You can put panels on exterior walls too. It's not just the roofing.
Potentially roads and walkways can be collectors.
Most planets don't have or need arcologies, but sure A large percentage of people might live in them. You won't use solar in lots of specific circumstances.
Conversely, it's a lot more attractive on an asteroid - just build solar collector towers out in all directions and use that unlimited area surrounding the rock as much as the population needs. When the sun is always shining (a feature of being in space) peak power management is simpler. Removing the need to gather and process fuel is a solid reason to consider solar.