Spectral Utilization II - Shockley-Queisser Limit

The Shockley-Queisser limit refers to the limit imposed on the maximum conversion efficiency of solar cells, at 48%, due to 3 types of losses.  Firstly, there's optical losses, where photon energy above the band gap is lost as heat, and where energy is also lost due to non-absorption of photons having less energy than the band gap (see diagram below, assuming EQE of 100%).

Secondly, there's energy losses due to black body radiation, because at a particular temperature, light will be emitted by the solar cell in the far infrared.  The loss is about 7% of the incident AM1.5 solar spectrum's energy.

Lastly, there's recombination losses, where in deriving the Shockley-Queisser limit, only radiative recombination is considered.  This means that the temperature of the solar cell is theoretically not allowed to increase (a thermodynamic approach), and that all incoming AM1.5 spectrum energy will leave the solar cell either through generating photocurrents, or by radiative recombination.

The diagram above shows the usable energy in single junction solar cells based on the Shockley-Queisser limit.  However, there is still a gap between the Shockley-Queisser limit and the actual best efficiencies at optimum band gaps of solar cells (see diagram below).

This is due to the missing optical losses, such as reflection and parasitic absorption, and electrical losses, such as SRH and Auger recombinations.  Thus, the Shockley-Queisser limit is most valid for direct band gap materials like GaAs.

One way to overcome the Shockley-Queisser limit is to have multi-junctions, where more than one semiconductor is used.  This means that there will be multiple band gaps to improve light absorption (see diagram below).

Reference:
3.3.3 Spectral Utilization II - Shockley-Queisser Limit, Delft University of Technology, https://www.youtube.com/watch?v=x4UP9m3O99w