# Do silicon solar cells act like an LED when you flip the voltage, or are we just seeing black body radiation?

In science communicator Steve Mould's Youtube video Why all solar panels are secretly LEDs, he supplies a voltage across a solar cell. Using an IR camera, we see IR emission coming from the solar cell, and he claims that is LED behavior.

Is it not more likely that the IR we're seeing is blackbody radiation? My understanding of LEDs is that indirect semiconductors (i.e. silicon) make for terrible light-emitters, and the energy will be emitted non-radiatively by a phonon.

Indeed, silicon does not act as an infrared LED. That is because the band gap is indirect, the maximum of the valence band and the minimum of the conduction band are not at the same point in $$k$$-space. Momentum must be conserved in recombination, and this is impossible because the infrared photon has negligible momentum.