New light on old instability of organic transistors

1 Dec 2009. NUS scientists at the Organic Nano Device Laboratory (ONDL) have uncovered a photo-doping mechanism that causes degradation of the plastic semiconductors used to make flexible transistor circuits and organic solar cells. In a report published recently in Advanced Materials (J.M.Zhuo et al, Advanced Materials 21 (2009) 4747-4752), the scientists shone light on an old mystery of why the electrical performance of devices based on regioregular poly(3-hexylthiophene) and poly[2,5-bis(3-tetradecylthiophen-2-yl)thieno(3,2-b)thiophene, for example, often degrade in air, and resolved the debate of whether oxygen or moisture was chiefly responsible. 

Now in a new series of experiments using Fourier-transform infrared spectroscopy with sufficient stability that chemical changes at the sub-1-mol% of repeat units can be detected, the scientists were able to show that moisture is involved together with light by uncovering the missing evidence that hydroxide ions are generated in the photochemical process. 

"The process is initially reversible, and the devices can recover to their original state when incubated in the glovebox.  This provides an important step forward in our understanding of the degradation of organic semiconductor devices,” said Jing-Mei Zhuo, lead author of the report.  “It shows the importance of keeping out at least either light or moisture.” 

"This research is an example of a relatively fundamental study that provides new insights that will likely lead to the development of more robust organic semiconductor systems for applications in light-emitting diodes, transistors and solar cells," said Prof Peter Ho, co-director of the ONDL.