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.     

Gate-planarized organic polymer thin film transistors

We report in this paper the fabrication and characterization of a new gate-planarized organic polymer thin-film transistor (GP OP-TFT). We describe in detail the effects of the measurement procedure on the GP OP-TFT electrical characteristics and extracted parameters and show that it is extremely critical to carefully control the electrical measurement conditions to obtain accurate and meaningful results, before any material optimization is undertaken. We also describe the importance of normalization of electrical characteristics and extracted parameters for a proper comparison of different devices. Finally, we report and analyze the gate voltage and channel length dependence of the TFT field-effect mobility.     

Photo-induced negative differential resistance of organic thin film transistors using anthracene derivatives

We fabricated organic thin film transistors (OTFTs) using soluble 5,5′-(2,6-Bis((4-hexylphenyl)ethynyl)anthracene-9,10-diyl)bis(ethyne-2,1-diyl)bis(2-hexylthieno[3,2-b]thiophene (HTT-ant-THB) as an active layer. We studied the photo-responsive and the gate field-dependent charge transport characteristics of the HTT-ant-THB-based OTFTs. When light (λ_ex = 505 nm) was irradiated on the OTFTs, negative differential resistance (NDR) behavior (i.e., negative slope of the current versus voltage curve) was observed in the reverse bias region of the source-drain current versus voltage characteristics. The NDR effect observed in this study is unique and is controlled by the wavelength and power of the incident light. The current hysteresis and NDR characteristics can be explained in terms of the trapping and releasing mechanism of the mobile charges at the interface between the electrodes and the organic layer. In addition, the NDR effect in the device disappeared on applying negative gate bias.     

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.     

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.     

Effect of selenophene in naphthalene-diimide-vinylene-based small molecules on n-type organic field-effect transistors

Two naphthalene diimide (NDI)-based small molecules, one with a thiophene-vinylene-thiophene linker donor unit (N-TVT-N) and the other with a selenophene-vinylene-selenophene linker donor unit (N-SVS-N), were newly synthesized for the purpose of serving as the active materials of n-type organic field-effect transistors (OFETs). We investigated the various characteristics of the synthesized small molecules to study how the type of donor unit would affect the charge transport of the resulting thin film. The monomeric molecular structure of N-SVS-N, i.e., that with selenium (Se) substituted for sulfur, was found to be tilted about twice as much as that of N-TVT-N, and to display lower backbone planarity. Unexpectedly, the thin films of N-SVS-N each showed a smoother and more uniform surface morphology and predominantly edge-on orientation in comparison with those of N-TVT-N. As the result, the optimally annealed OFETs containing N-SVS-N and N-TVT-N exhibited electron mobilities of up to 0.016 cm² V⁻¹ s⁻¹ and 6.6 × 10⁻³ cm² V⁻¹ s⁻¹, respectively. These results could be explained by the structural features of N-SVS-N facilitating interactions between the electron-rich selenophenes.     

Molecular-scale charge trap medium for organic non-volatile memory transistors

In this work, we introduce a molecular-scale charge trap medium for an organic non-volatile memory transistor (ONVMTs). We use two different types of small molecules, 2,3,6,7,10,11-hexahydroxytriphenylene (HHTP) and 2,3,6,7,10,11-hexamethoxytriphenylene (HMTP), which have the same triphenylene cores with either hydroxyl or methoxy end groups. The thickness of the small molecule charge trap layer was sophisticatedly controlled using the thermal evaporation method. X-ray photoelectron spectroscopy (XPS) and Fourier transform infrared (FTIR) analysis revealed that there were negligible differences in the chemical structures of both small molecules before and after thermal deposition process. The ONVMTs with a 1-nm-thick HHTP charge trap layer showed a large hysteresis window, approximately 20 V, under a double sweep of the gate bias between 40 V and −40 V. The HMTP-based structure showed a negligible memory window, which implied that the hydroxyl groups affected hysteresis. The number of trapped charges on the HHTP charge trap layer was measured to be 4.21 × 10¹² cm⁻². By varying the thickness of the molecular-scale charge trap medium, it was determined that the most efficient charge trapping thickness of HHTP charge trap layer was approximately 5 nm.     

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.     

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.     

Non-functionalized soluble diketopyrrolopyrrole: Simplest p-channel core for organic field-effect transistors

We report the synthesis, characterization and behavior in field-effect transistors of non-functionalized soluble diketopyrrolopyrrole (DPP) core with only a solubilizing alkyl chain (i.e. –C₁₆H₃₃ or –C₁₈H₃₇) as the simplest p-channel semiconductor. The characteristics were evaluated by UV–vis and fluorescence spectroscopy, X-ray diffraction, cyclic voltammetry (CV), thermal analysis, atomic force microscopy (AFM) and density functional theory (DFT) calculation. For top-contact field-effect transistors, two types of active layers were prepared either by a solution process (as a 1D-microwire) or thermal vacuum deposition (as a thin-film) on a cross-linked poly(4-vinylphenol) gate dielectric. All the devices showed typical p-channel behavior with dominant hole transports. The device made with 1D-microwiress of DPP-R18 showed field-effect mobility in the saturation region of 1.42 × 10^?2 cm²/V s with I_ON/I_OFF of 1.82 × 10³. These findings suggest that the non-functionalized soluble DPP core itself without any further functionalization could also be used as a p-channel semiconductor for low-cost organic electronic devices.     

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.     

High-resolution patterned nanoparticulate Ag electrodes toward all printed organic thin film transistors

High-resolution patterned nanoparticulate Ag electrode arrays and all printed organic thin film transistors (OTFTs) were demonstrated using a simple dip-casting and a photoresist-free, non-relief-pattern lithographic process. An octadecyltrichlorosilane self-assembled monolayer was deposited to provide low surface energy and patterned by deep ultraviolet light, resulting in reproducible periodic arrays of patterned hydrophilic domains separated from hydrophobic surroundings. Using a simple dip-casting with optimal withdrawal speed, viscosity, and solvent polarity, dot size and electrode width of less than 1 μm and 5 μm were obtained, respectively. All printed OTFTs were fabricated. Ink-jet printed 6,13-bis(triisopropyl-silylethynyl) pentacene OTFTs including high-resolution patterned nanoparticulate Ag source/drain electrodes (L < 10 μm) have shown similar performance to the OTFTs with photolithographically patterned electrodes.     

A tunable organic inverter based on groove patterned pentacene thin film transistors using soft-contact lamination

Organic zero drive load inverters based on pentacene thin film transistors with periodic groove patterned dielectrics are fabricated using nanoimprinting and soft-contact lamination methods. Depletion mode transistor behavior is achieved when the current direction is parallel to groove direction and enhancement mode transistor behavior is achieved when the directions are crossed. An organic inverter is created after connecting two soft-contact laminated transistors. The electrical performance of the drive transistor can be varied and the organic inverter is tunable. This is done by utilizing a PDMS stamp with the source-drain electrode and changing the angle between the current direction and groove direction. The gain and symmetry of the VTC is improved by using an enhancement mode transistor where the source-drain electrode formed by thermal evaporation instead of being a soft contact-laminated device.     

Field-effect transistors based on organic single crystals grown by an improved vapor phase method

High-quality organic single crystals are produced directly onto the substrates using an improved vapor phase method. Unlike the conventional vapor phase methods, the present method is characterized by forming a large-sized crystal to which semiconductor devices can readily be made. The relevant method requires small space of only a 10-cm cube in which a couple of plates are put in close proximity. The crystal growth is carried out nearly at the thermodynamic equilibrium within the narrow space surrounded with the two plates. Thin single crystals of several hundreds of micrometers in size are grown on one of those plates. For the organic materials to be crystallized, we have chosen 1,4-bis(5-phenylthiophen-2-yl)benzene (AC5) and 5,5-diphenyl-2,2′:5′,2″:5″,2:5,2-quinquethiophene (P5T) from among thiophene/phenylene co-oligomers. The resulting crystals are well-defined polygons, each side reflecting the specific crystallographic orientation. In particular, those grown on self-assembled monolayers are exceedingly flat and free from cracks. We have directly fabricated top-contact field-effect transistors on these crystals. The devices exhibit the excellent performance and keep it both in air and in vacuum for a maximum of a hundred days.     

Solution processable bilayered gate dielectric towards flexible organic thin film transistors

In this study, we have successfully explored the potential of a new bilayer gate dielectric material, composed of Polystyrene (PS), Pluronic P123 Block Copolymer Surfactant (P123) composite thin film and Polyacrylonitrile (PAN) through fabrication of metal insulator metal (MIM) capacitor devices and organic thin film transistors (OTFTs). The conditions for fabrication of PAN and PS-P123 as a bilayer dielectric material are optimized before employing it further as a gate dielectric in OTFTs. Simple solution processable techniques are applied to deposit PAN and PS-P123 as a bilayer dielectric layer on Polyimide (PI) substrates. Contact angle study is further performed to explore the surface property of this bilayer polymer gate dielectric material. This new bilayer dielectric having a k value of 3.7 intermediate to that of PS-P123 composite thin film dielectric (k ∼ 2.8) and PAN dielectric (k ∼ 5.5) has successfully acted as a buffer layer by preventing the direct contact between the organic semiconducting layer and high k PAN dielectric. The OTFT devices based on α,ω-dihexylquaterthiophene (DH4T) incorporated with this bilayer dielectric, has demonstrated a hole mobility of 1.37 × 10⁻² and on/off current ratio of 10³ which is one of the good values as reported before. Several bending conditions are applied, to explore the charge carrier hopping mechanism involved in deterioration of electrical properties of these OTFTs. Additionally, the electrical performance of OTFTs, which are exposed to open atmosphere for five days, can be interestingly recovered by means of re-baking them respectively at 90 °C.     

Effect of substrate temperature on vapor-phase self-assembly of n-octylphosphonic acid monolayer for low-voltage organic thin-film transistors

N-octylphosphonic acid (C₈PA) monolayer was self-assembled on aluminum oxide (AlO_x) from vapor in vacuum, while the substrate temperature was varied between 25 and 150 °C. The capacitance, water contact angle measurement, Fourier transform infrared (FTIR) spectroscopy, and atomic force microscopy (AFM) confirmed the presence of C₈PA on AlO_x for all growth temperatures. However, the structural and electrical properties of such monolayers depend on their growth temperature. The minimum surface roughness of 0.36 nm, the maximum water contact angle of 113.5° ± 1.4°, the lowest leakage current density of ∼10⁻⁷ A/cm² at 3 V, and the capacitance of 0.43 μF/cm² were obtained for AlO_x/C₈PA bi-layers with C₈PA deposited at 25 °C. The elevated temperature led to increased surface roughness, decreased water contact angle, increased leakage current, inferior molecular ordering, and lower molecular coverage; while the effect on the chemisorption of the phosphonate was minimal. Methyl and methylene FTIR vibrations associated with C₈PA aliphatic tails exhibited similar centre-peak wavenumbers to those observed for C₈PA monolayers assembled from solutions, presenting a viable ‘dry’ alternative to the existing solution process.     

High performance p-type organic thin film transistors with an intrinsically photopatternable,ultrathin polymer dielectric layer

A high-performing bottom-gate top-contact pentacene-based oTFT technology with an ultrathin (25–48 nm) and electrically dense photopatternable polymeric gate dielectric layer is reported. The photosensitive polymer poly((±)endo,exo-bicyclo[2.2.1]hept-5-ene-2,3-dicarboxylic acid, diphenylester) (PNDPE) is patterned directly by UV-exposure (λ = 254 nm) at a dose typical for conventionally used negative photoresists without the need for any additional photoinitiator. The polymer itself undergoes a photo-Fries rearrangement reaction under UV illumination, which is accompanied by a selective cross-linking of the macromolecules, leading to a change in solubility in organic solvents. This crosslinking reaction and the negative photoresist behavior are investigated by means of sol–gel analysis. The resulting transistors show a field-effect mobility up to 0.8 cm² V⁻¹ s⁻¹ at an operation voltage as low as −4.5 V. The ultra-low subthreshold swing in the order of 0.1 V dec⁻¹ as well as the completely hysteresis-free transistor characteristics are indicating a very low interface trap density. It can be shown that the device performance is completely stable upon UV-irradiation and development according to a very robust chemical rearrangement. The excellent interface properties, the high stability and the small thickness make the PNDPE gate dielectric a promising candidate for fast organic electronic circuits.     

High operational stability of solution-processed organic field-effect transistors with top-gate configuration

Electrical characteristics of top-gate field-effect transistors based on a wide range of solution-processed organic semiconductors are systematically investigated. The top-gate field-effect transistors based on different organic semiconductors—from an amorphous polymer semiconductor to a polycrystalline molecular semiconductor—exhibit higher operational stability compared with bottom-gate organic field-effect transistors reported in literature, in spite of significant difference in field-effect mobility. The correlation between charge transport and operational stability is discussed to gain insight into high operational stability of top-gate organic field-effect transistors.     

Electrical characterization of mica as an insulator for organic field-effect transistors

Electrical properties of conjugated polymer films, including poly(3-hexylthiophene)-2,5-diyl (P3HT), poly(3,3-didodecylquarterthiophene) (PQT-12), and poly(triarylamine) (PTAA), on mica substrates have been studied. The test structure was similar to a standard organic field-effect transistor but with a 150-μm-thick commercially available mica gate insulator/substrate, which allowed to obtain a field-effect mobility of P3HT as high as 0.08 cm²/Vs in the linear regime in ambient air. The influence of interface treatment, thermal annealing, and measurement conditions on the electrical properties of the P3HT films has been characterized and analyzed. We also studied the time dependence of the carrier concentration and mobility before and after a thermal annealing process. The results indicate that mica is a promising insulator for organic field-effect transistors, apart from already being one of the common thin-film materials widely used in electric capacitors.     

Improving performance of TIPS pentacene-based organic thin film transistors with small-molecule additives

This work demonstrates an effective approach to improve both charge transport and performance consistency in solution-processed organic thin-film transistors (OTFTs) by blending 6,13-bis(triisopropylsilylethynyl)pentacene (TIPS pentacene, or TP) with a series of small-molecule additives: 4-butylbenzoic acid (BBA), 4-hexylbenzoic acid (HBA), and 4-octylbenzoic acid (OBA). These three small molecules share a benzoic acid moiety, but have different length of hydrophobic tails. The self-assembled interfacial layer of small molecules on the gate oxide surface leads to uniform deposition of TP crystal seeds and facilitates TP to grow along the tilted orientation of substrate, which results in a film of enhanced crystal orientation and areal coverage. OTFTs based on TP/small molecule blends demonstrate greatly improved average hole mobility and performance consistency, which correlates with the length of hydrophobic tail of the small-molecule additives.