The photo above shows the solar cell in my shadow as I took the picture. Notice that the meter is reading about 6 microamps of current. The above photo shows the solar cell in the sunshine. Notice that the meter has jumped up to about 33 microamps of current. Sometimes it will go over 50 microamps, swinging the needle all the way over to the right.
Cuprous oxide is a type of material called a semiconductor .
A semiconductor is in between a conductor, where electricity can flow freely, and an insulator, where electrons are bound tightly to their atoms and do not flow freely. An electron cannot gain just a little bit of energy and move away from the atom's nucleus into the bandgap. An electron must gain enough energy to move farther away from the nucleus, outside of the bandgap. It must lose enough energy to fall past the bandgap into the area where electrons are allowed. When a shadow falls on the solar cell, fewer electrons move through the meter, and the needle dips back down.