Increasing temperature will reduce solar
panel output, but usually increased insolation in the summer periods will more than make up for this. The average output is generally highest in summer. There are exceptions in tropical areas, but this is more due to cloud build up in summer, rather than a temperature increase.
If you are looking for the highest instantaneous output this is not usually in summer. Cooler temperatures where the sun peaks out from a cloud in the "shoulder season" will generally produce the highest peak output (sometimes exceeding the STC output), but it is short lived and has little practical impact on the total output, which is more important to most cruisers.