High-resolution direct-writing of metallic electrodes on flexible substrates for high performance organic field effect transistors

We report on organic field-effect transistors (OFETs) with sub-micrometer channels fabricated on plastic substrates with fully direct-written electrical contacts. In order to pattern source and drain electrodes with high resolution and reliability, we adopted a combination of two digital, direct writing techniques: ink-jet printing and femtosecond laser ablation. First silver lines are deposited by inkjet printing and sintered at low temperature and then sub-micrometer channels are produced by highly selective femtosecond laser ablation, strongly improving the lateral patterning resolution achievable with inkjet printing only. These direct-written electrodes are adopted in top gate OFETs, based on high-mobility holes and electrons transporting semiconductors, with field-effect mobilities up to 0.2 cm²/V s. Arrays of tens of devices have been fabricated with high process yield and good uniformity, demonstrating the robustness of the proposed direct-writing approach for the patterning of downscaled electrodes for high performance OFETs, compatibly with cost-effective manufacturing of large-area circuits.     

All-Inkjet-Printed Bottom-Gate Thin-Film Transistors Using UV Curable Dielectric for Well-Defined Source-Drain Electrodes

Technological restrictions of the inkjet printing technology for printed electronics can hinder its application potential, mainly due to the limited resolution and layer homogeneity in comparison to conventional manufacturing techniques for electronics. The manufacturing of active devices such as thin-film transistors with appropriate performance using printing technologies is still one of the current challenges towards industrial applications. This work demonstrates the application of an ultraviolet (UV) curable ink as insulating material for the gate dielectric. The advantage of the UV curable ink is its fast curing and the smooth surface enabling high resolution patterns on top of it. In this way, all-inkjet-printed organic thin-film transistors (OTFTs) were fabricated with silver electrodes, UV curable gate dielectric, and 6,13-bis(triisopropylsilylethynyl)pentacene for the active semiconductor layer. By fine tuning of processing parameters and pattern geometries, a stable channel length of about 10 μm was obtained in the bottom-gate configuration without the need of additional steps, suggesting a way to build low-cost all-inkjet-printed OTFTs with well-defined source-drain electrodes and fast UV curable dielectric without any additional steps. The inkjet-printed device is characterized by an electron mobility of 0.012 cm² V^?1 s^?1 and on/off ratio of 10³.     

Photosintering and electrical performance of CuO nanoparticle inks

A copper oxide (CuO) nanoparticle ink was inkjet printed and photosintered in order to optimize electrical performance as a function of pattern dimension. For a given photosintering condition, electrical conductance varied strongly with line widths, ranging from 100 to 300 μm, illustrating the implications of printing and sintering complex circuit designs with varying feature sizes. By tuning the time delay between printing and sintering, exposure wavelength, radiant energy, pulse width and the distance between the light-source and substrate, photosintering conditions were optimized so that variations in sheet resistance for different line widths were minimized. Using optimized photosintering conditions, a sheet resistance value as low as 150 mΩ/□ (resistivity of 9 μΩ cm) and current carrying capacity of 280 mA for a 300 μm wide trace was achieved.     

Effect of active layer thickness on environmental stability of printed thin-film transistor

We have studied the effect of active layer thickness on the performance and environmental stability of the 6,13-bis(triisopropylsilylethynyl) pentacene (TIPS pentacene) thin-film transistor. The organic thin-film transistors (OTFTs) were fabricated by inkjet printing using a solution based TIPS pentacene. To get thick organic semiconductor, the surface of gate insulator was treated with n-octyltrichlorosilane (OTS-C₈) before jetting. The on-currents of the OTFT with ～1 μm active layer decreases a little in air, but the OTFT with 0.05 μm TIPS pentacene shows a significant degradation in drain currents.     

All-inkjet-printed low-pass filters with adjustable cutoff frequency consisting of resistors,inductors and transistors for sensor applications

Low-cost and flexible first and second order low-pass filters with adjustable cutoff frequency were designed and printed by inkjet printing technology. The all-inkjet-printed low-pass filters were characterized and an adjustable cutoff frequency feature in form of an inkjet-printed organic thin-film transistors (OTFTs) was added to the filters for application-oriented fine-tuning. The applicability of these small circuits was evaluated by signal filtering for sensor applications. As a result, low-pass filters with an adjustable cutoff frequency ranging from 82 Hz to 740 Hz were obtained, demonstrating their suitability in signal filtering and their promising applicability for tactile sensing characterized by low frequency signals.     

Tuning the viscosity of halogen free bulk heterojunction inks for inkjet printed organic solar cells

For the solution processing of organic photovoltaics on an industrial scale, the exclusion of halogenated solvents is a necessity. However, the limited solubility of most semiconducting polymer/fullerene blends in non-halogenated solvents results in ink formulations with low viscosities which poses limitations to the use of roll-to-roll compatible deposition processes, such as inkjet printing. We propose to add polystyrene as a rheological modifier to increase the viscosity of bulk heterojunction (BHJ) non-halogenated inks. The printing and performance of P3HT/PCBM photoactive layer inks are characterized as a function of polystyrene concentration and three different molecular weights. Addition of 1 wt% polystyrene provided a near two-fold gain in viscosity, with the largest viscosity gains coming from the polymer with the highest molecular weight. However, this coincided with greater viscoelastic behavior, which reduced the jetting performance of the inks. Differences in solvent compatibility of the polystyrene/P3HT/PCBM ternary blend resulted in phase separation upon layer drying, whereby polystyrene segregated to the layer-air interface to form an isolated domain or network like topology. Nevertheless, a 1.7-fold increase in dynamic viscosity was obtained for devices with printed BHJ layers containing polystyrene at the expense of a 20% reduction in OPV performance. The improved viscosity and good printing behavior achieved with small additions of polystyrene demonstrates its potential to overcome the limited viscosity resulting from typical non-halogenated ink formulations for semiconducting polymers. These results offer a step forward to the industrialization of inkjet printing as an effective deposition technique for functional layers of organic electronics.     

High-performance thin-film transistor with 6,13-bis(triisopropylsilylethynyl) pentacene by inkjet printing

We have studied the performance improvement of organic thin-film transistor (OTFT) with a solution based TIPS pentacene (6,13-bis(triisopropylsilylethynyl)pentacene) by inkjet printing. The TIPS pentacene with 1.0 wt.% solution in 1,2-dichlorobenzene was used for printing of an active layer of OTFT. The OTFT printed at room temperature shows a shoulder-like behavior but it disappears for the OTFT printed at the substrate temperature of 60 °C. The OTFT on plastic exhibited an on/off current ratio of ∼10⁷, a threshold voltage of −2.0 V, a gate voltage swing of 0.6 V/decade and a field-effect mobility of 0.24 cm²/Vs in the saturation region.     

Printing and preparation of integrated optical waveguides for optronic sensor networks

Printing processes are employed to create three-dimensional transparent structures which serve as multimode optical waveguides in intelligent systems. The additive manufacturing techniques deliver integrated elements for optical signal transmission in optical communications and novel optronic sensor systems. These networks of sources, sensors and detectors consist of polymers and rely entirely on the conversion of light for space resolved measurements. Measured quantities are temperature, strain and chemical concentration in application scenarios such as structural health monitoring and the life sciences.Results show the suitability of flexographic and inkjet printing to generate parabolic shaped waveguides with a minimum lateral dimension of 50 μm and maximum height of 110 μm. We describe the geometrical properties of printed waveguides as basis for further integration of functional elements.End facets of waveguides serve as interfaces to adjacent functional elements. We compare the capabilities of several processes for end facet preparation of printed polymer waveguides. Stress-induced cleaving is presented as a highly effective procedure with a root mean square surface roughness smaller than 60 nm.     

Formation of low surface energy separators with undercut structures via a full-solution process and its application in inkjet printed matrix of polymer light-emitting diodes

In this paper, low surface energy separators with undercut structures were fabricated through a full-solution process. These low surface energy separators are more suitable for application in inkjet printed passive-matrix displays of polymer light-emitting diodes. A patterned PS film was formed on the P4VP/photoresist film by microtransfer printing firstly. Patterned Au-coated Ni film was formed on the uncovered P4VP/photoresist film by electroless deposition. This metal film was used as mask to pattern the photoresist layer and form undercut structures with the patterned photoresist layer. The surface energy of the metal film also decreased dramatically from 84.6 mJ/m² to 21.1 mJ/m² by modification of fluorinated mercaptan self-assemble monolayer on Au surface. The low surface energy separators were used to confine the flow of inkjet printed PFO solution and improve the patterning resolution of inkjet printing successfully. Separated PFO stripes, complement with the pattern of the separators, formed through inkjet printing. The separators also realized the patterning of cathodes. A passive-matrix display device was obtained through the assistant patterning of low surface energy separators.     

Photonic sintering of inkjet printed current collecting grids for organic solar cell applications

ITO-free organic solar cells with inkjet printed current collecting grids are demonstrated. For sintering those grids, thermal treatment and its faster alternative, photonic flash sintering, are applied and the characteristics of the resulting metal structures are compared with each other. The electrical potentials and resulting currents in the devices with different sintering conditions are calculated. The flash sintered current collecting grids exhibit clear advantages over thermally sintered grids in terms of geometry and conductivity. Similar conductivities are obtained after 5 s of flash sintering and 6 h of thermal sintering. This finding demonstrates the great potential of flash sintering for the roll-to-roll manufacturing of printed organic solar cells on flexible substrates.     

Fabrication of organic thin film transistors on Polyethylene Terephthalate (PET) fabric substrates

In this paper organic thin film transistors (OTFTs) are directly fabricated on fabric substrates consisting of Polyethylene Terephthalate (PET) fibers. A key process is coating the polymer layers on the fabric in order to reduce the large surface roughness of the fabric substrate. Two polymers, i.e. polyurethane (PU) and photo-acryl (PA), are used to reduce the large surface roughness and simultaneously improve the process compatibility of the layers with the subsequent OTFTs processes while also retaining the original flexibility of the fabric. The surface roughness of the PU/PA-coated fabric is significantly reduced to 0.3 μm. Furthermore, the original flexibility of the PET fabric remained after coating of the PU/PA polymer layers. The mobility of the OTFTs fabricated on the PU-PA coated fabric substrate is 0.05 ± 0.02 cm²/V s when three PA layers and 90 nm thick pentacene layer were used. The performance does not vary even after 30,000 bending test.     

Teflon/SiO2 bilayer passivation for improving the electrical reliability of pentacene-based organic thin-film transistors

We demonstrate a bilayer passivation method using a Teflon and SiO₂ combination layer to improve the electrical reliability of pentacene-based organic thin-film transistors (OTFTs). The Teflon was deposited as a buffer layer using a thermal evaporator that exhibited good compatibility with the underlying pentacene channel layer, and can effectively protect the OTFTs from plasma damage during the SiO₂ deposition process, resulting in a negligible initial performance drop in OTFTs. Furthermore, because of the excellent moisture barrier ability of SiO₂, the OTFTs exhibited good time-dependent electrical performance, even after 168 h of aging in ambient air with 60–80% relative humidity.     

Solution processable bilayered gate dielectric towards flexible organic thin film transistors

In this study, we have successfully explored the potential of a new bilayer gate dielectric material, composed of Polystyrene (PS), Pluronic P123 Block Copolymer Surfactant (P123) composite thin film and Polyacrylonitrile (PAN) through fabrication of metal insulator metal (MIM) capacitor devices and organic thin film transistors (OTFTs). The conditions for fabrication of PAN and PS-P123 as a bilayer dielectric material are optimized before employing it further as a gate dielectric in OTFTs. Simple solution processable techniques are applied to deposit PAN and PS-P123 as a bilayer dielectric layer on Polyimide (PI) substrates. Contact angle study is further performed to explore the surface property of this bilayer polymer gate dielectric material. This new bilayer dielectric having a k value of 3.7 intermediate to that of PS-P123 composite thin film dielectric (k ∼ 2.8) and PAN dielectric (k ∼ 5.5) has successfully acted as a buffer layer by preventing the direct contact between the organic semiconducting layer and high k PAN dielectric. The OTFT devices based on α,ω-dihexylquaterthiophene (DH4T) incorporated with this bilayer dielectric, has demonstrated a hole mobility of 1.37 × 10⁻² and on/off current ratio of 10³ which is one of the good values as reported before. Several bending conditions are applied, to explore the charge carrier hopping mechanism involved in deterioration of electrical properties of these OTFTs. Additionally, the electrical performance of OTFTs, which are exposed to open atmosphere for five days, can be interestingly recovered by means of re-baking them respectively at 90 °C.     

Low temperature fabrication of Ni–P metallic patterns on ITO substrates utilizing inkjet printing

A low temperature process to fabricate high resolution metallic lines on indium tin oxide (ITO) substrates using inkjet printing and subsequent electroless plating is described in this study. In this method, a thermo-sensitive (styrene-co-NIPAAm)/Pd (St-co-NIPAAm/Pd) nanoparticle-based ink was printed onto ITO substrates to create patterned catalytic sites, where nickel is subsequently deposited by electroless plating to form metal lines with desired width and conductivity. The inkjet printing variables such as droplet spacing and printing voltage, as well as the Ni electroless deposition variables such as deposition time and temperature were systematically investigated to obtain the optimum parameters. The adhesion of the deposited Ni–P coating to the ITO substrate was evaluated by a scotch tape test method. Optical microscope observation shows that a continuous pattern was formed with a printing voltage of 37 V and a droplet spacing of 50 μm. The irritating coffee ring effect was significantly suppressed by raising the substrate temperature to 50 °C and increasing the electroless plating temperature to 75 °C. High-resolution conductive metal lines can be easily and successfully fabricated using our method, which shows good potential for preparing the metallic lines as front or back electrodes in solar cells.     

Photo-patternable polyimide gate insulator with fluorine groups for improving performance of 2,7-didecyl[1]benzothieno[3,2-b][1]benzothiopene (C10-BTBT) thin-film transistors

Surface properties of gate insulators strongly affect the device performance of organic thin-film transistors (OTFTs). To improve the performance of OTFTs, we have developed photo-sensitive polyimide gate insulator with fluorine groups. The polyimide gate insulator film could be easily patterned by selective UV exposure without any photoinitiator. The polyimide gate insulator film, fabricated at 130 °C, has a dielectric constant of 2.8 at 10 kHz, and leakage current density of <1.6 × 10^?10 A/cm² while biased from 0 to 90 V. To investigate the potential of the polyimide with fluorine groups as a gate insulator, we fabricated C₁₀-BTBT TFTs. The field-effect mobility and the on/off current ratio of the TFTs were measured to be 0.76 ± 0.09 cm²/V s and >10⁶, respectively.     

Fully-printed,all-polymer,bendable and highly transparent complementary logic circuits

In this contribution we show a simple approach for the development of all-polymer based complementary logic circuits fabricated by printing on plastic, at low temperature and in ambient conditions. This is achieved by patterning, with a bottom-up approach, solely synthetic carbon-based materials, thus incorporating earth-abundant elements and enabling in perspective the recycling – a critical aspect for low-cost, disposable electronics. Though very simple, the approach leads to logic stages with a delay down to 30 μs, the shortest reported to date for all-polymer circuits, where each single component has been printed. Moreover, our circuits combine bendability and high transparency, favoring the adoption in several innovative applications for portable and wearable large-area electronics.     

Soluble pentacene thin-film transistor using a high solvent and heat resistive polymeric dielectric with low-temperature processability and its long-term stability

Solution processable organic thin-film transistors (OTFTs) were fabricated using 6,13-bis(triisopropyl-silylethynyl) pentacene (TIPS-pentacene) and low-temperature processable polyimide gate dielectric. The TIPS-pentacene OTFT with the dielectric was found to have a field-effect mobility of 0.15 cm²/Vs, which is comparable to that of OTFT with an inorganic dielectric. The OTFTs with the polyimide dielectric did not show any significant performance degradation as time passed. A field-effect mobility of the OTFTs in 60 days was found to be almost identical to that of pristine OTFT. The combination of TIPS-pentacene and our polyimide gate dielectric can be one of the potential candidates for the fabrication of stable OTFTs for large-area flexible electronics.     

Transverse charge transport in inkjet printed poly(3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS)

Transverse (z) alignment of PEDOT grains was demonstrated in inkjet printed PEDOT:PSS. This explained the superior transverse charge conduction mode in inkjet printed PEDOT:PSS films, best fitted by the Efros-Shklovskii 1D-VRH (variable range hopping) model in this study compared with spin coated PEDOT:PSS films, which have demonstrated layers of generally in-plane aligned PEDOT:PSS grains. The findings of this study, regarding the microstructure of inkjet printed PEDOT:PSS films and their transverse charge transport model, justify measurements of the transverse conductivity of inkjet printed films in this study being 600 times higher than that of spin coated films. In addition, it was found that the addition of 5 wt% DMSO in the printing PEDOT:PSS ink lowers the workfunction by 3% approximately.     

Bottom gate organic thin-film transistors fabricated by ultraviolet transfer embossing with improved device performance

Ultraviolet transfer embossing is optimized to fabricate bottom gate organic thin-film transistors (OTFTs) on flexible plastic substrates, achieving significant improved device performance (μ = 0.01–0.02cm²/Vs; on/off ratio = 10⁴) compared with the top gate OTFTs made previously by the same method (μ = 0.001–0.002 cm²/Vs; on/off ratio = 10²). The performance improvement can be ascribed to the reduced roughness of the dielectric-semiconductor interface (R_rms = 0.852 nm) and thermally cross-linked PVP dielectric which leads to reduced gate leakage current and transistor off current in the bottom-gated configuration. This technique brings an alternative great opportunity to the high-volume production of economic printable large-area OTFT-based flexible electronics and sensors.     

Ionic self-assembled monolayer for low contact resistance in inkjet-printed coplanar structure organic thin-film transistors

To reduce the contact resistance in inkjet-printed organic thin-film transistors (OTFTs), the use of a newly synthesized ionic self-assembled monolayer (SAM) consisting of an anchoring group, a linker group, and an ionic functional group, is investigated. According to the gated transmission line method (TLM) measurements of a series of OTFT devices, where one type has no charge injection layer, another type having a pentafluorobenzenethiol (PFBT) injection layer, and a third type containing a (6-mercaptohexyl)trimethylammonium bromide (MTAB) ionic SAM, the latter exhibits the lowest contact resistance value of ∼3.1 K Ω cm. The OTFTs without charge injection layer and with the PFBT SAM have relatively higher contact resistance values of ∼6.4 K Ω cm and ∼5.0 K Ω cm, respectively. The reduced contact resistance in the OTFTs with ionic SAMs is attributed to the large charge carrier density induced by the ionic SAM, which allows sufficient tunneling-assisted injection of the carriers from the metal electrode to the polymer semiconductor. These results suggest that the use of appropriate ionic SAM injection layer is an effective way to reduce the contact resistance, hence improving the charge transport characteristics of inkjet-printed OTFTs.