The evolution of PV technology

31 March 2011

PV technology has not just evolved but multiplied into several forms. The listing of technology types below can also be seen as a general technical progression, with more recent developments generally further down the list. Improvements in each technology involve both efficiencies of light capture and conversion, and efficiencies of manufacture – with the attendant implications for the cost of power they produce. Even with the classical silicon-based forms, the efficiency envelope is still being pushed upward, toward the theoretical limit of 29% for all forms of silicon. Meanwhile, the more recent types including those using more recently discovered hybrid materials, can be expected to see significant improvement in both areas and to contribute to the growing contribution of PV to supply.
• Single-crystal (monocrystalline) silicon. The first type broadly manufactured, it is commonly made from thin wafers of silicon, sliced out of a cylinder drawn slowly out of a molten pool of the element (the Czochralski method) in order to allow the atoms to settle into a single crystal. The most efficient of the silicon types (conversion efficiencies reported in the range of 30%, it saw early use in space applications, due to the initially high cost of manufacture where cost was not a factor. Cost has now declined to the point where it is used almost as widely as multicrystalline forms.
• Multicrystalline silicon. Cheaper to manufacture, but less efficient at turning light into electricity, due to resistance at crystal boundaries, among other things. Can be made either from cast blocks of multicrystalline silicon, or less efficient but cheaper still, drawn as a continuous ribbon from a melt.
• Amorphous or microcrystalline silicon. As the amorphous structure has a higher absorption rate of light than crystalline cells, the complete light spectrum can be absorbed with a very thin layer of photo-electrically active material.
• Thin film. A process that deposits any of a range of photovoltaic materials onto a substrate that serves both as material support and conductor. Materials include amorphous silicon, cadmium telluride, copper indium gallium selenide, and various dye-sensitized materials.
• Cells based on organic polymeric materials. These are especially cheap to manufacture; on the other hand conversion efficiencies are low. This technology is in the early stages of development, and much progress can be expected.
• Multi-junction cells have, typically, two or three layers of different material, each able to take advantage of a different part of the light spectrum. These employ relatively exotic materials with very precise formulations like indium gallium phosphide, gallium arsenide, and so on. Each layer, laid down again generally by some form of vapour deposition, is obviously thin enough to allow light to pass to the next. The most recent research involves organic dye-based multi-junction cells.
• Light concentrating cells. This approach can be applied to any of the above technologies. Techniques vary: some integrate a small lens directly into the construction of the cell, some, such as Toronto-based Morgan Solar, add a larger concentrator afterwards in the finished module.
• Building-integrated PV. This is a parallel development that can employ many of the above types, integrating large areas of PV directly into a building’s exterior surface, such as siding, cladding or shingles (hence distinct from separately-applied rooftop solar, for example). Photovoltaics that are sufficiently cheap to make, even if relatively inefficient (in the vicinity of 6%, say), can be cost-effectively integrated into a building’s roof or facing material. At least one manufacturer has managed to build PV cells into window frames, using the window material itself as a light-guide concentrator.

The evolution of PV technology

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