Ammonium cations with high pK a in perovskite solar cells for improved high-temperature photostability

Phenethylammonium (PEA+) and butylammonium (BA+) are widely used in three-dimensional (3D) perovskites to form two-dimensional perovskites at film surfaces and grain boundaries (GBs) for defect passivation and performance enhancement. Here we show that these cations are unstable with 3D formamidinium (FA+)-containing perovskites under high-temperature light soaking. PEA+ and BA+ are found to deprotonate to amines, which then react with FA+ to produce (phenethylamino)methaniminium (PEAMA+) and (butylamino)methaniminium (BAMA+), respectively, severely limiting device high-temperature photostability. Removing these cations greatly improves the photostability but compromises device efficiency by leaving non-fully passivated surfaces and GBs. Ammonium cations with a high acid dissociation constant (pK a), including PEAMA+ (pK a = 12.0) and BAMA+ (pK a = 12.0), can replace PEA+ or BA+ for passivation and are stable with FA-based perovskites due to their resistance to further deprotonation. P–i–n structure solar cells with PEAMA+ additive maintained over 90% of their initial efficiency after light soaking at open circuit and 90 °C for 1,500 hours