Tailored multifunctional amidines for crystallization regulation and grains boundary bridging in perovskite solar cells

Molecular engineering has demonstrated significant potential in modulating the crystallization and interfacial defect passivation of perovskite films. However, the deprotonation of conventional organic ammonium under light or thermal stress compromises the long-term operational stability of perovskite solar cells (PSCs). Here, we designed two multifunctional deprotonation-resistant cycloalkyl amidines with different heteroatoms, tetrahydro-2H-pyran-4-carboximidamide hydroiodide (TPCAI) and tetrahydro-2H-thiopyran-4-carboximidamide hydroiodide (TTCAI), which were used to precisely regulate the crystallization process and interfacial properties of perovskite films. The larger dipole moment and enhanced electronic properties of sulfur-substituted TTCAI than TPCAI strengthen its interaction with the perovskite lattice. This interaction markedly slows down the crystallization rate, promotes preferential growth along the (1 0 0) crystal plane, reduces defect density, and effectively suppresses non-radiative recombination. TTCAI meanwhile construction of passivation layers on the surface and grain boundaries of the perovskite film through multiple hydrogen-bond interactions, passivates grain boundary defects, which significantly improves the film's environmental stability. Consequently, the TTCAI-modified device achieved a high efficiency of 25.58 %, and the unencapsulated device retained 92 % of its initial efficiency after 1200 h of storage at 65 °C under air (RH 30–65 %). This study provides new insights into the rational design of multifunctional amidine ligands toward achieving efficient and stable PSCs.