Divergent Optoelectronic Tuning in Sky-Blue Quasi-2D Perovskites via Spacers Conjugation and Fluorination

Dimensional reduction mediated by organic spacers is a key strategy for achieving blue emission in perovskites. In the extensive reports so far, functionalized spacers typically enhance both optical and electrical properties concurrently through defect passivation, phase distribution control, and energy level regulation. Here, through the synthesis and application of novel conjugated spacers, including 3-phenylprop-2-yn-1-aminium bromide (PPYABr), 3-(4-fluorophenyl)prop-2-yn-1-aminium bromide (FPPYABr), and 3-(3,4,5-trifluorophenyl)prop-2-yn-1-aminium bromide (3FPPYABr), and compared them with conventional non-conjugated phenethylammonium bromide (PEABr) and low-conjugated (E)-3-phenylprop-2-en-1-aminium bromide (PPABr). Intriguingly, these spacers induced divergent tuning of optical and electrical properties in all-bromine quasi-two-dimensional perovskites. Photoluminescence (PL) performance degraded upon introducing double or triple bonds but rebounded with fluorination. This V-shaped PL trend arose from directional growth of three-dimensional phase with low exciton binding energy in PPYABr-based film, reducing exciton recombination efficiency. Fluorinated spacers further suppressed defects and optimized the phase distribution. In contrast, electroluminescence performance improved linearly with enhanced carrier (especially electron) mobility and film smoothness. The 3FPPYABr-based sky-blue device achieved a champion external quantum efficiency of 12.26%. Nuclear magnetic resonance confirmed π···H─N and π···Pb²⁺ interactions in the 3FPPYABr-based conjugated system. This discovery of distinct optimization pathways offers critical insights for designing functional perovskite additives.