Organic thin-film transistors (TFTs) can typically be fabricated at temperatures below about T = 100°C and thus not only on glass substrates, but also on a variety of unconventional substrates, such as plastics and paper. This makes organic TFTs potentially useful for the realization of flexible, large-area electronics applications, such as rollable or foldable information displays and conformable sensor arrays. In some of the more advanced applications envisioned for organic TFTs, such as the integrated row and column drivers of flexible active-matrix organic light-emitting diode (AMOLED) displays, the TFTs have to be able to control electrical signals of a few volts at frequencies of several megahertz. For portable applications powered by small batteries, an additional requirement is a very low power consumption, which implies a complementary circuit technology and thus the availability of both p-channel and n-channel TFTs with sufficient static and dynamic performance.
The first requirement for achieving high switching frequencies in organic TFTs is efficient charge transport in the organic semiconductor layer. To meet this requirement we have chosen small-molecule semiconductors that provide good molecular ordering and large field-effect mobilities even when processed at temperatures below 100°C. In the case of organic p‑channel TFTs, the alkylated thienoacene C10‑DNTT that was recently developed in the group of Kazuo Takimiya at
The second requirement for achieving high switching frequencies in organic TFTs is a small channel length. To meet this requirement, we have recently developed a TFT process in which high-resolution silicon stencil masks are employed for the patterning of the various transistor components. With this process, bottom-gate, top-contact organic TFTs with a channel length of 1µm can be fabricated on plastic substrates without exposing the organic semiconductors to potentially harmful organic solvents and photoresists [2,3].
Figure 1 shows two photographs and the measured current-voltage characteristics of a C10‑DNTT p‑channel TFT with a channel length of 1µm fabricated on a flexible, 125µm‑thick polyethylene naphthalate substrate. The small thickness (5.3nm) and large capacitance per unit area (800nF/cm2) of the AlOx/SAM gate dielectric allow the TFTs to operate with low voltages of about 3V. The TFTs have an effective hole mobility in the saturation regime of 1.2cm2/Vs, an on/off current ratio of 107, a subthreshold swing of 150 mV/decade, and a width-normalized transconductance of 1.2S/m . This is believed to be the largest width-normalized transconductance reported so far for organic TFTs on plastic substrates.
The third and final requirement for achieving high switching frequencies (in addition to a large field-effect mobility and a small channel length, as discussed above) is a small gate capacitance. One component of the gate capacitance is the parasitic capacitance that is formed by the geometric overlaps between the gate electrode and the source/drain contacts, so reducing not only the channel length, as discussed above, but also the gate overlap length can be useful in view of high-frequency TFT operation [2,3].
Figure 2 shows the circuit schematic and the photograph of an 11-stage ring oscillator with output buffer comprising unipolar inverters with saturated load based on C10‑DNTT p‑channel TFTs fabricated on a flexible PEN substrate. In the most aggressive design, the organic TFTs have a channel length of 1µm and a gate overlap of 5µm. In a more relaxed design, the channel length is 4µm and the gate overlap is 20µm. Also shown in Fig. 2 are the signal propagation delays per stage measured in these ring oscillators and plotted as a function of the supply voltage. For the more aggressive dimensions (channel length 1µm, gate overlap 5µm), the measured stage delay is 1.9µs at a supply voltage of 1V, 730ns at 2V, 420ns at 3V, and 300ns at 4V. These are believed to be the first organic TFTs fabricated on flexible plastic substrates demonstrating cutoff frequencies above 1 MHz at supply voltages below 10 V.
By choosing an organic semiconductor with a large electron affinity, i.e., with a large energy difference between the lowest unoccupied molecular orbital (LUMO) and the vacuum level, it is possible to realize organic n‑channel TFTs. An example is the recently developed core-chlorinated and fluoroalkyl-substituted naphthalene tetracarboxylic diimide NTCDI‑Cl2‑(CH2C3F7)2 . Figure 3(a) shows the chemical structure of this semiconductor, along with the measured current-voltage characteristics of an NTCDI‑Cl2‑(CH2C3F7)2 n‑channel TFT with a channel length of 1µm. The TFTs have an effective electron mobility in the saturation regime of 0.06cm2/Vs, an on/off ratio of 106, a subthreshold swing of 180 mV/decade, and a width-normalized transconductance of 0.06S/m. While these parameters are notably inferior to those of the C10‑DNTT p‑channel TFTs shown in Fig. 1, they represent the best performance currently achievable in air‑stable, low‑voltage organic n‑channel TFTs with such a small channel length.
The fact that the field-effect mobility of the n‑channel TFTs is significantly smaller than that of the p‑channel TFTs implies that the dynamic performance of complementary circuits based on organic n‑channel and p‑channel TFTs will be limited by the longer signal propagation delay of the n‑channel devices. Figure 3(b) shows the photograph of an 11-stage complementary ring oscillator based on NTCDI‑Cl2‑(CH2C3F7)2 n‑channel TFTs and DNTT p‑channel TFTs fabricated on a flexible PEN substrate, along with the signal delays measured for ring oscillators with minimum feature sizes of 1µm, 2µm and 4µm. For the most aggressive design (channel length 1µm), the measured stage delay is 17µs at a supply voltage of 2.6V, which is indeed slower by more than an order of magnitude compared with the signal delay of the unipolar all‑p‑channel ring oscillators shown in Fig. 2. Nevertheless, these results demonstrate the feasibility of realizing low‑voltage, low‑power complementary circuits on flexible plastic substrates.