A multijunction device is a photovoltaic device containing two or more cell junctions, each of which is optimized for a particular part of the solar spectrum, to achieve greater overall efficiency. This structure, also called a cascade or tandem cell, can achieve a higher total conversion efficiency by capturing a larger portion of the solar spectrum.
|A multijunction device is a stack of individual single-junction
cells in descending order of bandgap (Eg). The top cell captures the
high-energy photons and passes the rest of the photons on to be absorbed
by lower-bandgap cells
|This multijunction device has a top cell of gallium
indium phosphide, then a "tunnel junction" to allow the flow of electrons
between the cells, and a bottom cell of gallium arsenide
In the typical multijunction cell, individual cells with different bandgaps are stacked on top of one another. The individual cells are stacked in such a way that sunlight falls first on the material having the largest bandgap. Photons not absorbed in the first cell are transmitted to the second cell, which then absorbs the higher-energy portion of the remaining solar radiation while remaining transparent to the lower-energy photons. These selective absorption processes continue through to the final cell, which has the smallest bandgap.
A multijunction cell can be made in two different ways. In the mechanical stack approach, two individual solar cells are made independently, one with a high bandgap and one with a lower bandgap. Then the two cells are mechanically stacked, one on top of the other. In the monolithic approach, one complete solar cell is made first, and then the layers for the second cell are grown or deposited directly on the first.
Much of today's research in multijunction cells focuses on gallium arsenide as one (or all) of the component cells. These cells have efficiencies of more than 35% under concentrated sunlight – which is high for PV devices. Other materials studied for multijunction devices are amorphous silicon and copper indium diselenide.