Plastic transistors improve with age

‘Vintage’ and ‘electronics’ are words not normally seen or heard together, but a group of materials scientists at the Swiss Federal Institute of Technology in Zürich has found that plastic transistors improve in performance if simply left to sit for a week at room temperature.

Room temperature jostling between molecules of the organic semiconductor pentacene can over time remove defects created during the deposition process

Cheap to produce, and comparable in electronic performance with amorphous silicon, transistors made from the organic semiconductor pentacene are promising candidates for thin-film applications. But during deposition from thermal evaporation, pentacene molecules can misalign to form defects which trap electrons and therefore degrade performance.

This may not be such a big problem, however, as the research team led by Wolfgang Kalb has found that if a newly made pentacene transistor is left to sit in a vacuum, the defects disappear naturally. This peculiar effect is described as being a direct consequence of the weak intermolecular interaction characteristic of organic semiconductors.

The Swiss researchers used pentacene in their study, but the results may have wider implications. “The effect has been observed with the prototypical organic semiconductor pentacene, and is very likely to be of universal validity,” says Kalb. “The effect will have to be considered in industrial fabrication procedures of organic thin-film transistors.”

Kalb and his colleagues found that under high vacuum conditions, the effective field-effect mobility increases by up to a factor or two, the contact resistance falls by more than an order of magnitude, and there is a significant reduction in current hysteresis. Experiments show that the improvement is driven by a thermal process rather than chemical doping of the samples.

Self-healing of materials typically requires heat, and annealing of pentacene thin films at an elevated temperature of 50°C leads to further improvements in semiconductor performance. But it seems that, given time, the jostling between molecules at room temperature is enough to remove the defects.

Further reading: “Defect healing at room temperature in pentacene thin films and improved transistor performance”, Kalb et al., Phys. Rev. B 76, 184112 (2007).

Figure: Room temperature jostling between molecules of the organic semiconductor pentacene can over time remove defects created during the deposition process. The discovery has beneficial implications for the development of plastic electronics (source: Wolfgang Kalb/ETHZ).

Article first published in Nanomaterials News.