We report on the realization of flexible and transparent all-organic Ambipolar Field Effect Transistors. The devices were assembled on a flexible plastic foil, i.e. Mylar®, and the contacts were realized with poly(ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) and patterned by means of Soft Lithography, MicroContact Printing (μCP). As semiconductor layer we used organic bulk heterojunctions of pentacene/C60 realized either by co-deposition of the two different molecules or by a double layer structure in which pentacene was used as buffer layer at the interface with the gate dielectric. Interestingly, all devices (co-deposited and double layer), measured in air, worked in accumulation mode as ambipolar OFETs, however some interesting differences between the two structures can be pointed out. Supported by Atomic Force Microscopy, we demonstrated that growing C60 on a pre-deposited pentacene buffer layer leads to a clear improvement in the morphology and crystallinity of the deposited film allowing to improve n-type conduction by two orders of magnitude. This work is particularly interesting because on one hand it confirms the importance of the substrate properties for the ordered growth of organic semiconductors, which determines the transport properties of organic materials; moreover, it demonstrates, also for n-type and ambipolar transistors, the possibility of avoiding problems normally associated to metal contacts: the lack of mechanical robustness, flexibility, and the unfeasibility of realizing contacts with low cost techniques like printing or soft lithography. The flexibility and transparency of the final OFET structure and the simple low cost fabrication technique employed pave the way for an economic mass production of large area transparent “Plastic Electronics”.
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