  The effect of the orientation of the cation MA (i.e., CH3NH3) on the physical
properties of MAPbI3 are investigated using the density functional theory. We
report that the Fermi energy level exhibits a large variation with MA
orientation, and it is this Fermi level variation that makes this material
unique and so different from the non-organic perovskites. The Fermi level
variation with orientation of MA is proposed to be responsible for the
experimentally observed intrinsic open-circuit voltage V_OC, ambipolarity, and
long lifetime of the excitons. Based on our results, ferroelectric domains in
the low-temperature orthorhombic phase or clusters of MA molecules not rotating
in unison (as a consequence of e.g. impurities and/or defects) in the
higher-temperature tetragonal and cubic phases are proposed to be responsible
for ambipolarity. This is because any given two adjacent domains or clusters
with different MA s average orientations may have very different Fermi levels,
causing them to behave as an effective diode. Also, the measured significant
lifetime of excitons in MAPbI3 finds a natural explanation using this effective
diode. Thus, our work provides a unified manner for explaining at least these
three important properties in terms of the rotation-induced Fermi level
variations. We used the density functional theory to get our results.
