Preparation of Tortuous 3D γ‐CsPbI 3 Films at Low Temperature by CaI 2 as Dopant for Highly Efficient Perovskite Solar Cells

Inorganic cubic CsPbI 3 perovskite (α‐CsPbI 3 ) has been widely explored for perovskite solar cells (PSCs) due to its thermal stability and suitable bandgap of 1.73 eV. However, α‐CsPbI 3 usually requires high synthesis temperatures (>320 °C). Additionally, it usually undergoes phase transition to the nonperovskite structure phase (β‐CsPbI 3 ), which results in poor photoelectric performance in devices. In this study, it is first found that the tortuous 3D CsPbI 3 phase (γ‐CsPbI 3 ) can be prepared and used for PSCs by solution process without any additive at low temperature (60 °C). The γ‐CsPbI 3 exhibits suitable bandgap of 1.75 eV and favorable photoelectric properties. However, γ‐CsPbI 3 is a metastable phase and easily transforms into β‐CsPbI 3 in ambient moisture. In order to improve the stability of γ‐CsPbI 3 , calcium ions (Ca 2+ ) with a relatively small radius of 100 pm are used to partially substitute lead ions (119 pm). This research proves that Ca 2+ can effectively improve the stability of the γ‐CsPbI 3 at room temperature. By optimizing the doping concentration of Ca 2+ (CsPb 1− x Ca x I 3 , x is from 0% to 2%), the Ca 2+ ‐doped γ‐CsPbI 3 PSCs achieve a hysteresis‐free J – V curve and a maximum power conversion efficiency (PCE) of 9.20%.