Modeling and Mechanistic Analyses of Electrochemical Impedance Spectroscopy of Ion Transport in Photovoltaic Devices Containing Perovskite Active Layers

• Published in: Meeting abstracts (Electrochemical Society) Vol. MA2016-02; no. 15; p. 1407
• Main Authors: Tuominen, Mark T, Adhikari, Ramesh Y, Xu, Zhou, Bag, Monojit, Renna, Lawrence A, Cutting, Christie L, Venkataraman, Dhandapani
• Format: Journal Article
• Language: English
• Published: 01.09.2016
• ISSN: 2151-2043; 2151-2035
• DOI: 10.1149/MA2016-02/15/1407

Easy to make photovoltaic devices incorporating perovskite active-layer materials have been observed to exhibit high efficiency energy conversion. Unfortunately, the performance of these devices degrade over time, especially with regards to continued exposure of light. Recent work using electrochemical impedance spectroscopy (EIS) measurements at systematic values of dc voltage bias reveals that the degradation is related to photo-induced diffusive ionic motion in the perovskite layer [M. Bag, et al. J. Am. Chem. Soc 2015, 137, 13130]. This behavior is revealed in the EIS data as a distinct Warburg impedance, easily seen in Nyquist plots. In this work the perovskite active layers used are alkyl ammonium metal halides, containing either methylammonium (MA) or formamidinium (FA) ions, or MA x FA 1-x mixtures thereof, with lead triiodide. Importantly and fortunately, the experiments have several independent variables to control to probe the underlying physics: dc bias voltage, temperature, and light/dark conditions. Here we present detailed modeling and analyses of the EIS behavior for these perovskite systems, providing deeper understanding of the physical basis for the ionic motion, investigating the dependence of ionic motion on temperature and light intensity. This is particularly important to unpack physically, given that these are multilayered, multi-material devices (containing interfaces) in which both electron and ionic mobilities contribute to the net impedance. The insights and models discussed here yield guidance for the design of new perovskite and related materials for better device stability.