Probing the ionic defect landscape in halide perovskite solar cells

Metal halide perovskites are a remarkable class of materials with particular promise for application in optoelectronics. Like in any semiconductor, defects in perovskites play a critical role in determining their properties and performance in optoelectronic devices, however many open questions regarding the nature of ionic defects remain unanswered. In this work we apply impedance spectroscopy and deep-level transient spectroscopy to characterize the ionic defect landscape in methylammonium lead triiodide perovskites (MAPbI$_3$) in which defects were purposely introduced by fractionally changing the precursor stoichiometry. Our results indicate that variation of the ionic defect landscape has a profound influence on the electronic landscape, exemplified by its impact on the device built-in potential, and consequently, the open-circuit voltage. Moreover, we find that all measured ionic defects fulfill the Meyer-Neldel rule with a characteristic energy, which corresponds to the underlying ionic hopping process in perovskite materials. These findings allow a defect categorization of our data and literature values.