1. Field of the Invention
The present invention relates to application of lignin derivatives obtained by reactions of phenol compounds with lignin. More specifically the invention pertains to application of lignin derivatives to a semiconductor thin film electrode, to a photoelectric transducer, and to a solar cell.
2. Description of the Related Art
Solar cells are widely used as photoelectric cells for photoelectric energy conversion. The most common structure of the solar cell has the pn junction in the vicinity of the surface of a semiconductor crystal (for example, silicon) or amorphous plate. The electric current runs in an external circuit from the p-type semiconductor to the n-type semiconductor by radiation of visible rays. Such solar cells consume large energy and chemicals, such as harmful heavy metals, for their production. These solar cells accordingly have the energy and environment-related problems, as well as the rather high manufacturing cost.
Dye-sensitized solar cells have recently drawn attention, because of their less environmental load, lower manufacturing cost, and high energy conversion efficiency. Gratzel et al. developed an electrode of a porous titanium oxide thin film sensitized with ruthenium-bipyridine-dicarboxylate to attain the photoelectric conversion efficiency of 10.0%. Ruthenium complexes, other noble metal complexes, chlorophyll derivatives, and zinc porphyrin complexes have been proposed as the excitation center of these dye-sensitized solar cells.
Ruthenium and other noble metals as the excitation center are, however, subject to resource limitation. The materials of synthesizing the dye ligands are mainly derived from the oil resources and other chemical resources and are thus expected to have future resource constraints.
There are solar cells sensitized with only organic material, in addition to the conventional silicon solar cells and the organic metal complex-sensitized solar cells. One example of the proposed prganic material-sensitized or organic dye-sensitized solar cells includes an oxide semiconductor electrode sensitized by coumarin dye (see Chem. Commun., 2001, 6, 569-570). This organic dye-sensitized solar cell has the high photoelectric conversion efficiency of 6.0%. The organic dye-sensitized solar cells have advantages of the performances, the environmental load, and the manufacturing cost. Like the other proposed solar cells, however, the materials of synthesizing the organic dyes are also derived from the oil resources and other chemical resources and are thus expected to have further resource constraints.