A. Kay et al., ARTIFICIAL PHOTOSYNTHESIS .2. INVESTIGATIONS ON THE MECHANISM OF PHOTOSENSITIZATION OF NANOCRYSTALLINE TIO2 SOLAR-CELLS BY CHLOROPHYLL DERIVATIVES, Journal of physical chemistry, 98(3), 1994, pp. 952-959
The mechanism of photosensitization of colloidal TiO2 electrodes with
chlorin e(6) and copper chlorophyllin is deduced from static and time-
resolved fluorescence quenching as well as laser flash photolysis and
photocurrent/voltage transients in combination with cyclic voltammetry
and spectroelectrochemistry. The fluorescence of chlorin e(6) on TiO2
decays to 80% within 0.4 ns, indicating efficient electron injection
from the singlet excited state with k(et) = 2.2 X 10(9) s-(1). Copper
chlorophyllin emits only from the triplet state, due to subpicosecond
intersystem crossing in the presence of the paramagnetic CU2+ center.
Its phosphorescence is strongly enhanced by immobilization on a nonque
nching ZrO2 reference adsorbent, but quenched on TiO2, indicating elec
tron transfer from the triplet state with k(et) = 3 X 10(8) s(-1). The
energy levels of the excited photosensitizers and the acceptor state
density of TiO2 are determined by cyclic voltammetry. A strong tail of
shallow surface states on the colloidal TiO2 electrodes modifies the
classical picture of a conduction band edge. Transient absorption spec
tra of the electrodes show three equivalent contributions due to dye b
leaching, cation radical formation, and conduction band electrons, in
accordance with electron injection from the neutral excited dye. The d
ecay of these species by recombination is followed over 6 time decades
in the absence and presence of iodide as reducing agent. The recombin
ation kinetics as well as laser flash induced photocurrent and voltage
transients reveal the importance of surface states acting as shallow
traps for the injected electrons.