ATLANTA, July 31, 2012 — A theoretical framework that addresses the significant “charge trap” defect of plastic semiconductors has yielded trap-free plastic electronics with the potential for more efficient plastic LEDs and solar cells.
Plastic electronics hold the promise of cheap, mass-produced devices; however, plastic semiconductors have a significant defect: The electronic current is influenced by charge traps in the material. These traps have a negative influence on solar cells and plastic LEDs and are poorly understood.
That is, until now. Scientists from Georgia Institute of Technology and the University of Groningen in the Netherlands have exposed a common mechanism underlying these charge traps and have provided a theoretical framework for the design of trap-free plastic electronics.
A visualization of an electron traveling through a potential field with charge traps in plastic electronics. (Image: Gert-Jan Wetzelaer, University of Groningen)
Plastic semiconductors, composed of organic, carbon-based polymers, comprise a tunable forbidden energy gap. An electron current is inserted into a higher molecular orbital, just above the energy gap in a plastic LED. After insertion, the electrons move toward the center of the LED and fall down in energy across the forbidden energy gap, converting the energy loss into photons. The result is an electrical current that is converted into visible light.
However, a lot of electrons get caught up in traps in the material and cannot be converted into light during their passage through the semiconductor. In addition, this trapping process minimizes the electron current and shifts the place where electrons are transformed into photons outside the center of the device. This process reduces the quantity of light that the diode generates, said Herman Nicolai, first author of a paper describing the process.
The poorly understood traps are thought to be caused by kinks in the polymer chains or impurities in the material.
A white polymer light-emitting diode fabricated at the University of Groningen. Devices such as these suffer losses from charge traps in the materials. (Image: Herman Nicolai, University of Groningen)
“We’ve set out to solve this puzzle by comparing the properties of these traps in nine different polymers,” Nicolai said. “The comparison revealed that the traps in all materials had a very similar energy level.”
Because the traps have similar current rates, the expected electron current can be estimated in various plastic materials. The study also points the way to trap-free materials. The results hold promise for both plastic LEDs and plastic solar cells.
“In both cases, the electron current should not be hindered by charge trapping,” Nicolai said. “With our results, more efficient designs can be made.”
The study was reported in Nature Materials
For more information, visit: www.gatech.edu