Formation of a Secondary Phase in Thermally Evaporated MAPbI3 and Its Effects on Solar Cell Performance

Thermal evaporation is a promising deposition technique to scale up perovskite solar cells (PSCs) to large areas, but the lack of understanding of the mechanisms that lead to high-quality evaporated methylammonium lead triiodide (MAPbI(3)) films gives rise to devices with efficiencies lower than those obtained by spin coating. This work investigates the crystalline properties of MAPbI(3) deposited by the thermal coevaporation of PbI2 and MAI, where the MAI evaporation rate is controlled by setting different temperatures for the MAI source and the PbI2 deposition rate is controlled with a quartz crystal microbalance (QCM). Using grazing incident wide-angle X-ray scattering (GIWAXS) and X-ray diffraction (XRD), we identify the formation of a secondary orthorhombic phase (with a Pnma space group) that appears at MAI source temperatures below 155 degrees C. With synchrotron-based X-ray fluorescence (XRF) microscopy, we show that the changes in crystalline phases are not necessarily due to changes in stoichiometry. The films show a stochiometric composition when the MAI source is heated between 140 to 155 degrees C, and the samples become slightly MAI rich at 165 degrees C. Increasing the MAI temperature beyond 165 degrees C introduces an excess of MAI in the film, which promotes the formation of films with low crystallinity that contain low-dimensional perovskites. When they are incorporated in solar cells, the films deposited at 165 degrees C result in the champion power conversion efficiency, although the presence of a small amount of low-dimensional perovskite may lead to a lower open-circuit voltage. We hypothesize that the formation of secondary phases in evaporated films limits the performance of PSCs and that their formation can be suppressed by controlling the MAI source temperature, bringing the film toward a phase-pure tetragonal structure. Control of the phases during perovskite evaporation is therefore crucial to obtain high-performance solar cells.

Automated summary

This study investigates the crystalline properties of thermal evaporated MAPbI(3) films and reveals that the formation of secondary phases limits the performance of perovskite solar cells (PSCs). By controlling the MAI source temperature, high-performance PSCs with a phase-pure tetragonal structure can be obtained.