Multifunctional Ionic Liquid Enabling Cation Homogenization and Lead Chelation for Pure-Iodide Wide-Bandgap...

Wide-bandgap perovskite solar cells (WBG-PSCs) are essential for high-efficiency tandem photovoltaics, yet conventional mixed-halide formulations suffer from photoinduced halide segregation. While pure-iodide WBG-PSCs offer a fundamental solution, their development is hindered by the distinct crystallization kinetics of A-site cations, which lead to severe compositional inhomogeneity, coupled with lead toxicity. Herein, we report a multifunctional ionic liquid, 1-propylsulfonate-3-methylimidazolium trifluoromethanesulfonate ([SO₃H-PMIm][OTf]), that addresses these challenges. The additive interacts with CsI through sulfonate-Cs⁺ coordination bonds to increase CsI solubility, anchors methylammonium (MA) and dimethylammonium (DMA) cations via hydrogen bonding, and coordinates with undercoordinated Pb²⁺ ions through the ─SO₃ ^-. These synergistic interactions suppress colloidal aggregation in the precursor solution, while this coordination effect lowers the crystallization temperature of Cs-based perovskites, enabling synchronous crystallization with their organic-based counterparts. Consequently, homogeneous A-site cation distribution across both in-plane and out-of-plane dimensions is achieved. The resulting Cs_0.3DMA_0.2MA_0.5PbI₃ pure-iodide WBG-PSCs deliver a champion efficiency of 22.02% with an open-circuit voltage (V_OC) of 1.231 V and retain 90% of initial efficiency after 3000 h under the ISOS-D-1 protocol. Pb²⁺ chelation and hydrophobic ─CF₃ mitigate lead leakage by over 58%. This strategy provides a versatile strategy for overcoming cation inhomogeneity and lead toxicity, enabling efficient, stable, and safe pure-iodide perovskite photovoltaics.