Flexible and lightweight Cu(In,Ga)(S,Se)2 solar cell on a stainless steel substrate with different structures for improving band alignment

Abstract

Herein, the conduction band minimum (EC) difference between the absorbing and buffer layers (named CBO) and that between the absorbing layer and the transparent conductive oxide (named ΔEC-TA) for the total band alignment of flexible and lightweight Cu(In,Ga)(S,Se)2 (CIGSSe) solar cells on a stainless steel (SS) substrate with different structures were measured and optimized to improve their photovoltaic performances. Under different structures, the first buffer layers are Zn(O,S,OH)x, CdS, and (Cd,Zn)S, the second buffer layer is Zn0.84Mg0.16O, and the transparent conductive oxide (TCO) layer is Zn0.88Mg0.12O:Al, leading to different CBO and ΔEC-TA values. According to theoretical and experimental examinations, the replacement of ZnO:B TCO by Zn0.84Mg0.16O buffer/Zn0.88Mg0.12O:Al TCO results in the improvement of band bending (reduced carrier recombination) in the device, which is attributed to EC values of Zn0.84Mg0.16O buffer/Zn0.88Mg0.12O:Al TCO being shallower than that of ZnO:B, thus improving ΔEC-TA. Moreover, the Zn(O,S,OH)x and (Cd,Zn)S buffers give rise to the improved CBO in the device. Ultimately, experimental conversion efficiency of 18.1% for the flexible and lightweight CIGSSe solar cell on the SS substrate was obtained through the use of (Cd,Zn)S as the first buffer, Zn0.84Mg0.16O as the second buffer, and Zn0.84Mg0.16O, Zn0.88Mg0.12O:Al as the TCO layer, leading to the optimized CBO value of −0.01 eV and ΔEC-TA value of +0.16 eV, which reduced carrier recombination; this was well verified by the predicted numerical result.