Solar cells generate electric power thanks to a physical principle called the photovoltaic effect. They use semiconductor materials that are capable of absorbing the energy contained in photons, or “light particles”. When the electrons in the semiconductor gain this energy, they start moving and the result is electric current.
There are many photovoltaic materials, but most solar manufacturers use silicon due to its abundance. According to the US Department of Energy, crystalline silicon is used to make more than 95% of solar panels worldwide.
How Does a Photovoltaic Cell Generate Electricity?
As you might remember from elementary physics classes, electric current is caused by the movement of electrons – the negatively charged particles that surround atoms. A photovoltaic or PV cell is a device capable of generating current when it absorbs energy from a light source.
- Most solar panels use specially treated silicon to accomplish this function.
- Silicon atoms have four free electrons that can be used to transfer electric charge.
Crystalline silicon is mixed with small amounts of other elements, which modify its chemical structure to improve electrical conductivity. This process is known as doping, and there are two possible results:
- n-type silicon has more free electrons than pure silicon.
- p-type silicon has less free electrons, and the gaps are called “electron holes”
Silicon becomes p-type if doped with the element boron, and it becomes n-type if doped with the element phosphorus. A solar cell can be made by joining two layers of n-type and p-type silicon together, and this is called a PN junction.
Here is a step-by-step explanation of how a solar PV cell converts sunlight into electric power:
- When the PV cell is exposed to sunlight, the electrons surrounding the silicon atoms gain energy, and they start moving. Free electrons have negative charge.
- Each electron that moves leaves behind a gap or “hole” that can be filled by another electron. These “electron holes” have net positive charge.
- Free electrons accumulate on the “n” side of the PN junction, while “electron holes” accumulate on the “p” side.
- As a result, the “p” side becomes positively charged and the “n” type becomes negatively charged.
- The charge separation inside the PV cell produces a voltage output at its terminals.
- Solar cells have a typical voltage of 0.50 to 0.60 volts. The exact voltage value will vary depending on the manufacturer.
A solar PV cell requires a light source to sustain its voltage. At night when there is no sunlight, solar cells no longer have an energy source to separate positive and negative charge. As a result, they return to neutral charge and their voltage output disappears.
Actually, solar cells can generate a voltage output with any light source, not only sunlight. However, using artificial lighting to keep solar panels active at night makes no sense, since the electricity spent is more than the electricity generated.
How Many PV Cells Are Used in a Solar Panel?
Solar panels come in many sizes, but two configurations are the most common in residential and commercial applications.
- Home solar systems generally use 60-cell modules.
- Commercial and industrial solar systems generally use 72-cell modules.
- Some manufacturers offer an intermediate 66-cell size.
When a solar panel is assembled, all PV cells are wired into a series circuit. This means their voltages add up, and the total voltage you get at the panel terminals is the sum of the individual cell voltages. Considering the typical cell voltage of 0.5-0.6 V, a 60-cell solar panel can be expected to generate 30-37 V, while a 72-cell panel can generate 36-44 V.
Some solar manufacturers have developed panels that achieve a higher efficiency using “half-cells” instead of conventional square cells. However, the overall size of the panel does not change. Instead of 60/66/72 complete cells, each panel has 120/132/144 half-cells covering the same space.
Types of Solar Photovoltaic Cells
Solar panels can be classified into three main types based on the composition of their PV cells:
- Monocrystalline silicon modules
- Polycrystalline silicon modules
- Thin-film modules
As mentioned above, crystalline silicon panels account for over 95% of the global market, while thin-film panels have a market share of less than 5%.
Monocrystalline panels have solar cells that are each made of a single silicon crystal. These solar cells have a uniform material structure, which means that electrons and “electron holes” move around more easily. In other words, mono solar cells are more conductive and efficient. However, processing mineral silicon into single-crystal ingots is a complex process, and monocrystalline panels are more expensive as a result.
Polycrystalline or multicrystalline panels have solar cells made of multiple crystals, which means their microscopic structure is less uniform. Poly cells are less efficient than mono cells, but their manufacturing process is simpler and less expensive. Molten silicon can be simply cast in square crucibles, and there is no need to grow single-crystal ingots.
- There is a common misconception that polycrystalline solar panels have a lower product quality, but this is not the case.
- Their lower efficiency is simply a result of different material properties. Both types of crystalline solar cells are manufactured from high-purity silicon.
Thin-film panels represent less than 5% of the global market, and they use other semiconductor materials to achieve the photovoltaic effect. Some of the most common thin-film materials are cadmium telluride (CdTe), copper indium gallium diselenide (CIGS) and silicon in noncrystalline form – known as amorphous silicon.
Do All Solar Panels Need an Inverter?
PV cells generate direct current (DC), while home appliances use alternating current (AC). The electricity output of your solar panels must be converted from DC to AC before it can be used, and this function is accomplished by the inverter. There are two possible configurations:
- Wiring solar panels into string circuits, which are connected to a common string inverter.
- Adding a microinverter to each solar panel. This device converts electric power from DC to AC at the source.
String inverters are more affordable (per watt of capacity), but microinverters achieve higher efficiency by optimizing the power output of each panel. Some string inverters are compatible with DC power optimizers, which are installed directly on the panels to boost their output.
All types of inverters add up the electricity output of solar panels, combining them into a single power generation system. However, there is an important difference in how the panels are wired together in each case:
- When using string inverters, the panels are wired as a DC series circuit. The same DC current travels through all the panels, and their voltages add up.
- When using microinverters, the panels are wired together as an AC parallel circuit. All panels are connected to the same AC voltage, and their current outputs add up.
Solar panels are designed to be used in both system configurations. The overall wiring changes, but the basic building blocks are the same with all inverter types. You can use solar panels with different inverter types in the same property, but they cannot be wired together. In other words, you will have different solar arrays operating independently.
As a quick example, assume you own a 6-kW solar system with a string inverter that is currently used at maximum capacity. If you want to add 4 kW of solar capacity for a home expansion, you can use microinverters for the new panels, without wiring them to the existing system.
When Were Solar Photovoltaic Cells Invented?
Photovoltaic cells became popular in recent decades, but the underlying technology was first developed more than a century ago. The photovoltaic effect was discovered by French scientist Edmond Becquerel in 1839, and the first solar panels were built by New York inventor Charles Fritts in 1883.
Fritts made his solar panels by applying a thin layer of gold on a selenium base, and their energy conversion efficiency was only 1-2%. As of June 2023, commercially available solar panels reach up to 22.8% efficiency, and the Fraunhofer Institute for Solar Energy Systems has achieved 47.6% efficiency under controlled laboratory conditions.
