Fabrication of single crystal field-effect transistors with phenacene-type molecules and their excellent transistor characteristics

Single crystal field-effect transistors (FETs) using [6]phenacene and [7]phenacene show p-channel FET characteristics. Field-effect mobilities, μs, as high as 5.6 × 10^?1 cm² V^?1 s^?1 in a [6]phenacene single crystal FET with an SiO₂ gate dielectric and 2.3 cm² V^?1 s^?1 in a [7]phenacene single crystal FET were recorded. In these FETs, 7,7,8,8-tetracyanoquinodimethane (TCNQ) was inserted between the Au source/drain electrodes and the single crystal to reduce hole-injection barrier heights. The μ reached 3.2 cm² V^?1 s^?1 in the [7]phenacene single crystal FET with a Ta₂O₅ gate dielectric, and a low absolute threshold voltage |V_TH| (6.3 V) was observed. Insertion of 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane (F₄TCNQ) in the interface produced very a high μ value (4.7–6.7 cm² V^?1 s^?1) in the [7]phenacene single crystal FET, indicating that F₄TCNQ was better for interface modification than TCNQ. A single crystal electric double-layer FET provided μ as high as 3.8 × 10^?1 cm² V^?1 s^?1 and |V_TH| as low as 2.3 V. These results indicate that [6]phenacene and [7]phenacene are promising materials for future practical FET devices, and in addition we suggest that such devices might also provide a research tool to investigate a material’s potential as a superconductor and a possible new way to produce the superconducting state.     

Intermolecular interactions in organic semiconductors based on annelated β-oligothiophenes and their effect on the performance of organic field-effect transistors

A series of derivatives based on annelated β-oligothiophenes were synthesized and characterized as active layer in organic field-effect transistors (OFETs). Highest field-effect mobility of 0.52 V^?1 s^?1 for 2,5-dibiphenyl-dithieno[2,3-b:3′,2′-d]thiophene (DBP-DTT), 2.2 cm² V^?1 s^?1 for 2,5-distyryl-dithieno[2,3-b:3′,2′-d]thiophene (DEP-DTT), and 0.16 cm² V^?1 s^?1 for 1,4-di[2-dithieno[2,3-b:3′,2′-d] thiophen-2-yl-vinyl]benzene (DDTT-EP) were obtained, while 2,5-diphenyl-dithieno [2,3-b:3′,2′-d]thiophene (DP-DTT) presents no field-effect behaviors. Their thermal, optical and electrochemical properties, topographical and X-ray diffraction patterns of films, and the single crystal structures were also investigated. With the end-capping groups changing in these materials, the intermolecular interactions could transform from S–S in DP-DTT to S–C in DBP-DTT, to S–π in DEP-DTT, and to the coexisting of S–S and S–π in DDTT-EP. According to the device performances and the results of transfer integral calculations, it was revealed that S–π intermolecular interaction benefits not only improving the mobility but also reducing the threshold voltage (V_T), while S–S intermolecular interaction is not favorable for promoting the mobility.     

Distributed feedback lasing from thin organic crystal based on active waveguide grating structures

A simple method has been proposed to fabricate active waveguide grating structures as distributed feedback configuration for organic crystal lasers. Organic single crystals, 2,5-bis(4-biphenyl)bithiophene (BP2T), act as both the gain medium and the waveguide. The distributed feedback structures are fabricated separately on top of crystals through interference lithography. The lasing emissions centered at 566 nm with FWHM of 1.1 nm have been observed when the pump intensity exceeds 19 μJ cm^?2. This method features simple, nondestructive, and it is expected to be applied in the electronically pumped laser devices.     

1 Volt organic transistors with mixed self-assembled monolayer/Al2O3 gate dielectrics

Control of the threshold voltage and the subthreshold swing is critical for low voltage transistor operation. In this contribution, organic field-effect transistors (OFETs) operating at 1 V using ultra-thin (∼4 nm), self-assembled monolayer (SAM) modified aluminium oxide layers as the gate dielectric are demonstrated. A solution-processed donor–acceptor semiconducting polymer poly(3,6-di(2-thien-5-yl)-2,5-di(2-octyldodecyl)-pyrrolo[3,4-c]pyrrole-1,4-dione)thieno[3,2-b]thiophene) (PDPP2TTT) is used as the active layer. It is shown that the threshold voltage of the fabricated transistors can be simply tuned by carefully controlling the composition of the applied SAM. The optimised OFETs display threshold voltages around 0 V, low subthreshold slopes (150 ± 5 mV/dec), operate with negligible hysteresis and show average saturated field-effect mobilities in excess of 0.1 cm²/V s at 1 V.     

Effects of thermal annealing on the photophysical properties of bisfluorene-cored dendrimer films

Annealing is widely used in the processing of organic semiconductors, and can modify their film morphology and photophysical properties. A study of the effect of annealing on films made from a blue emitting bisfluorene-cored dendrimer is reported. Annealing causes a 15 nm blue-shift in the photoluminescence (PL) spectrum and an 11 nm blue-shift in the amplified spontaneous emission (ASE) spectrum. It causes the PL efficiency to decrease only slightly from 0.92 to 0.83. The radiative decay rate of 1.3 × 10⁹ s^?1, the ASE threshold of 1.5 × 10¹⁸ cm^?3 and the singlet–singlet exciton annihilation rate of 5.5 × 10^?10 cm³ s^?1 are unaffected by annealing. The results indicate a scope for colour adjustment of dendrimer light-emitting diodes and lasers without affecting their efficiencies. Investigation by spectroscopic ellipsometry shows that on annealing, the films become anisotropic, with larger values of the refractive index and extinction coefficient observed for light polarised in the plane of the film than the corresponding out-of-plane values in the absorption region of the bisfluorene core. This anisotropy indicates a preferential in-plane orientation of bisfluorene cores upon annealing.     

Channel width effect for organic thin film transistors using TIPS-pentacene employed as a dopant of poly-triarylamine

The effects of the physical channel width on the characteristics of organic thin film transistors (OTFTs), made with 6,13-bis(triisopropyl-silylethynyl)-pentacene (TIPS-pentacene) embedded into poly-triarylamine (PTAA, hole conductor within an active channel), have been examined in this paper. The devices are estimated by measuring the drain-source current (I_DS) for different contact metals such as Au and Ag, at fixed gate and drain voltages. The results show that the threshold voltage (V_T) and I_DS increase with increasing channel width. Furthermore, it has been observed that the field effect mobility is dependent on V_T, which is influenced by the channel width. The OTFTs, produced using Au and Ag contacts, exhibited the highest values of mobility in the saturation regime, namely 5.44 × 10^?2 and 1.33 × 10^?2 cm²/Vs, respectively.     

Highly efficient green phosphorescent organic light emitting diodes with simple structure

A series of simple structures is investigated for realization of the highly efficient green phosphorescent organic light emitting diodes with relatively low voltage operation. All the devices were fabricated with mixed host system by using 1,1-bis[(di-4-tolylamino)phenyl]cyclohexane (TAPC) and 1,3,5-tri(p-pyrid-3-yl-phenyl)benzene (TpPyPB) which were known to be hole and electron type host materials due to their great hole and electron mobilities [μ_h(TAPC): 1 × 10^?2 cm²/V s and μ_e(TpPyPB): 7.9 × 10^?3 cm²/V s] [1]. The optimized device with thin TAPC (5–10 nm) as an anode buffer layer showed relatively high current and power efficiency with low roll-off characteristic up to 10,000 cd/m². The performances of the devices; with buffer layer were compared to those of simple devices with single layer and three layers. Very interestingly, the double layer device with TAPC buffer layer showed better current and power efficiency behavior compared to that of three layer device with both hole and electron buffer layers (TAPC, TpPyPB, respectively).     

Low-voltage,high speed inkjet-printed flexible complementary polymer electronic circuits

We report the development of high-performance inkjet-printed organic field-effect transistors (OFETs) and complementary circuits using high-k polymer dielectric blends comprising poly(vinylidenefluoride-trifluoroethylene) (P(VDF-TrFE)) and poly(methyl methacrylate) (PMMA) for high-speed and low-voltage operation. Inkjet-printed p-type polymer semiconductors containing alkyl-substituted thienylenevinylene (TV) and dodecylthiophene (PC12TV12T) and n-type P(NDI2OD-T2) OFETs showed high field-effect mobilities of 0.1–0.4 cm² V^?1 s^?1 and low threshold voltages down to 5 V. These OFET properties were modified by changing the blend ratio of P(VDF-TrFE) and PMMA. The optimum blend – a 7:3 wt% mixture of P(VDF-TrFE) and PMMA – was successfully used to realize high-performance complementary inverters and ring oscillators (ROs). The complementary ROs operated at a supplied bias (V_DD) of 5 V and showed an oscillation frequency (f_osc) as high as ～80 kHz at V_DD = 30 V. Furthermore, the f_osc of the complementary ROs was significantly affected by a variety of fundamental parameters such as the electron and hole mobilities, channel width and length, capacitance of the gate dielectrics, V_DD, and the overlap capacitance in the circuit configuration.     

Indolo[3,2-b]carbazole: Promising building block for highly efficient electroluminescent materials

Two luminescent materials based on indolo[3,2-b]carbazole have been designed and synthesized. They were highly fluorescent both in solution and in the solid state. High-performance electroluminescent devices with indolo[3,2-b]carbazole luminescent derivatives as the emissive materials were fabricated for the first time with low turn-on voltage of 2.65 V, high luminescence efficiency of 7.92 lm W^?1, and high brightness of 68729 cd m^?2. The results demonstrated that indolo[3,2-b]carbazole has great potentials as promising building block for highly efficient electroluminescent materials.     

Tailoring chromatic dispersion in chalcogenide–tellurite microstructured optical fiber

We report fabrication of a highly nonlinear hybrid microstructured optical fiber composed of chalcogenide glass core and tellurite glass cladding. The flattened chromatic dispersion can be achieved in such an optical fiber with near zero dispersion wavelength at telecommunication wavelengths λ = 1.35–1.7 μm, which cannot be achieved in chalcogenide glass optical fibers due to their high refractive index, i.e. n > 2.1. We demonstrate a hybrid 4-air hole chalcogenide–tellurite optical fiber (Δn = 0.25) with flattened chromatic dispersion around λ = 1.55 μm. In optimized 12-air hole optical fiber composed of the same glasses, the chromatic dispersion values were achieved between −20 and 32 ps/nm/km in a broad wavelength range of 1.5–3.8 μm providing the fiber with extremely high nonlinear coefficient 86,000 km⁻¹W⁻¹. Hybrid chalcogenide/tellurite fibers pumped with the near infrared lasers give good promise for broadband optical amplification, wavelength conversion, and supercontinuum generation in the near- to mid-infrared region.     

A highly efficient OLED based on terbium complexes

A neutral ligand 9-(4-tert-butylphenyl)-3,6-bis(diphenylphosphineoxide)-carbazole (DPPOC) and its complex Tb(PMIP)₃DPPOC (A, where PMIP stands for 1-phenyl-3-methyl-4-isobutyryl-5-pyrazolone) were synthesized. DPPOC has a suitable lowest triplet energy level (24,691 cm^?1) for the sensitization of Tb(III) (⁵D₄: 20,400 cm^?1) and a significantly higher thermal stability (glass transition temperature 137 °C) compared with the familiar ligand triphenylphosphine oxide (TPPO). Experiments revealed that the emission layer of the Tb(PMIP)₃DPPOC film could be prepared by vacuum co-deposition of the complex Tb(PMIP)₃(H₂O)₂ (B) and DPPOC (molar ratio = 1:1). The electroluminescent (EL) device ITO/N,N′-diphenyl-N,N′-bis(1-naphthyl)-1,1′-diphenyl-4,4′-diamine (NPB; 10 nm)/Tb(PMIP)₃ (20 nm)/co-deposited Tb(PMIP)₃DPPOC (30 nm)/2,9-dimethyl-4,7-diphenyl-1,10-phenanthroline (BCP; 10 nm)/tris(8-hydroxyquinoline) (AlQ; 20 nm)/Mg_0.9Ag_0.1 (200 nm)/Ag (80 nm) exhibited pure emission from terbium ions, even at the highest current density. The highest efficiency obtained was 16.1 lm W^?1, 36.0 cd A^?1 at 6 V. At a practical brightness of 119 cd m^?2 (11 V) the efficiency remained above 4.5 lm W^?1, 15.7 cd A^?1. These values are a significant improvement over the previously reported Tb(PMIP)₃(TPPO)₂ (C).     

Theoretical analysis of characteristics for 2 μm Tm3+:Ho3+ co-doped silica fiber laser pumped by a 1550 nm fiber laser

We established a theoretical model of 2 μm Tm³⁺:Ho³⁺ co-doped silica fiber laser pumped by a 1550 nm fiber laser based on the rate-equation theory and performed the numerical simulation using Runge–Kutta algorithm and Newton–Raphson algorithm. The intracavity power distributions of both pump and laser of the Tm³⁺:Ho³⁺ co-doped silica fiber laser based on the Tm³⁺:Ho³⁺ co-doped silica fiber supplied by the National Optics Institute in Canada (NOIC) were obtained. The effects of the output reflectivity R₄(λ_s) at the output laser wavelength λ_s and the concentrations of Tm³⁺ and Ho³⁺ in the fiber on laser output performance were analyzed. In order to achieve a high laser output power, the optimal R₄(λ_s) of 0.13 was verified and the optimal Tm:Ho ratio of 1:2.4 was proposed. Finally, better output performance for the fiber laser based on the optimized Tm³⁺:Ho³⁺ co-doped silica fiber was obtained than the laser using the fiber supplied by the NOIC. This theoretical model and numerical simulation results will guide the fabrication of 2 μm Tm³⁺:Ho³⁺ co-doped all-fiber lasers pumped by 1600-nm-band (1500–1750 nm) Er³⁺:Yb³⁺ co-doped silica fiber lasers.     

Efficient solution-processed electrophosphorescent devices using ionic iridium complexes as the dopants

Efficient solution-processed electrophosphorescent devices using two blue-emitting ionic iridium complexes (complex 1 and complex 2) were fabricated, with poly(N-vinylcarbazole) (PVK):1,3-bis(5-(4-tert-butylphenyl)-1,3,4-oxadiazol-2-yl)benzene (OXD-7) as the host and Cs₂CO₃/Al as the cathode. Using complex 1 as the dopant, we obtained efficient blue-green electrophosphorescence from single-layer devices with a maximum efficiency of 12.2 cd A^?1, a maximum brightness of 12,600 cd m^?2 and CIE (Commission Internationale de l’Éclairage) coordinates of (0.19, 0.45). And the maximum efficiency of the device based on complex 1 can be further improved to 20.2 cd A^?1, when a thin 1,3,5-tris(1-phenyl-1H-benzo[d]imidazol-2-yl)benzene (TPBI) layer was inserted between the light-emitting layer and the cathode. Using complex 2 as the dopant, we obtained deep-blue electrophosphorescence with the emission peak at 458 nm and CIE coordinates of (0.16, 0.22). Our work suggests that ionic iridium complexes are promising phosphors for obtaining efficient electrophosphorescence in the blue region.     

A new method to simulate random dopant induced threshold voltage fluctuations in sub-50 nm MOSFET’s with non-uniform channel doping

In this paper, based on a precise and efficient analytical function of relatively realistic dopant fluctuations, a new method is proposed to simulate the threshold voltage variation of MOSFET’s with non-uniform channel doping due to random dopant fluctuations. Both the number and position fluctuations of dopants are taken into account. Using this method, 2500 microscopically different devices under certain process conditions that cover the range of channel length L from 35 nm to 90 nm, oxide thickness T_ox from 1 nm to 4 nm and channel surface doping concentration N_A from 1 × 10¹⁸ to 5 × 10¹⁸ cm⁻³ are simulated to show how our method works.     

All-conjugated diblock copolymer of poly(3-hexylthiophene)-block-poly(3-phenoxymethylthiophene) for field-effect transistor and photovoltaic applications

The electronic properties, morphology and optoelectronic device characteristics of conjugated diblock copolythiophene, poly(3-hexylthiophene)-block -poly(3-phenoxymethylthiophene) (P3HT-b-P3PT), are firstly reported. The polymer properties and structures were explored through different solvent mixtures of chloroform (CHCl₃), dichlorobenzene (DCB), and CHCl₃:DCB (1:1 ratio). The absorption maximum (λ_max) of P3HT-b-P3PT prepared from DCB was around 554 nm with a shoulder peak indicative for the highly crystalline structure around 604 nm while that from CHCl₃ was 516 nm without the clear shoulder peak. The field-effect hole mobility of P3HT-b-P3PT increased from ～6.0 × 10^?3, ～8.0 × 10^?3 to ～2.0 × 10^?2 cm² V^?1 s^?1 as the DCB content in the solvent mixture enhanced. The AFM images suggested that the highly volatile CHCl₃ processing solvent led to the amorphous structure, on the other hand, less volatile DCB resulted in the largely crystalline structure of the P3HT-b-P3PT. Such difference on the polymer structure and hole mobility led to the varied power conversion efficiency (PCE) of the photovoltaic cells fabricated from the blend of P3HT-b-P3PT/[6,6]-phenyl-C₆₁-butyric acid methyl ester (PCBM) (1:1, w/w): 1.88 (CHCl₃), 2.13 (CHCl₃:DCB (1:1)), and 2.60% (DCB). The PCBM blend ratio also significantly affected the surface structure and the solar cell performance. The PCE of polymer/PCBM could be improved to 2.80% while the ratio of polymer to PCBM went to 1:0.7. The present study suggested that the surface structures and optoelectronic device characteristics of conjugated diblock copolymers could be easily manipulated by the processing solvent, the block segment characteristic, and blend composition.     

Modeling of Ga1−xInxAs1−y−zNySbz/GaAs quantum well properties for near-infrared lasers

The aim of this work is to model the properties of GaInAsNSb/GaAs compressively strained structures. Indeed, Ga_1?xIn_xAs_1?y?zN_ySb_z has been found to be a potentially superior material to GaInAsN for long wavelength laser dedicated to optical fiber communications. Furthermore, this material can be grown on GaAs substrate while having a bandgap smaller than that of GaInNAs. The influence of nitrogen and antimony on the bandgap and the transition energy is explored. Also, the effect of these two elements on the optical gain and threshold current density is investigated. For example, a structure composed of one 7.5 nm thick quantum well of material with In=30%, N=3.5%, Sb=1% composition exhibits a threshold current density of 339.8 A/cm² and an emission wavelength of 1.5365 μm (at T=300 K). It can be shown that increasing the concentration of indium to 35% with a concentration of nitrogen and antimony, of 2.5% and 1%, respectively, results in a decrease of the threshold current density down to 253.7 A/cm² for a two well structure. Same structure incorporating five wells shows a threshold current density as low as 221.4 A/cm² for T=300 K, which agrees well with the reported experimental results.     

A blue light emitting perylene derivative with improved solubility and aggregation control: Synthesis,characterisation and optical limiting properties

We report the synthesis and characterisation of 3,4,9,10-tetra-(12-alkoxycarbonyl)-perylene, a blue emitting organic material with controlled aggregation and improved solubility. The dye has a UV–Vis maximum absorption peak at 472 nm. Room temperature photoluminescence reveal a blue emission peak at 489 nm. Thermo gravimetric analysis (TGA) performed in air show the product to be thermally stable up to 300 °C. Dynamic scanning calorimetry (DSC) did not show a liquid crystalline phase normally present in 3,4,9,10-tetra-(n-alkoxycarbonyl)-perylenes for 2 ? n ? 9 where n is the number of carbon atoms in the alkyl chain. Atomic force microscopy (AFM) and scanning electron microscopy (SEM) were employed for aggregation studies of the dye showing that molecular stacking is not favoured. The Tauc relation applied to the UV–Vis absorbance cut-off wavelength at 500 nm was used to evaluate the optical band gap of the material as 3.81 eV and an exciton binding energy of 1.2 eV. Open-aperture Z-scan proved the molecule to be nonlinear optically active with an optical limiting extinction coefficient of 3.0 × 10^?10 cm W^?1. Experimental evidence shows that the optical limiting ability is attributed to nonlinear scattering.     

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.     

Active layer thickness effect on the recombination process of PCDTBT:PC71BM organic solar cells

We investigated the effect of active layer thickness on recombination kinetics of poly[N-9″-hepta-decanyl-2,7-carbazole-alt-5,5-(4′,7′-di-2-thienyl-2′,1′,3′-benzothiadiazole)] (PCDTBT) and [6,6]-phenyl C71-butyric acid methyl ester (PC₇₁BM) based solar cells. Analysis of the fitted Lambert W-function of illuminated current density–voltage (J–V) characteristics revealed increased recombination processes with increased active layer thicknesses. The ideality factor extracted from PCDTBT:PCBM solar cells continuously increased from 1.89 to 3.88 when photoactive layer thickness was increased from 70 to 150 nm. We found that such increase in ideality factor is closely related to the defect density which is increased with increased photoactive layer thickness beyond 110 nm. Therefore, the different density of defect states in PCDTBT:PCBM solar cells causes the different recombination paths where solar cells with a thicker active layer (?110 nm) are considered to undergo coupled trap-assisted recombination processes while single-defect trap-assisted recombination is dominant for thinner (70–90 nm) PCDTBT:PCBM solar cells. As a result, we found that the optimal efficiencies of PCDTBT:PC₇₁BM solar cells were limited to the active layers between 70 and 90 nm. Particularly, when PCDTBT:PC₇₁BM solar cells were optimized with an active layer thickness of 70 nm, energy conversion efficiency reached 6.5% while an increase in thickness led to the reduction of efficiency to 4.7% at 133 nm but then an increase to 5.02% at 150 nm.     

A cross-linked poly(methyl methacrylate) gate dielectric by ion-beam irradiation for organic thin-film transistors

The electrical characteristics of pentacene organic thin-film transistors (OTFTs) using cross-linked poly(methyl methacrylate) (PMMA) as the gate dielectric are reported. Ultra-thin films of cross-linked PMMA could be obtained by spin-coating and subsequent irradiation using a 1.515 MeV ⁴He⁺ ion beam. The resulting film, with a thickness of 33 nm, possessed a low leakage current density of about 10^?6 A cm^?2 for fields up to 2 MV cm^?1. OTFTs incorporating the cross-linked dielectric operated at relatively low voltages, <10 V, and exhibited a mobility of 1.1 cm² V^?1 s^?1, a threshold voltage of ?1 V, a sub-threshold slope of 220 mV per decade and an on/off current ratio of 1.0 × 10⁶.