Forum for Science, Industry and Business

Sponsored by:     3M 
Search our Site:


Overlooked resistance may inflate estimates of organic-semiconductor performance


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 | EurekAlert!

More articles from Materials Sciences:

nachricht From ancient fossils to future cars
21.10.2016 | University of California - Riverside

nachricht Study explains strength gap between graphene, carbon fiber
20.10.2016 | Rice University

All articles from Materials Sciences >>>

The most recent press releases about innovation >>>

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

Im Focus: New 3-D wiring technique brings scalable quantum computers closer to reality

Researchers from the Institute for Quantum Computing (IQC) at the University of Waterloo led the development of a new extensible wiring technique capable of controlling superconducting quantum bits, representing a significant step towards to the realization of a scalable quantum computer.

"The quantum socket is a wiring method that uses three-dimensional wires based on spring-loaded pins to address individual qubits," said Jeremy Béjanin, a PhD...

Im Focus: Scientists develop a semiconductor nanocomposite material that moves in response to light

In a paper in Scientific Reports, a research team at Worcester Polytechnic Institute describes a novel light-activated phenomenon that could become the basis for applications as diverse as microscopic robotic grippers and more efficient solar cells.

A research team at Worcester Polytechnic Institute (WPI) has developed a revolutionary, light-activated semiconductor nanocomposite material that can be used...

Im Focus: Diamonds aren't forever: Sandia, Harvard team create first quantum computer bridge

By forcefully embedding two silicon atoms in a diamond matrix, Sandia researchers have demonstrated for the first time on a single chip all the components needed to create a quantum bridge to link quantum computers together.

"People have already built small quantum computers," says Sandia researcher Ryan Camacho. "Maybe the first useful one won't be a single giant quantum computer...

Im Focus: New Products - Highlights of COMPAMED 2016

COMPAMED has become the leading international marketplace for suppliers of medical manufacturing. The trade fair, which takes place every November and is co-located to MEDICA in Dusseldorf, has been steadily growing over the past years and shows that medical technology remains a rapidly growing market.

In 2016, the joint pavilion by the IVAM Microtechnology Network, the Product Market “High-tech for Medical Devices”, will be located in Hall 8a again and will...

Im Focus: Ultra-thin ferroelectric material for next-generation electronics

'Ferroelectric' materials can switch between different states of electrical polarization in response to an external electric field. This flexibility means they show promise for many applications, for example in electronic devices and computer memory. Current ferroelectric materials are highly valued for their thermal and chemical stability and rapid electro-mechanical responses, but creating a material that is scalable down to the tiny sizes needed for technologies like silicon-based semiconductors (Si-based CMOS) has proven challenging.

Now, Hiroshi Funakubo and co-workers at the Tokyo Institute of Technology, in collaboration with researchers across Japan, have conducted experiments to...

All Focus news of the innovation-report >>>



Event News

#IC2S2: When Social Science meets Computer Science - GESIS will host the IC2S2 conference 2017

14.10.2016 | Event News

Agricultural Trade Developments and Potentials in Central Asia and the South Caucasus

14.10.2016 | Event News

World Health Summit – Day Three: A Call to Action

12.10.2016 | Event News

Latest News

Resolving the mystery of preeclampsia

21.10.2016 | Health and Medicine

Stanford researchers create new special-purpose computer that may someday save us billions

21.10.2016 | Information Technology

From ancient fossils to future cars

21.10.2016 | Materials Sciences

More VideoLinks >>>