Efficient and stable wide bandgap perovskite solar cells through surface passivation with long alkyl chain organic cations

Defects on perovskite surfaces acting as charge-carrier-traps are a key factor limiting the performance of perovskite solar cells (PSCs). Here we studied the defect passivation effect of three bromide-containing alkylammonium organic cations with increasing alkyl-chain-length: n-butylammonium bromide, n-octylammonium bromide and n-dodecylammonium bromide on a perovskite composition with 1.72 eV bandgap. Long-alkyl-chain organic cations were found to have a greater passivation effect compared to their shorter counterparts due to greater reduction in surface defects and substantial changes in the electronic structure of the passivated perovskite films. The efficiency of 1.72 eV PSCs was improved to 19.1% with an excellent open-circuit-voltage of over 1280 mV. The long-alkyl-chain passivation significantly improved the moisture and light stability of PSCs as the unencapsulated devices retained >90% of the initial performance after 144 h at 70–85% relative-humidity and >93% of the initial performance after operating under light for 80 h. The study has paved the way for efficient and stable wide bandgap perovskite top cells used in perovskite-silicon tandem solar cells.