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 (electrons) 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, therefore reducing the charge separation and reducing the voltage. Power, which is related to efficiency, is equal to the voltage times the current. It works out that voltage decreases faster than current increases, so 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. temperature in their product specs. Here are three examples.

Suntech:
suntech
Mitsubishi:
Mitsubishi
ET Solar:
ET Solar

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.

When solar cells are hotter they are less effective at producing electricity. When a photon of light is absorbed by a solar panel it energizes an electron which causes it to jump up to a higher energy level. The starting energy level and higher energy level are separated by a "bandgap" of energies where the electron isn't allowed to exist. Once the electron is collected by a wire it can be used as electrical energy. But the higher the temperature the more likely it is that the electron will fall back down to the lower energy level before it is collected, in which case that energy is not turned into electricity.

But during the summer it is brighter and the days are longer so you still probably get more energy in the summer than in the winter because the solar cells will be less effective per unit of sunlight but there will be a lot more sunlight.