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The Photovoltaic Solar Cell: Basic Principles
The photovoltaic solar cell needs to be part of a complete circuit before it can generate any electric current. Electricity is the result of electrons moving in ONE direction through a circuit. A circuit is a closed loop of wire; there are no loose ends in a circuit. When we use electricity electrons are not gained or lost - their number remains constant. It's the energy they carry that gets used. Electrons coming from the power supply are like loaded trucks. They get unloaded at the point where we use the energy, then they return to the power source to be loaded up again. The more energy the electrons have the more work we can do with them. A power supply pushes energy rich electrons into the wire. When they return to the power source they are tired, depleted of energy. It is that energy that has powered the device we want to operate. For each electron that the power supply sends OUT one comes IN, like a hose filled with marbles. How does a photovoltaic solar cell use sunlight to make these energized electrons move? I'll start with a very basic view of the solar cell and then expand it. Each cell is made of two layers with a barrier in between them. The first layer (Layer A) contains electrons that are free to move to the second layer (Layer B). The Layer B wants these electrons more than the first layer.
These Layer A electrons will migrate to Layer B automatically, no sunlight needed. Layer B now has the extra electrons. However, while layer B has a better grip on these electrons that Layer A, it can still lose them. This is due to the pull of the nucleus on the electrons. So when sunlight hits Layer B the electrons are dislodged. Their natural response is to try to go back to the now positively charged Layer A; they do this because they can now move.This is where the barrier layer comes in. This is a one-way gate for the electrons to go from Layer A to Layer B but NOT the other way. The only way for the electrons to get back to Layer A is through the wire joining the two. So a photovoltaic solar cell or panel of cells will only generate electricity if it is connected to a circuit; otherwise it will just heat up in the sun. Once the electrons have done some work, they will have lost the energy that the sunlight provided them with and they will return, exhausted, to ayer A where they go back to their parent metal atoms. But wait! As soon as they get there they feel the pull of Layer B and off they go again, for another round. DOES HARVESTING SOLAR ENERGY ADD TO THE AMOUNT OF HEAT ON THE PLANET? Lets take a specific example. What if I run an air conditioner on Solar Power? Air conditioners use a lot of electricity and generate quite a bit of waste heat...isn't that polluting the environment?
The good news is that it is not! The electricity to power it has come from sunlight that would otherwise have been either turned into heat anyway or reflected off the surface into the atmosphere or into space. All I am doing with the air conditioner is moving that heat from where it would have been, to the back of my air conditioner. The photovoltaic solar cells do this with no Carbon Dioxide emissions, no extra heat or pollutants...NO NETT EFFECT ON THE ENVIRONMENT!! |
