Using d-limonene as the non-aromatic and non-chlorinated solvent for the fabrications of high performance polymer light-emitting diodes and field-effect transistors

Aiming to environment protection, green solvents are crucial for commercialization of solution-processed optoelectronic devices. In this work, d-limonene, a natural product, was introduced as the non-aromatic and non-chlorinated solvent for processing of polymer light-emitting diodes (PLEDs) and organic field effect transistors (OFETs). It was found that d-limonene could be a good solvent for a blue-emitting polyfluorene-based random copolymer for PLEDs and an alternating copolymer FBT-Th₄(1,4) with high hole mobility (μ_h) for OFETs. In comparisons to routine solvent-casted films of the two conjugated polymers, the resulting d-limonene-deposited films could show comparable film qualities, based on UV–vis absorption spectra and observations by atomic force microscopy (AFM). With d-limonene as the processing solvent, efficient blue PLEDs with CIE coordinates of (0.16, 0.16), maximum external quantum efficiency of 3.57%, and luminous efficiency of 3.66 cd/A, and OFETs with outstanding μ_h of 1.06 cm² (V s)⁻¹ were demonstrated. Our results suggest that d-limonene would be a promising non-aromatic and non-chlorinated solvent for solution processing of conjugated polymers and molecules for optoelectronic device applications.     

Highly branched green phosphorescent tris-cyclometalated iridium(III) complexes for solution-processed organic light-emitting diodes

A series benzoimidazole-based dendritric complexes of iridium dendrimers containing Fréchet-type dendrons with peripheral fluorenyl surface groups have been synthesized. These iridium dendrimers are green-emitting with high phosphorescence quantum yield, and can be spin-coated as films of good quality. From cyclic voltammograms (CV), high onset potentials at 1.42–1.58 V due to the peripheral fluorene group were observed. Device from a second generation dendrimer 17 with structure of ITO/PEDOT:PSS/CBP: 20 wt% 17/TPBI/LiF/Al (PEDOT:PSS = poly(ethylene dioxythiophene): polystyrenesulfonate and CBP = bis(N-carbazolyl)biphenyl) has the best performance: maximum external quantum efficiency of 13.58% and maximum current efficiency of 45.7 cd/A. Space-charge-limited current (SCLC) flow technique was used to measure the mobility of charge carriers in the blend films of the compounds in CBP. Blend films of higher generation dendrimers have lower hole mobility, albeit with higher device efficiencies.     

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.     

Multihydroxylated aryl amine as a novel alcohol-processable hole-transport molecular glass exhibiting remarkable resistance to weakly polar solvents

Multihydroxylation of a small-molecule aryl amine leads to a contrasting solubility in polar and weakly polar solvents. The resulting compound shows an intrinsic amorphous morphology with a high T_g of 190 °C and is capable of affording uniform smooth and transparent films, spin-cast from 2-propanol. The fitting of the space-charge-limited current characteristics reveals a hole mobility of ～4.6 × 10^?6 cm² V^?1 s^?1 at low-voltages. Incorporating this new compound as a hole-transport layer into conventional bottom-anode organic light-emitting diodes that consisted of a solution-processed emitter provides promising performance.     

Raman spectra of CdTe/ZnTe self-assembled quantum dots

In this paper, we present Raman scattered spectra and results of photoluminescence (PL) measurements of CdTe/ZnTe self-assembled quantum dots (SAQD). The PL spectrum displayed two main emission peaks, both connected with the existing of QDs. One presents direct deexcitation to ground state and the other is optical phonon (ω=204.2 cm^?1)-assisted deexcitation. The registered multiphonon (MP) emission process depends on temperature. At low temperature, one-phonon spectra shows line at 200.4 and 210.3 cm^?1 (ZnTe LO mode confined by degenerate superlattices), 217 cm^?1 (TA+LO in ZnTe) and 386 cm^?1 (LO+TO(Γ) in ZnTe).     

Vacuum-processed polyethylene as a dielectric for low operating voltage organic field effect transistors

We report on the fabrication and performance of vacuum-processed organic field effect transistors utilizing evaporated low-density polyethylene (LD-PE) as a dielectric layer. With C₆₀ as the organic semiconductor, we demonstrate low operating voltage transistors with field effect mobilities in excess of 4 cm²/Vs. Devices with pentacene showed a mobility of 0.16 cm²/Vs. Devices using tyrian Purple as semiconductor show low-voltage ambipolar operation with equal electron and hole mobilities of ～0.3 cm²/Vs. These devices demonstrate low hysteresis and operational stability over at least several months. Grazing-angle infrared spectroscopy of evaporated thin films shows that the structure of the polyethylene is similar to solution-cast films. We report also on the morphological and dielectric properties of these films. Our experiments demonstrate that polyethylene is a stable dielectric supporting both hole and electron channels.     

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.     

High field-effect mobility from poly(3-hexylthiophene) thin-film transistors by solvent–vapor-induced reflow

Room-temperature exposure of spin-coated poly(3-hexylthiophene) (P3HT) films to ortho-dichlorobenzene vapor increases the field-effect mobility of the P3HT organic thin-film transistors (OTFTs). The mobility increases moderately with unsaturated vapor exposure, owing to increased crystallinity of the P3HT films; on the other hand, the mobility increases abruptly with saturated vapor exposure, to 0.11 cm²/V s. The saturated vapor exposure causes the P3HT films to reflow, leaving in the active area approximately 2–3 P3HT monolayers whose molecular ordering is enhanced by the flow-generated shear against the gate dielectric. Although the reflowed OTFTs degrade in air much faster than do the non-reflowed OTFTs due to the susceptibility of the ultra-thin reflowed films, they become highly stable when encapsulated, obtaining a lifetime of more than 3000 h.     

A novel blue-light organic electroluminescence material derived from 8-hydroxyquinoline lithium

A novel blue-light organic electroluminescence material (OEM) derived from 8-hydroxyquinoline lithium (Liq), bis[(8-hydroxyquinolin)lithium·8-hydroxyquinolin] sodium ([Liq·q^?]₂·2Na⁺), was synthesized and investigated. For stereo molecular structure and presence of sodium ions, [Liq·q^?]₂·2Na⁺ exhibits higher quantum yield and longer fluorescence lifetime than Liq. When used as light-emitting layer in organic light-emitting diode (OLED), [Liq·q^?]₂·2Na⁺ shows the maximum emission peak at 489 nm in electroluminescence spectra and emits bluer light with higher current efficiency stability than Liq. Moreover, [Liq·q^?]₂·2Na⁺ exhibits excellent thermal stability and film formability.     

Development of a new diindenopyrazine–benzotriazole copolymer for multifunctional application in organic field-effect transistors,polymer solar cells and light-emitting diodes

A new donor–acceptor (D?A) copolymer (PIPY–DTBTA) containing 6,12-dihydro-diindeno[1,2-b;1′,2′-e]pyrazine donor and benzotriazole acceptor was synthesized and characterized for multifunctional applications in organic field-effect transistors (OFETs), polymer solar cells (PSCs) and polymer light-emitting diodes (PLEDs). The polymer exhibits high molecular weights, excellent film-forming ability, a deep HOMO energy level, and good solution processability. Solution-processed thin film OFETs based on this polymer revealed good p-type characteristic with a high hole mobility up to 0.0521 cm² V^?1 s^?1. Bulk-heterojunction PSCs comprising this polymer and PC₆₁BM gave a power conversion efficiency (PCE) of 0.77%. The single-layer PLEDs based on PIPY–DTBTA emitted a yellow–red light with a maximum brightness of 385 cd m^?2 at the turn-on voltage of 6 V.     

Organic light-emitting diodes based on an ambipolar single crystal

Slice-like organic single crystals of 1,4-bis(2-cyano-2-phenylethenyl)benzene (BCPEB) are grown by the physical vapor transport (PVT) method, and exhibit a very high photoluminescence quantum efficiency (Φ_PL) of 75%. The ambipolar behavior of BCPEB single crystals are confirmed using the time of flight technique. The high efficiency and balanced (μ_h = 0.059 cm²/Vs and μ_e = 0.070 cm²/Vs) carriers’ mobility imply that the BCPEB single crystal is a promising light-emitting layer in the diodes structure. Intense green electroluminescence (EL) from a diode has been successfully demonstrated at an applied electric field of 2 × 10⁵ V/cm.     

Using pyridal[2,1,3]thiadiazole as an acceptor unit in a low band-gap copolymer for photovoltaic applications

In this report, we explore the optoelectronic properties of a low band-gap copolymer based on the alternation of electron rich (thiophene and thienothiophene units) and electron deficient units (pyridal[2,1,3]thiadiazole (Py)). Initial density functional theory calculations point out the interest of using the Py unit to optimize the polymer frontier orbital energy levels. A high molecular weight (M_n = 49 kg/mol) solution-processable copolymer, based on Py, thiophene and thienothiophene units, has been synthesized successfully. From cyclic-voltammetry and UV–visible absorption measurements a relatively deep HOMO level (−5.1 eV) and an optical band-gap (1.48 eV) have been estimated. Charge transport both in horizontal and vertical directions were extracted from field-effect transistors and space charge limited current diodes, respectively, and led to a relatively high in-plane hole mobility in pure polymer films (0.7 × 10⁻² cm² V⁻¹ s⁻¹). GIWAXS results showed almost identical in-plane lamellar morphologies, with similar average size and orientation of the polymer crystalline domains in both, pure polymer films and polymer:fullerene blends. Also, the gate-voltage dependence of the field-effect mobility revealed that the energy disorder in the polymer domains was not altered by the introduction of fullerenes. The nevertheless significantly higher out-of-plane hole mobility in blends, in comparison to pure polymer films, was attributed to the minor amorphous polymer phase, presumably localized close to the donor/acceptor interface, whose signature was observed by UV–vis absorption. Promising photovoltaic performances could be achieved in a standard device configuration. The corresponding power conversion efficiency of 4.5% is above the value achieved previously with a comparable polymer using benzo [2,1,3]thiadiazole instead of Py as acceptor unit.     

Optoelectronic study in porous silicon thin films

A photoconductivity (PC) study in as deposited porous silicon (PS) thin films is presented in this work. PS thin films were produced by the electrochemical anodizing method at different anodizing times. The films surfaces were characterized by SEM and porosity was determined by gravimetric methods. Photoluminescence and PC measurements were taken at room temperature. The maximum of the photoluminescence spectra are located around 650 nm, whereas those of PC are placed around 400 nm. The maximum of the photoluminescence signal shifts toward short wavelengths as the quantum dimension of the material skeleton diminishes, while any spectral displacement of the photocurrent signal as the porosity of the material increases is not observed. The spectral position of the PC signal does not change because it is strongly affected by the large quantity of defects present in the sample surface which diminishes the mean free path of the carriers to reach the electrodes. In all the samples photocurrent is small around 10^?1 μA and the intensity of the signal goes down as the porosity increases. Two mechanisms exist that compete with one another, the carrier generation and recombination through light emission centers which diminish the photocurrent.     

Colloidal quantum-dot LEDs with a solution-processed copper oxide (CuO) hole injection layer

Solution-processed copper oxide (CuO) thin films are introduced as a hole injection layer (HIL) for quantum dot-based light-emitting diodes (QD-LEDs). AFM, XPS and UPS measurements are utilized for the characterization of the thermally-annealed CuO films. The optimized CuO-based QD-LEDs exhibited an external quantum efficiency (EQE) of 5.37% with a maximum brightness over 70,000 cd/m². The key parameters including the current efficiency and power efficiency of CuO-based QD-LEDs are comparable to the commonly-used PEDOT:PSS-based QD-LEDs using the same structure, further demonstrating that CuO is an effective hole injection layer for QD-LED applications.     

Enhanced organic ferroelectric field effect transistor characteristics with strained poly(vinylidene fluoride-trifluoroethylene) dielectric

Poly(vinylidene fluoride-trifluoroethylene) (70–30 mol%) was used as the functional dielectric layer in organic ferroelectric field effect transistors (FeFET) for non-volatile memory applications. Thin P(VDF-TrFE) film samples spin-coated on metallized plastic substrates were stretch-annealed to attain a topographically flat-grain structure and greatly reduce the surface roughness and current leakage of semi-crystalline copolymer film, while enhancing the preferred β-phase of the ferroelectric films. Resultant ferroelectric properties (P_R = |10| μC/cm², E_C = |50| MV/m) for samples simultaneously stretched (50–70% strain) and heated below the Curie transition (70 ^oC) were comparable to those resulting from high temperature annealing (>140 ^oC). The observed enhancements by heating and stretching were studied by vibration spectroscopy and showed mutual complementary effects of both processes. Organic FeFET fabricated by thermal evaporating pentacene on the smooth P(VDF-TrFE) films showed substantial improvement of semiconductor grain growth and enhanced electrical characteristics with promising non-volatile memory functionality.     

Low power flexible organic thin film transistors with amorphous Ba0.7Sr0.3TiO3 gate dielectric grown by pulsed laser deposition at low temperature

We deposited amorphous Ba_0.7Sr_0.3TiO₃ (BST) on silicon and plastic substrate under 110 °C by pulsed laser deposition (PLD) and use it as the dielectric of the organic transistor. Depends on the thickness of BST layer, the highest mobility of the devices can achieve 1.24 cm² V^?1 s^?1 and 1.01 cm² V^?1 s^?1 on the silicon and polyethylene naphthalate (PEN) substrate, respectively. We also studied the upward and downward bending tests on the transistors and the dielectric thin films. We found that the BST dielectric pentacene transistor can maintain the mobility at 0.5 cm² V^?1 s^?1 or higher while the bending radius is around 3 mm in both upward and downward bending. Our finding demonstrates the potential application of PLD growth high-k dielectric in the large area organic electronics devices.     

Development of transparent conducting copper and iron co-doped cadmium oxide films: Effect of annealing in hydrogen atmosphere

Hydrogenated (annealed in hydrogen atmosphere) cadmium oxide (CdO) thin films co-doped with iron (Fe) of different levels and fixed (2.5%) copper (Cu) amount were deposited on glass and silicon wafer substrates by thermal evaporation. The films were characterised with X-ray fluorescence, X-ray diffraction, optical spectroscopy, and dc-electrical measurements. The obtained results show important improvements in the conductivity, mobility, and carrier concentration compared to un-doped and non-hydrogenated CdO. Hydrogenated CdO doped with 2.5% Cu and 1.3% Fe improved the conductivity (2293.6 S/cm) by ~46 times, mobility (78.31 cm²/V s) by ~11 times, and carrier concentration (1.82×10²⁰ cm⁻³) by ~4 times. This suggests the possibility of using CdO:Cu:Fe–H as transparent-conducting-oxide and dilute-magnetic-semiconductor field of applications.     

Bulky rigid substitutions: A route to high electron mobility and high solid-state luminescence efficiency of perylene diimide

In this paper, we report that a kind of perylene diimide derivative with bulky rigid substituents, 1,7-bis(p-tert-butylphenoxy)-N,N′-dicyclohexyl-perylene-3,4,9,10-tetracarboxylic diimide (TBPCHPDI) possesses both high electron mobility (1.8 cm² V⁻¹s⁻¹) and high fluorescence quantum yield (0.32) in the solid state. Through X-ray diffraction (XRD), UV–Vis absorption and fluorescence spectra, and differential scanning calorimetry (DSC) measurements, it is demonstrated that the above phenomenon can be ascribed to the unique crystal structure of TBPCHPDI: due to steric hindrance of bulky rigid substituents, the intermolecular π–π actions are neglectable, providing high luminescence efficiency; in the mean time, the spacing between perylene chromophores is still very short (3.47 Å), which is favorable for the hopping transportation of charge carrier from one molecule to neighboring molecule. Therefore, our finding would help design and synthesize novel organic semiconductive materials with potential applications in electrically pumped lasers which require high emission efficiency when large current density is applied.     

Characterization of solution processed,p-doped films using hole-only devices and organic field-effect transistors

We report a solution processed, p-doped film consisting of the organic materials 4,4′,4″-tris(3-methylphenylphenylamino)triphenylamine (MTDATA) as the electron donor and 2-(3-(adamantan-1-yl)propyl)-3,5,6-trifluorotetracyanoquinodimethane (F3TCNQ-Adl) as the electron acceptor. UV–vis–NIR absorption spectra identified the presence of a charge transfer complex between the donor and acceptor in the doped films. Field-effect transistors were used to characterize charge transport properties of the films, yielding mobility values. Upon doping, mobility increased and then slightly decreased while carrier concentration increased by two orders of magnitude, which in tandem leads to conductivity increasing from 4 × 10^?10 S/cm when undoped to 2 × 10^?7 S/cm at 30 mol% F3TCNQ-Adl. The hole density was calculated based on mobility values extracted from OFET data and conductivity values extracted from bulk I–V data for the MTDATA: x mol% F3TCNQ-Sdl films. These films were then shown to function as the hole injection/hole transport layer in a phosphorescent blue OLED.     

Properties of CdTe films brush plated on high temperature substrates

CdTe thin films were brush plated on substrates maintained at temperatures in the range 30–90 °C from the precursors. The films exhibited cubic structure. Optical band gap of 1.45 eV was obtained. XPS measurements indicated the formation of CdTe. AFM studies indicated the formation of fine grains of the order of 50 nm, for the films deposited on room temperature substrates. Hot probe measurements indicated films to be n-type. A mobility in the range of 5–60 cm² V⁻¹ s⁻¹ and a carrier density of 10¹⁵ cm⁻³ was obtained.