Crystallization Kinetics and Morphology Control of Formamidinium–Cesium Mixed‐Cation Lead Mixed‐Halide Perovskite via Tunability of the Colloidal Precursor Solution

The meteoric rise of the field of perovskite solar cells has been fueled by the ease with which a wide range of high‐quality materials can be fabricated via simple solution processing methods. However, to date, little effort has been devoted to understanding the precursor solutions, and the role of additives such as hydrohalic acids upon film crystallization and final optoelectronic quality. Here, a direct link between the colloids concentration present in the [HC(NH 2 ) 2 ] 0.83 Cs 0.17 Pb(Br 0.2 I 0.8 ) 3 precursor solution and the nucleation and growth stages of the thin film formation is established. Using dynamic light scattering analysis, the dissolution of colloids over a time span triggered by the addition of hydrohalic acids is monitored. These colloids appear to provide nucleation sites for the perovskite crystallization, which critically impacts morphology, crystal quality, and optoelectronic properties. Via 2D X‐ray diffraction, highly ordered and textured crystals for films prepared from solutions with lower colloidal concentrations are observed. This increase in material quality allows for a reduction in microstrain along with a twofold increase in charge‐carrier mobilities leading to values exceeding 20 cm 2 V −1 s −1 . Using a solution with an optimized colloidal concentration, devices that reach current–voltage measured power conversion efficiency of 18.8% and stabilized efficiency of 17.9% are fabricated.