Fabrication of 6,13‐bis(triisopropyl‐silylethynyl)–pentacene thin‐film transistors with the silver ink transfer method using a polymer stamp

We fabricated 6,13‐bis(triisopropylsilylethynyl)–pentacene (TIPS–pentacene) thin film transistors using a direct metal transfer method. Using different metals, such as Au and Ag ink, electrode patterns are formed from the relief region of the polymer mold. TIPS–pentacene TFTs using the Ag ink transfer method show a similar performance to those using the Au metal transfer method. This method has advantages over the Au metal transfer method because it does not require vacuum equipment and a dry etching process. The self‐assembled monolayer (SAM) treated device exhibits a carrier mobility of 9.5 × 10^–2 cm²/V · s, and an on/off ratio of 4.6 × 10⁴. (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Optimization and improvement of TIPS–pentacene transistors (OTFT) with UV–ozone and chemical treatments using an all-step solution process

We fabricated an organic thin-film transistor (OTFT) using an all-step solution process. The printed layers, in which the electrode (silver), dielectric layer (BaTiO₃–PMMA), source–drain layer, and semiconductor 6,13-Bis(triisopropylsilylethynyl)pentacene(TIPS–pentacene), were optimized using roll-to-roll, an inkjet printer, and drop-casting. After coating the source–drain layer, we applied ultraviolet (UV)–ozone and self-assembled monolayer (SAM) treatments to the composite layer. The OTFTs treated with the UV–ozone and SAM treatments were found to exhibit excellent performance and good properties in comparison to silicon-based OTFTs.     

Laser forward transfer of silver electrodes for organic thin-film transistors

Organic thin-film transistors (OTFTs) with top- and bottom-contact configurations were fabricated using silver nano-inks printed by laser forward transfer for the gate and source/drain electrodes with pentacene and poly-4-vinylphenol as the organic semiconductor and dielectric layers, respectively. The volume of the laser-printed Ag pixels was typically in the subpicoliter (0.2–0.4 pl) range. The top-contact OTFTs resulted in lower contact resistance compared to those obtained from the bottom-contact OTFTs, and showed improved overall device performance. The top-contact OTFTs exhibited field-effect mobilities of ∼0.16 cm² V⁻¹ s⁻¹ and on/off current ratios of ∼10⁵.     

Roughness influence on the sheet resistance of the PEDOT:PSS printed on paper

The use of paper as a platform to manufacture organic electronic devices, electronic paper, has expanding potential for many applications because of several properties offered. In this work, we show a study of PEDOT:PSS printed by inkjet on bond paper, vegetal paper and sheets of PET. The relation between the surface density of the deposited material, morphology and resistivity was investigated for samples printed with a commercial Hewlett-Packard(HP)^® printer and Microsoft Word^® software. The amount of material deposited, i.e. surface density, was controlled using the print number in the same position and changing the gray scale used in the image formation. Changing the surface density of printed PEDOT:PSS, it is possible to produce a continuous film permeating the papers fibers. Sheet resistances obtained, when 7.0 mg cm⁻² of PEDOT:PSS were deposited on the surfaces, were: (a) 413.2 kΩ/Sq for bond paper, (b) 5.6 kΩ/Sq for vegetable paper and (c) 2.3 kΩ/Sq for PET. The exponential dependence of sheet resistance with the surface density of printed material allows us to evaluate the strong influence of substrate roughness on PEDOT:PSS conductivity and to predict, for each one, conditions to minimize it.     

Fabrication of printed memory device having zinc-oxide active nano-layer and investigation of resistive switching

An all printed resistive memory device, a 9-bit memristor, has been presented in this study consisting of 3 × 3 memristor crossbars deposited via electrohydrodynamic inkjet printing process at room conditions. Transparent zinc oxide active nano-layers, directly deposited by electrospray process, are sandwiched between the crossbars to complete the metal–insulator metal structure consisting of copper–zinc oxide–silver, where Cu and Ag are used as bottom and top electrodes respectively. The 9-bit memristor device has been characterized using current–voltage measurements to investigate the resistive switching phenomenon thereby confirming the memristive pinched hysteresis behavior signifying the read–write and memory characteristics. The memristor device showed a current bistability due to the existence of metal–oxide layer which gives rise to oxygen vacancies upon receiving the positive voltage hence breaking down into doped and un-doped regions and a charge transfer takes place. The maximum ON/OFF ratio of the current bi-stability for the fabricated memristor was as large as 1 × 10³, and the endurance of ON/OFF switchings was verified for 500 read–write cycles. The metal–insulator–metal structure has been characterized using X-ray diffraction, X-ray photoelectron spectroscopy and scanning electron microscope techniques.     

Multi-nozzle electrohydrodynamic inkjet printing of silver colloidal solution for the fabrication of electrically functional microstructures

In this paper, multi-nozzle electrohydrodynamic (EHD) inkjet printing of a colloidal solution containing silver nanoparticles in a fully controlled fashion is reported. For minimizing interaction, i.e. cross-talk, between neighboring jets, the distance between the nozzles was optimized numerically by investigating the magnitude of the electric field strength around the tip of each nozzle. A multi-nozzle EHD inkjet printing head consisting of three nozzles was fabricated and successfully tested by simultaneously printing electrically conductive lines of a colloidal solution containing silver nanoparticles onto a glass substrate. The printed results show electrical resistivity of 5.05×10⁻⁸ Ω m, which is almost three times larger than that of bulk silver. These conductive microtracks demonstrate the feasibility of the multi-nozzle EHD inkjet printing process for industrial fabrication of microelectronic devices.     

Electrohydrodynamic micropatterning of silver ink using near-field electrohydrodynamic jet printing with tilted-outlet nozzle

This paper introduces for the first time near-field electrohydrodynamic jet printing with tilted-outlet nozzle to obtain the fine and highly conductive patterns of silver (Ag) ink. Line widths produced by near-field electrohydrodynamic jet printing are less than 6 μm, which is approximately twenty times smaller than that of inkjet printing. Under optimized Ag ink annealing ranges 3–9 min for 30 wt% at 150°C, we observed Ag line pattern resistivities as low as 7×10⁻⁶ Ω⋅cm. Ag ink conduction mechanisms were brought to light from microstructure analysis and post-thermal-annealing examination of electrical characteristics.     

Crystal growth of small-molecule organic semiconductors with nucleation additive

In this study, we employ a nucleation additive 4-octylbenzoic acid (OBA) with an optimized solvent evaporation method to regulate crystal orientation and grain width of small-molecule organic semiconductors. When 6,13-bis(triisopropylsilylethynyl) pentacene (TIPS pentacene) was utilized as a benchmark material to mix with the additive, a self-assembled OBA interfacial layer was formed and promoted uniform deposition of nucleation seeds. As a result, the TIPS pentacene/OBA blend crystalline film exhibited crystal alignment in long range order, attributing to a 11-fold reduction of the crystal misorientation angle and a 4-fold increase of the grain width. We further discussed the important correlation between the effective hole mobility, grain boundaries, grain width and length, and nucleation sites. Organic thin film transistors were fabricated to test charge transport, yielding a hole mobility of up to 0.17 cm²/V. This work provides a new pathway to modulate the nucleation and crystallization events of organic semiconductors, and can potentially be applied to optimize the thin film morphology and electrical performance of organic semiconducting materials in general.     

Star-shaped oligofluorene nanostructured blend materials: controlled micro-patterning and physical characteristics

Star-shaped oligofluorene consists of highly-fluorescent macromolecules of considerable interest for organic electronics. Here, we demonstrate controlled micro-patterning of these organic nanostructured molecules by blending them with custom-synthesized photo-curable aliphatic polymer matrices to facilitate solventless inkjet printing. The printed microstructures are spherical with minimum dimensions of 12 μm diameter and 1 μm height when using a cartridge delivering ∼1 pL droplets. We evaluate the physical characteristics of the printed structures. Photoluminescence studies indicate that the blend materials possess similar fluorescence properties to neat materials in solid films or toluene solution. The fluorescence lifetime consists of two components, respectively 0.68±0.01 ns (τ ₁) and 1.23±0.12 ns (τ ₂). This work demonstrates that inkjet printing of such blends provides an attractive method of handling fluorescent nano-scaled molecules for photonic and optoelectronic applications.     

ITO-free inverted organic solar cells fabricated with transparent and low resistance ZnO/Ag/ZnO multilayer electrode

Inverted organic solar cells (OSCs) based on poly (3-hexylthiophene) (P3HT):[6,6]-phenyl-C61 butyric acid methyl ester (PCBM) bulk heterojunctions (BHJ) were fabricated with optimized ZnO/Ag/ZnO multilayer and conventional indium–tin oxide (ITO) cathode electrodes and their performance was compared. The ZnO/Ag/ZnO multilayer films showed sheet resistances in the range 3.6–3.9 Ω/sq, while ITO exhibited 14.2 Ω/sq. On the one hand, the carrier concentration gradually decreased from 1.74 × 10²² to 4.33 × 10²¹ cm⁻³ as the ZnO thickness increased from 8 to 80 nm, respectively. The transmittance of the ZnO(40 nm)/Ag(19 nm)/ZnO(40 nm) films was ∼95% at 550 nm, which is comparable to that of ITO (∼96%). The multilayer films were smooth with a root mean square (RMS) roughness of 0.81 nm. OSCs fabricated with the ZnO(40 nm)/Ag(19 nm)/ZnO(40 nm) film showed a power conversion efficiency (2.63%) comparable to that of OSCs with a conventional ITO cathode (2.71%).     

A high mobility C60 field-effect transistor with an ultrathin pentacene passivation layer and bathophenanthroline/metal bilayer electrodes

C₆₀ field-effect transistor (OFET) with a mobility as high as 5.17 cm²/V·s is fabricated. In our experiment, an ultrathin pentacene passivation layer on poly-(methyl methacrylate) (PMMA) insulator and a bathophenanthroline (Bphen)/Ag bilayer electrode are prepared. The OFET shows a significant enhancement of electron mobility compared with the corresponding device with a single PMMA insultor and an Ag electrode. By analysing the C₆₀ film with atomic force microscopy and X-ray diffraction techniques, it is shown that the pentacene passivation layer can contribute to C₆₀ film growth with the large grain size and significantly improve crystallinity. Moreover, the Bphen buffer layer can reduce the electron contact barrier from Ag electrodes to C₆₀ film efficiently.     

Room-temperature and solution-processed vanadium oxide buffer layer for efficient charge injection in bottom-contact organic field-effect transistors

We introduce a room temperature and solution-processible vanadium oxide (VO_x) buffer layer beneath Au source/drain electrodes for bottom-contact (BC) organic field-effect transistors (OFETs). The OFETs with the VO_x buffer layer exhibited higher mobility and lower threshold voltages than the devices without a buffer layer. The hole mobility with VO_x was over 0.11 cm²/V with the BC geometry with a short channel length (10 μm), even without a surface treatment on SiO₂. The channel width normalized contact resistance was decreased from 98 kΩ cm to 23 kΩ cm with VO_x. The improved mobility and the reduced contact resistance were attributed to the enhanced continuity of pentacene grains, and the increased work function and adhesion of the Au electrodes using the VO_x buffer layer.     

The effect of Ag layer thickness on the properties of WO3/Ag/MoO3 multilayer films as anode in organic light emitting diodes

Transparent conductive WO₃/Ag/MoO₃ (WAM) multilayer electrodes were fabricated by thermal evaporation and the effects of Ag layer thickness on the optoelectronic and structural properties of multilayer electrode as anode in organic light emitting diodes (OLEDs) were investigated using different analytical methods. For Ag layers with thickness varying between 5 and 20 nm, the best WAM performances, high optical transmittance (81.7%, at around 550 nm), and low electrical sheet resistance (9.75 Ω/cm²) were obtained for 15 nm thickness. Also, the WAM structure with 15 nm of Ag layer thickness has a very smooth surface with an RMS roughness of 0.37 nm, which is suitable for use as transparent conductive anode in OLEDs. The current density?voltage?luminance (J?V?L) characteristics measurement shows that the current density of WAM/PEDOT:PSS/TPD/Alq₃/LiF/Al organic diode increases with the increase in thickness of Ag and WO₃/Ag (15 nm)/MoO₃ device exhibits a higher luminance intensity at lower voltage than ITO/PEDOT:PSS/TPD/Alq₃/LiF/Al control device. Furthermore, this device shows the highest power efficiency (0.31 lm/W) and current efficiency (1.2 cd/A) at the current density of 20 mA/cm², which is improved 58% and 41% compared with those of the ITO-based device, respectively. The lifetime of the WO₃/Ag (15 nm)/MoO₃ device was measured to be 50 h at an initial luminance of 50 cd/m², which is five times longer than 10 h for ITO-based device.     

Thermal and electrical conductivities of silver–indium–tin alloys

The variations of thermal conductivity with temperature for the Ag–[x] wt% Sn–20 wt% In alloys (x=8, 15, 35, 55 and 70) were measured using a radial heat flow apparatus. From the graphs of thermal conductivity versus temperature, the thermal conductivities of solid phases at their melting temperature for the Ag–[x] wt% Sn–20 wt% In alloys (x=8, 15, 35, 55 and 70) were found to be 46.9±3.3, 53.8±3.8, 61.2±4.3, 65.1±4.6 and 68.1±4.8 W/Km, respectively. The variations of electrical conductivity of solid phases versus temperature for the same alloys were determined from the Wiedemann–Franz equation using the measured values of thermal conductivity. From the graphs of electrical conductivity versus temperature, the electrical conductivities of the solid phases at their melting temperatures for the Ag–[x] wt% Sn–20 wt% In alloys (x=8, 15, 35, 55 and 70) alloys were obtained to be 0.036, 0.043, 0.045, 0.046 and 0.053 (×10⁸/Ωm), respectively. Dependencies of the thermal and electrical conductivities on the composition of Sn in the Ag–Sn–In alloys were also investigated. According to present experimental results, the thermal and electrical conductivities for the Ag–[x] wt% Sn–20 wt% In alloys linearly decrease with increasing the temperature and increase with increasing the composition of Sn.     

Infra-red assisted sintering of inkjet printed silver tracks on paper substrates

In this study, conductive silver features using inkjet printing have been successfully prepared and their sintering studied. Regarding conductivity, metallic inks are the most efficient available conductive inks, even if important drawbacks regarding the use of such inks in inkjet still exist. Indeed, the sintering step is an important limiting factor for the productivity and the substrate choice. An infrared (IR) drying method was experimented to optimize the sintering treatment time. Trials with glass and paper substrates were performed and proved that IR drying is interesting to optimize sintering. Indeed a similar level of resistance was obtained for conventional heating (200 °C, 5 min) and for IR radiations within a shorter treatment time (3 min). Moreover, the substrate temperature during the IR sintering treatment was controlled. The substrate appears to be a relevant parameter to optimize sintering because its thermal behaviour directly impacts on the treatment duration. And for the first time a sheet resistance of 1.9 Ω sq⁻¹ was obtained on paper substrate after only 2 min of IR treatment without any observation of substrate degradation. The evaluation of an electrical treatment on the sintering of a nanoparticle film was also performed using a corona pre-treatment. This lead appears to be interesting because the study proves that an electrical treatment can initiate the sintering of silver nanoparticles. The current flow generated by the corona electrodes certainly generates a local heating by dissipation of the conductive pattern. Finally, the solutions presented in this article allow reducing the sintering time of silver conductive inkjet inks. However mainly, it proves that using paper as substrate for such inks is now possible.     

Photo-curable epoxy functionalized cyclotetrasiloxane as a gate dielectric for organic thin film transistors

We synthesized a new photo-curable organic/inorganic hybrid material, cyclotetrasiloxane (CTS) derivative containing cyclohexene-1,2-epoxide functional groups (CTS-EPOXY), and its characteristics are compared with a prototypical organic gate insulator of poly(4-vinylphenol) (PVP) in the organic thin film transistors (OTFTs) using pentacene as an active p-type organic semiconductor. Compared with PVP, CTS-EPOXY shows better insulating characteristics and surface smoothness. A metal/insulator/metal (MIM) device with the 300-nm-thick CTS-EPOXY film shows more than two orders of magnitude lower current (less than 40 nA/cm² over the voltage range up to 60 V) compared with PVP. In addition, the pentacene TFT with CTS-EPOXY as a gate dielectric layer shows slightly higher field-effect mobility of μ_FET = 0.20 cm²/V s compared to that with PVP.     

Organic passivation layers for pentacene organic thin-film transistors

We studied the passivation layers for pentacene organic thin-film transistors (OTFTs) that were used to drive the active-matrix organic-light-emitting-diodes (AMOLEDs) fabricated by inkjet process. Conventional polyvinyl acetate (PVA) passivation layer could not protect OTFT channel from poly(3,4-ethylenedioxythiophene):poly(styrene sulfonic acid) (PEDOT:PSS) inkjet process so that the performance was degraded critically after the process. By applying PVA/PVA/photoacryl (PA) multi-passivation layers, we could get OTFT arrays with switching ratio over 10⁶ even after PEDOT:PSS process. Using these OTFTs, we could drive AMOLEDs made by inkjet process.     

Ag on Mo(110) studied by AES and STM

Auger electron spectroscopy (AES) and scanning tunnelling microscopy (STM) have been used to investigate the growth behaviour of ultra-thin Ag films on a Mo(110) surface at room temperature. An analysis of AES and STM measurements indicates that three-dimensional (3D) growth of a Ag film is observed. For submonolayer coverage, the growth of Ag is mediated by a two-dimensional step-flow mechanism. During the initial stage of this growth, the first Ag layer nucleates and creates islands (average size of islands is about 180 ± 20 nm²) at Mo step edges. In the monolayer coverage range, the decoration of substrate steps by Ag can be distinguished by the presence of a fractional step of p₁ = 0.86 ± 0.6 Å height at the Ag–Mo boundary. As the sample is post-annealed to 700 K, the morphology of the surface changes. Step-flow growth in this case gives rise to a regular Ag nanostripe network attached to Mo(110) step edges. The corrugation profiles reveal the protrusion of silver nanostripes of thicknesses p₁ = 0.98 ± 0.16 Å and p₂ = 0.39 ± 0.06 Å for submonolayer and monolayer coverage ranges, respectively, above each single step of a Mo terrace morphology.     

AZO/Ag/AZO multilayer films prepared by DC magnetron sputtering for dye-sensitized solar cell application

Ag films were deposited on Al-doped ZnO (AZO) films and coated with AZO to fabricate AZO/Ag/AZO multilayer films by DC magnetron sputtering on glass substrates without heating of glass substrates. The best multilayer films have low sheet resistance of 19.8 Ω/Sq and average transmittance values of 61% in visible region. It was found that the highest figure of merit (F_TC) is 6.9 × 10⁻⁴ Ω⁻¹. For the dye-sensitized solar cell (DSSC) application, the multilayer films were used as transparent conductive electrode (multilayer films/ZnO + Eosin-Y/LiI + I₂/Pt/FTO). The best DSSC based on the multilayer films showed that open circuit voltage (V_oc) of 0.47 V, short circuit current density (J_sc) of 2.24 mA/cm², fill factor (FF) of 0.58 and incident photon-to-current conversion efficiency (η) of 0.61%. It was shown that the AZO/Ag/AZO multilayer films have potential for application in DSSC.     

Optimization of transmittance and resistance of indium gallium zinc oxide/Ag/indium gallium zinc oxide multilayer electrodes for photovoltaic devices

We report on the optimization of the optical and electrical properties of IGZO/Ag/IGZO multilayer films as a function of IGZO thickness. The transmission window slightly widened and shifted toward lower energies with increasing IGZO thickness. The IGZO(39 nm)/Ag(19 nm)/IGZO(39 nm) showed transmittance 88.7% at 520 nm. The optical transmittance spectra were examined by finite-difference time-domain (FDTD) simulations. The carrier concentration decreased from 1.73 × 10²² to 4.99 × 10²¹ cm⁻³ with increasing the IGZO thickness, while the charge mobility insignificantly changed from 19.07 to 19.62 cm²/V. The samples had sheet resistances of 4.17–4.39 Ω/sq with increasing IGZO thickness, while the resistivity increased from 1.89 × 10⁻⁵ to 6.43 × 10⁻⁵ Ω cm. The 39 nm-thick IGZO multilayer sample had a smooth surface with a root mean square roughness of 0.63 nm. The IGZO(39 nm)/Ag(19 nm)/IGZO(39 nm) multilayer showed a Haacke's FOM of 49.94 × 10⁻³ Ω⁻¹.