Thermodynamics and physics of photoconversion: a review and a look at photosynthesis

A number of devices have been constructed which directly convert light into electrical work or into a flow of chemical products with a free energy higher than the starting materials. Solar Concentrators have been developed which increase the irradiance on the converter material to levels above that used without the device even if the incident light is diffuse. In contrast to man made converter systems, natural photosynthesis seems, at first, to rely on entirely different conversion mechanisms and physics. In this paper is presented a unification of the physics and the thermodynamics of all quantum solar converters. It will be shown that the Planck equation may describe the upper limit to the performance of systems as diverse as the Fluorescent Solar Concentrator, the novel Ruthenium dye sensitized TiO2 photoelectrochemical cells of Dr. Graetzel, silicon solar cells, Chlorophyll, and, perhaps, the unique `protonic' solar conversion found in the purple membrane of Halobacterium halobium.