Leonard Ariel A, Mosquera Martín A, Jones Leighton O, Cai Zhengxu, Fauvell Thomas J, Kirschner Matthew S, Gosztola David J, Schatz George C, Schaller Richard D, Yu Luping, Chen Lin X
Department of Chemistry , Northwestern University , 2145 Sheridan Road , Evanston , Illinois 60208 , USA . Email:
Beijing Key Laboratory of Construction Tailorable Advanced Functional Materials and Green Applications , School of Materials Science & Engineering , Beijing Institute of Technology , Beijing 100081 , China.
Chem Sci. 2020 Jul 1;11(27):7133-7143. doi: 10.1039/d0sc02862b. eCollection 2020 Jul 21.
Perylenediimide (PDI) derivatives have been widely studied as electron acceptor alternatives to fullerenes in organic photovoltaics (OPVs) because of their tunable absorption in the visible range, inexpensive synthesis, and photochemical stability. A common motif for improving device efficiency involves joining multiple PDIs together through electron-rich linkers to form a twisted acceptor-donor-acceptor molecule. Molecular features such as ring fusion are further employed to modify the structure locally and in films. These synthetic efforts have greatly enhanced OPV device efficiencies, however it remains unclear how the increasingly elaborate structural modifications affect the photophysical processes integral to efficient photon-to-charge conversion. Here we carry out a systematic study of a series of PDI dimers with thienoacene linkers in which the twist angle, linker length, and degree of ring fusion are varied to investigate the effects of these structural features on the molecular excited states and exciton recombination dynamics. Spectroscopic characterization of the dimers suggest that ring fusion causes greater coupling between the donor and acceptor components and greatly enhances the lifetime of a thienoacene to PDI charge transfer state. The lifetime of this CT state also correlates well with the linker-PDI dihedral angle, with smaller dihedral angle resulting in longer lifetime. DFT and two-photon absorption TDDFT calculations were developed in-house to model the ground state and excited transitions, providing theoretical insight into the reasons for the observed photophysical properties and identifying the charge transfer state in the excited state absorption spectra. These results highlight how the longevity of the excited state species, important for the efficient conversion of excitons to free carriers in OPV devices, can be chemically tuned by controlling ring fusion and by using steric effects to control the relative orientations of the molecular fragments. The results provide a successful rationalization of the behavior of solar cells involving these acceptor molecules.
苝二酰亚胺（PDI）衍生物因其在可见光范围内可调谐的吸收、廉价的合成方法以及光化学稳定性，作为有机光伏（OPV）中富勒烯的电子受体替代物受到了广泛研究。提高器件效率的一个常见策略是通过富电子连接基团将多个PDI连接在一起，形成扭曲的受体-供体-受体分子。诸如环融合等分子特征进一步用于局部修饰结构和薄膜结构。这些合成努力极大地提高了OPV器件的效率，然而，目前尚不清楚日益精细的结构修饰如何影响高效光子到电荷转换所必需的光物理过程。在此，我们对一系列带有噻吩并苯连接基团的PDI二聚体进行了系统研究，其中改变了扭转角、连接基团长度和环融合程度，以研究这些结构特征对分子激发态和激子复合动力学的影响。二聚体的光谱表征表明，环融合导致供体和受体组分之间的耦合增强，并极大地延长了噻吩并苯到PDI电荷转移态的寿命。该电荷转移态的寿命也与连接基团-PDI二面角密切相关（二面角越小，寿命越长）。我们自行开发了密度泛函理论（DFT）和双光子吸收含时密度泛函理论（TDDFT）计算方法，以模拟基态和激发态跃迁，为观察到的光物理性质的原因提供理论见解，并确定激发态吸收光谱中的电荷转移态。这些结果突出了激发态物种的寿命对于OPV器件中激子有效转化为自由载流子的重要性，并且可以通过控制环融合以及利用空间效应控制分子片段的相对取向来进行化学调节。这些结果成功地解释了涉及这些受体分子的太阳能电池的行为。
