Thermodynamically Matched Molten‐Salt‐Assisted Phase Engineering of 2D/3D Perovskite Heterostructures for Efficient and Stable Solar Cells

ABSTRACT The prevailing architecture in the best‐performing perovskite solar cells (PSCs) generally leverages 2D/3D heterostructures that synergize the environmental stability of 2D perovskites with the superior charge‐carrier dynamics of 3D counterparts. However, conventional fabrication strategies for such heterostructures emphasize 2D phase formation, with insufficient attention paid to the thermodynamic compatibility with the phase transition of perovskites. Herein, we introduce a molten‐salt‐assisted 2D/3D heterophase approach guided by thermodynamic compatibility between perovskite crystallization and molten‐salt phase transition. The selected molten salt of 4‐(4,6‐dimethoxy‐1,3,5‐triazin‐2‐yl)‐4‐methylmorpholinium chloride (DMTMM), reveals an appropriate melting temperature that aligns with the phase transition window of perovskite materials. Molten DMTMM serves as an excellent solvent for the 2D perovskite, which suppresses random nucleation and extends the crystal growth period, thereby facilitating the growth of a highly ordered 2D phase and reducing residual PbI 2 phase at the heterointerface. Upon solidification, the multiple ether groups of DMTMM passivate under‐coordinated Pb 2+ , further enhancing lattice stability. This thermodynamically matched molten‐salt‐assisted growth strategy enables precise interfacial control, boosting the power conversion efficiency from 23.43% to 25.73% with improved thermal stability. This work provides a fundamental thermodynamic perspective for constructing high‐quality 2D/3D heterointerfaces, paving a new avenue for advancing the performance and robustness of perovskite photovoltaics.