Designed bi-directional cation/anion interfacial distribution of perovskite solar cells

Pseudo-halide anion formate (HCOO-) in halide perovskite has been widely employed to improve film quality without altering the band gap. However, there is a lack of understanding regarding the effect on perovskite films depending on the chemical bonding state of formate. Herein, we demonstrated the distinguishable crystallization behavior of FAPbI3 film by comparing the formate salts with different bonding states. Furthermore, we found that distinct passivation mechanisms depended on the cations of formate salts, which resulted in the differences in device performance. Notably, sodium-based formate exhibited simultaneous bi-directional behavior of cation and anion, distinguishing it from other formate salts. Consequently, the device exhibited an enhanced power conversion efficiency (PCE) of 25.6% with a significantly high open-circuit voltage (Voc) of 1.18 V. This work provides insights into the effect on the chemical bonding state of formate, as well as different passivation mechanisms depending on the cations of formate salts.