Organic thin-film transistors (TFTs) are of interest for flexible, large-area electronics applications, such as rollable or foldable active-matrix displays, conformable sensor arrays and plastic circuits. Some of these applications will benefit from the availability of both p-channel and n‑channel organic TFTs, since this will allow the realization of complementary circuits that provide greater noise margins and smaller static power consumption compared to unipolar circuits. Owing to the continued development of novel conjugated organic semiconductors and improved device fabrication methods, the performance of both p-channel  and n-channel  organic TFTs and thus the prospects of low-power organic complementary circuits have greatly improved over the past few years. Here we report on the successful fabrication of fast organic complementary circuits on flexible plastic substrates using the two recently developed high-performance organic semiconductors diphenyl-dinaphtho-[2,3-b:2’,3’-f]thieno[3,2-b]thiophene)  (DPh-DNTT) for the p-channel TFTs and bis(heptafluorobutyl)-dicyano-perylene tetracarboxylic diimide (Polyera ActivInkTM N1100) for the n-channel TFTs .
Figure 1 shows the schematic cross-section of the TFTs, which were fabricated on flexible polyethylene naphthalate (PEN) substrates. In the first step, aluminum gate electrodes with a thickness of 30 nm were deposited onto the substrate by thermal evaporation in vacuum. The gate dielectric is composed of an oxygen-plasma-grown AlOx layer with thickness of 3.6 nm and a solution-processed self-assembled monolayer (SAM) of pentadecafluoro-octadecylphosphonic acid. The SAM has a thickness of 2.1 nm and the AlOx/SAM gate dielectric has a capacitance per unit area of 700 nF/cm2. 20 nm thick layers of the organic semiconductors DPh-DNTT (for the p-channel TFTs) and N1100 (for the n-channel TFTs) were deposited onto the AlOx/SAM gate dielectrics by sublimation in vacuum, and the TFTs were completed by the vacuum deposition of gold source and drain contacts. The metal and semiconductor layers were patterned using high-resolution silicon stencil masks. All measurements were performed in ambient air at room temperature.
Static transistor performance
Figure 2 shows the measured current-voltage characteristics of DPh-DNTT p-channel and N1100 n-channel TFTs having channel lengths of 10 µm, 1 µm and 0.5 µm. For a channel length of 10 µm, the TFTs have effective field-effect mobilities of 0.5 cm2/Vs (DPh-DNTT) and 0.1 cm2/Vs (N1100). Importantly, reducing the channel length from 10 µm to 0.5 µm produces virtually no degradation in the off-state drain current, the threshold voltage or the subthreshold slope of the TFTs. Even at a channel length of 0.5 µm, the on/off current ratio is still greater than 106 and the output curves still display reasonably good current saturation. Using the transmission line method , we have measured a contact resistance of 0.38 kWcm for the DPh-DNTT p-channel TFTs and 15 kWcm for the N1100 n-channel TFTs. The significant difference between the contact resistances is evident in the output characteristics of the TFTs with the shortest channel length (0.5 µm), where the drain current at small drain-source voltages shows the desirable linear dependence on the drain-source voltage in the case of the DPh-DNTT TFTs, but is significantly suppressed in the case of the N1100 TFTs.
Dynamic circuit performance
To evaluate the dynamic performance of the TFTs, we fabricated 11-stage complementary ring oscillators based on DPh-DNTT p-channel and N1100 n-channel TFTs having channel lengths of 4 µm, 2 µm and 1 µm. Figure 3 shows a photograph of one of the ring oscillators and the measured signal propagation delay per stage as a function of the supply voltage. Owing to the small thickness and large capacitance of the AlOx/SAM gate dielectric, these circuits can be operated with supply voltages as low as 1 V. Also, since that the gate capacitance scales linearly with the lateral TFT dimensions, smaller channel lengths lead to faster circuit operation. The shortest stage delay is 3.1 µs, measured on a ring oscillator based on TFTs with a channel length of 1 µm and being operated with a supply voltage of 5 V. This is the second-shortest stag delay reported to date for flexible organic complementary ring oscillators.