Efficiency enhancement of Cu2ZnSnS4 thin film solar cells by chromium doping

We report for the first time the effect of a chromium-doped absorber layer on the performance of Cu2ZnSnS4 (CZTS) thin film solar cells. CZTS:Cr films were deposited on Soda-Lime glass substrates through a double step method that includes sulfurization of a sputtered stack of Zn-Cr/Sn/Cu metallic layers. Cr was introduced as a dopant in the Zn layer, and the electro-optical properties of the CZTS:Cr film were investigated as a function of the Cr concentration, showing a large absorption increase for wavelengths above 850 nm. Films with different Cr concentration have been used as absorber layers to investigate their effect on efficiency and performance of the solar cells. Finally, an efficiency increase from 1.86% to 3.96% was obtained with a Cr concentration of 0.04%. This 113% increase ratio is mainly due to enhancement of the current density assigned to a double step absorption of low energy photons and decrease in deep acceptor type defects. Empirical simulations show that this enhancement is also ascribable to the reduction of the acceptor-type defect density. These findings open the way to highly efficient CZTS solar cells.