Materials: sheet of copper flashing (1/2 sq. ft.), 2 alligator clips, micro-ammeter (read 10-50 mutlimeters), electric stove (to temper copper), large clear plastic bottle, table salt, sand paper on electric drill, tap water, sheet metal sheers (to cut copper)
How it’s done:

Preparation:
Take the copper piece and cut a piece about the size of your stove’s burner. Wash your hands and the piece of copper you cut thoroughly so neither has any grease or oil on it. Now take the sand paper to the copper sheet to remove any light corrosion or sulphide is removed. Next, place the cleaned and dried copper sheet on the burner and turn the burner to its highest setting. As the copper starts to heat up, you will see beautiful oxidation patterns begin to form. Oranges, purples, and reds will cover the copper. As the copper gets hotter, the colors are replaced with a black coating of cupric oxide. This is not the oxide we want, but it will flake off later, showing the reds, oranges, pinks, and purples of the cuprous oxide layer underneath. The last bits of color disappear as the burner starts to glow red. When the burner is glowing red-hot, the sheet of copper will be coated with a black cupric oxide coat. Let it cook for a half an hour, so the black coating will be thick. This is important, since a thick coating will flake off nicely, while a thin coat will stay stuck to the copper. After the half hour of cooking, turn off the burner. Leave the hot copper on the burner to cool slowly. If you cool it too quickly, the black oxide will stay stuck to the copper. As the copper cools, it shrinks. The black cupric oxide also shrinks. But they shrink at different rates, which makes the black cupric oxide flake off. The little black flakes pop off the copper with enough force to make them fly a few inches. This means a little more cleaning effort around the stove, but it is fun to watch. When the copper has cooled to room temperature (this takes about 20 minutes), most of the black oxide will be gone. A light scrubbing with your hands under running water will remove most of the small bits. Resist the temptation to remove all of the black spots by hard scrubbing or by flexing the soft copper. This might damage the delicate red cuprous oxide layer we need to make to solar cell work.

Cut another sheet of copper about the same size as the first one. Bend both pieces gently, so they will fit into the plastic bottle or jar without touching one another. The cuprous oxide coating that was facing up on the burner is usually the best side to face outwards in the jar, because it has the smoothest, cleanest surface.
Attach the two alligator clip leads, one to the new copper plate, and one to the cuprous oxide coated plate. Connect the lead from the clean copper plate to the positive terminal of the meter. Connect the lead from the cuprous oxide plate to the negative terminal of the meter.
Now mix a couple tablespoons of salt into some hot tap water. Stir the saltwater until all the salt is dissolved. Then carefully pour the saltwater into the jar, being careful not to get the clip leads wet. The saltwater should not completely cover the plates -- you should leave about an inch of plate above the water, so you can move the solar cell around without getting the clip leads wet.That’s how it’s done.

Why it worksCuprous 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. Similarly, 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 dips back down.