Visualization of an electron traveling through a potential field with charge traps in plastic electronics. Credit: Gert-Jan Wetzelaer, University of Groningen |
Plastic
electronics hold the promise of cheap, mass-produced devices. But
plastic semiconductors have an important flaw: the electronic current is
influenced by “charge traps” in the material. These traps, which have a
negative impact on plastic light-emitting diodes and solar cells, are
poorly understood.
However,
a new study by a team of researchers from the University of Groningen
and the Georgia Institute of Technology reveals a common mechanism
underlying these traps and provides a theoretical framework to design
trap-free plastic electronics. The results are presented in an advance
online publication of the journal Nature Materials.
Plastic
semiconductors are made from organic, carbon-based polymers, comprising
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 down 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, first author of the Nature Materials paper.
A white polymer light-emitting diode fabricated at the University of Groningen. Devices such as this suffer losses from charge traps in the materials. Credit: Herman Nicolai, 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 explained. “The comparison
revealed that the traps in all materials had a very similar energy
level.”
The
Georgia Tech group, led by Jean-Luc Bredas, investigated
computationally 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
explains 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 Nicolai 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 estimate 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.
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.
A white polymer light-emitting diode fabricated at the University of Groningen. Devices such as this suffer losses from charge traps in the materials. Credit: Herman Nicolai, University of Groningen |
“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.
The
experimental work for this study was done in the Zernike Institute of
Advanced Materials (ZIAM) at the faculty of Mathematics and Natural
Sciences, University of Groningen, the Netherlands. The theoretical work
to identify the nature of the trap was carried out at the School of
Chemistry and Biochemistry and Center for Organic Photonics and
Electronics at the Georgia Institute of Technology, Atlanta, USA .
The
work at the University of Groningen was supported by the European
Commission under contract FP7-13708 (AEVIOM). The work at Georgia Tech
was supported by the MRSEC program of the National Science Foundation
under award number DMR-0819885.
Unification of trap-limited electron transport in semiconducting polymers