Intermediate-Phase Engineering via Dimethylammonium Cation Additive for Stable Perovskite Solar Cells

Achieving the long-term stability of perovskite solar cells is arguably the most critical challenge to enabling their widespread commercialization. Understanding the perovskite crystallization process and its direct impact on device stability is essential to achieve this goal. Surprisingly, we find that intermediate phases that occur during the crystallization process strongly influence the long-term perovskite device stability. The commonly employed dimethyl formamide/dimethyl sulfoxide (DMF/DMSO) solvent system preparation method results in poor crystal quality and microstructure of the polycrystalline perovskite films. In this work, we introduce a high-temperature DMSO-free processing method that utilizes dimethylammonium chloride (DMACl) as an additive to control the perovskite intermediate precursor phases accurately. By precisely controlling the 2H to 3C perovskite phase crystallization sequence, we tune the grain size, texturing, orientation (corner-up vs face-up), and crystallinity of the formamidinium (FA)yCs1-yPb(IxBr1-x)3 perovskite system. Encapsulated devices show significantly improved operational stability, with a champion device showing a T80 of 490 hours under simulated sunlight at 85 °C in air, under open circuit conditions. Our work introduces a new processing method that allows higher overall perovskite device stability by controlling the intermediate phase domains during the perovskite formation. This work highlights the importance of material quality to achieve long-term operational stability of perovskite optoelectronic devices.