Low-Voltage Organic Thin-Film Transistors with Improved Stability and Large Transconductance

Abstract

Pentacene is among the most popular organic semiconductors for organic thin-film transistors (TFTs), due to its relatively large carrier mobility [1,2]. However, the stability of pentacene TFTs under continuous dynamic operation may be insufficient for future applications. Here we compare the static and dynamic performance and the operational stability of low-voltage organic TFTs based on pentacene and a recently synthesized organic semiconductor, di(phenylvinyl)anthracene (DPVAnt). DPVAnt was synthesized by a Suzuki coupling reaction [3], and pentacene was purchased from commercial sources. Both materials were purified by temperature-gradient sublimation. TFTs and circuits were fabricated on glass substrates using an inverted staggered TFT structure with evaporated aluminum gates, a thin gate dielectric based on a solution-processed self-assembled monolayer (0.7 iF/cm2), an evaporated organic semiconductor layer, and evaporated gold top contacts [4]. All layers were patterned using manually aligned shadow masks. The device structure and the chemical structures of the semiconductors are shown in Figures 1 and 2. All electrical measurements were carried out in ambient air. Figures 3 and 4 show the electrical characteristics of a pentacene TFT and a DPVAnt TFT before, during, and after an operational stability test. During the test a continuous square-wave signal with a period of 10 sec is applied to the gate electrode, and the drain current is monitored using a Semiconductor Parameter Analyzer. As can be seen, the performance of both TFTs changes during the test. In particular, carrier mobility and on-current of the pentacene TFT are reduced by more than an order of magnitude (mobility from 0.4 cm2/Vs to 0.02 cm2/Vs, on-current from 2 pA to 80 nA), while the threshold voltage appears to be unchanged. On the other hand, the mobility of the DPVAnt TFT is stable at 0.3 cm2/Vs. The threshold voltage of the DPVAnt TFT has shifted by about -0.5 V during the test, reducing the on-current by about 50% (from 1.3 pA to 700 nA), and the subthreshold behavior of the DPVAnt TFT is somewhat degraded. Overall, the DPVAnt TFT shows better operational stability compared with the pentacene TFT. In order to obtain TFTs with a high cut-off frequency, it is necessary to reduce the parasitic capacitances and increase the transconductance. The latter can be accomplished by reducing the channel length, but it also requires a small contact resistance. Figure 5 shows a shadow-mask-patterned pentacene TFT with a channel length of 10 ptm, a channel width of 100 ptm, a total gate capacitance of about 15 pF, and a transconductance of 4 iS (40 pS/mm) at low voltage (VDS =-1.5 V, VGS -2.0 .. -2.5 V). For DPVAnt TFTs we have obtained a transconductance of 30 pS/mm. Analyzing pentacene and DPVAnt TFTs with channel length ranging from 50 ptm to 10 ptm we have extracted the gate-bias-dependent contact resistance (sum of drain and source resistance [5]) in the linear regime and found a value of 850 Q.cm for pentacene and 1.3 kQ.cm for DPVAnt (see Fig...