Ionically generated built in equilibrium space charge zones -- a paradigm change for lead halide perovskite interfaces

Methylammonium lead iodide (MAPI) is the archetype of the intensively researched class of perovskites for photovoltaics. Nonetheless, even equilibrium aspects are far from being fully understood. Here we discuss equilibrium space charge effects at the MAPI/TiO2 and MAPI/Al2O3 interfaces, which are of paramount significance for solar cells. Different from the photovoltaic literature in which such built-in potentials are considered as being generated solely by electronic charge carriers, we will apply a generalized picture that considers the equilibrium distribution of both ionic and electronic carriers. We give experimental evidences that it is the ions that are responsible for the equilibrium space charge potential in MAPI, the reason being a pronounced ion adsorption at the contacts. The occurrence of equilibrium space charge effects generated by ionic redistribution has not been considered for photovoltaic materials and as such provides a novel path for modifying charge-selective interfaces in solar cells, as well as a better understanding of the behavior in mesoporous systems.