Leveraging the Ambipolar Transport in Polymeric Field-Effect Transistors via Blending with Liquid-Phase Exfoliated Graphene

摘要

Emerging technological demands require more and more the development of semiconducting materials suitable for ambipolar field-effect transistors (FETs) as potential active components for organic complementary metal-oxide semiconductor (CMOS) to ultimately enable the development of robust, low-noise, low-power organic electronics. For a simple device fabrication process, single metal source and drain electrodes are preferred. Thus efficient ambipolar FETs can be ideally achieved by using symmetric electrodes and sophisticated single component semiconducting molecules with narrow bandgaps, i.e., lower than 2 eV, designed in such a way to be capable of transporting both electrons and holes under suitable biasing conditions and device configuration. Conversely, asymmetric electrodes having different work function (WF) can be employed to inject different charges in the active layer of the device, although the fabrication procedure is more cumbersome since it requires multiple steps. An alternative approach relies on the use of bilayers of semiconductors with high and low electron affinity (so called n-type and p-type) by choosing the materials carefully according to the relative position of their highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbitals (LUMO) levels. The combination of semiconductors in a bilayer provides a great opportunity to overcome the high injection barrier for one type of charge carrier when a single electrode material is employed, and renders the synthesis of the building block less challenging. However, the main disadvantage remains the need of controlled deposition of two layers on the top of each other. Alternatively, ambipolar transport was achieved by blending unipolar n- and p-type materials either by co-evaporation or co-deposition from solutions resulting in an interpenetrating network of both materials. The concept of blending materials was not only used for obtaining ambipolar devices but also for improving the device performance benefiting from the properties brought by each component, as well as to incorporate more functions in a single device. Graphene is one among the most interesting candidates that can be embedded in polymer matrices to improve the polymer performances. Graphene is a well-defined functional, 2D ambipolar material with a mobility for charge carriers exceeding 15 000 cm~2 V~(-1) s~(-1) even under ambient conditions, thus outperforming any organic semiconductor or even silicon. Despite its exceptional optical, mechanical and electronic properties, the greatest challenge in graphene research remains the ability to process it in large quantities and high quality making use of up-scalable methods. Among the various proposed methods, liquid-phase exfoliation is an extremely versatile and easily accessible procedure to produce graphene with a high yield starting from graphite powder. Moreover, graphene is a zero-bandgap semiconductor: to impart to this 2D material a capacity to switch on and off, one can either confine it in space by generating graphene nanoribbons, or having it interacting with molecular components that can act as dopant. In this regard, graphene has been used for application in FETs by modulating its properties via blend with p-type semiconductors.