How to avoid traps in plastic electronics
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 study by a team of researchers of the University of Groningen, supervised by Paul Blom who is now at MPIP, and Georgia Tech reveals a common mechanism underlying these traps and provides a theoretical framework to design trap-free plastic electronics. The results are presented in the journal Nature Materials (vol. 11, p. 882, 2012).
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 towards 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 cannot be converted into light anymore. In addition, this trapping process greatly reduces the electron current and moves the place where electrons are converted into photons away from the center of the device. ‘And this reduces the amount of light the diode can produce,’ explains Herman Nicolai, first author of the Nature Materials paper. The traps are poorly understood. It has been suggested that they are caused by kinks in the polymer chains or impurities in the material.
By comparing the properties of these traps in nine different polymers it was revealed that the traps in all materials had a very similar energy level. Theoretical calculations showed that this level compares well with that of a water-oxygen complex. Such a complex could easily be introduced during the manufacturing of the semiconductor material, even if this is done under controlled conditions. The fact that the traps have a similar energy level means that it is now possible to calculate the expected electron current in different plastic materials. And it also points the way to trap-free materials. The results of this study are therefore important for both plastic LEDs and plastic solar cells.