Question

What is the physical mechanism in multi-junction solar cells that allows them to exceed the Shockley-Queisser efficiency limit by capturing photons that would otherwise cause thermalization losses?

Accepted Answer

The Shockley-Queisser limit establishes a maximum efficiency for a single-junction solar cell because it is limited by a fixed bandgap, which is the minimum energy required to excite an electron from the valence band to the conduction band to create current. When a solar cell absorbs a photon with energy higher than its bandgap, the excess energy is lost as heat through a process called thermalization, where the electron quickly drops to the band edge. When a photon has energy lower than the bandgap, it cannot be absorbed at all and passes through the cell unused. Multi-junction solar cells bypass these losses by stacking multiple semiconductor layers, each with a different, precisely tuned bandgap. The top layer consists of a material with a high bandgap that absorbs high-energy, short-wavelength photons while allowing lower-energy photons to pass through. Subsequent layers beneath it have progressively smaller bandgaps, allowing them to capture the photons that passed through the layers above. By matching the bandgap of each layer to a specific segment of the solar spectrum, the cell significantly reduces thermalization losses because the excess energy of each captured photon is minimized. Simultaneously, it increases the total current and voltage generated by capturing a much broader range of the sun&#x27;s electromagnetic radiation than any single material could alone.

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What is the physical mechanism in multi-junction solar cells that allows them to exceed the Shockley-Queisser efficiency limit by capturing photons that would otherwise cause thermalization losses?