For space applications, you can use the aforementioned solar sail, or charge an ion engine (though this still requires you to relinquish some matter). For air

For space applications, you can use the aforementioned solar sail, or charge an ion engine (though this still requires you to relinquish some matter). For air applications, the old-fashioned propeller is your ticket to thrust. Something like a dirigible or a hot-air balloon, which is able to keep itself in the air without continuous thrust of a minimum force, would be less difficult to make work. Interestingly enough, weight is more important than efficiency when dealing with aircraft PV. A company I spoke with that was working on "stratellites" (basically high-altitude blimps that stayed in one place indefinitely) used thin film PV instead of high efficiency cells because of their flexibility and high power-per-weight rating.