Decoupling Structure and Composition of CH3NH3PbI3-xBrx Films Prepared by Combined One-Step and Two-Step Deposition

Perovskite solar cells (PSCs) continue to attract great interest from chemists, physicists, and materials scientists due to their facile preparation and outstanding properties. The solution-based methods used to prepare PSCs usually involve one-step (1-s) or two-step (2-s) deposition. Here, we investigate mixed halide perovskite films (MAPbI(3-x)Br(x), where MA is CH3NH3) prepared using a new solution-based method that combines 1-s and 2-s deposition. This new combined method is denoted as 1 & 2-s deposition. Mixed halide films prepared using 1 & 2-s deposition are shown to have structures and optoelectronic properties that depend strongly on the order of addition of the reactants. The results suggest a tunable competition occurred between crystal growth and anion exchange. The XRD and XPS data show that the structures and compositions of MAPbI,,Brx prepared using 1-s and 2-s deposition can be decoupled. The composition of MAPbI(3-x)Br(x) can be either stoichiometrically controlled or diffusion controlled depending on the addition sequence of MAI and MABr. The stoichiometrically controlled MAPbI3,Brx films gave devices with the highest power conversion efficiency (PCE), thermal stability, and device stability. Optimization gave a maximum PCE of 15.11%, which is 70% higher than those for PSCs prepared previously using the 1 & 2-s method. The key advantage of the 1 & 2-s deposition method is that it separates perovskite growth into a structure-forming first stage and an alloy-forming second stage which provide enhanced versatility for perovskite film architecture and composition control. The 1 & 2-s method should be applicable to other solution processed PSCs.