Modulating Crystallization in Semitransparent Perovskite Films Using Submicrometer Spongelike Polymer Colloid Particles to Improve Solar Cell Performance

Semitransparent perovskite solar cells (PSCs) have excellent potential for solar window applications. A major challenge exists, however, in achieving uniform coverage for thin perovskite films. Unfortunately, uncontrolled pinhole formation is a common problem for such films that obstructs development, especially for large area devices. In this study, we used very small (submicrometer) swellable polymer colloid particles (microgels) as additives to prepare uniform thin CH3NH3PbI3 (MAPI) perovskite films. Microgels (MGs) are good film-formers and promoted formation of semitransparent (ST) perovskite films with improved coverage. The MGs act as colloidal sponges and delayed release of perovskite precursors, thereby delaying perovskite crystallization. The ST films prepared using MGs had fewer pinholes compared to the MG-free control films. X-ray photoelectron spectroscopy showed evidence of Pb coordination by the MGs and they were shown to passivate MAPI. Remarkably, the submicrometer MGs used in this study decreased light scattering for the ST films. Planar devices constructed using a 10 nm ST film with an average visible transmittance of 46.8% gave an average power conversion efficiency (PCE) of 7.69%, which compares favorably to literature values. The average PCE increased to 9.62% upon inclusion of a thin meso-TiO2 layer. These PCE values are significantly higher than that achieved for the MG-free ST control (4.93%). The MGs and approaches used here are scalable and should apply to other ST perovskite films, solar cells, and, potentially, tandem devices.