The widespread commercialization of metal halide perovskites in semiconductor applications is still hindered due to the instability of the perovskite itself[1,2]. The intrinsic instability primarily arises from the influence mobile ions in the film, which cannot be trivially circumvented[3]. However, ionic reactivity is not always detrimental; these ions are responsible for the well-known “perovskite healing” effects and can even result in perovskites exhibiting memory-like behaviour[4-6]. Understanding the precise mechanisms and influences of the different ionic species is therefore crucial for optimizing device performance for PV and LEDs, and can open up new technological pathways which exploit the positive ionic effects (examples include perovskite computing and perovskite memory).  

To date, key techniques used to study ion dynamics are predominantly electrical; techniques include impedance spectroscopy, transient ion drift, etc.[7-9]. These methods – indeed, all electrical methods - are inherently limited by the requirement of electrical contacts, restricting their applicability exclusively to operational devices. This is a significant drawback as the electrical response in perovskite devices often is dominated by the interfaces and contacts, rather than from the ions in the bulk[10,11]. This makes it challenging to disentangle the intrinsic ionic properties from surface and interfacial effects. Moreover, such electrical methods are not applicable for contact-free applications of perovskites, such as phosphors. Finally, it is challenging to conduct spatial mapping of properties with electrical measurements, which can be useful to understand local variations and inhomogeneities.

To overcome such drawbacks, we present an alternative fully optical technique which is sensitive to ionic processes across all relevant timescales. We present an overview of our technique and the results obtained for a triple-cation, double-halide perovskite. We show how the ionic properties, such as the characteristic lifetimes, resistances and more can be extracted using our method. Furthermore, we present the added benefits of analyzing ionic processes purely optically; mapping is possible using our technique, and half-stacks or bare films can be measured. These benefits greatly simplify the interpretation of the data, which is otherwise generally complex in the electrically equivalent techniques. With our approach, we resolve at least two processes in our perovskite film, with measured lifetimes of 2 ms and 6 s, likely corresponding to the diffusion times of the iodide vacancies and a mobile cation species, respectively[10]. Finally, compare our results with electrical measurements on corresponding perovskite solar cells and find that the ionic features we observe optically correlate to those seen with impedance spectroscopy.