H. Klauk, U. Zschieschang, R.T. Weitz, F. Ante and D. K¨alblein Flexible thin-film transistors based on
room-temperature-processable organic semiconduc-tors first emerged in the late 1980s as some-what of a laboratory curiosity. The subject of an ever-growing world-wide research effort for 20 years, organic transistors are now beginning to make their way into consumer products. First to market will be the Readius, a digital mo-bile device with a 5-inch foldable active-matrix display enabled by 76800 pentacene transistors (see Fig. 44(a)). The Readius, which is sched-uled to go into mass production in 2008, is expected to be followed by an A5-size flex-ible electronic newspaper device that utilizes more than one million polymer transistors (see Fig. 44(b)). Both devices have been developed and will be manufactured in Europe.
Next-generation flexible devices based on or-ganic thfilm transistors will likely feature in-creasingly larger displays with higher resolu-tion, brighter colors, and more and more
ad-ditional functions. This will translate into in-creasing power consumption, and much like to-day’s mobile phones and laptop computers, fu-ture flexible-display devices will either suffer from limited battery life or will require larger, heavier, more expensive battery packs.
One reason for this is that the transistors that drive each of the many pixels in an active-matrix display typically require operating volt-ages in the range of 10 V to 20 V. To alleviate the power problem we are developing organic transistors that can be operated with much lower voltages of only 2 V to 3 V. The key innovation to enable low-voltage thin-film transistor oper-ation is a very thin gate dielectric that is based on a combination of an oxygen-plasma-grown aluminum oxide layer and a solution-processed molecular self-assembled monolayer. Both lay-ers are prepared at room temperature, so that the transistors are suitable for flexible polymeric substrates.
Figure 44: Left: Thefirst consumer product that employs organic transistors is the Readius developed by Polymer Vision and expected to become available in 2008 (www.polymervision.com). Right: Plastic Logic is developing an electronic newspaper device enabled by more than one million polymer transistors on a flexible substrate (www.plasticlogic.com).
Figure 45: Characteristics of a low-voltage pen-tacene 5-stage ring oscillator. The drive TFTs have a channel length of 10μm and a channel width of 100μm, the load TFTs have a channel length and width of 50μm.
The total dielectric thickness is only about 6 nm, so that a gate voltage of only 2 V to 3 V is suf-ficient to induce a carrier channel in the or-ganic semiconductor. Most importantly, owing to the high quality of the self-assembled mono-layer, the leakage currents through these ultra-thin gate dielectrics are very small, less than 10μA/cm2 at 3 V. Consequently, organic tran-sistors with ultrathin gate dielectrics based on self-assembled monolayers are well-suited for low-voltageflexible electronic applications [1].
Figure 45 summarizes our recent results on or-ganic integrated circuits based on low-voltage pentacene transistors. Our inverters show sharp switching with rail-to-rail output swings and negligible hysteresis. Ring oscillators operate with supply voltages between 2 V and 5 V and with a minimum signal propagation delay of 200μs per stage [2].
A commonly encountered problem with organic transistors is their limited stability when oper-ated or stored in air. This limited stability is most likely caused by the diffusion of oxidiz-ing species, such as water, oxygen, or ozone into the organicfilms. In principle, this problem can be eliminated by encapsulating the transis-tors, but from a manufacturing cost perspective it is more desirable to develop organic semi-conductors that are less sensitive to oxidation.
A promising strategy to the development of air-stable organic semiconductors is the synthesis of conjugated compounds with a larger ioniza-tion potential, i.e., with a smaller conjugated core. However, a smaller conjugated core will also lead to poor molecular ordering in thin films, and this will reduce the carrier mobility of the semiconductor. One solution is the synthesis of molecules that consist of several small con-jugated cores separated by vinyl groups. Such molecules have a larger ionization potential and thus better stability compared with molecules with just one large conjugated core, but they fa-cilitate a similar degree of molecular ordering and thus are expected to provide similar carrier mobility [3].
Figure 46: Compared with pentacene transistors (a), transistors using the newly synthesized di(phenylvinyl)anthracene have similar carrier mobility, but show much less rapid degradation when ex-posed to air (b). This is due to the molecular structure in which three smaller conjugated cores are separated by vinyl groups.
An example for this new approach is shown in Fig. 46. As can be seen from Fig. 46(a), the mobility of a transistor based on pen-tacene (a commercially available semiconduc-tor with a single large conjugated core) de-grades rapidly when the transistor is exposed to air. After 100 days in air the mobility has de-creased from 0.4 cm2/Vs to 0.005 cm2/Vs. On the other hand, when pentacene is replaced by di(phenylvinyl)anthracene (a newly synthesized compound with three smaller conjugated cores separated by vinyl groups), the initial mobility is essentially the same as for the pentacene
tran-sistor (0.3 cm2/Vs), but the mobility degrades much less rapidly compared with the pentacene transistor (from 0.3 cm2/Vs to 0.2 cm2/Vs after 100 days in air).
[1] Klauk, H., U. Zschieschang, J. Pflaum and M. Halik.
Nature445, 745–748 (2007).
[2] Klauk, H., U. Zschieschang and M. Halik.Journal of Applied Physics102, 074514 (2007).
[3] Klauk, H., U. Zschieschang, R.T. Weitz, H. Meng, F. Sun, G. Nunes, D.E. Keys, C.R. Fincher and Z. Xiang.Advanced Materials19, 3382–3384 (2007).