High mobility organic field-effect transistors based on defect-free regioregular poly(3-hexylthiophene-2,5-diyl)

We report on organic field-effect transistors (OFETs) prepared using defect free (100% regioregular) poly(3-hexylthiophene-2,5-diyl) (DF-P3HT) as semiconductor and cross-linked poly(vinyl alcohol) (cr-PVA) as gate insulator. High field-effect mobility (μ_FET) of 1.2 cm² V⁻¹ s⁻¹ is obtained and attributed to the absence of regioregularity defects. These transistors have transconductance of 0.35 μS and the DF-P3HT film shows larger crystallites (∼80 Å) than a highly regioregular (>98%) material (∼32 Å). Devices with increased μ_FET (2.8 cm² V⁻¹ s⁻¹) could be obtained at the expense of the On-Off current ratio, which was reduced by one order of magnitude, when poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) treatment was applied to the dielectric surface. Our results suggest that the interaction of charged sites at the dielectric surface with regioregularity defects of the P3HT is an important factor degrading μ_FET even at very low concentration of regioregularity defects.     

High-performance diketopyrrolopyrrole-based organic field-effect transistors for flexible gas sensors

We demonstrate high-performance flexible polymer OFETs with P-29-DPP-SVS in various geometries. The mobilities of TG/BC OFETs are approximately 3.48 ± 0.93 cm²/V s on a glass substrate and 2.98 ± 0.19 cm²/V s on a PEN substrate. The flexible P-29-DPP-SVS OFETs exhibit excellent ambient and mechanical stabilities under a continuous bending stress of 1200 times at an R = 8.3 mm. In particular, the variation of μ_FET, V_Th and leakage current was very negligible (below 10%) after continuous bending stress. The BG/TC P-29-DPP-SVS OFETs on a PEN substrate applies to flexible NH₃ gas sensors. As the concentration of NH₃ increased, the channel resistance of P-29-DPP-SVS OFETs increased approximately 100 times from ∼10⁷ to ∼10⁹ Ω at V_SD = −5 V and V_GS = −5 V.     

Ultra-high mobility in defect-free poly(3-hexylthiophene-2,5-diyl) field-effect transistors through supra-molecular alignment

Performance enhancement of organic field-effect transistors (OFETs) based on solution-processable conjugated polymers (CPs) holds critical significance for the realization of cost-effective commercial applications such as organic light-emitting diode displays. One of the most critical performance parameters is the charge-carrier field-effect mobility (μ_FET) that is significantly influenced by the molecular arrangement in a CP. In this article, floating film transfer method (FTM) is utilized for the deposition of a CP––defect-free poly(3-hexylthiophene-2,5-diyl) (DF-P3HT)––which results in the formation of aligned supra-molecular assemblies. When applied as the active layer in OFET devices, μ_FET reaching as high as 8.0 cm²/V.s (6.3 cm²/V.s on average) is obtained. The value of μ_FET observed in the current study is the highest value reported so far for P3HT based OFETs (∼5 times higher as compared to when DF-P3HT is deposited using spin coating).     

Air-stable polythiophene-based thin film transistors processed using oxidative chemical vapor deposition: Carrier transport and channel/metallization contact interface

Oxidative chemical-vapor-deposition (oCVD) provides a facile route to polymerize and deposit insoluble monomers in thin film form. Here, we report on oCVD polythiophene (PT)-based organic thin film transistors (OTFTs) that present both high mobility and excellent stability over time in air. The measured field effect mobility (μ_FE) is ∼0.02 cm²/V sec with the low threshold voltage between −1 V and 0.3 V. Additionally the PT OTFTs show no evidence of performance degradation after 3 months exposure in air. The transmission line model (TLM) enables the determination of the specific contact resistance (ρ_C) of oCVD PT channel/metallization interface and reveals that ρ_C is improved with increasing gate bias. The oCVD PT channel conductivity (σ_ch) and carrier density (p) were evaluated from more than 100 devices using TLM measurements and the relation of σ_ch = qpμ_FE. Carrier transport analysis suggests that the charge screening effect governs hole carrier mobility in the carrier density regime below approximately 10¹⁸/cm³ where an increase in carrier density leads to higher mobility. We also demonstrate photo-conductivity of oCVD PT through an increase in device on-state current and the field effect mobility when the PT OTFT is illuminated. Strategies to further enhance the performance of the materials and devices are also suggested.     

Role of geometry, substrate and atmosphere on performance of OFETs based on TTF derivatives

We report a comparative study of OFET devices based on zone-cast layers of three tetrathiafulvalene (TTF) derivatives in three configurations of electrodes in order to determine the best performing geometry. The first testing experiments were performed using SiO₂/Si substrates. Then the optimum geometry was employed for the preparation of flexible OFETs using Parylene C as both substrate and dielectric layer yielding, in the best case, to devices with μ_FET = 0.1 cm²/V s. With the performed bending tests we determined the limit of curvature radius for which the performance of the OFETs is not deteriorated irreversibly. The investigated OFETs are sensitive to ambient atmosphere, showing reversible increase of the source to drain current upon exposition to air, what can be explained as doping of TTF derivative by oxygen or moisture.     

Performance enhancement in mechanically stable flexible organic-field effect transistors with TIPS-pentacene:polymer blend

Flexible organic field-effect transistors (OFETs) with TIPS-pentacene: polystyrene (PS) blend are demonstrated to exhibit enhanced mobility and significantly improved electrical stability compared to neat TIPS-pentacene on poly(4-vinylphenol) (PVP) dielectric (bi-layer OFETs), along with high mechanical stability. Due to merit of high quality dielectric-semiconductor interface, pristine TIPS-pentacene: PS blend OFETs exhibited maximum mobility of 0.93 cm² V⁻¹ s⁻¹ with average of 0.44(±0.25) cm² V⁻¹ s⁻¹ compared to 0.14(±0.10) cm² V⁻¹ s⁻¹ for bi-layer OFETs with high current on-off ratios on the order 10⁵ for both. Both types of devices exhibited high electrical stability upon bending with increasing magnitude of strain or its duration up to 5 days. However, significant differences in electrical stability of devices were observed upon applying constant bias-stress for 40 min to 1 h. Pristine blend devices exhibited outstanding electrical stability with very low drain current decay of <5% compared to ∼30% for bi-layer devices. Even upon bias-stress after 5 days of bending, the drain current decay levels were only changed to <10% and ∼50% for blend and bi-layer devices respectively.     

Solution-processed field-effect transistors based on polyfluorene –cesium lead halide nanocrystals composite films with small hysteresis of output and transfer characteristics

We have designed and investigated electrical and optical properties of solution-processed organic field-effect transistors (OFETs) based on conjugated polymer PFO and perovskite –cesium lead halide nanocrystals (CsPbI₃) composite films. It was shown that OFETs based on PFO:CsPbI₃ films exhibit current-voltage (I-V) characteristics of OFETs with dominant hole transport and saturation current behavior at temperatures 200–300 K. It was found that PFO:CsPbI₃ OFETs have a negligible hysteresis of output and transfer characteristics especially at temperatures below 250 K. The values of the hole mobility estimated from I-Vs of PFO:CsPbI₃ OFETs were found to be ∼2.4 10⁻¹ cm²/Vs and ∼1.9 10⁻¹ cm²/Vs in saturation and low fields regimes respectively at 300 K; the hole mobility dropped down to ∼6 10⁻³ cm²/Vs and 2.8 10⁻³ cm²/Vs respectively at 200 K, and then down to 5.5 10⁻⁵ cm²/Vs at 100 K (in low field regime), which is characteristic of hopping conduction. The effect of sensitivity to light and light-emitting effect were found under application of negative source-drain and gate pulse voltages to PFO:CsPbI₃ OFETs at 300 K. The mechanism of charge carrier transport in OFETs based on PFO:CsPbI₃ hybrid films is discussed.     

Role of Schottky barrier height at source/drain contact for electrical improvement in high carrier concentration amorphous InGaZnO thin film transistors

We report the fabrication of bottom-gate thin film transistors (TFTs) at various carrier concentrations of an amorphous InGaZnO (a-IGZO) active layer from ~10¹⁶ to ~10¹⁹ cm⁻³, which exceeds the limit of the concentration range for a conventional active layer in a TFT. Using the Schottky TFTs configuration yielded high TFT performance with saturation mobility (μ_sat), threshold voltage (V_TH), and on off current ratio (I_ON/I_OFF) of 16.1 cm²/V s, −1.22 V, and 1.3×10⁸, respectively, at the highest carrier concentration active layer of 10¹⁹ cm⁻³. Other carrier concentrations (<10¹⁹ cm⁻³) of IGZO resulted in a decrease of its work function and increase in activation energy, which changes the source/drain (S/D) contact with the active layer behavior from Schottky to quasi Ohmic, resulting in achieving conventional TFT. Hence, we successfully manipulate the barrier height between the active layer and the S/D contact by changing the carrier concentration of the active layer. Since the performance of this Schottky type TFT yielded favorable results, it is feasible to explore other high carrier concentration ternary and quaternary materials as active layers.     

All ink-jet printed low-voltage organic field-effect transistors on flexible substrate

In this work, all ink-jet printed (IJP) low-voltage organic field-effect transistors (OFETs) on flexible substrate are reported. The OFETs use IJP silver (Ag) for source/drain/gate electrodes, poly(4-vinylphenol) (PVP) for gate dielectric, 6,13-bis(triisopropylsilylethynyl)-pentacene (TIPS-pentacene) blended with polystyrene (PS) as the semiconducting layer and CYTOP for encapsulation layer. All the printing processes were carried out in ambient air environment using a single laboratory ink-jet printer Dimatix DMP-2831. The all IJP device presents state-of-the-art performance with low operation voltage down to 3 V, small subthreshold swing (SS) of 0.155 V/decade, mobility of 0.26 cm² V⁻¹s⁻¹, threshold voltage (V_th) of −0.17 V and on/off ratio of 3.1 × 10⁵, along with a yield of 62.5%. Through interface engineering and proper process optimization, this work demonstrates a promising low-voltage all IJP device platform for low-cost flexible printed electronics.     

Top-gate organic field-effect transistors fabricated on paper with high operational stability

We report on top-gate organic field-effect transistors (OFETs) fabricated on specialty paper, PowerCoat™ HD 230 from Arjowiggins Creative Papers coated with a buffer layer composed of a polyvinyl alcohol (PVA) and polyvinylpyrrolidine (PVP) blend. OFETs operate at low voltages and display average carrier mobility values of 1.7 ± 1.1 × 10⁻¹ cm²/Vs, average threshold voltage values of −1.4 ± 0.2 V, and average on/off current ratio of 10⁵. OFETs also display excellent operational stability demonstrated by stable 1000 scans of the transfer characteristics and by stable on-currents displaying less than 6% change during a DC bias stress test at V_DS = V_GS = −10 V for 1 h. Furthermore, OFETs on paper display a decrease of only 7% in their on-state current during a bending test. The performance of these OFETs on paper is comparable to that displayed by top-gate OFETs with the same geometry fabricated on glass substrates.     

Solution processable PEDOT:PSS based hybrid electrodes for organic field effect transistors

We report high performance solution processed conductive inks used as contact electrodes for printed organic field effect transistors (OFETs). Poly(3,4-ethylenedioxythiophene): polystyrene sulfonate (PEDOT:PSS) electrodes show highly improved very low sheet resistance of 65.8 ± 6.5 Ω/square (Ω/□) by addition of dimethyl sulfoxide (DMSO) and post treatment with methanol (MeOH) solvent. Sheet resistance was further improved to 33.8 ± 8.6 Ω/□ by blending silver nanowire (AgNW) with DMSO doped PEDOT:PSS. Printed OFETs with state of the art diketopyrrolopyrrole-thieno[3,2-b]thiophene (DPPT-TT) semiconducting polymer were demonstrated with various solution processable conductive inks, including bare, MeOH treated PEDOT:PSS, single wall carbon nanotubes, and hybrid PEDOT:PSS-AgNW, as the source and drain (S/D) electrode by spray printing using a metal shadow mask. The highest field effect mobility, 0.49 ± 0.03 cm² V⁻¹ s⁻¹ for DPPT-TT OFETs, was obtained using blended AgNW with DMSO doped PEDOT:PSS S/D electrode.     

Raman characterization and electrical properties of poly(3-hexylthiophene) doped electrochemically in an ionic liquid-gated transistor geometry

Using Raman spectroscopy, we observed carriers, polarons and bipolarons formed in an ionic-liquid-gated P3HT electrochemical transistor with an ionic liquid [BMIM][TFSI] as a gate dielectric. The relationships between the source−drain current (I_D), the gate voltage (V_G) at a constant source−drain voltage (V_D), and injected charges at each V_G were investigated. An increase in I_D is attributed to the formation of positive polarons, whereas a decrease in I_D corresponded to positive bipolarons. Thus, positive polarons are efficient carriers in P3HT electrochemical transistors. Charge densities, doping levels, electrical conductivities, and mobilities of polarons in P3HT were calculated from the electrochemical measurements. Only positive polarons exist below the dopant level x = 27 mol%, whereas at higher doping levels, polarons and bipolarons coexist. The mobility of polarons was dependent on the doping level. The highest mobility was 0.31 cm² V⁻¹ s⁻¹ at x = 15 mol%.     

Multi-level non-volatile organic transistor-based memory using lithium-ion-encapsulated fullerene as a charge trapping layer

We report on multi-level non-volatile organic transistor-based memory using pentacene semiconductor and a lithium-ion-encapsulated fullerene (Li⁺@C₆₀) as a charge trapping layer. Memory organic field-effect transistors (OFETs) with a Si⁺⁺/SiO₂/Li⁺@C₆₀/Cytop/Pentacene/Cu structure exhibited a performance of p-type transistor with a threshold voltage (V_th) of −5.98 V and a mobility (μ) of 0.84 cm² V⁻¹ s⁻¹. The multi-level memory OFETs exhibited memory windows (ΔV_th) of approximate 10 V, 16 V, and 32 V, with a programming gate voltage of 150 V for 0.5 s, 5 s, and 50 s, and an erasing gate voltage of −150 V for 0.17 s, 1.7 s, and 17 s, respectively. Four logic states were clearly distinguishable in our multi-level memory, and its data could be programmed or erased many times. The multi-level memory effect in our OFETs is ascribed to the electron-trapping ability of the Li⁺@C₆₀ layer.     

Controllable thin-film morphology and structure for 2,7-dioctyl[1]benzothieno[3,2-b][1]benzothiophene (C8BTBT) based organic field-effect transistors

Organic field-effect transistors (OFETs) based on organic semiconductor material 2,7-dioctyl[1]benzothieno[3,2-b] benzothiophene (C8BTBT) as the active layer were fabricated by using organic molecular beam deposition (OMBD) and solution-processed methods, in which the C8BTBT thin-film morphology could be well controlled. In OMBD method, C8BTBT thin-film morphology could be controlled by the thickness of organic semiconductor layer and the deposition rate, of which the high-quality C8BTBT thin film was obtained at a thickness of about 20 nm and at a deposition rate of 1.2 nm/min, resulting in an obvious mobility improvement from 2.8 × 10⁻³ cm² V⁻¹ s⁻¹ to 1.20 cm² V⁻¹ s⁻¹. While in the solution-processing, C8BTBT thin-film morphology and thickness are related to the spin-coating speed and the substrate position in spin coater, i.e., in-centre and off-centre position. The off-centre spin-coating with an optimized speed produced large-size domain C8BTBT thin film and accordingly resulted in a mobility of 1.47 cm² V⁻¹ s⁻¹. Furthermore, an additive polystyrene (PS) was added into C8BTBT solution could further improve the thin-film morphology with more metal-stable phase as well as improve the interface contact with the substrate SiO₂, resulting in the highest mobility up to 3.56 cm² V⁻¹ s⁻¹. The research suggested that C8BTBT-based OFETs with the mobility over 1.20 cm² V⁻¹ s⁻¹ could be fabricated by using both OMBD and solution-processed methods through the thin-film morphology and structure optimization, which shows the potential applications in high-performance flexible and printed electronics.     

Solution-processed small molecules based on indacenodithiophene for high performance thin-film transistors and organic solar cells

Solution-processed indacenodithiophene (IDT)-based small molecules with 1,3-indanedione (ID) as terminal acceptor units and 3,3′-hexyl-terthiophene (IDT-3Th-ID(I)) or 4,4′-hexyl-terthiophene (IDT-3Th-ID(II)) as π-bridges, have been designed and synthesized for the application in organic field-effect transistors (OFETs) and organic solar cells (OSCs). These molecules exhibited excellent solubility in common organic solvents, good film-forming ability, reasonable thermal stability, and low HOMO energy levels. For the OFETs devices, high hole motilities of 0.52 cm² V⁻¹ s⁻¹ for IDT-3Th-ID(I) and 0.61 cm² V⁻¹ s⁻¹ for IDT-3Th-ID(II) were achieved, with corresponding high I_ON/I_OFF of ca. 10⁷ and ∼10⁹ respectively. The OSCs based on IDT-3Th-ID(I)/PC₇₀BM (2:1, w/w) and IDT-3Th-ID(II)/PC₇₀BM (2:1, w/w) without using any treatment of solvent additive or thermal annealing, showed power conversion efficiencies (PCEs) of 3.07% for IDT-3Th-ID(I) and 2.83% for IDT-3Th-ID(II), under the illumination of AM 1.5G, 100 mW/cm². The results demonstrate that the small molecules constructed with the highly π-conjugated IDT as donor unit, 3Th as π-bridges and ID as acceptor units, could be promising organic semiconductors for high-performance OFETs and OSCs applications.     

Organic field-effect transistor and its photoresponse using a benzo[1,2-b:4,5-b′]difuran-based donor–acceptor conjugated polymer

Organic field-effect transistors (OFETs) were fabricated through a solution process with a donor–acceptor (D–A) conjugated polymer poly{4,8-bis(2′-ethylhexylthiophene)benzo [1,2-b;3,4-b′]difuran-alt-5,5-(4′,7′-di-2-thienyl-5′,6′-dioctyloxy-2′,1′,3′-benzothiadiazole)} (PBDFTDTBT) as the active layer, which is a highly efficient D–A conjugated polymer as a donor in polymer solar cells with a power conversion efficiency (PCE) over 6.0%. The OFET devices showed a hole mobility of 0.05 cm²/Vs and an on/off ratio of 4.6 × 10⁵. Those are one of the best performance parameters for OFETs based on D–A conjugated polymers including benzo[1,2-b:4,5-b′]dithiophene (BDT) or benzo[1,2-b:4,5-b′]difuran (BDF) unit. The photoresponse of OFETs was investigated by modulating light with various intensities. The devices produced a photosensitivity (I_light/I_dark) of 1.2 × 10⁵ and a photoresponsivity of 360 mA W⁻¹ under white light illumination. The drain current in saturation region increases gradually with increasing illumination intensity. The threshold voltage exhibited a positive shift from −15.6 V in darkness to 27.8 V under illumination, which can be attributed to the well-known photovoltaic effect resulting from the transport of photogenerated holes and trapping of photogenerated electrons near the source electrode in organic phototransistors. Meanwhile, the devices showed good stability and with no obvious degeneration for 3 months in air. The study suggests that D–A conjugated polymers including BDF unit can be potentially applied in OFETs and organic phototransistors in addition to highly efficient polymer solar cells.     

Forming semiconductor/dielectric double layers by one-step spin-coating for enhancing the performance of organic field-effect transistors

We report one-step formation of the gate dielectric and conduction channel for enhancing the performance of organic field effect transistors (OFETs). The resulting OFET with the semiconductor/dielectric bi-layers spun in ambient conditions exhibits μ_FET up to 1.6 cm²/V s and on–off ratio higher than 10⁶, no additional treatment needed. Contact angle measurements and absorption spectra reveals that a well-defined semiconductor-top and dielectric-bottom film form after spin-coating the mixture of the two components, which is due to the surface induced self-organized phase separation. Compared to the single layer semiconductor film, the staggered film exhibits over 5 times higher mobility and nearly 90% reduced hysteresis in OFET. The higher performance is attributed to the simultaneous optimization in the dielectric interface and semiconductor crystallization. The approach is significant for the fabrication of low cost, easy processed and high performance OFETs.     

Source/drain metallization effects on the specific contact resistance of indium tin zinc oxide thin film transistors

We report on the specific contact resistance of interfaces between thin amorphous semiconductor Indium Tin Zinc Oxide (ITZO) channel layers and different source/drain (S/D) electrodes (Al, ITO, and Ni) in amorphous oxide thin film transistors (TFTs) at different channel lengths using a transmission line model. All the contacts showed linear current–voltage characteristics. The effects of different channel lengths (200–800 μm, step 200 μm) and the contact resistance on the performance of TFT devices are discussed in this work. The Al/ITZO TFT samples with the channel length of 200 μm showed metallic behavior with a linear drain current-gate voltage (I_D–V_G) curve due to the formation of a conducting channel layer. The specific contact resistance (ρ_C) at the source or drain contact decreases as the gate voltage is increased from 0 to 10 V. The devices fabricated with Ni S/D electrodes show the best TFT characteristics such as highest field effect mobility (16.09 cm²/V·s), ON/OFF current ratio (3.27×10⁶), lowest sub-threshold slope (0.10 V/dec) and specific contact resistance (8.62 Ω·cm² at V_G=0 V). This is found that the interfacial reaction between Al and a-ITZO semiconducting layer lead to the negative shift of threshold voltage. There is a trend that the specific contact resistance decreases with increasing the work function of S/D electrode. This result can be partially ascribed to better band alignment in the Ni/ITZO interface due to the work function of Ni (5.04–5.35 eV) and ITZO (5.00–6.10 eV) being somewhat similar.     

Contact resistivity of amorphous and crystalline GeCu2Te3 to W electrode for phase change random access memory

We have investigated the contact resistivity of GeCu₂Te₃ (GCT) phase change material to a W electrode using the circular transfer length method (CTLM). The contact resistivity ρ_c of as-deposited amorphous GCT to W was 3.9×10⁻² Ω cm². The value of ρ_c drastically decreased upon crystallization and crystalline GCT that annealed at 300 °C showed a ρ_c of 4.8×10⁻⁶ Ω cm². The ρ_c contrast between amorphous (as-deposited) and crystalline (annealed at 300 °C) states was larger in GCT than in conventional Ge₂Sb₂Te₅ (GST). Consequently, it was suggested from a calculation based on a simple vertical structure memory cell model that a GCT memory cell shows a four times larger resistance contrast than a GST memory cell.     

Interface effect in pentacene field-effect transistors from high energy proton beam irradiation

We report the effect of irradiation using 10 MeV high energy proton beams on pentacene organic field-effect transistors (OFETs). The electrical characteristics of the pentacene OFETs were measured before and after proton beam irradiation with fluence (dose) conditions of 10¹², 10¹³, and 10¹⁴ cm⁻². After proton beam irradiation with fluences of 10¹² or 10¹³ cm⁻², the threshold voltage of the OFET devices shifted to the positive gate voltage direction with an increase in the current level and mobility. In contrast, for a high proton beam fluence condition of 10¹⁴ cm⁻², the threshold voltage shifted to the negative gate voltage direction with a decrease in the current level and mobility. It is evident from the electrical characteristics of the pentacene OFETs treated with a self-assembled monolayer that these experimental observations can be attributed to the trapped charges in the dielectric layer and pentacene/SiO₂ interface. Our study will enhance the understanding of the influence of high energy particles on organic field-effect transistors.