Effect of trap states, ion migration and interfaces on carrier transport in single crystal, polycrystalline and thick film devices of halide perovskites CH₃NH₃PbX₃ (X= I, Br, Cl)

The understanding of the mixed ionic-electronic nature of charge transport in Metal Halide Perovskites (MHPs) and the role of morphological and interface defects is crucial for improving the performance of MHP based photovoltaic devices. We present results of a parallel study on MAPbX$_3$ (X = I, Br and Cl), synthesized as solution processed polycrystalline powders, and as single crystals grown by a facile low temperature assisted technique. We have studied ionic-electronic charge transport in single-crystal and polycrystalline (pressed pellet and thick film) samples in order to compare the effect of defects and trap states associated with halide ion migration, device morphology and interfaces at grain boundaries as well as at electrodes. The mobility of halide ions and associated Coulomb capture of electrons or holes was determined by dielectric and space charge limited current (SCLC) dark I-V measurements, and also simulated using an ionic-electronic model. The defect capture cross section of electronic charge was found to be proportional to the simulated halide ion density Nion, which varied in the range $10^{16}$ cm$^{-3}$ - $10^{22}$ cm$^{-3}$ depending on the halide ion. The trap state density from I-V measurements, Ntrap ~ $10^9$ - $10^{10}$ cm$^{-3}$, was found to be lower than previous reports. Single crystal MAPbI3 devices exhibited a low capture cross section ($\sigma$ ~ $10^{-16}$ cm$^{-2}$), high mobility ($\mu$ ~ 196 cm$^2$/V-s) and large diffusion length (LD ~ 6 $\mu$m). The study shows that non-radiative energy loss and carrier trapping are suppressed and transport properties enhanced by reducing grain boundary effects, along with interface engineering to prevent halide ion accumulation at the electrodes.