Dual additives balance phase distribution in all-bromide quasi-2D perovskites for spectrally stable pure-blue light-emitting diodes

All-bromide quasi-two-dimensional (quasi-2D) perovskites exhibit remarkable potential for application in blue perovskite light-emitting diodes (PeLEDs) due to their superior optoelectronic properties. However, confining quasi-2D phases to <i>n</i> ≤ 3 and establishing an optimal gradient distribution for efficient carrier injection remain challenging. Herein, we present a dual-additive strategy, wherein 2-(4-fluorophenyl)ethylamine hydrobromide (<i>p</i>-F-PEABr) serves as a strong confinement additive that forms quasi-2D phases with <i>n</i> ≤ 3, while ethylammonium bromide (EABr) functions as a weak confinement additive that can disrupt the π-π stacking of <i>p</i>-F-PEABr and enable precise <i>n</i>-value modulation within the <i>n</i> ≤ 3 regime, consequently forming a gradient phase distribution. Therefore, through the dual additives with strong and weak confinement, efficient energy transfer channels and smooth high-quality films are constructed, resulting in pure-blue (470 nm) PeLEDs with a maximum external quantum efficiency (EQE) of 4.26% and stable electroluminescence (EL) under operating voltages of 3-7 V without spectral shift. Notably, the Commission Internationale de l'Éclairage (CIE) coordinates of the EL are (0.138, 0.09), satisfying the National Television System Committee (NTSC) pure-blue standard with a negligible offset (ΔCIE is only (0.014, 0.005)) upon increasing the voltage to 7 V. This work provides a novel approach for developing spectrally stable pure-blue quasi-2D PeLEDs for wide colour gamut display applications.