Improving the efficiency of Cu2ZnSn(S,Se)4 solar cell by using localized K-doped precursor film

In this paper, we prepared Na–K co-doped Cu2ZnSn(S,Se)4 (CZTSSe) films by solution method. By optimizing the distribution of the K-doping (uniform or localized K-doping) in the precursor films, the efficiencies of CZTSSe solar cells were improved. The effect of the K-doping distribution on the properties of CZTSSe films and solar cells were investigated. Compared to the solar cells without K-doping, the uniform K-doping in the precursor film increased the solar cell efficiency from 10.4% to 11.7%. In comparison, the localized K-doping in the bottom and in the top of the precursor films yielded solar cells with efficiencies of 12.4% and 10.1%, respectively. It was determined that the K-doping distribution primarily had an influence on carrier recombination in the depletion regions of CZTSSe solar cells. Whereas k-doping at the top of the precursor film greatly enhances the carrier recombination in the depletion region, the bottom doping has the opposite effect. The reason could be that the variation of the K-doping distribution modified the crystallization of the CZTSSe films. As the result, the defect properties of the CZTSSe films were altered. Moreover, the effect of the bottom K-doping on the back contact properties of CZTSSe solar cells was investigated by impedance spectroscopy. Though the K-doping can reduce the back contact barrier of the solar cell, the back contact barrier still achieved the height of 228 meV. The results indicated that the back contact barrier would be an important limiting factor for the efficiency of CZTSSe solar cell.