Understanding the photostability of perovskite solar cell

Joshi, Pranav
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Global climate change and increasing energy demands have led to a greater focus on cheaper photovoltaic energy solutions. Perovskite solar cells and organic solar cells have emerged as promising technologies for alternative cheaper photovoltaics.

Perovskite solar cells have shown unprecedentedly rapid improvement in power conversion efficiency, from 3% in 2009 to more than 21% today. High absorption coefficient, long diffusion lengths, low exciton binding energy, low defect density and easy of fabrication has made perovskites near ideal material for economical and efficient photovoltaics.

However, stability of perovskite and organic solar cells, especially photostability is still not well understood. In this work, we study the photostability of organic solar cells and of perovskite solar cells.

In the first part of this work, effect of modification of sidechain of polymer in high efficiency organic solar cell was investigated. It was observed that PTB7-Th polymer with alkyl thiophene sidechain degrades more than PTB7 with alkoxy sidechain.

Next, we study the stability of perovskite cells. When the perovskite cells are exposed to light under open circuit condition, very intriguing results were observed. Isc decreased during exposure, whereas surprisingly, Voc of the device increased. Even more intriguing phenomena were observed after exposure. After exposure, Isc of the device recovered very quickly whereas increased Vocà ¬ of the device decreased to a value lower than initial value after around 30 minutes of exposure. Voc of the device then slowly recovered to initial value. Capacitance, dark IV of the device also showed intriguing but similar changes.

We explain all these observations using a model based on ionic migration. We consider intrinsic thermally generated ions and generation of more ions during light exposure and then recombination of these ions when light exposure is turned off. This model can be used to explain all anomalous observation during light exposure and recovery after exposure. Few more experiments were done, effect of intensity on degradation, effect of temperature on after degradation recovery, which confirm the model presented. Experiment on devices with different grain size suggest that ion migration is grain boundary assisted phenomena, such that for higher grain size films, degradation is slower.

degradation, ion migration, organic solar cell, perovskite solar cell, photostability, sidechain modification