低驱动力有机太阳能电池激子分离机制的理论研究

Reducing the driving force for exciton dissociation is one of effective solutions to minimize energy loss and thus improve power conversion efficiency for organic solar cells. Traditionally, the driving force, namely, the energy offset for the LUMO/HOMO between donor and acceptor should be larger than 0.3 eV to achieve efficient exciton dissociation. However, recent experiments have shown that excitons can be effectively dissociated although the energy offsets are extremely small, but the exciton dissociation mechanisms with such low driving forces are not understood yet. In this project, we are going to develop and apply reliable computational methods to obtain an accurate characterization of exciton binding energies and exciton dissociation reorganization energies with considering the electronic polarization and delocalization effects, and analyze the relationship of the driving forces with the exciton binding energies and reorganization energies to elucidate the mechanisms of exciton dissociation with low driving forces. Furthermore, to reveal the influence of molecular electronic nature on the driving force for exciton dissociation, we will systematically investigate various combinations of organic photovoltaic donors and acceptors, which is expected to be helpful for rational design of high-performance organic solar cells with low energy losses.

减小激子分离的驱动力是降低能量损失提高有机太阳能电池光电转换效率的有效途径之一。传统认为,驱动力即有机给/受体间的能级差至少要大于0.3eV。然而，最近实验发现即使能级差很小，激子也能有效分离，但其机制尚不清楚。本项目拟考虑电子极化与离域效应，发展和运用可靠方法精确计算激子束缚能和激子分离的重组能，分析它们与驱动力的相关性，阐明低驱动力激子分离的根源。进一步，通过综合分析不同给受体组合的有机光伏体系，揭示分子电子性质对激子分离驱动力的影响，为理性设计低能量损失的有机光伏材料提供指导。

减小激子分离的驱动力是降低能量损失提高有机太阳能电池光电转换效率的有效途径之一。传统认为，驱动力即有机给/受体间的能级差至少要大于0.3 eV。然而，最近实验发现即使能级差很小，激子也能有效分离，但其机制尚不清楚。本项目通过准确计算影响驱动力的重要参数，深入研究低驱动力激子解离机制。主要取得了以下进展：（1）发展并运用迭代自洽的量子力学/嵌入电荷方法计算固态极化效应，从而实现了在第一性原理水平上计算静电作用和诱导效应，且不依赖于任何经验参数，并可以考虑分子堆积结构的影响。（2）揭示低驱动力有机太阳能电池发生激子分离的机制。通过对系列非富勒烯体系计算发现，激子分离的驱动力与激子束缚能线性相关，为降低驱动力减小能量损失指明了方向。（3）运用可靠方法研究了非富勒烯晶体中分子间不同排布方式对激子束缚能的影响，实现晶体分子排列结构对激子束缚能的调控。并阐明了电子极化和离域效应、薄膜中结构无序对激子束缚能的影响。（4）建立分子内重组能与激子解离及能量损失的关系。低重组能有利于促进激子扩散和电荷传输，抑制由于激子驰豫、电荷转移态和双分子复合带来的能量损失。我们的研究为理性设计低能量损失的有机光伏材料提供指导。

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