This is interesting. There’s a new thing being tested (invented): it’s called a negative illumination diode. It generates electricity in a way similar to the way a photovoltaic cell does, but instead of generating current from the incoming photons (from e.g. the sun), it generates electricity from the outgoing photons. Outgoing photons, you ask? There are always outgoing photons, on earth, because space is cold and the earth radiates heat (infrared photons) from all its surfaces, including from the diode in question. See here.