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Abstract 2D hybrid organic–inorganic perovskites are potentially promising materials as passivation layers that can enhance the efficiency and stability of perovskite photovoltaics. The ability to suppress ion transport is proposed as a stabilization mechanism, yet an effective characterization of relevant modes of halide diffusion in 2D perovskites is nascent. In light of this knowledge gap, molecular dynamics simulations with enhanced sampling and experimental validation to systematically characterize how ligand chemistry in seven (R‐NH₃)₂PbI₄systems impacts halide diffusion, particularly in the out‐of‐plane direction is combined. It is found that increasing stiffness and length of ligands generally inhibits ion transport, while increasing ligand polarization generally enhances it. Structural and energetic analyses of the migration pathways provide quantitative explanations for these trends, which reflect aspects of the disorder of the organic layer. Overall, this mechanistic analysis greatly enhances the current understanding of halide migration in 2D hybrid organic–inorganic perovskites and yields insights that can inform the design of future passivation materials. 

Abstract Organic small molecules that exhibit second‐scale phosphorescence at room temperature are of interest for potential applications in sensing, anticounterfeiting, and bioimaging. However, such materials systems are uncommon—requiring millisecond to second‐scale triplet lifetimes, efficient intersystem crossing, and slow rates of nonradiative recombination. Here, a simple and scalable approach is demonstrated to activate long‐lived phosphorescence in a wide variety of molecules by suspending them in rigid polymer hosts and annealing them above the polymer's glass transition temperature. This process produces submicron aggregates of the chromophore, which suppresses intramolecular motion that leads to nonradiative recombination and minimizes triplet–triplet annihilation that quenches phosphorescence in larger aggregates. In some cases, evidence of excimer‐mediated intersystem crossing that enhances triplet generation in aggregated chromophores is found. In short, this approach circumvents the current design rules for long‐lived phosphors, which will streamline their discovery and development. 

While typical perovskite solar cells (PSCs) with doped Spiro-OMeTAD as a hole transport material (HTM) have shown rapid increase in their power-conversion efficiencies (PCEs), their poor stability remains a big concern as the dopants and additives used with Spiro-OMeTAD have a strong tendency to diffuse into and degrade the perovskite active layer under normal operating conditions. Aiming to push forward the development of PSCs, many dopant-free small-molecular HTMs have been reported based on energetic considerations for charge transfer and criteria for charge transport. However, the PCEs of the state-of-the-art PSCs with dopant-free small-molecular HTMs are still inferior to those using doped Spiro-OMeTAD, and little attention has been paid to the interactions between the HTM and perovskite absorber in PSCs. Here, we report a facile design concept to functionalize HTMs so that they can passivate perovskite surface defects and enable perovskite active layers with lower density of surface trap states and more efficient charge transfer to the hole transport layer. As a consequence, perovskite solar cells with a functionalized HTM exhibit a champion PCE of 22.4%, the highest value for PSCs using dopant-free small molecular HTMs to date, and substantively improved operational stability under continuous illumination. With a T 80 of (1617 ± 7) h for encapsulated cells tested at 30 °C in air, the PSCs containing the functionalized HTM are among the most stable PSCs using dopant-free small-molecular HTMs. The effectiveness of our strategy is demonstrated in PSCs comprising both a state-of-the-art MA-free perovskite and MAPbI, a system having more surface defects, and implies the potential generality of our strategy for a broad class of perovskite systems, to further advance highly efficient and stable solar cells.