Multi-Junction Solar Cells

 

Multi-Junction Solar Cells

Iftikar Ahmed, PG Student,

Department of Renewable Energy, MAKAUT

 

A multi-junction solar cell is a tandem solar cell with more than one p-n junction. In practice, this means that there are multiple layers of different semiconductor materials, each of which produces electric currents in response to different wavelengths of light. This means that, theoretically, multi-junction solar cells are capable of converting more sunlight that hits them to electricity when compared to single-junction cells. Multi-junction solar cells are capable of absorbing different wavelengths of incoming sunlight by using different layers, making them more efficient at converting sunlight into electricity than single-junction cells. While they have the potential to be many times more efficient than traditional solar cells, 

The materials that go into a photovoltaic cell make a large difference on the cell's efficiency, as the band gap varies based on the materials and the dopants within the material that make the pn junction. For traditional single-junction cells, monocrystalline silicon is used, as it is abundant and relatively cheap; in addition it has a gap of 1.11 eV, quite close to the optimal 1.4 eV. For multi-junction cells, the most common material used is Gallium Arsenide, GaAs, as it has a band gap of 1.43 eV, which is extremely close to the optimal band gap range. As the optimum material, GaAs generally is in the middle layers, commonly in between Indium Gallium Phosphide and Germanium, with band gaps of 1.85eV and 0.67eV respectively. GaInP utilizes the high energy photons while Ge utilizes the much lower energy photons and GaAs utilizes those in between.

In terms of theoretical efficiency, multi-junction solar cells have the potential to significantly outperform traditional single-junction solar cells. According to the Department of Energy, multi-junction solar cells with three junctions have theoretical efficiencies over 45 percent, while single-junction cells top out at about 33.5%. Adding more junctions (potentially up to 5 or 6 junctions) could boost efficiency over 70 percent. For reference, the most efficient solar panels available today have efficiencies around 22 percent.

The benefits of multijunction III-V solar cells include:

●        Spectrum matching: High-efficiency cells (>45%) can be fabricated by matching sections of the solar spectrum with specific absorber layers having specific bandgaps.

●        Crystal structure: The various combinations of III-V semiconductors have similar crystal structures and ideal properties for solar cells, including long exciton diffusion lengths, carrier mobility, and compatible absorption spectra.

The cost of producing solar cells based on compounds of III-V elements has confined such devices to niche applications including drones and satellites, where low weight and high efficiency are more pressing concerns than costs, in relation to the energy produced.