Defect Spectroscopy in Halide Perovskites Is Dominated by Ionic Rather than Electronic Defects

Abstract

Atomic defects in semiconductors such as vacancies, interstitials, or dopants can lead to additional electronic states within the bandgap. These atomic defects are considered immobile in classical semiconductor theory, one of the reasons why a p–n junction with its ionized dopants remains stable over time. For mixed ionic–electronic conductors, the assumption of immobile defects is not valid. Halide perovskites (HaPs) are a class of materials in which ions and the related atomic defects have been shown to be mobile and migrate in response to external stimuli such as voltage or light. (1−3) We refer to them as ionic defects. Using iodide partial pressure investigations, it was deduced that the halide ion is the most mobile. (4,5) Furthermore, it was shown that the mobile ions follow the Meyer–Neldel rule (6,7) and that HaPs are capable of self-healing after being damaged. (8,9) As a result, Rakita et al. (10) stated that HaPs “are likely an example of a class of materials that cannot support static bulk defect densities significantly above thermodynamically-dictated densities. The reasons are (a) the free energy to form HaPs (from binary halides) is less than the formation energies of (static) defects in them and (b) the small kinetic stabilization of such defects.” These dynamic defects have significant implications for defect spectroscopy measurements. (11,12) In defect spectroscopy, the observed electronic defects (in the bandgap) that trap (localize) mobile charge carriers are usually assumed to be caused by immobile ionic defects. <em>In the case of mobile ionic defects, it is instead possible to observe the migration of the ionic defects themselves, rather than the capture/emission of charge carriers into/from the electronic defects created by these ionic defects.</em>

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