Modulation of surface solubility and wettability for high-performance inkjet-printed organic transistors

The surface solubility and wettability of photosensitive layers of polystyrene (PS) were engineered to evaluate its effect on the crystalline microstructure and film morphology of inkjet-printed 6,13-bis(triisopropylsiylethynyl) pentacene (TIPS-pentacene). UV exposure proved to be a simple and effective method for modulating the solubility of PS films with controllable crosslinking. As compared with the untreated PS film, the film morphology of the printed semiconductor on the UV-irradiated crosslinked PS films was significantly optimized. The optimal degree of crosslinking and solubility of the PS film were achieved by UV irradiation at a dose of 0.417 J cm⁻². Field-effect transistors fabricated using well-organized crystals on the optimal crosslinked PS film exhibited a maximum mobility of 0.48 cm² V⁻¹ s⁻¹ and an average value of 0.19 cm² V⁻¹ s⁻¹. The performance is clearly superior to that of devices prepared on a pristine PS film (0.02 cm² V⁻¹ s⁻¹).     

Self-assembled monolayer modification of silver source-drain electrodes for high-performance pentacene organic field-effect transistors

We demonstrated excellent performance improvement of bottom-contact pentacene-based organic thin film transistors (OTFTs) fabricated at room-temperature with silver electrodes modified by self-assembled monolayers (SAMs) of binary mixtures of n-alkanethiol (n-decanethiol, HDT) and the fluorinated analog (3,3,4,4,5,5,6,6,7,7,8,8,9,9,10,10,10-heptadecafluoro-1-decanethiol, FDT). The OTFTs with modified silver (Ag) electrodes exhibit carrier mobility of 0.21 cm²/V s, which is faster than most of bottom-contact pentacene-based OTFTs fabricated at room-temperature with gold (Au) electrodes. The threshold voltage is reduced from −30 V of the devices with Au electrodes to −5.4 V of the devices with modified Ag electrodes. The hole injection barrier is also reduced with modified Ag as indicated by ultraviolet photoemission spectroscopy. The enhancement of the saturation current and the mobility of the devices are due to both the reduction of hole injection barriers and the continuous grain size of pentacene on top of electrodes and dielectrics.     

Large scale,flexible organic transistor arrays and circuits based on polyimide materials

Polyimide (PI) materials are lightweight, flexible, resistant strongly to heat and chemicals, and have been widely used in electronics industry such as working as electronic packaging materials in large-scale integrated circuits. In this letter, PI materials, for the first time, are introduced into organic field-effect transistors (OFETs) and circuits as insulator layers in order to be compatible with the photolithography process. Moreover, a novel method is developed to make the PI films strong enough to endure the critical processes of photolithography (e.g., the influence of developer on polyimide layer). Based on the intact PI insulator and the modified photolithographic technique, large scale, flexible transistor arrays and circuits were fabricated with high resolution and high performance (mobility up to 0.55 cm² V⁻¹ s⁻¹ for bottom-contact bottom-gate OFETs). It provides a new way for the fabrication of large-area organic devices and circuits beyond solution printed techniques, especially for the application of organic semiconductors with poor solubility, e.g., pentacene.     

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.     

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.     

A diketopyrrolopyrrole containing molecular semiconductor: Synthesis,characterization and solution-processed 1D-microwire based electronic devices

Highly oriented 1D-microwires of a diketopyrrolopyrrole (DPP) based semiconductor i.e. DPP(CBZ)₂ (Carbazole capped DPP) were synthesized, characterized and applied to the fabrication of organic optoelectronic devices. 1D-microwires of DPP(CBZ)₂ were prepared by solution processing on capillary tubes serving to pin solution. A bottom-gate, top-contact field-effect transistor employing 1D-microwire and polymeric gate dielectric showed a hole mobility of 1.24 × 10⁻² cm²/V s, an on-to-off drain current ratio (I_on/I_off) of 4.7 × 10³ and subthreshold slopes of 4 V/dec under ambient conditions. Under white light, a photosensitivity of 800 at V_G = −40 V and photoresponsivity of 830 mW/A were achieved. This work demonstrates the potential of this new molecule and the solution method for use in various opto-electronic devices such transistors, photosensors and photovoltaics.     

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.     

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.     

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.     

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.     

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.     

Air-stability and bending properties of flexible organic field-effect transistors based on poly[N-9′-heptadecanyl-2,7-carbazole-alt-5,5-(4′,7′-di-2-thienyl-2′,1′,3′-benzothiadiazole)]

We have fabricated flexible field-effect transistors (FETs) using poly[N-9′-heptadecanyl-2,7-carbazole-alt-5,5-(4′,7′-di-2-thienyl-2′,1′,3′-benzothiadiazole)], PCDTBT, as an active channel, poly(methyl methacrylate) (PMMA) as gate dielectric and biaxially oriented polyethyleneterephthalate (BOPET) as supporting substrate. The output and transfer characteristics of the devices were measured as a function of channel length. It has been observed that various OFET parameters viz. on–off ratio (∼10⁵), mobility (μ ∼ 10⁻⁴ cm² V⁻¹ s⁻¹), threshold voltage (V_th ∼ −14 V), switch-on voltage (V_so ∼ −6 V), subthreshold slope (S ∼ 7 V/decade) and trap density (N_it ∼ 10¹⁴ cm⁻² V⁻¹) are almost independent of the channel length, which suggested a very high uniformity of the PCDTBT active layer. These devices were highly stable under atmospheric conditions (temperature: 20–35 °C and relative humidity: 70–85%), as no change in mobility was observed on a continuous exposure for 70 days. The studies on the effect of strain on mobility revealed that devices are stable up to a compressive or tensile strain of 1.2%. These results indicate that PCDTBT is a very promising active layer for the air stable and flexible FETs.     

Electrical responses of short-channel organic transistor prepared by solution-processed organic crystal wire mask

We report a formation of a solution-grown single crystal wire mask for the fabrication of short-channel organic field-effect transistor with enhanced dynamic response time. The various channel length, ranging from submicrometer to a few micrometers, were obtained by controlling the concentration of solution and processing conditions. We fabricated p- and n-channel bottom-contact organic field-effect transistors using pentacene and PTCDI-C₁₃, respectively, and static and dynamic electrical characteristics of the devices were investigated. The highest and average field-effect hole mobility values were found to be 0.892 cm²/V s and 0.192 cm²/V s, respectively. The load type inverter based on the short-channel transistor connected with a 2 MΩ resistor showed a clear switching response when square wave input signals up to 1 kHz were applied at V_DD = −60 V.     

Dibenzothiophene-S,S-dioxide based medium-band-gap polymers for efficient bulk heterojunction solar cells

Medium-band-gap polymers based on indacenodithiophene (IDT) and dibenzothiophene-S,S-dioxide (SO) derivatives, PIDT-SO and PIDT-DHTSO, were synthesized via a microwave assisted Stille polycondensation. The polymers have the maximum absorption ∼500 nm, high absorption coefficients above 0.6 × 10⁻² nm⁻¹, and medium band gaps of ∼2.2 eV. Their hole mobilities are around 2 × 10⁻⁴ cm² V⁻¹ s⁻¹ as measured by field effect transistors. The photovoltaic performances of the polymers were investigated on the inverted bulk heterojunction (BHJ) devices of ITO/PFN/PIDT-DHTSO:PC₇₁BM (1:3, w/w)/MoO₃/Al, and a power conversion efficiency (PCE) of 3.81% with an open-circuit voltage (V_oc) of 0.95 V, a short-circuit current (J_sc) of 8.20 mA cm⁻² and a fill factor (FF) of 48% were achieved. Those results indicated that dibenzothiophene-S,S-dioxide derivatives could be an excellent electron-deficient building block for medium-band-gap electron-donor polymers.     

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.     

Mobility enhancement by using silk fibroin in F16CuPc organic thin film transistors

High performance n-type F₁₆CuPc organic thin-film transistors (OTFTs) were fabricated on polyethylene terephthalate (PET) using silk fibroin as the gate dielectric. The average field-effect mobility (μ_FE) value in the saturation regime is 0.39 cm² V⁻¹ s⁻¹ approximately one order of magnitude higher than the reported values in the literature. A typical F₁₆CuPc OTFT exhibits an on/off current ratio of 9.3 × 10², a low threshold voltage of 0.65 V, and a subthreshold swing value of 730 mV/decade. The enhancement of μ_FE results from very good crystal quality of F₁₆CuPc on silk fibroin, supported by grazing incidence X-ray diffraction (GIXD) data.     

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.     

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.     

Cyclopentadithiophene–benzothiadiazole oligomers: Synthesis via direct arylation,X-ray crystallography,optical properties,solution casted field-effect transistor and photovoltaic characteristics

This article reports the synthesis, crystallographic structure and OFET and OPV performance of the conjugated oligomer of cyclopentadithiophene (CPDT) with benzothiadiazole (BT). Synthesis of the oligomer composed of the CPDT-BT-CPDT sequence is accomplished using direct arylation reactions. Theoretical and experimental X-ray single crystallography confirms that two CPDT-BT-CPDT molecules are not entirely disordered, but are actually stacking directly across each other at the central BT units with an intermolecular distance of 3.61 Å, providing valuable insight into the polymer bulk structure. The performance of the oligomer in OFET devices is investigated by fabricating bottom gate top contact devices and demonstrates a hole mobility of 5.0 × 10⁻³ cm² V⁻¹ s⁻¹. OPV devices of the oligomer blended with PC₆₁BM and PC₇₁BM show power conversion efficiency (PCE) of 1.61%. One potential use for the oligomer could be as a sensitiser in a ternary blend with P3HT–PC₆₁BM or PCPDTBT–PC₆₁BM OPVs; the PCE can be relatively increased by 3–9% depending on concentration, primarily as a result of increased short circuit current density.     

An easily made thienoacene comprising seven fused rings for ambient-stable organic thin film transistors

A novel easily made thienoacene-based organic semiconductor, i.e., dinaphtho[3,4-d:3′,4′-d′]benzo[1,2-b:4,5-b′]dithiophene (Ph5T2), was synthesized in high yield, and its thermal stability, electrochemical properties, thin-film morphology and field-effect mobility were investigated. Ph5T2 exhibit excellent thermal stability with a decomposition temperature (T_d) of 427 °C. Thin-film X-ray diffraction (XRD) and atomic force microscopy (AFM) characterizations indicate that Ph5T2 can form highly ordered films with large domain size on the para-sexiphenyl (6P)-modified substrates. Organic thin-film transistors (OTFTs) with top-contact geometry based on Ph5T2 exhibit mobilities up to 1.2 cm² V⁻¹ s⁻¹ in ambient. The devices are highly stable and exhibit almost no performance degradation during 3 months storage under ambient conditions with relative humidity up to 80%.