Molecular and Self-Trapped Excitonic Contributions to the Broadband Luminescence in Diamine-Based Low-Dimensional Hybrid Perovskite Systems

The present solid state lighting (SSL) technology is based on using a combination of phosphors to give the desired white light emitting devices. The property of broadband emission from a single phosphor is not only difficult to achieve but also poses a challenge in device fabrication. Hybrid organic-inorganic perovskites especially in low dimensions (2D/1D) are being widely explored for their optoelectronic properties. Few of these materials exhibit broadband emission upon ultraviolet excitation, providing a scope for synthetic engineering in achieving commercially viable single-phosphor materials. In this work, three interesting diammonium-based low-dimensional hybrid perovskites for broadband photoluminescence (PL) are examined. The doubly protonated ethylenediamine-configured monoclinic (P2(1)/n) 1D ribbon assembly (H3NCH2CH2NH3)(8)(Pb4Br18)Br-6 (1) and the orthorhombic (Pbcm) 2D-twisted octahedral (H3NCH2CH2NH3)(Pb2Cl6) (2) show white luminescence, while the doubly protonated piperazine-configured orthorhombic (Pnnm) 0D dual-octahedral (C4N2H12)(4)(Pb2Br11)(Br)(H2O)(4) (3) exhibits bluish-white luminescence. Based on the PL of the organic diammonium salt, the time-resolved PL, Raman signatures, and density functional theory (DFT) calculations, it is shown that the broadband luminescence has dual origin: one around 400 nm from diammonium-related molecular fluorescence and another around 516 nm from self-trapped excitons. The structure-specific relative contributions and interplay between the two define the overall character of the broadband luminescence.