ARTIFICIAL PHOTOSYNTHESIS .2. INVESTIGATIONS ON THE MECHANISM OF PHOTOSENSITIZATION OF NANOCRYSTALLINE TIO2 SOLAR-CELLS BY CHLOROPHYLL DERIVATIVES

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 nonquenching ZrO2 reference adsorbent, but quenched on TiO2, indicating electron 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 spectra of the electrodes show three equivalent contributions due to dye bleaching, cation radical formation, and conduction band electrons, in accordance with electron injection from the neutral excited dye. The decay of these species by recombination is followed over 6 time decades in the absence and presence of iodide as reducing agent. The recombination 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.