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.

钙钛矿太阳能电池在运行中的稳定性是一个关键的挑战性问题，特别是在极端温度或温度变化下。据报道，钙钛矿太阳能电池在 –40 至 85 °C（热循环的标准测试协议）之间进行 200-300 次热循环后即可保持初始效率。快速退化很大程度上归因于多层器件堆叠内的机械残余应力。大多数研究都集中在通过改进设备中的界面来缓解应力，众所周知，这些界面容易分层或电池的封装。对钙钛矿层的关注较少。现在，Min Chen、Joseph Luther 及其来自美国和瑞士的同事表明，钙钛矿中残余应变的最小化可以提供更好的热循环应力稳定性。

研究人员在钙钛矿前体溶液中添加了烷基铵添加剂——正辛基碘化铵。退火后，铵分子分布在晶界和界面处，抑制钙钛矿薄膜中的残余拉伸应变。这意味着太阳能电池的热循环稳定性得到提高。当完成每个循环的时间为 6 小时时，初始效率可保持超过 500 个循环；在加速测试中，每个循环持续 5 分钟，可保持超过 2,500 个循环。研究团队表明，不同器件设计以及电池和模块的稳定性都得到了增强，证实了该方法的可重复性。通过分析压力测试后的设备，Chen 等人。证明拉伸应变降低的钙钛矿薄膜裂纹更少，不易分层，确保器件更稳定。未来的工作可以建立在钙钛矿薄膜的应力工程以及其他现有的界面和封装策略的基础上，以进一步提高钙钛矿太阳能电池抵抗热应力的稳定性。