Forum for Science, Industry and Business

Sponsored by:     3M 
Search our Site:

 

Overlooked resistance may inflate estimates of organic-semiconductor performance

10.03.2016

If unaccounted for, 'non-ideal' behaviors can inflate estimates of charge-carrier mobility

It's hardly a character flaw, but organic transistors--the kind envisioned for a host of flexible electronics devices--behave less than ideally, or at least not up to the standards set by their rigid, predictable silicon counterparts. When unrecognized, a new study finds, this disparity can lead to gross overestimates of charge-carrier mobility, a property key to the performance of electronic devices.


A circuit made from organic thin-film transistors is fabricated on a flexible plastic substrate. A team of NIST, Wake Forest, and Penn State University researchers has identified an overlooked source of electrical resistance that can exert a dominant influence on organic-semiconductor performance.

Credit: Patrick Mansell/Penn State

If measurements fail to account for these divergent behaviors in so-called "organic field-effect transistors" (OFETs), the resulting estimates of how fast electrons or other charge carriers travel in the devices may be more than 10 times too high, report researchers from the National Institute of Standards and Technology (NIST), Wake Forest University and Penn State University. The team's measurements implicate an overlooked source of electrical resistance as the root of inaccuracies that can inflate estimates of organic semiconductor performance.

Their article appears in the latest issue of Nature Communications.

Already used in light-emitting diodes, or LEDs, electrically conductive polymers and small molecules are being groomed for applications in flexible displays, flat-panel TVs, sensors, "smart" textiles, solar cells and "Internet of Things" applications. Besides flexibility, a key selling point is that the organic devices--sometimes called "plastic electronics"--can be manufactured in large volumes and far more inexpensively than today's ubiquitous silicon-based devices.

A key sticking point, however, is the challenge of achieving the high levels of charge-carrier mobility that these applications require. In the semiconductor arena, the general rule is that higher mobility is always better, enabling faster, more responsive devices. So chemists have set out to hurry electrons along. Working from a large palette of organic materials, they have been searching for chemicals--alone or in combination--that will up the speed limit in their experimental devices.

Just as for silicon semiconductors, assessments of performance require measurements of current and voltage. In the basic transistor design, a source electrode injects charge into the transistor channel leading to a drain electrode. In between sits a gate electrode that regulates the current in the channel by applying voltage, functioning much like a valve.

Typically, measurements are analyzed according to a longstanding theory for silicon field-effect transistors. Plug in the current and voltage values and the theory can be used to predict properties that determine how well the transistor will perform in a circuit.

Results are rendered as a series of "transfer curves." Of particular interest in the new study are curves showing how the drain current changes in response to a change in the gate electrode voltage. For devices with ideal behavior, this relationship provides a good measure of how fast charge carriers move through the channel to the drain.

"Organic semiconductors are more prone to non-ideal behavior because the relatively weak intermolecular interactions that make them attractive for low-temperature processing also limit the ability to engineer efficient contacts as one would for state-of-the-art silicon devices," says electrical engineer David Gundlach, who leads NIST's Thin Film Electronics Project. "Since there are so many different organic materials under investigation for electronics applications, we decided to step back and do a measurement check on the conventional wisdom."

Using what Gundlach describes as the semiconductor industry's "workhorse" measurement methods, the team scrutinized an OFET made of single-crystal rubrene, an organic semiconductor with a molecule shaped a bit like a microscale insect. Their measurements revealed that electrical resistance at the source electrode--the contact point where current is injected into the OFET-- significantly influences the subsequent flow of electrons in the transistor channel, and hence the mobility.

In effect, contact resistance at the source electrode creates the equivalent of a second valve that controls the entry of current into the transistor channel. Unaccounted for in the standard theory, this valve can overwhelm the gate--the de facto¬ regulator between the source and drain in a silicon semiconductor transistor--and become the dominant influence on transistor behavior.

At low gate voltages, this contact resistance at the source can overwhelm device operation. Consequently, model-based estimates of charge-carrier mobility in organic semiconductors may be more than 10 times higher than the actual value, the research team reports.

Hardly ideal behavior, but the aim of the study, the researchers write, is to improve "understanding of the source of the non-ideal behavior and its impact on extracted figures of merit," especially charge-carrier mobility. This knowledge, they add, can inform efforts to develop accurate, comprehensive measurement methods for benchmarking organic semiconductor performance, as well as guide efforts to optimize contact interfaces.

###

Article: E.G. Bittle, J.I. Basham, T.N. Jackson, O.D. Jurchescu, and D.J. Gundlach, "The effect of gated contacts on organic field-effect transistor operation and parameterization," Nature Communications. DOI: 10.1038/NCOMMS10908

Media Contact

Mark Bello
mark.bello@nist.gov
301-975-3776

 @usnistgov

http://www.nist.gov 

Mark Bello | EurekAlert!

More articles from Materials Sciences:

nachricht New design improves performance of flexible wearable electronics
23.06.2017 | North Carolina State University

nachricht Plant inspiration could lead to flexible electronics
22.06.2017 | American Chemical Society

All articles from Materials Sciences >>>

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

Supersensitive through quantum entanglement

28.06.2017 | Physics and Astronomy

X-ray photoelectron spectroscopy under real ambient pressure conditions

28.06.2017 | Physics and Astronomy

Mice provide insight into genetics of autism spectrum disorders

28.06.2017 | Health and Medicine

VideoLinks
B2B-VideoLinks
More VideoLinks >>>