Basically, you need a sun-facing shield that is perfectly reflective, and diamond fins with a perfectly black surface coating, forever in its shadow. An effect

Basically, you need a sun-facing shield that is perfectly reflective, and diamond fins with a perfectly black surface coating, forever in its shadow. An effective radiator is an equally effective absorber, so management of light and shadow is critical. But it's still sort of moot when you consider atmospheric friction on the way down. You're not going to get much power out of the gradient. Even if you cooled an object in space to near absolute zero, space probes must necessarily be low mass to get into orbit from Earth. The low mass of the whole object, and the low heat capacity of most space-grade building materials combines to make it a really awful cold sink. You would essentially need to harvest water and ammonia from somewhere outside of a gravity well, use your radiator to cool it off as much as possible, and then drop it in the heat well. That's an awful lot of trouble to go to for the amount of power you will then get out of it.