You are correct about thermodynamic efficiency. I interpreted it to be about economic efficiency. In which case one or more doublings of kWh output from dollar input is still a stretch but plausible.

Modern rooftop solar modules haven't even doubled efficiency from 40 years ago, but they have improved cost per kilowatt hour by more than 100x.

Yeah, I tend to agree with you here. The difference I see is that solar has already been heavily invested in and developed recently, so it's probably near the limit of today's material science.

My personal philosophy about technological progress is based heavily on the idea that material science is the principle barrier to what we can accomplish. That's why humanity's progress is recounted in "ages" named by material.

So, I think solar panels can get maybe 2-10x more economical, whereas other options like new hydrocarbon sources, fission, and fusion can probably all get at least 10x more economical than the maximum possible with solar, purely due to the physical limitations of the technology.

I can believe significant price improvements to come for solar, fission, and fusion since their fuel costs are zero or tiny. What new hydrocarbon sources could be 10x cheaper than coal or natural gas?

Well, I'm not really supposed to talk about this but we're working on a hydrocarbon byproduct of our fission reactors. Can't extrapolate but point is, there are unconventional ways to make hydrocarbons that become economically viable when they are a byproduct of an already profitable process.