Things You’ll Need:
Electric stove or hot plate
Micro-ammeter capable of reading currents lower the 50 microamperes
2 alligator-clip leads
Clear plastic bottle
2 Tbsp. salt
Cut 2 rectangular pieces of copper sheeting, each approximately the size of the stove burner.
Wash 1 piece of the copper thoroughly, and then use the sandpaper to clean off any corrosion.
Lay the copper sheet down on the burner.
Turn the burner to maximum heat.
Monitor the copper carefully. It will change colors as it begins to oxidize, slowly transforming to black.
Leave the copper on the burner for at least 40 minutes, or until it has a thick coat of oxidized black material.
Turn off the heat on the burner and allow the copper to cool in the air. This should take another 20 to 30 minutes.
Gently scrub the copper with your hands under running water to take off the most easily removed pieces of the oxidized coating. Do not try to remove all of the coating or scrub too hard. Your goal is to take off what can easily be removed, but not to damage the remaining copper below the oxidized layer, which is required to build the solar panel.
Cut off the top of the plastic bottle so that it forms a makeshift bucket.
Take the second piece of copper (the one you didn't heat up), and bend it slightly to fit the contours of the bottle.
Slide the second piece of copper into place along the inside of the bottle.
Repeat the process with the first piece of copper (the one you heated up). The 2 pieces of copper should not be touching each other.
Connect an alligator-clip lead to each piece of copper.
Connect the alligator-clip lead from the second (uncooked) piece of copper to the positive terminal of the micro-ammeter.
Connect the alligator clip lead from the first (cooked) piece of copper to the negative terminal of the micro-ammeter.
Stir the salt into several cups of hot water until the salt is completely dissolved.
Slowly pour the salt water into the bottle, taking care to keep the 2 alligator-clip leads dry. Make sure you leave at least an inch of the copper pieces above the water line.
Check the needle on the micro-ammeter. The apparatus you've constructed is essentially a battery, so there will be some charge whether it is in light or darkness.
Place the apparatus in sunlight and observe how the charge increases as the cell converts the sun's energy to electricity.
How does it do that?
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. In a semiconductor, there is a gap, called a bandgap between the electrons that are bound tightly to the atom, and the electrons that are farther from the atom, which can move freely and conduct electricity.
Electrons cannot stay inside the bandgap. 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.
In a same way, an electron outside the bandgap cannot lose a little bit of energy and fall just a little bit closer to the nucleus. It must lose enough energy to fall past the bandgap into the area where electrons are allowed. When sunlight hits the electrons in the cuprous oxide, some of the electrons gain enough energy from the sunlight to jump past the bandgap and become free to conduct electricity.
The free electrons move into the saltwater, then into the clean copper plate, into the wire, through the meter, and back to the cuprous oxide plate. As the electrons move through the meter, they perform the work needed to move the needle. When a shadow falls on the solar cell, fewer electrons move through the meter, and the needle move back down.