  Hybrid halide perovskites such as methylammonium lead iodide (CH3NH3PbI3)
exhibit unusually low free carrier concentrations despite being processed at
low-temperatures from solution. We demonstrate, through quantum mechanical
calculations, that the origin of this phenomenon is a prevalence of ionic over
electronic disorder in stoichiometric materials. Schottky defect formation
provides a mechanism to self-regulate the concentration of charge carriers
through ionic compensation of charged point defects. The equilibrium charged
vacancy concentration is predicted to exceed 0.4% at room temperature. This
behaviour, which goes against established defect conventions for inorganic
semiconductors, has implications for photovoltaic performance.
