solar cell efficiency

A solar cell's efficiency is the percentage of the sunlight landing on it that comes out as electricity. The rest is lost, mostly as heat, and the losses are not a sign of sloppy engineering. They follow from physics.

Sunlight arrives as photons across a wide range of energies, while a given material absorbs only photons above a particular threshold, its bandgap. Photons below it pass through unused, and photons well above it give up their surplus as heat. The US Department of Energy puts the resulting ceiling precisely: the maximum theoretical efficiency that a single-bandgap solar cell can achieve with non-concentrated sunlight is about 33.5 percent, a limit known after the physicists who derived it as the Shockley-Queisser limit.

That number is the reference point a reader needs. Ordinary commercial silicon panels convert something in the low twenties, so a laboratory single-layer cell approaching the low thirties is close to the maximum its design can ever reach. Getting past it requires changing the design rather than refining it, by stacking layers with different bandgaps so each captures a different slice of the spectrum. The Department of Energy notes such multijunction devices reach recorded efficiencies above 45 percent. This is also why efficiency claims are quoted under standardised test conditions: without a common reference spectrum and temperature, two percentages would not describe the same thing.