Hole delocalization as a driving force for charge pair dissociation in organic photovoltaics
Hole delocalization as a driving force for charge pair dissociation in organic photovoltaics
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DOI：
10.1039/c8mh01204k
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发表时间：
2019-06-01
影响因子：
13.3
通讯作者：
Samuel, Ifor D. W.
中科院分区：
文献类型：
--
作者：
Matheson, Andrew B.;Ruseckas, Arvydas;Samuel, Ifor D. W.
利用宽带瞬态吸收光谱研究了低激发密度下共轭聚合物PTB7与电子受体PC71BM的光电共混物中载流子的光产生。在优化后的混合物中，我们观察到在500 ps时间尺度上空穴极化子吸收的增加，这意味着当光产生的电荷对解离成自由电荷时，空穴离域增加。这一概念得到了聚合物薄膜电化学氧化时极化子吸收饱和的观察结果的支持，以及由于束缚电荷转移（CT）状态而在低效混合中观察到的瞬态吸收光谱的显著差异。我们的研究结果表明，空穴极化子在聚合物链上的离域和熵通过降低空间分离电荷对的自由能，为电荷分离提供了驱动力。由于载流子离域的存在，对CT状态的重构存在潜在的障碍，这也有助于减少非双生载流子的重组。
Charge carrier photogeneration is studied in photovoltaic blends of the conjugated polymer PTB7 with the electron acceptor PC71BM at low excitation densities using broadband transient absorption spectroscopy. In the optimized blend we observe a rise of hole polaron absorption on a 500 ps time scale which implies an increase of hole delocalization when photo-generated charge pairs dissociate into free charges. This concept is supported by the observed saturation of polaron absorption with electrochemical oxidation of polymer films, and the significant differences in transient absorption spectra observed in an inefficient blend due to bound charge transfer (CT) states. Our results suggest that hole polaron delocalization on polymer chains and entropy provide driving force for charge separation by lowering the free energy of the spatially separated charge pair. A potential barrier to the reformation of CT states appears as a result of carrier delocalization which also helps to reduce non-geminate carrier recombination.