How does mariner 10 generate electricity?why can/cant mariner 10 use solar cells?

Mariner 10 used solar panels to generate electricity during its operational life. It can make use of them because it was heading to the inner planets Venus and Mercury, so there would be ample sunlight. In fact it would be able to generate more electricity the closer it got to its target as the sunlight would get steadily brighter. Probes heading to the outer solar system cannot use solar panels because the further away they get the less sunlight they receive, so their power would slowly dwindle as they got closer to their objective.