Microtuning of the Wide-Bandgap Perovskite Lattice Plane for Efficient and Robust High-Voltage Planar Solar Cells Exceeding 1.5 V

Iodide-free tribromide-based perovskites, with their wide bandgap of over 2.0 eV, are highly regarded as potential candidates for a photoelectrochemical water splitting system and the topmost cell in tandem solar cell. Herein, we report on the importance of microtuning of the crystal lattice by cesium incorporation into the A-site on low temperature processed formamidinium lead tribromide (CH(NH2)2PbBr3 = FAPbBr3) perovskite films. The partial incorporation of cesium bromide (CsBr) into the FAPbBr3 film tunes crystal-lattice interactions, resulting in a high-purity cubic crystal system with preferred orientation. An entirely low temperature processed planar photovoltaic device assembled with FAPbBr3 containing 8% Cs (Cs0.08FA0.92PbBr3) exhibited an optimum PCE (power conversion efficiency) of 8.56% with a Voc (open-circuit voltage) of 1.516 V, which is higher than the PCE of 7.07% and Voc of 1.428 V of the FAPbBr3 device. Photoluminescence-intensity and temporal-imaging measurements were conducted by laser scanning confocal time-resolved microscopy (LCTM), which revealed that CsBr incorporation into a FAPbBr3 film significantly suppresses the nonradiative recombination pathways and homogenizes the spatial distribution of photoluminescence. It was visualized that the incorporation of CsBr in FAPbBr3 directly affects the bulk defect and photoluminescence properties, which provides evidence that Cs ions surely alleviate the segregation and aggregation of ions in the perovskite film. Notably, the Cs0.08FA0.92PbBr3 film, with a carrier lifetime of about 270 ns, exhibited a 1.37-fold longer radiative recombination time than that (210 ns) observed for the FAPbBr3 film. Furthermore, aging experiments without encapsulation under ambient (in air for 2000 h) and severe (65 °C and 65% RH for 500 h) conditions revealed that the Cs0.08FA0.92PbBr3 devices were more robust than the FAPbBr3 devices.