Long Exciton Diffusion Length in Single Crystal Halide Perovskites*

Hybrid inorganic-organic halide perovskites have garnered an incredible amount of attention over the last few years, thanks to power conversion efficiencies climbing from 3% to now 22.7% in less than a decade. The majority of published work thus far has focused on creating higher quality thin films, while the underlying carrier transport processes are not well understood. By studying single crystal methylammonium lead iodide (MAPbI₃) samples, we avoid the issues caused by thin film grain boundaries and shed light on the inherent carrier transport mechanisms of this revolutionary material. We determined the carrier diffusion lengths via scanning photocurrent microscopy (SPCM). We observed excitonic behavior through electric field independent photocurrent distributions as well as a measured exciton diffusion length that increased abruptly below the phase transition temperature from the room temperature tetragonal phase to the low temperature orthorhombic phase. The low temperature phase diffusion reached up to 100 μm at 80 K. We attributed the long exciton diffusion length to the reduced carrier scattering because of the suppressed ion rotation in the orthorhombic phase.