Solvent-vapor induced self-assembly of a conjugated polymer: A correlation between solvent nature and transistor performance

A systematical investigation on solvent-vapor annealing in polymer thin film transistors is performed using a thiazolothiazole-bithiazole conjugated polymer as the active layer. Film morphology, packing order and device performance are closely related to polarity and solubility parameter of the annealing solvent and annealing time. The formation of highly ordered and closely connected fibrillar domains is realized by using a solvent with similar solubility parameter and polarity to the conjugated polymer. Field-effect transistors based on pristine polymer films exhibit a highest charge carrier mobility of 0.0067 cm² V⁻¹ s⁻¹. After solvent vapor annealing with THF for 48 h, the mobility boosts up to 0.075 cm² V⁻¹ s⁻¹. This correlation between solvent polarity, solubility parameter and film morphology, packing order and mobility provides a useful guideline towards high performance polymer thin film transistors with solvent-vapor annealing method.     

Interface optimization using diindenoperylene for C60 thin film transistors with high electron mobility and stability

C₆₀-based organic thin film transistors (OTFTs) with high electron mobility and high operational stability are achieved with (1 1 1) oriented C₆₀ films grown by using template effects of diindenoperylene (DIP) under layer on the SiO₂ gate insulator. The electron mobility of the C₆₀ transistor is significantly increased from 0.21 cm² V⁻¹ s⁻¹ to 2.92 cm² V⁻¹ s⁻¹ by inserting the template-DIP layer. Moreover much higher operational stability is also observed for the DIP-template C₆₀ OTFTs. A grazing incidence X-ray diffraction and ultrahigh-sensitivity photoelectron spectroscopy measurements indicate that the improved electron mobility and stability arise from the decreased density of trap states in the C₆₀ film due to increased (1 1 1) orientation of C₆₀-grains and their crystallinity on the DIP template.     

Flexible thin-film transistors using multistep UV nanoimprint lithography

A multistep imprinting process is presented for the fabrication of a bottom-contact, bottom-gate thin-film transistor (TFT) on poly(ethylene naphthalate) (PEN) foil by patterning all layers of the metal–insulator–metal stack by UV nanoimprint lithography (UV NIL). The flexible TFTs were fabricated on a planarization layer, patterned in a novel way by UV NIL, on a foil reversibly glued to a Si carrier. This planarization step enhances the dimensional stability and flatness of the foil and thus results in a thinner and more homogeneous residual layer. The fabricated TFTs have been electrically characterized as demonstrators of the here developed fully UV NIL-based patterning process on PEN foil, and compared to TFTs made on Si with the same process. TFTs with channel lengths from 5 μm down to 250 nm have been fabricated on Si and PEN foil, showing channel length-dependent charge carrier mobilities, μ, in the range of 0.06–0.92 cm² V⁻¹ s⁻¹ on Si and of 0.16–0.56 cm² V⁻¹ s⁻¹ on PEN foil.     

Materials and interfaces issues in pentacene/PTCDI-C8 ambipolar organic field-effect transistors with solution-based gelatin dielectric

Gelatin is a natural protein, which works well as the gate dielectric for pentacene/N,N-dioctyl-3,4,9,10-perylene tetracarboxylic diimide (PTCDI-C₈) ambipolar organic field-effect transistors (OFETs) in air ambient and in vacuum. An aqueous solution process was used to form the gelatin gate dielectric film on poly(ethylene terephthalate) (PET) by spin-coating and subsequent casting. Pentacene morphology and interface roughness are two major factors affecting the electron and hole field-effect mobility (μ_FE) values of pentacene/PTCDI-C₈ ambipolar OFETs in vacuum and in air ambient. In contrast, water absorption in gelatin has higher contribution to the electron and hole μ_FE values in air ambient. The ambipolar performance of pentacene/PTCDI-C₈ ambipolar OFETs depends on their layer sequence. For example, when PTCDI-C₈ is deposited onto pentacene, i.e. in the structure of PTCDI-C₈/pentacene, unbalanced ambipolar characteristics appear. In contrast, better ambipolar performance occurs in the structure of pentacene/PTCDI-C₈. The optimum ambipolar characteristics with electron μ_FE of 0.85 cm² V⁻¹ s⁻¹ and hole μ_FE of 0.95 cm² V⁻¹ s⁻¹ occurs at the condition of pentacene (40 nm)/PTCDI-C₈ (40 nm). Surprisingly, water absorption plays a crucial role in ambipolar performance. The device performance changes tremendously in pentacene/PTCDI-C₈ ambipolar OFETs due to the removal of water out of gelatin in vacuum. The optimum ambipolar characteristics with electron μ_FE of 0.008 cm² V⁻¹ s⁻¹ and hole μ_FE of 0.007 cm² V⁻¹ s⁻¹ occurs at the condition of pentacene (65 nm)/PTCDI-C₈ (40 nm). The roles of layer sequence, relative layer thickness, and water absorption are proposed to explain the ambipolar performance.     

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.     

Bottom-contact small-molecule n-type organic field effect transistors achieved via simultaneous modification of electrode and dielectric surfaces

Low-voltage, n-type organic field effect transistors (OFETs) with simultaneously modified bottom-contact (BC) electrodes and dielectric were compared to their top-contact (TC) counterparts. The devices modified with 6-phenoxyhexylphosphonic acid (Ph6PA) self-assembled monolayer (SAM) showed similar performance, morphology, and contact resistance. Electron mobility of C₆₀ devices were 0.212 and 0.320 cm² V⁻¹ s⁻¹ and [6,6]-phenyl-C₆₁-butyric acid methyl ester (PCBM) devices were 0.04 and 0.06 cm² V⁻¹ s⁻¹ for TC and BC devices, respectively. Low contact resistance between 11 and 45 kΩ cm was found regardless of device architecture or n-type semiconductor used. This work shows it is possible to fabricate solution processable low-voltage bottom-contact devices with performance that is similar or better than their top-contact counterparts without the addition of complex and time-consuming processing steps.     

Electrical characterization of poly(amide-imide) for application in organic field effect devices

The admittance spectra and current–voltage (I–V) characteristics are reported of metal–insulator–metal (MIM) and metal–insulator–semiconductor (MIS) capacitors employing cross-linked poly(amide–imide) (c-PAI) as the insulator and poly(3-hexylthiophene) (P3HT) as the active semiconductor. The capacitance of the MIM devices are constant in the frequency range from 10 Hz to 100 kHz, with tan δ values as low as 7 × 10⁻³ over most of the range. Except at the lowest voltages, the I–V characteristics are well-described by the Schottky equation for thermal emission of electrons from the electrodes into the insulator. The admittance spectra of the MIS devices displayed a classic Maxwell–Wagner frequency response from which the transverse bulk hole mobility was estimated to be ∼2 × 10⁻⁵ cm² V⁻¹s⁻¹ or ∼5 × 10⁻⁸ cm² V⁻¹s⁻¹ depending on whether or not the surface of the insulator had been treated with hexamethyldisilazane (HMDS) prior to deposition of the P3HT. From the maximum loss observed in admittance-voltage plots, the interface trap density was estimated to be ∼5 × 10¹⁰ cm⁻² eV⁻¹ or ∼9 × 10¹⁰ cm⁻² eV⁻¹ again depending whether or not the insulator was treated with HMDS. We conclude, therefore, that HMDS plays a useful role in promoting order in the P3HT film as well as reducing the density of interface trap states. Although interposing the P3HT layer between the insulator and the gold electrode degrades the insulating properties of the c-PAI, nevertheless, they remain sufficiently good for use in organic electronic devices.     

Implication of molecular orientation on charge transport and gas sensing characteristics of cobalt–phthalocyanine thin films

Charge transport and gas sensing characteristics of cobalt phthalocyanine films deposited along (ATB) and perpendicular (PTB) to the natural twin boundaries of (0 0 1) LaAlO₃ substrate have been investigated. The charge carrier mobility of ATB films (∼5 cm² V⁻¹ s⁻¹) is five orders of magnitude higher compared to that of PTB films (∼7 × 10⁻⁵ cm² V⁻¹ s⁻¹), suggesting that twin boundaries acts like a template for ordering of molecules. The ATB films on exposure to ammonia showed a reversible increase of resistance, with fast response and recovery. In contrast PTB films showed same sensitivity, but exhibits base resistance drift along with sluggish response.     

Synthesis,size-dependent optoelectronic and charge transport properties of thieno(bis)imide end-substituted molecular semiconductors

The synthesis of two new thieno(bis)imide (TBI, N) end functionalized oligothiophene semiconductors is reported. In particular, trimer (NT3N) and pentamer (NT5N) have been synthesized and characterized. Two different synthetic approaches for their preparation were tested and compared namely conventional Stille cross coupling and direct arylation reaction via C–H activation. Theoretical calculations, optical and electrochemical characterization allowed us to assess the role of the π-conjugation extent, i.e., of the oligomer size on the optoelectronic properties of these materials. In both TBI ended compounds, due to the strong localization of the LUMO orbital on the TBI unit, the LUMO energy is almost insensitive to the oligomer size, this being crucial for the fine-tailoring of the energy and the distribution of the frontier orbitals. Surprisingly, despite its short size and contrarily to comparable TBI-free analogues, NT3N shows electron charge transport with mobility up to μ_N = 10⁻⁴ cm² V⁻¹ s⁻¹, while increasing the oligomer size to NT5N promotes ambipolar behavior and electroluminescence properties with mobility up to μ_N = 0.14 cm² V⁻¹ s⁻¹ and to μ_P = 10⁻⁵ cm² V⁻¹ s⁻¹.     

Using unsorted single-wall carbon nanotubes to enhance mobility of diketopyrrolopyrrole-quarterthiophene copolymer in thin-film transistors

Organic thin film transistors (OTFTs) were fabricated for the first time using a semiconductor copolymer of diketopyrrolopyrrole-quarterthiophene (DPP-QT) and unsorted single walled carbon nanotubes (SWCNTs). Three different SWCNTs having different tube diameters, length, and shape were used to investigate the effects of carbon nanotubes’ properties on dispersion of the SWCNTs in DPP-QT polymer, as well as the mobility and current on/off ratio of the OTFTs. The DPP-QT polymer was able to selectively disperse two types of SWCNTs. An optimal SWCNT loading was found to be 1.5–2.5 wt% for these SWCNTs, before the on/off ratio fell below 10⁵ due to increased metallic tube content of the film. At this optimal loading, the field effect mobility was improved by a factor of two, with the maximum mobility reaching 1.3 cm² V⁻¹ s⁻¹, when the SWCNTs with a short length and small tube diameter were used.     

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.     

Toward High Mobility Green Solvent-Processable Conjugated Polymers: A Systematic Study on Chalcogen Effect in Poly(Diketopyrrolopyrrole-alt-Terchalcogenophene)s

Nine diketopyrrolopyrrole (DPP)-based conjugated polymers (CPs), that is, poly(diketopyrrolopyrrole-alt-terchalcogenophene)s, via combinations of furanyl-(FDPP), thienyl-(TDPP), selenophenyl-DPP (SeDPP) and furan, thiophene, selenophene comonomers, are synthesized to explore the chalcogen effect on the solubility, film morphology/microstructure, and charge transport property of the resultant polymers. All polymers except for SeDPP-Se are soluble in non-chlorinated solvents such as o-xylene and tetralin. Flanking of DPP with furan in FDPP-F, FDPP-T, and FDPP-Se enables even good solubility in green solvent anisole. TDPP-Se exhibits the highest reliable hole mobility over 10 cm² V⁻¹ s⁻¹ in organic thin film transistors (OTFTs) bar-coated from o-xylene/tetralin (20/80 v/v) solution. With anisole as the processing solvent, FDPP-F-based bar-coated OTFTs displays a reliable hole mobility up to 3.50 cm² V⁻¹ s⁻¹. This is the first report on green solvent processed OTFTs with mobility above 1 cm² V⁻¹ s⁻¹. Charge transport property of all the polymers is correlated with the film morphology and microstructure that are noticeably influenced by the type and position of chalcogenophenes. The current work sheds light on the design of high mobility CPs processable with green solvents.     

BODIPY derivatives as n-type organic semiconductors: Isomer effect on carrier mobility

In the present work, two dipyrro-boradiazaindacenes (BODIPY) derivatives functioning as novel high-performance organic semiconductors are investigated by theoretical method. These two isomeric complexes are demonstrated to have large electron-transfer mobility, which means they are favor to be n-type organic semiconductors. The highest electron-transfer mobility appears at the same packing style in two crystals. The intermolecular distances of the packing style are nearly same, 4.994 Å in crystal 1 and 5.283 Å in crystal 2. However, their electron-transfer mobility changes significantly. The mobility of crystal 2 with better planar molecular structure is 0.291 cm² V⁻¹ s⁻¹, which is 13 times larger than that of crystal 1 as 0.022 cm² V⁻¹ s⁻¹. The significant difference of carrier mobility is ascribed to the little structural difference of these two isomers. It has been demonstrated that both crystal 1 and 2 show remarkable anisotropic behavior. This study will undoubtedly provide a new understanding of isomerization on designing novel organic semiconductors.     

A simple droplet pinning method for polymer film deposition for measuring charge transport in a thin film transistor

Thin-film field-effect transistors (FETs) are widely used to evaluate charge transport properties of semiconducting polymers. Discovery of high performance materials require design and synthesis of new polymers. However, most polymers require multi-step synthesis and are difficult to be obtained in a large scale for comprehensive device evaluations. Here, we report a simple method to cast semiconducting polymer films from solutions with polymer concentration as low as 0.5 mg/mL, which is substantially less than typical values (∼10 mg/mL) used in conventional spin coating method. Here, we demonstrate that using this method, our cast films of a previously-reported polymer (PDPP-TT2T) exhibited field-effect mobility (μ_hole = 0.89 ± 0.13 cm² V⁻¹ s⁻¹, μ_e = 0.025 ± 0.005 cm² V⁻¹ s⁻¹), which is comparable to the reported values using the same device geometry. Furthermore, we extend this method to examine cast films of a pair of polymers (PDPP-3T-Ref, PDPP-3T-Si) to study the effect of siloxane substitution in the side chains on the molecular packing and their subsequent FET performance. We observed that shorter π-stacking distance (3.61 Å) for the siloxane-terminated polymer, when compared to that for the reference polymer (3.73 Å), resulted in improved FET performance (e.g., μ_hole = 0.63 ± 0.046 cm² V⁻¹ s⁻¹ for PDPP-3T-Si vs μ_hole = 0.17 ± 0.062 cm² V⁻¹ s⁻¹ for PDPP-3T-Ref). Taken together, this work presents an efficient alternative film-casting approach to produce polymer FETs that consumes much less material for their fabrication, lending viability for evaluation of various polymeric materials.     

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.     

A new diketopyrrolopyrrole-based co-polymer for ambipolar field-effect transistors and solar cells

A new thieno[3,2-b]thiophenediketopyrrolopyrrole-benzo[1,2-b:4,5-b′]dithiophene based narrow optical gap co-polymer (PTTDPP-BDT) has been synthesized and characterized for field-effect transistors and solar cells. In field-effect transistors the polymer exhibited ambipolar charge transport behaviour with maximum hole and electron mobilities of 10⁻³ cm² V⁻¹ s⁻¹ and 10⁻⁵ cm² V⁻¹ s⁻¹, respectively. The respectable charge transporting properties of the polymer were consistent with X-ray diffraction measurements that showed close molecular packing in the solid state. The difference in hole and electron mobilities was explained by density functional theory calculations, which showed that the highest occupied molecular orbital was delocalized along the polymer backbone with the lowest unoccupied molecular orbital localized on the bis(thieno[3,2-b]thiophene)diketopyrrolopyrrole units. Bulk heterojunction photovoltaic devices with the fullerene acceptor PC₇₀BM were fabricated and delivered a maximum conversion efficiency of 3.3% under AM1.5G illumination.     

Laser-assisted patterning of solution-processed oxide semiconductor thin film using a metal absorption layer

We present a method to pattern solution-processed oxide semiconductor thin films by all laser process. A metal thin film is first photoetched by a spatially-modulated pulsed Nd-YAG laser beam and this layer is then covered with a semiconductor film. Uniform irradiation by the same laser generates a thermo-elastic force on the underlying metal layer and this force serves to detach it from the substrate, leaving only a patterned semiconductor structure. Sharp-edged zinc-tin oxide (ZTO) patterns at the micrometer scales could be fabricated over a few square centimeters by a single pulse of 850 mJ. A mobility of 7.6 × 10⁻² cm² V⁻¹ s⁻¹, an on/off ratio higher than 10⁶, and an off-current of 1.91 × 10⁻¹¹ A were achieved from a thin film transistor (TFT) with the patterned ZTO channel. These values were similar to those from a reference TFT, demonstrating the feasibility of this patterning process for electronic devices.     

Hydrated bovine serum albumin as the gate dielectric material for organic field-effect transistors

Bovine serum albumin (BSA) is a natural protein with good hydration ability which contains acidic and basic amino acid residues of ca. 34% in total. In vacuum, pentacene organic field-effect transistors (OFETs) with BSA as the gate dielectric exhibits a field-effect mobility value (μ_FE,sat) of 0.3 cm² V⁻¹ s⁻¹ in the saturation regime and a threshold voltage (V_TH) of ca. −16 V. BSA is easy to be hydrated in air ambient. Electrical properties of BSA in vacuum and hydrated BSA in air ambient are characterized. Similar to polyelectrolyte, hydrated BSA may act the gate dielectric with the formation of electric double-layer capacitors (EDLCs) to improve the device performance. In a relative humidity of 47%, the μ_FE,sat value increases to 4.7 cm² V⁻¹ s⁻¹ and the V_TH reduces to −0.7 V. Generation of mobile ions in hydrated BSA and the formation of EDLCs are discussed.     

Downscaling of n-channel organic field-effect transistors with inkjet-printed electrodes

In this contribution we demonstrate for the first time a downscaled n-channel organic field-effect transistors based on N,N′-dialkylsubstituted-(1,7&1,6)-dicyanoperylene-3,4:9,10-bis(dicarboximide) with inkjet printed electrodes. First we demonstrate that the use of a high boiling point solvent is critical to achieve extended crystalline domains in spin-coated thin films and thus high electron mobility >0.1 cm² V⁻¹ s⁻¹ in top-gate devices. Then inkjet-printing is employed to realize sub-micrometer scale channels by dewetting of silver nanoparticles off a first patterned gold contact. By employing a 50 nm crosslinked fluoropolymer gate dielectric, ∼200 nm long channel transistors can achieve good current saturation when operated <5 V with good bias stress stability.     

The influence of assisted ion beam bombardment on structure and electrical characteristics of HfSiO dielectric synthesized by DIBSD

We have investigated the influence of assisted ion beam bombardment on structure and electrical properties of HfSiO dielectrics deposited on Si (1 0 0) substrate by dual-ion beam sputtering deposition (DIBSD). The X-ray photoelectron spectroscopy (XPS) analysis indicates that assisted ion beam bombardment could suppress the formation of Si clusters and partial SiO bonds. The excellent electrical properties with maximum dielectric constant (18.6) and the smaller oxide-charge density (7.2 × 10¹¹ cm⁻²) and leakage current (2.8 × 10⁻⁷ A/cm² at (V_fb−1) V) were obtained for HfSiO film by assisted ion beam bombardment at AIE = 100 eV, which provide a initial energy for the formation of film, activate the substrate surface atoms, enhance the polarization rate and improve the film surface compact and adhesion.