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

Inorganic cubic CsPbI3 perovskite (α‐CsPbI3) has been widely explored for perovskite solar cells (PSCs) due to its thermal stability and suitable bandgap of 1.73 eV. However, α‐CsPbI3 usually requires high synthesis temperatures (>320 °C). Additionally, it usually undergoes phase transition to the nonperovskite structure phase (β‐CsPbI3), which results in poor photoelectric performance in devices. In this study, it is first found that the tortuous 3D CsPbI3 phase (γ‐CsPbI3) can be prepared and used for PSCs by solution process without any additive at low temperature (60 °C). The γ‐CsPbI3 exhibits suitable bandgap of 1.75 eV and favorable photoelectric properties. However, γ‐CsPbI3 is a metastable phase and easily transforms into β‐CsPbI3 in ambient moisture. In order to improve the stability of γ‐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 γ‐CsPbI3 at room temperature. By optimizing the doping concentration of Ca2+ (CsPb1−xCaxI3, x is from 0% to 2%), the Ca2+‐doped γ‐CsPbI3 PSCs achieve a hysteresis‐free J–V curve and a maximum power conversion efficiency (PCE) of 9.20%. [ABSTRACT FROM AUTHOR]