Technical approach can improve stability of perovskite solar cells under reverse bias conditions

Perovskite-based solar cells, a class of versatile materials with promising optoelectronic properties, are gradually becoming commercialized. Although these solar cells may offer notable advantages over existing solar cell designs, including higher power conversion efficiency and lower manufacturing costs, their performance has been found to be significantly impaired under reverse bias conditions.

Reverse bias occurs when one cell in a series-connected solar panel becomes shaded and generates less power. The remaining illuminated cells apply a reverse voltage to the shaded cell, trying to force current through it in the wrong direction. This can cause serious degradation of the shaded cell.

Researchers from the University of Washington, the University of Colorado (UC) Boulder, Rice University, and the University of Oxford have recently developed a new strategy that could help improve the stability of perovskite solar cells under strong reverse bias. Their proposed approach, published in Natural energyrelies on a unique device architecture that combines a polymer hole transport layer with an electrochemically stable back electrode.

“We had seen Mike McGehee give an inspiring talk about the importance of reverse bias stability and his team’s work to understand it,” David S. Ginger, a professor of chemistry at the University of Washington and lead author of the paper, told Tech Xplore. “We were at a point where Fangyuan Jiang was finishing up a project and looking for a new one, and we agreed that this question might be a good fit for some of our research, particularly our surface passivation to stabilize perovskite/electrode interfaces and reduce ionic conductivity.”

Initially, the researchers tried to stabilize perovskite solar cells using a surface passivation technique, but that approach proved ineffective. Jiang, the study’s lead author, then came up with the idea behind their proposed new strategy, which they first tested at the UW and then analyzed in more detail in collaboration with Michael D. McGehee and his team at UC Boulder, as well as the teams of professors Aditya Mohite at Rice and Henry Snaith at Oxford.

“We designed our solar cells in two steps: using a robust polymer material for hole transport to block electron injection; and using an electrochemically stable electrode material to prevent metal oxidation,” Jiang explains. “We believe that the early degradation of cells under reverse bias is an electrochemical process triggered by the injected charge carriers.”

The new engineering approach designed by Jiang, Ginger and their colleagues allows them to regulate the electrochemical reactions that lead to early degradation of solar cells under reverse bias. This then allows the cells to be stabilized in the presence of relatively high reverse bias.

“Our approach would probably be more suitable for a solar module design that incorporates bypass diodes, as is the case for many silicon solar cell modules,” Ginger explained. “I think it gives some hope that we could design all-perovskite cells that are also stable under reverse current, although that might be more difficult.”

The researchers used their engineering strategy to create new pin-based solar cells and evaluated their performance. They found that the cells were stable under high reverse voltages, comparable to those that conventional silicon-based cells can withstand.

“We’ve shown that perovskite solar cells are not inherently unstable to reverse voltages,” Ginger said. “I think this is a real mindset shift for many. What’s more, our study shows that we need to think holistically: it’s not just one interface or another that’s key, but the design of the entire device stack.”

The promising results obtained by Ginger, Jiang and colleagues could encourage other teams to experiment with the design strategy they propose. This could contribute to the development of more reliable and stable perovskite solar cells under reverse bias conditions.

“I think the significance of our study also extends beyond solar cells, as our engineering strategy could serve as an inspiration for other optoelectronics such as photodetectors and light-emitting diodes (LEDs),” Jiang said. “I am also proud of the rigorous presentation of our work, thanks to a great team. I hope that the fundamental knowledge in electrochemistry can also inspire future studies on reverse bias.”

The team’s recent work could contribute to the future commercialization of perovskite-based photovoltaic (PV) cells while informing the development of other optoelectronic devices. In parallel, Jiang, Ginger, and their colleagues plan to conduct further research into the mechanisms underlying the degradation of perovskite cells under reverse bias and/or high reverse current.

“We would now like to understand the remaining failure mechanisms under reverse current flow and see if we can find ways to enable them to transmit reverse current close to the maximum power current operating in sunlight, but without irreversible damage,” Ginger added.

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