Consumers worldwide are demanding more environmentally friendly energy sources. Therefore, the optimization of the performance and economy of solar cells is an important research focus. Improving the efficiency of perovskite solar cells was a particular priority. However, there has been less emphasis on understanding why cell performance deteriorates. The latest findings from researchers at Tsukuba University are now providing a microscopic examination of perovskite solar cells to close the knowledge gap.
Hybrid organic-inorganic perovskites are attractive materials for use in solar cells because they are easy and inexpensive to manufacture and absorb light over a wide range of wavelengths. Solar cells that use perovskite layers as a photoactive material are continuously being improved with a special focus on their energy conversion efficiency (PCE), which can now exceed 25%.
Focusing on improving PCEs alone, however, could result in researchers missing out on the significant advances that could result from a more detailed understanding of the underlying mechanisms. For example, why the performance of perovskite solar cells deteriorates is an important question that has not been fully answered.
It is known that external factors such as oxygen and moisture in the air affect the perovskite layers. However, the internal changes that affect cell performance are not that well known. The researchers therefore examined the mechanism of deterioration using electron spin resonance spectroscopy (ESR).
“During operation, we performed ESR spectroscopy on perovskite solar cells, which provided us with a real-time image of the changes at the molecular level,” explains the author of the study, Professor Kazuhiro Marumoto. “In particular, we observed the charges and defects and the associated spin states in the solar cell layers while measuring the current-voltage properties of the solar cells. This enabled us to understand the relationships between these factors.”
This in-depth investigation of perovskite solar cells in operation showed that changes in the spin states are due to changes in hole transport and the formation of electrical interface dipole layers. It was therefore concluded that the deterioration of the cells can be prevented by improving the charge mobility in the hole transport material and preventing the formation of an electric dipole layer.
“Finding that changes in spin states correlate with device performance has greatly expanded our understanding of perovskite solar cells,” says Professor Marumoto. “We hope that our results provide a valuable new starting point for the further development of solar cells and help accelerate the reality of lower-cost green energy.”
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