Phase regulation via dual additives for pure-blue emitting quasi-2D perovskite light-emitting diodes

Solution-processed mixed-halide quasi-2-dimensional (quasi-2D) perovskites offer precise bandgap tunability and high color purity, and their moderate exciton binding energies enable efficient inter-phase energy funneling, from wide-bandgap lower-n (where n denotes the number of inorganic octahedral layers in the quasi-2D lattice) phases to narrower-bandgap larger-n phases, that concentrates excitations in the emissive phase and supports high emission efficiencies. Achieving pure-blue emission, however, often requires heavy spacer loading (e.g., phenethylammonium, PEA⁺), which overproduces low-n (n ≤ 3) domains with excessively high exciton binding energies, thereby intensifying Auger recombination and diminishing inter-phase funneling. Compounding these effects, ion migration/halide redistribution, halide-vacancy-induced under-coordinated Pb²⁺ centers, and charge-injection imbalance collectively undermine the stability of blue devices. This study exploits the synergy of bis(triphenylphosphine)iminium chloride (PPNCl) and cesium trifluoroacetate (CsTFA) to tune spacer-perovskite binding while passivating Pb²⁺ centers and halide vacancies. Cl^- widens the bandgap of large-n phases, enabling pure-blue emission, and the [PPN]⁺ cation acts as an electron donor that forms hydrogen bonds with PEA⁺, thereby strengthening spacer-perovskite interactions. This interaction reconstructs the quasi-2D n-phase distribution. Similarly, TFA^-, enabled by the high electronegativity of its F substituents, forms hydrogen bonds with PEA⁺ and promotes n-phase redistribution. Together, these effects markedly deplete low-dimensional phases (n = 2, 3), the primary contributors to non-radiative recombination, and suppress the emergence of bulk 3D phases with low exciton binding energy. They also enhance inter-phase energy transfer, as confirmed by transient absorption spectroscopy (TAS). Additionally, the additives effectively passivate halide vacancies and suppress metallic Pb formation, further improving device performance. Compared with devices relying solely on PbBr₂/PbCl₂ ratio tuning, this approach delivers superior spectral stability. The resulting devices achieve pure-blue emission at 473 nm, with external quantum efficiency (EQE) = 8%, peak luminance = 350 cd m^-2, and a narrow full width at half maximum (FWHM) of 21 nm; notably, electroluminescence peak shifts are limited to ≤ 3 nm during operation, ensuring excellent spectral stability. These results represent a significant advancement in quasi-2D mixed-halide perovskite LEDs, offering a promising pathway for the development of stable and efficient pure-blue perovskite light-emitting diodes.