The sun is 5778K and Earth is ~290K. Using the sun as the hot reservoir and earth as a cold reservoir we get 95% Carnot efficiency. However, the solar power efficiency limit is only 86%, see: http://www.energy.udel.edu/pdf/Honsberg_UDEI_Symposium.pdf
This is not just because of our atmosphere or a non-ideal sun. There is a more fundamental constraint: Carnot heat engines don't care how quickly energy is transferred from the hot to the cold. In our case, however, we are force-fed energy and must accept it as fast as possible. If energy is lost back to the sun we lose efficiency. Thus the term "single-shot": we only have one chance to generate power.
What is the single-shot efficiency limit as a function of temperature ratio? Assume you are surrounded with black body radiation from all directions at the cold temperature (otherwise you wouldn't even need a sun to get energy!) and an unlimited supply of coolant. The "sun" is a hot black body radiator that takes up a small angle of the sky. Efficiency is (useful power)/(intercepted radiation power). This is a theoretical calculation, there are no restrictions on the design (solar thermal, PV, hybrid, etc).