Effect of functional groups in passivating materials on stability and performance of perovskite solar cells

Abstract

Post-treatment of formamidinium lead triiodide (FAPbI₃) with ammonium halogen salts, such as phenethylammonium or octylammonium iodide, has been widely used to improve the photovoltaic performance and stability of perovskite solar cells (PSCs) via surface defect passivation. Although the passivation is expected to be influenced by functional groups (ammonium versus amidinium and aryl versus alkyl), little study on this effect has been reported. Here, we report on the dependence of stability and photovoltaic performance on cation groups of iodide passivating materials. Amidinium (–C(NH₂)₂⁺) and ammonium (–NH₃⁺) in a given aryl group and aryl and alkyl group with similar steric size in a given amidinium group were compared using phenylamidinium (benzamidinium) iodide (PhADI), benzylammonium iodide (BnAMI), and tert-butyl amidinium iodide (tBADI). It was found from the photovoltaic parameters that amidinium was better than ammonium in the given aryl group, and aryl was better than alkyl in the given amidinium moiety. Post-treatment with PhADI shows the best photovoltaic performance among the studied series of passivating materials. Open-circuit voltage was mainly improved from 1.11 V to 1.16 V by the PhADI treatment due to the reduced trap density from 2.53 × 10¹⁶ to 1.54 × 10¹⁶ cm⁻³, resulting in an improvement of power conversion efficiency (PCE) from 21.27% to 23.24%. Moreover, the PhADI-treated device maintained over 99% of its initial PCE after 400 h, while inferior maintenance of 65% was observed from the device without post-treatment.