The stability of perovskite solar cells under operation is a critical challenging issue, especially under extreme temperatures or temperature variations. Perovskite solar cells have been reported to retain the initial efficiency over just 200–300 thermal cycles between –40 to 85 °C — the standard testing protocol for thermal cycling. The rapid degradation has been largely attributed to mechanical residual stresses within the multilayer device stacks. Most studies have focussed on stress relief by improving the interfaces in the devices, which are known to be prone to delamination, or the cell’s encapsulation. Less attention has been paid to the perovskite layer. Now, Min Chen, Joseph Luther and colleagues across the United States and Switzerland show that the minimization of residual strain in the perovskite affords better stability against thermal cycling stresses.
The researchers add an alkyl ammonium additive — n-octylammonium iodide — into the perovskite precursor solution. Upon annealing, the ammonium molecules distribute at grain boundaries and interfaces, suppressing residual tensile strain in the perovskite film. This translates into solar cells with improved stability against thermal cycling. The initial efficiency is retained over 500 cycles, when the time to complete each cycle is 6 hours, and over 2,500 cycles in accelerated tests in which each cycle lasts 5 minutes. The research team shows that stability is enhanced for different device designs and both in cells and modules, confirming the reproducibility of the approach. By analysing the devices after the stressing test, Chen et al. demonstrate that the perovskite film with reduced tensile strain has fewer cracks and is less prone to delamination, ensuring more stable devices. Future work can build on stress engineering of the perovskite film and other existing strategies for interfaces and encapsulation to further improve the stability of perovskite solar cells against thermal stresses.
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