Organic Transistors Based on Di(phenylvinyl)anthracene: Performance and Stability**

The electrical performance of organic thin-film transistors (TFTs) often degrades when the devices are exposed to air. This is generally ascribed to the generation of trap states, [1] possibly as a result of the oxidation of the organic semiconductor. [2] One strategy to improve the stability of p-channel organic TFTs is the synthesis of conjugated semiconductors with a relatively large ionization potential. [3–8] However, most of the TFTs based on organic semiconductors with large ionization potentials reported up till now have shown carrier mobilities that are smaller than that of pentacene. Here, we report on a new organic semiconductor, di(phenylvinyl)anthracene (DPVAnt), [9] that combines large carrier mobility (similar to that of pentacene) with increased ionization potential and improved stability as compared to pentacene. DPVAnt has been synthesized by a Suzuki coupling reaction between 2,6-dibromoanthracene and 4,4,5,5-tetramethyl2-[2-phenylvinyl]-[1,3,2]dioxaborolane [9] with a yield of 85%. Pentacene has been purchased from Fluka. Both semiconductors have been purified by temperature gradient sublimation in a stream of inert gas. Cyclic voltammetry indicates a highest occupied molecular orbital (HOMO) energy of –5.4 eV for DPVAnt, as compared to –5.0 eV for pentacene. From UV-vis absorption spectroscopy we have determined an optical bandgap of 2.6 eV for DPVAnt and 1.8 eV for pentacene. These results are consistent with the general observation that molecules characterized by a smaller conjugated p-system have more negative HOMO energies and larger bandgaps. Simple TFT test structures have been prepared on heavily doped silicon substrates (serving as the gate electrode) with a thermally grown SiO2 gate dielectric. The dielectric surface has been treated with octadecyltrichlorosilane (OTS), [10] and the organic semiconductor has been vacuum deposited onto the substrate. Gold source/drain contacts have been thermally evaporated throughashadowmask(Fig. 1a).Duringthedeposition of the semiconductor, the substrates are held at a temperature of 60°C for pentacene and 80°C for DPVAnt. The carrier mobilities extracted from the transfer characteristics measured in air are 1 cm 2 V –1 s –1 for pentacene and 1.3 cm 2 V –1 s –1 for DPVAnt (Fig. 1b). Both TFTs have an on/off current ratio of 10 7 and a subthreshold swing of 500 mV decade –1 . Perhaps the most striking differences between the two devices are the much more negative turn-on and threshold voltages of the DPVAnt transistor (Vturn-on =– 14 V,Vth = –16 V) as compared to the pentacene TFT (Vturn-on =– 2 V,Vth = –5 V). The exact reason for this difference is not known, but it may be related to the more negative HOMO energy of DPVAnt as compared to pentacene. As shown by the atomic force microscopy (AFM) images in Figure 1c and d, both semiconductors form well-ordered polycrystalline films, which is a prerequisite for obtaining large carriermobilities.