Nanocrystalline Thin-Film PV Cells
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MRS BULLETIN/OCTOBER 1993
tical implementation of such devices must overcome formidable obstacles if the goal is to develop molecular systems that convert sunlight to electricity with an efficiency comparable to that of silicon photovoltaic devices. We now will outline our approach in more detail, drawing attention to analogies of the nanocrystalline device with the harvesting and conversion of solar light energy by natural photosynthesis, i.e., the green leaf.
Nanocrystalline Photovoltaics and Artificial Photosynthesis Our newly developed nanocrystalline photovoltaic cell replicates the most important principles of its prototype, photosynthesis. In any case, the components of the synthetic system must be selected to satisfy the high stability requirements encountered in practical applications. A photovoltaic system must remain in service for 20 years without significant loss of performance. In living systems, this stability is less significant, since unstable components are continuously renewed. Because chlorophyll, and likewise the lipid membrane, are labile in vitro, they cannot be adopted directly. In artificial photosynthesis, chlorophyll is therefore substituted by a more stable sensitizer molecule (S). One of the most remarkable achievements of research in inorganic chemistry during the last two decades has been the development of a great variety of transition-metal complexes,1 mainly of the elements osmium and ruthenium, which are exceptionally stable and display good absorption in the visible range. We have submitted some of these sensitizers to long-time illuminations where they sustained as much as 107 redox cycles under light without noticable decomposition. The redox potentials of these complexes can be adjusted to the desired value by a suitable choice of the ligands and their substituents. The role of the sensitizer is the same as that of chlorophyll: It must absorb the incident sunlight and exploit the light energy to induce a vectorial electron transfer re-
action. In place of the biological lipid membrane, a titanium dioxide ceramic membrane is employed. Titanium dioxide is a semiconductor which does not absorb visible light because of its large (about 3 eV) bandgap. It is a harmless, environmentally friendly material, remarkable for its high stability. It occurs in nature as ilmenite, and is used in quantity as a white pigment and as an additive in toothpaste. World annual production is of the order of a million tons, at a price of about $l/kg. Since the membrane is about five microns thick, about 10 g of titanium dioxide is used per square meter of solar collector surface, representing an investment of only one cent per square meter. The role of the titanium dioxide film is to provide a support for the sensitizer, which must be applied to the surface of the membrane as a monomolecular layer. Furthermore, the conduction band of the titanium dioxide accepts the electrons from the electronically excited sensitizer. The electron injected into the conduction band travels rapidly across the membran
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