Department of Physics, Cavendish Laboratory, University of Cambridge , J.J. Thompson Avenue, Cambridge CB3 0HE, United Kingdom.
Center for Polymers and Organic Solids, University of California , Santa Barbara, California 93106, United States.
ACS Nano. 2016 Dec 27;10(12):10736-10744. doi: 10.1021/acsnano.6b06211. Epub 2016 Nov 3.
Donor-acceptor organic solar cells often show high quantum yields for charge collection, but relatively low open-circuit voltages (V) limit power conversion efficiencies to around 12%. We report here the behavior of a system, PIPCP:PCBM, that exhibits very low electronic disorder (Urbach energy less than 27 meV), very high carrier mobilities in the blend (field-effect mobility for holes >10 cm V s), and a very low driving energy for initial charge separation (50 meV). These characteristics should give excellent performance, and indeed, the V is high relative to the donor energy gap. However, we find the overall performance is limited by recombination, with formation of lower-lying triplet excitons on the donor accounting for 90% of the recombination. We find this is a bimolecular process that happens on time scales as short as 100 ps. Thus, although the absence of disorder and the associated high carrier mobility speeds up charge diffusion and extraction at the electrodes, which we measure as early as 1 ns, this also speeds up the recombination channel, giving overall a modest quantum yield of around 60%. We discuss strategies to remove the triplet exciton recombination channel.
供体-受体有机太阳能电池通常表现出高的电荷收集量子产率，但相对较低的开路电压（V）限制了功率转换效率约为 12%。我们在这里报告一个系统，PIPCP:PCBM，它表现出非常低的电子无序（Urbach 能小于 27 meV），在混合物中具有非常高的载流子迁移率（空穴的场效应迁移率大于 10 cm V s），以及非常低的初始电荷分离驱动力（50 meV）。这些特性应该会有很好的性能，事实上，V 相对于供体的能量间隙很高。然而，我们发现整体性能受到复合的限制，供体上较低的三重态激子的形成占复合的 90%。我们发现这是一个双分子过程，发生在 100 ps 这样短的时间尺度上。因此，尽管没有无序和相关的高载流子迁移率加速了在电极处的电荷扩散和提取，我们在 1 ns 时就已经测量到了这一点，但这也加速了复合通道，导致整体量子产率约为 60%。我们讨论了消除三重态激子复合通道的策略。
