Liquid–crystalline perylene tetracarboxylic acid bisimide bearing oligosiloxane chains with high electron mobility and solubility

Liquid–crystalline perylene tetracarboxylic acid bisimide derivatives 1 and 2 bearing 1,1,1,2,2,3,3-heptamethyltrisiloxane chains at the end of alkyl side-chains have been synthesized. Compound 1 having four trisiloxane chains exhibits a hexagonal columnar phase at room temperature. Time-of-flight measurement reveals that the electron mobility in the columnar phase of compound 1 exceeds 10^?3 cm² V^?1 s^?1 at room temperature. In contrast, compound 2 bearing two trisiloxane chains is not mesomorphic. These two compounds are soluble in various organic solvents except for alcohols and thin films can be produced by a spin-coating method.     

Charge transport properties and microstructure of polythiophene/polyfluorene blends

We present a combined charge transport and X-ray diffraction study of blends based on regioregular poly(3-hexylthiophene) (P3HT) and the polyfluorene co-polymer poly((9,9-dioctylfluorene)-2,7-diyl-alt-[4,7-bis(3-hexylthien-5-yl)-2,1,3-benzothiadiazole]-2′,2′′-diyl) (F8TBT) that are used in efficient all-polymer solar cells. Hole mobility is observed to increase by nearly two orders of magnitude from less than 10^?7 cm² V^?1 s^?1 for as spin-coated blends to 6 × 10^?6 cm² V^?1 s^?1 for blends annealed at 453 K at a field of 2.7 × 10⁵ V/cm, but still significantly below the time-of-flight mobility of unblended P3HT of 1.7 × 10^?4 cm² V^?1 s^?1. The hole mobility of the blends also show a strong negative electric-field dependence, compared with a relatively flat electric-field dependence of unblended P3HT, suggestive of increased spatial disorder in the blends. X-ray diffraction measurements reveal that P3HT/F8TBT blends show a phase separation of the two components with a crystalline part attributed to P3HT and an amorphous part attributed to F8TBT. In as-spun and mildly annealed blends, the measured d-values and relative intensities of the 100, 200 and 300 P3HT peaks are noticeably different to unblended P3HT indicating an incorporation of F8TBT in P3HT crystallites that distorts the crystal structure. At higher anneal temperatures the blend d-values approach that of unblended P3HT suggesting a well separated blend with pure P3HT crystallites. P3HT crystallite size in the blend is also observed to increase with annealing from 3.3 to 6.1 nm, however similar changes in crystallite size are observed in unblended P3HT films with annealing. The lower mobility of P3HT/F8TBT blends is attributed not only to increased P3HT structural disorder in the blend, but also due to the blend morphology (increased spatial disorder). Changes in hole mobility with annealing are interpreted in terms of the need to form percolation networks of P3HT crystallites within an F8TBT matrix, with a possible contribution due to the intercalation of F8TBT in P3HT crystallites acting as defects in the as-prepared state.     

Comparative study of soluble naphthalene diimide derivatives bearing long alkyl chains as n-type organic thin-film transistor materials

In this study, several naphthalene tetracarboxylic acid diimide (NTCDI) derivatives substituted at the N and N′ positions with long normal alkyl chains of different lengths were evaluated as soluble n-type organic thin-film transistor (TFT) materials. NTCDI derivatives with diundecyl (NTCDI-C11), didodecyl (NTCDI-C12), and ditridecyl (NTCDI-C13) exhibited acceptable solubility in chloroform, and their TFTs showed typical n-type TFT performance with relatively high field effect electron mobility (～0.2 cm²/V s) after annealing at a workable temperature of 150 °C. Although NTCDI with dioctyl (NTCDI-C8) showed good solubility in chloroform, the TFT performance of this material was highly inferior to that of NTCDI-C11, NTCDI-C12, or NTCDI-C13. We could not anneal NTCDI-C8 thin films at workable temperatures in vacuo because of sublimation of the material from the substrates. In contrast, NTCDI with dipentadecyl (NTCDI-C15) and dioctadecyl (NTCDI-C18) exhibited both poor solubility for chloroform and poor TFT performance. In short, these compounds are not suitable as soluble n-type organic TFT materials.     

Solution processable poly(2,5-dialkyl-2,5-dihydro-3,6-di-2-thienyl-pyrrolo[3,4-c]pyrrole-1,4-dione) for ambipolar organic thin film transistors

Solution processable diketopyrrolopyrrole (DPP)-bithiophene polymers (PDBT) with long branched alkyl side chains on the DPP unit are synthesized. These polymers have favourable highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) energy levels for the injection and transport of both holes and electrons. Organic thin film transistors (OTFTs) using these polymers as semiconductors and gold as source/drain electrodes show typical ambipolar characteristics with very well balanced high hole and electron mobilities (μ_h = 0.024 cm² V^?1 s^?1 and μ_e = 0.056 cm² V^?1 s^?1). These simple and high-performing polymers are promising materials for ambipolar organic thin film transistors for low-cost CMOS-like logic circuits.     

Side chains contributions to characteristics of resistive memory based on water-soluble polyfluorenes: Effects of structure and length of side pendant group

This paper investigates the effects of side chains, which are important structural constituents, on the characteristics of organic resistive memory devices with water-soluble polyfluorene (WPF) derivatives. The WPF derivatives have either an ethylene oxide (EO) or an alkyl side chain the lengths of the EO side chains are 2, 4, or 6 molecules. WPFs exhibit typical bipolar switching behaviors with reliable non-volatile characteristics and good device-to-device uniformity under ambient conditions. WPFs with the EO side chains showed better memory characteristics than those of the alkyl side chains of similar length. In addition, as the EO unit lengthened, the ON/OFF ratio of the memory device gradually increased from 5 × 10² to 10⁵, and the threshold voltage (V_th) progressively decreased from 4 to 3.5 V. The retention times for WPF-hexyl, WPF-2O, WPF-4O, and WPF-6O are 10⁴, 200, 10⁴, and 10⁴ s, respectively. The excellent switching properties of WPF-4O and WPF-6O are believed to be mainly attributed to highly localized current pathways and the low trap density.     

Dependence of annealing on stability of transparent amorphous InGaZnO thin film transistor

Bottom-gate transparent IGZO–TFT had been successfully fabricated at relatively low temperature (200 °C). The devices annealing for 4 h at 200 °C exhibit good electrical properties with saturation mobility of 8.2 cm²V^?1s^?1, subthreshold swing of 1.0 V/dec and on/off current ratio of 5×10⁶. The results revealed that the stability of TFT devices can be improved remarkably by post-annealing treatment. After applying positive gate bias stress of 20 V for 5000 s, the device annealing for 1 h shows a larger positive V_th shift of 4.7 V. However, the device annealing for 4 h exhibits a much smaller V_th shift of 0.04 V and more stable.     

Synthesis and characterization of the conjugated polymers tethered with dipolar side chains containing a benzothiadiazole entity for bulk heterojunction solar cells

New conjugated copolymers (P1?P3) containing dipolar side chains connected to the main chain via triphenylamine donors have been synthesized and characterized. The side chains of these polymers have an electron deficient benzothiadiazole moiety in the spacer, but with different acceptors at the end. By changing the acceptor moieties of the side chain, the absorption spectra and HOMO/LUMO gaps of the polymers can be fine-tuned, ranging from 1.86 to 1.59 eV. Solution processed bulk heterojunction (BHJ) solar cells using these polymers as the donor and [6,6]-phenyl-C₆₁-butyric acid methyl ester (PCBM) as the acceptor were fabricated and measured under 100 mW cm^?2 of AM 1.5 illumination. The cell based on the blend of P1/PCBM (1:1, w/w) exhibited the highest power conversion efficiency of 1.78%, with open circuit voltage (V_oc) = 0.79 V, short circuit current (J_sc) = 6.63 mA cm^?2 and fill factor (FF) = 0.34, respectively.     

High electron mobility triazine for lower driving voltage and higher efficiency organic light emitting devices

The triazine compound 4,4′-bis-[2-(4,6-diphenyl-1,3,5-triazinyl)]-1,1′-biphenyl (BTB) was developed for use as an electron transport material in organic light emitting devices (OLEDs). The material demonstrates an electron mobility of ∼7.2 × 10⁻⁴ cm² V⁻¹ s⁻¹ at a field of 8.00 × 10⁵ V cm⁻¹, which is 10-fold greater than that of the widely used material tris(8-hydroxyquinoline) aluminum (AlQ₃). OLEDs with a BTB electron transport layer showed a ∼1.7–2.5 V lower driving voltage and a significantly increased efficiency, compared to those with AlQ₃. These results suggest that BTB has a strong potential for use as an OLED electron transport layer material.     

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.     

High performance inkjet-printed C60 fullerene thin-film transistors: Toward a low-cost and reproducible solution process

We have demonstrated high performance inkjet-printed n-channel thin-film transistors (TFTs) using C₆₀ fullerene as a channel material. Highly uniform amorphous C₆₀ thin-film patterns were fabricated on a solution-wettable polymer gate dielectric layer by inkjet-printing and vacuum drying process. Fabricated C₆₀ TFTs shows great reproducibility and high performance; field-effect mobilities of 2.2–2.4 cm² V^?1 s^?1, threshold voltages of 0.4–0.6 V, subthreshold slopes of 0.11–0.16 V dec^?1 and current on/off ratio of 10⁷–10⁸ in a driving voltage of 5 V. This is due to the efficient annealing process that extracting the solvent residue and the formation of low trap-density gate dielectric surface.     

Solution-processed single-crystalline organic transistors on patterned ultrathin gate insulators

Single-crystalline organic transistors of 3,11-didecyl-dinaphtho[2,3-d:2′,3′-d′]benzo[1,2-b:4,5-b′]dithiophene (C₁₀-DNBDT-NW) and 2,9-didecyl-dinaphtho[2,3-b:2′,3′-f]thieno[3,2-b]thiophene (C₁₀-DNTT) were fabricated by solution processes on top of the patterned hybrid ultrathin gate dielectrics consisting of 3.6 nm-thick aluminum oxide and self-assembled monolayers (SAMs). Due to the excellent crystallinity of the channel films, bottom-gate and top-contact field-effect transistors exhibited the average field-effect mobility of 3.7 cm²/V s and 4.3 cm²/V s for C₁₀-DNBDT-NW and C₁₀-DNTT, respectively. These are the first successful devices of solution-processed single-crystalline transistors on ultrathin gate dielectrics with the mobility above 1 cm²/V s, opening the way to develop low-power-consumption and high-performance printed circuits.     

Germanium on sapphire by wafer bonding

This paper describes the creation of a germanium on sapphire platform, via wafer bonding technology, for system-on-a-chip applications. Similar thermal coefficients of expansion between germanium (5.8 × 10^?6 K^?1) and sapphire (5 × 10^?6 K^?1) make the bonding of germanium to sapphire a reality. Germanium directly bonded to sapphire results in microvoid generation during post bond annealing. Inclusion of an interface layer such as silicon dioxide layer by plasma enhanced chemical vapour deposition, prior to bonding, results in a microvoid free bond interface after annealing. Grinding and polishing of the subsequent germanium layer has been achieved leaving a thick germanium on sapphire (GeOS) substrate. Submicron GeOS layers have also been achieved with hydrogen/helium co-implantation and layer transfer. Circular geometry transistors exhibiting a field effect mobility of 890 cm²/V s have been fabricated onto the thick germanium on sapphire layer.     

Functionalized terfluorene for solution-processed high efficiency blue fluorescence OLED and electrophosphorescent devices

A new multifunctional blue-emitting terfluorene derivative (TFDPA) featured with triphenylamine groups for hole-transportation and long alkyl chains for solution processability on the conjugation inert bridge centers was reported. TFDPA can give homogeneous thin film by solution process and exhibits high hole mobility (μ_h ≈ 10^?3 cm² V^?1 s^?1) and suitable HOMO for hole injection. Particularly, TFDPA performs efficient deep-blue emission with high quantum yield (～100% in solution, 43% in thin film) and suitable triplet energy (E_T = 2.28 eV), making solution-processed OLED devices of using TFDPA as blue emitter and as host for iridium-containing phosphorescent dopants feasible. The solution-processed nondoped blue OLED device gives saturated deep-blue electroluminescence [CIE = (0.17, 0.07)] with EQE of 2.7%. TFDPA-hosted electrophosphorescent devices performed with EQE of 6.5% for yellow [(Bt)₂Ir(acac)], 9.3% of orange [Ir(2–phq)₃], and 6.9% of red [(Mpq)₂Ir(acac)], respectively. In addition, with careful control on the doping concentration of [(Bt)₂Ir(acac)], a solution-processed fluorescence–phosphorescence hybrided two-color-based WOLED with EQE of 3.6% and CIE coordinate of (0.38, 0.33) was successfully achieved.     

Phase transition and surface morphology effects on optical,electrical and lithiation/delithiation behavior of nanostructured Ce-doped V2O5 thin films

Cerium doped V₂O₅ thin films were prepared by the sol−gel process. X-ray diffraction analysis revealed the phase transition from α-V₂O₅ orthorhombic to β-V₂O₅ tetragonal structure by annealing at 400 °C. The SEM and AFM images revealed that annealing temperature changed the surface morphology of the V₂O₅ films from fiber like wrinkle network to elongated sheets. Also, the particle shape was significantly influenced by Ce doping and a nanorod-like morphology was formed at 1.5 mol% Ce−doped V₂O₅. Power spectral density analysis indicated that surface roughness and fractal dimension of β−V₂O₅ increase by Ce doping. Optical measurement showed that the band gap narrowing (from 2.68 to 2.28 eV) occurred when the annealing temperature and dopant concentration increased. The variation of activation energy of the films was explained based on the small polaron hopping mechanism. The α−V₂O₅ film showed enhanced lithium−ion storage capacity compared to pristine β−V₂O₅ film and 1 mol% Ce−doped α−V₂O₅ thin film revealed the best ion storage capacity (Q_a=207.19 mC/cm², I_max=4.13 mA/cm² at scan rate of ν=20 mV/s).     

Effects of RTA temperatures on conductivity and micro-structures of boron-doped silicon nanocrystals in Si-rich oxide thin films

In this work, the B-doped Si rich oxide (SRO) thin films were deposited and then annealed using rapid thermal annealing (RTA) to form SiO₂-matrix silicon nanocrystals (Si NCs). The effects of the RTA temperatures on the structural properties, conduction mechanisms and electrical properties of B-doped SRO thin films (BSF) were investigated systematically using Hall measurements, Fourier transform infrared spectroscopy and Raman spectroscopy. Results showed that the crystalline fraction of annealed BSF increased from 41.3% to 62.8%, the conductivity was increased from 4.48×10⁻³ S/cm to 0.16 s/cm, the carrier concentration was increased from 8.74×10¹⁷ cm⁻³ to 4.9×10¹⁸ cm⁻³ and the carrier mobility was increased from 0.032 cm² V⁻¹ s⁻¹ to 0.2 cm² V⁻¹ s⁻¹ when the RTA temperatures increased from 1050 °C to 1150 °C. In addition, the fluctuation induced tunneling (FIT) theory was applicable to the conduction mechanisms of SiO₂-matrix boron-doped Si-NC thin films.     

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.     

D–A copolymer with high ambipolar mobilities based on dithienothiophene and diketopyrrolopyrrole for polymer solar cells and organic field-effect transistors

Donor–acceptor (D–A) type conjugated polymers have been developed to absorb longer wavelength light in polymer solar cells (PSCs) and to achieve a high charge carrier mobility in organic field-effect transistors (OFETs). PDTDP, containing dithienothiophene (DTT) as the electron donor and diketopyrrolopyrrole (DPP) as the electron acceptor, was synthesized by stille polycondensation in order to achieve the advantages of D–A type conjugated polymers. The polymer showed optical band gaps of 1.44 and 1.42 eV in solution and in film, respectively, and a HOMO level of 5.09 eV. PDTDP and PC₇₁BM blends with 1,8-diiodooctane (DIO) exhibited improved performance in PSCs with a power conversion efficiency (PCE) of 4.45% under AM 1.5G irradiation. By investigating transmission electron microscopy (TEM), atomic force microscopy (AFM), and the light intensity dependence of J_SC and V_OC, we conclude that DIO acts as a processing additive that helps to form a nanoscale phase separation between donor and acceptor, resulting in an enhancement of μ_h and μ_e, which affects the J_SC, EQE, and PCE of PSCs. The charge carrier mobilities of PDTDP in OFETs were also investigated at various annealing temperatures and the polymer exhibited the highest hole and electron mobilities of 2.53 cm² V⁻¹ s⁻¹ at 250 °C and 0.36 cm² V⁻¹ s⁻¹ at 310 °C, respectively. XRD and AFM results demonstrated that the thermal annealing temperature had a critical effect on the changes in the crystallinity and morphology of the polymer. The low-voltage device was fabricated using high-k dielectric, P(VDF-TrFE) and P(VDF-TrFE-CTFE), and the carrier mobility of PDTDP was reached 0.1 cm² V⁻¹ s⁻¹ at V_d = −5 V. PDTDP complementary inverters were fabricated, and the high ambipolar characteristics of the polymer resulted in an output voltage gain of more than 25.     

Investigation of structural and optoelectronic properties of annealed nickel phthalocyanine thin films

Thin films of nickel phthalocyanine (NiPc) were prepared by thermal evaporation and the effects of annealing temperature on the structural and optical properties of the samples were studied using different analytical methods. Structural analysis showed that the grain size and crystallinity of NiPc films improved as annealing temperature increased from 25 to 150 °C. Also, maximum grain size (71.3 nm) was obtained at 150 °C annealing temperature. In addition, NiPc films annealed at 150 °C had a very smooth surface with an RMS roughness of 0.41 nm. Optical analysis indicated that band gap energy of films at different annealing temperatures varied in the range of 3.22–3.28 eV. Schottky diode solar cells with a structure of ITO/PEDOT:PSS/NiPc/Al were fabricated. Measurement of the dark current density–voltage (J–V) characteristics of diodes showed that the current density of films annealed at 150 °C for a given bias was greater than that of other films. Furthermore, the films revealed the highest rectification ratio (23.1) and lowest barrier height (0.84 eV) demonstrating, respectively, 23% and 11% increase compared with those of the deposited NiPc films. Meanwhile, photoconversion behavior of films annealed at 150 °C under illumination showed the highest short circuit current density (0.070 mA/cm²) and open circuit voltage of (0.55 V).     

Oriented growth of rubrene thin films on aligned pentacene buffer layer and its anisotropic thin-film transistor characteristics

Oriented growth of polycrystalline rubrene thin film on oriented pentacene buffer layer was investigated. The oriented pentacene buffer layer was created by thermal evaporation of pentacene on a rubbed polyvinylalcohol (PVA) surface. The pentacene layer in turn induced the oriented growth of rubrene crystals upon thermal deposition. The structures of successive layers were characterized by using grazing incidence X-ray diffraction (GIXD) and atomic force microscopy. Highly oriented rubrene crystallites with the a-axis aligning along the surface normal and the (0 0 2) plane preferentially oriented 45° away from the rubbing direction were found. In contrast, the rubrene thin film deposited on PVA or rubbed-PVA substrate without a pentacene buffer layer only gave amorphous phases. With the aligned pentacene/rubrene film as the active layer of organic field-effect transistor, anisotropic mobilities were observed. The highest field-effect mobility (0.105 cm²/V s) was observed along the direction 45° away from the rubbing direction and is ∼4 times higher than that for similar device prepared on unrubbed PVA. The direction was consistent with the GIXD observation that a large number of rubrene crystallites are having their [0 0 2] direction aligning in this direction. A favourable C–H⋯π interaction between an oriented pentacene layer and the rubrene layer on the control of molecular orientation in the conduction channel of the OFET is suggested.     

Two-dimensional π-conjugated molecules based-on 2,6,9,10-tetra(prop-1-yn-1-yl)anthracene and their application to solution-processed photovoltaic cells

Novel two-dimensional π-conjugated molecules, i.e., 5′,5″′-((9,10-bis((4-hexylphenyl)ethynyl)anthracene-2,6-diyl)bis(ethyne-2,1-diyl))bis(5-hexyl-2,2′-bithiophene) (2,6-HBT) and 5′,5″′-((2,6-bis((4-hexylphenyl)ethynyl)anthracene-9,10-diyl)bis(ethyne-2,1-diyl))bis(5-hexyl-2,2′-bithiophene) (9,10-THB) were successfully synthesized and utilized as highly soluble p-type organic semiconductors for organic thin film transistors (TFTs) and solar cells. From the TFTs, the high hole mobility of the pristine film of 9,10-THB was measured to be 0.07 cm² V⁻¹ s⁻¹ (I_on/off = 10⁶–10⁷), which is mainly attributed to slip-stacked charge-transport behavior in J-aggregation-induced crystallites. Further, a solution-processed solar cell made of 9,10-THB and PC₆₁BM exhibited very promising and reproducible power conversion efficiencies of 3.30% and 2.53% with composition 1:1 and 1:2 w/w ratio, respectively.