If solar cells currently absorb 20% - 30% of the suns energy, what spectrum
More like 10%-15%, unless you count the heat...
does this fall under? does the limitation of the spectrum captured the
reason for such low efficiency?
Solar photo-voltaic cells work because the incoming photon knocks loose
an electron. One definition of electric current is flowing electrons,
thus the photon causes current to flow.
This only works if the incoming photon has enough energy to knock the
electron loose. So only the higher energy light is converted to
electricity, the lower energy light just results in heat. (ultraviolet
end of the spectrum is higher energy than the infrared end.)
You only get one electron per photon, so if a photon has more energy
than needed, the rest of the energy results in, you guessed it, heat.
Research continues into materials with a lower bandgap energy (so lower
energy light can knock out electrons) and also higher bandgap energy, as
well as making multi-layer cells so that different photons can knock
loose electrons from different layers.
All alternatives are fantastically expensive compared to the old standby
-- silicon. Thus silicon remains the material of choice for commercial
solar PV cells / modules / panels.
sdb