[论文解读] A novel graph-based formulation for characterizing morphology with application to organic solar cells

Olga Wodo, Srikanta Tirthapura+2 more|arXiv (Cornell University)|Jun 17, 2011

Organic Electronics and Photovoltaics参考文献 44被引用 6

一句话总结

本文提出了一种基于图的新颖框架，将体异质结有机太阳能电池的形貌建模为带标签、加权、无向图，以定量分析关键光物理过程——光吸收、激子扩散、电荷分离和电荷传输。通过利用图论，该方法高效计算形貌描述符及效率上限，揭示在热退火过程中电荷传输是初始瓶颈，最优形理由 $\tilde{t}=0.144$ 时达到 84% 的内量子效率。

ABSTRACT

Organic solar cells have the potential for widespread usage due to their promise of low cost, roll-to-roll manufacturability, and mechanical flexibility. However, deployment is impeded by their relatively low power conversion efficiencies. The last decade has seen significant progress in enhancing the power conversion of these devices through various strategies. One such approach is based on morphology control. This is because morphology affects all phenomena involved in solar conversion: light absorption and electron-hole pair (exciton) generation; exciton diffusion and dissociation into free charges; and transport of charges to the electrodes. Progress in experimental characterization and computational modeling now allow reconstruction and imaging of the thin film morphology. This opens up the possibility of rationally linking fabrication with morphology, as well as morphology with performance. In this context, a comprehensive set of computational tools to rapidly quantify and classify the heterogeneous internal structure of thin films will be invaluable in linking process, structure and property. We present a novel graph-based framework to efficiently construct a broad suite of physically meaningful morphology descriptors. These morphology descriptors are further classified according to the physical subprocesses within an organic solar cells. The approach is motivated by the equivalence between a discretized morphology and a labeled, weighted, undirected graph. We utilize this approach to pose key questions related to structure characterization. We subsequently construct estimates and upper bounds of various efficiencies. The approach is showcased by characterizing the effect of thermal annealing on time-evolution of a thin film morphology. We conclude by formulating natural extensions to characterize crystallinity and anisotropy of the morphology using the framework.

研究动机与目标

• 开发一种全面且计算高效的框架，用于定量表征体异质结有机太阳能电池的复杂二维/三维形貌。
• 通过具有物理解释意义的描述符，将制备工艺（例如热退火）与形貌及其最终器件性能联系起来。
• 通过分别分解这些过程，解决形貌对光吸收、激子解离和电荷传输产生耦合且相互矛盾影响的挑战。
• 为每个子过程提供严格的效率上限和估计值，以实现性能基准化。
• 通过丰富图的顶点和边属性，将该框架扩展至表征结晶度和各向异性。

提出的方法

• 将二维/三维薄膜形貌表示为带标签、加权、无向图，其中顶点代表材料区域，边代表连通性与距离。
• 使用标准图算法计算对每个光物理子过程至关重要的基于距离和连通性的度量。
• 提出六个关键表征问题，以系统分析四个子过程（吸收、激子扩散、电荷分离和传输）的形貌。
• 通过图论公式推导每个子过程效率的上限，例如 $\eta_{abs}^{upp}$、$\eta_{diss}^{upp}$ 和 $\eta_{out}^{upp}$。
• 通过吸收深度模型校正光吸收效率：$\eta_{abs}^{est} = \frac{I_0 \int_0^{h_{tot}} M(x) e^{-x/H_d} dx}{I_0 \int_0^{h_{tot}} dh}$。
• 将框架扩展至三维和周期性结构，并通过额外的顶点和边属性引入结晶度和各向异性。

实验结果

研究问题

• RQ1 如何系统地表征有机太阳能电池运行四个子过程（光吸收、激子扩散、电荷分离和电荷传输）中的形貌？
• RQ2 哪些关键图论度量能够捕捉形貌对每个子过程的物理相关性？
• RQ3 在热退火过程中，每个子过程效率的上限如何演变？它们揭示了哪些性能瓶颈？
• RQ4 在内量子效率方面，最优形貌是什么？其出现的时间点为何？
• RQ5 如何将图框架扩展以纳入形貌中的结晶度和各向异性？

主要发现

• 在热退火过程中，电荷传输是初始性能瓶颈，其效率提升快于激子解离。
• 当 $\tilde{t} > 0.144$ 时，激子解离成为主导瓶颈，尽管电荷传输持续改善，整体效率仍受限制。
• 在 $\tilde{t} = 0.144$ 时达到最优形貌，其内量子效率为 84%，与最佳实验报道值一致。
• 上限分析显示，在最优点处 $\eta_{abs}^{upp} = 50\%$、$\eta_{diss}^{upp} = 98\%$ 和 $\eta_{out}^{upp} = 86\%$，表明仍有显著提升潜力。
• 具有颗粒状结构的形貌其估计效率显著低于互穿网络结构，强调了连续通路的重要性。
• 该基于图的框架具有维度无关性，并可通过丰富顶点和边属性自然扩展至三维、周期性、结晶性和各向异性形貌。