Understanding the doping effect in CsPbI2Br solar cells: crystallization kinetics, defect passivation and energy level alignment

• Inhibit undesirable iodide-rich perovskite intermediate phases by LiAc doping. • Tune the Fermi level of CsPbI 2 Br to enhance interfacial charge extraction. • Passivate uncoordinated Pb 2+ defects to reduce trap density. • A remarkable PCE of 16.05% is achieved in LiAc-doped CsPbI 2 Br PSCs. Additive engineering is an efficient approach to improve the photovoltaic performance of all-inorganic CsPbI 2 Br perovskite. However, rare attention has been paid to the CsBr intermediate, which has a significant effect on the perovskite crystallization process and thus the quality of final perovskite films. Herein, we find that the intermediate CsBr is formed during spin-coating of the CsPbI 2 Br precursor solution, which leads to the generation of iodide-rich perovskite (CsPbI 2+ x Br 1- x ) phases in the precursor film. This finally results in low-quality perovskite film after thermal annealing. To suppress the CsBr formation, lithium acetate (LiAc) was added into the CsPbI 2 Br precursor solution. We find that the intermediate CsBr is significantly suppressed after doping of LiAc, which results in less phase segregations in the precursor film and thus high-quality CsPbI 2 Br film after thermal annealing. The LiAc-doped perovskite film shows higher crystallinity, larger grain size and more preferential orientation than the pristine perovskite film. Furthermore, Ac‾ coordinates with Pb 2+ to passivate uncoordinated Pb 2+ defects, and Li + aggregates at the perovskite surface to upwardly shift the Fermi level of CsPbI 2 Br closer to the conduction band edge, which leads to the suppressed trap-assisted recombination losses and the enhanced interfacial charge extraction in the LiAc-doped perovskite solar cells (PSCs). As a result, a remarkable power conversion efficiency (PCE) of 16.05% is achieved in LiAc-doped CsPbI 2 Br PSCs. Moreover, the devices exhibit superior thermal stability with almost no PCE degradation after 300 h of thermal aging at 85 °C. Our results provide deep insights into the doping effect of additive, especially on perovskite crystallization kinetics, which are important for the future optimization of high-performance all-inorganic PSCs.