Voyager 1 is powered by a RTG, or "radioisotope thermoelectric generator". This works by converting the heat released from radioactive decay into electricty. Solar cells would be almost useless since the craft is very far from the Sun. The specific RTG power plant of both Voyager crafts (each use three of them) can produce up to 160W of continuous power from 2.4kg of Plutonium 238; the devices weight around 39kg. So they're not really "handheld".