https://today.uconn.edu/2013/02/uconn-professors-patented-te...
Look at the picture of the electrode with triangles and the other rectangular. The distance between the tip of the triangle and the other electrode is required to be on the order of a nm (in this reference here 1.5nm). This allows electrons to tunnel from one electrode to the other. A charge on an electrode tries to spread out, and the density is higher near pointed protrusions. So in the cycle the triangle-electrode is negatively charged, the electrons can tunnel to the other electrode, but in the other half of the cycle the electrodes prefer to spread out differently on the rectangular electrode. That's how the diode is implemented.
Edit: just adding, so the asymmetry of the diode is due to the asymmetry between electrodes and the asymmetry between electron and hole mobilities
A green light would create a AC current at 476 THz. We put this through a diode to capture the pulses of current that are moving in the right direction?
Also, the voltage won't be very high. The diodes have to be efficient in the millivolt regime.