Liu Xiang-Yang, Li Zi-Wen, Fang Wei-Hai, Cui Ganglong
College of Chemistry and Material Science, Sichuan Normal University, Chengdu 610068, P. R. China.
Key Laboratory of Theoretical and Computational Photochemistry, Ministry of Education, College of Chemistry, Beijing Normal University, Beijing 100875, P. R. China.
J Phys Chem A. 2020 Sep 17;124(37):7388-7398. doi: 10.1021/acs.jpca.0c05865. Epub 2020 Sep 8.
Interface orientation between zinc phthalocyanine (ZnPc) and fullerene (C) affects their interfacial charge separation dynamics; however, the underlying physical origin is still elusive. In this work, we have employed the time-dependent density functional theory (TDDFT) method to explore excited-state properties of ZnPc and C heterojunctions with both face-on and edge-on configurations. Spectroscopically bright absorption is from locally excited (LE) singlet excitons within ZnPc. In the face-on configuration, LE excitons are much higher in energy than charge-transfer (CT) excitons, thereby making charge separation process favorable. However, in the edge-on configuration, LE excitons are the lowest ones and CT ones are higher in energy; thus, charge separation is not efficient. Subsequently, we have carried out TDDFT-based nonadiabatic dynamics method to simulate photoinduced exciton and charge separation dynamics of ZnPc and C heterojunctions with both edge-on and face-on configurations. In the former, there are no exciton transfer and charge separation processes observed within 300 fs simulation time; while, in the latter, fragment-based electronic transition density matrix analysis reveals that only LE excitons |CZnPC*⟩ and CT excitons |CZnPC⟩ are involved. The exciton transfer from |CZnPC*⟩ to |CZnPC⟩ is completed within about 100 fs in which charge separation takes place with electron-hole distances increasing from 1.0 to 4.5 Å. This exciton transfer process is essentially in company with electron transfer from ZnPc to C but almost not involving hole transfer. These gained insights not only rationalize experiments but also enrich our knowledge to design donor-acceptor orientations to optimize organic photovoltaic performance.
酞菁锌（ZnPc）与富勒烯（C）之间的界面取向会影响它们的界面电荷分离动力学；然而，其潜在的物理根源仍然难以捉摸。在这项工作中，我们采用了含时密度泛函理论（TDDFT）方法来探究具有面对面和边缘对边缘构型的ZnPc与C异质结的激发态性质。光谱上明亮的吸收来自ZnPc内的局域激发（LE）单重态激子。在面对面构型中，LE激子的能量比电荷转移（CT）激子高得多，从而使电荷分离过程变得有利。然而，在边缘对边缘构型中，LE激子能量最低，CT激子能量较高；因此，电荷分离效率不高。随后，我们进行了基于TDDFT的非绝热动力学方法，以模拟具有边缘对边缘和面对面构型的ZnPc与C异质结的光致激子和电荷分离动力学。在前者中，在300 fs的模拟时间内未观察到激子转移和电荷分离过程；而在后者中，基于片段的电子跃迁密度矩阵分析表明，仅涉及LE激子|CZnPC*⟩和CT激子|CZnPC⟩。从|CZnPC*⟩到|CZnPC⟩的激子转移在大约100 fs内完成，在此过程中发生电荷分离，电子 - 空穴距离从1.0 Å增加到4.5 Å。这种激子转移过程本质上伴随着电子从ZnPc转移到C，但几乎不涉及空穴转移。这些见解不仅解释了实验结果，还丰富了我们关于设计供体 - 受体取向以优化有机光伏性能的知识。
