NREL scientists have conducted microscopic analysis of perovskite solar cells, and made new discoveries regarding the degradation mechanisms occurring within the material. The results, according to NREL, could help scientists in focusing their research on key areas for improvement.
The research, conducted by NREL and the University of Texas, Austin, is published in the journal of Nature Communications. The team examined two different perovskite thin film materials, using a novel microscopic technique termed light stimulated microwave impedance microscopy (MIM).
The two thin films examined were both made from methylammonium lead iodide, a perovskite material commonly used in solar cell research, and differed in terms of the size of the crystal grains. Both materials were coated with a layer of protective plastic (polymethyl methacrylate). The cells were illuminated from below by a focused laser beam, and examined using MIM, which allowed researchers to map photoconductivity of the materials at the nano scale.
“The general assumption is that degradation starts with grain boundaries [the areas where individual crystals touch each other],” says co-author and NREL Senior Scientist Kai Zhu. “We were able to show that degradation is not really starting from the visible boundaries between grains. It is coming from the grain surface.”
The two materials were tested over the course of a week, under conditions of 23°C and 35% humidity. After around three days, the photoconductivity began to drop as water molecules moved through the plastic coating.
The researchers observed that the degradation was caused by disintegration of the grains, and not the grain boundaries as has previously been assumed. The paper notes that, in the material tested with larger grains, the boundaries ‘are relatively benign’, and that perovskite films with better crystallinity should be the direction of future research.