Enhancement of photo-conversion efficiency in Cu2ZnSn(S,Se)4 thin-film solar cells by control of ZnS precursor-layer thickness
Cu2ZnSn(S,Se)4 (CZTSSe) is a promising absorber layer materials for solar cells which is able to be used as a substitute for Cu(In,Ga)Se2 (CIGS). CZTSSe is composed of Zn and Sn, which are more abundant and less expensive than In and Ga. CZTSSe offers many advantages, including a high absorption coefficient (104 cm-1) and a tunable band gap by either changing the ration of S and Se or by applying Ge. To date, the propitious composition of CZTSSe thin-film solar cells has not been clearly defined. Generally, high-efficiency CZTSSe thin-film solar cells have been reported to have Cu-poor and Zn-rich compositions. However, the optimal composition of Zn is controversial as Zn-rich composition easily forms Zn(S,Se) secondary phases that negatively impact the performance of the device. On the other hand, it can also improve the p-type conductivity and reduce the formation of Cu2Sn(S,Se)3 (CTSSe) secondary phases.
In this research, we carried out a dimpling etching method for Kelvin probe force microscopy (KPFM) to detect secondary phases in the CZTSSe thin-film solar cells. As the thickness of the ZnS precursor increases, the device factors improved. This could be because of the reduction in the MoSe2 phase. The work function distribution obtained by KPFM results is considerably different relaying on the thickness of ZnS precursor of the CZTSSe thin films. The highest efficiency for the CZTSSe thin films shows a uniform work function which is distributed at the surface, and the MoSe2 and ZnSe phase appeared over sider regions when compared with those of other circumstances. We also verified the exact location of the phases and assume the amount of secondary phases by analyzing the work function distribution. The CZTSSe thin films with the best performance is shown to have a thinner MoSe2 secondary phase. The optical and electronic properties of the secondary phases can help us to resolving issues related to the secondary phases and to obtain a higher conversion efficiency for the CZTSSe thin-film solar cells. Therefore, the fine ZnS precursor thickness in the CZTSSe films affects the secondary phase formation and in particular, the VOC and JSC as well.
Figure 1. Cu2ZnSn(S,Se)4 (CZTSSe) thin-film solar cell structure. CZTSSe is one of the most promising materials for a light-absorbing layer in thin-film solar cells.
Figure 2. Work function distribution according to the spots of the CZTSSe thin films in the depth profiles. (a) 330 nm, (b) 337 nm and (c) 344 nm of ZnS precursor. The highest efficiency is achieved by the 337-nm film in terms of the uniform work function peak in the top spot (pure CZTSSe work function ~4.7 eV). We obtained a work function for the secondary phases of the ZnSe and MoSe2 at the middle depths.
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“Enhancement of photo-conversion efficiency in Cu2ZnSn(S,Se)4 thin-film solar cells by control of ZnS precursor-layer thickness”, Progress in Photovoltaics: Research and Applications, 24, 292 (2016).
“Effects of the compositional ratio distribution with sulfurization temperatures in the absorber layer on the defect and surface electrical characteristics of Cu2ZnSnS4 solar cells”, Progress in Photovoltaics: Research and Applications, 23, 1771 (2015).