Tandem Cells

One method to increase the efficiency of a solar cell is to split the spectrum and use a solar cell that is optimised to each section of the spectrum.

tandem solar cell

Series connected tandem solar cell. Adding more devices allows for each device to be optimized to a narrower spectrum giving a higher overall efficiency.

Tandem solar cells can either be individual cells or connected in series. Series connected cells are simpler to fabricate but the current is the same though each cell so this contrains the band gaps that can be used. The most common arrangement for tandem cells is to grow them monolithically so that all the cells are grown as layers on the on substrate and tunnel junctions connect the individual cells.

Ideal efficiency of a two junction tandem

The maximum efficiency for a two junction tandem under the AM1.5G spectrum and without concentration is 47 %. At the peak efficiency the top cell has a bandgap of 1.63 eV and the bottom cell has a bandgap of 0.96 eV.

As the number of bandgaps increases the efficiency of the stack also potentially increases. In reality, the semiconductor materials do not exist to allow for arbitrary materials with a specific bandgap and of high quality.

tandem cell efficiency as a funciton of the number of bandgaps

Efficiency of a an ideal stack of solar cells as a function of the number of bandgaps [1]. The spectrum used here is the direct spectrum (AM 1.5D) where the diffuse radiation is excluded.