> The principle they operate on suggests no temperature gradient is required A temperature gradient is absolutely required. Ignoring materials problems (such a

> The principle they operate on suggests no temperature gradient is required A temperature gradient is absolutely required. Ignoring materials problems (such as melting) a photovoltaic cell ultimately does work off a temperature differential because it has to absorb photons. It cannot do that if its temperature is the same as the black body temperature of the lightsource illuminating it, otherwise they would be at an emission/absorption equilibrium. This is also why PV cells could theoretically work in reverse mode at night, emitting IR into space. They'd just do that paltry power ratings because ΔT(earth, cosmic background) is much smaller than ΔT(photosphere, earth). And they'd have to be made of a material with a much smaller bandgap.