Ag, Ge Dual Gradient Substitution for Low-Energy Loss and High Efficiency Kesterite Solar Cells

A major obstacle hindering the further development of kesterite Cu2ZnSn(S,Se)4 (CZTSSe) solar cells is the inherent recombination losses, both via bulk defects and charge extraction interfaces. Herein, we propose a generic Ag, Ge dual gradient absorber architecture to improve device performance involving the downshift of absorber VBM at front contact and upshift of absorber CBM at back contact. We experimentally show that the substitution of Cu by Ag allows for larger band bending at p-n junction, and the substitution of Sn by Ge imposes an additional drift field within quasi-neutral region. Additionally, the diffused Ag and Ge ions into bulk absorber would passivate the deleterious CuZn and CuSn deep level defects. In return, the accelerated charge extraction, transport and minimized recombination loss offer surprising gains in Voc and Jsc, and the dual gradient device achieves maximum efficiency of 12.26% with improved Voc,deficit of 553 mV. This Ag, Ge graded-substitution approach offers an alternative absorber architecture to improve Voc,deficit for future high efficiency kesterite PV.