a promising absorber material due to its merits and promising photovoltaic performance compared with that of Cu(In,Ga)Se2 (CIGS) [1–6]. To date, over-11%efficient CZTSSe solar cells using CdS layer as the buffer
layer have been reported [7–9]. The application of a CdS buffer layer in CZTSSe solar cells enables good efficiency due to the appropriate conduction band offset (CBO) at the CdS and CZTSSe heterojunction [10,11]. However, the toxic and hazardous cadmium in CdS buffer layers remains a serious problem [12]. To solve this problem, Cd-free buffer layers, such as (Zn,Mg)O, Zn(O,S), and Zn1−xSnxO (ZTO), have been proposed to replace the CdS layer [13–15]. Among those Cd-free buffer layers, the Zn(O,S) buffer layer has been widely studied due to its controllable bandgap, low-cost, and low-toxicity characteristics. Recently, many groups have demonstrated over-5% CZTSSe solar cells using Zn(O,S) as a buffer layer [16,17]. Neuschitzer et al. [18] reported a 6.5%-efficient CZTSe solar cell with a 10-nmthick Zn(O,S) buffer layer by adjusting the sulfur content in the Zn(O,S) buffer layer. However, making an excessively thin Zn(O,S) buffer layer with uniform coverage remains an unsolved problem. In ord
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