Bogner Lea, Yang Zechao, Corso Martina, Fitzner Roland, Bäuerle Peter, Franke Katharina J, Pascual José Ignacio, Tegeder Petra
Freie Universität Berlin, Fachbereich Physik, Arnimallee 14, D-14195 Berlin, Germany.
Phys Chem Chem Phys. 2015 Oct 28;17(40):27118-26. doi: 10.1039/c5cp04084a.
Dicyanovinyl (DCV)-substituted oligothiophenes are promising donor materials in vacuum-processed small-molecule organic solar cells. Here, we studied the structural and the electronic properties of DCV-dimethyl-pentathiophene (DCV5T-Me2) adsorbed on Au(111) from submonolayer to multilayer coverages. Using a multi-technique experimental approach (low-temperature scanning tunneling microscopy/spectroscopy (STM/STS), atomic force microscopy (AFM), and two-photon photoemission (2PPE) spectroscopy), we determined the energetic position of several affinity levels as well as ionization potentials originating from the lowest unoccupied molecular orbitals (LUMO) and the highest occupied molecular orbitals (HOMO), evidencing a transport gap of 1.4 eV. Proof of an excitonic state was found to be a spectroscopic feature located at 0.6 eV below the LUMO affinity level. With increasing coverage photoemission from excitonic states gains importance. We were able to track the dynamics of several electronically excited states of multilayers by means of femtosecond time-resolved 2PPE. We resolved an intriguing relaxation dynamics involving four processes, ranging from sub-picosecond (ps) to several hundred ps time spans. These show a tendency to increase with increasing coverage. The present study provides important parameters such as energetic positions of transport levels as well as lifetimes of electronically excited states, which are essential for designing organic-molecule-based optoelectronic devices.
二氰基乙烯基（DCV）取代的寡聚噻吩是真空处理的小分子有机太阳能电池中有前景的供体材料。在此，我们研究了从亚单层到多层覆盖度吸附在Au(111)上的DCV - 二甲基 - 五噻吩（DCV5T - Me2）的结构和电子性质。使用多技术实验方法（低温扫描隧道显微镜/光谱（STM/STS）、原子力显微镜（AFM）和双光子光电子能谱（2PPE）），我们确定了几个亲和能级的能量位置以及源自最低未占据分子轨道（LUMO）和最高占据分子轨道（HOMO）的电离势，证明了1.4 eV的传输能隙。发现激子态的证据是位于LUMO亲和能级以下0.6 eV处的一个光谱特征。随着覆盖度增加，来自激子态的光发射变得更加重要。我们能够通过飞秒时间分辨2PPE追踪多层膜中几个电子激发态的动力学。我们解析了一个涉及四个过程的有趣弛豫动力学，时间跨度从亚皮秒（ps）到几百皮秒。这些显示出随着覆盖度增加而增加的趋势。本研究提供了诸如传输能级的能量位置以及电子激发态寿命等重要参数，这些对于设计基于有机分子的光电器件至关重要。
