2, National Institute for Materials Science (NIMS), Tsukuba, , Japan
Very high values of carrier mobility have been recently reported in newly developed polymer semiconductors for field-effect transistors (FETs). However, many of them show the non-ideal current-voltage characteristics that may lead to make the not suitable for direct applications in logic or driving circuits or sensing applications. We investigate how contact problems and bulk defects induce deviations from ideal current-voltage characteristics.
We derived quantitative methods to extract contact and channel properties from FETs by fast and simple characterizations. The presented methods tell how the metal-polymer contact works in FETs and reveal how carrier diffusion limits the device performance. Then we investigated how much contact resistance a polymeric FET can tolerate to allow the conventional current-voltage equations, which is derived for no contact resistance. We contend that mobility in transistors with resistive contact can be underestimated with the presence of the injection barrier, whereas mobility in transistors with gated Schottky contact can be overestimated by more than ten times. This is because the band bending and injection barrier experience a complicated evolution on account of electrostatic doping in the semiconducting polymers. For precision, carrier mobility should be presented against gate voltage and should be examined by other recommended extraction methods. [1,2,3]
Then, we demonstrate how to use bulk doping and interface modification to enhance charge injection rate as well as charge transport and to obtain almost ideal FET with advanced polymers. [4,5] In particular, when using the organic dopant bis(cyclopentadienyl)-cobalt(II) (cobaltocene, CoCp2) at a low concentration, the FET mobility with P(NDI2DO-T2) was increased and the threshold voltage was decreased from 32.7 V to 8.8 V. Deviating from previous discoveries, we found that mobility increases first and then decreases drastically beyond a critical value of molecule ratio. Meanwhile, the intensity and width of the main peak of in-plane X-ray diffraction starts to decrease at the same critical molecule ratio, showing that dopants also induce crystallization in polymers. In addition, we found self-assembled dielectrics can provide high carrier concentrations as well as low interfacial dipolar disorders to enhance device performances.