Solar cell efficiency gets better with colder temperatures, pretty much linearly down to very cold temperatures. The Qpeak G6+ panels being used in current Tesla installations have a temperature coefficient of -0.36%/C. I think this is typical of fairly high end panels.
One also should consider how the atmospheric conditions affect the solar insolation. I believe it tends to also fall with increasing air temperature, perhaps due in part to higher water vapor content in the air. The optical density of the atmosphere is highest in the summer--I'm not sure why. It's a several percent effect.