Since exposing a pn junction diode to light produces an electric current just like a solar battery, the reverse should also hold true, i.e. applying an external electric current in the opposite direction should cause light to emit from the pn junction.

This phenomenon does in fact occur. Making the n-type silicon the cathode and the p-type silicon the anode produces light. This is known as a light-emitting diode (LED). However, light emission from such rudimentary LEDs is inefficient, making them ill suited for practical applications. Only after creating pn junctions using semiconductor materials made of the compounds gallium arsenide, gallium phosphide, and gallium arsenide phosphide did LEDs become practical.