Solar cells contain electrons generated by n-type and p-type silicon semiconductors which create a flow of electricity when they interact with light. This is what is known as the photoelectric effect. First discovered in 1887, the photoelectric effect describes what happens when electrons meet photons.
But what exactly are electrons or photons? Can a solar panel run out of electrons? In this guide, we zoom in on solar panels to see how they work at an atomic level.
What Is An Electron?
An electron is a subatomic particle with a charge of negative electricity. All atoms contain electrons as elementary particles. They are the primary carrier of electricity in solids. Electrons can be bound to the nucleus of the atom or occupy the space around the nucleus as free electrons.
In an electrical conductor, the movement of electrons from atom to atom or from negative to positive poles creates a current flow. In a semiconducting material like silicon, current flows from the movement of electron deficiencies.
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Electron deficient atoms in semiconductors are called holes so it might be more accurate to say electrons fill up holes to create current.
What is a photon?
A photon is a bundle of electromagnetic or light energy. They are particles of light that carry energy. A photon is also a “quantum” or unit of electromagnetic radiation. While the single largest source of photons is the sun, photons are everywhere from the screen on your device to the magnets on your refrigerator.
Now you know what electrons and photons are. But how do they combine to produce electricity?
How Do Electrons Flow In A Solar Panel?
Electrons flow in a solar panel through the photoelectric effect. Sunlight carries electromagnetic radiation in the form of photons. When photons are absorbed in a solar panel, electrons are energized which causes them to move. It is this movement that creates an electric current.
To get a more complete definition of how the movement of electrons in a solar panel occurs, we need to understand how a solar cell is designed.
What is in a solar panel?
A solar panel is made from silicon. But not just any old silicon is used. Pure silicon is a poor conductor of electricity so impurities are injected into the silicon to make it more conductive. This is what is known as doping.
Doping is the foundational element of solar panel technology that allows us to alter a material’s ability to conduct electricity. The most common elements used to dope silicon are:
- Phosphorous
- Boron
- Gallium
- Indium
When you dope a material, you’re increasing the number of charge carriers or electrons in that material to produce a semiconductor. The most common type of semiconductors are n-type and p-type semiconductors.
N-Type or Negative type semiconductor
With an N-type semiconductor, phosphorous is injected into the silicon. An N-type semiconductor is the electron donor as they have more electrons than holes.
P-Type or Positive Type semiconductor
With P-type conductors, silicon is doped with boron or gallium. A P-type semiconductor is the electron acceptor as it has more holes than electrons.
In a solar cell, the N-type and P-type semiconductors are layered together. The edges where they meet form a single junction. This is called a P-N junction or positive to a negative junction and it is where our electrons and electron holes meet.
When sunlight hits a solar panel, photons are absorbed and the free electrons in the N-type layer of the P-N junction move across the junction to combine with electron holes in the P-type layer. The electron holes in the P-type layer move the other way to combine with electrons in the N-type layer. This is how you get the flow of electrons that creates a flow of current.
Do Solar Panels Run Out Of Electrons?
A solar panel cannot run out of electrons. The electrons in a solar panel form a closed circuit with the device or battery they are connected to. Electrons flow between the solar panels and the device but they never leave the circuit.
A circuit can be as simple as a light bulb connected to a battery via two wires. There is a negative wire on the negative terminals and a positive wire on the positive terminals. If you remove one or both wires then there is no conductor through which electrons will flow. The circuit is open.
When you place the wires on the terminals, the circuit is closed so electrons flow back and forth between the battery in an endless loop. Source
Picture it as a watertight pipe with no outlets. The water circulates within the pipe, but because there are no outlets and the pipe is watertight, the pipe never runs out of water. In this case, the solar panels are the pipe and the electrons are the water.
At this point, you might have a simple question in your mind. If electrons never run out, why do solar panels become less efficient, or why are they not 100% efficient? If the number of electrons does not get depleted, shouldn’t solar panels retain their efficiency?
The drop in the efficiency of a solar panel is partially explained by factors that are outside the photoelectric effect. For example, solar panel efficiency declines due to the natural wear and tear of a solar panel’s components over time.
Part of why solar panels are not 100% efficient is because of a flaw in the photoelectric effect. The flaw is the effect can also occur in reverse.
Instead of having electrons combining with electron holes, you have electrons combining with photons. This is called recombination and it is the reason why solar panels have limited efficiency.
We highly recommend that you check out our post Why Are Solar Panels So inefficient? Factors & Comparision To Other Energy Sources!
How Are Electrons Replenished In Solar Panels?
Electrons in a solar panel are not replenished because they do not deplete. They move around in a circuit at a constant rate and never run out.
What Carries Electrons In A Photovoltaic Cell?
Electrons in a photovoltaic cell are carried by busbars. Solar cell busbars are thin metallic wires that are typically made from copper or aluminum. Copper or aluminum are good conductors of electricity so they carry electrons from a solar cell into the connectors and through to the rest of the system.