Self-aggregates of synthetic zinc chlorins as the photosensitizer on carbon paste electrodes for a novel solar cell

To investigate a dye-based solar cell, a naturally occurring chlorophyll analogue (chlorin 1) modified electrode was prepared. Synthetic zinc chlorin 1 easily self-aggregated to form supramolecules (1)n in non-polar organic solvents as well as in the thin film. The photocurrent of the electrode modified with self-aggregated chlorin (1)n (CE1) was measured in the range from −300 to 300 mV (vs. Ag ∣ Ag+) by irradiation with >510 nm light. Below 100 mV of the bias potential, cathodic photocurrents of CE1 were observed in an aqueous solution of 0.1 M 2-(N-morpholino)ethanesulfonic acid+0.1 M Na2SO4 at pH 6.7. The photocurrent action spectrum of CE1 was similar to the visible absorption spectrum of (1)n on a Pyrex plate, indicating that the photoexcited state of zinc chlorin aggregates (1)n* played a role as a photosensitizer on CE1 with a wide band of longer wavelength light. An oxygen molecule dissolved in an aqueous solution was an electron acceptor; electron transfer from (1)n* to O2 at the interface between the carbon paste electrode and the aqueous solution yielded the cathodic photocurrent. The quantum yield for CE1 was estimated to be 0.09% (730 nm). The value was larger than the quantum yield for the carbon paste electrode modified with monomeric zinc chlorin 3 (CE3), 0.01% (660 nm). These results indicate that self-aggregation of zinc chlorin (1)n made an efficient photosensitizer of solar cells and utilized up to 800 nm light.