By Chris Deziel | Updated Aug 30, 2022
Photo credit: luchschen/iStock/GettyImages
Solar panels are essentially arrays of photovoltaic cells that convert sunlight into electricity. While commercial panels use doped silicon, it’s both educational and impressive to build your own cells from everyday materials. With a sheet of copper flashing, a little saltwater, and a clear bottle, you can generate a measurable current from sunlight—enough, when combined in series, to charge a battery or light a small bulb.
When Heinrich Hertz first observed the photoelectric effect, he worked with metal plates—often oxidized copper—because silicon wafers were not yet available. Oxidized copper, specifically cuprous oxide (Cu₂O), behaves as a semiconductor. When immersed in an electrolyte and exposed to light, electrons are excited and flow from the copper into the surrounding solution. A clean copper plate serves as the anode, while the oxidized plate acts as the cathode, creating a voltage difference that drives current through an external circuit.
Start with approximately half a square foot of copper flashing, which can be found at most hardware stores. Using tin snips, cut the sheet into two equal pieces. Wash both pieces with soap and water to remove any grease or residue.
Place one sheet on the burner of an electric stove, ensuring it covers the surface. Set the heat to its highest setting and observe the color change—first brightening, then darkening as a thin layer of cupric oxide (CuO) forms. Allow the copper to reach a black appearance and continue heating for an additional 30 minutes. Turn off the heat and let the plate cool on the burner.
As the plate cools, the copper and cupric oxide shrink at different rates, causing the black coating to flake. Once the plate is completely cool, gently brush away any loose flakes. Beneath the flaked layer remains a red band of cuprous oxide—this is the semiconducting layer that is essential for the cell’s operation. Do not remove this red layer.
Use a clear plastic one‑liter bottle as the container. Cut the bottle near the middle, remove the top, and open the bottom so it becomes a shallow tray. Bend the heated copper sheet into a semicircle and place it inside the bottle so it rests against one side, with the oxidized side facing outward. Repeat with the unheated copper sheet, positioning it on the opposite side. Ensure the plates do not touch.
In a separate glass, dissolve two tablespoons of table salt into about two cups of warm water. Pour the saline solution into the bottle, filling it roughly ¾ full. The tops of the copper plates should remain above the liquid, allowing you to attach alligator clips.
Take the cell outdoors and place it on a flat surface, orienting the oxidized plate toward direct sunlight. Connect a multimeter to the plates using alligator clips. Set the meter to read microamps (µA). In full sun, the meter should register between 33 µA and 50 µA. Switch the meter to voltage mode; you should observe about 0.25 V. Calculating the maximum power yields:
P = V × I = 0.25 V × 0.00005 A = 0.0000125 W, or 12.5 µW.
Solar panels are arrays of cells wired in series, which adds their voltages while keeping current constant. To build a simple series array, connect the clean copper plate (anode) of one bottle cell to the oxidized plate (cathode) of the next cell with a wire and alligator clips. Measure the voltage across the remaining two free plates; the voltage should double to approximately 0.5 V. Each additional cell in series adds another 0.25 V. By chaining enough cells, you can achieve the voltage or power needed to charge a battery or power a small LED.
• Handle hot copper plates with care to avoid burns.
• Use insulated alligator clips to prevent short circuits.
• When scaling up, consider using a larger container or a more robust electrolyte to improve efficiency.
