How is Photovoltaic used?

Photovoltaic cells can convert sunlight into energy.

Silicon

Semi conductors are used, usually silicon doped with some other material, which produce a low dc voltage. That's just the physics of how it works.

The only answer that comes to mind is "Airships". Hydrogen, the lightest element, was used in airships, where excess weight was undesirable. However, accidents like the Hindenburg disaster showed...

A model aeroplane is used for presentations and testing. Model planes are also used for fun. They are used model-plane clubs, aeroplane company headquarters and studios. (Bedroom, in my house)