A new class of materials based on salt crystal halide perovskites has been in the spotlight of the solar cell community in recent years. Only a few years after their discovery, solar cells made from these perovskite materials already reach efficiencies close to that of the best silicon solar cells. Unlike silicon solar cells, however, perovskite solar cells are easy to manufacture, for example by printing. They can be printed because they are made from salts, which can form an ink.
The downside is that these salts are ionic materials, which means that the ions inside the material are able to migrate. Ion migration is an important degradation pathway impeding commercial applications. For the fabrication of stable perovskite devices, it is thus crucial to understand ion migration. So far, however, scientific measurements and theoretical predictions of ion migration vary widely, which does not help to clarify the matter.
To precisely measure the migration of ions in these perovskites, the researchers used a method originally developed more than 25 years ago, which has since received only moderate attention. This so-called transient ion drift method is based on the measurement of the capacitance change caused by the drift of mobile ions after applying an electrical pulse. By measuring the change in capacitance at different temperatures, important physical properties, such as activation energy, diffusion coefficient, and concentration of mobile ions can be measured.
By studying the prototype perovskite (methylammonium lead iodide), the research team showed that this technique is a fast and accurate method to quantify mobile ions in perovskite devices with high precision. In this perovskite material, both organic (methylammonium) and inorganic (iodide) ions are migrating, but on completely different timescales. These measurements help to clarify uncertainties in the literature over how and which ions migrate in these materials and it shows that many theoretical calculations cannot be verified experimentally. The measurements will be important to guide future investigation of ion migration and hence the degradation pathways of these promising new solar cells.
Reference
Moritz H. Futscher, Ju Min Lee, Lucie McGovern, Loreta A. Muscarella, Tianyi Wang, Muhammad Irfan Haider, Azhar Fakharuddin, Lukas Schmidt-Mende and Bruno Ehrler. Quantification of ion migration in CH3NH3PbI3 perovskite solar cells by transient capacitance measurements, Materials Horizons, 2019 DOI: 10.1039/c9mh00445a
