Enhancing electroluminescence performance of ultra-deep-blue through-space charge transfer emitters with CIEy ≈ 0.05 via methyl-modification†

Abstract

Achieving efficient solution-processed ultra-deep-blue OLEDs remains a challenge. Herein, a methyl-modification strategy is proposed to overcome weak intramolecular charge transfer and the large energy gap between the singlet and triplet states (ΔE_ST) of ultra-deep-blue through-space charge transfer (TSCT)-thermally activated delayed fluorescence (TADF) emitters. In this way, the reverse intersystem crossing (RISC) process is found to be effective from T₁ to S₁ and can be accelerated with the assistance of T₂. As a result, the ultra-deep-blue TSCT emitter 3MeCz-BO exhibits a minimized ΔE_ST of 0.02 eV, and an enhanced RISC rate of 3.71 × 10⁵ s⁻¹. Additionally, this modification can improve the solubility, enabling the fabrication of solution-processed organic light-emitting diodes (OLEDs). The maximum external quantum efficiency of the 3MeCz-BO-based solution-processed OLED reaches 10.1%, with a Commission Internationale de L'Eclairage (CIE) coordinates of (0.151, 0.051) and a luminance of 1334 cd m⁻². This work is the first instance of developing high-performance ultra-deep-blue solution-processed TSCT-TADF OLEDs, which show comparable performance to vacuum-deposited OLEDs. Furthermore, the 3MeCz-BO-based OLED fits well within the standard Red Green Blue (sRGB) of CIE coordinates (0.15, 0.06), and is close to the CIE coordinates (0.131, 0.046) for the Rec. 2020 standard, implying its potential application in colorful display devices.