Guanidinium thiocyanate selective Ostwald ripening induced large grain for high performance perovskite solar cells

Organic-inorganic lead halide perovskite has become one of the most attractive materials for future low-cost high-efficiency solar technology. However, the polycrystalline nature of perovskite thin-film often possesses an exceptional density of defects, especially at grain boundaries (GBs) and film surface, limiting further improvement in the power conversion efficiency (PCE) of the perovskite device. Here, we report a simple method to reduce GBs and to passivate the surface of a methylammonium lead tri-iodide (MAPbI3) film by guanidinium thiocyanate (GUTS)-assisted Ostwald ripening post treatment. High-optoelectronic quality MAPbI3 film consisting of micron-sized grains were synthesized by post-treating a MAPbI3 film with GUTS/isopropanol solution (4 mg/mL, GUTS-4). Analysis of the electrochemical impedance spectra (EIS) of the solar cells showed that interfacial charge recombination resistance of the device based on a GUTS-4 post-treated MAPbI3 absorber film was increased by a factor of 1.15 to 2.6, depending on light illumination intensity, compared to the control MAPbI3 cell. This is consistent with results of the open-circuit voltage (Voc) decay and the light intensity dependent photovoltage evolution which shows device with GUTS treatment had one order longer charge carrier lifetime and was more ideal (ideality factor = = 1.25). Further characterization by Kelvin probe force microscope indicated that GUTS-4 treatment shifted the energetics of the MAPbI3 film by ~100 meV towards better energy level alignment with adjacent SnO2 electron transport layer, leading to a more favorable charge extraction process at the MAPbI3/SnO2 interface. As a result, the PCE of PSCs was enhanced from 14.59% to 16.37% and the hysteresis effect was mitigated.