Hello! This is a great question. Currently, the record for the space mission that has traveled the furthest from the Sun using solar power is held by the Juno mission to Jupiter. Before Juno, all of the spacecraft that visited the outer planets used radioisotope thermoelectric generators (RTGs) to power the instruments. RTGs generate power through the natural radioactive decay of plutonium dioxide, which produces a large amount of heat. This heat is converted directly to electrical power using devices called thermocouples. Although solar panel technology keeps improving, NASA still uses RTGs for some missions. The
New Horizons mission to Pluto used an RTG to power the spacecraft.