Ishino Yuuta, Miyata Kiyoshi, Sugimoto Toshiki, Watanabe Kazuya, Matsumoto Yoshiyasu, Uemura Takafumi, Takeya Jun
Department of Chemistry, Graduate School of Science, Kyoto University, Kyoto 606-8502, Japan.
Phys Chem Chem Phys. 2014 Apr 28;16(16):7501-12. doi: 10.1039/c3cp54157f.
Ultrafast dynamics of excitons in organic semiconductors is essential for a deep understanding of the working mechanism of plastic opto-electronic devices. In this work, excited state dynamics in dinaphtho[2,3-b:2'3'-f]thieno[3,2-b]-thiophene thin films has been studied with femtosecond transient absorption and time-resolved photoluminescence spectroscopy. Upon the excitation with a femtosecond pulse at 400 nm, a broad positive absorption band at 1.5-2.4 eV is observed that contains two components: one decays with a time constant of a few ps and the other with 67 ± 7 ps. Because the decay curve of the latter coincides with that of photoluminescence, the slow decay component is ascribed to the lowest singlet exciton. The former fast decay component is ascribed to mixed states between charge transfer (CT) and Frenkel excitons, because it is accompanied by a feature due to the Stark effect caused by transient charged species: a combination of bleach and positive absorption at hνprobe > 2.4 eV which looks like derivative modulations of the ground state absorption spectrum. A pronounced polarization dependence is observed for the derivative-like features; this is due to anisotropic distributions of the dipole moments formed by the CT excitons. The derivative-like feature changes its shape after the decay of the mixed Frenkel-CT exciton and grows with a pump-probe delay time of up to 1 ns due to a thermal effect. The decay rate of the mixed Frenkel-CT exciton strongly depends on its density because of exciton-exciton annihilation at high density.
有机半导体中激子的超快动力学对于深入理解塑料光电器件的工作机制至关重要。在这项工作中，利用飞秒瞬态吸收和时间分辨光致发光光谱研究了二萘并[2,3 - b:2'3'- f]噻吩并[3,2 - b]噻吩薄膜中的激发态动力学。用400 nm的飞秒脉冲激发后，观察到在1.5 - 2.4 eV处有一个宽的正吸收带，它包含两个成分：一个以几皮秒的时间常数衰减，另一个以67±7皮秒衰减。因为后者的衰减曲线与光致发光的衰减曲线一致，所以慢衰减成分归因于最低单重态激子。前者的快速衰减成分归因于电荷转移（CT）激子和弗伦克尔激子之间的混合态，因为它伴随着由瞬态带电物种引起的斯塔克效应的一个特征：在hνprobe > 2.4 eV处的漂白和正吸收的组合，看起来像基态吸收光谱的导数调制。对于类似导数的特征观察到明显的偏振依赖性；这是由于CT激子形成的偶极矩的各向异性分布。在混合的弗伦克尔 - CT激子衰减后，类似导数的特征改变其形状，并由于热效应在高达1 ns的泵浦 - 探测延迟时间内增长。混合的弗伦克尔 - CT激子的衰减速率由于高密度下的激子 - 激子湮灭而强烈依赖于其密度。
