Room Temperature Phase Transition in Methylammonium Lead Iodide Perovskite Thin Films Induced by Hydrohalic Acid Additives

Although reactive additives have been employed in perovskite solar cells to enhance film morphology and significantly increase device performance, little is known about the effect of these additives on perovskite structural and optical properties. Here we report a systematic study of how the properties of methylammonium lead iodide perovskite (CH3NH3PbI3) are influenced by hydrohalic acid additives (HX; X=I, Br, Cl) in the precursor solution. Detailed structural and optical spectroscopic analysis reveals that all three acids affect the optical properties and alter the unit cell lattice parameters. Depending on the identity and concentration of HX, optical bandgaps widen or compress: addition of HBr yields a wider bandgap, whereas HI compresses the gap at high concentrations; HCl, on the other hand, has no significant effect on the bandgap. These changes can be understood by correlating them with the types of defects present in polycrystalline perovskite thin films in combination with the structural strain induced in very small crystallites. The presence of extra halides from HX in the precursor solution enables filling of the lattice vacancies in the perovskite, thereby altering metal–halogen–metal bond connectivity and consequently cell volumes and optical bandgaps. Remarkably, a room temperature tetragonal→cubic phase transition is observed for CH3NH3PbI3 films treated with high HX concentrations. Further insights into this anomalous phase transformation are obtained from in situ variable‐temperature X‐ray diffraction in the 25–55 °C (298–328 K) range, revealing a monotonic fall in transition temperature with increasing precursor solution HX concentration. Additive influence: Detailed structural and optical analysis reveals that hydrohalic acids (HX), frequently used as additives in perovskite precursor solutions, affect the perovskite optical properties and alter the unit cell lattice parameters. A decline in the tetragonal→cubic phase transition temperature is observed as a function of HX concentration, resulting in stabilization of the cubic phase at room temperature for perovskite films treated with high HX concentrations.