Effect of Hammett substituent constant of para-substituted benzoic acid on the perovskite/SnO2 interface passivation in perovskite solar cells

Abstract

It is critical to design the bifunctional passivation molecules to passivate simultaneously both charge transport layer and perovskite layer at the charge transport layer/perovskite interface in perovskite solar cells (PSCs). In this report, we investigate effect of para-substituted benzoic acid with different Hammett constant (σ) on photovoltaic performance of PSCs. Two passivation molecules of 4-aminomethylbenzoic acid (4-AMBA) and 4-sulfamoylbenzoic acid (4-SABA) are used to passivate the SnO2 surface with carboxylic acid and the perovskite with para-substituent electron-donating -CH2NH2 (σ = ca. -0.02) and electron-withdrawing -SO2NH2 (σ = ca. +0.60). Compared with non-passivated PSC, the passivation improves power conversion efficiency (PCE) mainly due to the increased open-circuit voltage (VOC) and fill factor (FF), where -SO2NH2 substituent is better in improving the photovoltaic performance than -CH2NH2 one. Trap density is more reduced and charge extraction ability is more improved by 4-SABA than by 4-AMBA, which indicates that a weak electron-withdrawing nature of para-substituent like -SO2NH2 is better for passivation of the bottom perovskite than a weak electron-donating -CH2NH2 substituent. Consequently, the passivation with 4-SABA enhances the PCE from 22.27% to 23.64%, along with improved long-term stability. This work highlights for the first time the role of Hammett constant in surface passivation of PSCs.