Lead(II) Propionate Additive and a Dopant-Free Polymer Hole Transport Material for CsPbI₂Br Perovskite Solar Cells

All-inorganic perovskites (CsPbI3 and CsPbI2Br), owing to their greater thermal stability compared to organic–inorganic hybrid perovskites, are becoming popular in perovskite photovoltaics, but the problem that remains with CsPbI2Br (or CsPbI3) is the humidity-assisted phase transformation. Herein, we report on the formation of CsPbI2Br α-phase and improvement of its phase stability under ambient atmosphere (20–30% relative humidity) by Pb(II) propionate additive in the CsPbI2Br precursor. Solar cells employing a CsPbI2Br film with an optimum concentration of the additive (1 mol %) and a donor–acceptor type polymer (synthesized by us) as dopant-free hole transport material that has a better energy level matching with CsPbI2Br (compared to other polymers like P3HT, PTAA, and asy-PBTBDT) work with a champion power conversion cell efficiency of 14.58%. A continuous increase in the open-circuit voltage, reaching 1.36 V for 5 mol % Pb(II) propionate, indicates a remarkable defect-passivation effect by the additive.