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
