Organic complementary-like inverters employing methanofullerene-based ambipolar field-effect transistors

Abstract

We demonstrate a complementary-like inverter comprised of two identical ambipolar field-effect transistors based on the solution processable methanofullerene [6,6]-phenyl-C 61 -butyric acid methyl ester (PCBM). The transistors are capable of operating in both the p-and n-channel regimes depending upon the bias conditions. However, in the p-channel regime transistor operation is severely contact limited. We attribute this to the presence of a large injection barrier for holes at the Au/PCBM interface. Despite this barrier the inverter operates in both the first and third quadrant of the voltage output versus voltage input plot exhibiting a maximum gain in the order of 20. Organic field-effect transistors (OFETs) are currently the focus of intense research efforts in numerous academic and industrial research laboratories around the world. Over the past decade OFETs have emerged as promising candidates for electronic device applications requiring low cost and large area coverage, mechanical flexibility, and low temperature processing. 1 Several groups have demonstrated OFETbased circuits with performances sufficient for practical applications. Few examples of such applications include switching devices for flat panel displays 2-4 and integrated circuits. [4][5][6] To date the largest organics-based electronic circuit reported is the standard logic based 32-stage shift registers that consists of 1888 transistors capable of operating at a frequency of 5 kHz. 4 As the number of transistors per circuit increases there is an increasing need for circuits characterized by low power dissipation, high noise margin, and greater operation stability. In silicon-based microelectronics such requirements are met through the use of complementary metal-oxide-semiconductor (CMOS) logic where an n-and a p-type transistor are combined to built logic circuits. Indeed, examples of organic CMOS logic have show that it is possible to make large-scale integrated circuits (up to 864 transistors per circuit) with much lower power dissipation. 6 In the work by Crone et al., however, the transistor channels had to be spatially separated in order to facilitate the separate vacuum deposition of the two semiconductors (an n-and a p-type), thus making circuit fabrication difficult and potentially expensive.Recently an alternative approach towards organic CMOS circuits has been proposed. 7 In their work Meijer et al. have demonstrated that by employing identical ambipolar OFETs based on polymer-small molecule interpenetrating networks as well as narrow band gap polymers, CMOS-like voltage inverters can be fabricated. This approach makes full use of the attractive processing properties of polymers while it simplifies device fabrication by utilizing a single semiconductor layer. However, despite these promising preliminary results there is still a drawback associated with most ambipolar OFETs reported to date, which is the relatively low carrier mobility. [7][8][9][10] For example, in the work by Meijer et al. the maximum ambipolar carrier mobilities...

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Ambipolar charge transport in organic field-effect transistors

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