Why Isn't Carbon Used as a Semiconductor in PV Cells Despite 4 Valence Electrons?

h1. Why is carbon not used as a semiconducting material in pv cells?
Carbon also has four electrons in its valence band but the barrier potential is a bit high . But still the condition for generating an electron hole pair by the photons is satisfied. so why is carbon not used in the fabrication of diodes.
h2. In plants the leaves absorb photons for their photo chemical reaction. so what material is used to absorbs these photons?
and why cant we use this material in fabricating a pv cell?

There is work being done with carbon based PV cells. However, the physics at present is as far as they have got. There is an enormous step between producing a laboratory sample, and moving it into full scale factory production. Efficiency is a big issue, as it has been for silicon. If you look back, silicon has taken many many years to get to the point where it is now, and improvements are continuing.
Wkipedia has coverage of the photosynthesis process, some work has been done in mimicking the photosynthesis process to produce a workable PV cell. Again, this is only at the laboratory stage.
https://en.wikipedia.org/wiki/Photosynthesis
If you want more detailed information, then google "google scholar", and search there, you should find some in depth papers,
cheers,
richard