Exploring the science behind photovoltaics
Solar panels convert light into electricity. It’s a complex process that involves physics, chemistry, and electrical engineering. With solar panels becoming an increasingly important part of the push against fossil fuels, it’s vital to learn just how a solar panel converts sunlight into usable energy. Interestingly enough, the same concepts that allow solar panels to power our homes are also driving the technological revolution. The secret lies in the silicon wafer, the building block of modern electronics.
To put it simply, sunlight strikes the panel and excites electrons in the silicon crystal. The photons give the electrons enough energy to move freely through the silicon. The silicon wafer is infused with impurities to create a natural electric field, which directs the movement of the electrons. Metal gridlines on the solar cell capture the electrical energy and transport it towards your inverter, then into your home.
What Is Energy?
We need energy to do work. Whether it’s to move our bodies, grow our crops, or power our homes, energy powers our world. Energy can take several forms, including light, motion, electricity, chemical reactions, and heat. The first law of thermodynamics states that energy cannot be created or destroyed, only change form. This is inherent to the problem of the needs of humanity, as energy itself is abundant but often doesn’t exist in a form that can be directly applied.
When we install solar panels, we are harnessing light energy from the sun. When the light strikes the surface of the semiconductor material, a reaction takes place, which converts the light energy into electrical energy. But since solar panels aren’t 100% efficient, some of this light energy becomes heat.
Once the energy is converted to electricity, metal gridlines on the panel carry the electricity out of the panel and toward your battery storage. The energy is then converted into chemical energy, where it is stored until it’s ready to be converted back to electricity for domestic use.
The Photovoltaic Effect
The photovoltaic effect is what allows sunlight to be captured and converted into electrical energy. The phenomenon was discovered by French physicist Edmond Becquerel in 1839 when he was experimenting in his father’s laboratory with platinum electrodes in an electrolyte solution. He noticed that when light was shined on the solution, the electric current was enhanced. The first rooftop solar array followed soon after.
Light is made of photons, which carry energy. The energy in a photon is proportional to the frequency of light. The photovoltaic effect is triggered when photons strike a photoelectric surface, which absorbs the photon’s energy and excites electrons within the material. An electric current is created when enough electrons are stimulated. Depending on the material, the frequency necessary to trigger the effect can vary. In photovoltaic solar panels, semiconductors are the photoelectric medium used to convert sunlight to electricity.
A semiconductor is a material that conducts electricity more than an insulator, like glass or wood, but less than a conductor, like copper or gold. The conductivity of semiconductors can be altered via doping, or the addition of impurities, to reach a conduction value that suits the needs of a particular application. They can be found in computers, cars, smartphones, and home appliances. Silicon is the most common semiconductor, usually taking the form of silicon wafers. The advent of crystallized silicon was the main driver of the digital revolution of the past 50 years, hence the use of the term Silicon Valley to refer to the Bay Area, home to the world’s largest tech companies.
The wafers can be positively doped (p-type) or negatively doped (n-type). A p-type and n-type can even exist within the same crystal, which is the case with PV panels. The p-type has atoms that lack an electron, called electron holes, where the n-type has atoms that have an excess of electrons. The electrons and holes are collectively known as charge carriers. The two meet in a boundary layer within the crystal, which is called the p-n junction.
The crystal structure of silicon wafers is integral to its function. In the crystal lattice, the electrons are bound in place, unable to move freely. When an input of energy excites the electrons to sufficient energy levels, they’re able to break free and move throughout the lattice structure. The electrons then diffuse through the p-n junction, filling in the electron holes and neutralizing both charge carriers. This creates an area of neutral material called the depletion zone. Eventually, the movement towards the p-n junction reaches a state of equilibrium, and an electric field forms around the depletion zone. The n-side boundary becomes positively charged, and the p-side boundary becomes negatively charged, creating a force that acts opposite to the movement toward the p-n junction. This stops the flow of electrons across the p-n junction, and the wafer remains in this equilibrium state until the energy level in the system changes.
Semiconductors are limited by their band gap, an energy range where the movement of electrons won’t occur. The light energy striking the surface of the solar panel must be above the band gap of the semiconductor, or else no electricity will be produced.
Just as in electronics, silicon is the most common semiconductor for solar panels. Silicon panels come in three types:
- Monocrystalline (MonoSi)
- Polycrystalline (PolySi)
- Amorphous Silicon (a-Si)
Several other types of semiconductors are used in the PV industry, though they tend to be less common. A few types are listed below.
- Cadmium Telluride (CdTe)
- Copper Indium Gallium Selenide (CIGS)
- Gallium Arsenide (GaAs)
Although this article focuses on the working mechanism behind silicon solar panels, most semiconductors work on the same principles.
Sunlight to Electricity
Now that we’ve explored the various concepts and processes that allow your solar panels to generate electricity, let’s take a closer look at what actually happens inside your PV array.
You wake up in the morning, and the sun rises above the horizon. As you begin your morning routine, sunlight washes over your roof, bringing energy to your home. The sun has a broad energy spectrum and emits photons over a large range of energy values. Remember that PV semiconductors have a band gap, and the photons striking the surface of your panels must be above the band gap in order to stimulate the conductivity of the material.
One of three things can happen when a photon interacts with your solar panel:
- The photons might be reflected off the surface of the panel.
- If the photon’s energy level is below the band gap, it will pass right through the panel.
- If the photon’s energy level is at or above the band gap, it will interact with the semiconductor.
The architecture of the solar cell plays an essential role in the movement of electrons. The n-doped layer is very thin and is placed directly under the glass, on top of the much thicker p-doped layer. This means the sunlight penetrates the n-side and reaches the p-n junction. The increased thickness of the p-side also creates a much larger depletion zone than if the two were equal in size. The energy from the photons is transferred to the electrons, giving them the energy to move across the depletion zone and into the p-side. The electrons recombine with the electron holes on the p-side, while the sunlight perpetually stimulates new electron-hole pairs in the depletion zone. This constant movement is the source of the electric current. The silicon remains in this electrically charged state as long as the sun is striking the panel. When the sun goes down, the silicon returns to its equilibrium state, and the depletion zone returns to its original width.
Whereas the depletion zone prevented an electric current from being generated, the energy input from sunlight gave the charge carriers enough energy to overcome the neutral layer. Since many of the photons that interact with the silicon have energy values higher than the band gap, the excess energy is dissipated as heat.
With the electrons free to move through the silicon, all that’s needed is a path for the electrical energy to make its way out of the panel. Each solar cell has two sets of metal gridlines connected to its surface, called fingers and busbars. The electricity is collected in the fingers, which are the very thin set of metal gridlines that run up and down the solar cell. The fingers route the electricity to the busbars, which run perpendicular to the fingers. The busbars are much thicker than the fingers, and most solar cells have two busbars spanning the length of the cell.
The busbars are connected via copper wires to the back of the next solar cell, and they are wired in series to each other, generating electricity and running it down the sequence of cells. Several series of cells are then wired parallel to each other, forming a solar panel. The solar panel is then wired to several other panels, creating a solar array.
The photovoltaic processes generate a direct current, so an inverter is needed to convert the DC power to AC power. The electricity is then stored in a battery, where the energy is stored as chemical bonds until it is ready to be discharged.
While humanity has been harnessing the sun’s energy as heat for centuries, solar PV has allowed us to directly capitalize on the sun’s rays. Although the technology has been slow to take off, the idea of harnessing sunlight for energy has revolutionized the energy industry. The prospect of ditching fossil fuels for the limitless energy from the sun has changed how we look at electricity. Photovoltaic panels draw upon the unique properties of silicon semiconductors to convert light energy to electrical energy. The physical and chemical properties of crystallized silicon allow the material to react to light in a way that it generates an electric charge. Metal gridlines carry the electrical energy out of the panel and toward your home. It’s a complex process, one that can potentially bring energy to sun-washed cities all over the globe.