Effects of film microstructure on the bias stability of pentacene field-effect transistors

In this report, the effects of film microstructure on the bias stability of pentacene field-effect transistors (FETs) were investigated. To control the microstructure of pentacene film, substrate temperature was changed from 25 to 90 °C during pentacene deposition. As the substrate temperature increased, pentacene grain size increased (or grain boundary (GB) decreased) because of the elevated surface diffusion of pentacene molecules. Accordingly, field-effect mobility increased up to 1.52 cm²/V. In contrast, bias stability showed totally different characteristics: samples prepared at high substrate temperatures exhibited the lowest degree of bias stability. This GB independent charge trapping phenomenon was solved by examining molecular scale ordering within the intragrain regions. The pentacene film grown at 90 °C showed the largest percentage of pentacene molecules with bulk crystalline structures. This inhomogeneity in the pentacene microstructure induces crystal mismatch within intragrain region, thereby providing deep trap sites for gate-bias stress driven instability. Our study shows that GB is not the main sites for bias stress related charge trapping, rather the molecular orientation within intragrain region is responsible for the charge trapping events. In this regard, the control of molecular scale ordering is important to obtain OFETs with a high bias stability.     

Origin of low-frequency noise in pentacene field-effect transistors

Measurements of power spectral density (PSD) of low-frequency noise (LFN) in pentacene field-effect transistors reveal the preponderance of a 1/f-type PSD behavior with the amplitude varying as the squared transistor gain and increasing as the inverse of the gate surface area. Such features impose an interpretation of LFN by carrier number fluctuations model involving capture/release of charges on traps uniformly distributed over the gate surface. The surface slow trap density extracted by the noise analysis is close to the surface states density deduced independently from static I(V) data, which confirms the validity of the proposed LFN interpretation. Further, we found that the trap densities in bottom-contact (BC) devices were higher than in their top-contact (TC) counterparts, in agreement with observations of a poorer crystal structure of BC devices, in the contact regions in particular. At the highest bias the noise originating from the contact resistance is also shown to be a dominant component in the PSD, and it is well explained by the noise originating from a gate-voltage dependent contact resistance. A gate area scaling was also performed, and the good scaling and the dispersion at the highest bias confirm the validity of the applied carrier number fluctuations model and the predominant contact noise at high current intensities.     

Ultra-thin and graded sliver electrodes for use in transparent pentacene field-effect transistors

The authors report the fabrication of efficient and transparent pentacene field-effect transistors (FETs) using a graded structure of ultra-thin silver (Ag) source and drain (S–D) electrodes. The S–D electrodes were prepared by thermal evaporation with a controlled deposition rate to form Ag layer with a graded structure, leading to a reduced injection barrier and smoothing the contact surface between the electrode and the pentacene channel. The sheet resistance of such Ag electrode was found to be as low as 9 Ω/sq. In addition, a hole-only behavior of device with Ag electrode characterized by current–voltage measurement and conductive atomic-force microscopy shows the injection property of high current flowing as compared with device using Au electrode, resulting in an efficient injection condition existing at the interface of the graded Ag/pentacene. Device characterization indicates the transparent pentacene FET with a graded ultra-thin Ag electrode and organic capping layer of N,N′-di(1-naphthyl)-N,N′-diphenylbenzidine exhibits a high transmission rate of ∼75% in the range of visible light from 400 to 550 nm, a threshold voltage of −6.0 V, an on–off drain current ratio of 8.4 × 10⁵, and a field-effect mobility of 1.71 cm²/V s, thus significantly outperforming pentacene FETs with multilayer oxide electrodes or other transparent thin metal layers.     

The color change in polychromatic organic light-emitting field-effect transistors: Optical filtering versus reemission

In this contribution the color conversion process of a polychromatic organic light-emitting field-effect transistor (OLET) is revisited on the basis of an analytic device model. The device of interest consists of a color conversion layer out of rubrene on top of a monochromatic light-emitting transistor based on poly(9,9-di-n-octyl-fluorene-alt-benzothiadiazole) (F8BT). The model describes the relation of color coordinate and emission intensity – set by the applied drain and gate biases – linking the optoelectronic response of the employed monochromatic OLET to the optical processes occurring in the color conversion layer. The model shows that the color shift is rather due to partial absorption of the F8BT emission by rubrene than, as was claimed earlier, due to a color conversion process by absorption and reemission in the conversion layer. In addition to the earlier publication, it will be demonstrated that such a device allows for an independent electrical tunability of emission intensity and color coordinate within the color span of the F8BT and the rubrene spectrum being a unique feature of such a polychromatic light-emitting field-effect transistor.     

Controllable threshold voltage of a pentacene field-effect transistor based on a double-dielectric structure

The authors report controllable threshold voltage (V_th) in a pentacene field-effect transistor based on a double-dielectric structure of poly(perfluoroalkenyl vinyl ether) (CYTOP) and SiO₂. When a positive switching voltage is applied to the gate electrode of the transistor, electrons traverse through the pentacene and CYTOP layers and subsequently trapped at the CYTOP/SiO₂ interface. The trapped electrons induce accumulation of additional holes in the pentacene conducting channel, resulting in a large V_th shift from ?4.4 to +4.6 V. By applying a negative switching voltage, the trapped electrons are removed from the CYTOP/SiO₂ interface, resulting in V_th returning to an initial value. The V_th shift caused by this floating gate-like effect is reversible and very time-stable allowing the transistor to be applicable to a nonvolatile memory that has excellent retention stability of stored data.     

Performance comparison of pentacene organic field-effect transistors with SiO2 modified with octyltrichlorosilane or octadecyltrichlorosilane

Performance of pentacene organic field-effect transistors (OFETs) is significantly improved by treatment of SiO₂ with octyltrichlorosilane (OTS-8) compared to octadecyltrichlorosilane (OTS-18). The average hole mobility in these OFETs is increased from 0.4 to 0.8 cm²/Vs when treating the dielectric with OTS-8 versus OTS-18 treated devices. The atomic force microscope (AFM) images show that the OTS-8 treated surface produces much larger grains of pentacene (∼500 nm) compared to OTS-18 (∼100 nm). X-ray diffraction (XRD) results confirmed that the pentacene on OTS-8 is more crystalline compared to the pentacene on OTS-18, resulting in higher hole mobility.     

Light emitting field-effect transistors with vertical heterojunctions based on pentacene and tris-(8-hydroxyquinolinato) aluminum

We report a top-contact light emitting field-effect transistor based on an asymmetric vertical heterojunction using pentacene as a field-effect layer and tris-(8-hydroxyquinolinato) aluminum (Alq₃) as an electron transport and luminescent material, which is fabricated on an indium tin oxide (ITO)-coated glass substrate with poly (methyl methacrylate) (PMMA) as a gate dielectric. The Alq₃ layer underneath the drain electrode roughly occupies one half of the pentacene surface forming an asymmetric heterojunction with pentacene. A hole transport layer N,N′-Di(1-naphthyl)-N,N′-diphenyl-(1,1′-biphenyl)-4,4′-diamine (NPB) is introduced to occupy the other half of the pentacene surface underneath the source electrode to allow vertical hole transport in the device. We have realized the electrical switching functionality of a field-effect transistor (FET) and the control of electroluminescence (EL) simultaneously under ambient atmosphere. The device exhibits typical p-channel characteristics and green emission from Alq₃ is observed adjacent to the drain electrode. A working principle of the device is discussed in detail. Furthermore, this device configuration enables high-spatial-resolution fluorescence imaging of device operation, which is a simple and powerful tool for studying organic luminescent materials.     

Bottom-contact organic field-effect transistors based on single-crystalline domains of 6,13-bis(triisopropylsilylethynyl) pentacene prepared by electrostatic spray deposition

Large crystalline domains (a few hundred micrometers in size) of 6,13-bis(triisopropylsilylethynyl) pentacene (TIPS pentacene) were prepared by electrostatic spray deposition (ESD) and used as the active layers of bottom-contact organic field-effect transistors. The TIPS pentacene active layers were directly patterned via a shadow mask in the ESD process. The device, which had a 5-μm-long channel composed of a single-crystalline domain, exhibited a high field-effect mobility of more than 0.1 cm²/V s but resulted in a high threshold voltage of −17 V. The threshold voltage could be lowered to −6.4 V by reducing the thickness of the BC electrodes from 30 to 10 nm; this threshold voltage lowering was probably due to an improvement in the charge injection from the source electrode to the active layer.     

Dielectric surface-polarity tuning and enhanced operation stability of solution-processed organic field-effect transistors

The electrical performance of triethylsilylethynyl anthradithiophene (TES-ADT) organic field-effect transistors (OFETs) was significantly affected by dielectric surface polarity controlled by grafting hexamethyldisilazane and dimethyl chlorosilane-terminated polystyrene (PS-Si(CH₃)₂Cl) to 300-nm-thick SiO₂ dielectrics. On the untreated and treated SiO₂ dielectrics, solvent–vapor annealed TES-ADT films contained millimeter-sized crystals with low grain boundaries (GBs). The operation and bias stability of OFETs containing similar crystalline structures of TES-ADT could be significantly increased with a decrease in dielectric surface polarity. Among dielectrics with similar capacitances (10.5–11 nF cm⁻²) and surface roughnesses (0.40–0.44 nm), the TES-ADT/PS-grafted dielectric interface contained the fewest trap sites and therefore the OFET produced using it had low-voltage operation and a charge-carrier mobility ∼1.32 cm² V⁻¹ s⁻¹, on–off current ratio >10⁶, threshold voltage ∼0 V, and long-term operation stability under negative bias stress.     

Contactless charge carrier mobility measurement in organic field-effect transistors

With the increasing performance of organic semiconductors, contact resistances become an almost fundamental problem, obstructing the accurate measurement of charge carrier mobilities. Here, a generally applicable method is presented to determine the true charge carrier mobility in an organic field-effect transistor (OFET). The method uses two additional finger-shaped gates that capacitively generate and probe an alternating current in the OFET channel. The time lag between drive and probe can directly be related to the mobility, as is shown experimentally and numerically. As the scheme does not require the injection or uptake of charges it is fundamentally insensitive to contact resistances. Particularly for ambipolar materials the true mobilities are found to be substantially larger than determined by conventional (direct current) schemes.     

Directed self-assembly of organic semiconductors via confined evaporative capillary flows for use in organic field-effect transistors

We fabricated well-defined 6,13-bis(triisopropylsilylethynyl) pentacene (TIPS-PEN) crystal arrays for use in electronic applications via a simple but effective method, the confined evaporative capillary flow (CEC) method. This has been accomplished by systematically controlling the contact line pinning at the edge of glass stylus and the outward hydrodynamic flow within the drying droplet with various processing solvents and surface properties of the substrate during solidification. We found that after CEC coating of TIPS-PEN solution dissolved into toluene onto SiO₂ surface, ribbon-shaped TIPS-PEN crystals were well developed with a width of 20–100 μm and length of 300 μm – 2 mm, which is presumably owing to optimized capillary evaporation. Specifically, TIPS-PEN crystals present highly preferred crystal orientation along the (l 0 0) axis, which can lead to efficient charge transport in a lateral direction. Thus, TIPS-PEN field-effect transistors (FETs) exhibited a good hole mobility of 0.72 cm²/Vs.     

Probing a two-step channel formation process in injection-type pentacene field-effect transistors by time-resolved electric-field-induced optical second-harmonic generation measurement

By using time-resolved electric-field-induced optical second-harmonic generation measurement, we studied carrier motion in pentacene field-effect transistors (FETs) with poly-4-vinylphenol (PVP) and with polyimide (PI) gate-insulator whose active layers were depleted by pre-biasing. Upon removal of the pre-biasing, channel formation proceeded as a two-step process in FETs with PVP gate-insulator and a conduction channel was formed eventually. On the other hand, no conduction channel was formed in FETs with PI gate-insulator but two-step carrier propagation was observed in a similar way. Results showed that a local electric field induced on the gate-insulator surface gives a significant effect on the carrier injection and the following carrier transport.     

Controlling organization of conjugated polymer films from binary solvent mixtures for high performance organic field-effect transistors

We investigate the effect of a binary solvent blend as a solvent for poly{[N,N′-bis(2-octyldodecyl)-1,4,5,8-naphthalenediimide-2,6-diyl]-alt-5,5′-(2,2′-bithiophene)} P(NDI2OD-T2) on the characteristics of n-channel organic field-effect transistors (OFETs). To make the binary solvent blend, the low-boiling-point non-solvent propylene glycol methyl ether acetate (PGMEA, b.p ∼146 °C) is added to the high-boiling-point good solvent 1,2-dichlorobenzene (O-DCB, b.p ∼180 °C) at various mixing ratio from 0 to 40 v%. UV–vis spectra of P(NDI2OD-T2) solution dissolved in the binary solvent clearly show the formation of polymer aggregates through a gradual red shift of the intramolecular charge transfer band with the addition of high concentrations of non-solvent PGMEA. Higher edge-on oriented crystallinity is observed for P(NDI2OD-T2) films spin-coated from the binary solvent with 5–10 v% PGMEA by out-of-order x-ray diffraction. P(NDI2OD-T2) films are applied as the active layer in top-gate/bottom-contact OFETs. Improved n-type field-effect mobility of the P(NDI2OD-T2) semiconducting layer up to 0.59 cm²/Vs was achieved for on-center spin coated films compared to 1.03 cm²/Vs for off-center (parallel alignment) spin-coated films respectively employing the binary solvent with 10 v% PGMEA.     

Using d-limonene as the non-aromatic and non-chlorinated solvent for the fabrications of high performance polymer light-emitting diodes and field-effect transistors

Aiming to environment protection, green solvents are crucial for commercialization of solution-processed optoelectronic devices. In this work, d-limonene, a natural product, was introduced as the non-aromatic and non-chlorinated solvent for processing of polymer light-emitting diodes (PLEDs) and organic field effect transistors (OFETs). It was found that d-limonene could be a good solvent for a blue-emitting polyfluorene-based random copolymer for PLEDs and an alternating copolymer FBT-Th₄(1,4) with high hole mobility (μ_h) for OFETs. In comparisons to routine solvent-casted films of the two conjugated polymers, the resulting d-limonene-deposited films could show comparable film qualities, based on UV–vis absorption spectra and observations by atomic force microscopy (AFM). With d-limonene as the processing solvent, efficient blue PLEDs with CIE coordinates of (0.16, 0.16), maximum external quantum efficiency of 3.57%, and luminous efficiency of 3.66 cd/A, and OFETs with outstanding μ_h of 1.06 cm² (V s)⁻¹ were demonstrated. Our results suggest that d-limonene would be a promising non-aromatic and non-chlorinated solvent for solution processing of conjugated polymers and molecules for optoelectronic device applications.     

Stable growth of large-area single crystalline thin films from an organic semiconductor/polymer blend solution for high-mobility organic field-effect transistors

We developed an effective and steady solution-processing technique for a small molecule–type semiconductor, C₁₀–DNBDT–NW, by adding an amorphous PMMA polymer to produce stable growth of a two-dimensional large-area single-crystalline thin film by effective phase separation at a crucially faster processing speed compared to the case without the addition of a polymer. By using this solution-processing technique, it is noteworthy that the single-crystalline films of C₁₀–DNBDT–NW/PMMA exhibit the highest and average mobilities of 17 and 10.6 cm²/Vs, respectively. Furthermore, we also show the limitations of two-dimensional continuous growth of a single-crystalline film in terms of the solution technique.     

Effect of electron-donating unit on crystallinity and charge transport in organic field-effect transistors with thienoisoindigo-based small molecules

We report the effect of an electron-donating unit on solid-state crystal orientation and charge transport in organic field-effect transistors (OFETs) with thienoisoindigo (TIIG)-based small molecules. End-capping of different electron-donor moieties [benzene (Bz), naphthalene (Np), and benzofuran (Bf)] onto TIIG (giving TIIG-Bz, TIIG-Np, and TIIG-Bf) is resulted in different electronic energy levels, solid-state morphologies and performance in OFETs. The 80 °C post-annealed TIIG-Np OFETs show the best device performance with a best hole mobility of 0.019 cm² V⁻¹ s⁻¹ and threshold voltage of −8.6 ± 0.9 V using top gate/bottom contact geometry and a CYTOP gate dielectric. We further investigated the morphological microstructure of the TIIG-based small molecules by using grazing incidence wide angle X-ray scattering, atomic force microscopy and a polarized optical microscope. The electronic transport levels of the TIIG-based small molecules in thin-film states were investigated using ultraviolet photoelectron spectroscopy to examine the charge injection properties of the gold electrode.     

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.     

Maskless metal patterning by meniscus-confined electrochemical etching and its application in organic field-effect transistors

Conductive micropatterns is an essential part for operation of electronic devices in both industrial and academic fields. Conventional mask-based photolithography and vacuum deposition are inadequate to meet the demands of convenience and simplicity due to their complicated operation, costly instrumentations and relatively low resolution (for vacuum deposition). Development of simple and efficient mask-less fabrication techniques of conductive micropatterns is highly expected. Here we report a facile meniscus-confined electrochemical etching (MCEE) approach to fabricate metal micropatterns with resolution down to at least 1.0 μm. Both the applied bias and the moving velocity directly influence the patterning resolution. MCEE process is developed to fabricate source and drain electrodes in organic transistors on both rigid and flexible substrates. Being a maskless direct writing method, the width and morphology of the etched channel can be easily modulated by the bias and the velocity. The organic transistor with top-contact configuration presents better electrical performance with device on/off ratio of 1.1 × 10⁵ and maximum carrier mobility of 1.07 cm²V⁻¹s⁻¹, which implies that MCEE operation doesn't result in the degradation of the already deposited semiconducting film. This mask-less MCEE approach provides a potential complementary to conventional mask-based techniques for the fabrication of microscale metal patterns.     

Large plate-like organic crystals from direct spin-coating for solution-processed field-effect transistor arrays with high uniformity

Solution-processed organic crystals are important in field-effect transistors because of their highly ordered molecular packing and ease of device fabrication. For practical applications, the patterning of organic crystal transistor arrays is critical. However, uniformity, which concerns the variation in electrical performance among devices fabricated simultaneously on the same substrate, is a common consideration in the commercial applications of the solution-processed organic crystal transistor arrays. Here, a simple approach for fabricating field-effect transistor arrays based on organic plate-like crystals is reported. Through this method, a direct spin-coating process from a mixture solution of organic semiconductor and polymer dielectric can produce organic plate-like crystals. The grain size of the crystals is observed to be hundreds of micrometers. By controlling the concentrations of the active materials, the transistor arrays exhibit high uniformity and good device performance. The results presented in this work promise that this approach is a comparable technology to hydrogenated amorphous silicon-based FETs and is a great candidate for practical applications in electronic devices.     

Synergistic effect in organic field-effect transistor nonvolatile memory utilizing bimetal nanoparticles as nano-floating-gate

A solution-processed bimetal nano-floating-gate, with a combination of stabilized Ag and Pt nanoparticles, is utilized to achieve high-performance organic field-effect transistor nonvolatile memories. The device based on the Ag–Pt nano-floating-gate shows the synergistic superiority in memory performance compared with the corresponding Ag-only and Pt-only devices. The Ag and Pt nanoparticles are found to prefer hole and electron trapping, respectively. Upon the blending of the Ag and Pt nanoparticles, both hole and electron trapping are significantly enhanced and thus realize a large memory window. The dipole enhancement induced local work function change for both Ag and Pt is proposed to be responsible for the synergistic effect, and this physical picture is supported by the electronic structure results. It is concluded that using a hybrid nano-floating-gate is a promising strategy to optimize the device performance of organic field-effect transistor nonvolatile memories.