Organic light-emitting diodes (OLEDs) are a revolutionary light-emitting display technology that has been successfully commercialized in mobile phones and televisions^1,2. The injected charges form both singlet and triplet excitons, and for high efficiency it is important to enable triplets as well as singlets to emit light. At present, materials that harvest triplets by thermally activated delayed fluorescence (TADF) are a very active field of research as an alternative to phosphorescent emitters that usually use heavy metal atoms^3,4. Although excellent progress has been made, in most TADF OLEDs there is a severe decrease of efficiency as the drive current is increased, known as efficiency roll-off. So far, much of the literature suggests that efficiency roll-off should be reduced by minimizing the energy difference between singlet and triplet excited states (ΔE_ST) to maximize the rate of conversion of triplets to singlets by means of reverse intersystem crossing (k_RISC)^{5,6,7,8,9,10,11,12,13,14,15,16,17,18,19,20}. We analyse the efficiency roll-off in a wide range of TADF OLEDs and find that neither of these parameters fully accounts for the reported efficiency roll-off. By considering the dynamic equilibrium between singlets and triplets in TADF materials, we propose a figure of merit for materials design to reduce efficiency roll-off and discuss its correlation with reported data of TADF OLEDs. Our new figure of merit will guide the design and development of TADF materials that can reduce efficiency roll-off. It will help improve the efficiency of TADF OLEDs at realistic display operating conditions and expand the use of TADF materials to applications that require high brightness, such as lighting, augmented reality and lasing.

有机发光二极管 (OLED) 是一种革命性的发光显示技术，已在手机和电视中成功商业化^1,2。注入的电荷形成单线态激子和三线态激子，为了获得高效率，使三线态激子和单线态激子发光非常重要。目前，通过热激活延迟荧光 (TADF) 收获三重态的材料是一个非常活跃的研究领域，作为通常使用重金属原子的磷光发射器的替代品^3,4。尽管已经取得了巨大的进步，但在大多数 TADF OLED 中，随着驱动电流的增加，效率会严重下降，称为效率滚降。到目前为止，许多文献表明，应通过最小化单重态和三重态激发态之间的能量差（Δ E _ST ）来减少效率滚降，以通过反向系间窜越（k _RISC）^{5,6,7,8,9,10,11,12,13,14,15,16,17,18,19,20}。我们分析了各种 TADF OLED 的效率滚降，发现这些参数都不能完全解释所报告的效率滚降。通过考虑 TADF 材料中单线态和三线态之间的动态平衡，我们提出了材料设计的品质因数以减少效率滚降，并讨论了其与 TADF OLED 报告数据的相关性。我们的新品质因数将指导 TADF 材料的设计和开发，以减少效率下降。它将有助于提高 TADF OLED 在实际显示操作条件下的效率，并将 TADF 材料的使用扩展到需要高亮度的应用，例如照明、增强现实和激光。