Highly Efficient Deep-Blue Organic Light-Emitting Diodes Based on Rational Molecular Design and Device Engineering

There is increasing interest in thermally activated delayed fluorescence (TADF) in materials, and to understand its mechanism in the excited state dynamics. Recent challenges include color purity, efficient deep-blue emission, fast exciton decay lifetimes, high reverse intersystem crossing rates (k_RISC), low-efficiency roll-off in organic light-emitting diodes (OLEDs), and long device lifetimes. Here, a series of compounds having benzonitrile and carbazole rings are examined, that provide a detailed understanding of the excited states, and a guideline for high-performance TADF. A dense alignment of the excited states with several different characters within a small energy range results in high k_RISC of >2 × 10⁶ s⁻¹, while maintaining radiative rate constants (k_r) >10⁷ s⁻¹. OLEDs based on the optimum compound exhibit a low-efficiency roll-off and a CIEy (y color coordinate of Commission Internationale de l'Éclairage) <0.4. TADF-assisted fluorescence (TAF) OLED exhibits a maximum external quantum efficiency of 22.4% with CIE coordinates (0.13,0.15). This work also provides insights for device engineering and molecular designs.