ARTIFICIAL PHOTOSYNTHESIS .1. PHOTOSENSITIZATION OF TIO2 SOLAR-CELLS WITH CHLOROPHYLL DERIVATIVES AND RELATED NATURAL PORPHYRINS

Colloidal TiO2 electrodes were photosensitized with derivatives of chlorophyll and related natura porphyrins resulting in light harvesting and charge separation efficiencies comparable to those in natural photosynthesis. The photocurrent action spectra of the electrodes correlate well with the absorption spectra of the dyes in solution. Incident photon to current efficiencies up to 83% are reached in the Soret peak at 400 nm with a 12-mum-thick TiO2 film sensitized by copper mesoporphyrin IX, which corresponds to nearly unity quantum efficiency of charge separation when light reflection losses are taken into account. Photocurrent/voltage curves of TiO2 solar cells sensitized with copper chlorophyllin show an energy conversion efficiency of 10% for the red peak at 630 nm. Under simulated sunlight illumination, an open circuit photovoltage of 0.52 V and a short circuit current density of 9.4 mA/cm2 are measured. The overall energy conversion efficiency of the cell is 2.6% under these conditions, in part limited by ohmic losses at such high current densities. The comparison of different chlorophyll derivatives indicates that free carboxyl groups are important for adsorption and sensitization on TiO2. However, conjugation of the carboxyl groups with the pi electron system of the chromophore is not necessary for efficient electron transfer. Free bases, zinc, and even the nonfluorescent copper complexes of chlorophyllins and mesoporphyrin IX are efficient sensitizers for TiO2. Cholanic acids as coadsorbates were found to be unique in improving both photocurrent and voltage of copper chlorophyllin sensitized cells. This effect is discussed by comparison with other coadsorbates.