Maximum theoretical efficiency of a solar panel is less than 50% because a panel is a forward biased diode in essence; only incremental gains will be made in the future and they will be insignificant compared to what we already get.
It's not the least charm of a theory that it is refutable. The hundred-times-refuted theory of "free will" owes its persistence to this charm alone; some one is always appearing who feels himself strong enough to refute it - Friedrich Nietzsche