Two-dimensional (2D) perovskite surface passivation is an effective strategy for suppressing nonradiative recombination in perovskite solar cells (PSCs). However, conventional Ruddlesden–Popper (RP) interlayers often suffer from structural instability, whereas the more robust Dion–Jacobson (DJ) counterparts can impede charge transport owing to their low n values. Here, we identify thermally driven n-value evolution as a versatile strategy for engineering DJ 2D interlayers and demonstrate its effectiveness in achieving highly efficient and stable PSCs.

By investigating DJ perovskites incorporating diammonium spacers with different alkyl-chain lengths, we reveal that their structural evolution is strongly dependent on annealing temperature and spacer chemistry. The distinct thermal responses suggest spacer-dependent kinetic barriers governing the formation of higher-n DJ phases. In particular, controlled post-deposition annealing of hexane-1,6-diammonium diiodide (HDA)-treated perovskites enables a well-defined structural transformation from n = 1 to n = 2. X-ray diffraction and grazing-incidence wide-angle X-ray scattering reveal the evolution of the layered crystal structures, while transient absorption spectroscopy confirms the corresponding changes in optical characteristics. Notably, the transformation proceeds without detectable decomposition of the underlying perovskite under optimized conditions, highlighting the feasibility of controlling DJ dimensionality through a simple thermal process.

Although both n = 1 and n = 2 DJ layers effectively suppress nonradiative recombination, their charge-extraction characteristics differ substantially. The n = 1 phase introduces an unfavorable energetic barrier at the perovskite/hole-transport interface, whereas the n = 2 phase provides improved energy-level alignment and facilitates hole extraction. These differences are supported by ultraviolet photoelectron spectroscopy, time-resolved photoluminescence, and transient photocurrent measurements. By implementing the tailored n = 2 DJ interlayer in PSCs employing PTAA as the hole-transport material, we achieve a power conversion efficiency of 25.56%, with an independently certified efficiency of 25.59%, primarily through improvements in open-circuit voltage and fill factor. Beyond photovoltaic performance, the engineered DJ interlayer exhibits superior structural robustness compared with its RP counterpart, maintaining crystalline integrity under thermal stress while providing enhanced interfacial adhesion. The resulting devices also demonstrate excellent durability against thermal cycling, damp-heat exposure, and prolonged illumination. Our findings establish thermally driven n-value engineering as a powerful approach to overcoming the charge-transport limitations of DJ passivation while preserving its structural advantages, offering a promising route toward highly efficient and durable perovskite photovoltaics.