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.
Femtosecond time-resolved two-photon photoemission spectroscopy is utilized to determine the electronically excited states dynamics at the α-sexithiophene (6T)/Au(111) interface and within the 6T film. We found that a photoinduced transition between the highest occupied molecular orbital and lowest unoccupied molecular orbital is essential in order to observe exciton population, which occurs within 100 fs. In thin 6T films, the exciton exhibits a lifetime of 650 fs. On a time scale of 400 fs, an energetic stabilization is observed leading to the formation of a polaron or electron trapping at defect states. The lifetime of this state is 6.3 ps. Coverage-dependent measurements show that apart from the excited state decay within the film, a substrate-mediated relaxation channel is operative. The present study demonstrates that two-photon photoemission spectroscopy is a powerful tool to investigate the whole life cycle from creation to decay of excitons in an organic semiconductor.
Dicyanovinyl (DCV)-substituted oligothiophenes 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-Me2). We ascribe this behavior to weaker intermolecular couplings in the DCV6T film. Despite the faster exciton decay, DCV5T-Me2 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-Me2 molecular structures.
By combining complementary optical techniques, photoluminescence and time-resolved excited state absorption, we achieve a comprehensive picture of the relaxation processes in the organic/inorganic hybrid system SP6/ZnO. We identify two long-lived excited states of the organic molecules of which only the lowest energy one, localized on the sexiphenyl backbone of the molecule, is found to efficiently charge separate to the ZnO conduction band or radiatively recombine. The other state, most likely localized on the spiro-linked biphenyl, relaxes only by intersystem crossing to a long-lived, probably triplet state, thus acting as a sink of the excitation and limiting the charge separation efficiency.An efficient light harvesting device relies on high carrier mobilities, low charge injection and ejection barriers and strong light-matter interaction. Inorganic semiconductors, on which current technology is predominantly based, fulfill the first two requirements (1) but their optical bandgap limits the amount of energy that can be converted (2). Strong light-matter coupling, instead, occurs in organic semiconductors and chemical design allows for flexible adjustment of absorption and emission spectra (3). Unfortunately, organic-based devices often suffer from low mobilities (4,5) and charge recombination can occur before extraction. The combination of inorganic and organic semiconductors into hybrid structures promises to lead to a new generation of devices that exploit the advantages of both material classes to increase light conversion efficiency (6,7). Efficient hybrid devices are based on a careful choice of the two materials; the energy level alignment at the interface determines the occurrence of charge and energy transfer processes. Furthermore, the device performance is affected by the relative probability of photoinduced energy relaxation processes that, by competing with the desired energy or charge transfer process at the hybrid interface, lead to an overall loss of harvested energy. In the molecular film, for example, these processes include a) intramolecular vibrational relaxation (IVR), b) internal conversion (IC), c) (non-) radiative recombination to the electronic ground state, d) triplet formation via intersystem crossing (ISC) or e) separation into a charge transfer state, among others. By reducing the exciton lifetime, these mechanisms shorten the exciton diffusion length and lower the probability for charge and energy transfer to occur. Similarly, the diffusion is likely to be less efficient in the case of strongly localized long-lived excited states such as charge transfer and triplet excitons. The relevance of a given relaxation pathway with respect to the others depends on the relative rate of the processes, which is in turn affected by the energetic separation of the involved energy levels. One prominent example is given by Kasha's spectroscopic rule stating that light emission always occurs from the lowest electronic excited state independent of the excitation density, since for all hi...
The adsorption behavior of α-octithiophene (8T) on the Au(111) surface as a function of 8T coverage has been studied with low-temperature scanning tunneling microscopy, high resolution electron energy loss spectroscopy as well as with angle-resolved two-photon photoemission and ultraviolet photoemission spectroscopy. In the sub-monolayer regime 8T adopts a flat-lying adsorption geometry. Upon reaching the monolayer coverage the orientation of 8T molecules changes towards a tilted configuration, with the long molecular axis parallel to the surface plane, facilitating attractive intermolecular π-π-interactions. The photoemission intensity from the highest occupied molecular orbitals (HOMO and HOMO - 1) possesses a strong dependence on the adsorption geometry due to the direction of the involved transition dipole moment for the respective photoemission process. The change in molecular orientation as a function of coverage in the first molecular layer mirrors the delicate balance between intermolecular and molecule/substrate interactions. Fine tuning of these interactions opens up the possibility to control the molecular structure and accordingly the desirable functionality.
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