Suppressing interface recombination via element diffusion regulation towards high-efficiency Cd-free Cu(In,Ga)Se2 solar cells

Magnetron-sputtered Zn1-xMgxO (ZMO) films suffer from severe elemental diffusion to Cu(In,Ga)Se2 (CIGS), which limits the further improvement of device performance. Herein, we introduced Zn(O,S) (ZOS) as a barrier layer, and we controlled the diffusion of Zn within an appropriate range by adjusting the annealing temperature of ZOS. The results indicate that Zn can occupy copper vacancies (VCu) in CIGS and easily diffuse through VCu. A moderate amount of Zn at the interface acts as a charge compensator, reducing the band tailing effect and promoting the carrier collection efficiency of photovoltaic (PV) devices. However, excessive Zn doping induces additional defects dominated by ZnCu, ZnGa and ZnIn, leading to severe nonradiative recombination. Consequently, the Cd-free CIGS solar cell produced based on these insights exhibited a 49.4-mV reduction in the Voc deficit (Voc,def) as well as a notable increase in the power conversion efficiency (PCE) to 18.0%. This study is the first to reveal the mechanism of defects caused by Zn diffusion, and these findings provide theoretical support for the study of Cd-free thin-film solar cells.