A similar question was answered in ScienceLine a
few years ago. You can find it here.
The brief science explanation is that when you
increase the temperature of a solar cell, its current
increases but its voltage decreases. When a
semiconductor is heated, electrons become more mobile
and current can therefore increase. Voltage
depends on a separation of negative charges
and positive charges (holes) across an energy band
gap in a semiconductor. When temperature
increases, electrons are more likely to jump
across the band gap and recombine with holes,
reducing the charge separation and reducing the
voltage. Power, which is related to efficiency, is
to the voltage times the current. It works out
that voltage decreases faster than current
increasing temperature decreases power and
therefore efficiency as well.
I did a quick search and found that most solar
cell manufacturers have a graph of power vs.
in their product specs. Here are three examples.
For all of these products, there’s about a 20%
power decrease between 25C and 75C (77F-167F). Colder
temperatures have higher efficiency but sometimes
less power, because colder often means less direct
sunlight! I wish you luck with your installation.
Thanks for embracing alternative energy!