外电场调控共轭分子电子转移效率的机理研究

To investigate the influence of external electric field on electron transfer process of conjugated molecule systems is of both fundamental and practical significance in governing the power conversion efficiency of organic photo-voltaic (OPV) cells. However, it is important to recognize that how does the external electric field, generated by the electrodes, enable efficient dissociation of the photogenerated excitons into separate charges, is still subject remained unanswered. Based on our preliminary groundwork, this project is to theorize a novel diabatic Marcus-Levich-Jortner model for describing the electron transfer reaction of C60/C70-Polymer-Porphyrin and C60/C70oooPolymer under external electric field, by employing the quantum mechanics perturbation method, nonadiabatic transition theory and static electric field corrected Coulombic interaction. In an attempt to reveal the physical mechanism of the influence of external electric field on electron transfer rate of these conjugated systems, the external electric field dependent electric coupling, Gibbs free energy and reorganization energy have been investigated from the point view of π-electron redistribution, electron-hole coherence and energy band structure of electronic states, which are adjusted by the external electric field. It is expected that results of this project could give more rational explanations to current experimental findings on external electric field dependent exciton dissociation and charge recombination processes of these systems. Meanwhile, it will provide a valid theoretical route to promote theoretical analysis on the parameters and conditions for improving the power conversion efficiencies of OPV as well as will offer crucial hints for the rational designing of novel conjugated molecule systems of organic photo-active layer in BHJ solar cell.

探索外电场对共轭分子体系电子转移过程的调控机理，对提高有机光伏电池（OPV）光电转化效率的研究具有重要的现实意义。如何在外电场作用下，高效地把激子解离成自由电荷，是目前亟待解决的前沿课题之一。本项目拟在前期工作基础上，以不同共轭结构和耦合方式的富勒烯-噻吩聚合物为研究对象，采用量子力学微扰方法和非绝热跃迁理论，发展外电场下Marcus-Levich-Jortner电子转移非绝热理论模型，从外电场调节π电子云极化分布、电子相干性和电子态能级结构等角度，分析电子耦合、反应自由能和重整能对外电场的依赖关系，揭示外电场调控共轭分子电子转移效率的物理机制。期望通过本项目的研究对已有的实验结果给出更为合理的解释，为探索提高共轭分子电子转移效率的外部电场和内部结构等因素提供有效的理论途径，为设计与合成新型高效OPV光活性层共轭分子体系提供更充分的理论依据。

外部静电场(Fext)会明显改变共轭分子电子转移激发态的势能面结构，这导致有机光伏电池（OPV）光活性材料受光激发后的激子产生、激子解离和电荷复合等动力学过程都受到Fext的影响。因此，探索外电场调控共轭分子体系电子转移反应的机理，对提高OPV光电转化效率的研究具有重要的现实意义。. 本项目以提高共轭分子体系的光诱导激子解离效率为目标，采用量子力学方法和非绝热跃迁理论，通过偶极近似把静电偶极场作为微扰引入体系的Hamiltonian，同时考虑外电场对绝热态/非绝热态偶极矩的影响以及体系电子态之间跃迁能对外电场的依赖关系，建立了外电场依赖的电子耦合非绝热理论模型VDA(Fext)；通过把外电场的影响作为修正因子引入到自由电子和空穴之间的相互作用，同时考虑由外电场引起的反应物能量和产物能量的改变量，修正了计算外电场条件下反应自由能ΔG(Fext)和重整能λ(Fext)的解析表达式形式；在不同外电场条件下选取一个频率为ωeff的有效振动模式来代替相关的高频振动，进而确定了有效Huang-Rhys因子Seff=λi/hνeff。基于以上外电场依赖的参数表达式，发展了外电场条件下模拟电子转移过程中的激子解离和电荷复合的理论模型，为研究外电场对体系激子解离和电荷复合过程的影响提供了理论依据。选择具有不同共轭结构和耦合方式的D-B-A和D•••A型共轭分子体系（C60/C70-Polymer- Porphyrin 和C60/C70•••Polymer）为研究目标体系，通过分析外电场对体系电子转移反应前后π电子重新分布、电子相干性、电子态能级结构、反应能垒、激发态核构型变化等的调节，揭示了外电场调控共轭分子体系VDA(Fext)、ΔG(Fext)和λ(Fext)的物理机制，为更好地帮助实验工作者解释已有的实验现象提供了理论途径。通过考察这三个参数对体系光诱导激子解离和电荷复合速率的影响程度，确定了外电场调控体系电子转移效率的核心参数；进一步研究了外电场与体系空间对称性和共轭结构之间的依赖关系，探索了有效提高共轭分子体系激子解离和抑制电荷复合效率的外部电场和内部结构等因素，以及外电场对共轭分子电子转移效率的调控机理，为设计高效的OPV光活性材料提供了理论参考。