Shedding light on solar cell technology

It's clear that solar energy harvesting is poised to go mainstream, but it's not clear which of the many diverse technologies vying for dominance in this emerging market will take precedence. From bulk and thin-film silicon to more exotic compound materials such as III-Vs and new printable organics, the designer has many choices but, as we explain here, each has its own pride of place on the totem pole of conversion efficiency, availability, reliability and cost.

People in the semiconductor industry use the term photovoltaics, or PV, interchangeably with solar power. PV refers to a class of power-generation devices that convert light directly into electrical current. Photons (preferably from the sun, given our hope for renewable energy) are converted into electron-hole pairs that are subsequently extracted into useful electrical current. The non-semiconductor part of the solar power field is solar thermal-power generation, where radiant energy from the sun produces steam that subsequently drives a turbine similar to the turbines used in conventional coal or nuclear power plants. PV is actually a subset--albeit it probably the larger portion--of the more general field of solar power generation.

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PV is categorized broadly into four generations:

» First-generation solar cells are similar to the earliest practical silicon cells designed in the 1950s on bulk silicon wafers.

» Second-generation devices are the thin-film type. Thin is a plus because less material means lower cost. The smaller volume also means there's less chance of losing an electron because of impurities, so you can use a lower-grade material.

» Third-generation solar cells, which are just beginning to find their way into pilot production, go beyond conventional junction semiconductors to include photoelectrochemical cells, polymers and dye-sensitized cells.

» Fourth-generation devices include future technologies, such as quantum dots and nanowires. Multi-quantum well (MQW) cells are fourth-generation devices, but they're closer to production, thanks to Imperial College spinoff Quantasol (U.K.).

Solar thermal plants depend on mirrors or lenses to focus sunlight to achieve higher temperatures in a working fluid that will eventually boil water to drive a turbine. Higher temperatures mean higher system efficiencies. Sunlight concentrators are also used for photovoltaic systems, forming concentrated PV (CPV). The idea behind CPV is to use higher-efficiency cells and focus a larger area of incident sunlight onto the smaller cells. The exotic materials used in CPV cells are more expensive than alternatives. Because a very small area of cell is used, CPV systems often stack several different PV materials together into a multijunction cell to capture more of the incident wavelengths and therefore extract more energy from the sunlight.

The focusing optics are inexpensive compared with the cell material and concentrate solar energy from a large surface area. Most CPV systems can follow the sun during the day to keep incident light from striking the PV cell at the steepest possible angle. These systems increase overall system cost because mechanical actuators and associated sun-tracking and control electronics are required to keep the CPV pointed directly at the sun throughout daylight hours. However, these costs are at least partially offset by CPV's higher power-output density.

A subcategory of CPV systems that is starting to make headlines is based on light-guiding rather than light-focusing optics. MIT startup Covalent Solar and Morgan Solar from Canada are developing systems that channel higher-energy, shorter-wavelength light to the edge of the panel, where an exotic, high-efficiency multijunction cell is placed. The overall efficiency of these systems is relatively high because they extract energy from a wide spectrum of incident sunlight. While the shorter, high-energy wavelengths are guided to the high-efficiency cell at the edge, longer wavelengths pass directly through the panel. Solar cells produced from less costly materials cover the larger area directly below the panel plate to extract the unguided long-wavelength radiation. Covalent's technology is based on luminescent dye coatings, while Morgan Solar uses glass or acrylic waveguide plates.