Triplet Harvesting with 100% Efficiency by Way of Thermally Activated Delayed Fluorescence in Charge Transfer OLED Emitters
Triplet Harvesting with 100% Efficiency by Way of Thermally Activated Delayed Fluorescence in Charge Transfer OLED Emitters
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DOI：
10.1002/adma.201300753
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发表时间：
2013-07-19
影响因子：
29.4
通讯作者：
Monkman, Andrew P.
中科院分区：
文献类型：
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作者：
Dias, Fernando B.;Bourdakos, Konstantinos N.;Monkman, Andrew P.
有机发光二极管 (OLED) 的性能受到电荷复合时产生的发射单线态数量 (25%) 的限制。最近，提出了一种新策略，基于热激活三重态到单重态的上转换，产生延迟荧光（TADF），大大增强了电致发光。这种反向系间穿越机制的能垒与单重态和三重态之间的交换能（E-ST）成正比；因此，具有分子内电荷转移（ICT）状态的材料（已知交换能量很小）是完美的候选者。然而，这里表明，即使在室温下 E-ST 超过 20 kT（其中 k 是玻尔兹曼常数，T 是温度）的材料中，也可以通过 TADF 以 100% 的效率收获三重态。阐明了孤对电子在一系列 ICT 分子中实现如此高效率中所发挥的关键作用。结果显示了高效 TADF 材料的复杂光物理学，并为设计新型发射器提供了明确的指导。
Organic light-emitting diodes (OLEDs) have their performance limited by the number of emissive singlet states created upon charge recombination (25%). Recently, a novel strategy has been proposed, based on thermally activated up-conversion of triplet to singlet states, yielding delayed fluorescence (TADF), which greatly enhances electroluminescence. The energy barrier for this reverse intersystem crossing mechanism is proportional to the exchange energy (E-ST) between the singlet and triplet states; therefore, materials with intramolecular charge transfer (ICT) states, where it is known that the exchange energy is small, are perfect candidates. However, here it is shown that triplet states can be harvested with 100% efficiency via TADF, even in materials with E-ST of more than 20 kT (where k is the Boltzmann constant and T is the temperature) at room temperature. The key role played by lone pair electrons in achieving this high efficiency in a series of ICT molecules is elucidated. The results show the complex photophysics of efficient TADF materials and give clear guidelines for designing new emitters.