Photosynthesis is terribly inefficient, something like 5-6% of the light is actually used to make glucose out of CO2 because the process is quite complex and humans don't have a huge skin surface.
Remember, it doesn't need to be terribly efficient. Just efficient enough. I agree that its probably 100-200 years off though (if it can be done at all).
That's easy to solve. We just start growing leaves.