Fullerene Based n-type Organic Thin-Film Transistors
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Fullerene Based N-type Organic Thin-Film Transistors Joshua Haddock, Benoit Domercq and Bernard Kippelen Center for Organic Photonics and Electronics School of Electrical and Computer Engineering Georgia Institute of Technology 777 Atlantic Dr NW, Atlanta, GA 30332, USA ABSTRACT Significant progress has been made in the area of p-type organic field effect transistors while progress in developing n-type materials and devices has been comparatively lacking, a limiting factor in the pursuit to develop complementary organic electronic circuits. Given the need for n-type organic semiconductors we have carried out studies using two different fullerene molecules, C60 and C70. Here, we report mobilities for C60 ranging from 0.02 cm2/Vs up to 0.65 cm2/Vs (depending on channel length), and mobilities from 0.003 cm2/Vs up to 0.066 cm2/Vs for C70. All devices were fabricated with organic films deposited under high vacuum but tested at ambient pressures under nitrogen. INTRODUCTION Organic field-effect transistors (OFETs) hold promise for electronic applications where low cost, large area, simplified processing and mechanical flexibility are of interest. Individual transistors with field-effect mobilities greater than 1 cm2/Vs, threshold voltages near 0 V, and on/off current ratios greater than 106 are routinely achieved in thin films of the p-type (hole transport) organic semiconductor pentacene [1]. This level of performance is comparable to that of amorphous silicon thin-film transistors used to drive some liquid-crystal displays. However, in order to realize applications such as organic complementary logic circuits [2], both p-type and n-type organic semiconductors are needed and should exhibit comparable electrical performance. In contrast, electron field-effect mobilities have only recently exceeded 1 cm2/Vs; where the highest value reported in a thin-film device was 1.7 cm2/Vs using the perylene-based compound PTCDI-C8 [3]. The fullerenes are another well known class of n-type organic semiconductor that have previously been used for OFETs. In thin films of the fullerene C60, reported fieldeffect mobility values have increased over time from 0.08 cm2/Vs, reported by Haddon et. al. in 1995 [4] to 0.56 cm2/Vs, reported by Kobayashi et. al. in 2003 [5, 6]. In both cases threshold voltages were approximately 15-20 V, and on/off current ratios increased from >106 for Haddon to >108 for Kobayashi. Thin films of the fullerene C70 have also been studied in OFET devices by Haddon [7] and the field effect mobility was two orders lower than C60, 0.002 cm2/Vs. Compared to Haddon’s C60 OFETs, these devices also exhibited higher threshold voltages (27 V compared to 15 V) and on-off current ratios that were one order of magnitude lower (105 compared to 106). Because n-type organic semiconductors are susceptible to the effects of oxygen [8], devices exhibiting the highest mobility values [4, 5, 6, 7] have been both fabricated and tested under ultra-high vacuum (UHV) conditions. One report of stable operation of C60 OFETs in air u
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