Cavendish Laboratory, University of Cambridge, Cambridge CB3 0HE, UK.
Nature. 2013 Aug 22;500(7463):435-9. doi: 10.1038/nature12339. Epub 2013 Aug 7.
In biological complexes, cascade structures promote the spatial separation of photogenerated electrons and holes, preventing their recombination. In contrast, the photogenerated excitons in organic photovoltaic cells are dissociated at a single donor-acceptor heterojunction formed within a de-mixed blend of the donor and acceptor semiconductors. The nanoscale morphology and high charge densities give a high rate of electron-hole encounters, which should in principle result in the formation of spin-triplet excitons, as in organic light-emitting diodes. Although organic photovoltaic cells would have poor quantum efficiencies if every encounter led to recombination, state-of-the-art examples nevertheless demonstrate near-unity quantum efficiency. Here we show that this suppression of recombination arises through the interplay between spin, energetics and delocalization of electronic excitations in organic semiconductors. We use time-resolved spectroscopy to study a series of model high-efficiency polymer-fullerene systems in which the lowest-energy molecular triplet exciton (T1) for the polymer is lower in energy than the intermolecular charge transfer state. We observe the formation of T1 states following bimolecular recombination, indicating that encounters of spin-uncorrelated electrons and holes generate charge transfer states with both spin-singlet ((1)CT) and spin-triplet ((3)CT) characters. We show that the formation of triplet excitons can be the main loss mechanism in organic photovoltaic cells. But we also find that, even when energetically favoured, the relaxation of (3)CT states to T1 states can be strongly suppressed by wavefunction delocalization, allowing for the dissociation of (3)CT states back to free charges, thereby reducing recombination and enhancing device performance. Our results point towards new design rules both for photoconversion systems, enabling the suppression of electron-hole recombination, and for organic light-emitting diodes, avoiding the formation of triplet excitons and enhancing fluorescence efficiency.
在生物复合物中，级联结构促进光生电子和空穴的空间分离，防止它们复合。相比之下，有机光伏电池中的光生激子在施主和受主半导体的混合混合物中形成的单一施主-受主异质结处解离。纳米级形态和高电荷密度使得电子-空穴遭遇的速率很高，原则上应该导致形成自旋三重态激子，就像在有机发光二极管中一样。尽管如果每次遭遇都导致复合，有机光伏电池的量子效率会很差，但最先进的例子仍然展示了接近 1 的量子效率。在这里，我们表明这种复合的抑制来自于有机半导体中电子激发的自旋、能量和离域之间的相互作用。我们使用时间分辨光谱学研究一系列模型高效聚合物-富勒烯体系，其中聚合物的最低能量分子三重态激子（T1）比分子间电荷转移态的能量低。我们观察到双分子复合后 T1 态的形成，表明自旋非相关电子和空穴的遭遇生成具有自旋单重态（1CT）和自旋三重态（3CT）特征的电荷转移态。我们表明，三重态激子的形成可能是有机光伏电池中的主要损耗机制。但是，我们还发现，即使在能量上有利，（3CT）态弛豫到 T1 态也可以通过波函数离域强烈抑制，从而允许（3CT）态回解离回到自由电荷，从而减少复合并提高器件性能。我们的结果指向新的设计规则，既适用于光转换系统，能够抑制电子-空穴复合，也适用于有机发光二极管，避免形成三重态激子并提高荧光效率。
