Cation‐Diffusion‐Based Simultaneous Bulk and Surface Passivations for High Bandgap Inverted Perovskite Solar Cell Producing Record Fill Factor and Efficiency

High bandgap perovskite solar cells are integral to perovskite-based multi-junction tandem solar cells with efficiency potentials over 40%. However, at present, high bandgap perovskite devices underperform compared to their mid bandgap counterparts in terms of voltage outputs and fill factors resulting in lower than ideal efficiencies. Here, the low fill factor aspect of high bandgap perovskite is addressed by developing a cation-diffusion-based double-sided interface passivation scheme that simultaneously provides bulk passivation for a 1.75 eV perovskite cell that is also compatible with a p-i-n cell architecture. The champion cell achieves a record fill factor of 86.5% and a power conversion efficiency of 20.2%. Results of ionic distribution profiling, Fourier transform infrared spectroscopy, and X-ray diffraction crystallography reveal evidence of cation diffusion from the surface perovskite passivation layer into bulk. The diffused cations reduce Shockley-Read-Hall recombination in the perovskite bulk and at the surfaces with the latter being more dominant as confirmed by light-intensity dependent and temperature-dependent open-circuit voltage measurements as well as thermal admittance spectroscopy. This concurrent bulk and surface passivation scheme renders record fill factor and efficiency in the double-side passivated cells. This provides new insights for future passivation strategies based on ionic diffusion of functionalized materials.