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Organic-inorganic hybrid perovskite solar cells have attracted much attention as one of the most important next-generation solar cells. The solar-cell performances are known to be changed under operation; however, the root cause of the instability has not yet been understood sufficiently. Here, we report improvement mechanism in the open-circuit voltage, VOC, of perovskite solar cells at the initial stage of device operation that were firstly investigated by electron spin resonance (ESR). ESR study reveals reverse electrons transfer at the interface from the perovskite layer to the hole-transport layer not only under dark conditions but also under light irradiation where electrons are subsequently trapped in the hole-transport layer. Thereby, an electron barrier is enhanced at the perovskite/hole-transport-layer interface, improving field-effect passivation at the interface, and thus the VOC improves. These findings are crucial for understanding the mechanisms of device performance changes under operation, showing a relation between reverse electrons transfer and performance improvement, which is valuable for the development of further efficient perovskite solar cells.

Organic-inorganic hybrid perovskite solar cells have attracted much attention as one of the most important next-generation solar cells. The solar-cell performances are known to be changed under operation; however, the root cause of the instability has not yet been understood sufficiently. Here, we report an improvement in the open-circuit voltage, VOC, of inverted perovskite solar cells at the initial stage of device operation and its mechanisms that were first investigated by electron spin resonance spectroscopy. Operando ESR measurements indicate that electrons travel from the perovskite layer to the hole-transport layer under irradiation and are subsequently trapped there. Thereby, an electron barrier is formed at the perovskite/hole-transport-layer interface, and thus the VOC improves. These findings are crucial for understanding the mechanisms of device performance changes at the initial stage of operation and, additionally, demonstrate that electron spin resonance spectroscopy is a powerful tool for investigating perovskite solar cells.