Preparation of Tortuous 3D γ-CsPbI3 Films at Low Temperature by CaI2 as Dopant for Highly Efficient Perovskite Solar Cells

Inorganic cubic CsPbI3 perovskite (alpha-CsPbI3) has been widely explored for perovskite solar cells (PSCs) due to its thermal stability and suitable bandgap of 1.73 eV. However, alpha-CsPbI3 usually requires high synthesis temperatures (>320 degrees C). Additionally, it usually undergoes phase transition to the nonperovskite structure phase (beta-CsPbI3), which results in poor photoelectric performance in devices. In this study, it is first found that the tortuous 3D CsPbI3 phase (gamma-CsPbI3) can be prepared and used for PSCs by solution process without any additive at low temperature (60 degrees C). The gamma-CsPbI3 exhibits suitable bandgap of 1.75 eV and favorable photoelectric properties. However, gamma-CsPbI3 is a metastable phase and easily transforms into beta-CsPbI3 in ambient moisture. In order to improve the stability of gamma-CsPbI3, calcium ions (Ca2+) with a relatively small radius of 100 pm are used to partially substitute lead ions (119 pm). This research proves that Ca2+ can effectively improve the stability of the gamma-CsPbI3 at room temperature. By optimizing the doping concentration of Ca2+ (CsPb1-xCaxI3, x is from 0% to 2%), the Ca2+-doped gamma-CsPbI3 PSCs achieve a hysteresis-free J-V curve and a maximum power conversion efficiency (PCE) of 9.20%.