Dimitrov S D, Azzouzi M, Wu J, Yao J, Dong Y, Tuladhar P Shakya, Schroeder B C, Bittner E R, McCulloch I, Nelson J, Durrant J R
SPECIFIC, College of Engineering , Swansea University , Bay Campus , Swansea SA1 8EN , United Kingdom.
Department of Chemistry , University College , London WC1H 0AJ , United Kingdom.
J Am Chem Soc. 2019 Mar 20;141(11):4634-4643. doi: 10.1021/jacs.8b11484. Epub 2019 Mar 8.
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.
尽管有机光伏电池的性能有所提升，但有机供体-受体界面处光致电子-空穴分离的机制仍未得到充分理解。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）态的作用尚不明确，一种模型认为它们限制了分离，而另一种模型则认为它们易于解离。在此，研究了具有对比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，薄膜结构从由分子混合的聚合物-富勒烯域组成变为由分子混合域和富勒烯域两者组成。瞬态吸收光谱表明，产生分离的电子-空穴对的CT态解离仅在具有富勒烯域的高能量偏移共混物中有效。在所有其他共混物（低偏移或主要是分子混合域）中，观察到纳秒级的双电子-空穴复合，揭示了空间局部化的电子-空穴对（束缚CT态）在电子-空穴动力学中的重要性。使用二维晶格激子模型来模拟模型系统的激发态光谱作为微观结构和能量偏移的函数。结果可以重现实验电致发光光谱的主要特征，表明随着能量偏移和富勒烯域密度的增加，电子-空穴对的束缚变弱且空间分离增加。电致发光光谱和光致发光光谱之间的差异可以通过CT光致发光由更多束缚态主导来解释，这反映了双复合过程，而CT电致发光优先探测逃脱双复合的较少束缚的CT态。这些结果表明，可以通过同一薄膜中同时存在束缚和非束缚CT态来解释关于电子-空穴分离的明显矛盾的研究，这是一系列界面结构的结果。
