Solution-processed small molecule thin films and their light-emitting devices

Thin films of N,N′-bis-(3-Naphthyl)-N,N′-biphenyl-(1,1′-biphenyl)-4,4′-diamine (NPB), tris-(8-hydroxyquinoline)-aluminum (Alq₃) and their blends prepared by spin-coating process were investigated. Experimental results revealed that the NPB films prepared by spin-coating process have smoother surface than that of Alq₃, which was attributed to their different molecular structures. Organic light-emitting devices (OLEDs) with emitting layer prepared by spin-coating the blends of NPB and Alq₃ exhibited a maximum luminance and a current efficiency over 10,000 cd/m² and 3.8 cd/A respectively, and when 10-(2-benzothiazolyl)-2,3,6,7-tetrahydro-1,1,7,7-tetramethyl-1H,5H,11H-[l]benzopyrano[6,7,8-ij]quinolizin-11-one was doped in, a current efficiency of 8 cd/A can be obtained. Comparative device performance to the vapor-deposited OLEDs suggested that solution-process could be an alternative route for the fabrication of OLEDs based on Alq₃.     

Characteristics of organic inverters using vertical- and lateral-type organic transistors

The characteristics of vertical-type organic static induction transistors (OSITs) were compared with those of lateral-type organic field effect transistors (OFETs). From these experiments, it was confirmed that the OSITs can operate at a voltage one order less than that required for OFETs. We also fabricated two types of organic inverter based on OSITs and OFETs and investigated their transfer characteristics. These results demonstrate that it is possible to decrease the operational voltage of organic inverters from ± 20 V to ± 2 V by using two OSITs with higher on/off ratios.     

Structures and electronic properties of thin-films of polycyclic aromatic hydrocarbons

We report the fabrication and characterization of organic thin-film transistors (TFTs) using several polycyclic aromatic hydrocarbons (PAHs). Pentacene, ovalene, dibenzocoronene and hexabenzocoronene were deposited as organic semiconductors on silicon wafers with gold electrodes as the bottom-contact configuration of the TFTs. The pentacene TFT showed the highest field-effect mobility of more than 0.1 cm²/Vs in comparison with the other PAHs. The results clarified that the high field-effect mobility of the pentacene thin film is due to large grain size and intrinsic electronic properties.     

Ambipolar organic heterojunction transistors with various p-type semiconductors

Ambipolar transport has been realized in organic heterojunction transistors with metal phthalocyanines, phenanthrene-based conjugated oligomers as the first semiconductors and copper-hexadecafluoro-phthalocyanine as the second semiconductor. The electron and hole mobilities of ambipolar devices with rod-like molecules were comparable to the corresponding single component devices, while the carrier mobility of ambipolar devices with disk-like molecules was much lower than the corresponding single component devices. The much difference of their device performance was attributed to the roughness of the first semiconductor films, which was original from their distinct growth habits. The flat and continuous films for the first semiconductors layer can lead to a smooth heterojunction interface, and obtained a high device performance for ambipolar organic heterojunction transistors.     

Organic field effect transistors with ferroelectric hysteresis

The ferroelectric copolymer Poly(vinylidene fluoride trifluoroethylene) is used as insulating material for capacitor structures and organic field effect transistors. For capacitors, we find the typical hysteresis in the capacitance-voltage characteristic upon increasing the voltage scan window. A writing process with adequate electric fields causes shifts in the flatband voltage. Based on these results, we fabricate organic transistors with regioregular poly(3-hexylthiophene) as organic semiconductor. The transistors are constructed in bottom gate architecture with thin layers (100 nm) of spincoated copolymer as gate insulation. The drain source current of the transistor is reversible affected by the polarized gate, which gives opportunities for fabrication of organic nonvolatile memory elements.     

Organic thin-film transistors using thin ormosil-based hybrid dielectric

We synthesized a novel thermally-crosslinkable ormosil-based hybrid material as a solution-processable dielectric layer for organic thin-film transistors (OTFTs). Dielectrics with a thickness of 50-260 nm were fabricated via spin-coating in order to evaluate their applicability as an ultra-thin gate dielectric. It was observed that the capacitance of the hybrid dielectric increases with decreasing film thickness. Hybrid dielectrics with a thickness of 260 nm and 160 nm, respectively, exhibited adequate leakage current behavior. Coplanar-type OTFTs were fabricated using each of the hybrid dielectrics (i.e., thickness of 260 nm and 160 nm). The off-current, threshold voltage, and field-effect mobility of both transistors were analyzed to investigate the effects of capacitance and film thickness on the electrical performance of the transistors.     

The influences of substrate temperature on ambipolar organic heterojunction transistors

Organic heterojunction thin-film transistors are fabricated based on copper phthalocyanine (CuPc) and hexadecafluorophtholocyaninatocopper (F₁₆CuPc) as double active layers, which exhibit typical ambipolar conduction. Several substrate temperatures are utilized to tune film morphology, which results in a remarkable change on the electric characteristics of organic transistors. The highest balanced mobility value of 2.91 × 10⁻² cm²/V s for hole and 1.04 × 10⁻² cm²/V s for electron are obtained by depositing F₁₆CuPc at 150 °C and CuPc at 200 °C, respectively, which are comparable to those conventional single-layer devices. This result demonstrates that the growth conditions of organic heterojunctions play a crucial role in ambipolar devices.     

Aging effects and electrical stability in pentacene thin film transistors

We have studied the transfer characteristic variations induced by aging effects and applied voltage in top contact pentacene thin film transistors (OTFTs) fabricated by using Polymethylmetacrylate buffer layer. The electrical stability of pentacene OTFTs was tested by applying prolonged bias stress (up to 10⁴ s) with gate voltage V_gstress = − 30 V and + 30 V. The environmental effects were analysed by measuring the degradation of electrical characteristics of OTFT exposed to air. The results have been analysed in terms of trap state model, evaluating the channel conductance using a one-dimensional approach. This allows us to correlate the transfer characteristics variations to changes in localised state distribution.     

Nanostructural depth-profile and field-effect properties of poly(alkoxyphenylene-thienylene) Langmuir-Schäfer thin-films

The correlations between morphological features and field-effect properties of poly(alkoxyphenylene-thiophene) thin Langmuir-Schäfer film deposited on differently terminated gate dielectric surfaces, namely bare and methyl functionalized thermal silicon dioxide (t-SiO₂), have been systematically studied. The film morphology has been investigated at different film thickness by Scanning Force Microscopy. Films thicker than a few layers show comparable morphology on both dielectric surfaces while differences are seen for the ultra-thin polymer deposit in close proximity to the substrate. Such deposit is notably more heterogeneous on bare t-SiO₂, while a more compact and uniform nanogranular structure is observed on the silylated t-SiO₂. As to the field-effect properties, the methyl-terminated gate dielectric surface leads to a two order of magnitude mobility enhancement along with a field-effect thickness independent conductance.     

Low-voltage-operating organic complementary circuits based on pentacene and C60 transistors

Low-voltage-operating organic complementary inverters and ring oscillators were fabricated using high field effect mobility pentacene and C₆₀ thin-film transistors (TFTs). The mobilities of pentacene and C₆₀ TFTs were 0.44 and 0.61 cm²/V s, respectively. The complementary inverters composed of these TFTs operated in the voltage range of 2-10 V with large gain values up to 65. The inverter yields 5-stage ring oscillators with a high oscillation frequency of 80 Hz at 10 V.     

Electric characteristics of organic thin-film transistors and logic circuits with a ferroelectric gate insulator

Organic electronic devices using a pentacene have improved importantly in the last several years. We fabricated pentacene organic thin-film transistors (OTFTs) with dielectric SiO₂ and ferroelectric Pb(Zr_0.3,Ti_0.7)O₃ (PZT) gate insulators. The organic devices using SiO₂ and PZT films had the field-effect mobility of approximately 0.1 and 0.004 cm²/V s, respectively. The drain current in the transfer curve of pentacene/PZT transistors showed a hysteresis behavior originated in a ferroelectric polarization switching. In order to investigate the polarization effect of PZT gate dielectrics in a logic circuit, the simple voltage inverter using SiO₂ and PZT films was fabricated and measured by an output-input measurement. The gain of inverter at the poling-down state was approximately 7.2 and it was three times larger than the value measured at the poling-up state.     

Synthesis and characteristics of a solution-processable fullerene derivative for n-type organic field-effect transistors

A fullerene molecule, C60TH2-DcB, was prepared by Prato reaction which provides better product yields and purity compared with the reaction for the synthesis of [60]methanofullerene analogues. The C60TH2-DcB is readily soluble in common organic solvents and shows good thermal properties. Field-effect transistors fabricated by solution processing exhibit good n-channel characteristics with a maximum mobility of 1.5 × 10⁻³ cm² V⁻¹ s⁻¹ with a threshold voltage of 11 V and on/off current ratio of 1.0 × 10⁵.     

Field-effect transistors of the block co-oligomers based on thiophene and pyridine

We have investigated the structural and electrical characteristics of two kinds of block co-oligomers, 5,5″-Bis(2-pyridyl)-2,2′:5′,2″-terthiophene (5A) and 2,5-Bis(2-(2′-thienyl)-pyridine-5-yl)thiophene (5B), which are composed of electron-donating thiophene and electron-withdrawing pyridine rings. At the view of building block units, the amount of the module units included in these molecules is completely equivalent to each other. X-ray diffraction patterns of 5A and 5B thin films grown on atomically flat α-Al₂O₃ (0001) substrates indicated a high degree of orientation along the c-axis. Field-effect transistors (FETs) of 5A and 5B thin films were fabricated and evaluated without exposure to air. The FETs based on 5A active layers exhibited p-type behavior with a mobility of ~ 10⁻³ cm²/V·s and an on-off ratio of 10⁴, while no switching behavior was observed in FETs based on 5B active layers. The lower highest occupied molecular orbital (HOMO) of 5B than that of 5A in the quantum-chemical calculation might explain these FET behaviors. Thus, HOMO, lowest unoccupied molecular orbital (LUMO) levels and FET actions of the co-oligomer molecules that consist of the same amount of building blocks can be controlled by the sequence of electron-donating and electron-withdrawing building blocks.     

Geometrical effect in submicrometer channel organic field effect transistors

The electrical behaviors of submicrometer bottom-gate bottom-contact organic field effect transistors (OFETs) with submicrometer channel lengths and channel widths were investigated. Short-channel effects (SCEs) were observed for devices with shorter channel lengths and wider channel widths. The SCEs were effectively suppressed by reducing the channel width to 50 nm. The relationship between the drain current density and the drain voltage normalized by their respective channel lengths revealed that the drain current characteristics of shorter length channels fall into two types: parasitic contact resistances at lower drain voltage and SCEs caused by the space charge limiting current at higher drain voltages. The carrier mobility was also investigated, and found to be enhanced in the narrower channel width.     

Control of threshold voltage of organic field-effect transistors by space charge polarization

We demonstrate organic field-effect transistors (OFETs) with an ion-dispersed polymer for the gate dielectrics. By applying external electric field (V_ex), the dispersed ions can migrate by electrophoresis and separated ion pairs form space charge polarization in the gate dielectrics. After V_ex was applied, the drain current is increased over 7 times and threshold voltage is decreased from − 12.9 V to − 2.9 V. The shift direction of V_th is controllable by the polarity of the V_ex. Results of ultraviolet/visible differential absorption study reveal that the active layer of OFETs is charged not only electrostatically but electrochemically with increasing the time after V_ex was applied.     

Flexible organic field-effect transistors and complementary inverters based on a solution-processable quinoidal oligothiophene derivative

We report on the fabrication and characterization of ambipolar organic field-effect transistors based on the solution-processable quinoidal oligothiophene [QQT(CN)4] and using a new fluorinated polymer (AL-X601) with a dielectric constant of 3.1 as dielectric material layer. As-prepared devices show ambipolar transport with hole and electron field-effect mobilities of 6 × 10⁻² and 5 × 10⁻³ cm²/V s respectively as well as an on and off state current ratio higher than 10³. Influence of a thermal annealing on the device performances was investigated and was found to lead to a majority carrier type conversion from a p-type to an n-type dominant behavior. QQT(CN)4 based field-effect transistors and complementary inverters fabricated on flexible substrates and using Al-X601 as gate dielectric material show high performance and good mechanical stability.     

Performance enhancement of organic thin-film transistors with improved copper phthalocyanine crystallization by inserting ultrathin pentacene buffer

Organic thin-film transistors (OTFTs) with high crystallization copper phthalocyanine (CuPc) active layers were fabricated by inserting an ultrathin pentacene buffer layer between the dielectric and CuPc layers. Comparing with the OTFTs without a pentacene buffer layer, the charge carrier mobility of the OTFT with a buffer layer presented a much higher value of ~ 0.20 cm²/V s. Meanwhile, by investigating the morphology of the CuPc active layer with an ultrathin pentacene buffer layer through scanning electron microscopy and X-ray diffraction, the high crystallization of the CuPc film with a larger grain size and less grain boundaries can be observed. As a result, the resistance of the conducting channel was decreased, leading to a performance improvement of the OTFTs.     

Flexible high capacitance nanocomposite gate insulator for printed organic field-effect transistors

Ceramic-polymer nanocomposite dielectric consisting of an epoxy solution with propylene glycol methyl ether acetate as the solvent and barium titanate nanoparticles with capacitance in excess of 60 pF/mm² was developed and utilized as the gate insulator for organic field-effect transistors (OFETs). The high relative permittivity (κ = 35), bimodal nanocomposite utilized had two different filler particle sizes 200 nm and 1000 nm diameter particles. Bottom gate organic filed-effect transistors were demonstrated using a commercially available printing technology for material deposition. A metal coated plastic film was used as the flexible gate substrate. Solution processable, p-type arylamine-based amorphous organic semiconductor was utilized as the active layer. Fabricated OFETs with the solution processed nanocomposite dielectric had a high field-induced current and a low threshold voltage; these results suggest that the low operating voltage was due to the high capacitance gate insulator. In this paper, we review the characteristics of the nanocomposite dielectric material and discuss the processing and performance of the printed organic devices.     

From One‐ to Three‐Dimensional Organic Semiconductors: In Search of the Organic Silicon?

Organic semiconductors based on π‐conjugated systems are the focus of considerable interest in the emerging area of soft or flexible photonics and electronics. Whereas in recent years the performances of devices such as organic light‐emitting diodes (OLEDs), organic field‐effect transistors (OFETs), or solar cells have undergone considerable progress, a number of technical and fundamental problems related to the low dimensionality of organic semiconductors based on linear π‐conjugated systems remain unsatisfactorily resolved. This low dimensionality results in an anisotropy of the optical and charge‐transport properties, which in turn implies a control of the material organization/molecular orientation during or after device fabrication. Such a constraint evidently represents a problem when device fabrication by solution‐based processes, such as printing techniques, is envisioned. The aim of this short Review is to illustrate possible alternative strategies based on the development of organic semiconductors with higher dimensionality, capable to exhibit isotropic electronic properties.