Self-assembled monolayer modification of silver source-drain electrodes for high-performance pentacene organic field-effect transistors

We demonstrated excellent performance improvement of bottom-contact pentacene-based organic thin film transistors (OTFTs) fabricated at room-temperature with silver electrodes modified by self-assembled monolayers (SAMs) of binary mixtures of n-alkanethiol (n-decanethiol, HDT) and the fluorinated analog (3,3,4,4,5,5,6,6,7,7,8,8,9,9,10,10,10-heptadecafluoro-1-decanethiol, FDT). The OTFTs with modified silver (Ag) electrodes exhibit carrier mobility of 0.21 cm²/V s, which is faster than most of bottom-contact pentacene-based OTFTs fabricated at room-temperature with gold (Au) electrodes. The threshold voltage is reduced from −30 V of the devices with Au electrodes to −5.4 V of the devices with modified Ag electrodes. The hole injection barrier is also reduced with modified Ag as indicated by ultraviolet photoemission spectroscopy. The enhancement of the saturation current and the mobility of the devices are due to both the reduction of hole injection barriers and the continuous grain size of pentacene on top of electrodes and dielectrics.     

Electrical stability of pentacene thin film transistors

The influence of environmental conditions on the device operation and the stability of polycrystalline pentacene thin film transistors (TFTs) were investigated. Electrical in-situ and ex-situ measurements of staggered pentacene TFTs were carried out to study the influence of dry oxygen and moisture on the device stability. The transistors were fabricated by organic molecular beam deposition on thermal oxide dielectrics. Oxygen exposure of the pentacene films lead to the creation of acceptor-like states in the bandgap. The acceptor-like states cause a shift of the onset of the drain current towards positive gate voltages. The charge carrier mobility and the on/off ratio of the transistor are not affected by the acceptor-like states. Furthermore, the acceptor-like states have an influence on the stability of the TFTs. Devices exposed to oxygen exhibit a shift of the threshold voltage upon prolonged biasing. Transistors characterized under vacuum conditions (no oxygen exposure) do not exhibit a shift of the threshold voltage (bias stress effect) as a consequence of prolonged biasing. The experimental results show a clear correlation between the device behavior upon oxygen exposure and the stability of the devices. The shift of the onset voltage upon oxygen exposure correlates with the shift of the threshold voltage upon prolonged bias. The influence of dry oxygen on the onset voltage, the threshold voltage, and the electrical stability will be described. Furthermore, the influence of bias stress on the operation of organic circuits like an active matrix addressed OLED displays will be discussed.     

Inkjet printed silver source/drain electrodes for low-cost polymer thin film transistors

Silver tracks for source/drain (S/D) electrodes in low-cost polymer thin film transistors (TFTs) have been realized through inkjet printing technique, using heavily n-doped silicon wafer with thermally grown silicon dioxide as the substrate and poly(3-hexylthiophene) (P3HT) as the channel material. Spin coating a layer of poly-4-vinylphenol (PVPh) onto the substrate was found to enhance the silver track uniformity and lower the cure temperature (from 300 to 210 °C). The surface roughness of the PVPh film was optimized to improve the device performance. The fabricated P3HT TFT with a channel length of 20 μm exhibited a saturation mobility of 3.5 × 10⁻2 cm²/V/s which was three times higher than that obtained in P3HT TFTs with gold S/D electrodes.     

Self-patterning of high-performance thin film transistors

We have developed a technique for the preparation of thin film transistors (TFTs) through the self-patterning of various organic and inorganic materials via solution processing using a wide range of solvents. To obtain selectively self-patterned layers, we treated the oxide dielectric with two-phase patterned self-assembled monolayers of hexamethyldisilazane (HMDS) and octyltrichlorosilane. The conducting polymer poly(3,4-ethylenedioxythiophene) doped with poly(styrene sulfonic acid) in water and the dielectric polymer poly(vinyl phenol) in propylene glycol methyl ether acetate were both selectively deposited and patterned on the HMDS regions with high-quality feature shapes. When source and drain electrodes were patterned on the bottom-gate oxide wafer, we also self-patterned organic and inorganic semiconductors around the channel (HMDS) regions. These TFT devices exhibited moderate to good electronic characteristics. This method has great potential for the economical full solution processing of large-area electronic devices. The selectivity in the patterning phenomena can be understood in terms of surface energy interactions.     

Environmental and operational stability of solution-processed 6,13-bis(triisopropyl-silylethynyl) pentacene thin film transistors

We report operational and environmental stability of solution-processed organic thin film transistors (OTFTs) using the small molecule organic semiconductor 6,13-bis(triisopropyl-silylethynyl) pentacene (TIPS-pentacene). Typical drop-cast TIPS-pentacene OTFTs show strong molecular ordering and relatively stable characteristics with air and illumination compared to vapor-deposited pentacene OTFTs. For short channel length devices, TIPS-pentacene OTFTs undergo significant degradation with bias-stress, possibly due to operation at large drive currents and large local power dissipation as well as built-in charges in the channel area.     

Optimization of electrohydrodynamic-printed organic electrodes for bottom-contact organic thin film transistors

In this study, we investigate the optimization of printed (3,4-ethylenedioxythiophene):poly(4-styrenesulfonate) (PEDOT:PSS) as source/drain electrodes for organic thin film transistors (OTFTs) through electrohydrodynamic (EHD) printing process. The EHD-printed PEDOT:PSS electrodes should fulfill the prerequisites of not only high conductivity but also optimum surface tension for successful jetting. The conductivity of PEDOT:PSS was dramatically enhanced from 0.07 to 352 S/cm by the addition of dimethylsulfoxide (DMSO). To use the DMSO-treated PEDOT:PSS solution in the EHD printing process, its surface tension was optimized by the addition of surfactant (Triton X-100), which was found to enable various jetting modes. In the stable cone-jet mode, the patterning of the modified PEDOT:PSS solution was realized on the surface-functionalized SiO₂ substrates; the printed line widths were in the range 384 to 81 μm with a line resistance of 8.3 × 10³ Ω/mm. In addition, pentacene-based OTFTs employing the EHD-printed PEDOT:PSS as source and drain electrodes were found to exhibit electrical performances superior to an equivalent vacuum-deposited Au-based device.     

Leakage current variation during two different modes of electrical stressing in undoped hydrogenated n-channel polysilicon thin film transistors (TFTs)

Leakage current evolution during two different modes of electrical stressing in hydrogenated-undoped n-channel polysilicon thin film transistors (TFTs) is studied in this work. On-state bias stress (high drain bias and positive gate bias) and off-state bias stress (high drain bias and negative gate bias) were performed in order to study the degradation of the leakage current. It is found that during off-state bias stress the gate oxide is more severely damaged than the SiO₂-polySi interface. In contrast, during on-state bias stress, two different degradation mechanisms were detected which are analyzed.     

One-pot surface modification of poly(ethylene-alt-maleic anhydride) gate insulators for low-voltage DNTT thin-film transistors

This study investigates the one-pot surface modification of poly(ethylene-alt-maleic anhydride) (PEMA) gate insulators crosslinked with 1,5-naphthalenediamine (1,5-NDA) for enhancing the device performance of low-voltage dinaphtho[2,3-b:2′,3′-f]thieno[3,2-b]thiophene (DNTT) organic thin-film transistors (OTFTs). Surface properties of the PEMA gate insulator could be easily modified by adding poly(maleic anhydride-alt-1-octadecene) (PMAO) to the coating solution. The surface energy of the gate insulator is strongly correlated with the growth of organic semiconductors and the charge carrier transport at the interface between the semiconductor and gate insulator. The results indicate that the device performance of low-voltage DNTT OTFTs can be improved by one-pot surface modification of the PEMA gate insulator.     

Fabrication of Au/PEDOT stacked electrodes for organic thin film transistors by imprinting technology

In this work, the Au/PEDOT stacked source/drain electrodes of OTFTs were fabricated by combining the micro-contact inking and reversal imprinting. The PEDOT was inked on the mold by the micro-contact process and the Au/PEDOT stacked layer was transferred on pentacene by imprinting technology. The threshold voltage, and on-off ratio, carrier mobility, and source/drain contact resistance of organic TFTs were all improved by the proposed process.     

High performance of pentacene organic thin film transistors by doping of iodine on source/drain regions

Contact doping was conducted by iodine in a top contact configuration in a pentacene organic thin film transistor (OTFT), to investigate its effects on contact resistance and the resulting electrical performance. Iodine doping in the pentacene film caused the change of pentacene structure, thus leading to an increase in electrical anisotropy, i.e. ratio of lateral to vertical resistivity. The two resistive components of doped pentacene film underneath the Au contacts were major contributors to the contact resistance, and a model to explain the dependence of contact resistance on iodine doping was presented. Finally, OTFTs fabricated on iodine doped source/drain contacts exhibited high mobility of 1.078 cm²/V s, two times that of OTFTs with undoped contacts, due to the low contact resistance.     

A review of recent advances in transparent p-type Cu2O-based thin film transistors

One of the crucial challenges that face the wide-spread implementation of flexible and transparent electronics is the lack of high performance p-type semiconductor material. Cu₂O in thin-film form is a potentially attractive material for such applications because of its native p-type semi-conductivity, transparency, abundant availability, non-toxic nature, and low production cost. This review summarizes recent research on using copper oxide Cu₂O thin films to produce p-type transparent thin-film transistors (TFTs) and complementary metal–oxide–semiconductor (CMOS) devices. After a short introduction about the main advantages of Cu₂O semiconductor material, different methods for depositing and growing Cu₂O thin films are discussed. The hi-tech development, along with the associated obstacles, of the Cu₂O-based thin-film transistors is reviewed, with special emphasis on those made of sputtered Cu₂O films. Finally, the bilayer scheme as one of the most exciting and promising technique for both TFTs and CMOS devices will be considered.     

Facile conversion of polymer organic thin film transistors from ambipolar and p-type into unipolar n-type using polyethyleneimine (PEI)-modified electrodes

We report here a successful polarity conversion of organic thin film transistors (OTFTs) based on several polymer semiconductors with low-lying LUMO (lowest unoccupied molecular orbital) energy levels (?−4 eV) from ambipolar and even p-type into unipolar n-type devices using an ultrathin layer (∼2–5 nm) of polyethyleneimine (PEI) to modify the source and drain contacts. The work function of gold is substantially reduced with the PEI layer on its surface, which effectively suppresses the injection of holes and thus enables electron-only charge transport of these polymers in OTFTs. This general approach of electrode work function modification broadens the scope of available polymer semiconductors for use in printed electronics where n-channel OTFTs are needed.     

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.     

TiO2-poly(4-vinylphenol) nanocomposite dielectrics for organic thin film transistors

We report on the fabrication of organic thin film transistors (OTFTs), which operate at low voltages, by incorporating a nanocomposite gate insulator material consisting of titania (TiO₂) nanoparticles used as fillers and poly(4-vinyl phenol) (PVP) used as matrix. The surface of the nanoparticles was modified by the ligands, 4-hydroxybenzoic acid, to enhance their compatibility with the polymer. The structure of the ligand is similar to that of the repeat units in the polymer. Once the nanoparticles were homogeneously dispersed in the polymer matrix, they were immobilized by cross-linking PVP with poly(melamine-co-formaldehyde) methylated/butylated (cross-linker). Consequently, no significant aggregation of the nanoparticles, even at a concentration of 31 wt%, was found in the nanocomposites, as observed by transmission electron microscopy (TEM). As a result, the nanocomposite exhibited a low leakage current density (∼10⁻⁸ A/cm⁻²). With an increase in the concentration of TiO₂ nanoparticles added, the dielectric constant of the nanocomposites also increased proportionately as compared to that of pristine PVP. The performance of the OTFTs in terms of the charge carrier mobility, on/off ratio, threshold voltages, and hysteresis was evaluated. In addition, the relationship between the concentration of TiO₂ nanoparticles and the device performance is discussed in detail.     

Preferred orientation of Cu(In,Ga)Se2 thin film deposited on stainless steel substrate

The influences of process parameters and Fe diffusing into Cu(In,Ga)Se₂ (CIGS) films on the orientation of CIGS absorbers grown on the stainless steel (SS) foils are investigated. The structural properties, morphology, and elemental profiles are characterized using X‐ray diffraction, scanning electron microscopy, and second ion mass spectroscopy, respectively. The orientation of CIGS thin films on the SS substrates strongly depends on the texture of the (In,Ga)₂Se₃ precursor, determined by the substrate temperature at the first stage (Ts1) and the flux ratio of Se to (In + Ga). Among these factors, Ts1 is the prerequisite to achieve [300]‐oriented IGS layer, which will yield [200]‐oriented CIGS thin film in the later process. The results indicate that through the comparison of CIGS thin films on the Mo/SS substrates and on the Mo/ZnO/SS substrates and combined with simply calculation, Fe diffusing into the CIGS layer will hinder the growth of the CIGS grains along [112] orientation. The grazing‐incidence X‐ray diffraction results suggest that the surface of the [220]‐textured CIGS thin film on the SS substrate still has [220] predominance, whereas the surface texture of the [220]‐texture CIGS thin film on the Mo/soda‐lime glass substrate became [112] predominant, which is due to the different compensation ability between Fe and Na elements. Finally, the relations between the device parameters and the degrees of the preferred orientation of CIGS absorbers are investigated. Copyright © 2012 John Wiley & Sons, Ltd.     

Optimization of nanocomposite gate insulators for organic thin film transistors

Nanocomposite gate insulators consisting of (Ba, Sr)TiO₃ (barium strontium titanate; BST) nanoparticles and crosslinked poly(4-vinyl phenol) (PVP) polymers were fabricated. Well-dispersed nanocomposite films were prepared by optimizing the BST nanoparticle size sorting process (ultrasound crushing and centrifuge method). The size-sorted BST nanoparticles (∼30 nm in size) were homogeneously mixed in the PVP host polymer in various BST contents, from 0 to 70 wt%, to tune the dielectric constant (κ) of the resulting nanocomposite films. The composite films exhibit three-fold increase in the κ value from 3.9 to 11.3. The physical properties including leakage current and surface roughness of the composites were also measured as a function of the BST loading content and particle dispersion. The relationship between these properties and the electrical performance of the corresponding organic thin film transistor were explored.     

ZnO nanorod arrays as an antireflective coating for Cu(In,Ga)Se2 thin film solar cells

A ZnO nanorod antireflective coating has been prepared on Cu(In,Ga)Se₂ thin film solar cells. This coating leads to a decrease of the weighted global reflectance of the solar cells from 8.6 to 3.5%. It boosts the solar cells short‐circuit current up to 5.7% without significant effect on their open‐circuit voltage and fill factor (FF), which is comparable to a conventional optimized single layer MgF₂ antireflective coating. The ZnO nanorod antireflective coating was electrochemically prepared from an aqueous solution at 80°C. The antireflective capability of ZnO nanorod arrays (ZNAs) may be further improved by optimization of growth conditions and their geometry. Copyright © 2010 John Wiley & Sons, Ltd.     

Performance improvement of organic thin film transistors with carbon nanotube/metal hybrid electrodes for S/D contacts

Electrode contact resistance is an important factor that seriously affects the performance of organic thin film transistors (OTFTs). In this paper, new low contact resistance carbon nanotube (CNT) based hybrid electrodes are introduced for the source and drain electrodes of OTFTs. The hybrid electrodes consist of solution-processed CNTs and a metal (Al; CNT/Al or Au; CNT/Au) layer evaporated on the CNTs. The contact resistance of the CNT/Al and CNT/Au hybrid electrodes was found to vary depending on the thickness of the Al and Au layer. The contact resistance of the CNT/Al hybrid electrodes exhibited a minimum of 2.9 kΩ cm at an Al thickness of 5 nm. It is notable that the minimum contact resistance of the CNT/Au was 0.9 kΩ cm at an Au thickness of 5 nm, and is the lowest value ever reported. It was lower than the 13 kΩ cm of the bare CNT electrodes, and tremendously less than the 4 MΩ cm of the pure Au electrode. The mobility of the OTFTs, which used pentacene as the semiconductor and polyvinylphenol as the gate dielectric, also followed the same dependence on metal thickness as the contact resistance. The maximum mobility of the OTFTs using CNT/Al and CNT/Au electrodes was 0.76 cm²/V sec and 1.0 cm²/V sec, respectively, at the same metal thickness of 5 nm, which was larger than 0.3 cm²/V sec of the bare CNT electrodes. The major origin of these enhancements was found to be the small energy difference between the work function of the CNT/metal hybrid electrodes and pentacene HOMO (5.1 eV), which was obtained at the metal thickness of 5 nm.     

Supramolecular control over the morphology of bio-inspired poly(3-hexylthiophene) for organic thin film transistors

We have developed a strategy for modifying the channel layer of organic thin film transistors (OTFTs) through side-chain induced self-organization into a well-ordered film. To obtain selectively self-patterned layers, we treated an adenine-functionalized poly(3-hexylthiophene) (PAT) with adenosine triphosphate (ATP). Using this strategy, interchain charge transport resulting from π-conjugation was selected to control the polymer morphology, without the need of additional chemical synthetic processing. The side chain–induced self-organization can be understood in terms of supramolecular interactions. The π-electrons were delocalized among the thiophene rings, thereby improving the interchain charge transport ability; the resulting planar π-electron system in PAT:ATP resulted in closer intermolecular π–π distances, facilitating enhanced charge carrier mobility within a fibrillar structure. The PAT:ATP-based OTFT device exhibited moderate to improved electronic characteristics, with an average field mobility of 3.2 × 10⁻⁴ cm² V⁻¹ s⁻¹ at −30 V and a threshold voltage (V_th) of 5 V, and an on/off current ratio of 10⁶. This method has great potential for inducing selective intermolecular interactions in fully solution processed electronic devices.     

H-aggregation mode in triple-decker phthalocyaninato-europium semiconductors. Materials design for high-performance air-stable ambipolar organic thin film transistors

Two new tris(phthalocyaninato) europium complexes Eu₂(Pc)[Pc(OPh)₈]₂ (1) and Eu₂[Pc(OPh)₈]₃ (2) [Pc = unsubstituted phthalocyaninate; Pc(OPh)₈ = 2,3,9,10,16,17,23,24-octaphenoxyphthalocyaninate], were designed and synthesized. Introduction of different number of electron-withdrawing phenoxy substituents at the phthalocyanine periphery within the triple-decker complexes not only ensures their good solubility in conventional organic solvents, but more importantly successfully tunes their HOMO and LUMO levels into the range of air-stable ambipolar organic semiconductor required on the basis of electrochemical studies over both 1 and 2, meanwhile fine controlling of aggregation mode (H vs. J) in solution-based film for improving OTFT performance is also achieved. Measurements over the OTFT devices fabricated from these sandwich compounds by a solution-based quasi–Langmuir–Shäfer (QLS) method reveal their ambipolar semiconductor nature associated with suitable HOMO and LUMO energy levels. Due to the H-aggregation mode employed by the heteroleptic triple-decker molecules in the QLS film, excellent performances with the electron and hole mobility in air as high as 0.68 and 0.014 cm² V⁻¹ s⁻¹, respectively, have been revealed for the OTFT devices of heteroleptic triple-decker 1. This represents the best performance so far for solution-processable ambipolar single-component phthalocyanine-based OTFTs obtained under ambient conditions. In good contrast, homoleptic analogue 2 prefers to J-type aggregation and this results in relatively lower electron and hole mobility, around 0.041 and 0.0026 cm² V⁻¹ s⁻¹ in air, respectively, for the devices fabricated. In particular, the performance of the devices fabricated based on 1 was found to remain almost unchanged in terms of both the carrier mobilities and on/off ratio even after being stored under ambient for 4 months.