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An excited electron also leaves a hole in the underlying valence band -- a mobile vacancy that can be moved through the material like a positive charge carrier.

The electrons then fall back into the valence band and become worthless in terms of electricity generation.

In a solar cell, electrons are dislodged by photons and raised to a higher energy level from the valence band to the conduction band.

The team used a laser to create a cloud of excitons at a corner of the pyramid's base, bouncing electrons out of the valence band of a semiconductor into the conduction band -- but the negatively charged electrons are still attracted to the positively charged holes left behind in the valence band.

The electron moves to the valence band and merges with the hole, releasing a photon. b, However, a process called non-radiative Auger recombination can instead occur.