Unraveling the Chain of Reactions

Perovskite solar cells, made from a family of materials with a unique crystal structure, have long held promise for their potential to enable highly efficient and cost-effective solar energy conversion. However, these devices have been plagued by issues of stability and performance degradation, particularly in those containing high levels of bromide.

A new study, published in Nature Energy, offers a potential solution to this problem through the use of redox mediators with redox and defect passivating capabilities. These additives are able to suppress halide migration, a key factor in the degradation of perovskite solar cells.

The research, led by Alex K.-Y. Jen and colleagues at City University of Hong Kong and The Hong Kong Polytechnic University, demonstrates a high open circuit voltage of 1.35 V in wide band gap perovskite solar cells, resulting in an improved efficiency of perovskite-organic tandem solar cells, which reaches 25.22% (certified efficiency of 24.27%).

The use of redox shuttles to mitigate halide segregation through cyclical defect recovery has been gaining recognition in recent years. However, the use of redox additives in perovskite solar cells has been scarce. The researchers take it further by introducing ammonium or phenylethylammonium groups as counter-cations, resulting in a not only suppression of photoinduced halide segregation by reducing iodide oxidation but also enhancement of the open circuit voltage by passivating defects and reducing nonradiative recombination.

The researchers investigate the impact of the defect passivating function by comparing anthraquinone-based redox mediators with different counter-cations. They show that redox mediators containing ammonium or phenethylammonium species result in a more significant suppression of nonradiative recombination and larger increase in open circuit voltage, with the best performance obtained with the phenethylammonium counter-cation.

The study represents a significant step forward in the development of high-efficiency, high-stability tandem solar cells. The approach of simultaneously tackling detrimental redox reactions and defects in perovskite solar cells has the potential to improve the device stability of these other perovskite solar cells, and could be combined with other optimization strategies in tandem cells to further improve performance.

While the work of Jen and team illustrates the importance of this approach, there is still work to be done to optimize the use of redox mediators and defect passivation in perovskite solar cells. The combined use of redox mediators and defect passivation can be explored to improve the device stability of these other perovskite solar cells. In tandem cells, the redox and passivation approach developed by Jen and team could be combined with other optimization strategies, such as optimization of the interconnect layer, perovskite/charge transport layer interfaces, or organic sub-cell, to further improve the performance of the devices.

Source:
Djurišić, A.B. (2024) Breaking the reaction chain. Nat Energy (2024). <https://doi.org/10.1038/s41560-024-01503-z>