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Keywords:

  • organic field-effect transistors;
  • gravure printing;
  • zone-casting;
  • polymer dielectrics;
  • organic semiconductors

One of the key advantages of organic field-effect transistors (OFETs) is their ability to form flexible, conformable and lightweight electronic devices, e.g. radio frequency identification (RFID) tags,[1] microprocessors[2] and flexible displays.[3] These require fabrication over large-areas on flexible plastic substrates, the poor dimensional stability of such substrates creating the additional demand of low-temperature processing (<200 °C).[4] While high performance source, drain and gate electrodes and interconnects require metal evaporation under vacuum, ideally the dielectric and organic semiconductor (OSC) should be processed from solution under ambient conditions to reduce fabrication costs. Regarding device architecture, OFETs with a bottom-gate (BG) bottom-contact (BC) geometry (Figure 1c) have an advantage in that the organic semiconducting layer is deposited last.[5] This affords easy fabrication and patterning of micron-scale OFET channels, electrodes and interconnects by conventional photolithographic methods, whilst avoiding exposure of the active OSC material to UV radiation and aggressive or solubilising chemicals. Furthermore, this architecture is compatible with vacuum sublimation or vapour phase techniques for OSC deposition, allowing access to a wide range of high-performance materials. Such OFETs can form the building blocks of high performance, low-cost electronic circuitry.

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Figure 1. (a) Optical image of a plastic substrate, 50 × 50 mm, with arrays of OFETs and capacitors (bottom right). (b) Optical micrograph of a single OFET on the plastic substrate. The dielectric is a square patch (3 × 3 mm) surrounded by source, drain and gate electrodes. (c) Schematic representation of bottom-gate bottom-contact OFET device stack; (d) chemical structures of the corresponding materials.

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For flexible OFETs, both inorganic and organic dielectrics have been used. However, inorganic oxide dielectrics such as SiO2,[6] Al2O3,[7] Ta2O5[8] and high-k HfO2[9] can be brittle and can require high processing temperatures[10] which are incompatible with low-cost plastic substrates. This is not an issue for organic polymer dielectrics such as poly(2-hydroxyethyl methacrylate) (PHEMA), polyvinylpyrrolidone (PVP), poly(methyl methacrylate) (PMMA), poly(perfluorobutenylvinylether) (CytopTM) and polystyrene (PS).[11-13] These can be processed by techniques such as spin-casting, gravure printing and ink-jet printing at room temperature in ambient conditions. To avoid significant gate leakage current these polymer dielectrics have typically been deposited with film thicknesses ≥ 500 nm, resulting in high OFET operating voltages ≥20 V. To achieve saturation in a transistor — a requirement for digital logic — the channel length must be at least 20 to 50 times the dielectric thickness; such thick dielectrics will therefore limit the channel length, and thus device speed. Although thermal crosslinking has been used,[11, 12] the majority of polymer dielectrics are also highly soluble, and can be damaged by further solution processing steps. An additional issue is that in both bottom and top gate architectures, the dielectric must be patterned to allow electrical access to the gate or source and drain contacts, an important requirement for circuit integration. Furthermore, in BG BC OFETs, the surface of the dielectric is critical. This is because of the way the OSC molecules organize, pack and crystallize on this surface directly determines the device performance. Indeed, one complex strategy has been to match OSCs with particular dielectrics.[14]

Here we report BG BC OFETs on plastic substrates (Figure 1a–c) using an organic dielectric deposited by either gravure contact printing or spin-casting and photopatterned by UV cross-linking. The dielectric layer is 100–130 nm thick, affords low-voltage device operation (≤10 V), and withstands the subsequent photolithographic processing of gold source and drain electrodes. An ultra-thin film of poly(α-methylstyrene) (PαMS) is used as a surface modification to optimise packing and orientation of OSCs deposited using three different methods. Vacuum sublimation of pentacene and zone-casting of its solution-processable derivative 6,13-bis(triisopropylsilylethynyl) pentacene (TIPS-pentacene) (Figure 1d)[15-18] result in state-of-the-art flexible p-type BG BC OFETs with average mobility values of 0.6 ± 0.1 and 0.3 ± 0.1 cm2 V−1 s−1, respectively.

To fabricate the BG BC OFETs, non-birefringent plastic substrates coated with a transparent conducting oxide (TCO) were used as required for flexible display applications. Gate electrodes, interconnects and pads were first patterned in the TCO by photolithography and etching. This was followed by deposition and photopatterning of the dielectric (see below). Gold source and drain electrodes, interconnects and pads were then patterned on top by photolithography and lift-off. A self-assembled monolayer (SAM) of pentafluorobenzenthiol (PFBP) was then deposited on the gold source and drain electrodes. Finally, a nanoscale layer of PαMS was spin-cast on the substrates. This dewets from the thiol-treated gold but covers the dielectric surface.[16] This was then followed by either pentacene or TIPS-pentacene deposition.

The dielectric was deposited by gravure contact printing, a high volume, reel-to-reel compatible, large-area coating technique;[12, 17] this gave ca. 110 nm thick films which are the thinnest printed dielectric films reported to date.[11] The dielectric formulation was based on a triacrylate cross-linker blended with high molecular weight PMMA as a viscosity modifier.[18] A radical photoinitiator for photopatterning, whose absorption band overlapped with the UV wavelength of the mask aligner at 365 nm, was added to the dielectric formulation. The dielectric gravure formulation had a high boiling point component (ca. 150 °C) to improve ink levelling on the substrate after printing.[19] After printing, the dielectric was photopatterned in ambient on top of the TCO gate electrodes. After a washing step to remove unexposed material, crosslinking was completed in a UV chamber filled with nitrogen. This formulation was also spin-cast, at 130 nm thickness, onto the substrate and then processed as per the gravure-printed films, to allow comparison between the two deposition methods. The spin-cast and gravure printed dielectric had a similar root-mean-square (RMS) surface roughness of 2 nm as measured by AFM (Figure S1). Optical interferometry and microscopy confirmed print homogeneity over large areas (Figure S2). Electrical testing of the films in capacitors gave a similar dielectric constant εr of 2.9 and similarly low leakage currents, <10–7 A mm−2, in parallel capacitors (Figure 2a, b and Figure S3). Breakdown voltages of the spin-cast and printed films were typically >2.4 MV cm−1 (Figure S4). Therefore, we conclude that gravure printing can produce dielectric films with comparable surface properties and homogeneity as spin-casting.

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Figure 2. Leakage current characteristics of capacitors with (a) gravure printed and (b) spin-cast dielectric. (c) Saturation mobility, onset voltage, sub-threshold swing voltage (SSV) and On/Off ratio of OFETs with and without PαMS on gravure printed and spin-cast dielectric. The data was extracted from the saturation regime at Vds = −10 V. Error bars are ±1 standard deviation. Numerical values for this figure are tabulated in Table S1. ODTS = octadecyltrichlorosilane; PPA = phenylphosphonic acid.

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For pentacene OFETs, the OSC was deposited by vacuum-sublimation and gave state-of-the-art performance. The transfer characteristics for OFETs (Figure 3a) showed no hysteresis and an AFM micrograph of the channel region reveals large crystalline domains (Figure 3b). The layer of PαMS (6 nm thick) acts as a non-covalent surface modification of the dielectric with pentacene[7, 20] improving interfacial OSC packing and morphology. Semiconductor crystallinity and charge-carrier mobility in BG BC OFETs is known to be affected by the dielectric surface energy, increasing with increasing water contact angle.[21] The water contact angle of the dielectric surface with and without PαMS was found to be 90 ± 2° and 69 ± 2°, respectively. Pentacene OFETs with PαMS achieved an average saturation mobility of 0.6 ± 0.1 cm2 V−1 s−1 and an onset voltage of about 1 V. This mobility is the highest yet reported for this semiconductor on flexible substrates with the BG configuration, exceeding the previously reported value of 0.3 cm2 V−1 s−1 using an inorganic dielectric[22] (Figure 2c). Compared to the bare dielectric, PαMS increased the saturation mobility ten-fold, indicating improved crystal growth and orientation on the less polar surface. Without PαMS, the devices have a higher average turn-on voltage of about 5 V (Figure S5c), indicating more localised traps at the interface from grain boundaries and crystal defects.[23] AFM data supported this theory, showing that the grain size of pentacene deposited on the bare dielectric was about five-fold smaller (Figure S8) than pentacene on the PαMS surface (Figure S9a, b).

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Figure 3. (a–c) Transfer characteristics in saturation (Vds = −10 V) and linear (Vds = −2 V) regimes for OFETs with gravure printed dielectric and (a) evaporated pentacene, (b) zone-cast TIPS-pentacene and (c) spin-cast TIPS-pentacene. The source-drain current, Ids, is normalised to channel width and length, W and L, respectively. (d–f) AFM micrographs of the corresponding OSC crystals in the OFET channel for (a–c) respectively; (g–i) cross-polarised optical micrographs of transistor channels for (a–c) respectively.

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For TIPS-pentacene OFETs, the OSC was deposited by spin-casting from anisole or zone-casting from toluene. Figure 3 shows the transfer characteristics for OFETs with gravure printed dielectric, AFM and optical micrographs of the channel region. Devices with spin-cast TIPS-pentacene achieved an average saturation mobility value of 0.08 ± 0.04 cm2 V−1 s−1 and an average onset voltage of about 0.6 V. The nanoscale layer of PαMS (10 nm thick) would dissolve into the semiconducting formulation during spin-casting, so we expect it to form a blended layer with the semiconductor.[15, 24] The presence of PαMS made little difference to OFET performance in terms of mobility and onset voltage, but it did improve the sub-threshold swing voltage (SSV) from 2 to 1 V dec−1 (Figure 2c). The SSV can be associated with the ratio between mobile and trapped charge-carriers at the semiconductor–dielectric interface as carriers first enter the channel when the OFET is switched from off- to on-state.[25] The presence of PαMS therefore reduces the density of interfacial traps responsible for the SSV in TIPS-pentacene. As a reference experiment, pure anisole solvent was spin-cast on top of the PαMS layer on the dielectric to simulate the conditions for solvent exposure during TIPS-pentacene spin-casting. The resultant surface had a water contact angle of 75 ± 2°, lying halfway between that of the bare dielectric (67 ± 2°) and the dielectric coated with PαMS (90 ± 2°). Hence, the solvent of the OSC formulation redissolves the PαMS layer during spin-casting of TIPS-pentacene, but a certain fraction of the PαMS polymer was redeposited.

To deposit a layer of TIPS-pentacene with higher crystallinity we then used solution-based zone-casting deposition and achieved state-of-the-art performance. Zone-casting is a large area, roll-to-roll compatible method in which a crystalline material is deposited from melt or solution. In this case, a heated TIPS-pentacene solution was deposited using a previously described recipe[26] onto heated substrates with a PαMS layer. Zone-casting TIPS-pentacene resulted in crystal growth in the casting direction and lead to macroscopically-sized crystalline domains (Figure 3h). The PαMS layer was dissolved by the hot OSC solution and incorporated into the crystal growth front. The polymer binder can be clearly seen on the AFM micrograph of the zone-cast film as globular structures between or on top the semiconducting crystals (Figure 3e). An average saturation mobility value of 0.3 ± 0.1 cm2 V−1 s−1 and an average onset voltage of about 0.8 V were achieved.

We note that all PαMS coated dielectric devices with pentacene and TIPS-pentacene have a similar onset and sub-threshold swing voltages. The dielectric and PαMS layer are therefore good at minimizing the formation of interfacial traps irrespective of the semiconductor deposition method.

OFETs with spin-cast and gravure printed dielectric show equal electrical performance (Figure S5 and 6). The charge-carrier mobility, onset voltage, SSV and on/off values for the spin-cast and gravure printed dielectric lie within ±1 standard deviation of each other (Figure 2c) for pentacene and TIPS-pentacene devices with PαMS. The physical morphology of the OSCs is also the same regardless of the dielectric deposition (Figure S9 and 10). Therefore, the triacrylate based dielectric can be processed using large-area compatible coating techniques such as gravure to give equally high performance devices as when it is spin-cast.

In conclusion, we have demonstrated that it is possible to fabricate state-of-the-art flexible small molecule BG BC OFETs on plastic foil using a large-area scalable platform. A flexible and photopatternable polymer dielectric has been demonstrated which can be gravure printed and spin-cast, gives low leakage current, high breakdown voltage, and low surface roughness. It is also resistant to subsequent processing with photolithographic chemicals and solvents. Small molecule OFETs could be fabricated with low onset voltage and sub-threshold slope, and high mobility and on/off ratio by modifying the wetting properties of the dielectric surface with a nanoscale thick layer of PαMS. This system is compatible with multiple small molecule deposition methods. Evaporated pentacene OFETs achieved an average mobility of 0.6 cm2 V−1 s−1, which is state-of-the-art for this device geometry for flexible transistors on plastic. We also demonstrated that zone-casting can be used to fabricate small molecule OFETs on plastic with a polymer dielectric, TIPS-pentacene achieving an average mobility of 0.3 cm2 V−1 s−1, which is state-of-the-art for this device geometry for flexible devices.

Experimental Section

  1. Top of page
  2. Experimental Section
  3. Acknowledgments
  4. Supporting Information

Materials: 6,13-bis(triisopropylsilylethynyl) pentacene (TIPS-pentacene) was purchased from High Force Research (UK). Polymer Source (Canada) with Mw of 632 kg mol−1 and PDI of 1.10. Both chemicals were used without further purification. Pentacene (for fluorescence, >95.0%) was purchased from Sigma-Aldrich (UK) and purified by gradient sublimation.

Device Fabrication: Commercially available poly carbonate (PC) plastic substrate coated with transparent conducting oxide (TCO, sheet resistance 30 Ω sq−1) was used as a substrate. The gate electrode of TCO was patterned by conventional photolithography and etching in hydrochloric acid (6 mol L−1). An experimental dielectric formulation supplied by BASF was either gravure printed or spin-cast. Photoinitiator Irgacure 369 from BASF was dissolved in the dielectric formulation at 10 mg mL−1 prior to film deposition. The dielectric film was photopatterned with a photo mask and Karl Süss MJB3 mask aligner; the UV dose was 1.44 J cm−2 at 365 nm. Afterwards, the dielectric was developed and further cured with UV for 10 min in Lightning Enterprises ELC-500 chamber under nitrogen. Gold source and drain electrodes were made of thermally evaporated gold with 30–50 nm thickness. The gold layer was photopatterned on top of the dielectric via a standard lift-off process using the same mask aligner. This step involved positive photoresist Microposit S1813 G2 (Chestech Ltd, UK); the developer was MF-26 A (Chestech Ltd, UK) and the lift-off solvent was acetone at 40 °C. The channel lengths were in the range 3–50 μm; the channel width was in the range 0.25–10 mm. The gold contacts were perfluorinated with PFBT using a 10 mmol L−1 solution in ethanol. PαMS was spin-cast from toluene solution with 3 and 10 mg mL−1 concentration to give 6 and 10 nm thick films, respectively; spin speed and acceleration were 4000 rpm and 10,000 rpm min−1 in both cases. The water contact angles on the bare dielectric and PαMS were recorded with Krüss DSA 100 under ambient conditions. TIPS-pentacene (10 mg mL−1) was dissolved in anisole (spin-casting) and toluene (zone-casting). Solutions were filtered through 0.45 μm PTFE filter. The semiconductor was spin-cast at 2000 rpm for 60 s with 10,000 rpm min−1 acceleration in air. Pentacene was evaporated onto the substrates in an ultrahigh vacuum chamber (10–8 Torr) at 0.25 Å s−1, while the substrate was kept at 68 °C.

Capacitor Structures: The dielectric films are characterised in semiconductor–insulator–metal (SIM) capacitor structures 0.04–2 mm2 in area. The bottom electrode is TCO. The top electrode is 30–50 nm thick thermally-evaporated gold. The dielectric capacitance was measured with Schlumberger IS 1260 impedance analyser in the range 10–106 Hz and 0.1 V AC-modulation. The capacitors were connected to a 433 kΩ resistor in parallel for these measurements. The data was modelled as an equivalent circuit with capacitor and resistor in parallel and a resistor in series connected to them. The latter accounts for the non-ideality of the connecting lines. Each capacitor was measured at 0 and 1 V DC-bias.

Gravure Printing: Norbert Schläfli Maschinen Labratester I gravure printer was used to print the dielectric at 44 m min−1 web speed. The nip pressure was ca. 100 N cm−2 and we used a metal doctor blade. The films were printed with stylus engraved cliché with print screen density of 110 line cm−1, which affords approximately 10 nL mm−2 of ink loading. Slower web speeds and larger print screen densities were observed to result in film non-uniformity.

Zone-casting of TIPS-Pentacene. Aligned, large-area films were deposited using zone-casting.[26] Solutions were loaded into a syringe chamber heated at 60 °C and injected at a flow rate of 18 μL s−1 onto the horizontal plastic substrates heated at 70 °C. The substrates were translated at a controlled speed of 0.5 mm s−1 during solution injection to form aligned crystalline films. The set-up was in a fume hood in air. For information on zone-casting optimisation see Supporting Information (Figure S11).

Characterisation of OFETs: Transistor transfer characteristics were recorded with Agilent 4156C SPA in continuous ramp mode. All OFETs with TIPS-pentacene were measured in ambient air whereas the pentacene devices were tested in nitrogen-filled glove box — water and oxygen content ≤0.1 ppm. The on/off ratio values for OFETs were limited by the noise level of the electrical set-up (0.1–1 nA), because the measurements were unshielded and with coaxial cabling.

Atomic force microscopy (AFM) images were recorded using a Bruker Multimode 8 in Tapping or ScanAssyst mode. Optical microscopy images were recorded using Olympus BX51 with cross-polariser. Film thicknesses were measured with Tencor Instruments AlphaStep 200.

Acknowledgments

  1. Top of page
  2. Experimental Section
  3. Acknowledgments
  4. Supporting Information

The authors would like to thank Steve Smout (imec) for pentacene evaporation. The research leading to these results has received funding from the European Community's Seventh Framework Programme (FP7/2007–2013) under grant agreement n°247978.

Supporting Information

  1. Top of page
  2. Experimental Section
  3. Acknowledgments
  4. Supporting Information

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