How does temperature affect photovoltaics (PV) efficiency? I know that photovoltaic panels are more efficient at lower temperatures: As the temperature increases, the output voltage decreases. I am looking for an explanation of the mechanism behind this effect. What is going on in terms of electron energy level transitions, the band gap, etc?
 A: From Sze (chapter 14 in the second edition):
"As the temperature increases, the diffusion lengths in Si and GaAs will increase, because the diffusion constant stays the same or increases with temperature, and the minority lifetime increases with temperature.  The increase in minority-carrier diffusion length causes an increase in J_L.  However, Voc will rapidly decrease because of the exponential dependence of the saturation current on temperature.  The increase in the "softness" (roundness) in the knee of the I-V curve as temperature increases will also degrade the fill factor.  Therefore, the overall effect causes a reduction of efficiency as the temperature increases."
Further details may be found in Sze (or other semiconductor device physics books).
A: I see you already have a answer at the device physics level, so here is the answer from the electronics point of view.
The forward drop of a semiconductor junction (which is what a solar cell is) is inversely proportional to temperature.  In fact, there have been integrated temperture sensors built on this principle.  The current thru a solar cell is proportional to insolation.  This same current is produced at a higher voltage at lower temperature, hence yielding more power.
A: And a third way of looking at it: At higher temperatures more conduction band states are occupied. This will increase recombination / increase dark current.
