Stabilizing the α-Phase of CsPbI3 Perovskite by Sulfobetaine Zwitterions in One-Step Spin-Coating Films

Abstract

Inorganic cesium lead halide perovskites (CsPbI3) are promising materials for efficient wide-bandgap perovskite solar cells, but they suffer from a phase transition from black α phase to yellow δ phase at room temperature. Here, we report a facile method to stabilize the α-phase CsPbI3 films via a single-step film deposition process. A small amount (∼1.5 wt %) of sulfobetaine zwitterion mixed in CsPbI3 precursor solution could facilitate the formation of black-phase CsPbI3 films that show significantly improved phase stability in air. The black-phase stabilization can be explained by the formation of small CsPbI3 grains with average size of ∼30 nm, which increased the grain surface area to stabilizes the α phase. The zwitterions were found to impede the crystallization of CsPbI3 perovskite films via electrostatic interaction with the ions and colloids in the CsPbI3 precursor solution. Solar cells using these zwitterion-stabilized perovskite films showed stabilized power conversion efficiency of 11.4% under 1-sun illumination. Due to the superior photovoltaic performance and enormous potential for commercialization, perovskite materials have attracted much attention in the past years. Particularly, CsPbI3 is a cutting-edge material for efficient wide-bandgap solar cells that can potentially boost the efficiency of silicon solar cells with tandem structure. However, CsPbI3 material suffers from a notorious phase transition from black α phase to yellow δ phase at room temperature, which dramatically reduces the absorbed sun light to reduce photovoltaic performance. Here, sulfobetaine zwitterions were mixed in CsPbI3 precursor solution to stabilize the α phase of CsPbI3 films. Solar cells with these α-CsPbI3 films exhibited excellent performance and stability. The α-phase CsPbI3 films may also be used to fabricate efficient light-emitting diodes or photodetectors. The method of phase manipulation may be used in stabilizing the black phase of other perovskite photovoltaic materials, such as FAPbI3 and CsSnI3. Stabilizing the α phase of CsPbI3 has become one of the most critical prerequisites for its photovoltaic application. We found that mixing a small amount of sulfobetaine zwitterions in CsPbI3 precursor solution could stabilize the α phase of CsPbI3 films at room temperature. The interaction of zwitterion with CsPbI3 impeded the fast crystallization of CsPbI3, which reduced CsPbI3 grain size to stabilize the α phase. Solar cells with these α-phase CsPbI3 films showed stabilized efficiency of 11.4% under 1-sun illumination.