The Importance of Grain Boundaries for the Time-Dependent Mobility Degradation in Organic Thin-Film Transistors

The relationships between organic semiconductor morphology and device stability of organic field-effect transistors (FETs) are very complex and not yet fully understood. Especially for obtaining high-performance, air-stable n-channel FETs, gaining a deep insight into possible degradation mechanisms can help improve their air stability. Here, we investigate the performance and stability of organic n-channel FETs based on solution-grown single-crystalline ribbons of the conjugated semiconductor bis(1 H,1 H -perfluorobutyl)-dicyano-perylene tetracarboxylic diimide (C 3 F 7 CH 2 −PTCDI-(CN) 2 ). The FETs show an electron mobility of 0.25 cm 2 /(V s) in air and an on/off ratio up to 1 × 10 7 . Their mobility does not significantly change when devices are stored in air for more than 5 weeks. This excellent air stability stands in contrast to FETs based on thin evaporated polycrystalline films of the same compound that degrade by more than an order of magnitude during the same 5 week period. We attribute this striking difference in air stability to the grain boundaries in the polycrystalline films and discuss different possible degradation mechanisms.