Formamidinium Acetate additive for antisolvent-free crystallization of wide bandgap hybrid perovskites

Mixed halide perovskite’s (HP) poor thermal stability remains a fundamental barrier that can limit their use to a low temperature range. Although Formamidinium (FA) based HP are known to be less susceptible to oxidation/evaporation issues than their Methylammonium (MA) counterparts, α-FAPbI₃ is metastable and transform to the non-photoactive δ-phase over time. The black photoactive α-phase can be stabilized by replacing some of the FA in the lattice with Cesium. Most of these wide bandgap HPs are deposited using an anti-solvent quenching process - adding an extra layer of complexity to the production process. This work utilizes FA based ionic liquid FA Acetate (FAAc) additive in wide band-gap perovskite FA_0.75Cs_0.25Pb(I_0.80Br_0.20)₃ (Cs25Br20) ink that assists in nucleation and grain growth of a continuous pinhole free layer without the need of anti-solvent or gas quenching. The spin-casted thin films were analyzed using XRD, AFM and SEM to confirm the crystal structure, morphology, and grain size respectively. We also fabricated single junction solar cells with FAAc additive yielding a power conversion efficiency of 15.93%.