Optically and electrically excited intermediate electronic states in donor:acceptor based OLEDs
Optically and electrically excited intermediate electronic states in donor:acceptor based OLEDs
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DOI：
10.1039/c9mh01475f
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发表时间：
2020-04-01
影响因子：
13.3
通讯作者：
Dyakonov, Vladimir
中科院分区：
文献类型：
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作者：
Bunzmann, Nikolai;Weissenseel, Sebastian;Dyakonov, Vladimir
由供体和受体分子组成的热激活延迟荧光 (TADF) 发射器对于电致发光 (EL) 应用来说可能非常有趣。它们的强荧光发射被认为是由于反向系间窜越（RISC）所致，其中涉及能量接近的三重态和单重态电荷转移（CT）态，也称为激基复合物态。为了区分不同的机制和所涉及的激发态，对有机发光二极管（OLED）和薄膜进行温度相关的自旋敏感测量至关重要。在我们的工作中，我们将连续波 (cw) 和时间分辨 (tr) 光致发光 (PL) 光谱以及自旋敏感 EL 和 PL 检测磁共振应用于由三种不同供体：受体组合制成的薄膜和 OLED 器件。我们的结果清楚地表明，在电驱动 OLED 和光激发薄膜中，三重态激基复合物态已形成，并通过 RISC 促进延迟荧光 (DF)。在同一样本集中，我们还发现了分子三重态激子，这种激子仅出现在光学激发下的 PL 实验中，并且对于某些材料系统仅在低温下出现。我们得出的结论是，在所有研究的分子系统中，供体：受体界面处形成的激基复合物状态是导致 OLED 中具有不同活化能的 TADF 的原因。分子（局部）三线态激子态也是可检测的，但仅在光激发下才可检测到，而当激发态产生电时，在 OLED 中则无法检测到它们。我们认为，弱束缚发射激基复合物态和强束缚非发射分子三重激发态共存于TADF发射器中，必须区分光学和电产生路径，因为它们可能涉及不同的中间激发态。
Thermally activated delayed fluorescence (TADF) emitters consisting of donor and acceptor molecules are potentially highly interesting for electroluminescence (EL) applications. Their strong fluorescence emission is considered to be due to reverse intersystem crossing (RISC), in which energetically close triplet and singlet charge transfer (CT) states, also called exciplex states, are involved. In order to distinguish between different mechanisms and excited states involved, temperature-dependent spin-sensitive measurements on organic light-emitting diodes (OLEDs) and thin films are essential. In our work we apply continuous wave (cw) and time-resolved (tr) photoluminescence (PL) spectroscopy as well as spin-sensitive EL and PL detected magnetic resonance to films and OLED devices made of three different donor:acceptor combinations. Our results clearly show that triplet exciplex states are formed and contribute to delayed fluorescence (DF) via RISC in both electrically driven OLEDs and optically excited films. In the same sample set we also found molecular triplet excitons, which occurred only in PL experiments under optical excitation and for some material systems only at low temperatures. We conclude that in all investigated molecular systems exciplex states formed at the donor:acceptor interface are responsible for TADF in OLEDs with distinct activation energies. Molecular (local) triplet exciton states are also detectable, but only under optical excitation, while they are not found in OLEDs when excited states are generated electrically. We believe that the weakly bound emissive exciplex states and the strongly bound non-emissive molecular triplet excited states coexist in the TADF emitters, and it is imperative to distinguish between optical and electrical generation paths as they may involve different intermediate excited states.