Plastic semiconductors offer the potential of inexpensive mass-produced devices incorporating plastic solar cells and plastic light-emitting diodes. But plastic electronics have one key flaw- the electronic current is subjected to “charge traps” in the material. These traps, which have a negative impact on plastic light-emitting diodes and solar cells, are not yet well understood.
Now, a study by researchers from the University of Groningen and the Georgia Institute of Technology reveals a common mechanism of these traps and gives a theoretical framework to the design of trap-free plastic electronics. The results are presented in the journal Nature Materials.
Made from organic, carbon-based polymers, semiconducting polymers represent a tunable forbidden energy gap. In a plastic light-emitting diode (LED), an electron current is injected into a higher molecular orbital, situated just above the energy gap.
After injection, the electrons move toward the middle of the LED and fall in energy across the forbidden energy gap, converting the energy loss into photons. As a result, an electrical current is converted into visible light.
However, during their passage through the semiconductor, a lot of electrons get stuck in traps in the material and can no longer be converted into light. In addition, this trapping process greatly reduces the electron current and moves the location where electrons are converted into photons away from the center of the device.
“This reduces the amount of light the diode can produce,”
explained Herman Nicolai, one of the authors of the paper.
Mystery Traps
Visualization of an electron traveling through a potential field with charge traps in plastic electronics.
Credit: Gert-Jan Wetzelaer, University of Groningen
The traps are poorly understood, and it has been suggested that they are caused by kinks in the polymer chains or impurities in the material.
“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.”
The Georgia Tech group, led by Professor Jean-Luc Bredas, computationally researched the electronic structure of a wide range of possible traps.
“What we found out from the calculations is that the energy level of the traps measured experimentally matches that produced by a water-oxygen complex,”
said Bredas.
Nicolai says that “such a complex could easily be introduced during the manufacturing of the semiconductor material, even if this is done under controlled conditions.” The devices he studied were fabricated in a nitrogen atmosphere, “but this cannot prevent contamination with minute quantities of oxygen and water,” he noted.
The fact that the traps have a similar energy level means that it is now possible to make an approximation of the expected electron current in different plastic materials. And it also points the way to trap-free materials.
“The trap energy lies in the forbidden energy gap,”
Nicolai explained.
Forbidden Energy Gaps
This energy gap represents the difference in energy of the outer shell in which the electrons circle in their ground state and the higher orbital to which they can be moved to become mobile charge carriers. When such a mobile electron runs into a trap that is within the energy gap it will fall in, because the trap has a lower energy level.
“But if chemists could design semiconducting polymers in which the trap energy is above that of the higher orbital in which the electrons move through the material, they couldn’t fall in,” he suggested. “In this case, the energy level of the trap would be higher than that of the electron.”
The results of this study are, therefore, important for both plastic LEDs and plastic solar cells.
“In both cases, the electron current should not be hindered by charge trapping. With our results, more efficient designs can be made,”
Nicolai concluded.
Reference: H. T. Nicolai, M. Kuik, G. A. H.Wetzelaer1, B. de Boer1, C. Campbell, C. Risko, J. L. Brédas and P.W. M. Blom. Unification of trap-limited electron transport in semiconducting polymers. Nature Materials, online: 29 July 2012 | DOI: 10.1038/NMAT3384
