Highly transparent organic thin-film transistors

Transparent thin-film transistors have been attracting considerable attention recently since some promising applications, such as see-through active-matrix displays, could be realized by using this technology.¹ Meanwhile, organic materials have been considered as promising candidates for next generation electronics, since organic thin-film transistors (OTFTs) are potentially flexible, lightweight, and inexpensive.² Transparent OTFTs can be used as pixel switching and/or driving elements for flexible active-matrix displays.^3–5 Current Si thin film transistors (TFTs) are opaque. If clear OTFTs could be used to fabricate a backplane, then visible light could travel directly through the transparent circuits in liquid crystal displays (LCDs). Consequently, a high aperture ratio could be easily achieved and the power consumption increased.

In order to fabricate transparent OTFTs, we need transparent electrodes instead of the opaque electrodes used in common OTFTs. A step in this direction was achieved by J. M. Choi and coworkers, who used NiO_x as the electrodes.³ However, their device was still not quite transparent. On the other hand, while the most common transparent conductor, indium-tin-oxide (ITO), can be used as the source or drain electrode directly, the device performance is unfortunately limited by the very large contact resistance of the interface between ITO and the organic semiconductors. To overcome this problem, we recently developed a one-electrode architecture consisting of ITO and a transition metal oxide. We used this architecture to fabricate transparent OTFTs.⁶

Metal oxides, such as MoO₂, may undergo a ‘self-doping’ process, in which other components such as MoO and free Mo are generated during the thermal evaporation. These components behave as dopants.⁷ Therefore, a highly-doped metal-oxide layer is formed. With this layer, closer-to-ideal contact can be achieved, overcoming the problem of the high contact resistance. This design simultaneously achieves high transparency and improved device performance.

Recently, we studied the device structure shown in the inset of Figure 1. ITO on a glass substrate was used as the gate electrode. To make the transparent dielectrics, a solution—consisting of poly-4-vinylphenol (11% by weight) and poly(melamine-co-formaldehyde) methylated (4% by weight) in propylene glycol monomethyl ether acetate—was spin-coated onto ITO-patterned glass substrates. The resulting film was thermally annealed at 200° C. A 60nm-thick layer of pentacene was then thermally evaporated onto the surface as the semiconducting layer. Before sputtering 50nm of ITO as the source and drain electrodes, the metal oxide, MoO₂, was evaporated onto pentacene to reduce the contact resistance.

Figure 1. The transfer characteristics of the organic thin film transistors (OTFTs) with (solid line) and without MoO₂ (dashed line). The thickness of the pentacene layer is 60 nm. Inset shows the transparent OTFT device structure. ITO: indium tin oxide.

The transfer characteristics of the OTFTs with and without the MoO₂ layer are shown in Figure 1. For the device using only ITO as the electrodes, the mobility and on-off ratio are 0.015cm²/Vs and 1.8×10³, respectively. The threshold voltage is -13.4V and the subthreshold slope is 17.6V/decade. On the other hand, the device with MoO₂ has an improved performance. The mobility and on-off ratio of the device are 0.12cm²/Vs and 3.3x10⁴, respectively. Meanwhile, the threshold voltage becomes 5.0V and the subthreshold slope is improved to 8.5V/decade. Clearly, the device after the modification with MoO₂ shows much stronger field-effect behavior.

Since the absorption of photons comes primarily from the pentacene layer, we reduced the thickness of this active layer to increase transmittance. Figure 2 shows the resulting transmittance spectra of the device with a 20nm-thick layer of pentacene. An average transmittance of 72.2% has been achieved. The inset of Figure 2 shows our OTFT on top of the color letters and emblem of our institute. The features of these patterns can be observed very clearly. The mobility and on-off ratio remain as good as 0.01cm²/Vs and 1.7×10⁴, respectively. The subthreshold slope is even improved to 8.0V/decade.

Figure 2. The transmittance spectra of the device with a thinner pentacene layer (20nm) than in Figure 1. The inset: a photo of the transparent OTFT on top of color letters and emblem of our institute.

In summary, inserting a layer of metal oxide between the source/drain electrodes and the semiconductor dramatically enhanced the performance of transparent OTFTs. Furthermore, our device is highly transparent: it achieved a transmittance of more than 70% in the visible region. We anticipate that transparent OTFTs could be good driving circuits for LCDs as well as other ‘invisible electronics’ products in the future. Next, we plan to further develop transparent devices with suppressed photoactivity.⁸ Because the absorption of the visible light by the active material is inevitable, it is necessary to obtain stable electrical performance under different light intensity conditions.

The author would like to acknowledge the financial support from AU Optronics Corp. (AUO), the National Science Council, and the Ministry of Education ATU Program.