Employing ∼ 100% Excitons in OLEDs by Utilizing a Fluorescent Molecule with Hybridized Local and ChargeTransfer Excited State
Employing ∼ 100% Excitons in OLEDs by Utilizing a Fluorescent Molecule with Hybridized Local and ChargeTransfer Excited State
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DOI：
10.1002/adfm.201301750
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发表时间：
2014-03-01
影响因子：
19
通讯作者：
Ma, Yuguang
中科院分区：
文献类型：
--
作者：
Li, Weijun;Pan, Yuyu;Ma, Yuguang
原则上，如果在OLED中由注入的电子和空穴的复合产生的激子（束缚电子-空穴对）具有足够弱的结合能，则可以预期电致发光（EL）与光致发光（PL）的最大量子效率的比率（phi）接近1。然而，在OLED中很少报道phi > 25%的例子，因为自然界中大多数有机半导体的激子都是强束缚的。在这里，一个扭曲的供体-受体三苯胺-噻二唑分子（TPA-NZP）通过杂化的本地和电荷转移激发态（HLCT），这是从荧光溶致变色实验和量子化学计算证明的荧光发射。HLCT态具有两个组合和兼容的特性：从局部激发（LE）态的大跃迁矩和从电荷转移（CT）态的弱束缚激子。前者有助于高效率的荧光辐射，而后者负责产生高比例的单重态激子。使用TPA-NZP作为OLED中的发光层，实现了93%（在低亮度下）和50%（在高亮度下）的高phi值，反映了OLED中激子的充分利用。EL器件的表征显示出饱和的深红色发光，CIE坐标为（0.67，0.32），伴随着相当优异的性能，最大亮度为4574 cd m（-2），最大外量子效率（ext）接近2.8%。HLCT态是实现电致发光器件高效化的一种新途径。
In principle, the ratio (phi) of the maximum quantum efficiencies for electroluminescence (EL) to photoluminescence (PL) can be expected to approach unity, if the exciton (bound electron-hole pair) generated from the recombination of injected electrons and holes in OLEDs has a sufficiently weak binding energy. However, seldom are examples of phi > 25% reported in OLEDs because of the strongly bound excitons for most organic semiconductors in nature. Here, a twisting donor-acceptor triphenylamine-thiadiazol molecule (TPA-NZP) exhibits fluorescent emission through a hybridized local and charge-transfer excited state (HLCT), which is demonstrated from both fluorescent solvatochromic experiment and quantum chemical calculations. The HLCT state possesses two combined and compatible characteristics: a large transition moment from a local excited (LE) state and a weakly bound exciton from a charge transfer (CT) state. The former contributes to a high-efficiency radiation of fluorescence, while the latter is responsible for the generation of a high fraction of singlet excitons. Using TPA-NZP as the light-emitting layer in an OLED, high phi values of 93% (at low brightness) and 50% (at high brightness) are achieved, reflecting sufficient employment of the excitons in the OLED. Characterization of the EL device shows a saturated deep-red emission with CIE coordinates of (0.67, 0.32), accompanied by a rather excellent performance with a maximum luminance of 4574 cd m(-2) and a maximum external quantum efficiency ((ext)) of approximate to 2.8%. The HLCT state is a new way to realize high-efficiency of EL devices.