Enhanced performance of CH3NH3PbI3 perovskite solar cells by excess halide modification

• The effects of excess halide on the crystallization of perovskite are investigated. • The effects of modification of perovskite films by excess lead halides depend on the halogen elements. • 5 mol% excess PbI 2 and PbBr 2 improve the quality of perovskite film. • The mechanism of crystallization process with excess halide modification is proposed. Metal halide perovskite solar cells (PSCs) become a research hotspot owing to their remarkable power conversion efficiency, simple preparation process and low cost. The properties of perovskite films play a decisive role in the photovoltaic performance of PSCs. This work systematically investigates the effects of excess halide modification on the crystallization quality of perovskite precursors (MAPbI 3 ) using cation halides (CsI and CH 3 NH 3 I) or lead halides (PbI 2 , PbBr 2 , PbCl 2 ). Modification by excess cation halides leads to low coverage of the films on the substrate and agglomeration at grain boundaries. The impact of lead halides on the crystallization process depends on the types of the halogen elements. Modification by PbCl 2 leads to enlarged grains and increased film roughness. In contrast, PbI 2 and PbBr 2 passivate defects at the grain boundaries and interfaces, improving the quality of the perovskite films. We propose crystallization mechanisms to explain the effects of modification using excess cation halides and lead halides on the quality of the crystallized perovskite films. The performance of PSCs fabricated using the modified films are tested. The PSCs with PbI 2 - and PbBr 2 -modified films exhibit optimal PCE of 16.5% and 19.3% respectively, whereas the pristine device shows an inferior PCE of 14.3%.