Forum for Science, Industry and Business

Sponsored by:     3M 
Search our Site:

 

Solar energy gets a boost

09.07.2014

UC Riverside chemists' work on 'singlet fission' can increase solar cell efficiency by as much as 30 percent

A perspective article published last month by University of California, Riverside chemists in the Journal of Physical Chemistry Letters was selected as an Editors Choice—an honor only a handful of research papers receive. The perspective reviews the chemists' work on "singlet fission," a process in which a single photon generates a pair of excited states. This 1->2 conversion process, as it is known, has the potential to boost solar cell efficiency by as much as 30 percent.


Singlet fission is a process in which a single photon generates a pair of excited states. This 1->2 conversion process has the potential to boost solar cell efficiency by as much as 30 percent.

Credit: Bardeen Lab, UC Riverside.

Applications of the research include more energy-efficient lighting and photodetectors with 200 percent efficiency that can be used for night vision. Biology may use singlet fission to deal with high-energy solar photons without generating excess heat, as a protective mechanism.

Currently, solar cells work by absorbing a photon, which generates an exciton, which subsequently separates into an electron-hole pair. It is these electrons that become solar electricity. The efficiency of these solar cells is limited to about 32 percent, however, by what is called the "Shockley-Queisser Limit." Future solar cells, also known as "Third Generation" solar cells, will have to surpass this limit while remaining inexpensive, requiring the use of new physical processes. Singlet fission is an example of such a process.

"Our research got its launch about ten years ago when we started thinking about solar energy and what new types of photophysics this might require," said Christopher Bardeen, a professor of chemistry, whose lab led the research. "Global warming concerns and energy security have made solar energy conversion an important subject from society's point-of-view. More efficient solar cells would lead to wider use of this clean energy source."

Research details

When a photon is absorbed, its energy takes the form of an exciton inside the material. Bardeen explained that excitons come in two "flavors," defined by the electron spins in them. One flavor is singlet, where all spins are paired. The other flavor is triplet, where two electrons are unpaired. In organic semiconductors, these two types of excitons have different energies.

"If a triplet exciton has half the energy of a singlet, then it is possible for one singlet exciton, generated by one photon, to split into two triplet excitons," Bardeen said. "Thus, you could have a 200 percent yield of excitons — and hopefully, electrons — per absorbed photon."

He explained that the Shockley-Queisser Limit involves photon absorption to create an exciton, which is basically a bound electron (- charge) and hole (+ charge) pair. In order to get useful electron flow (photocurrent), these excitons must be dissociated. Ideally, one exciton produces one electron (hole) and thus current to run, say, a light bulb.

"To absorb a photon, the photon energy has to be greater than the bandgap of the semiconductor, so you already miss part of the solar spectrum," Bardeen said. "But if you absorb a photon with energy higher than the bandgap, it has too much energy, and that excess energy is usually wasted as heat. The trick is to take that high energy exciton and split the energy into two excitons, rather than dissipating it as heat."

Bardeen explained that the singlet exciton spontaneously splits into the two triplets, through a mechanism that is still under active investigation.

"The exact mechanism is unknown, but it does happen quickly — at the sub-nanosecond timescale — and with high efficiency," he said. "Our work has shown that it is very sensitive to the alignment and position of the two molecules — at least two are required, since we have two excitons — involved in singlet fission. Recent work at MIT has already demonstrated an organic photovoltaic cell with more than 100 percent external quantum efficiency based on this effect. It may be possible to integrate this effect with inorganic semiconductors and use it to raise their efficiencies."

Next, Bardeen's lab will look for new materials that exhibit singlet fission, figure out how to take the triplet excitons and turn them into photocurrent efficiently, and look at how the spin properties of the electrons affect the exciton dynamics.

###

The research was supported by a grant to Bardeen from the National Science Foundation. He was joined in the research by Geoffrey B. Piland, Jonathan J. Burdett and Robert J. Dillon at UC Riverside.

The University of California, Riverside is a doctoral research university, a living laboratory for groundbreaking exploration of issues critical to Inland Southern California, the state and communities around the world. Reflecting California's diverse culture, UCR's enrollment has exceeded 21,000 students. The campus opened a medical school in 2013 and has reached the heart of the Coachella Valley by way of the UCR Palm Desert Center. The campus has an annual statewide economic impact of more than $1 billion. A broadcast studio with fiber cable to the AT&T Hollywood hub is available for live or taped interviews. UCR also has ISDN for radio interviews. To learn more, call (951) UCR-NEWS.

Iqbal Pittalwala | Eurek Alert!

More articles from Power and Electrical Engineering:

nachricht How protons move through a fuel cell
22.06.2017 | Empa - Eidgenössische Materialprüfungs- und Forschungsanstalt

nachricht Fraunhofer IZFP acquires lucrative EU project for increasing nuclear power plant safety
21.06.2017 | Fraunhofer-Institut für Zerstörungsfreie Prüfverfahren IZFP

All articles from Power and Electrical Engineering >>>

The most recent press releases about innovation >>>

Die letzten 5 Focus-News des innovations-reports im Überblick:

Im Focus: Can we see monkeys from space? Emerging technologies to map biodiversity

An international team of scientists has proposed a new multi-disciplinary approach in which an array of new technologies will allow us to map biodiversity and the risks that wildlife is facing at the scale of whole landscapes. The findings are published in Nature Ecology and Evolution. This international research is led by the Kunming Institute of Zoology from China, University of East Anglia, University of Leicester and the Leibniz Institute for Zoo and Wildlife Research.

Using a combination of satellite and ground data, the team proposes that it is now possible to map biodiversity with an accuracy that has not been previously...

Im Focus: Climate satellite: Tracking methane with robust laser technology

Heatwaves in the Arctic, longer periods of vegetation in Europe, severe floods in West Africa – starting in 2021, scientists want to explore the emissions of the greenhouse gas methane with the German-French satellite MERLIN. This is made possible by a new robust laser system of the Fraunhofer Institute for Laser Technology ILT in Aachen, which achieves unprecedented measurement accuracy.

Methane is primarily the result of the decomposition of organic matter. The gas has a 25 times greater warming potential than carbon dioxide, but is not as...

Im Focus: How protons move through a fuel cell

Hydrogen is regarded as the energy source of the future: It is produced with solar power and can be used to generate heat and electricity in fuel cells. Empa researchers have now succeeded in decoding the movement of hydrogen ions in crystals – a key step towards more efficient energy conversion in the hydrogen industry of tomorrow.

As charge carriers, electrons and ions play the leading role in electrochemical energy storage devices and converters such as batteries and fuel cells. Proton...

Im Focus: A unique data centre for cosmological simulations

Scientists from the Excellence Cluster Universe at the Ludwig-Maximilians-Universität Munich have establised "Cosmowebportal", a unique data centre for cosmological simulations located at the Leibniz Supercomputing Centre (LRZ) of the Bavarian Academy of Sciences. The complete results of a series of large hydrodynamical cosmological simulations are available, with data volumes typically exceeding several hundred terabytes. Scientists worldwide can interactively explore these complex simulations via a web interface and directly access the results.

With current telescopes, scientists can observe our Universe’s galaxies and galaxy clusters and their distribution along an invisible cosmic web. From the...

Im Focus: Scientists develop molecular thermometer for contactless measurement using infrared light

Temperature measurements possible even on the smallest scale / Molecular ruby for use in material sciences, biology, and medicine

Chemists at Johannes Gutenberg University Mainz (JGU) in cooperation with researchers of the German Federal Institute for Materials Research and Testing (BAM)...

All Focus news of the innovation-report >>>

Anzeige

Anzeige

Event News

Plants are networkers

19.06.2017 | Event News

Digital Survival Training for Executives

13.06.2017 | Event News

Global Learning Council Summit 2017

13.06.2017 | Event News

 
Latest News

Study shines light on brain cells that coordinate movement

26.06.2017 | Life Sciences

Smooth propagation of spin waves using gold

26.06.2017 | Physics and Astronomy

Switchable DNA mini-machines store information

26.06.2017 | Information Technology

VideoLinks
B2B-VideoLinks
More VideoLinks >>>