A solar panel is a seemingly complex technology of silicon wafers, that absorb photons from the sun and transform that energy into an electrical current of electrons.
Apr 24, 2023
In a nut shell a solar panel consists of solar cells that harness photon energy radiated by the sun. The photon energy excites electrons inside a semiconductor material, commonly doped silicon. These excited electrons can create an electrical current that is then harnessed and converted into electricity.
At first, that short explanation might not make a whole lot of sense, but it’s more simple than we might realize.
In just a few short minutes, that explanation of how a solar panel works will make complete sense. By the end of this article, you will understand how solar panels harness the sun’s energy to create the electricity that runs everything.
The main principles and concepts that I will breakdown are:
How is solar energy created?
What are photons
What are electrons
What is silicon doping?
Creating the flow of electrons for electrical current
The physical components of a solar panel
What solar efficiency means
The theoretical limit of solar efficiency
The types of solar panels currently available
Firstly it is important to understand that a solar cell is the individual unit of a solar module, and multiple solar modules together are known as a solar panel. Check out this illustration to better understand the components of a solar system. To truly appreciate how a solar panel works, we first need to understand its smallest part, the solar cell.
In its most basic essence, solar power is the capture of energy donated by the radiated photons of the sun inside solar cells. For a photovoltaic panel, this radiation is harnessed by the doped silicon in a solar cell. I’ll explain what doped silicon is in a minute, so stick with me.
The harnessing of energy is achieved by photons hitting the silicon and then exciting the electrons with energy. The electrons get so excited they’re knocked out of their orbit around an atom and become free electrons.
These free and excited electrons are then drawn to another semiconductor material strategically placed nearby, creating a flow of electrons through a conductor, like a copper wire, to form an electrical current. This process of starting with photons hitting a solar cell and ending in an electrical current is known as the photovoltaic effect. This effect is also why solar panels are sometimes referred to as photovoltaic panels or PV panels.
If you’re like me, that in itself was a lot to take in, so let’s take one step back and take a minute to understand how photons and electrons magically generate electricity through this PV process.
The amount of photon energy reaching the earth’s surface is around 7000 to 8000 times the global energy consumption per year. If only 0.1% of this energy were converted into electricity with 15% efficiency, it would provide around 15,000 gigawatts of energy, enough to power the whole planet.
In perhaps the broadest understanding, photons are electromagnetic energy packets that ride the wave/particle/excitation of light. To fully unpack a solid understanding of photons is an investigation with no conclusive end as the science around photons continues today.
To understand how solar panels work, you need to know that the sun is emitting photons at the solar panel, where a semiconductor material absorbs the energy of the photons. This energy excites the electrons inside the semiconductor material to move around, and then by using unique materials in a specific pattern, an electrical current can be generated.
If you want to learn more about photons, here is a great history and breakdown of how we currently understand photons.
Now that we have a foundational knowledge of photons, let’s discover a bit more about how electrons behave in the context of a solar cell.
An electron is a tiny negatively charged particle inside an atom that exists around the nucleus. The electrons living on the outer band of the atom are known as the valence electrons. These valence electrons in doped silicon are the type of electrons that become excited by photons hitting them with energy, and most of the time will readily become loose enough to leave the valence band.
Electrons are essential because they willfully move from negatively charged parts of a solar cell to the positively charged and, in doing so, create a current of electricity. This flow of electrons is possible due to the doping technique of two silicon wafers inside the solar cell.
Doping is a technique that changes the elemental composition of the silicon semiconductors so the separate layers of silicon have an imbalance of electrons, known as extrinsic semiconductors. Doping silicon can vary the number of electrons in the semiconductor, so one silicon layer could have more electrons than pure silicon, and another layer could have fewer.
Having this imbalance of electrons is essential during the photovoltaic effect.
On one side of the solar cell, we have a silicon wafer with an overabundance of electrons, on the other side is another wafer with a deficit of electrons.
On the deficit side, there is a tiny hole where an electron would exist if the material were pure silicon. Like electrons, these holes can also freely move around inside the semiconductor, yes, magic holes.
The electron-rich layer of silicon wafer is known as the n-type and is commonly treated with phosphorus to give it that extra electron. N-type has an extra electron because phosphorus is a group V element on the periodic table, meaning it has five valence electrons.
When a phosphorus atom is added to the silicon through doping, it adds an extra valence electron. This additional electron donation occurs because silicon is a group IV element on the periodic table, meaning it normally only has four valence electrons in its orbit.
Inversely to create a silicon layer that lacks an electron, we can treat the silicon with boron or gallium, which are group III elements. Thus, the gallium-doped silicon is now missing an electron and has a hole in its place. This electron-deficient layer is known as the p-type.
Hopefully, you can see where this is going now. One side doped with an extra election, and one side doped with a missing electron.
Doping silicon also makes it easier for the extra valence electron to move from the valence band of the atom to the conduction band. The conduction band is the outer outer band that allows electrons to move freely. Once in the conduction band, the electrons can move freely and begin to move around inside the semiconductor material.
The space between the valence band and the conduction band is known as the bandgap. The smaller the bandgap, the less energy the electron requires to leave the valence band and enter the conduction band. Generally, the bandgap should decrease after doping, making it easier for electrons to move into the conductive band, known as bandgap narrowing.
To learn more about how doping is achieved in silicon for solar panel use cases, I would recommend taking a look at this article about the silicon doping process.
In between these two doped silicon surfaces is the p-n junction, which is, you guessed it, where the p-type doped wafer and n-type doped wafer mentioned earlier meet.
When an electron abundant and an electron-deficient layer meet, they create a layer of an imbalance of charge. The phosphorus layer will become positively charged as it loses electrons. Inversely the gallium (or boron) layer will become negatively charged.
Eventually, this layer will reach an equilibrium of charge, and electrons and holes will stop flowing. This region can also be referred to as the depletion region, as all holes and electrons have been paired. The depletion region’s creation is also beneficial as it only allows electrons to flow in one direction due to its consistent charge.
It is in this p-n junction where the photons of light are knocking electrons free. Once free, the electron can move to the top n-type layer while the magic hole is drawn towards the bottom p-type layer.
Then by connecting wires and metal plates to the top of the n-type layer and the bottom of the p-type layer, the free electrons are strategically given a path to avoid the depletion region and reunite with the holes in the p-type layer.
The electrons passing through the wired circuit to avoid the depletion region create the electrical current and can be directed to power electronics or batteries.
Now that the electricity is generated inside the solar cell, it needs to be channelled and harnessed. Two primary components that are essential in this part of the process are cell fingers and busbars. There is a grand photo of the difference between fingers and busbars.
Cell fingers, also known as grid or contact fingers, are dozens or hundreds of tiny little thin strips of conductive metals printed onto the front and back of a solar cell. These little thin strips carry direct electrical currents to the larger busbars to be utilized as electricity.
Busbars are much larger than cell fingers and are another metallic strip used to channel the electrical current of the semiconductors in the desired direction. Busbars are typically made from a combination of copper, aluminium, and silver to optimize conductivity.
Tab wires are soldered to the busbars to keep the electrical current flowing between each solar cell. Eventually, when enough solar cells have been soldered together, the voltage is high enough to reach the bus wires.
The bus wires are thicker and wider versions of the tab wires and can carry more current. Bus wires are made to deliver the cumulative current from all the cells in the panel to a PV junction box.
The junction box is a small device that sits on the back of a solar panel that keeps electrical current flowing in one direction while preventing current from flowing back into the solar panels by utilizing bypass and blocking diodes.
I like to think of the junction box as traffic police making sure the electrical current is constantly flowing in the right direction.
Until this point, the solar cell has been creating direct current electricity. If a house or business requires a useable alternating current, then it must be converted.
A solar inverter’s primary purpose in life is to convert direct current (DC) into alternating current (AC). Alternating current is desirable as the electrical grid in your town is most definitely built using alternating current, and many of our home appliances are powered by it.
Once the current has been converted to AC, a few other smaller components can be installed for a residential solar setup. The complete resedential setup would entail a revenue-grade meter to see how much electricity is being produced. An AC disconnect for when the current needs to be turned off. Then finally, a nearby main service panel where the current is hooked into your existing home power board.
The efficiency of a solar panel is a measurement to compare the performance of one solar cell to another. Solar efficiency is the ratio of energy output to the energy input from the sun.
Depending on the geographic location and the time of the year, the amount of solar radiation hitting a particular part of the earth can fluctuate dramatically. For example, in Denver, Colorado (near 40° latitude), a solar cell can receive nearly three times more solar energy in June than December. So to measure solar efficiency, all solar cells are tested under the same conditions at simulated peak performance in a lab.
The Shockley Queisser limit is the maximum possible efficiency limit for solar panels with a single p-n junction. Depending on the intensity of the light hitting the solar panel, this limit is between 30% to 33.7%.
However, it is possible to use multiple layers of silicon to form more than one p-n junction in the solar cell, which can double the theoretical limit of solar efficiency. Various companies are already trying to push the boundaries on this technique, and results have been promising so far.
Today, most solar panels achieve an efficiency level between 15% to 20%. Innovations in solar technology, however, have reached higher limits. The combination of multiple p-n junctions and newer semiconductor materials like perovskite lab-tested efficiencies have reached as high as 50%.
There are three types of silicon solar cells widely used for residential and commercial purposes. Two methods are mono and polycrystalline, and the third is a super thin-film amorphous solar cell.
Monocrystalline silicon produces far more efficient solar cells, though it is also the most expensive. Monocrystalline silicon is cut from a single refined crystal of silicon using the czochralski method. Once the crystal is formed, it is thinly sliced to form the silicon wafers. A monocrystalline solar cell can be easily identified as the silicon cell will have rounded or cut corners, and the cell will be a dark, almost blackish color.
Polycrystalline is formed from fragments of silicon that are melted together to form the shape of the silicon cell. The efficiency of a polysilicon cell is not as high as the mono process, but poly cells are cheaper to produce since the science is less exact. Polycrystalline solar cells are identifiable from their square or rectangular shapes and their light blue color.
Thin-film has a much lower efficiency level than poly and monocrystalline cells. These cells are only used because of their incredibly flexible, thin, and durable nature. The thin film of the solar cell is placed on a glass or plastic surface to power low-energy devices. Thin-film solar is commonly used in small devices like calculators or for recreational uses like camping.
At this point, you should have a general understanding of all the main components of how sunlight hitting the earth is miraculously turned into electricity by a solar cell. Each section in this article outlined the primary factors of how the photovoltaic effect works and how this leads to a solar panel harnessing the sun’s energy.
To summarise, some of the essential key terms and principles I discussed were:
Electromagnetic energy packets that ride the wave/particle/excitation of light.
Tiny little negatively charged particles inside an atom that exist orbiting around the nucleus.
Material that is capable of electricity conduction but is not completely conductive.
The process of treating silicon with another element to either add or detract electrons from its atomic orbit.
The distance between the valence band and the conduction band for electrons travel when excited by the energy of photons.
The p-n junction is the meeting point of the two doped semiconductor materials, resulting in a depletion gap where a positive and negative charge equilibrium is created.
Solar Cell Fingers
The tiny thin strips printed onto the solar cell channel the electrical current towards the busbars.
They are the more prominent, normally silver-coated, conductive wires running across a solar cell to channel the electrical current further.
By utilizing bypass and blocking diodes, the junction box keeps electrical current flowing in one direction while preventing current from flowing back into the solar panels.
Since the electricity created from a solar cell is direct current, the electricity needs to be changed to alternating current by the inverter for most everyday applications.
The theoretical limit for solar cell efficiency is between 30% to 33.7% if only one p-n junction is formed inside the solar cell.
This is the process of creating pure silicon crystals by using the Czochralski method, which produces a higher efficiency but more expensive solar cell than other methods.
This type of solar cell is created by melting solar fragments together to form a less efficient but cheaper solar cell.
A low-efficiency solar cell that is primarily used for its durability and flexible nature.
Suppose we reflect on the first paragraph of this article where I gave a brief description of how a solar panel works. The definition, at first, may not have been so intuitive. So now, let’s look back on that first definition but go a step further with even more of the terminology and concepts we just learned in this article.
A solar panel is a group of solar modules made up of individual solar cells. The solar cells within a solar panel are commonly made of monocrystalline or polycrystalline silicon. A solar cell converts the energy of photons radiated by the sun into excited electrons. A solar cell is doped with other elements to either provide an extra electron or leave a hole in the valence band. Through this process, the pure silicon wafers will become a semiconductor.
Once an excited electron exits the valence band of their atomic orbits, it crosses the bandgap to enter the conduction band. At which point the excited free electron can move across a circuit of cell fingers, busbars, bus wires, and tab wires through a junction box to create an electrical current. This electrical current can be converted from DC into AC for practical use.
Congrats, you now understand the basic principles of a solar panel. With this knowledge reading about solar panel technology, innovations and debates should make all the more sense. If you have any comments or want to chat about solar, feel free to reach out.