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有机半导体材料的高激子束缚能限制了电荷分离效率, 研究聚合物给体和非富勒烯受体分子激发态特性对优化材料设计和提升有机光伏器件性能至关重要. 本文通过紧束缚量子模型, 对比两类材料的激发态性质, 发现非富勒烯比聚合物的晶格畸变更小, 带隙更窄, 束缚能更低, 并可以通过降低电子-晶格耦合强度来进一步减小聚合物和非富勒烯分子晶格畸变、带隙及束缚能. 除此之外, 还发现通过增强非富勒烯分子中间基团的给电子能力或端基吸电子能力, 可以优化能级结构, 进一步降低束缚能, 实现有效的电荷分离. 结果表明, 聚合物/非富勒烯有机光伏体系有效的电荷转移与分离源于其分子激发态特性的差异, 通过协同调控电子-晶格耦合作用和非富勒烯分子推拉式电子结构, 可设计高性能的有机光电材料并开发新型非富勒烯有机光伏器件.The high exciton binding energy of organic semiconductor materials limits charge separation efficiency. Investigating the excited state characteristics and modulation mechanisms of polymer donor and non-fullerene acceptor molecules is crucial for optimizing material design and enhancing the performance of organic photovoltaic devices. Therefore, this study investigates the excited state characteristics in polymer and non-fullerene organic materials. The tight-binding quantum mechanical method is used to systematically compare the excited state characteristics (including lattice geometry, band structure, and binding energy) between polymer donor and non-fullerene acceptor molecules, with particular emphasis on the role of electron-phonon coupling in modulating these excited state characteristics. The results indicate that non-fullerene acceptor molecules exhibit smaller lattice distortion, narrower bandgap, and lower binding energy than polymer donor molecules. It is precisely due to the different excited state characteristics of the polymer donor and non-fullerene acceptor molecules that the exciton binding energy in the organic photovoltaic system they constitute can be effectively reduced, while also providing a favorable energy-level shift for exciton dissociation. This significantly enhances the efficiency of charge transfer and separation. Furthermore, the decrease of electron-lattice coupling strength can further reduce these parameters in both polymer donor and non-fullerene acceptor molecules. By enhancing the electron-donating capability of central groups or the electron-withdrawing capacity of end groups in non-fullerene acceptor molecules, the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) energy levels can be shifted upward or downward. The upshifted HOMO and LUMO energy levels are accompanied by an increase in molecular polarizability and a decrease in reorganization energy, while the downshifted HOMO and LUMO energy levels lead to an enhanced molecular dipole moment and improved electron affinity. This optimized energy-level structure further reduces the binding energy and achieves efficient charge separation. These findings demonstrate that the efficient charge transfer and separation in polymer/non-fullerene organic photovoltaic systems originate from their distinct molecular excited state characteristics. This basic understanding enables the rational design of high-performance organic optoelectronic materials and the development of novel organic photovoltaic devices by strategically adjusting the electron-phonon coupling strength and push-pull electronic structures of non-fullerene acceptors.
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Keywords:
- polymer /
- non-fullerene /
- organic photovoltaic /
- excited state
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图 1 (a) 聚合物(P3HT)和非富勒烯(Y6)的化学式; (b) 聚合物和非富勒烯简化的分子模型
Fig. 1. (a) Chemical structures of the polymer donor (P3HT) and the non-fullerene acceptor (Y6); (b) simplified molecular models of the polymer donor and the non-fullerene acceptor.
图 2 不同电子-晶格相互作用常数时, 聚合物和非富勒烯分子激发态的晶格位形 (a) $ \alpha {\text{ = 3}}{\text{.9 eV/{\AA}}} $; (b) $ \alpha {\text{ = 4}}{\text{.1 eV/{\AA} }} $; (c) $ \alpha {\text{ = 4}}{\text{.3 eV/{\AA} }} $
Fig. 2. Lattice configurations of excited states in the polymer and non-fullerene molecule for different electron-lattice interaction constant: (a) $ \alpha {\text{ = 3}}{\text{.9 eV/{\AA}}} $; (b) $ \alpha {\text{ = 4}}{\text{.1 eV/{\AA} }} $; (c) $ \alpha {\text{ = 4}}{\text{.3 eV/{\AA} }} $.
图 3 聚合物和非富勒烯分子激发态的带隙(Eg)随电子-晶格常数(α)的变化
Fig. 3. The bandgap (Eg) of excited states in the polymer and non-fullerene molecule as a function of the electron-lattice constant (α).
图 4 聚合物和非富勒烯分子激发态的束缚能(EB)随电子-晶格常数(α)的变化
Fig. 4. The binding energy (EB) of excited states in the polymer and non-fullerene molecule as a function of the electron-lattice constant (α).
图 5 不同电子-晶格相互作用常数时, 非富勒烯分子的HOMO和LUMO能级随中间基团给电子能力($ {\Delta _{{\text{on}}}} $)的变化 (a) $ \alpha {\text{ = 3}}{\text{.9 eV/{\AA}}} $; (b) $ \alpha {\text{ = 4}}{\text{.1 eV/{\AA} }} $; (c) $ \alpha {\text{ = 4}}{\text{.3 eV/{\AA} }} $
Fig. 5. HOMO and LUMO energy levels of the non-fullerene molecule as a function of the electron-donating ability of the central group ($ {\Delta _{{\text{on}}}} $) for different electron-lattice interaction constant: (a) $ \alpha {\text{ = 3}}{\text{.9 eV/{\AA}}} $; (b) $ \alpha {\text{ = 4}}{\text{.1 eV/{\AA} }} $; (c) $ \alpha {\text{ = 4}}{\text{.3 eV/{\AA} }} $.
图 6 不同电子-晶格相互作用常数时, 非富勒烯分子的HOMO和LUMO能级随端基吸电子能力($ {\Delta '_{{\text{on}}}} $)的变化 (a) $ \alpha {\text{ = 3}}{\text{.9 eV/{\AA}}} $; (b) $ \alpha {\text{ = 4}}{\text{.1 eV/{\AA} }} $; (c) $ \alpha {\text{ = 4}}{\text{.3 eV/{\AA} }} $
Fig. 6. HOMO and LUMO energy levels of the non-fullerene molecule as a function of the electron-withdrawing ability of the end group ($ {\Delta '_{{\text{on}}}} $) for differnet electron-lattice interaction constant: (a) $ \alpha {\text{ = 3}}{\text{.9 eV/{\AA}}} $; (b) $ \alpha {\text{ = 4}}{\text{.1 eV/{\AA} }} $; (c) $ \alpha {\text{ = 4}}{\text{.3 eV/{\AA} }} $.
图 7 电子-晶格相互作用常数分别为$ \alpha {\text{ = 3}}{\text{.9 eV/{\AA}}} $、$ \alpha {\text{ = 4}}{\text{.1 eV/{\AA} }} $和 $ \alpha {\text{ = 4}}{\text{.3 eV/{\AA} }} $时, 非富勒烯分子激发态的束缚能(EB)随(a)中间基团给电子能力($ {\Delta _{{\text{on}}}} $)和(b)端基吸电子能力($ {\Delta '_{{\text{on}}}} $)的变化
Fig. 7. Binding energy (EB) of excited states in the non-fullerene molecule as a function of (a) the electron-donating ability of the central group ($ {\Delta _{{\text{on}}}} $) and (b) the electron-withdrawing ability of the end group ($ {\Delta '_{{\text{on}}}} $) when the electron-lattice interaction constants is $ \alpha {\text{ = 3}}{\text{.9 eV/{\AA}}} $, $ \alpha {\text{ = 4}}{\text{.1 eV/{\AA} }} $, and $ \alpha {\text{ = 4}}{\text{.3 eV/{\AA} }} $, respectively.
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