I've read through the explanation and can not understand what they are saying in terms of the underlying physics. My hunch is that this is a demonstration of a well established thermoelectric effect made to look more profound through obscuration, but I could be wrong.
Question: Is this photodiode-based "energy harvester using the coldness of outer space" just based on established thermoelectric effects?
Wikipedia's article on the Thermoelectric Effect says:
The term "thermoelectric effect" encompasses three separately identified effects: the Seebeck effect, Peltier effect, and Thomson effect.
The paper in question appears as an Applied Physics Letter: Experimental demonstration of energy harvesting from the sky using the negative illumination effect of a semiconductor photodiode and the abstract reads:
We experimentally demonstrate electric power generation from the coldness of the universe directly, using the negative illumination effect when an infrared semiconductor diode faces the sky. Our theoretical model, accounting for the experimental results, indicates that the performance of such a power generation scheme is strongly influenced by the degree of matching between the responsivity spectrum and the atmospheric transparency window, as well as the quantum efficiency of the diode. A Shockley-Queisser analysis of an ideal optimized diode, taking into consideration the realistic transmissivity spectrum of the atmosphere, indicates the theoretical maximum power density of 3.99 W/m2 with the diode temperature at 293 K. The results here point to a pathway towards night-time power generation.
For reference, Phys.org's Experimental device generates electricity from the coldness of the universe says:
An international team of scientists has demonstrated for the first time that it is possible to generate a measurable amount of electricity in a diode directly from the coldness of the universe. The infrared semiconductor device faces the sky and uses the temperature difference between Earth and space to produce the electricity.
Spacecraft and planetary rovers use the "coldness of the universe" all the time. Their Radioisotope Thermoelectric Generators pass heat through a large array of thermocouple junctions with the cold side facing the cold of space. Is this new result any different than this?