Forum for Science, Industry and Business

Sponsored by:     3M 
Search our Site:

 

Research highlights potential for improved solar cells

12.02.2009
Certain nanocrystals shown to generate more than one electron

A team of Los Alamos researchers led by Victor Klimov has shown that carrier multiplication—when a photon creates multiple electrons—is a real phenomenon in tiny semiconductor crystals and not a false observation born of extraneous effects that mimic carrier multiplication.

The research, explained in a recent issue of Accounts of Chemical Research, shows the possibility of solar cells that create more than one unit of energy per photon.

Questions about the ability to increase the energy output of solar cells have prompted Los Alamos National Laboratory researchers to reassess carrier multiplication in extremely small semiconductor particles.

When a conventional solar cell absorbs a photon of light, it frees an electron to generate an electrical current. Energy in excess of the amount needed to promote an electron into a conducting state is lost as heat to atomic vibrations (phonons) in the material lattice. Through carrier multiplication, excess energy can be transferred to another electron instead of the material lattice, freeing it to generate electrical current—thereby yielding a more efficient solar cell.

Klimov and colleagues have shown that nanocrystals of certain semiconductor materials can generate more than one electron after absorbing a photon. This is partly due to strengthened interactions between electrons squeezed together within the confines of the nanoscale particles.

In 2004, Los Alamos researchers Richard Schaller and Klimov reported the first observations of strong carrier multiplication in nanosized crystals of lead selenide resulting in up to two electron-hole pairs per absorbed photon. A year later, Arthur Nozik and coworkers at the National Renewable Energy Laboratory reproduced these results. Eventually, spectroscopic signatures of carrier multiplication were observed in nanocrystals of various compositions, including silicon.

Recently, the claims in carrier multiplication research have become contentious. Specifically, some recent studies described low or negligible carrier multiplication efficiencies, which seemed to run contrary to earlier findings. To sort out these discrepancies, Los Alamos researchers analyzed factors that could have led to a spread in the reported carrier multiplication results. These factors included variations between samples, differences in detection techniques, and effects mimicking the signatures of carrier multiplication in spectroscopic measurements.

To analyze how a particular detection technique might affect an outcome, John McGuire, a postdoctoral researcher on Klimov's team, investigated carrier multiplication using two different spectroscopic techniques—transient absorption and time-resolved photoluminescence. The results obtained by these two methods were in remarkable agreement, indicating that the use of different detection techniques is unlikely to explain discrepancies highlighted by other researchers. Further, although these measurements revealed some sample-to-sample variation in carrier multiplication yields, these variations were much smaller than the spread in reported data.

After ruling out these two potential causes of discrepancies, the researchers focused on effects that could mimic carrier multiplication. One such effect is photoionization of nanocrystals.

"When a nanocrystal absorbs a high-energy photon, an electron can acquire enough energy to escape the material," Klimov explained. "This leaves behind a charged nanocrystal, which contains a positive 'hole.' Photogeneration of another electron by a second photon results in a two-hole, one-electron state, reminiscent of one produced by carrier multiplication, which can lead to false positives," he said.

To evaluate the influence of photoionization, the Los Alamos researchers conducted back-to-back studies of static and stirred solutions of nanocrystals. Stirring removes charged nanocrystals from the measured region of the sample. Therefore, when crystals are subjected to light, the stirring eliminates the possibility that charged nanocrystals will absorb a second photon. While stirring of some samples did not affect the results of the measurements, other samples showed a significant difference in the apparent carrier multiplication yields measured under static and stirred conditions. Since most previous studies were performed on static samples, these results suggest that discrepancies noted by other researchers arise at least in part from uncontrolled photoionization, which stirring seeks to eliminate.

The Los Alamos researchers re-evaluated carrier multiplication efficiencies when photoionization was suppressed. The results are encouraging.

While the newly measured electron yields are lower than previously reported, the efficiency of carrier multiplication is still greater than in bulk solids. Specifically, both the energetic onset and the energy required to generate an extra electron in nanocrystals are about half of those in bulk solids.

These results indicate significant promise for nanosized crystals as efficient harvesters of solar radiation.

"Researchers still have a lot of work to do," Klimov cautioned. "One important challenge is to figure out how to design a material in which the energetic cost to create an extra electron can approach the limit defined by a semiconductor band gap. Such a material could raise the fundamental power conversion limit of a solar cell from 31 percent to above 40 percent."

James E. Rickman | EurekAlert!
Further information:
http://www.lanl.gov
http://quantumdot.lanl.gov/

More articles from Life Sciences:

nachricht Individual Receptors Caught at Work
19.10.2017 | Julius-Maximilians-Universität Würzburg

nachricht Rapid environmental change makes species more vulnerable to extinction
19.10.2017 | Universität Zürich

All articles from Life Sciences >>>

The most recent press releases about innovation >>>

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

Im Focus: Neutron star merger directly observed for the first time

University of Maryland researchers contribute to historic detection of gravitational waves and light created by event

On August 17, 2017, at 12:41:04 UTC, scientists made the first direct observation of a merger between two neutron stars--the dense, collapsed cores that remain...

Im Focus: Breaking: the first light from two neutron stars merging

Seven new papers describe the first-ever detection of light from a gravitational wave source. The event, caused by two neutron stars colliding and merging together, was dubbed GW170817 because it sent ripples through space-time that reached Earth on 2017 August 17. Around the world, hundreds of excited astronomers mobilized quickly and were able to observe the event using numerous telescopes, providing a wealth of new data.

Previous detections of gravitational waves have all involved the merger of two black holes, a feat that won the 2017 Nobel Prize in Physics earlier this month....

Im Focus: Smart sensors for efficient processes

Material defects in end products can quickly result in failures in many areas of industry, and have a massive impact on the safe use of their products. This is why, in the field of quality assurance, intelligent, nondestructive sensor systems play a key role. They allow testing components and parts in a rapid and cost-efficient manner without destroying the actual product or changing its surface. Experts from the Fraunhofer IZFP in Saarbrücken will be presenting two exhibits at the Blechexpo in Stuttgart from 7–10 November 2017 that allow fast, reliable, and automated characterization of materials and detection of defects (Hall 5, Booth 5306).

When quality testing uses time-consuming destructive test methods, it can result in enormous costs due to damaging or destroying the products. And given that...

Im Focus: Cold molecules on collision course

Using a new cooling technique MPQ scientists succeed at observing collisions in a dense beam of cold and slow dipolar molecules.

How do chemical reactions proceed at extremely low temperatures? The answer requires the investigation of molecular samples that are cold, dense, and slow at...

Im Focus: Shrinking the proton again!

Scientists from the Max Planck Institute of Quantum Optics, using high precision laser spectroscopy of atomic hydrogen, confirm the surprisingly small value of the proton radius determined from muonic hydrogen.

It was one of the breakthroughs of the year 2010: Laser spectroscopy of muonic hydrogen resulted in a value for the proton charge radius that was significantly...

All Focus news of the innovation-report >>>

Anzeige

Anzeige

Event News

ASEAN Member States discuss the future role of renewable energy

17.10.2017 | Event News

World Health Summit 2017: International experts set the course for the future of Global Health

10.10.2017 | Event News

Climate Engineering Conference 2017 Opens in Berlin

10.10.2017 | Event News

 
Latest News

Electrode materials from the microwave oven

19.10.2017 | Materials Sciences

New material for digital memories of the future

19.10.2017 | Materials Sciences

Physics boosts artificial intelligence methods

19.10.2017 | Physics and Astronomy

VideoLinks
B2B-VideoLinks
More VideoLinks >>>