  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.
