Earth Abundant Element Cu₂Zn(Sn_1−<i>x</i>Ge_<i>x</i>)S₄Nanocrystals for Tunable Band Gap Solar Cells: 6.8% Efficient Device Fabrication

Abstract

Cu 2 Zn(Sn 1− x Ge x )S 4 nanocrystals have been synthesized via batch reaction in oleylamine with no additional surfactants present. The nanocrystals are knife-coated on molybdenum substrates and then selenized to form a dense layer of Cu 2 Zn(Sn 1− x Ge x )(S,Se) 4, which is then used as the photoabsorbing layer in a thin film solar cell. The band gaps of the nanocrystals and the resulting solar cells are demonstrated to be controlled by adjusting the Ge/(Ge+Sn) ratio of the nanocrystal synthesis precursors. Solar cells fabricated from Cu 2 ZnGeS 4 nanocrystal films yielded a power conversion efficiency of 0.51%. However, Cu 2 Zn( Sn x Ge 1− x )S 4 nanocrystals with a Ge/(Ge+Sn) ratio 0.7 yielded devices with an efficiency of 6.8% when synthesized to be Cu-poor and Zn-rich. This result opens the possibility of forming Ge gradients to direct minority carriers away from high recombination interfaces and significantly improve the device efficiency of CZTSSe-based solar cells.