Single junction solar cell efficiency by solar cell junction bandgap (public domain image by Sbyrnes321)

Because of the spectrum of sunlight, no single junction solar cell can achieve an efficiency of over 33.16% (the Shockley-Queisser limit).

To be absorbed by the solar cell junction, photons have to have an energy above the junction band gap energy.  The band gap energy is the difference in energy between electrons in the normal, unexcited, highest energy state (HOMO – highest occupied molecular orbital) and electrons in the lowest excited energy state (LUMO – lowest unoccupied molecular orbital).

The single junction cell band gap energy, and thus the photon absorption minimum energy and maximum wavelength, can be tuned by doping or choice of materials, to respond to photons of any maximum wavelength.  Every photon absorbed above this minimum energy produces one excited electron with a constant excitation energy equal to the band gap.  Any excess energy, from individual photons with a higher energy than required for absorption, is wasted as heat.  In external terms, the band gap sets the output voltage of the solar cell at zero current.

Thus, a low cell bandgap can tap into photons from more of the solar spectrum, but each photon absorbed produces one free electron with a constant, low excitation energy.  A high cell bandgap results in a higher electron excitation energy per photon, but taps into less of the spectrum.  The happy medium of optimum efficiency is a band gap of 1.34 eV which allows a maximum single junction energy efficiency of 33.16% – the Shockley-Quiesser limit.

Note that the efficiency limit only applies to simple, single-junction solar cells exposed to sunlight.

The Shockley-Queisser limit can be circumvented by the following:-

• Multiple junctions (described below), for which the May 2022 record was 47.6% under the illumination of 667 suns from a Fraunhofer-ISE four-junction solar cell.\\   \\ The “multiple junction” solar cell category also includes “tandem” (dual junction) perovskite/silicon solar cells for which the December 2022 record is a respectable 32.5%  – just below the Shockley–Queisser limit for a single junction solar cell.  And there is also more to come from tandem perovskite/silicon solar cell technology, whose efficiency will exceed the Shockley-Queisser theoretical single junction limit soon.  Manufacturing of such “tandem” cells is straightforward, so costs are relatively low compared to three, four and more junction solar cells.
• “Ratchet” techniques – having multiple bandgaps within a single junction allowing single high energy photons to kick an electron to the higher bandgap energy level and two (or more) lower energy photons to promote a single electron to the same high energy level in two lower-bandgap stages.  Effectively, a “ratchet” bandgap single junction solar cell is an artificial multiple-junction cell.
• Illumination by pure laser light of a single wavelength, for which the record single junction GaAs solar cell efficiency record, set in June 2021, is 68.9% when illuminated by laser light of 858 nm.  The high efficiency of single junction solar cells, when illuminated by monochromatic laser light, is important for the overall efficiency of power transfer systems using maximum efficiency lasers and solar cells.  Perhaps strictly, if a laser, rather than sunlight, illuminates a cell, the cell should be called a “photovoltaic” cell rather than a “solar” cell, as the sun is not involved!