Bi-Directional Simultaneous Cation/Anion Engineering of Formate Complex for Holistic Management of Perovskite Solar Cells

Pseudo-halide anion formate (HCOO^-) has been widely recognized as crystallization agent to control the crystal growth without inducing any changes in the band-gap. However, there is a lack of understanding regarding the effect on perovskite films depending on the chemical bonding state of formate. In this work, we demonstrated the distinguishable crystallization behavior of FAPbI₃ film by comparing the formate complexes with different bonding state. In addition, we found that the distinct passivation mechanisms depending on the cations of formate complexes, which resulted in the differences in device performance. Notably, sodium formate (NaFo) showed bi-directional behavior of cation and anion, eliciting a distinction from other formate complexes. As a result, the NaFo device showed an enhanced power conversion efficiency (PCE) of 25.6% with a high open-circuit voltage (<i>V_oc</i>) of 1.18 V, achieving 95% of the Shockley-Qeuisser limit at the PV band-gap of 1.53 eV. This work provides insights of the effect of crystallization by the chemical bonding state of formate as well as different passivation mechanism depending on the cations of formate complexes.