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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?

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No, the diode under negative illumination does not generate electricity through any of the thermoelectric effects. The TE effect that generates electrical power is the Seebeck effect, by the way, and it requires a temperature difference across a material (or a pair of materials, but not necessarily) to manifest itself as a potential difference that can be used to create a current.

From what I read in https://web.stanford.edu/group/fan/publication/Santhanam_PRB_93_161410_2016.pdf , the principle of an illuminated diode to create power comes from photon fluxes generating currents in the material. These photon fluxes are generated because of a temperature difference between the diode itself and a cold source (space in your examples).

Edit: A thermoelectric generator and the diode under negative illumination happen to be heat engines. And the radioisotope one happens to share the same cold source than the diode, i.e. space.

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  • $\begingroup$ Hmm... "What are photon fluxes generated because of a temperature difference between the diode itself and a cold source?" is going to have to be a follow-up question. To double check, you are ruling out all three, correct? (Seebeck effect, Peltier effect, and Thomson effect) Thanks! $\endgroup$ – uhoh May 7 at 11:11
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    $\begingroup$ Yes I am ruling out the three TE effects. $\endgroup$ – thermomagnetic condensed boson May 7 at 11:25
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    $\begingroup$ Okay I've had a quick look at your link to Santhanam & Fan 2016 and I'm getting the idea now, will read further, thank you! The paper and references therein are quite helpful. "...the presence of Auger recombination and its time-reversed process electron impact ionization (sometimes called Auger generation)." Carriers are maintained by the heat source. Occasionally there is spontaneous emission as one drops into the valence band. Since the photodiode is facing a cold source, there are fewer returning photons so the inverse process can not balance. That's at least part of what's going on. $\endgroup$ – uhoh May 8 at 4:31

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