Mercouri Kanatzidis1
Northwestern University1
Understanding and tailoring the physical behavior of halide perovskites under external stimuli is critical for designing efficient optoelectronic devices. We found that continuous light illumination leads to a contraction in the out-of-plane direction in two-dimensional hybrid perovskites, which is reversible and strongly dependent on the specific superlattice packing. Constant light illumination causes the accumulation of positive charges in the terminal iodine atoms, thereby enhancing the bonding character of inter-slab I-I interactions across the organic barrier and activating out-of-plane contraction. This results in a threefold increase in carrier mobility and conductivity, consistent with an increase in the electronic band dispersion predicted by first-principles calculations. Flux-dependent space-charge-limited current measurement reveals that light-induced interlayer contraction activates interlayer charge transport. The enhanced charge transport boosts the photovoltaic efficiency of two-dimensional perovskite solar cells by increasing the fill factor and open-circuit voltage. The dynamic behavior of MAPbI<sub>3</sub>, under optical stimulation using X-ray diffraction, also revealed unexpected phenomena. We identified several nonequilibrium structures and phenomena that have not been shown in studies where the structure is measured in equilibrium. Using ultrafast X-ray diffraction techniques, we identified and characterized the dynamic, photo-excited structures of MAPbI<sub>3</sub>. We observed anisotropic distortions and rates of change along specific directions of the lattice as the material transitioned from the starting phase to a higher-symmetry, metastable excited structure over 1 ns following photoexcitation. The dynamics of these nonequilibrium structures demonstrate the flexible lattice response of metal halide perovskites, which facilitates favorable properties and provides further insight to enable new device architectures.