Cuprous oxide is a semiconductor, just like doped silicon, and was one of the first materials experimented with as a solar cell. But its efficiency was found to be so low, around 1%, that people stopped trying, though the occasional set of experiments are still done in research labs and papers published. It was also used to make commercially sold diodes and may still be, since that's what a solar cell is - a photosensitive diode.
Here are some research papers on cuprous oxide solar cells:
And before you comment that it's a battery... At 2:30
the cell is getting full sunlight and it is around 10 microamps. At 2:35
I block the sun and it is around 0 microamps. Then at 2:41
I unblock the sun so the cell is getting full sunlight again and it is around 10 microamps again. If it was a battery then the current wouldn't be going back and forth when I block and unblock the sun. So it's solar.
This shows how to make and test a solar cell using a sheet of copper. These solar cells are basically sheets of copper with a layer of cuprous oxide (Cu2O) on them. The cuprous oxide is a semiconductor. You won't get much electricity from it but it's fun to make!
Also watch my video "How to Make Solar Cells using Transistors (2N3055)" showing how to open these transistors and the testing I did, including with a fresnel lens:
And see my "Make Solar Cell Transistor Powered Calculator using 2n3055" video where I make a solar panel out of transistor cells to power a calculator:
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How to make a homemade solar cell with a sheet of copper on WonderHowTo.