Recent increases in the cost of silicon have encouraged the development of alternative processes. Thin-film technologies have the potential for substantial cost advantages because of such factors as lower material use, fewer processing steps, and simpler manufacturing procedures for large-area modules. The most common materials used for thin films are amorphous silicon, cadmium telluride, and copper indium gallium diselenide and related alloys. Future directions for thin-film technologies include multi-junction assemblies aimed at significantly higher efficiencies, transparent and better-conducting oxide electrodes, and thin polycrystalline silicon films.

Another new technology, which takes advantage of photochemistry, is the dye-sensitized solar cell, in which organic dye molecules are adsorbed onto nanocrystalline titanium dioxide films (O’Regan and Grätzel, 1991). The dye molecules then absorb solar photons to create an excited molecular state that injects electrons into the film, the electrons are collected on a transparent electrode, and the dye is then reduced back to its initial state by accepting the electrons, which completes the circuit and generates electrical power in the external load. This type of solar cell is attractive because of its low cost and simplicity in manufacturing, but the device’s efficiency and stability will need to be closely monitored before large-scale deployment is possible.

In organic solar cells, which also are in the early developmental stage, the sunlight creates an exciton, which separates into an electron on one side and a hole on the other side of a material interface within the device. This allows the cell to be thinner, which significantly reduces cost in at least four ways: inexpensive constituent elements (which do not require pure silicon), decreased material use, modest conversion efficiency, and high-volume production techniques. Some examples of organic solar cells include quantum dots embedded in an organic polymer, liquid crystal cells, and small-molecule chromophore cells.

Nanotechnology too could become a useful tool for making PV cells because it can tune the optical and electronic properties of the PV materials by precisely controlling their particle sizes and shapes. Nanoparticles produced by chemical solution methods may streamline the manufacturing process, but their long-term stability must be tested.

Solar PV technologies are at various stages of development. Silicon flatplate PV cells are mature and are actively being deployed today. Reductions in the production costs of the cells and increases in efficiency and reliability will be needed, however, to make them more attractive to potential customers. Thin-film technologies, which have great potential to reduce module cost, are in a relatively



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