Solar cells are devices that turn sunlight into electricity. This technology offers a more Earth-friendly way to produce power than burning coal and other fossil fuels. But panels of solar cells need a lot of open, sunny space to harvest that sunlight. Where space is limited, people might have to choose between solar panels and fields to grow food. But what if you could produce crops and electricity at the same time? For Yang Yang, the solution is clear. Clear solar cells, that is.
Yang is a physicist at the University of California, Los Angeles. He studies solar cells, also known as photovoltaic (Foh-toh-vol-TAY-ik) cells. Many are made using the element silicon. Yang instead works with carbon-based versions. These solar cells are known as OPVs, short for organic photovoltaics. OPVs are flexible and simple to make. Recently, several groups of scientists made OPVs that are transparent, or clear.
Because they let light pass through, one of Yang’s students suggested that they use OPVs on a greenhouse roof. This inspired Yang’s team to create and test a solar cell that could both make electricity and let enough light pass through to grow plants.
Sunlight is made up of many colors, or wavelengths, of light. The colors we see — from violet through red — are called visible light. Their wavelengths range from 400 to 700 nanometers. (A nanometer is a billionth of a meter.) Yang’s solar cells let visible light pass through, which makes them look clear. But solar cells must also absorb some light to generate electricity. Yang’s absorb infrared light, which has wavelengths between 700 nanometers and 1 millimeter. About half of the light that comes from the sun is infrared. People can’t see it, but some animals — such as snakes and bats — can sense infrared.
Plants don’t need infrared light. In fact, they need only a very small range of visible light to grow. Most plants absorb red and blue light but reflect green light. (That’s why most plants look green.) Yang’s team wanted to see whether the visible light that passes through the clear solar cells would be enough for plants to grow.
His team didn’t have enough materials to build a whole greenhouse. So they did experiments on a small scale instead. First, they put dirt into beakers and planted mung beans in them. They grew the beans in natural sunlight under three conditions. One group of beakers was uncovered. One group had aluminum foil on the sides and a clear OPV on top. Beakers in the final group were fully covered with aluminum foil to block all light.
After 13 days, the scientists looked at how well the seeds had sprouted. In the foil-covered beakers, plants grew poorly. But seeds with a clear solar cell on top grew about as well as seeds in uncovered beakers. That means the solar cells might work on the roof of a greenhouse.
The team published its findings online January 3 in ACS Nano.
Room for growth
Whether enough light can pass through a solar cell isn’t the only feature scientists need to think about. There’s also efficiency. Common rooftop solar cells, which absorb visible light, have an efficiency of about 18 percent. That means they produce 18 watts of power for every 100 watts of sunlight they absorb. Yang’s test OPV has close to 10 percent efficiency, which is still quite good. "But our lab's cell is very small," Yang says. "When the solar cell is made with a large area, the efficiency typically drops a little bit."
Yang thinks greenhouses might become more popular for farming in the future. As climate change makes growing conditions less predictable, greenhouses could give farmers more control. Yang’s team would like to “turn the greenhouse into a powerhouse to generate electricity,” he says. “That would be very useful.”
But for people to use this technology on a wide scale, “everything comes down to cost,” he adds. “Right now, this technology is very expensive.” Yang hopes that one day a large company would be willing to mass-produce the technology. That might help to drop its cost.
Luis Campos thinks using the solar cells for both power generation and farming is clever. A chemist, he works at Columbia University in New York City. Because most solar cells absorb visible light, only transparent OPVs like Yang’s could be used for this purpose. “It is certainly exciting to see the multiple applications that renewable energy devices can serve,” he says.
Campos shares Yang’s concern that bigger versions of the cells might not work as well. Making devices larger can create new challenges, he says, and their efficiencies can fall. Still, he finds Yang’s use of solar cells “an impressive accomplishment.”
When today’s students grow up, Yang hopes some of them will help make this type of technology better and less costly. That would boost its appeal and potential uses. “Science is fun and a challenge,” he says, “and I hope the young kids will join us.”
aluminum A metallic element, the third most abundant in Earth’s crust. It is light and soft, and used in many items from bicycles to spacecraft.
application A particular use or function of something.
carbon The chemical element having the atomic number 6. It is the physical basis of all life on Earth. Carbon exists freely as graphite and diamond. It is an important part of coal, limestone and petroleum, and is capable of self-bonding, chemically, to form an enormous number of chemically, biologically and commercially important molecules.
climate change Long-term, significant change in the climate of Earth. It can happen naturally or in response to human activities, including the burning of fossil fuels and clearing of forests.
electricity A flow of charge, usually from the movement of negatively charged particles, called electrons.
element A building block of some larger structure. (in chemistry) Each of more than one hundred substances for which the smallest unit of each is a single atom. Examples include hydrogen, oxygen, carbon, lithium and uranium.
fossil fuels Any fuels — such as coal, petroleum (crude oil) or natural gas — that have developed within the Earth over millions of years from the decayed remains of bacteria, plants or animals
greenhouse A light-filled structure, often with windows serving as walls and ceiling materials, in which plants are grown. It provides a controlled environment in which set amounts of water, humidity and nutrients can be applied — and pests can be prevented entry.
infrared A type of electromagnetic radiation invisible to the human eye. The name incorporates a Latin term and means “below red.” Infrared light has wavelengths longer than those visible to humans. Other invisible wavelengths include X-rays, radio waves and microwaves. Infrared light tends to record the heat signature of an object or environment.
mass A number that shows how much an object resists speeding up and slowing down — basically a measure of how much matter that object is made from.
nano A prefix indicating a billionth. In the metric system of measurements, it’s often used as an abbreviation to refer to objects that are a billionth of a meter long or in diameter.
online (n.) On the internet. (adj.) A term for what can be found or accessed on the internet.
organic (in chemistry) An adjective that indicates something is carbon-containing; a term that relates to the chemicals that make up living organisms. (in agriculture) Farm products grown without the use of non-natural and potentially toxic chemicals, such as pesticides.
photovoltaic An adjective that describes the ability of certain technologies to convert sunlight into electricity.
physicist A scientist who studies the nature and properties of matter and energy.
range The full extent or distribution of something. For instance, a plant or animal’s range is the area over which it naturally exists. (in math or for measurements) The extent to which variation in values is possible. Also, the distance within which something can be reached or perceived.
renewable energy Energy from a source that is not depleted by use, such as hydropower (water), wind power or solar power.
silicon A nonmetal, semiconducting element used in making electronic circuits. Pure silicon exists in a shiny, dark-gray crystalline form and as a shapeless powder.
solar cell A device that converts solar energy to electricity.
watt A measure of the rate of energy use, flux (or flow) or production. It is equivalent to one joule per second. It describes the rate of energy converted from one form to another — or moved — per unit of time. For instance, a kilowatt is 1,000 watts, and household energy use is typically measured and quantified in terms of kilowatt-hours, or the number of kilowatts used per hour.
wavelength The distance between one peak and the next in a series of waves, or the distance between one trough and the next. It’s also one of the “yardsticks” used to measure radiation. Visible light — which, like all electromagnetic radiation, travels in waves — includes wavelengths between about 380 nanometers (violet) and about 740 nanometers (red). Radiation with wavelengths shorter than visible light includes gamma rays, X-rays and ultraviolet light. Longer-wavelength radiation includes infrared light, microwaves and radio waves.