Spectroscopic Investigation of the Effect of Microstructure and Energetic Offset on the Nature of Interfacial Charge Transfer States in Polymer: Fullerene Blends
Spectroscopic Investigation of the Effect of Microstructure and Energetic Offset on the Nature of Interfacial Charge Transfer States in Polymer: Fullerene Blends
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DOI：
10.1021/jacs.8b11484
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发表时间：
2019-03-20
影响因子：
15
通讯作者：
Durrant, J. R.
中科院分区：
文献类型：
--
作者：
Dimitrov, S. D.;Azzouzi, M.;Durrant, J. R.
尽管有机光伏材料的性能有所提高，但人们对有机给体-受体界面光诱导电子-空穴分离的机制仍然知之甚少。不确定的实验和理论结果产生了相互矛盾的电子-空穴分离模型，其中界面电荷转移（CT）状态的作用尚不清楚，一个模型认为它们是限制分离的，另一个模型认为它们是容易解离的。本文研究了具有不同光电流特性和驱动分离的焓偏移的聚合物-富勒烯共混物。通过改变组成，薄膜结构由分子混合聚合物-富勒烯结构域变为分子混合聚合物-富勒烯结构域。瞬态吸收光谱显示，CT态解离产生分离的电子-空穴对仅在与富勒烯结构域的高能偏移共混中有效。在所有其他共混（低偏移或主要是分子混合域）中，观察到纳秒双生电子-空穴复合，揭示了空间定域电子-空穴对（束缚CT态）在电子-空穴动力学中的重要性。采用二维晶格激子模型模拟了模型系统的激发态谱随微观结构和能量偏移的变化。结果可以再现实验电致发光光谱的主要特征，即随着能量偏移量和富勒烯畴密度的增加，电子-空穴对的束缚减小，空间分离增大。电致发光光谱和光致发光光谱的差异可以解释为CT的光致发光以多结合态为主，反映了双结合态重组过程，而CT的电致发光则优先探测逃避双结合态重组的低结合态。这些结果表明，表面上相互矛盾的电子空穴分离研究可以解释为，由于一系列界面结构，在同一膜中存在束缚和非束缚的CT态。
Despite performance improvements of organic photovoltaics, the mechanism of photoinduced electron-hole separation at organic donor-acceptor interfaces remains poorly understood. Inconclusive experimental and theoretical results have produced contradictory models for electron-hole separation in which the role of interfacial charge-transfer (CT) states is unclear, with one model identifying them as limiting separation and another as readily dissociating. Here, polymer-fullerene blends with contrasting photocurrent properties and enthalpic offsets driving separation were studied. By modifying composition, film structures were varied from consisting of molecularly mixed polymer-fullerene domains to consisting of both molecularly mixed and fullerene domains. Transient absorption spectroscopy revealed that CT state dissociation generating separated electron-hole pairs is only efficient in the high energy offset blend with fullerene domains. In all other blends (with low offset or predominantly molecularly mixed domains), nanosecond geminate electron-hole recombination is observed revealing the importance of spatially localized electron-hole pairs (bound CT states) in the electron-hole dynamics. A two-dimensional lattice exciton model was used to simulate the excited state spectrum of a model system as a function of microstructure and energy offset. The results could reproduce the main features of experimental electroluminescence spectra indicating that electron-hole pairs become less bound and more spatially separated upon increasing energy offset and fullerene domain density. Differences between electroluminescence and photoluminescence spectra could be explained by CT photoluminescence being dominated by more-bound states, reflecting geminate recombination processes, while CT electroluminescence preferentially probes less-bound CT states that escape geminate recombination. These results suggest that apparently contradictory studies on electron-hole separation can be explained by the presence of both bound and unbound CT states in the same film, as a result of a range of interface structures.