Artificial photosynthesis. 1. Photosensitization of titania solar cells with chlorophyll derivatives and related natural porphyrins

Abstract

Colloidal TiO[sub 2] electrodes were photosensitized with derivatives of chlorophyll and related natural 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-[mu]m-thick TiO[sub 2] 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 TiO[sub 2] 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/cm[sup 2] 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 TiO[sub 2]. However, conjugation of the carboxyl groups with the [pi] electron system of the chromophore is notmore » necessary for efficient electron transfer. Free bases, zinc, and even the nonfluorescent copper complexes of chlorophyllins and mesoporphyrin IX are efficient sensitizers for TiO[sub 2]. 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. 23 refs., 3 figs., 1 tab.« less