By Nick Gilbert
Plastic electronics are promising affordable, mass-produced devices; however, plastic semiconductors have a significant defect: in the material, the electronic current is affected by “charge traps”. These traps have a negative influence on solar cells and plastic light-emitting diodes and are poorly understood.
Visualization of an electron traveling through a potential field with charge traps in plastic electronics. Credit: Gert-Jan Wetzelaer, University of Groningen
But, a new study carried out by a team of scientists from the Georgia Institute of Technology and the University of Groningen exposes a prevalent mechanism underlying these charge traps and provides an ideological framework for designing trap-free plastic electronics. The results of this new study have been published online in the Nature Materials journal.
Plastic semiconductors are made up of organic, carbon-based polymers and they 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 in energy over against the forbidden energy gap and convert the energy loss in the process into photons. As a consequence, an electrical current is changed into visible light.
But, a lot of electrons get caught up in traps in the material and cannot be converted into light during their journey through the semiconductor. Furthermore, this trapping process minimizes the electron current and shifts the place where electrons are transformed into photons outside the center of the device. The first author of Nature Materials, Herman Nicolai explained that this process reduces the quaintly of light the diode can generate.
The traps are caused by impurities in the material or kinks in the polymer chains. The Georgia Tech group, headed by Professor Jean-Luc Bredas from the School of Chemistry & Biochemistry, examined the electronic structure of various possible traps.
Since the traps have similar current rates, the expected electron current can be estimated in different plastic materials. The study also points the method to trap-free materials. The results of this investigation are important for both plastic solar cells and plastic LEDs.