Solar Battery

This is the kind of energy most people think of when you mention ‘alternative energy’, and for good reason! Without the sun, none of anything you see around you could be here. Plants have known forever how to take the energy and turn it into usable stuff… so why can’t we?

The truth is that we can. While normally it takes factories the size of a city block to make a silicon solar cell, we’ll be making a copper solar cell after a quick trip to the hardware store. We’re going to modify the copper into a form that will allow it to react with sunlight the same way silicon does. The image shown here is the type of copper we’re going to make on the stovetop.

This solar cell is a real battery, and you’ll find that even in a dark room, you’ll be able to measure a tiny amount of current. However, even in bright sunlight, you’d need 80 million of these to light a regular incandescent bulb.

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You’ll need to gather these materials together:

• ½ sq. foot of copper flashing sheet (check the scrap bin at a hardware store)
• Alligator clip leads
• Multimeter
• Electric stove (not gas)
• Large plastic 2L soda bottle
• ¼ cup salt
• Sandpaper & sheet metal shears

Here’s what you need to do:

Download Student Worksheet & Exercises

How does that work? Do you remember learning about the photoelectric effect in Unit 9? This cuprous oxide solar cell ejects electrons when placed in UV light – and sunlight has enough UV light to make this solar cell work. Those free electrons are now free to flow – which is exactly what we’re measuring with the volt meter.

Semiconductors are the secret to making solar cells. A semiconductor is a material that is part conductor, part insulator, meaning that electricity can flow freely and not, depending on how you structure it. There are lots of different kinds of semiconductors, including copper and silicon.

In semiconductors, there’s a gap (called the bandgap) that’s like a giant chasm between the free electrons (electrons knocked out of its shell) and bound electrons (electrons attached to an atom). Electrons can be either free or attached, but it costs a certain amount of energy to go either way (like a toll both).

When sunlight hits the semiconductor material in the solar cell, some of the electrons get enough energy to jump the gap and get knocked out of their shell to become free electrons. The free electrons zip through the material and create a low of electrons. When the sun goes down, there’s no source of energy for electrons to get knocked out of orbit, so they stay put until sunrise.

Does it really matter what angle the solar cell makes with the incoming sunlight? If so, does it matter much? When the sun moves across the sky, solar cells on a house receive different amounts of sunlight. You’re going to find out exactly how much this varies by building your own solar vehicles.

Exercises Answer the questions below:

1. The sunlight causes the electrons to flow from the cuprous oxide because of:
   1. Photosynthesis
   2. The electromagnetic spectrum
   3. The photoelectric effect
   4. The photochemical principle
2. What material do most solar cells use instead of copper?

1. What part of the electromagnetic spectrum is most active in this experiment?
   1. Visible Light
   2. Ultraviolet Light
   3. Gamma Rays
   4. Microwaves
2. When you read amps, you read:
   1. Current
   2. Voltage
   3. Power Draw
   4. Work

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