My son asked me the other day, “How do solar panels work?”
I’ve come to realize lately that if I can’t break something down into simple enough concepts for my four year old to understand, I don’t understand it well enough myself.
I knew the basics enough to say that energy from the sun is converted by the solar panels into electrical energy that can be used to light our houses.
I was waiting for him to ask “But how do they do that?” – I was lucky…this time.
So, in my endeavour to be able to answer my inquisitive child, I have now done some research into what happens within the panels themselves. Here’s what I’ve learned…
Solar cells versus photovoltaic cells
A solar panel is simply a series of solar cells wired together and packaged in a frame. You are more likely to see the term photovoltaic cell rather than solar cell, simply because it is the more technically correct term.
Photovoltaic (PV) literally means the conversion of energy from light (photo) to electrical energy (voltaic). The light energy doesn’t always have to be sunlight and therefore photovoltaic is more broadly used.
However, if you are referring to solar panels on the roof of your house, the light they will be receiving will always be solar energy, so they can be called more specifically solar cells.
The layers of the solar panel
The inner most layer of the solar panel is the solar cells. The solar cells are wired together as one individual cell is simply not enough to produce any amount of useable electricity.
Moving outward, the next layer from the solar cells is usually some form of anti-reflective coating that is added to ensure electrons are not lost from the system.
Finally, the outermost layer of the solar panel is a glass plate cover to protect the cells. These layers are all combined into a frame, which is known as a solar panel.
Inside the solar cell: getting down to the molecular level
Essentially, a solar cell needs to consist of a semiconductor, capable of transporting electrons. Silicone is the most common element used to build these semiconductors as it is one of the most common elements on Earth; it’s sand.
Unfortunately, silicone in a pure form is too stable as a semiconductor. If you remember back to high school chemistry, you’ll recall that electrons orbit about the nucleus of the atom in a series of valence shells. Each valence shell has a maximum capacity of electrons and a filled shell is a happy shell as it has a lower energy. Unfilled shells make for anxious atoms (higher energy). These atoms with unfilled shells create bonds with other atoms so they can share electrons and therefore lower their energy level.
Hopefully that wasn’t too painful. I’m reminding you of all this because it’s critical to understanding the inner workings of a PV cell.
The silicone atom has four electrons in its outermost shell, which is only half the number of electrons that fit in this shell. This means it will bond readily to other atoms in order to satisfy its electron needs!
Silicone in its pure form will bond with four other silicone atoms, sharing their electrons and therefore creating stable bonds that build a matrix of silicone atoms. It then takes quite a bit of energy to break these bonds.
So, to make the silicone matrix a better conductor of electrons, impurities in the form of other atoms are introduced through a process called doping.
Phosphorous is introduced to silicone as it has five electrons in the outer valence shell. The phosphorous will then bond with four silicone atoms but it will have an extra electron, which can be broken free from the atom with relatively little energy as it remains unbound. Silicone doped with phosphorous therefore has a prevalence of free electrons, making it an effective conductor with an overall negative charge. This gives it the name N-type silicone.
On the other hand, silicone doped with boron crates a positively charged conductor, known as P-type silicone. Boron only has three electrons in its outer shell, which means that when it minds with four silicone atoms, there remains space for one electron in its outer shell.
When positives and negatives connect
Inside the solar cell, a layer of N-type (negative) silicone is placed on top of a layer of P-type (positive) silicone so that they come in contact with one another. Some free electrons from the negative side travel to the positive side quickly and easily. However, those closest to the barrier get there first and essentially block the path for those free electrons that are further from where the two silicone types meet. This essentially creates a wall where electrons can pass from the positive side to the negative side, but not vice versa, which is an electric field separating the two sides.
Electrons get excited by the sun
When the sun hits a PV cell, the energy (known as a photon) easily breaks those extra phosphorous electrons free in the negative silicone layer. This free electron then travels through the negative field looking for a hole to fill. It is unable to cross the electric field and so if an external circuit path is provided in the form of metal conductors, the electron will travel through it toward the positive field. This movement of electrons is called current and the electric field is called voltage. Together, they are electricity.
Clear as mud? Perhaps this video will help put some visuals to my words.
The electrons travelling through the conductors as DC current will finally be converted into useable AC current for your home by an inverter. This is the final step in converting solar energy into electric energy for the home.
So, now you should be able to answer the question “how do solar panels work” with sufficient detail to amaze or daze anyone who’s asking.