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Electric transport properties of chemically modificated carbon nanotubes (CNT) using Si-containing organic molecules and polymers were investigated by means of the field effect transistors (FET) technique. From the results of FET measurements for each chemically surface modified CNT, it was shown that p-type semiconducting CNT can be converted to n-type ones by physical adsorption of Si-containing organic molecules and polymers having Ph-groups. It is suggested that the electron carrier are doped into CNT from the adsorbed molecules and polymers, and it was also confirmed by the results of adsorption spectra. That is, it can be said that the electronic properties of CNT can be controlled by chemically modifications of outer surface.
We report high-mobility rubrene single-crystal field-effect transistors with ionic-liquid electrolytes used for gate dielectric layers. As the result of fast ionic diffusion to form electric double layers, their capacitances remain more than 1.0 μF/cm2 even at 0.1 MHz. With high carrier mobility of 9.5 cm2/Vs in the rubrene crystal, pronounced current amplification is achieved at the gate voltage of only 0.2 V, which is two orders of magnitude smaller than that necessary for organic thin-film transistors with dielectric gate insulators. The results demonstrate that the ionic-liquid/organic semiconductor interfaces are suited to realize low-power and fast-switching field-effect transistors without sacrificing carrier mobility in forming the solid/liquid interfaces.
Since the invention of organic electroluminescent devices, a great deal of effort has been made to improve their performance. Reducing the barrier and optimizing charge injection is crucial for efficient and bright Organic Light Emitting Diodes (OLEDs). We report the performance of OLEDs with ITO/TPD/Alq3/Al structure by inserting LiF both at electrode-organic interfaces and organic-organic interface. We elucidate the mechanism of the LiF buffer layer inserted at different interfaces. The device with LiF as a cathode injection layer shows improved luminescence and steeper IV characteristics.
Calculated results for spin injection, transport, and magneto-resistance (MR) in organic semiconductors sandwiched between two ferromagnetic contacts are presented. The carrier transport is modeled by spin dependent device equations in drift-diffusion approximation. In agreement with earlier results, spin injection from ferromagnetic contacts into organic semiconductors can be greatly enhanced if (spin-selective) tunneling is the limiting process for carrier injection. Modeling the tunnel processes with linear contact resistances yields spin currents and MR that tend to increase with increasing bias. We also explore the possibility of bias dependent contact resistances and show that this effect may limit MR to low bias.
We studied the formation of phenylalkyltrichlorosilane self-assembled monolayers on native oxide covered silicon. After a first chemisorption step in the monolayer growth, the presence of the short alkyl chain (3-4 carbon atoms) is responsible for a second growth step which corresponds to the arrangement between molecules. We found that this packing step is accelerated by replacing phenyl by pentafluoro-phenyl rings, possibly due to quadrupolar interactions between fluorinated cycles. Furthermore we demonstrate that mixing phenyl and pentafluoro-phenyl molecules leads to an even faster packing step which is accounted for by hydrogen bonding CH...FC in a face to face phenyl/pentafluoro-phenyl arrangement. We believe these results allow improving charge delocalization over conjugated molecular domains. In a second part, we studied the phase separation between phenyl-alkyltrichlorosilane and octadecyltrichlorosilane (OTS) molecules. Improving the phase separation was studied using ring to ring interactions afore-analyzed. We show phase separation is improved and OTS islands are smaller with phenyl species that involve stronger ring to ring interactions. The best case is obtained with mixing phenyl and pentafluoro-phenyl rings using hydrogen bonds for packing together the aromatic species. These results demonstrate improved control of SAM composition and morphology essential to further use the obtained islands for building molecular devices.
Organic single crystals offer the interesting and unique opportunity to investigate the intrinsic electrical behaviour of organic materials, excluding hopping phenomena due to grain boundaries and structural imperfections. Their structural asymmetry permits also to investigate the correlation between their three-dimensional order and their charge transport characteristics. Here we report on millimeter-sized, solution-grown organic single crystals, based on 4-hydroxycyanobenzene (4HCB), which exhibit three-dimensional anisotropic electrical properties along the three crystallographic axes a, b (constituting the main crystal flat face) and c (the crystal thickness), measured over several different samples. The carrier mobility was determined by means of space charge limited current (SCLC) and air-gap field effect transistors fabricated with 4HCB single crystals and the measured value well correlate with the structural packing anisotropy of the molecular crystal. A differential analysis of SCLC curves allowed to determine the distribution and the concentration of the dominant electrically active density of states within the gap.
Single wall carbon nanotube enabled vertical field effect transistors (VFETs) are studied and the dependence of the on/off ratio on the relative number of electron traps is investigated. Current versus voltage measurements on several VFETs with varying interfacial trap densities in the vicinity of the nanotube network/polymer active layer junction are taken. It is found that the on/off ratio of the VFET changes from 1600 to 20 for typical operational currents as the onset gate voltage in the off-to-on transfer curve shifts from 94 V to 72 V. Such a strong dependence on trapped charge motivates future work to uncover the mechanism of charge trapping.
How to accurately determine carrier mobility and density in organic semiconducting materials is a very important subject for their optoelectronic applications including light-emitting diodes, solar cells, and thin film field-effect transistors. In this work, we report on a unique data analysis procedure for space-charge limited currents to simultaneously obtain the carrier density and mobility in semiconducting organic-materials. This procedure has been used for a few newly synthesized perylene tetracarboxylic diimide (PDI) derivatives with tunable π-stack structures without altering the electronic characteristic of individual molecules. How π-stack structural variation and residual carrier density affect electron transport performance will be discussed.
Studies of gate dielectrics in organic field effect transistors (OFETs) have been attractive because the electric properties of OFETs are susceptibly affected by the choice of the gate dielectrics. Here, we demonstrate a tunable threshold voltage in an organic field effect transistor (OFET) using an ion-dispersed gate dielectrics. By applying external electric field (Vex) to the gate dielectrics, the dispersed ions in the gate dielectrics are separated by electrophoresis and form space charge polarization. The drain current of the OFET increased over 1.9 times and the threshold voltage (Vth) decreased 22 V (from -35.1 V to -13.1 V).
The shift direction of Vth was easily tuned by the polarity of the external voltage. The dielectric permittivity of the gate dielectrics and mobility of the active layer were unchanged after the polarization of the gate dielectrics. The UV-VIS differential absorption spectra of the OFETs indicate that there is no chemical doping in the active layer of the OFETs. These results indicated the shifts of threshold voltages were originated from the polarization of gate dielectrics.
We report the increase in open-circuit voltage (Voc) by inserting of MoO3 layer on ITO substrate to improve built-in potential of organic solar cells (OSCs). In the OSCs using 5,10,15,20-tetraphenylporphyrine (H2TPP) as a p-type material and C60 as a n-type material, the Voc effectively increased from 0.57 to 0.97 V as increasing MoO3 thickness. The obtained highest Voc (0.97 V) is consistent with the theoretical value estimated from the energy difference between the LUMO (−4.50 eV) of C60 and the HOMO (−5.50 eV) of H2TPP layer. Importantly, the enhancement in the Voc was achieved without affecting the short-circuit current density (Jsc) and the fill-factor (FF). Thus, the power conversion efficiency of the device linearly increased from 1.24% to 1.88%. We also demonstrated that a MoO3 buffer layer enhances the stability of OSCs after photo-irradiation. We have investigated the stability of OSCs using H2TPP and N,N′-di(1-naphthyl)-N,N′-diphenylbenzidine as a p-type layer. The both devices with MoO3 layer showed improved stability. These results clearly suggest that the interface at ITO/p-type layer affects the device stability.
Three-dimensional organic field-effect transistors are developed with multiple vertical channels of organic semiconductors to gain high output current and high on-off ratio. High-mobility and air-stable dinaphtho[2,3-b:2′,3′-f]thieno[3,2-b]thiophene thin films deposited on horizontally elongated vertical sidewalls have realized unprecedented high output current per area of 2.6 A/cm2 with the application of drain voltage -10 V and gate voltage -20 V. The on-off ratio is as high as 2.7×106. Carrier mobility of the organic semiconductor deposited on the vertical sidewalls is typically 0.30 cm2/Vs. The structure is built also on plastic substrates, where still considerable current modulation is preserved with high output current per area of 70 mA/cm2 and with high on-off ratio of 8.7×106. The performance exceeds practical requirements for applications in driving organic light-emitting diodes in active-matrix displays. The technique of gating with electric double layers of ionic liquid is also introduced to the three-dimensional transistor structure.
Herein we report on the synthesis of perylene diimide (PDI) based P1 and P2 conjugated polymers via Suzuki polymerization. The chemical structure of the polymers was elucidated using GPC, 1H, 13C NMR and elemental analysis. The absorption spectra of polymers were in the visible region from 250 – 800 nm in solution and in solid state. The optical band gap was (Egopt) found to be between 1.60 – 1.83 eV in solid state.
Simplicity of construction and operation are advantages of iTMC (ionic transition metal complex) OLEDs compared with multi-layer OLED devices. Unfortunately, lifetimes do not compare favorably with the best multi-layer devices. We have previously shown for Ru(bpy)3(PF6)2 based iTMC OLEDs that electrical drive produces emission-quenching dimers of the active species. We report evidence here that a chemical process may also be implicated in degradation of devices based on Ir(ppy)2(dtb-bpy)PF6 albeit by a very different mechanism. It appears that degradation of operating devices made with this Ir-based complex is related to current-induced heating of the organic layer, resulting in loss of the dtb-bpy ligand. (The dtb-bpy ligand is labile compared with the cyclometallated ppy ligands.) Morphological changes observed in electrically driven Ir(ppy)2(dtb-bpy)PF6 OLEDs provide evidence of substantial heating during device operation. Evidence from UV-vis spectra in the presence of an electric field as well as MALDI-TOF mass spectra of the OLED materials before and after electrical drive add support for this model of the degradation process.
OLED with non-constant dopant concentration profiles have been processed by means of organic vapour phase deposition (OVPD) and were compared with regard to their luminous current efficiencies. Especially when driven at ultra-high luminance (>10,000 cd/A), OLED with a dopant concentration profile starting with a rather high dopant concentration on the anode side of the emissive layer showed improved luminous current efficiencies compared to their conventional counterparts.
To further investigate this effect, the width and location of the recombination zone have been simulated for all investigated concentration profiles by numerical solution of the semiconductor device equations using experimentally determined doping-dependent charge carrier mobilities. The obtained theoretical results are discussed with regard to the accomplished experiments.
We have shown that hole mobilities of a wide variety of organic thin films can be estimated using a steady-state space-charge-limited current (SCLC) technique due to formation of Ohmic hole injection by introducing a very thin hole-injection layer of molybdenum oxide (MoO3) between an indium tin oxide anode layer and an organic hole-transport layer. Organic hole-transport materials used to estimate hole mobilities are 4,4′,4″-tris(N-3-methylphenyl-N-phenyl-amino)triphenylamine (m-MTDATA), 4,4′,4″-tris(N-2-naphthyl-N-phenyl-amino)triphenylamine (2-TNATA), rubrene, N,N′-di(m-tolyl)-N,N′-diphenylbenzidine (TPD), and N,N′-diphenyl-N,N′-bis(1-naphthyl)-1,1′-biphenyl-4,4′-diamine (α-NPD). These materials are found to have electric-field-dependent hole mobilities. While field dependence parameters (β) estimated from SCLCs are almost similar to those estimated using a widely used time-of-flight (TOF) technique, zero field SCLC mobilities (μ0) are about one order of magnitude lower than zero field TOF mobilities.
In this work, ambipolar rubrene single crystal field-effect transistors (FETs) with PMMA modification layer and Au/Ca as electrodes were fabricated. The electron mobility was studied in these devices. PMMA modification layer on the surface of SiO2 is necessary for electron behavior. We found that the device with PMMA modified insulator and Au-Ca asymmetric metals possessed hole mobility and electron mobility of 1.27 and 0.017 cm−2/Vs, respectively. Furthermore, the shift of light emitting with applied gate voltage was observed in this device.
Thermal conductivity of rubrene single crystals is measured for both bulk and film-like crystals down to 0.5 K in order to estimate quantitatively density of crystalline defects through their phonon mean free paths. The temperature profile of the bulk rubrene crystals exhibit pronounced peak at ∼ 10 K in the thermal conductivity as the result of very long mean-free paths of their phonons which indicates extremely low-level defect density in the region of 1015-1016 cm−3 depending on different growth methods. The crystals grown from gas phase tend to have less defects than those grown from solution. It turned out that the film-like crystals have a few times more defect density as the result of the measurement by using newly developed devices for minute crystals.
The current and luminous efficacy of a red phosphorescent organic light emitting diode (OLED) with sharp interfaces between each of the organic layers can be increased from 18.8 cd/A and 14.1 lm/W (at 1,000 cd/m2) to 36.5 cd/A (+94%, 18% EQE) and 33.7 lm/W (+139%) by the introduction of a layer cross-fading zone at the hole transport layer (HTL) to emission layer (EL) interface. Layer cross-fading describes a procedure of linearly decreasing the fraction in growth rate of an organic layer during deposition over a certain thickness while simultaneously increasing the fraction in growth rate of the following layer. For OLED processing and layer cross-fading organic vapor phase deposition (OVPD) is used. The typical observation of a roll-off in current efficacy of phosphorescent OLED to higher luminance can be reduced significantly. An interpenetrating network of a prevailing hole and a prevailing electron conducting material is created in the cross-fading zone. This broadens the recombination zone and furthermore lowers the driving voltage. The concept of layer cross-fading to increase the efficacies is suggested to be useful in multi-colored OLED stacks as well.
In this paper the optical absorption properties of n-type C60 and PTCDA, and p-type CuPc small molecule semiconductors are investigated by optical transmission and Photothermal Deflection Spectroscopy (PDS). The results show the usual absorption bands related to HOMO-LUMO transitions in the high absorption region of the transmission spectra. PDS measurements also evidences exponential absorption shoulders with different characteristic energies. In addition, broad bands in the low absorption level are observed for C60 and PTCDA thin-films. These bands have been attributed to contamination due to air exposure. In order to get deeper understanding of the degradation mechanisms single and co-evaporated thin-films have been characterized by PDS. The dependence of the optical coefficient on exposure to light and air have been studied and correlated to the structural properties of the films (as measured by X-Ray Diffraction Spectroscopy). The results show that CuPc and PTCDA are quite stable against light and air exposure, while C60 shows important changes in its absorption coefficient. The bulk heterojunctions show stability in agreement with what observed for single layers, since the absorption coefficient of CuPc:PTCDA is almost not altered after the degradation treatments, while CuPc:C60 shows changes for low energy values.
The formation of nanocrystal-molecule-nanocrystal nanostructures via controlled mixing of Au nanocrystals and bifunctional Re linkers is reported. UV-visible absorbance data, coupled with histogram analysis of nanostructures measured using Scanning Electron Microscopy has shown a characteristic optical response at wavelengths close to 600 nm following formation of dimer and trimer nanostructures. Directed assembly processes based on dielectrophoretic trapping have also been developed for electrical interfacing of these nanostructures between top-down nanoelectrode pairs for electrical characterization.