UC Davis

We investigated temperature and gate-dependent photogenerated carrier diffusion in single-crystal micro-structures of the hybrid organic-inorganic perovskite (HOIP) methylammonium lead trihalide MAPbX3, (where X = I, Br) via scanning photocurrent microscopy (SPCM). In both materials, carrier diffusion lengths (L_D) were found to increase abruptly across the tetragonal to orthorhombic phase transition, coincident with an abrupt increase in exciton binding energy (E_B), and reached over 100 um by 80 K. Combined with relatively short carrier lifetimes, the low temperature L_D measurements implied an enormous carrier mobility (10^4 Vs/cm^2) in both materials, too high in fact to be typical electron or hole diffusion. Thus, we attributed this fascinating behavior to fast, efficient transport of charge-neutral excitons, where the dipolar nature of the excitons massively reduces their optical phonon scattering, allowing them to diffuse unhindered through these materials. We also discovered the ability to tune the low temperature exciton diffusion via an applied gate voltage (V_G). Depending on the material, as well as the sign and magnitude of V_G, L_D could be increased or decreased by a significant margin. In addition, MAPbBr3's intrinsically larger E_B made this value directly observable through photocurrent spectroscopy. The measured E_B values were temperature-dependent (E_B increased as T decreased) with the sharpest change occurring at the low temperature phase transition. Finally, we branched out from our HOIP studies to investigate polarization-dependent anisotropic photoluminescence of self-trapped excitons (STEs) in the one-dimensional (1D) organic metal halide hybrid (OMHH) C4N2H14PbBr4. 1D materials can exhibit strongly anisotropic optical properties and highly efficient light emission, making them promising candidates for novel photodetection and lighting applications. We discovered that the sample emission intensity can shift between being maximum under parallel-to-1D chain versus perpendicular-to-1D chain excitation, depending on the excitation wavelength (lambda_EX). We attributed this lambda_EX-dependent emission to fast surface recombination, supported by first-principles calculations of optical absorption and a fast emission decay component seen with time-resolved photoluminescence (TRPL) only when absorption was located near the surface.