Substituent engineering in tertiary phosphine oxides for passivating defects of perovskite solar cells

Defect passivation based on Lewis acid–base chemistry has been regarded as an effective strategy to improve the photovoltaic performance and stability of perovskite solar cells (PSCs). Here, we report on tertiary phosphine oxides (R₃PO) as materials for defect passivation, where photovoltaic performance was investigated depending on the substituents R. Electron-donating ability of the substituents in R₃PO was found to play an important role in passivation. Cyclohexyl substituent was better in achieving photovoltaic performance than linear hexyl substituent. The heterocyclic morpholine substituent bearing oxygen and nitrogen in cyclohexyl form further improved photovoltaic performance due to its enhanced electron-donating ability. Compared with an untreated PSC, the trimorpholinophosphine oxide (TMPPO)-treated PSC improved the power conversion efficiency from 21.95% to 23.72%. Additionally, the dark-storage stability test with an unencapsulated device showed that the TMPPO-treated device maintained 92.7% of its initial PCE after 1250 h, while 86.8% was maintained for the untreated device. Three hundred hour-light-soaking of the encapsulated devices revealed that the operational stability of the TMPPO-treated PSC was superior to the untreated device.