Featured Research

from universities, journals, and other organizations

Light-emitting Diodes: Understanding Factors That Influence Efficiency Of Organic-based Devices

Date:
July 11, 2008
Source:
Georgia Institute of Technology
Summary:
Organic-based devices, such as organic light-emitting diodes, require a transparent conductive layer with a high work function, meaning it promotes injection of electron holes into an organic layer to produce more light. New research provides insight into factors that influence the injection efficiency.

Research conducted by Jean-Luc Brédas, a professor in the Georgia Institute of Technology's School of Chemistry and Biochemistry, aims to understand factors that influence the efficiency of organic-based devices.
Credit: Georgia Tech Photo: Gary Meek

Organic-based devices, such as organic light-emitting diodes, require a transparent conductive layer with a high work function, meaning it promotes injection of electron holes into an organic layer to produce more light.

Research presented on July 8 at the International Conference on Science and Technology of Synthetic Metals in Brazil provides insight into factors that influence the injection efficiency. A balanced injection of positive and negative charge carriers into the organic layer is important to achieve high quantum efficiency, but the interface between the metallic coating and organic layer where the injection occurs is poorly understood.

Placing an organic layer on top of the conductive layer modifies each layer's individual work function, or the minimum energy needed to extract the first electron from the metal.

"Measuring the work functions independently for each layer does not provide an indication of how their energy levels match when they touch each other," explained Jean-Luc Brédas, a computational materials chemist, professor in the Georgia Institute of Technology's School of Chemistry and Biochemistry and Georgia Research Alliance Eminent Scholar.

The energy levels for each layer should align when attached; otherwise, a barrier will form and a higher voltage will be required to send current in.

With funding from the Office of Naval Research, Brédas first developed a theoretical model of the interface between conventional metals and a single layer of organic molecules forming a self-assembled monolayer on the metal. His goal was to determine how the metal work function could be modified by depositing the self-assembled monolayer.

Brédas and postdoctoral research fellow Georg Heimel, who is now at the Humboldt University in Berlin, looked for changes in the work function of gold when they modified the chemical nature of the head group of the organic molecules in the self-assembled monolayer and the nature of the docking group, which directly connected the organic layer and metal.

The study, published in the April 2007 issue of Nano Letters, showed that changing the head group of the organic molecules located far from the surface and changing the docking group provided two nearly independent ways to modify the metal work function.

While studying two metal substrates -- gold and silver -- the researchers found that even though the chemical interface between the metal and thiol-based self-assembled monolayer were different, the organic-covered metals had virtually identical work functions.

Postdoctoral research fellow Pavel Paramonov, who is now an assistant research professor at the University of Akron, expanded the original work to model the interface between a self-assembled monolayer and indium tin oxide, the conducting material commonly used as the transparent electrode in liquid crystal displays and organic light-emitting diodes.

"Researchers frequently cover the hydrophilic indium tin oxide surface with a self-assembled monolayer containing a hydrophobic subgroup pointing away from the surface, providing much better adherence and compatibility with the active organic layer that comes on top," said Brédas.

The cover layer also prevents the indium from diffusing into the active organic layer and degrading the device, but adding this layer also provides a way to fine-tune the work function.

With funding from the Solvay Group, Paramonov modeled the indium tin oxide surface, which was a complex task because indium tin oxide is not stoichiometric -- every vendor's indium tin oxide is somewhat different. Then he modeled the binding of a self-assembled monolayer of phosphonic acid to the indium tin oxide surface. Paramonov's first goal was to determine how the oxygen and phosphorus atoms of the self-assembled monolayer bind to the indium tin oxide surface.

In collaboration with Seth Marder, a professor in the Georgia Tech School of Chemistry and Biochemistry, and Neal Armstrong, a professor in the Department of Chemistry at the University of Arizona, they were able to characterize the main binding modes of the phosphonic acid molecules on indium tin oxide. This work has led to further research characterizing the impact of the self-assembled monolayer on the indium tin oxide work function, according to Brédas.

"More theoretical work needs to be done to study conducting oxides used as transparent electrodes in organic solar cells and organic transistors," added Brédas. "On the experimental side, the quality of the self-assembled monolayer coverage also needs to be improved."

Researchers usually design devices with potentially well-aligned energy levels when the layers are measured individually, but they should be examining the layers when they are attached, according to Brédas. This is because the reorganization of the chemical, electronic and geometric structures of the two layers at the interface has a major impact on the overall device characteristics.


Story Source:

The above story is based on materials provided by Georgia Institute of Technology. Note: Materials may be edited for content and length.


Cite This Page:

Georgia Institute of Technology. "Light-emitting Diodes: Understanding Factors That Influence Efficiency Of Organic-based Devices." ScienceDaily. ScienceDaily, 11 July 2008. <www.sciencedaily.com/releases/2008/07/080708105351.htm>.
Georgia Institute of Technology. (2008, July 11). Light-emitting Diodes: Understanding Factors That Influence Efficiency Of Organic-based Devices. ScienceDaily. Retrieved October 22, 2014 from www.sciencedaily.com/releases/2008/07/080708105351.htm
Georgia Institute of Technology. "Light-emitting Diodes: Understanding Factors That Influence Efficiency Of Organic-based Devices." ScienceDaily. www.sciencedaily.com/releases/2008/07/080708105351.htm (accessed October 22, 2014).

Share This



More Matter & Energy News

Wednesday, October 22, 2014

Featured Research

from universities, journals, and other organizations


Featured Videos

from AP, Reuters, AFP, and other news services

Chameleon Camouflage to Give Tanks Cloaking Capabilities

Chameleon Camouflage to Give Tanks Cloaking Capabilities

Reuters - Innovations Video Online (Oct. 22, 2014) — Inspired by the way a chameleon changes its colour to disguise itself; scientists in Poland want to replace traditional camouflage paint with thousands of electrochromic plates that will continuously change colour to blend with its surroundings. The first PL-01 concept tank prototype will be tested within a few years, with scientists predicting that a similar technology could even be woven into the fabric of a soldiers' clothing making them virtually invisible to the naked eye. Matthew Stock reports. Video provided by Reuters
Powered by NewsLook.com
Jet Sales Lift Boeing Profit 18 Pct.

Jet Sales Lift Boeing Profit 18 Pct.

Reuters - Business Video Online (Oct. 22, 2014) — Strong jet demand has pushed Boeing to raise its profit forecast for the third time, but analysts were disappointed by its small cash flow. Fred Katayama reports. Video provided by Reuters
Powered by NewsLook.com
Internet of Things Aims to Smarten Your Life

Internet of Things Aims to Smarten Your Life

AP (Oct. 22, 2014) — As more and more Bluetooth-enabled devices are reaching consumers, developers are busy connecting them together as part of the Internet of Things. (Oct. 22) Video provided by AP
Powered by NewsLook.com
What Is Magic Leap, And Why Is It Worth $500M?

What Is Magic Leap, And Why Is It Worth $500M?

Newsy (Oct. 22, 2014) — Magic Leap isn't publicizing much more than a description of its product, but it’s been enough for Google and others to invest more than $500M. Video provided by Newsy
Powered by NewsLook.com

Search ScienceDaily

Number of stories in archives: 140,361

Find with keyword(s):
 
Enter a keyword or phrase to search ScienceDaily for related topics and research stories.

Save/Print:
Share:  

Breaking News:

Strange & Offbeat Stories

 

Space & Time

Matter & Energy

Computers & Math

In Other News

... from NewsDaily.com

Science News

Health News

Environment News

Technology News



Save/Print:
Share:  

Free Subscriptions


Get the latest science news with ScienceDaily's free email newsletters, updated daily and weekly. Or view hourly updated newsfeeds in your RSS reader:

Get Social & Mobile


Keep up to date with the latest news from ScienceDaily via social networks and mobile apps:

Have Feedback?


Tell us what you think of ScienceDaily -- we welcome both positive and negative comments. Have any problems using the site? Questions?
Mobile iPhone Android Web
Follow Facebook Twitter Google+
Subscribe RSS Feeds Email Newsletters
Latest Headlines Health & Medicine Mind & Brain Space & Time Matter & Energy Computers & Math Plants & Animals Earth & Climate Fossils & Ruins