Photovoltaic or photochemical (photosynthesis) processes are basically extracting energy from the temperature difference between the Sun and Earth. In practice they are only able to capture photons in rather narrow range of energies, so their efficiency is rather low. But assuming we found a way to capture most of the spectrum, what is theoretical limit of the process?
A solar powerplant using heating of a medium is limited by the Carnot cycle efficiency, but photovoltaic process does not seem to involve any cycle. So does it allow, theoretically, achieving higher efficiency?