28 March 2024 | S. Diesing123, L. Zhang123, E. Zysman-Colman252 & I. D. W. Samuel1325
The article discusses the efficiency roll-off issue in thermally activated delayed fluorescence (TADF)-based organic light-emitting diodes (OLEDs) and proposes a new figure of merit (FOM) to guide the design of materials that can reduce this phenomenon. The efficiency roll-off is a significant challenge in TADF OLEDs, where the efficiency decreases as the drive current increases. The authors analyze a wide range of TADF OLEDs and find that neither the energy difference between singlet and triplet excited states (ΔE<sub>ST</sub>) nor the reverse intersystem crossing rate constant (k<sub>RISC</sub>) fully accounts for the reported efficiency roll-off. They propose a new FOM, which combines the product of the radiative rate constant from triplet to singlet (k<sub>T</sub><sup>S</sup>) and the equilibrium constant (K<sub>eq</sub>) between singlet and triplet states, to better predict and reduce efficiency roll-off. This FOM is shown to have a stronger correlation with the efficiency roll-off compared to k<sub>RISC</sub>. The authors also discuss the implications of this FOM for material design and provide target values for different applications, such as displays, lighting, and beyond. They conclude that maximizing k<sub>T</sub><sup>S</sup> relative to k<sub>RISC</sub> and ensuring high k<sub>T</sub><sup>2</sup> are crucial for reducing efficiency roll-off.The article discusses the efficiency roll-off issue in thermally activated delayed fluorescence (TADF)-based organic light-emitting diodes (OLEDs) and proposes a new figure of merit (FOM) to guide the design of materials that can reduce this phenomenon. The efficiency roll-off is a significant challenge in TADF OLEDs, where the efficiency decreases as the drive current increases. The authors analyze a wide range of TADF OLEDs and find that neither the energy difference between singlet and triplet excited states (ΔE<sub>ST</sub>) nor the reverse intersystem crossing rate constant (k<sub>RISC</sub>) fully accounts for the reported efficiency roll-off. They propose a new FOM, which combines the product of the radiative rate constant from triplet to singlet (k<sub>T</sub><sup>S</sup>) and the equilibrium constant (K<sub>eq</sub>) between singlet and triplet states, to better predict and reduce efficiency roll-off. This FOM is shown to have a stronger correlation with the efficiency roll-off compared to k<sub>RISC</sub>. The authors also discuss the implications of this FOM for material design and provide target values for different applications, such as displays, lighting, and beyond. They conclude that maximizing k<sub>T</sub><sup>S</sup> relative to k<sub>RISC</sub> and ensuring high k<sub>T</sub><sup>2</sup> are crucial for reducing efficiency roll-off.