The sun is beaming with energy. Light and heat from the sun are what allows life on Earth to exist. Plants are already equipped to take advantage of the sun’s energy, using photosynthesis to support growth. Unfortunately for us, we can’t use the light energy from the sun in its current form, so we devised ways to convert the sun’s light into electricity. Enter the solar cell. Solar cells capture photons from the sun and use the light to generate electricity. Read on to learn about the complex processes that allow photovoltaic cells to provide energy to your home.
How do Photovoltaic Solar Cells Generate Electricity?
All light is made of photons, whether it comes from the sun or a lightbulb. Solar cells use these photons to generate electricity. Like batteries, PV cells have a positive and negative side. When the photons from the sun strike the surface of the PV cell, the energy in the photon causes the electrons within the semiconductor material’s crystal lattice to be released. The electrons then flow from the negative side to the positive side, which creates an electric current. Metal circuitry, or the metal gridlines in your solar panel, pick up the electric current and run it out of the panel and into your home. Solar cells generate DC power, so an inverter is necessary to convert the electricity into an AC current before you can use it domestically. A single solar cell only generates a few watts of power, but that power is compounded when multiple cells are combined into a solar panel.
Only a portion of the sun’s light is actually converted into electricity. Lower energy photons pass right through PV modules. Much of the sun’s energy is also reflected by the glass. Then, there’s the bandgap, which represents the energy value above which a photon can excite an electron and create a current. The photovoltaic response of the solar cell is limited to the photons that have enough energy to land above the band gap of the semiconductor material. Different materials have different band gap values. Because of these limitations, most solar panels have an efficiency between 15% and 25%.
Solar panel manufacturers have made a few workarounds to create modules that capture more of the sun’s energy, which results in higher efficiency. Anti-reflective coatings are applied to the glass, which decreases the number of reflected photons, allowing more light to reach the PV module. Multijunction solar cells stack different materials with varying bandgap values to capture photons across a wider range of energy levels. Multijunction photovoltaics are among the highest performing solar cells, with efficiencies as high as 40%.
The Photoelectric Effect
Solar cells are possible because of the photoelectric effect. The photoelectric effect is when a material creates an electric charge after being exposed to certain frequencies of light. While any material can demonstrate the photoelectric effect, its most commonly seen in conductors and semiconductors such as metals and silicates. The effect is directly tied to the bandgap of the material and the wavelength, or color, of the light.
The photoelectric effect was first observed in 1839 by French physicist Alexandre Becquerel. Almost 100 years later, Albert Einstien used the photoelectric effect to prove the particle nature of light, earning him the Nobel Peace Prize in 1921. Einstien found that only light of certain frequencies activates the photoelectric effect, proving the existence of photons. If light was a wave, then the effect would be triggered by light of any frequency after a certain amount of time.
What are Solar Cells made of?
Solar cells require specialized materials in order to function. More specifically, they utilize special semiconductors that demonstrate the photoelectric effect at a bandgap value that matches the photon energy of the sun. Solar cell manufacturers use a variety of semiconductors to harness the sun’s energy, though silicon-based solar cells dominate the market. The silicon semiconductor is overlaid which circuitry which directs the electric current out of the solar cell. The semiconductor is protected by a metal and plastic frame, as well as a glass laminate to allow the sun’s rays to shine through.
Types of Solar Cells
Solar cells are usually identified by their design semiconductor material. There are two main types of solar cell: crystalline silicon and thin-film. Crystalline silicon solar cells have a higher efficiency and last longer, whereas thin-film solar cells trade efficiency for flexibility and portability.
Crystalline silicon (sometimes written as c-Si) is by far the dominant semiconductor material, making up over 70% of the PV market. They’re often called first-generation solar cells, being the first type of PV cell. They remain the solar cell that offers the most efficiency for the lowest cost, beating out thin-film solar cells. Though the c-Si wafers have a high purity, they are usually doped with other elements, such as phosphorus or boron, to increase the electrical conductivity. Silicon wafers come in two types: monocrystalline (mono-Si), and polycrystalline (multi-Si).
Monocrystalline remain the most efficient solar cell type, though they’re more costly. Mono-Si wafers are made of one large silicon crystal which is sliced into thin wafers. The uniform crystal structure gives mono-Si panels more conductivity, and therefore more efficiency. Mono-Si solar cells have a dark and uniform appearance.
Multi-Si solar cells are made of silicon crystals that are crushed and melded together. They’re cheaper than mono-Si, though they are less efficient since the crystalline structure is non-uniform. They also have a lower lifespan. Polycrystalline solar cells blue in hue, are recognizable by their “blue-flake” effect, created by the different crystals melded together.
Thin-Film Solar Cells
Whereas c-Si solar cells are mounted within a metal and glass frame, thin-film solar cells have a fine layer of semiconductor material printed onto plastic or glass. This makes thin-film solar cells light, flexible, and versatile, though they lack efficiency and durability when compared to c-Si. When printed on glass, thin-film solar cells can be double-sided, where the semiconductor collects sunlight from both sides. Though thin-film solar panels only make up 15%-20% of the global PV market share, they are slowly becoming more efficient and desirable for large-scale operations.
Most thin-film solar cells are made with amorphous silicon (a-Si). They are the most environmentally friendly form of thin-film cell, lacking any hazardous materials. Amorphous silicon solar cells absorbs a wide range of light, with its functional range extending outside of the visible spectrum, absorbing infrared and ultraviolet light. This makes the solar cell incredibly efficient at generating power in low-light situations, like early mornings or evenings, and during cloudy weather. This is in contrast to c-Si panels, which perform badly in diffuse light, though a-Si solar cells are less efficient overall compared to c-Si, and sometimes experience a large drop in efficiency during the first six months of operation.
Cadmium telluride (CdTe) solar cells are rapidly overtaking a-Si in the thin-film market. CdTe solar cells are cheap and efficient, with the best solar cells rivaling multi-Si cells in efficiency, having efficiencies around 18%. CdTe is often marketed as having the smallest ecological footprint of any current PV technology, and is said to have the lowest energy payback time. Though the use of both cadmium and tellurium is a major environmental concern. Cadmium is highly toxic, though is much less so when in its crystallized for as cadmium telluride. Tellurium is extremely rare in the Earth’s crust, and it’s abundance is often compared to platinum. Numerous recycling programs exist for CdTe solar cells in an effort to preserve the both environment and the rare materials.
Copper indium gallium selenide solar cells, or CIGS for short, have remarkably high efficiency for thin-film solar cells, with efficiencies as high as 20%. Due to their high absorption coefficient, they use a thinner film of semiconductor material than other solar cells. CIGS outperform polysilicates at the molecular level, but researchers are working on translating that performance onto a larger scale. CIGS are not as popular as other thin-film solar cells, and many companies that attempted to produce them commercially have gone bankrupt.
Gallium arsenide (GaAs) is sometimes used as a semiconductor for thin-film solar cells. It’s very expensive but extremely efficient. It’s saved for experimental and high-tech operations. GaAs solar cells have the highest efficiency out of any single-junction solar cell, with a record efficiency of 28.8%. They are usually used in manufacturing multijunction solar cells.
Multijunction Solar Cells & Concentrator PV
Multijunction solar cells are a high-efficiency solar cell that uses more than one semiconductor material to capture a wider range of wavelengths. They may incorporate c-Si with other semiconductors, usually GaAs. The different materials are then stacked on top of one another. Multijunction solar cells are often paired with concentrator technology, which uses curved mirrors or lenses to focus the sunlight onto the solar cells. These high-efficiency solar cells have achieved efficiencies between 35% to 45%.
Solar cells capture energy from the sun and convert it to electrical energy. When photons from the sun strike the semiconductor material, electrons are freed and an electric current is created. Circuitry on the solar cells carry the energy out of the solar cell.
Solar cells are made of plastic, glass, metal, and most importantly, a semiconductor. The semiconductor is what captures the light energy. The semiconductor is laid within a metal and/or plastic frame, then covered with glass to make a solar cell.
Crystalline solar cells are the most common type of solar cell that are commercially available. They use silicon wafers layered with glass to collect sunlight. They remain the most cost-effective and efficient solar cell design. Thin-film solar panels use a paper-thin semiconductor material printed on plastic or glass. Thin-films are cheaper to produce, but lack efficiency. They’re highly flexible, making them versatile in their uses. Multijunction solar cells are used in high-tech operations, like the International Space Station. They combine different semiconductors to capture a wider range of the sun’s energy, making them highly efficient.
The photoelectric effect is the concept that certain materials produce an electric charge after being hit by certain wavelengths of light. The semiconductors used in solar cells exhibit the effect when exposed to energy that roughly matches the wavelengths of light most emitted by the sun.
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