Large synergy effects of doping, a site substitution, and surface passivation in wide bandgap Pb-free ASnI2Br perovskite solar cells on efficiency and stability enhancement

Tin (Sn) based perovskite solar cells (PSCs) are rapidly getting attention due to their relatively less toxic nature compared to lead-PSCs. However, Sn perovskites previously reported are narrow-bandgap materials. Wide-bandgap perovskites, which are the key materials for the top layer of lead-free tandem solar cells, are rarely researched up to now. Here, it is demonstrated that optoelectronic properties of the GA0.06(FA0.8Cs0.2)0.94SnI2Br based wide-bandgap perovskite can be improved by exploring (1) Germanium (II) Iodide (GeI2) doping, (2) incorporation of Ethylenediamine Bromide (EDABr2) at A site, (3) EDA passivation. The incorporation of big organic cation EDABr2 in the wide-bandgap tin perovskite crystal structure together with GeI2 doping made the carrier lifetime of perovskite absorber increase from 1.1 ns to 22.8 ns and the power conversion efficiency (PCE) was enhanced from 2.55% to 4.86% with an increment of Voc ˃ 100 mV. Further, we achieved the balanced charge transfer by using EDA passivation on the optimized perovskite surface film. As a result, the efficiency was improved from 4.86% to 7.50%, which is the highest efficiency among lead-free wide bandgap PSCs. This enhanced photovoltaic performance of the wide-bandgap tin perovskite device presents a wide application in lead-free tandem towards commercial development.