Solution-Processable Zirconium Oxide Gate Dielectrics for Flexible Organic Field Effect Transistors Operated at Low Voltages

Abstract

We investigate solution based fabrication of high-k ZrO2 thin films for low-voltage-operated organic field effect transistors (OFETs). An alternative UV curing method for the densification of Zr-based gel films, which allows for low-temperature processing, is compared to the conventional thermal annealing method. Elemental and microstructural analysis shows that UV-curing induces the decomposition of organic-metal bonds and causes the densification of the metal oxide film, just as the conventional thermal annealing of gel films does, resulting in a high-k dielectric layer from Zr-based solutions. Furthermore, we found that the low temperature associated with UV-curing prevents the interface layer from intermixing with the substrate. Fabricated ZrO2 films (5–6 nm in thickness) treated with an octadecylphosphonic acid self-assembled monolayer exhibit low leakage current density (below 10–6 to 10–7 A/cm2) at 3 V and high dielectric breakdown strength (V > 4 V). Using this dielectric layer, solution processable polymer OFETs with PBTTT-C-14 as the organic semiconductor function well at low voltage (below −3 V.) The effect of self-assembled monolayers (SAMs) on the morphology and microstructure of the organic semiconductor deposited on the ZrO2 dielectrics are investigated. Finally, we demonstrate solution-processable, low-temperature fabrication of OFETs on a flexible substrate.

UV‐Assisted Low Temperature Oxide Dielectric Films for TFT Applications

“…Assuming that the overall volume change of film occurs in the z-direction and that the density of the film does not vary during UV annealing, the thickness change will be proportional to the molar mass change, expected to be 53%. This is in broad agreement with the figure of 28% for the thickness change using zirconium acetylacetonate reported by Park et al, [12] which is consistent with the theoretical value (25.3%) noted by this group.…”

“…Since the nitrate group is much smaller than acetylacetonate, the resulting oxide dielectric layer will have fewer nano-pores, which should lead to lower leakage currents. This may be confirmed by thickness changes [12] and surface ellipsometric measurements [23] during UV-assisted thermal annealing. Furthermore, inorganic nitrate salts are low-cost and readily available.…”

“…[12,17] The dielectric constants, obtained using the measured film thicknesses, are shown in Figure 7. Table 1 and is 19.8, which is slightly higher than that of our observed ZrO 2 .…”

“…Desirable requirements for gate insulators include large permittivities, low leakage current densities, an optimized structure with smooth interfaces [6,7] and also the possibility of low temperature processing. [12] The availability of ultra-thin, highdielectric-constant (high-k) gate insulators will reduce significantly the operating voltage of solution processed oxide TFTs (currently a few tens of volts). [13] However, there are few reports of solution-processed gate insulators that fulfill the above requirements and that can be deposited at a sufficiently low temperature.…”

“…[21] Subsequently, Xu et al, [22] Park et al [12] and Lin et al [11] adopted the same method to demonstrate that low-voltage TFTs could be achieved by combining the low-temperature, solution-processed ZrO 2 dielectric layer with either ZnO or an organic semiconducting channel. However, during the conversion from metal acetylacetonate to metal oxide, nano-pores can be generated in the gate insulator layer.…”