Some sort of heat engine instead of solar cells is another option although that implies lot`s of mechanical parts and radiators which is a high potential for breakdown.
Not more so then that of a modern steam-powered power station.
You could design the heat engine to be simple (in relative terms) and easily accessible for maintainance by astronauts or teleoperated robots.
But then again building a space based solar power system capable of making significant dent in worlds energy budget by default means a commitment to large scale space program
The idea is that the power station would be built and launched from the Moon, using lunar factories built by bootstrap outposts using local materials in an effort to lessen the impact to the Terran environment. It would indeed entail a "large-scale" space program (compared to the one we have now), but, IMO, it's really the only space program worth having.
so having a rotating teams of technicians in GEO space station to do necessary maintenance and repairs might be a viable option.
Or you could use remotely-operated robots, which would be cheaper to operate since their survival needs are far less then those of humans. Humans
are far more capable then machines, but you could make up for it by making the vulnerable components easy to reach and replace.
Solar power sat repair would be an excuse to test out long-duration space technologies and such, and do science such as sun monitoring and astronomy.