Looking out on the National Mall from the offices at the Department of Energy on a brilliant summer day, it's easy to see why sunlight is a popular choice for producing electricity. However, that same view on a drizzly day in March illustrates a major problem. Clouds hamper solar panels. And it isn't just the weather.
The time of day affects panels, with more light beaming down around lunchtime than earlier or later in the day, and no energy produced at night. The calendar also plays a role; shorter winter days mean less energy. Add in the light-blocking nature of surrounding buildings, and the challenge becomes clear.
Yet these challenges have not kept solar power from the energy stage. Scientific innovations funded by the Department of Energy have helped lift the efficiency of conventional silicon-based panels to almost 20 percent, a dramatic step up from the first experiments in the 1880s. And the efficiency of panels is rising.
While improving the efficiency is vital, continuing the trend of lowering production costs is also important. As anyone who has written a budget knows, there are lots of ways to think about costs. Here we're looking at the energy payback — the time it takes for a solar panel to make as much energy as was used in manufacturing it. Payback lets scientists compare more than just the cost. The payback for today's silicon-based solar cells has dropped to around one to four years, depending on the design. Both efficiency and payback must be considered. A highly efficient panel made with difficult-to-acquire metals could be a better choice than a less-efficient one made from more abundant options. Or vice versa.
At national labs, universities, and specially designated collaborative centers called Energy Frontier Research Centers (EFRCs) where scientists dedicate several years to foundational questions, researchers funded by the DOE's Office of Science develop new ideas, new materials, and new designs to continue to push solar energy to take center stage. Here are five ways that these scientists are taking on the challenges?
1. Let the sunshine in. In countless images, you see light glinting off of solar panels. While it makes a cool photo, it means that the panel's surface is bouncing that energy away instead of absorbing it and using it to create electricity. Coatings can reduce the glare, but scientists at DOE's Brookhaven National Laboratory and Stony Brook University in New York bypassed films entirely and directly etched a texture onto the light-absorbing silicon. The texture was inspired by the eyes of common moths, which collect vast amounts of low light but don't reflect that light and give the creature's position away to nearby predators. Carved at the nanoscale, the nature-inspired texture stops reflections and results in cells that perform far better than those with a single antireflective film.
In Indiana, scientists at Notre Dame University took a different route. They created a "rainbow cell" that absorbs green, orange, and red light. Conventional silicon cells pick up fewer colors of light. The rainbow version relies on tiny dots created by varying levels of selenium, not silicon. Layered correctly, the dots are synergistic, working harder together than each did alone. The dots could be cheaper to manufacture than the smooth silicon wafers used today, reducing the energy payback.
2. Let it go. Sunlight strikes the silicon wafer in a solar cell and frees electrons that then zip to the nearest electrode, producing an electrical current that can power computers, brew coffee, and bake a croissant. However, not all of the freed electrons make it to the electrode. "Hot" electrons that are just a bit more excited by the sunlight than others fall back into the silicon and release a bit of heat and light, which hurts the overall efficiency. However, if that light escapes, it boosts the cell's output, or voltage.
"It is completely counterintuitive," admits Harry Atwater, who works at the Light-Material Interactions in Energy Conversion Center, an EFRC led by Caltech. After years of research, Atwater along with his colleagues Eli Yablonovitch and Owen Miller at Berkeley Lab showed that the best solar cells are also the best light emitters.
So, how do designers get the light out? They added a highly reflective mirror at the back of the cell and a simple gap at the right spot to give the internally generated light more chances to escape. Together, the gap and rear mirror raised the efficiency by more than 3 percent.
3. Walking on perovskite. Silicon, the go-to material in solar panels, is something of a prima donna. To work, the silicon must be shaped into an ultrapure wafer, an energy-intense process. Enter perovskite, a much more laid-back choice. While myriad reports exist on how to manufacture solar cells with new materials with this crystal structure, scientists at Cornell University along with their colleagues found a way to grow the perovskite films in just a few minutes by deleting the troubling iodine salts. The result is an ultrasmooth and almost pinhole-free film. Perovskite panels could reduce energy payback to a mere 2 to 3 months.
4. Get the lead out. Lead is the key in the leading [no pun intended] perovskite solar cells. At the Argonne-Northwestern Solar Energy Research Center, an EFRC led by Northwestern University, scientists took on the challenge of removing the element by revising the formula to use nontoxic tin. As an added bonus, the cells can be "tuned" to capture more colors of light than silicon. The new perovskite converts only 6 percent of the energy it receives into electricity, but scientists expect to get the efficiency well into the double digits fairly soon. That, coupled with a better manufacturing process, may result in perovskite materials becoming the new silicon.
5. One step over the line. If the electricity from a solar panel isn't used, it's lost. One option is to store the electricity as fuels. A fuel packs energy, such as intermittent solar energy, into a tight, dense space — a chemical bond — and releases that energy when needed. Converting solar-generated electricity into fuels essentially stores the sun's energy for use on a rainy day or any time it is needed. Scientists have tried creating such sunlight-to-fuel-producing cells, but the components are far from perfect with the silicon quickly oxidizing, essentially rusting, when it encounters water. Protective coatings help protect the silicon against humidity, but the coatings interfere with the light streaming in. At the Fuels from Sunlight Energy Innovation Hub at the Joint Center for Artificial Photosynthesis at Caltech, a team led by Nathan S. Lewis and Ke Sun built a thin nickel-oxide-based film that not only protects silicon and other types of absorbing layers, but it is highly transparent, antireflective, stable, doesn't interfere with the electrons, and helps produce oxygen from water, to enable systems to produce either hydrogen fuel or hydrocarbon fuels directly from sunlight.
At the Center for Solar Fuels, an EFRC based at the University of North Carolina at Chapel Hill, scientists reimagined the electricity-producing solar panel as a miniature fuel refinery. The refinery's design is reminiscent of an éclair with sprinkles. At the center is a tin-based material; it is wrapped in a thin shell made from the same compound that gives sunscreen its white pigment, titanium dioxide. The shell is sprinkled with molecules made of light-catching chromophores and catalysts that lower the amount of energy needed to create hydrogen fuel. When exposed to sunlight, all of the parts of this wrapped-up refinery work together to create hydrogen fuel from water, essentially storing sunlight's energy for those long winter nights.
More efficient, more affordable solar panels that could provide use-any-time fuel would benefit far more than you, me, and our local power company. Such panels would reduce the carbon pollution that causes climate change worldwide, reducing both human suffering and economic damage. These innovative panels will also help drive a domestic energy economy.
The Office of Science is the single largest supporter of basic energy research in the physical sciences in the United States and is working to address some of the most pressing challenges of our time. For more information please visit http://science.energy.gov.
Kristin Manke is a Communications Specialist at Pacific Northwest National Laboratory on detail to the U.S. Department of Energy's Office of Science, firstname.lastname@example.org.