Thermal Evolution of the Structure and Luminescence of the Hybrid-Cation-Stabilized [(4AMTP)PbBr₂]₂PbBr₄ Layered Perovskite

A typical layered hybrid perovskite, A2PbBr4, consists of organic A-site cations and the inorganic [PbBr4]2– perovskite layers. Alternatively, here the A-site cation itself is a hybrid one, namely, [(4AMTP)PbBr2]22+, containing a nonperovskite PbBr2 type lattice and 4AMTP (4-aminomethyltetrahydropyran cation). How does this hybrid A-site cation influence the structure and luminescence of a [(4AMTP)PbBr2]2PbBr4 2D layered perovskite? Here, we address this question by exploring crystal structure and photoluminescence (PL) in the temperature range 7–300 K. Centimeter-sized single crystals of [(4AMTP)PbBr2]2PbBr4 show a stable monoclinic P21/c space group in the entire temperature range, without showing any phase transition. The absence of a phase transition signifies higher structural rigidity brought in by the hybrid A-site cation, unlike typical A2PbBr4 with organic A-site cations that often exhibit a phase transition in this temperature range. PL of [(4AMTP)PbBr2]2PbBr4 at room temperature shows excitonic emissions similar to a typical A2PbBr4 with an organic A-cation because neither hybrid nor organic A-site cations contribute to the valence and conduction band edges. Interestingly, below 70 K, the excitonic emission suddenly red-shifts by 15 meV from 3.017 to 3.002 eV, along with an order of magnitude increase in lifetime. Similar temperature-induced PL changes in monoclinic-phase layered perovskites were previously attributed to spin-forbidden "dark" exciton emissions, which become significant at lower temperatures. The hybrid A-site cation in [(4AMTP)PbBr2]2PbBr4 stabilizes its monoclinic phase, influencing its luminescence characteristics. The hybrid A-site cations offer exciting prospects for tailoring the chemical composition, structure, and properties of layered perovskites, warranting the novel properties of halide perovskites.