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Golden, Colo. — Two recent innovations are boosting prospects for a new type of solar-energy technology. Both rely on a somewhat unusual type of crystal. Panels made from them have been in the works for about 10 years. But those panels had lots of limitations. New tweaks to their design might now lead to better and potentially less costly solar panels.
Photovoltaic (FOH-toh-voal-TAY-ik) panels convert sunlight into electricity. One tweak to the materials designed for use in the new type of panel would let them tap more of the energy in sunlight. A second advance makes it easier to stack layers of this material into a sandwich. Each layer is most sensitive to different wavelengths, or portions of sunlight. Stacking the layers should harvest more incoming light.
Researchers at the National Renewable Energy Laboratory (NREL) in Golden, Colo., have been leading efforts to develop the new solar technology. They unveiled new developments in October 2019 to reporters who came to the Society of Environmental Journalists annual meeting. It was held in nearby Fort Collins, Colo.
A big industry already exists to make solar panels. Today, almost all are made from thin but rigid wafers of silicon. Silicon, the basis of sand, is cheap. But making wafers from it is not. The wafers must be manufactured in carefully controlled conditions. And the finished solar panels won’t bend.
In September 2018, physicist Joe Berry (fourth from right) and others at the National Renewable Energy Laboratory reported how to make perovskite solar cells with roll-on printing. The process might one day make production of solar panels as fast as printing out newspaper pages today.DENNIS SCHROEDER/NREL
In contrast, the new solar panels are made with manufactured crystals called perovskites (Puh-RAHV-skytes). These crystals contain some element with properties like bromine or iodine, plus a metal and other ingredients. A liquid mixture of these can be painted or rolled onto any surface.
As the liquid quickly dries, crystals form. The crystals line up in a way that makes them work well as semiconductors — materials that sometimes conduct electricity. Yet they’re much easier and quicker to make than the crystals in panels of silicon-based solar cells.
So covering sheets with these crystals might one day be as fast as printing ink onto rolling panels of paper. But instead of ending up with a newspaper, you’d end up with solar panels — ones that might be as flexible as a magazine page. Or, the perovskite liquid might be painted onto a structural surface. This could turn the sun-facing wall of a building into a massive solar panel.
Photovoltaic materials usually work well with only certain wavelengths of sunlight. Which wavelengths work best depends on what the materials are made from. Lead-based perovskite crystals work well in the deep-red to near-infrared range.
Joe Berry is a physicist at NREL. He and others knew tin-based perovskites could produce power from lower-energy infrared wavelengths. But they weren’t very efficient. And the material broke down quickly. So his team looked at where the solar cells were losing efficiency. They found that the contact points between the crystals and other materials often develop defects.
They tried a number of fixes. The one that worked best: Add a chemical called guanidinium thiocyanate (Gwahn-ih-DIN-ee-um Thy-oh-SY-uh-nayt). Biochemists often use it in the lab to protect bits of genetic material. Here, Berry’s team added it to improve the structure of crystals that touch surfaces. This tweak also let the solar cells harvest sunlight for a bit longer. Both innovations boosted the ability of the solar panels to produce electricity.
“There was a lot of stuff that we tried,” Berry says. But such trial and error, he adds, “was required to come up with the ultimate solution.” Crystal panels made with just the tweaked tin material were 20.5 percent efficient in NREL’s tests. That means they harvested one-fifth of the incoming sunlight.
The team also tested multi-layered solar panels. One layer was made from the improved tin-based crystals. A second, lead-based layer was most sensitive to other wavelengths of light. The layers work in tandem. That is, they work together, side-by-side. This upped the panel’s overall efficiency to between 23 and 25 percent. Until then, the best a tin-lead combo had been was 16 to 17 percent efficient, says David Moore. He’s a materials scientist, also at NREL. The NREL team shared its new data in the May 3, 2019, issue of Science.
Moore, Berry and others also tackled another important problem. Most combo solar panels with the new crystals were made by pouring the solution for the top layer right over the bottom material. But often the liquid for the top layer messed up the bottom layer.
NREL researchers Ashley Marshall, Erin Sanehira and Joey Luther show liquids containing nanoscale bits of perovskite compounds. These perovskites glow red or green when exposed to ultraviolet light.DENNIS SCHROEDER/NREL
To solve the problem, the researchers added a nanometer-thick divider between the two layers. (A nanometer is one billionth of a meter.) They chose a polymer — a chemical made from long chains of repeating groups of atoms. (Many plastics are polymers.) This nano-divider helped prevent damage to the bottom layer as the top perovskite layer went on. The NREL team described this fix in the September 18, 2019, issue of Joule.
Both studies “make great progress in high-performance tandem solar cells,” says Zhiqun Lin. This materials scientist at the Georgia Institute of Technology in Atlanta did not work on either project. Tweaking the recipe for the tin-based crystals gave them more time to harvest sunlight. That was “novel” and should make them more “practical,” Lin says. He also lauds the nano-divider for overcoming that second problem in layered solar panels.
But big challenges still remain. “The biggest roadblock,” says Moore, is their lifetime. Most silicon solar panels now last 20 years or more. Perovskite solar cells are not so hardy. Moisture, oxygen damage and other factors gradually lower the crystals’ efficiency. Over time, they may stop working. Only a few years ago, these materials would break down after a few hours. Thanks to advances, now they can last about a year. But they have a long way to go to become practical for widespread use.
Lin recently found a way to boost the lifetime for light-emitting diodes (LEDs) made from perovskites. His team jacketed them with a silica shell and plastic hair-like structures. This coating repelled water. Perhaps a similar material might help the solar panels, he suggests.
Moore points out that science almost always moves slowly, “by a step-by-step process.” But there’s lots of motivation to work toward better and longer-lasting solar panels. They tend to be cleaner than fossil fuels and better for the environment. Beyond that, energy experts forecast that the world will need almost 50 percent more energy by 2050. With innovation, flexible solar technologies might one day play a big role there.
This is one in a series presenting news on technology and innovation, made possible with generous support from the Lemelson Foundation.
annual: Adjective for something that happens every year. (in botany) A plant that lives only one year, so it usually has a showy flower and produces many seeds.
atom: The basic unit of a chemical element. Atoms are made up of a dense nucleus that contains positively charged protons and uncharged neutrons. The nucleus is orbited by a cloud of negatively charged electrons.
chemical: A substance formed from two or more atoms that unite (bond) in a fixed proportion and structure. For example, water is a chemical made when two hydrogen atoms bond to one oxygen atom. Its chemical formula is H2O. Chemical also can be an adjective to describe properties of materials that are the result of various reactions between different compounds.
crystal: (adj. crystalline) A solid consisting of a symmetrical, ordered, three-dimensional arrangement of atoms or molecules. It’s the organized structure taken by most minerals. Apatite, for example, forms six-sided crystals. The mineral crystals that make up rock are usually too small to be seen with the unaided eye.
develop: To emerge or come into being, either naturally or through human intervention, such as by manufacturing.
development: (in engineering) The growth or change of something from an idea to a prototype.
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.
environment: The sum of all of the things that exist around some organism or the process and the condition those things create. Environment may refer to the weather and ecosystem in which some animal lives, or, perhaps, the temperature and humidity (or even the placement of things in the vicinity of an item of interest).
factor: Something that plays a role in a particular condition or event; a contributor.
fossil fuel: Any fuel — such as coal, petroleum (crude oil) or natural gas — that has developed within the Earth over millions of years from the decayed remains of bacteria, plants or animals.
genetic: Having to do with chromosomes, DNA and the genes contained within DNA. The field of science dealing with these biological instructions is known as genetics. People who work in this field are geneticists.
innovation: (v. to innovate; adj. innovative) An adaptation or improvement to an existing idea, process or product that is new, clever, more effective or more practical.
iodine: An element needed for the thyroid to produce the hormone used in growth, development and more. Some foods naturally have plenty. Others, principally table salt, may be fortified with this nutrient.
joule: The amount of energy needed to produce one watt for one second. Joule is a standard unit of energy.
light emitting diodes, or LED: A type of semiconductor device that produces light.
materials science: The study of how the atomic and molecular structure of a material is related to its overall properties. Materials scientists can design new materials or analyze existing ones. Their analyses of a material’s overall properties (such as density, strength and melting point) can help engineers and other researchers select materials that are best suited to a new application.
metal: Something that conducts electricity well, tends to be shiny (reflective) and malleable (meaning it can be reshaped with heat and not too much force or pressure).
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.
plastic: Any of a series of materials that are easily deformable; or synthetic materials that have been made from polymers (long strings of some building-block molecule) that tend to be lightweight, inexpensive and resistant to degradation.
polymer: A substance made from long chains of repeating groups of atoms. Manufactured polymers include nylon, polyvinyl chloride (better known as PVC) and many types of plastics. Natural polymers include rubber, silk and cellulose (found in plants and used to make paper, for example).
prospect: (n.) The vista (as in what’s in view) or the future of something (such as whether it’s going to be successful).
range: The full extent or distribution of something. For instance, a plant or animal’s range is the area over which it naturally exists.
renewable energy: Energy from a source that is not depleted by use, such as hydropower (water), wind power or solar power.
semiconductor A material that sometimes conducts electricity. Semiconductors are important parts of computer chips and certain new electronic technologies, such as light-emitting diodes.
silica: A mineral, also known as silicon dioxide, containing silicon and oxygen atoms. It is a basic building block of much of the rocky material on Earth and of some construction materials, including glass.
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.
solution: A liquid in which one chemical has been dissolved into another.
solvent: A material (usually a liquid) used to dissolve some other material into a solution.
spectrum: (in light and energy) The range of electromagnetic radiation types; they span from gamma rays to X rays, ultraviolet light, visible light, infrared energy, microwaves and radio waves.
technology: The application of scientific knowledge for practical purposes, especially in industry — or the devices, processes and systems that result from those efforts.
tin: A metallic element with the atomic number 50.
waste: Any materials that are left over from biological or other systems that have no value, so they can be disposed of as trash or recycled for some new use.
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.
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