(A) study of the single bromide quasi-2-dimensional perovskite crystal for high efficiency blue light emitting diodes

Abstract

Metal halide perovskites have emerged as promising candidates for next-generation display applications due to remarkable color purity and tunable bandgaps. Mixed halide perovskites allow for facile bandgap tunability through composition control but show emission spectral instability due to halide segregation during device operation. An alternative approach to widen the bandgap involves constructing a quasi-2-dimensional structure using a single halide anion, leveraging the confinement effect. However, this results in multi-bandgap phases, leading to a red-shifted emission due to energy funneling toward a phase with a bandgap lower than the target. Here, we synthesized (PBA)2Csn-1PbnBr3n+1 quasi-2D perovskite crystals where ‘n’ indicates the number of PbBr6 octahedral sheets in each repeating unit. To achieve pure-blue emission (460-470 nm), the target wavelength by ‘Rec. 2020’, the industry standard for ultra-high definition television standard, we manipulated the crystallization process of the quasi-2D perovskite. The manipulation involved controlling the ‘n’ phase distribution (i.e., bandgap), specifically focusing on ensuring the phase's dominance with the smallest bandgap, the target emission. We attained pure-blue photoluminescence(PL) at 461 nm with a relatively narrow full-width at half maximum(FWHM) of 25 nm through a two-step crystallization control process. Initially, we performed a coarse adjustment of the PL wavelength by changing the solute concentration and solvent polarity, which heavily influence the diffusion of cations, a determinant for the value of 'n'. Subsequently, we further enhanced the PL quantum yield(PLQY) to 51% through fine-tuning in the second-step crystallization process with trioctylphosphine oxide(TOPO) as an additive for the antisolvent treatment. TOPO affects slightly slower diffusion of precursors, resulting in halide-vacancy passivation, well-ordered crystals, and faster carrier transfer between phases. We successfully fabricated pure-blue light-emitting diodes, which exhibited a relatively low turn-on voltage of 3V and an external quantum efficiency of 2.5% at an emission peak of 465 nm with FWHM of 29 nm.