Ionic Liquid Assisting the Crystallization of CsPbI₃ for High-Quality Perovskite Solar Cells in Air Atmosphere

The CsPbI₃ perovskite with excellent photothermal stability and a suitable band gap has made rapid progress along with the gradual deepening of the study gradually. However, the component distribution is not homogeneous in the vertical direction of the CsPbI₃ film, which is often overlooked by numerous researchers. Here, 1-hexyl-3-methylimidazolium iodide (HMIMI) ionic liquid was adopted to regulate the crystallization process of CsPbI₃. The addition of HMIMI could effectively optimize the surface morphology and crystallization characteristics of the CsPbI₃ film. HMIMI could increase the existence time of DMAPbI₃ and upgrade the stability barrier of Cs₄PbI₆. The prolonged crystallization time greatly enhances the ion migration of the perovskite, improving the internal component uniformity of the perovskite film. There exist hydrogen bonding and Lewis acid-base interactions between HMIM⁺ and the perovskite. The hydrogen bond is the main factor delaying the crystallization of the perovskite. The Lewis acid-base interaction of N with uncoordinated Pb²⁺ could inhibit the formation of Pb⁰. The addition of HMIMI could effectively enhance the N-type characteristics and optoelectronic properties of the perovskite. The modified films exhibit outstanding humidity and thermal and light stability. We further prepare perovskite devices in an air atmosphere. The PCE of the modified device raises to 20.19%, with a high V_OC of 1.23 V. The unencapsulated device maintains 83.7% of its initial PCE after 3000 h of aging. Furthermore, under continuous illumination in ambient air (ISOS-L-1 protocol, 30 ± 5 °C, 40 ± 10% RH), the unencapsulated modified device retains over 80% of its initial PCE after 550 h of maximum power point (MPP) tracking. This work provides a novel insight into improving the long-term stability of CsPbI₃ perovskite.