Researchers from Rice University have developed a new way to extract more energy from solar radiation, potentially making photovoltaic cells more efficient than ever before.
Their setup harnesses the power of hot electrons, which are highly kinetic electrons created when photons from the sun strike material within a photovoltaic panel. Hot electrons usually decay within trillionths of a second after photons strike the panel, but Isabell Thomann, assistant professor of electrical and computer engineering and of chemistry and materials science and nanoengineering at Rice, has found a way to slow their decay and harness their energy.
Getting Power From Hot Electrons
The biggest challenge with capturing hot electron energy is how quickly these excited particles return to their lower energy levels. They jump out of a lower energy level to a higher one, leaving holes behind. To keep them from returning right back to the lower energy level, Thomann put more distance between these excited electrons and the holes they left behind.
Along with a team of graduate students, Thomann constructed a three-layer setup. The base consisted of a thin sheet of aluminum topped with a transparent layer of nickel oxide. On top of the nickel oxide, the team dropped light-activated gold nanoparticles in discs about 10 to 30 nanometers wide.
When sunlight hit the gold nanoparticles from the top or reflected back up from the aluminum base layer, the photons received a quick supercharge from the nanoparticles, releasing hot electrons. Essentially, the nanoparticles and nickel oxide acted as a filter, letting electron holes through and leaving hot electrons sitting on the nanoparticles. Aluminum drew the electron holes back down, leaving the hot electrons with nowhere to go, which made them available for chemical reactions.
Thomann’s team then covered their materials with a thin layer of water to test whether hot electrons could split water molecules into hydrogen and oxygen atoms. When measuring the current generated by their technique, Thomann’s team reported current efficiencies consistent with far more sophisticated — and expensive — PV technologies.
Solving Frustrating Efficiency Problems
Historically, the biggest obstacle to widespread solar power usage has always been cost. Innovation and government subsidies have made photovoltaics cheaper than ever before, putting solar within the hands of homeowners. Thanks to tax breaks, the U.S. has seen many entrepreneurs form LLCs dedicated to installing solar panels and photovoltaic systems. Even so, solar installations are still costly, and increased adoption depends on making solar cheaper.
When sunlight makes it to Earth’s surface, it’s already lost about one-third of the energy it had when it left the sun. Earth’s atmosphere reflects some light back into space, and it also absorbs some of the energy, which the atmosphere and creates wind currents. One of the best ways to get more from existing PV cells is to make them more efficient. Current solar technology, however, manages to harness between only 5 and 25 percent of the sun’s energy.
Hot electrons dissipate from most current PV cells in the form of heat loss. By capturing these particles and keeping them from going to lower energy levels, Thomann’s team extracts more energy from the sunlight. It’s impossible to say when or how the Rice team’s findings could be incorporated into PV cells, but their research has enormous potential to make solar power less expensive — and more attractive to everyday consumers.
Keeping Costs Down
At MIT, graduate student Qiong Ma has used graphene and hexagonal boron nitride with similar effects to Thomann’s nanoparticle sheet. Her experiments pair sheets of graphene with top and bottom electrodes of different sizes, with a top gate made from dielectric boron nitride.
Electrons from the sun are slow to bond with graphene’s lattice structure, keeping electrons at higher energy levels for a longer period and extracting more energy for electricity. Unfortunately, it costs a lot to isolate graphene from graphite and turn it into usable material. According to the International Business Times, some U.S. vendors sell copper foil covered with a layer of graphene for $60 per square inch.
The Rice University team’s method uses more cost-effective materials, although Thomann readily admits their methods need further study before widespread implementation. Even so, it’s a promising solution that could make solar more affordable, reducing the world’s dependence on fossil fuels.