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

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.