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

Achieving the long-term stability of perovskite solar cells is arguably the most important challenge required to enable widespread commercialization. Understanding the perovskite crystallization process and its direct impact on device stability is critical to overcoming this hurdle. Surprisingly, we find that intermediate phases that occur during the crystallization process are critical for long-term stability. The commonly employed dimethylformamide/ dimethyl sulfoxide (DMF/DMSO) solvent system for FA_yCs_1-yPb(I_xBr_1-x)₃ perovskite (FACs) results in poor crystal quality, microstructure and the retention of DMSO. That ultimately leads to inferior material stability compared to the DMF/dimethylammonium (DMF/DMA) processing method presented here. We replace DMSO with DMACl 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 FACs perovskite system.<br/><br/>A population of encapsulated devices showed a T80 lifetime (for the steady-state PCE) of 1190 h as the median value under simulated sunlight at 65 °C in air, under open-circuit conditions. In contrast to a median value of T80 = 780 hours for conventional DMF/DMSO devices. Our work introduces an innovative processing route that allows higher overall perovskite device stability with fewer defects by controlling the intermediate phase domains during the perovskite formation. This work highlights the importance of material quality in achieving long-term operational stability and shows that it is necessary to find alternative processing routes without DMSO. An important step towards stable multijunction devices. (1)<br/><br/>(1) McMeekin, D.P., Holzhey, P., et al., Nature Materials 2023, https://doi.org/10.1038/s41563-022-01399-8