Department of Chemistry and Centre for Plastic Electronics, Imperial College London, London SW7 2AZ, United Kingdom.
J Chem Phys. 2019 Mar 14;150(10):104704. doi: 10.1063/1.5079285.
Recent progress in organic photovoltaics (OPVs) has been enabled by optimization of the energetic driving force for charge separation, and thus maximization of open-circuit voltage, using non-fullerene acceptor (NFA) materials. In spite of this, the carrier dynamics and relative energies of the key states controlling the photophysics of these systems are still under debate. Herein, we report an in-depth ultrafast spectroscopic study of a representative OPV system based on a polymer donor PffBT4T-2OD and a small-molecule NFA EH-IDTBR. Global analysis of the transient absorption data reveals efficient energy transfer between donor and acceptor molecules. The extracted kinetics suggest that slow (∼15 ps) generation of charge carriers is followed by significant geminate recombination. This contrasts with the "reference" PffBT4T-2OD:PCBM system where bimolecular recombination dominates. Using temperature-dependent pump-push-photocurrent spectroscopy, we estimate the activation energy for the dissociation of bound charge-transfer states in PffBT4T-2OD:EH-IDTBR to be 100 ± 6 meV. We also observe an additional activation energy of 14 ± 7 meV, which we assign to the de-trapping of mobile carriers. This work provides a comprehensive picture of photophysics in a system representing new generation of OPV blends with a small driving force for charge separation.
近年来，通过优化用于电荷分离的能量驱动力（即开路电压），使用非富勒烯受体（NFA）材料，有机光伏（OPV）取得了显著进展。尽管如此，控制这些体系光物理性质的关键态的载流子动力学和相对能量仍存在争议。在此，我们报道了基于聚合物给体 PffBT4T-2OD 和小分子 NFA EH-IDTBR 的代表性 OPV 体系的深入超快光谱研究。瞬态吸收数据的全局分析表明供体和受体分子之间存在有效的能量转移。提取的动力学表明，载流子的缓慢（∼15 ps）生成随后是明显的复合。这与“参考”PffBT4T-2OD:PCBM 体系形成对比，其中双分子复合占主导地位。使用温度依赖的泵浦推动光电流光谱法，我们估计 PffBT4T-2OD:EH-IDTBR 中束缚电荷转移态解离的活化能为 100 ± 6 meV。我们还观察到 14 ± 7 meV 的额外活化能，我们将其归因于移动载流子的去陷阱化。这项工作提供了具有小电荷分离驱动力的新一代 OPV 共混物体系光物理的综合图景。
