Electron spin resonance sheds light on tin-based perovskite solar cell efficiency

Perovskite solar cells are attracting attention as next-generation solar cells. These cells have high efficiency, are flexible, and can be printed, among other features. However, lead was initially used in their manufacture, and its toxicity has become an environmental issue.

Therefore, a method for replacing lead with tin, which has a low environmental impact, has been proposed. Nevertheless, tin is easily oxidized; consequently, the efficiency and durability of tin perovskite solar cells are lower than those of lead perovskite solar cells.

To improve the durability of tin perovskite by suppressing tin oxidation, a method that introduces large organic cations into tin perovskite crystals to form a two-dimensional layered structure called Ruddlesden-Popper (RP) tin-based perovskites has been proposed. However, the internal state of this structure and the mechanism by which it improves performance have not been fully elucidated.

In this study, researchers at University of Tsukuba used electron spin resonance to investigate an RP perovskite solar cell's internal state during operation from a microscopic perspective. The research is published in the journal npj Flexible Electronics.

Perovskite solar cells have a structure in which holes and electron transport layers surround a perovskite crystal. First, when no light was irradiated on the RP perovskite solar cell, holes diffused from the hole transport layer to the RP perovskite. This led to the formation of an energy barrier at the hole transport layer-RP tin perovskite interface, which suppressed the backflow of electrons and hence improved the performance.

Second, under sunlight irradiation, electrons moved from the RP tin-based perovskite to the hole transport layer, attributable to the high-energy electrons produced by short-wavelength light, such as ultraviolet rays. Further, they found that this electron transfer increased the energy barrier at the hole transport layer-RP tin perovskite interface, further improving the device's efficiency.

Understanding the mechanism behind the performance improvements during device operation is crucial for developing highly efficient, long-lasting solar cells and will contribute to future research developments.