半结晶聚合物半导体中激子的二维空间相干性：分子量的影响
Two-dimensional spatial coherence of excitons in semicrystalline polymeric semiconductors: The effect of molecular weight
摘要：
大分子半导体薄膜的电子性质在很大程度上取决于它们的固态微结构，而固态微结构又受到所选加工条件以及聚合物化学性质和分子量的控制。具体而言，低分子量材料形成共面$π$堆叠分子的结晶域，而较高分子量聚合物的通常缠结性质导致由通过无定形区域互连的分子有序微晶组成的微结构。在这里，我们研究扩展激子状态之间的相互作用，沿着聚合物主链和跨聚合物链内的$π$堆栈离域，取决于分子量的结构发展。我们结合光学光谱，热探针，和理论建模，专注于纯聚（3-己基噻吩）（P3 HT），最广泛研究的聚合物半导体之一，重均分子量为3- 450\，kg/mol。链内的空间相干性显著降低（降低近30%）。这些观察结果提供了有价值的结构信息，他们建议，在高分子量P3 HT的聚集区域的大分子采用更平面的构象相比，低分子量的材料。这导致所观察到的链内激子相干性的增加。相比之下，较短的链似乎导致扭转更无序的架构。一个严格的，基本的描述初级光激发在$π$共轭聚合物，因此开发：二维激子定义的链长依赖的分子排列和互连的共轭大分子，导致分子内和分子间的空间相干性之间的相互作用。
英文摘要：
The electronic properties of macromolecular semiconductor thin films depend profoundly on their solid-state microstructure, which in turn is governed, among other things, by the processing conditions selected and the polymer chemical nature and molecular weight. Specifically, low-molecular-weight materials form crystalline domains of cofacially $π$-stacked molecules, while the usually entangled nature of higher molecular-weight polymers leads to microstructures comprised of molecularly ordered crystallites interconnected by amorphous regions. Here, we examine the interplay between extended exciton states delocalized along the polymer backbones and across polymer chains within the $π$-stack, depending on the structural development with molecular weight. We combine optical spectroscopies, thermal probes, and theoretical modeling, focusing on neat poly(3-hexylthiophene) (P3HT), one of the most extensively studied polymer semiconductors, of weight-average molecular weight of 3-450\,kg/mol. The spatial coherence within the chain is significantly reduced (by nearly 30\%). These observations give valuable structural information; they suggest that the macromolecules in aggregated regions of high-molecular-weight P3HT adopt a more planar conformation compared to low-molecular-weight materials. This results in the observed increase in intrachain exciton coherence. In contrast, shorter chains seem to lead to torsionally more disordered architectures. A rigorous, fundamental description of primary photoexcitations in $π$-conjugated polymers is hence developed: two-dimensional excitons are defined by the chain-length dependent molecular arrangement and interconnectivity of the conjugated macromolecules, leading to interplay between intramolecular and intermolecular spatial coherence.
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