The materials in most of today’s residential rooftop solar panels can store energy from the sun for only a few microseconds at a time. A new technology developed by chemists at UCLA is capable of storing solar energy for up to several weeks—an advance that could change the way scientists think about designing solar cells.
The findings are published June 19 in the journal Science.
The new design is inspired by the way that plants generate energy through photosynthesis.
“Biology does a very good job of creating energy from sunlight,” said Sarah Tolbert, a UCLA professor of chemistry and one of the senior authors of the research. “Plants do this through photosynthesis with extremely high efficiency.”
“In photosynthesis, plants that are exposed to sunlight use carefully organized nanoscale structures within their cells to rapidly separate charges—pulling electrons away from the positively charged molecule that is left behind, and keeping positive and negative charges separated,” Tolbert said. “That separation is the key to making the process so efficient.”
The scientists devised a new arrangement of solar cell ingredients, with bundles of polymer donors (green rods) and neatly organized fullerene acceptors (purple, tan). Credit: UCLA Chemistry
To capture energy from sunlight, conventional rooftop solar cells use silicon, a fairly expensive material. There is currently a big push to make lower-cost solar cells using plastics, rather than silicon, but today’s plastic solar cells are relatively inefficient, in large part because the separated positive and negative electric charges often recombine before they can become electrical energy.
“Modern plastic solar cells don’t have well-defined structures like plants do because we never knew how to make them before,” Tolbert said. “But this new system pulls charges apart and keeps them separated for days, or even weeks. Once you make the right structure, you can vastly improve the retention of energy.”
The two components that make the UCLA-developed system work are a polymer donor and a nano-scale fullerene acceptor. The polymer donor absorbs sunlight and passes electrons to the fullerene acceptor; the process generates electrical energy.
The plastic materials, called organic photovoltaics, are typically organized like a plate of cooked pasta—a disorganized mass of long, skinny polymer “spaghetti” with random fullerene “meatballs.” But this arrangement makes it difficult to get current out of the cell because the electrons sometimes hop back to the polymer spaghetti and are lost.
The UCLA technology arranges the elements more neatly—like small bundles of uncooked spaghetti with precisely placed meatballs. Some fullerene meatballs are designed to sit inside the spaghetti bundles, but others are forced to stay on the outside. The fullerenes inside the structure take electrons from the polymers and toss them to the outside fullerene, which can effectively keep the electrons away from the polymer for weeks.
“When the charges never come back together, the system works far better,” said Benjamin Schwartz, a UCLA professor of chemistry and another senior co-author. “This is the first time this has been shown using modern synthetic organic photovoltaic materials.”
In the new system, the materials self-assemble just by being placed in close proximity.
“We worked really hard to design something so we don’t have to work very hard,” Tolbert said.
The new design is also more environmentally friendly than current technology, because the materials can assemble in water instead of more toxic organic solutions that are widely used today.
“Once you make the materials, you can dump them into water and they assemble into the appropriate structure because of the way the materials are designed,” Schwartz said. “So there’s no additional work.”
The researchers are already working on how to incorporate the technology into actual solar cells.
Yves Rubin, a UCLA professor of chemistry and another senior co-author of the study, led the team that created the uniquely designed molecules. “We don’t have these materials in a real device yet; this is all in solution,” he said. “When we can put them together and make a closed circuit, then we will really be somewhere.”
Explore further: Improving the efficiency of solar energy cells
More information: Long-lived photoinduced polaron formation in conjugated polyelectrolyte-fullerene assemblies Science 19 June 2015: Vol. 348 no. 6241 pp. 1340-1343. DOI: 10.1126/science.aaa6850
Source:National Institute for Materials Science (NIMS)
Perovskite solar cells are promising low-cost and highly-efficient next-generation solar cells. The ad hoc Team on Perovskite PV Cells (Kenjiro Miyano, Team Leader) at the Global Research Center for Environment and Energy based on Nanomaterials Science (GREEN) (Kohei Uosaki, Director-General), NIMS (Sukekatsu Ushioda, President), successfully developed perovskite solar cells with good reproducibility and stability as well as exhibiting ideal semiconducting properties.
Lead-halide-based perovskite (hereinafter simply referred to as perovskite) has been used as a solar cell material since six years ago. Perovskite solar cells are promising low-cost and highly-efficient next-generation solar cells because they can be produced through low-temperature processes such as spin coating, and generate a large amount of electricity due to their high optical absorption together with the high open-circuit voltage. As such, the research on perovskite solar cells is making rapid progress. In order to identify the semiconducting properties of perovskites and formulate guidelines for the development of highly efficient solar cell materials, NIMS launched an ad hoc Team on Perovskite PV Cells last October led by the deputy director-general of GREEN.
While the conventional perovskite solar cells have demonstrated high conversion efficiency, they were not sufficiently stable plagued by their low reproducibility and the hysteresis in the current-voltage curves depending on the direction of the voltage sweeps. For this reason, the semiconducting properties of perovskites had not been identified. Researchers successfully created reproducible and stable perovskite solar cells as follows;
- They created perovskite solar cells with a simplified structure while strictly eliminating moisture and oxygen by employing the fabrication technique they had developed for the organic solar cells in the past.
- They found that the perovskite solar cells are stable and they observed no hysteresis in the current-voltage curve. Furthermore, they found that the perovskite solar cell material serves as an excellent semiconductor with ideal diode properties.
They proposed an equivalent circuit model that explains the semiconducting properties of perovskites based on analysis of the internal resistance of perovskite solar cells. This model indicated the existence of a charge transport process derived from an impurity level between the conduction and valence bands in the perovskite layer. Due to this transport process, the efficiency of perovskite solar cells may be suppressed to some extent.
In future studies, researchers will investigate into the cause of the impurity level and its influence on solar cells. In addition, they intend to remove the impurity level and improve the efficiency of the solar cells, thereby contributing to energy and environmental conservation.
This study was conducted at GREEN as a part of the MEXT-commissioned project titled “Development of environmental technology using nanotechnology.”
This study had been published in March 2015 in Applied Physics Letters, a journal issued by the American Institute of Physics.
- Kenjiro Miyano, Masatoshi Yanagida, Neeti Tripathi, Yasuhiro Shirai. Simple characterization of electronic processes in perovskite photovoltaic cells. Applied Physics Letters, 2015; 106 (9): 093903 DOI: 10.1063/1.4914086
26 Feb 2015
Magnetic nanoparticles can increase the performance of solar cells made from polymers – provided the mix is right. This is the result of an X-ray study at DESY’s synchrotron radiation source PETRA III.
Adding about one per cent of such nanoparticles by weight makes the solar cells more efficient, according to the findings of a team of scientists headed by Prof. Peter Müller-Buschbaum from the Technical University of Munich. They are presenting their study in one of the upcoming issues of the journal Advanced Energy Materials (published online in advance).
Polymer, or organic, solar cells offer tremendous potential: They are inexpensive, flexible and extremely versatile. Their drawback compared with established silicon solar cells is their lower efficiency. Typically, they only convert a few per cent of the incident light into electrical power. Nevertheless, organic solar cells are already economically viable in many situations, and scientists are looking for new ways to increase their efficiency.
One promising method is the addition of nanoparticles. It has been shown, for example, that gold nanoparticles absorb additional sunlight, which in turn produces additional electrical charge carriers when the energy is released again by the gold particles.
Müller-Buschbaum’s team has been pursuing a different approach, however. “The light creates pairs of charge carriers in the solar cell, consisting of a negatively charged electron and a positively charged hole, which is a site where an electron is missing,” explains the main author of the current study, Daniel Moseguí González from Müller-Buschbaum’s group. “The art of making an organic solar cell is to separate this electron-hole pair before they can recombine. If they did, the charge produced would be lost. We were looking for ways of extending the life of the electron-hole pair, which would allow us to separate more of them and direct them to opposite electrodes.”
This strategy makes use of a quantum physical principle which states that electrons have a kind of internal rotation, known as spin. According to the laws of quantum physics, this spin has a value of 1/2. The positively charged hole also has a spin of 1/2. The two spins can either add up, if they are in the same direction, or cancel each other out if they are in opposite directions. The electron-hole pair can therefore have an overall spin of 0 or 1. Pairs with a spin of 1 exist for longer than those with an overall spin of 0.
The researchers set out to find a material that was able to convert the spin 0 state into a spin 1 state. This required nanoparticles of heavy elements, which flip the spin of the electron or the hole so that the spins of the two particles are aligned in the same direction. The iron oxide magnetite (Fe3O4) is in fact able to do just this. “In our experiment, adding magnetite nanoparticles to the substrate increased the efficiency of the solar cells by up to 11 per cent,” reports Moseguí González. The lifetime of the electron-hole pair is significantly prolonged.
Adding nanoparticles is a routine procedure which can easily be carried out in the course of the various methods for manufacturing organic solar cells. It is important, however, not to add too many nanoparticles to the solar cell, because the internal structure of organic solar cells is finely adjusted to optimise the distance between the light-collecting, active materials, so that the pairs of charge carriers can be separated as efficiently as possible. These structures lie in the range of 10 to 100 nanometres.
“The X-ray investigation shows that if you mix a large number of nanoparticles into the material used to make the solar cell, you change its structure”, explains co-author Dr. Stephan Roth, head of DESY’s beam line P03 at PETRA III, where the experiments were conducted. “The solar cell we looked at will tolerate magnetite nanoparticle doping levels of up to one per cent by mass without changing their structure.”
The scientists observed the largest effect when they doped the substrate with 0.6 per cent nanoparticles by weight. This caused the efficiency of the polymer solar cell examined to increase from 3.05 to 3.37 per cent. “An 11 percent increase in energy yield can be crucial in making a material economically viable for a particular application,” emphasises Müller-Buschbaum.
The researchers believe it will also be possible to increase the efficiency of other polymer solar cells by doping them with nanoparticles. “The combination of high-performance polymers with nanoparticles holds the promise of further increases in the efficiency of organic solar cells in the future. However, without a detailed examination, such as that using the X-rays emitted by a synchrotron, it would be impossible to gain a fundamental understanding of the underlying processes involved,” concludes Müller-Buschbaum.
More information: Advanced Energy Materials, 2015; DOI: 10.1002/aenm.201401770
HyperSolar (www.hypersolar.com) has developed a breakthrough technology to make renewable hydrogen using sunlight and any source of water. Renewable hydrogen, the cleanest and greenest of all fuels, can be used as direct replacement for traditional hydrogen, which is usually produced by reforming CO2 emitting natural gas.
By optimizing the science of water electrolysis, our low cost photoelectrochemical process efficiently uses sunlight to separate hydrogen from any source of water to produce clean and environmentally friendly renewable hydrogen. Our innovative solar hydrogen generator eliminates the need for conventional electrolyzers, which are expensive and energy intensive. We believe that our solution will produce the lowest cost renewable hydrogen available in the market today.
Hydrogen is the most abundant element and cleanest fuel in the universe. Unlike hydrocarbon fuels, that produce harmful emissions, hydrogen fuel produces pure water as the only byproduct. Using our low cost method to produce renewable hydrogen, we intend to enable a world of distributed hydrogen production for renewable electricity and hydrogen fuel cell vehicles.
Hydrogen expert to join R&D team focused on increasing the water-splitting voltage of proprietary hydrogen technology
SANTA BARBARA, CA – February 18, 2015 –HyperSolar, Inc. (OTCQB: HYSR), the developer of a breakthrough technology to produce renewable hydrogen using sunlight and water, today announced that Dr. Wei Cheng, a post-doctoral researcher who has extensive experience in developing hydrogen production applications and previously served the Company during his time as visiting scholar at the University of California, Santa Barbara, will be joining HyperSolar’s research and development team at the University of Iowa.
Dr. Cheng focuses on developing a low-cost way to make photo-electrochemical devices for producing hydrogen in wastewater. Dr. Cheng received his bachelor’s degree in Materials Science and Technology from Nanjing University of Aeronautics and Astronautics, his master’s degree and PhD in Materials Physics and Chemistry from Shanghai Jiao Tong University, China. He is currently a post-doctoral researcher at the University of Iowa. His previous works include producing hydrogen using low voltage electro-oxidation of organic wastewater and preparing non-toxic metal sulfide semiconductors with low-cost materials such as tin monosulfide (SnS) and Cu2ZnSnS4.
As HyperSolar’s technology progresses, the market for hydrogen fuel continues to build momentum. Just recently, the “big 3” auto manufacturers in Japan – Nissan, Toyota, and Honda – jointly announced their goal of “working together to help accelerate the development of hydrogen station infrastructure for fuel cell vehicles (FCVs).” Among several topics, hydrogen fuel infrastructure with respect to fueling stations was emphasized throughout the announcement as being of utmost importance. HyperSolar believes that its hydrogen producing technology, which uses a completely renewable process capable of being implemented at or near the point of distribution, will support fueling infrastructure upon commercialization.
“We are thrilled that Dr. Cheng will be joining our University of Iowa team to focus on increasing the water-splitting voltage required for commercialization of real-world systems,” said Tim Young, CEO of HyperSolar. “Dr. Cheng’s background in producing hydrogen, along with his familiarity with HyperSolar technology, makes him an integral part of our research and development team. As hydrogen fuel solutions continue to garner attention from major corporations around the world, we are confident that our technology will serve many applications within both consumer and commercial industries.”
HyperSolar’s technology is based on the concept of developing a low-cost, submersible hydrogen production particle that can split water molecules using sunlight without any other external systems or resources – acting as artificial photosynthesis. A video of an early proof-of-concept prototype can be viewed at http://hypersolar.com/application.php.
Date: Wednesday, February 18, 2015