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

Abstract

It is critical to design bifunctional passivation molecules to simultaneously passivate the charge transport layer and perovskite layer at the charge transport layer/perovskite interface in perovskite solar cells (PSCs). In this study, we investigate the effect of para-substituted benzoic acid with different Hammett constants (σ) on the photovoltaic performance of PSCs. Two passivation molecules 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 the power conversion efficiency (PCE) mainly due to the increased open-circuit voltage (V_OC) and fill factor (FF), where the –SO₂NH₂ substituent is better in improving the photovoltaic performance than the –CH₂NH₂ one. The trap density is more reduced and the charge extraction ability is more improved by 4-SABA than by 4-AMBA, which indicates that the weak electron-withdrawing nature of a para-substituent such as –SO₂NH₂ is better for the 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 the Hammett constant in the surface passivation of PSCs.