Controlling the Formation Process of Methylammonium-Free Halide Perovskite Films for a Homogeneous Incorporation of Alkali Metal Cations Beneficial to Solar Cell Performances

Incorporating multiple cations of the Ia alkali metal column of the periodic table (K+/Rb+/Cs+) to prepare m-AMCs perovskite film is promising for boosting the photovoltaic properties. However, contrary to K+, both Cs+ and Rb+ suffer from non-uniformity at the origin of performance and stability losses. In this paper, Ammonium chloride (NH4Cl) additive is shown to address this concern. We analyze step-by-step the effects of NH4Cl Additive and Alkali Metal Cations (K+/Rb+/Cs+) on the one-step film formation process of methylammonium-free, formamidinium-based, halide perovskites. First, we highlight the action of NH4Cl additive on the spin-coating and the anti-solvent dripping processes. We show that it improves the solubility of PbI2 in solution by forming an intermediate and then favor the perovskite phase formation. Then, we investigate the annealing process by introducing depth profile evolution monitoring by the glow discharge optical emission spectroscopy (GD-OES) technique. The real-time distribution changes of m-AMCs in the film upon annealing at 155°C is visualized. We show that K at low concentration is homogeneously distributed throughout the film. Cs is more concentrated at the surface and Rb in the film depth. With NH4Cl additive, these two alkali metals are more homogeneously distributed. We show that NH4Cl can slow down the movement of m-AMCs so that they are better evenly distributed into the perovskite layer. Moreover, it changes the growth direction of the perovskite film changes, making the overall crystallization quality improved and the distribution more uniform. It results in perovskite films with large monolithic grains, permitting a high stabilized power conversion efficiency (PCE) over 21%. Finally, by combining AC additive and film surface treatment with n-propylammonium iodide (PAI), the performance was further upgraded at a stabilized PCE of 22.04%.