Photovoltaics - Multijunction Cells

Background

Today's most common PV devices use a single junction, or interface, to create an electric field within a semiconductor such as a PV cell. In a single-junction PV cell, only photons whose energy is equal to or greater than the band gap of the cell material can free an electron for an electric circuit. In other words, the photovoltaic response of single-junction cells is limited to the portion of the sun's spectrum whose energy is above the band gap of the absorbing material, and lower-energy photons are not used.

One way to get around this limitation is to use two (or more) different cells, with more than one band gap and more than one junction, to generate a voltage. These are referred to as "multijunction" cells (also called "cascade" or "tandem" cells). Multijunction devices can achieve a higher total conversion efficiency because they can convert more of the energy spectrum of light to electricity.

In other words, a multijunction device is a stack of individual single-junction cells in descending order of band gap. The top cell captures the high-energy photons and passes the rest of the photons on to be absorbed by lower-band-gap cells.

A Look Inside

In a typical multijunction PV cell, individual single-junction cells with different energy band gaps are stacked on top of one another. Sunlight then falls first on the material with the largest band gap, and the highest-energy photons are absorbed. Photons not absorbed in the first cell continue on to the second cell, which absorbs the higher-energy portion of the remaining solar radiation while remaining transparent to the lower-energy photons.

Much of today's research in multijunction cells focuses on gallium arsenide as one (or all) of the component cells. Such cells have reached efficiencies of around 35% under concentrated sunlight. Other materials studied for multijunction devices have been amorphous silicon and copper indium diselenide.

A typical multijunction device might use a top cell of gallium indium phosphide, "a tunnel junction," to aid the flow of electrons between the cells, and a bottom cell of gallium arsenide.

Source: U.S. Department of Energy Photovoltaics Program.

For more information on this source please visit National Renewable Energy Laboratory

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