A fuel cell large enough to provide a few hundred watts of power could be made to use whatever fuel a space probe uses for propulsion. Like the old gasoline-powered heaters in Volkswagen cars. Combustible fuels, like nuclear devices produce far, far more power than solar cells can, especially out where there is little sunlight (beyond Mars). Solar cells are best-suited for close-solar missions where large, complex and mechanically failure-prone solar panels aren't needed.