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Soluble Pentacene Precursors
Preparation of field-effect transistor devices using soluble pentacene precursors
In order to introduce unfunctionalized pentacene into devices such as organic field effect transistors (OFETs), soluble pentacene precursors, which are readily converted to pentacene when heated (~200°C), are crucial materials. The following describes the fabrication process of solution-deposited bottom-gate, bottom-contact natural and pentacene transistors and presents representative performance characteristics of the transistors.
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FABRICATION PROCEDURE
Highly doped n-type Si wafers (resistance <0.01 Ω-cm) having thermally grown SiO2?with a thickness of 250 nm were used as substrates, which formed the back-gate and gate dielectrics of the transistors, respectively. These 2x2cm2?wafers were treated with 50W oxygen plasma (Technics PlanarEtchII-350 plasma generator) for 10 minutes. The substrates were then immediately surface treated with HMDS (1,1,1,3,3,3-hexamethyldisilazane).HMDS alters the SiO2?surface and contact angle to improve transistor characteristics. Petri dishes containing HMDS (5 mL) were placed in a vacuum desiccator along with the substrate to be surface treated. The desiccator was pumped with an indoor vacuum and the substrate was exposed to HMDS vapor for 20 minutes. The source and drain of Au were then deposited on the substrate by electron beam evaporation through a shade.
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Dissolve the precursor of 13,6-N-sulfonyl acetamide pentacene in chloroform solvent (15 mg/mL) and filter with a 0.2 μ m PTFE syringe filter.?A few tenths of a milliliter of the solution was then placed on each substrate. The substrates were rotated at 1500 rpm (initial ramp rate of 500 rpm/sec) for 1 minute. The precursor films were then heat-treated in a nitrogen atmosphere glove box for 1 minute at 200°C on a preheated hot plate to convert the p-pentacene precursor to p-pentacene. The resulting thin film of juxtapentacene had a thickness of 100 nm. The manufacturing details are shown in Figure 1.
Figure 1. Schematic diagram of the solution deposition process, including spin coating, followed by thermal conversion of the pentacene/N-sulfinylacetamide adduct (red) to pentacene (blue) thin film transistor.
ELECTRICAL CHARACTERIZATION
Use the Agilent 4156C semiconductor parameter analyzer and Karl Suss PM5 probe station to perform electrical meter characterization of the device in an ambient atmosphere. The representative IV characteristics of the manufactured transistor are shown in Figure 2.
Figure 2. (A) For a pentacene transistor made of gold (15-20nm) metal contacts and channel sizes L=160 μ m and W=1.5mm, the relationship between drain current (ID) and source drain voltage (VDS) and (B) drain current and source gate voltage (VGS) at a constant drain source voltage of -50 V. The mobility is 0.189 cm2/Vs, and the current modulation ratio (Ion/Ioff) is 4.4 × 105, the threshold voltage (VT) is -26.8 V.
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Calculate the mobility (μ) and threshold voltage (VT) in a saturated state using the square law equation:
Among them, ID?is the drain current, VGS?is the gate source voltage, W is the channel width, L is the channel length, μ is the mobility, and VT is the threshold voltage. Through this program, flowability of approximately 0.1-0.2 cm2?V-1?s-1?can be achieved. As previously reported1,2, SiO2/pentacene interface plays a very important role in device characteristics, and through gas-phase HMDS surface treatment, the reported mobility can be further increased to~1 cm2?V-1?s-1.
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In addition to the soluble pentacene precursor used above, other precursors can also be selected, such as pentacene N-sulfinyl tert butyl carbamate, which can be photopatterned using ultraviolet light in the presence of a photoacid generator before being briefly heated at 130 ° C. Another method of introducing high-purity pentacene into organic electronic devices is to use TIPS pentacene, a soluble pentacene molecule. In summary, high-performance devices can be made from both soluble pentacene derivatives and soluble pentacene precursors. These methods are crucial as they enable researchers to overcome the insolubility of pentacene and incorporate high mobility pentacene into their terminal devices.
Reference
1.?Kagan CR, Afzali A, Graham TO. 2005. Operational and environmental stability of pentacene thin-film transistors. Appl. Phys. Lett.. 86(19):193505. https://doi.org/10.1063/1.1924890
2.?Afzali A, Dimitrakopoulos CD, Breen TL. 2002. High-Performance, Solution-Processed Organic Thin Film Transistors from a Novel Pentacene Precursor. J. Am. Chem. Soc.. 124(30):8812-8813. https://doi.org/10.1021/ja0266621
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