The role of spin in the kinetic control of recombination in organic photovoltaics

有机太阳能电池 激子 离域电子 单重态 有机半导体 化学物理 三重态 接受者 电子 分子间力 材料科学 自旋态 化学 原子物理学 分子物理学 光电子学 激发态 凝聚态物理 物理 聚合物 分子 量子力学 有机化学
作者
Akshay Rao,Philip C. Y. Chow,Simon Gélinas,Cody W. Schlenker,Chang‐Zhi Li,Hin‐Lap Yip,Alex K.‐Y. Jen,David S. Ginger,Richard H. Friend
出处
期刊:Nature [Nature Portfolio]
卷期号:500 (7463): 435-439 被引量:496
标识
DOI:10.1038/nature12339
摘要

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.
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