Center for Polymers and Organic Solids, University of California-Santa Barbara, Santa Barbara, California 93106, United States.
J Am Chem Soc. 2012 Dec 5;134(48):19828-38. doi: 10.1021/ja308949m. Epub 2012 Nov 19.
Understanding the charge generation dynamics in organic photovoltaic bulk heterojunction (BHJ) blends is important for providing the necessary guidelines to improve overall device efficiency. Despite more than 15 years of experimental and theoretical studies, a universal picture describing the generation and recombination processes operating in organic photovoltaic devices is still being forged. We report here the results of ultrafast transient absorption spectroscopy measurements of charge photogeneration and recombination processes in a high-performing solution-processed molecular BHJ. For comparison, we also studied a high-performing polymer-based BHJ material. We find that the majority of charge carriers in both systems are generated on <100 fs time scales and posit that excited state delocalization is responsible for the ultrafast charge transfer. This initial delocalization is consistent with the fundamental uncertainty associated with the photon absorption process (in the visible, λ/4π > 30 nm) and is comparable with the phase-separated domain size. In addition, exciton diffusion to charge-separating heterojunctions is observed at longer times (1-500 ps). Finally, charge generation in pure films of the solution processed molecule was studied. Polarization anisotropy measurements clearly demonstrate that the optical properties are dominated by molecular (Frenkel) exictons and delocalized charges are promptly produced (t < 100 fs).
了解有机光伏体异质结（BHJ）混合物中的电荷产生动力学对于提供必要的指导原则以提高整体器件效率非常重要。尽管经过了 15 多年的实验和理论研究，但对于在有机光伏器件中起作用的产生和复合过程，仍然没有一个通用的描述。我们在此报告了对高性能溶液处理分子 BHJ 中电荷光生和复合过程的超快瞬态吸收光谱测量的结果。为了进行比较，我们还研究了一种高性能聚合物基 BHJ 材料。我们发现，这两种体系中的大多数电荷载流子都是在<100fs 的时间尺度上产生的，我们认为激发态离域是超快电荷转移的原因。这种初始离域与与光子吸收过程（在可见光范围内，λ/4π>30nm）相关的基本不确定性一致，并且与相分离的畴尺寸相当。此外，在较长时间（1-500ps）观察到激子扩散到电荷分离异质结。最后，研究了溶液处理分子的纯膜中的电荷产生。偏振各向异性测量清楚地表明，光学性质主要由分子（Frenkel）激子主导，并且离域电荷很快产生（t<100fs）。
