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 the results of a parallel study on MAPbX3 (X = I, Br, 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/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 of 1016–1022 cm–3 depending on the halide ion. The trap state density from I–V measurements, Ntrap ∼ 109 to 1010 cm–3, was found to be lower than those of previous reports. Single-crystal MAPbI3 devices exhibited a low capture cross section (σ– ∼ 10–16 cm–2), high mobility (μ ∼ 196 cm2/V-s), and large diffusion length (LD ∼ 6 μm). The study shows that nonradiative energy loss and carrier trapping are suppressed and transport properties are enhanced by reducing grain boundary effects, along with interface engineering to prevent halide ion accumulation at the electrodes.