Low‐Voltage Organic Thin‐Film Transistors with High‐k Nanocomposite Gate Dielectrics for Flexible Electronics and Optothermal Sensors

The performance of organic thin-film transistors (OTFT) for flexible, low cost and disposable “plastic” electronic products advances rapidly: various organic semiconductors display hole or electron carrier mobilities that compare favorably with those of hydrogenated amorphous silicon, the inorganic counterpart for such applications as flexible displays, smart cards and radio frequency identification tags, nonvolatile memories and sensors. The possibility for tailoring functional organic materials, bears potential towards novel electronic products such as smart skins, smart textiles and “invisible electronics”, where multiple functionalities, portability and ubiquitous integration is requested. In this context diverse properties of organic thin-film devices are inevitable such as lightweight, low power consumption, low operationvoltage and compatibility with diverse substrates. Reducing the threshold voltage and the subthreshold swing is essential for operating OTFTs at low-voltage levels. When combined with very low gate leakage currents, OTFTs may also become a key element in high-end sensor applications, such as flexible touch pads and screens or thermal imaging tools for night vision, surveillance or for the detection of undesired heat loss paths in buildings. The aforementioned transistor parameters not only critically depend on the thickness and the dielectric properties of the gate insulator, but also on the trapped charge densities at the interface between these materials. The selection of semiconductors and gate insulators with excellent interface properties is currently the challenge in the quest for improving the performance of OTFTs. Here we show that bottom-gate OTFTs based on the organic semiconductor pentacene and high-k nanocomposite gate dielectrics, exhibit transistor performances with very low gate leakage currents, subthreshold swings close to the theoretical limit, and low-voltage battery operation. The subthreshold swings of OTFTs with different organic and hybrid gate dielectrics follow an inverse dependence on the gate capacitance as is expected by standard MOS theory. The trapped charge carrier density at the interface between the semiconductor and the dielectric surpasses that of the SiO2-pentacene interface, being close to the average trap densities in the SiO2–Si interface in metal oxide semiconductor transistors. [15] We also report the first application of these OTFTs in an optothermal light sensor. We describe the transistor, the temperature sensitive fluorinated polymer, their combination in an integrated circuit, and the application of this circuit as a thermal infrared sensor and as a switch that can be operated by a laser pointer. Figure 1 shows the structure of low-voltage organic transistors with high dielectric constant (high-k) oxide–polymer nanocomposites. Al2O3 or ZrO2 were chosen as high-k dielectric materials, combined with poly(a-methyl styrene) (PaMS) or poly(vinyl cinnamate) (PVCi) to form a smooth and dense nanocomposite gate dielectric. Pentacene is used as the organic semiconductor material, the gate electrode is based on Al, while Au source and drain electrodes are employed. Figure 1b, c, and d show atomic force microscopy (AFM) images of a ZrO2/PaMS nanocomposite gate dielectric based transistor. The bare ZrO2 metal oxide surface is displayed in Figure 1b, the nanocomposite in (c) and the pentacene layer grown on top of the nanocomposite dielectric in (d). The AFM images clearly reveal that the rough (surface rms-roughness = 1.5 nm) and less dense ZrO2 layer, which is composed of regularly clubbed grains (Fig. 1a), smoothens by forming the nanocomposite (rms-roughness = 0.4 nm). The substrate roughness critically influences the growth dynamics of pentacene molecules on top of dielectric surfaces, grain sizes typically increase with decreasing surface roughness. For rmsroughness values below 0.5 nm the pentacene morphology is characterized by dentritic crystallites of several microns C O M M U N IC A IO N