Heterojunction and Multi-Junction Solar Cells

Multi-junction, or heterojunction, solar cells are a type of thin-film solar cell. Most thin-film solar cells are inherently multi-junction. That's because a multi-junction cell consists of two or more thin layers of semiconductors, each designed to absorb a certain portion of the solar spectrum. The idea is that the combination of the semiconductors that form a multi-junction solar cell will absorb as close to the entire light spectrum as possible, thus creating maximum solar electricity from incident sunlight.

Examples of multi-junction cells include CIGS solar cells, which use layers of cadmium sulfide, copper indium selenide (CIS) and/or copper indium gallium selenide (CIGS), and gallium arsenide (GaAs) solar cells, which use twin layers of gallium arsenide and indium gallium phosphide.

The trick is to partner semiconductor materials with adjacent band gaps that will together absorb all available sunlight. In standard single-junction solar cells, such as silicon wafer cells, efficiency is limited because many of the photons either pass through the cell or are lost as heat within the cell.

Multi-junction solar cells are playing a key role in the quest for high-efficiency solar cells. So far, thin-film solar cells that use heterojunctions can absorb a vast amount of sunlight, but are relatively inefficient at converting that sunlight into usable electricity. CIGS solar cells are, however, continually showing improvement, having reached cell efficiencies near 20% and module efficiencies as high as 12.5%.

Thin-film heterojunction solar cells are typically much cheaper to produce than conventional silicon-based solar cells. So, if researchers can match their absorption rates with higher efficiencies, these cells will be ready to take over the solar panel market.

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