Improving the performance of lead-free Cs2AgBiBr6 double perovskite solar cells by passivating Br vacancies

Abstract

Cs₂AgBiBr₆ perovskite solar cells (PSCs) have attracted much attention in recent years. However, a large number of intrinsic point defects lead to their low photoelectric conversion efficiency (PCE). Herein, the density functional theory (DFT) calculations show that the formation enthalpy of the Br vacancy (V_Br) is very low and it is a deep-level defect for p-type Cs₂AgBiBr₆, which will have a negative impact on the performance of the PSCs. Accordingly, hydrobromic acid (HBr) is added to the perovskite precursor solution to passivate the V_Br. The hole-transporting-layer (HTL)-free and carbon electrode-based planar PSCs with 4% HBr added Cs₂AgBiBr₆ as the light-absorbing layer exhibit a 68% increase compared with the control. Both theoretical calculations and experimental characterization prove that the improvement of PCE is due to V_Br being occupied by Br atoms in HBr. This work provides new ideas for vacancy defect engineering from the aspect of the intrinsic point defect characteristics of Cs₂AgBiBr₆.