Mixed ionic-electronic conductivity is a defining feature of hybrid perovskites, and its understanding is key to open new frontiers in the design and optimization of perovskite based optoelectronics. [1] Progress in this direction relies on the development of appropriate experimental methods able to probe mixed conduction in these materials, and of models that can provide an accessible, yet accurate, interpretation of device function.

In this contribution, we present a systematic approach to the investigation of mixed conduction in methylammonium lead iodide (MAPI) thin films based on horizontal device structures. [2] The results highlight the importance of interfacial charging in the long time scale polarization behavior of MAPI devices close to equilibrium. Such polarization behavior is consistent with the dynamics of electric field screening in MAPI devices probed with non-destructive spectroscopic and optoelectronic methods. [3] Next, we address the interpretation of perovskite solar cells’ electrical response under light and/or voltage bias, a long-standing question in the field. We propose an equivalent circuit model that can describe the impedance of mixed conducting perovskite devices under bias, based on the previously introduced concept of ionic-to-electronic current amplification. [5] We demonstrate the analytical accuracy of the proposed model, by comparing results obtained with drift-diffusion simulations. [6] Based on the description of the ionic and electronic charge carrier dynamics, our analysis of calculated impedance spectra allows the clarification of several anomalous experimental observations. These include inductive behavior and multiple low frequency impedance features reported for perovskite solar cells. [7] Our study contributes to enabling the use of impedance spectroscopy as a routine characterization technique in perovskite photovoltaics.

We conclude by providing an outlook on possible strategies to address the defect chemical quasi-equilibrium of halide perovskites under light. By combining models describing their equilibrium defect chemistry [8] [9] with the relevant description of their optoelectronic properties, we are able to comment on the effect of light and halogen partial pressure on the steady-state defect concentrations. The results highlight the potential of controlling defect chemistry for the optimization of solar energy conversion devices based on mixed conductors.