  Functional properties of mixed ionic electronic conductors (MIECs) can be
radically modified by (de)insertion of mobile charged defects. A complete
control of this dynamic behaviour has multiple applications in a myriad of
fields including advanced computing, data processing, sensing or energy
conversion. However, the effect of different MIECs state-of-charge is not fully
understood yet and there is a lack of strategies for fully controlling the
defect content in a material. In this work we present a model-less technique to
characterize ionic defect concentration and ionic insertion kinetics in MIEC
materials: Optoionic Impedance Spectroscopy (OIS). The proof of concept and
advantages of OIS are demonstrated by studying the oxygen (de)insertion in thin
films of hole-doped perovskite oxides. Ion migration into/out of the studied
materials is achieved by the application of an electrochemical potential,
achieving stable and reversible modification of its optical properties. By
tracking the dynamic variation of optical properties depending on the gating
conditions, OIS enables to extract electrochemical parameters involved in the
electrochromic process. The results demonstrate the capability of the technique
to effectively characterize the kinetics of single- and even multi- layer
systems. The technique can be employed for studying underlying mechanisms of
the response characteristics of MIEC-based devices.
