Charge carrier chemistry in methylammonium lead iodide

Both electronic and ionic transport properties are key issues in the performance of perovskite solar cells. In order to understand the electrical behavior of organic-inorganic halide perovskites, and the possibilities to influence them, the elucidation of their equilibrium charge carrier chemistry as a function of stoichiometry and dopant concentration is a necessary and fundamental prerequisite. We provide here insight into the point defect chemistry of methylammonium lead iodide, whereby the decisive carriers are identified and their concentrations discussed as a function of stoichiometry (iodine partial pressure) and dopant content (oxygen partial pressure or Na addition). The experimental results indicate how ionic conductivity, which can be attributed to iodine vacancies, and electronic conductivity, which can be attributed to electron holes (or conduction electrons under reducing conditions), can be significantly and systematically altered by such treatments. Experimental results are discussed in the context of simple defect chemical models.