CsPbCl-Driven Low-Trap-Density Perovskite Grain Growth for >20% Solar Cell Efficiency.

Charge recombination in grain boundaries is a significant loss mechanism for perovskite (PVK) solar cells. Here, a new strategy is demonstrated to effectively passivate trap states at the grain boundaries. By introducing a thin layer of CsPbCl coating before the PVK deposition, a passivating layer of PbI is formed at the grain boundaries. It is found that at elevated temperature, Cl ions in the CsPbCl may migrate into the PVK via grain boundaries, reacting with MA to form volatile MACl and leaving a surface layer of PbI at the grain boundary. Further study confirms that there is indeed a small amount of PbI distributed throughout the grain boundaries, resulting in increased photoluminescence intensity, increased carrier lifetime, and decreased trap state density. It is also found that the process passivates only grain surfaces, with no observable effect on the morphology of the PVK thin film. Upon optimization, the obtained PVK-film-based solar cell delivers a high efficiency of 20.09% with reduced hysteresis and excellent stability.