HOPV12 - Blogs

4 th Hybrid and Organic Photovoltaic Conference -Uppsala 2012 70
B29 - Dynamics of Charge-Transfer Interfacial Excitons at Dye-Sensitized Donor/
Acceptor Hybrid Heterojunction
Jan C. Brauer, Arianna Marchioro,Jacques-E. Moser
Photochemical Dynamics Group, Ecole Polytechnique Fédérale de Lausanne, EPFL SB ISIC GR-MO, Station 6, CH-1015 Lausanne,
Switzerland
A combination of ultrafast laser transient absorption and optical pump-THz probe (OPTP)
spectroscopies was applied to scrutinize the details of interfacial photoinduced charge
separation in the dye-sensitization of wide bandgap oxides. When a suitable dye-sensitizer (S),
such as Ru II (H2dcbpy)(dnbpy)(NCS)2 (Z907), is adsorbed onto the surface of nanocrystalline
TiO2, optical absorbance of the dye cation (S + ) is readily observed upon photo-excitation of the
sensitizer and fully develops within < 20 fs. Provided all dye molecules are strongly achored on
the surface and no dye aggregates are formed, kinetics of the fs electron injection into the
conduction band of the solid appears to be independent of the medium (solvent, electrolyte,
solid hole conductor material, ...).
The charge injection process was monitored in identical samples by OPTP spectroscopy with
sub-ps time resolution. In the presence of various electrolytes, a multiphasic behaviour was
observed, where ~ 25 % of the total number of injected electrons contributed immediately to
the THz conduction, while the mobility of the remaining charges increased slowly with a halfprocess
time of ~ 100 ps. In a pure solvent, the slower kinetic part was missing, yielding a 4fold
abatement of the final conduction of photo-injected electrons. These results lead to the
conclusion that Coulomb attraction between injected electrons and positive charges left on
dye molecules must give rise to charge-transfer (CT) interfacial excitons. Such bound electronhole
pairs formed across the interface prevent a majority of injected charges to contribute to
the THz conduction. In the presence of an electrolyte, however, the mobility of conductionband
electrons in TiO2 nanoparticles increases markedly when anions are adsorbed onto the
surface, screening the Coulomb interaction and decreasing the exciton binding energy.
Figure 1 Formation and dissociation of a charge transfer interfacial exciton at the organic donor (D) | dye-sensitizer
(S) | metal oxide acceptor (A) heterojunction.
The system above involves the simplest type of CT interfacial exciton, namely, a free
electron in the semiconductor interacting with a localized hole in the sensitizer cation. This
picture is further complicated when the hole is injected into an organic donor material (D).
Similar to the effect of the electrolyte, filling the pores of a nanostructured TiO2 film with the
organic hole-conducting material spiro-OMeTAD gave rise to an additional slow component in
© SEFIN 2012