Controlling The Crystallization of Pure Bromide Quasi-2-Dimensional Perovskite Crystals for High Efficiency Pure-Blue Light-Emitting Diodes

Metal halide perovskites have emerged as promising candidates for next-generation display applications due to their remarkable color purity and tunable bandgaps by adjusting the composition of halide anions. Mixed halide perovskites allow for facile bandgap tunability through composition control, but they have limitations in terms of emission spectral stability due to halide segregation occurring 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 approach often 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)₂Cs<i>_n</i>_-1Pb<i>_n</i>Br_3<i>n</i>+1 quasi-2D perovskite crystals where ‘<i>n</i>’ indicates the number of PbBr₆ octahedral sheets in each repeating unit. To achieve pure-blue emission within the range of 460-470 nm, the target wavelength by the ITU-R Recommendation BT.2020 (Rec. 2020), the industry standard for 4K and 8K ultra-high definition television standard, we manipulated the crystallization process of the quasi-2D perovskite prepared by solution process. The manipulation involved controlling the distribution of different ‘<i>n</i>’ (i.e., different bandgap) phases, with a specific focus on ensuring the dominance of the phase with the smallest bandgap, which is also the target emission bandgap. 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, as these factors heavily influence the diffusion of cations, a crucial determinant for the value of '<i>n</i>'. Subsequently, we further enhanced the PL quantum yield (PLQY) from 31% to 51% through fine-tuning in the second-step crystallization process, which involved the incorporation of trioctylphosphine oxide (TOPO) as an additive during the antisolvent treatment. The antisolvent treatment with TOPO slightly slowed down the diffusion of precursors, resulting in halide-vacancy passivation and well-ordered crystals and leading to faster carrier transfer between phases. Based on these strategies, we successfully fabricated pure-blue light-emitting diodes (LEDs), 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 a FWHM of 29 nm.