Formation cross-sections of singlet and triplet excitons in π-conjugated polymers
Formation cross-sections of singlet and triplet excitons in π-conjugated polymers
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DOI：
10.1038/35054025
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发表时间：
2001-01
期刊：
影响因子：
64.8
通讯作者：
M. Wohlgenannt;Kunj Tandon;S. Mazumdar;S. Ramasesha;Z. Vardeny
中科院分区：
文献类型：
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作者：
M. Wohlgenannt;Kunj Tandon;S. Mazumdar;S. Ramasesha;Z. Vardeny
有机发光二极管中的电致发光是由注入的正电荷和负电荷之间的电荷转移反应引起的，通过电荷转移反应，它们结合形成单态激子，随后辐射衰减。这一过程的量子产额(每个注入的电子或空穴产生的光子数)通常被认为具有25%的统计上限。这是基于这样的假设，即单重态激子的形成截面σS近似等于三个等价的无辐射三重态激子的任何一个态的形成截面σT，即σS/σT≈1。然而，最近的实验和理论工作表明σS/σT可能大于1。本文报道了对大量σ共轭聚合物和低聚物的σS/πT的直接测量。我们发现σS/σ对有机材料的光学带隙有很强的系统依赖性，但不是单调的。我们给出了关联π电子电荷转移反应的详细物理图像，并用精确的价键计算对这个过程进行了量化。计算得到的σS/σ与实验观测的趋势一致。计算还表明，σS/σ对光学带隙的强烈依赖性是离散激子能谱的一个特征，其中较高能量的激子能级参与了电荷复合过程。
Electroluminescence in organic light-emitting diodes arises from a charge-transfer reaction between the injected positive and negative charges by which they combine to form singlet excitons that subsequently decay radiatively. The quantum yield of this process (the number of photons generated per electron or hole injected) is often thought to have a statistical upper limit of 25 per cent. This is based on the assumption that the formation cross-section of singlet excitons, σS, is approximately the same as that of any one of the three equivalent non-radiative triplet exciton states, σT; that is, σS/σT≈ 1. However, recent experimental and theoretical work suggests that σS/σTmay be greater than 1. Here we report direct measurements of σS/σTfor a large number of π-conjugated polymers and oligomers. We have found that there exists a strong systematic, but not monotonic, dependence of σS/σTon the optical gap of the organic materials. We present a detailed physical picture of the charge-transfer reaction for correlated π-electrons, and quantify this process using exact valence bond calculations. The calculated σS/σTreproduces the experimentally observed trend. The calculations also show that the strong dependence of σS/σTon the optical gap is a signature of the discrete excitonic energy spectrum, in which higher energy excitonic levels participate in the charge recombination process.