Step By Step Instructions for Building Your Own Homemade Solar Cell part 3

Step 3: Burning the copper
Turn the burner on to its highest setting and let it heat up thoroughly. Then place one piece of the copper sheeting on the burner. The copper will start to turn a variety of different colors as it heats up, and then it will start to become covered in a thick black coating known as cupric oxide. 
Once the copper is fully covered in the black cupric oxide, let the copper continue to heat on the burner for another thirty minutes. After 30 minutes you can shut the burner off, and then let the piece of copper cool for at least another thirty minutes before touching it. Don’t worry if some of the black cupric oxide layer flakes off during the heating or cooling process, as you will be removing this layer later anyways.

Step 4: Gently washing off the scalded layer
Once the copper has sufficiently cooled, tap it gently on its edge on a hard surface to knock off as much of the black cupric oxide as you can. After this you can gently wash it with soap and water to reveal a reddish rust colored layer of cuprous oxide underneath. 
Do not scrub or scour the copper sheeting as you don’t want to damage the layer of cuprous oxide. It’s not necessary to remove all of the black flakes of cupric oxide, just try to get as much of it off as possible without damaging the layer underneath.