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For the first half of 2023, renewable sources, including solar, wind, and hydro, produced 25% of America’s electricity. But how do solar panels make electricity out of light? Can they be a viable replacement for fossil fuels at scale? In this article, we’ll look at how solar power works and what it can—and can’t—do.
How Do Solar Panels Generate Electricity?
The Earth’s surface receives about a kilowatt of solar energy per square meter under optimal conditions, which means “at the equator, at solar noon.” The best solar cells can wring out nearly half that light energy as electricity.
It happens like this: A photon from the sun strikes the solar panel’s photoactive layer, which “knocks” an electron out the far side of the panel. This is possible because solar panels use semiconductors to transform light into electricity.
Silicon, like carbon, has a half-full outermost valence shell. Monocrystalline solar panels use layers of silicon doped with different types of atoms, like boron and phosphorus, that have opposing charges (think +1/-1). Doping in this way turns a regular silicon crystal lattice into n-type and p-type semiconductors. N-type semiconductors have mobile electrons; p-type semiconductors have mobile “holes.” It turns the whole system into an electrical turnstile, forcing electrons to move in one direction.
Credit: American Chemical Society
When sunlight strikes the outermost layer, the atoms absorb enough energy to punt them across the ‘band gap.’ (You can’t have, say, 85% of an electron, or 110%—the system will find the nearest stable energy level. The ‘band gap’ is the unstable range of energy levels between the stable ones.) To get back into a stable configuration, the system sheds energy as an electron. The n-type layer already has ‘extra’ electrons, so it’s disinclined to take them back. Instead, the extra charge is passed through an electrode.
What results is electricity in the form of direct current (DC). To be useful to AC appliances, that DC power must go through an inverter.
There are a few dominant types of solar panel technology. Perhaps best known are solar cells made of silicon, in allotropic arrangements from amorphous silicon to a rigid crystal lattice. But there are also solar dye cells, thin film solar cells, and solar panels made of a different type of crystal called perovskites. These approaches use semiconducting materials to produce photovoltaic energy—electricity harvested directly from sunlight.
Crystalline Silicon
Most standalone solar panels, including the classic, hard-panel 200W workhorse solar cells, are made of crystalline silicon. Embedded systems and devices, such as the humble solar walkway light and solar-powered calculators, also frequently use crystalline silicon solar cells. This type of solar cell can generate solar power at 15-20% efficiency, which means it outputs 15-20% of the energy it receives from the sun.
Tesla Solar Roof tiles use crystalline silicon cells.
Credit: Tesla
Crystalline silicon is often the material of choice for solar cells because of its durability. The windows on the space shuttles were made of pure fused crystalline silicon. Similarly, crystalline silicon panels can withstand more abuse and weathering than some thin-film solar cells.
Newer residential solar technologies like the Tesla Solar Panel and Solar Roof, GAF’s Timberline Solar Energy Shingles, CertainTeed Apollo II shingles and tiles, and Luma Solar’s solar shingles use crystalline silicon. Some manufacturers cover their panels with a layer of tempered glass to prevent damage from hail.
Thin-Film Solar Cells
Thin-film solar cells are what you get when, instead of using a rigid crystal wafer shot through with dopants, you vapor deposit photoactive semiconductor molecules onto a sliver of crystal, or a ribbon of flexible polymer.
Many thin-film cells use amorphous silicon, which can absorb energy from UV, visible, and infrared light. Others use polycrystalline silicon, vapor deposited in a thin film on glass, which can reduce reflections and better trap light.
Researchers at Sandia National Laboratories developed tiny glitter-sized photovoltaic cells that are only 14 to 20 micrometers thick (a human hair is approximately 70 micrometers thick), yet perform at about the same efficiency as solar devices that use 100 times more silicon.
Credit: energy.gov
Thin-film cells use less material than monocrystalline panels, so they may have a lower life-cycle ecological impact. Some thin-film systems can reach efficiencies of 47% by using multiple types of photoreactive materials, a little bit like how plants use different photopigments to catch different wavelengths of light. However, like perovskites, they experience a drop-off in electrical output during the first months of their useful life.
What Is a Perovskite, Anyway?
Perovskites are minerals of a specific crystal structure. Where crystalline silicon cells use dopant ions to facilitate electron handoffs between layers of positive and negative charge, perovskites’ own crystal lattice alternates between anions and cations. This means they can serve as a solid-state solar cell.
Perovskite solar cells have gathered some buzz recently because they are cheap and straightforward to manufacture. This cell type may be useful in ambiently powered embedded systems, such as the Internet of Things. There are a lot of different perovskite chemistries, many of which are outlined here:
Perovskites and other fun solar cell types are shown in red.
Credit: U.S. Department of Energy’s National Renewable Energy Laboratory (NREL)
Compared with monocrystalline silicon solar cells, some perovskite cells can generate more electrical power from the same amount of solar energy. However, perovskites are less durable; because their chemistry is unstable, they experience a characteristic drop-off in energy production over the first few years of operation. The best-understood perovskites also frequently use lead. While the crystals don’t just drop free lead ions, lead mining and processing present their own hazards. Tin will do the job but at a lower efficiency.
Photoreactive Pigments: ‘Artificial Photosynthesis’
Another fun solar power technology is the dye-sensitized solar cell. These flexible cells use a photoreactive dye mixed with semiconductor nanoparticles as the top layer. Usually, that mixture floats on a liquid electrolyte that acts like the p-type layer on a crystalline cell and its own electrode simultaneously.
Like perovskites, these flexible cells conduct their reactions throughout their volume instead of along a plane. This has led some experts to refer to dye cells as ‘artificial photosynthesis.’ But perovskites are crystals, and the same liquid electrolyte that makes solar dye cells so flexible makes them vulnerable to chemical degradation and temperature extremes.
How Does Solar Power Scale Up?
There are two principal ways that we currently use solar energy to generate power on a scale suitable for use as a public utility. One you’ve probably seen is the standard-issue solar farm. These facilities provide energy by parking rows and rows of solar panels on land usually not useful for much else—although recently, scientists have been researching ways to use the shaded ground beneath the panels for different purposes, such as energy storage in the form of batteries. One interesting proposal involves growing shortish, shade-loving plants like coffee and cacao.
Credit: Andres Siimon/Unsplash
Put bluntly, solar power only works when there’s sunlight to gather. This means power banks and diversified energy production are pretty important parts of utility-scale solar power production: It’s critical to store any extra energy to make hay while the proverbial sun shines. Tesla, in addition to its solar power panels and solar roof tiles, offers a battery backup called the Megapack. Arrays of dozens or hundreds of these power storage cells are already in operation, facilitating the use of solar power and other renewable energy sources.
Another way we’ve scaled up solar power is with concentrating solar plants, like Ivanpah in California. Instead of producing power via the photoelectric effect, these systems use mirrors or lenses to concentrate solar energy into a (relatively) focused beam pointed toward a central collecting tower. There, the solar energy drives a heat engine, often a steam turbine, which converts heat into electricity.
A concentrating solar thermal power plant in Jiquan City, China.
Credit: Li Damao/Unsplash
From June 2021 to June 2022, 17.6 GW of new utility-scale solar capacity came online in the United States. This brings us to about 66 GW of total utility-scale solar power. Last year, between power plants and distributed solar installations, solar power provided 12% of America’s electricity. While that’s just a small fraction of total domestic power generation, we’re adding more new capacity yearly. As renewables replace older non-renewable power plants, solar power is sure to have its day in the sun.