Electronic States and Exciton Dynamics in Dicyanovinyl-Sexithiophene on Au(111)
Electronic States and Exciton Dynamics in Dicyanovinyl-Sexithiophene on Au(111)
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DOI：
10.1021/acs.jpcc.6b07325
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发表时间：
2016-12-08
影响因子：
3.7
通讯作者：
Tegeder, Petra
中科院分区：
文献类型：
--
作者：
Bogner, Lea;Yang, Zechao;Tegeder, Petra
二氰基乙烯基 (DCV) 取代的低聚噻吩通常用作真空处理的小分子有机太阳能电池中的供体材料，其效率高达 10%。我们结合扫描隧道显微镜/光谱学和双光子光电子发射（2PPE），获得了吸附在 Au（111）表面上的 DCV-六噻吩（DCV6T）电子结构的完整图像。因此，我们表明输运能隙达到 1.4 eV。我们还确定了结合能为 0.6 eV 的激子态。使用飞秒时间分辨 2PPE，我们跟踪了不同分子层厚度下光激发电子态的动力学。在多层体系中，我们解决了激子态的衰变动力学，涉及从飞秒到几十皮秒的过程。激子态的衰减比 DCV-二甲基五噻吩 (DCV5T-Me-2) 慢得多。我们将此行为归因于 DCV6T 薄膜中较弱的分子间耦合。尽管激子衰减更快，但与 DCV6T 相比，DCV5T-Me-2 因太阳能电池效率更高而闻名。我们认为这是由于耦合良好的 DCV5T-Me-2 分子结构中伴随着更好的激子和载流子传输。
Dicyanovinyl (DCV)-substituted oligothio-phenes are often used as donor materials in vacuum-processed small-molecule organic solar cells, which exhibit promising efficiencies up to 10%. We combine scanning tunneling microscopy/spectroscopy and two-photon photoemission (2PPE) to obtain a complete picture of the electronic structure of DCV-sexithiophene (DCV6T) adsorbed on a Au(111) surface. We thus show that the transport gap amounts to 1.4 eV. We also identified an excitonic state possessing a binding energy of 0.6 eV. Using femtosecond time-resolved 2PPE, we followed the dynamics of optically excited electronic states at different molecular layer thicknesses. In the multilayer regime, we resolved the decay dynamics of excitonic states involving processes ranging from femtoseconds to several tens of picosseconds. The decay of the excitonic states is considerably slower than in DCV-dimethyl-pentathiophene (DCV5T-Me-2). We ascribe this behavior to weaker intermolecular couplings in the DCV6T film. Despite the faster exciton decay, DCV5T-Me-2 is known for a better solar cell efficiency compared to that of DCV6T. We suggest that this is due to the concomitant better exciton and charge carrier transport in well-coupled DCV5T-Me-2 molecular structures.