How do solar cells work — Why do they produce electricity?
What Is the Photoelectric Effect?
The photoelectric effect was discovered in 1887 by the German physicist Heinrich Rudolf Hertz. In connection with work on radio waves, Hertz observed that, when ultraviolet light shines on two metal electrodes with a voltage applied across them, the light changes the voltage at which sparking takes place.
Albert Einstein had a role to play in bringing the world’s attention to solar energy and its potential. In 1905, Einstein published a paper on the photoelectric effect and how light carries energy.This generated more attention and acceptance for solar power on a broader scale.
When light hits a metal surface, an electron that was previously bound to the metal is knocked loose. Each particle of light, called a photon, collides with an electron and uses some of its energy to dislodge it from the metal. The rest of the photon’s energy is transferred to the now free-roaming negative charge, called a photoelectron. In the absence of the junction-forming layers, these “free” electrons are in random motion, and so there can be no oriented direct current. The electric field gives a collective motion to the electrons that flow past the electrical contact layers into an external circuit where they can do useful work.
What are solar cells?
Devices based on the photoelectric effect have several desirable properties, including producing a current that is directly proportional to light intensity and a very fast response time. One basic device is the photoelectric cell, or photodiode.
A solar cell, also called photovoltaic cell is a device that directly converts the energy of light into electrical energy through the photovoltaic effect. The overwhelming majority of solar cells are fabricated from silicon — with increasing efficiency and lowering cost as the materials range from amorphous (noncrystalline) to polycrystalline to crystalline (single crystal) silicon forms.
A solar cell is a complex and precise organization of many different materials. The topmost layer of solar cells consists of glass with an anti-reflective coat. The glass protects the materials underneath it, while the anti-reflective coat helps more sunlight reach the semiconductors. When you look at a solar cell, you will see a small grid pattern. This is a grid of thin metallic strips beneath the glass. The glass, anti-reflective coat and metallic strips create the top layer of the cell.
The middle layer of the solar cell is the most important section. It is where solar energy is created through the photovoltaic effect and consists of two layers of semiconductors. The first layer is made up of n-type material. This is generally silicon mixed with small amounts of phosphorous, which makes the silicon negatively charged. The second layer is a p-type material. This material is positively charged, and usually made by mixing silicon with small amounts of boron.
The bottom layer of the solar cell has two parts. There is a rear metallic electrode directly beneath the p-type semiconductor. This rear electrode works with the metallic grid in the top layer to create an electric current. The final layer is a reflective layer to reduce the loss of sunlight in the system. Different solar cells may use different materials depending on their intended use and desired cost. They may also have additional layers to those already mentioned. However, all solar cells use this basic configuration.
How is solar electricity produced and transmitted?
PV solar panels generate direct current (DC) electricity. With DC electricity, electrons flow in one direction around a circuit.With AC (alternating current) electricity, electrons are pushed and pulled, periodically reversing direction, much like the cylinder of a car’s engine. Generators create AC electricity when a coil of wire is spun next to a magnet.
An inverter’s basic function is to “invert” the direct current (DC) output into alternating current (AC). AC is the standard used by all commercial appliances, which is why many view inverters as the “gateway” between the photovoltaic (PV) system and the energy off-taker.
Solar inverters may be classified into three broad types:
1.) String inverters:
Most small-scale solar energy systems use a string inverter, also known as a “centralized” inverter. In a solar PV system with a string inverter, each panel is wired together into “strings.” When they produce energy, it all gets sent to a single inverter, which is usually located on the side of the home, in a garage, or in your basement. The inverter will convert all of the electricity from solar panels into AC electricity.
String inverters are the lowest-cost inverter option.They are also the easiest to maintain, because they are in an easy-to-access location.
Using a string inverter, it will only produce as much useful electricity as its least productive solar panel. Electricity production for a system with a string inverter can “bottleneck,” or be dramatically reduced, if just one or two panels are in the shade or aren’t operating properly.
2.) Microinverters:
Solar PV systems with microinverters have a small inverter installed at the site of each individual solar panel. Rather than sending energy from every panel down to a single inverter, microinverter systems convert the DC solar energy to AC energy on the roof.
Microinverters are more efficient than string inverters at converting energy. Systems with microinverters will still produce energy, even if one or two panels in the system are underperforming. Microinverters also make it possible to monitor the performance of specific panels, which makes it easier to identify production issues if they should arise.
Microinverters will cost significantly more than a string inverter, and can be more difficult to maintain or repair in the event of a problem because they are located on the roof.
Best for systems with solar panels that face multiple directions, homeowners who want to maximize solar production in a small space, and properties that have “complicated” roofs with gables, chimneys, or other objects that can cause shade.
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