Most sources on the web say that 1.3 eV or 1.4 eV is 'ideal' for most solar cells...

What happens to lower-gap, gapless or overlap materials (conductors)? Are they ionized, rather than sending their electrons neatly into the conduction band?

P.S.: May I also ask if there are advantages, disadvantages to using 'direct' bandgap semiconductors vs. indirect band gap ones?

  • \$\begingroup\$ You only get about 0.5V (maybe 0.55V open circuit) from a Si cell. This is the one place where you really want a higher forward voltage in a diode. \$\endgroup\$
    – user16324
    Commented Nov 27, 2021 at 14:43

1 Answer 1


See these lecture slides (42 pages) Light Absorption and Thermalization:


Page 15 Thermal Efficiency vs Band Gap

For small band gaps, efficiency is limited by thermalization losses

For large band gaps, efficiency is limited by losses due to non- absorption of the solar spectrum

The tradeoff between thermalization and non-absorption losses results in the optimal band gap of a semiconductor of approximately 1.2 eV, and a maximum theoretical efficiency of close to 30%.

Page 36:

Light absorption in a direct semiconductor requires only photons

Light absorption in an indirect semiconductor requires a photon and a phonon, and is statistically less likely to occur.

Page 38:

Solar cells made from indirect semiconductors need to be thicker because we need to provide more opportunities for the transition to take place.

Page 39:

Absorption in indirect semiconductors is temperature-dependent, because phonons are needed and phonon population depends on temperature.

The direct materials may seem to have an advantage, however, the economics and design tradeoff in a given application are not discussed in the reference.


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