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

Additive engineering is an efficient approach to improve the photovoltaic performance of all-inorganic CsPbI2Br 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 inter-mediate CsBr is formed during spin-coating of the CsPbI2Br precursor solution, which leads to the generation of iodide-rich perovskite (CsPbI2+xBr1-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 CsPbI2Br 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 CsPbI2Br 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 Pb2+ to passivate uncoordinated Pb2+ defects, and Li+ aggregates at the perovskite surface to upwardly shift the Fermi level of CsPbI2Br 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 CsPbI2Br PSCs. Moreover, the devices exhibit superior thermal stability with almost no PCE degradation after 300 h of thermal aging at 85 degrees 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.