Bizarre night

In a world first, a team at the University of New South Wales (UNSW) has demonstrated measurable power generation from "the inverse of a conventional solar cell." It could eventually produce around one tenth as much power as a solar panel – but at night.

Solar panels, as we all know, absorb energy from sunlight and convert it into electricity. To grossly oversimplify, they use two differently treated silicon semiconductor layers in what's known as a P-N junction. The N layer is doped with extra electron "donor" impurities, the P layer is doped with "acceptor" impurities – spaces for the electrons to fit into – and in the middle there's a "depletion region," where those electrons and electron-accepting holes more or less eliminate one another, creating a barrier that stops all the N side's electrons from diffusing straight through to the P side.

When sunlight shines on the cell, thermal energy in the incoming photons is absorbed in the silicon, and if an electron in the depletion region receives enough energy to jump the bandgap between the two sides, it can pop out of its hole and be accelerated across to the N side, increasing the voltage potential between the two sides. Connecting the two sides together in an external circuit, you can run the electrons back around to the P side and do electrical work.

All of which is to say, it's the thermal energy in photons coming down from the sky that kicks the process off. But this isn't just a one-way process. As our planet spins around, solar radiation heats up the Earth during the daytime, but the Earth releases that energy again as infrared light into the cool of night. And it's this flow of infrared photons into colder air that one group of UNSW researchers is working on harnessing.