High‐Efficiency Perovskite Solar Cells Enabled by Guanylation Reaction for Removing MACl Residual and In‐Situ Forming 2D Perovskite

The thermodynamical deprotonation of methylammonium chloride (MACl) has several detrimental influences on the quality of formamidinium (FA⁺)-based perovskite, which limits both efficiency and stability of inverted perovskite solar cells (IPSCs). Herein, a new additive strategy was developed by introducing methyl carbamimidothioate hydroiodide (MCH) into perovskite precursor, where guanylation reaction occurred between MCH and MACl to form a new intermediate of methyl-substituted guanidine (MSG). MSG could then bond with undercoordinated Pb²⁺ to in situ form a two-dimensional (2D) perovskite, which would promote the growth and crystallization of three-dimensional (3D) perovskite with higher crystallinity, lower defect-states density and superior stability. Finally, the MCH-treated IPSC with a small area (0.09 cm²) achieved an impressive power conversation efficiency (PCE) of 26.81 % (certified as 26.02 %), which is one of the highest PCEs reported to date. The large area MCH-treated device (1.00 cm²) also obtained a high PCE of 24.36 %. Moreover, the unencapsulated and MCH-treated device exhibited excellent operational stability, maintaining 91.95 % and 97.06 % of their initial efficiencies after aging in air and a nitrogen-filled atmosphere at 85 °C for 1200 h. The encapsulated MCH-treated devices retained 94.25 % of its initial efficiency after continuously tracking at the maximum power-point for 1200 h in air.