A new Förster resonance energy transfer (FRET) concept for a multichromophoric organic sensitizer in a dye-sensitized solar cell is presented based on a phenyl base body that accommodates the separately linked donor and acceptor moieties. The whole assembly is attached to the surface of a ZnO nanorod electrode via a carboxylic anchor group. FRET activity was demonstrated with UV-VIS measurements, and the charge separation dynamics at the inorganic/organic interface were analyzed with fs transient absorption and terahertz pump/probe for the precursors and fully assembled FRET units.
The temporal relaxation of optically excited electrons at the In-rich reconstructed InP(100) surface was studied using
time-resolved two-photon-photoemission spectroscopy (TR-2PPE). High-energy carriers were generated at laser pump
energies chosen to populate hot electron bulk states or the well known C<sub>2</sub> surface state via resonant direct optical
excitation. The different relaxation pathways arising from these population schemes involve Γ-L-Γ intervalley scattering
and the transient occupation of an additional surface state, C<sub>1</sub>. The dynamics of these processes were recorded with a
novel experimental setup using two ultra-low power 150 KHz repetition rate sub-20 fs NOPAs enabling two-colour
pump-probe experiments in the linear regime. These experiments provide useful information in understanding the
dynamics of devices on the basis of this semiconductor medium such as solar cells and high-speed switching circuits.
Hot electron injection from the aromatic chromophore perylene into TiO<sub>2</sub> was measured with transient absorption signals for different rigid anchor-cum-spacer groups revealing 15 fs as the shortest and 4 ps as the longest injection time. The energetic position of the donor orbital of the chromophore with respect to the conduction band edge was determined at about 0.8 eV employing ultraviolet photoelectron spectroscopy (UPS)and simple absorption
spectroscopy.It is not clear in the case of rutile or anatase TiO<sub>2</sub> whether unoccupied surface states are involved
in the electron injection process as acceptor states. Since the surface reconstruction of TiO<sub>2</sub> is difficult to control
electron scattering between a well-defined surface state and isoenergetic unoccupied bulk states was studied with InP(100). Electron scattering was time-resolved employing two-color two-photon-photoemission (2PPE). Scattering from isoenergetic bulk states to the empty C<sub>1</sub> surface state was found to occur with a 35 fs time
constant, and the reverse process showed a time constant in the range of 200 fs. The latter was controlled by energy relaxation in bulk states, i.e. via the emission of longitudinal optical phonons in InP. In general, the injection of a hot electron from a molecular donor into electronic states of a semiconductor as to be distinguished
from consecutive electron scattering processes between surface states and bulk states. Distinguishing between the different processes may become difficult, however,if the electronic interaction becomes large for a small chromophore directly attached to the semiconductor.