Effect of doping of cesium carbonate on electron transport in Tris(8-hydroxyquinolinato) aluminum

Electron transport studies in Tris(8-hydroxyquinolinato) aluminum (Alq₃) is hindered due to lack of efficient electron injecting electrode. We demonstrate that an electron injection layer of Cs₂CO₃ forms ohmic contact with Alq₃, which enables the observation of SCLC. This allows us to directly determine the electron mobility in Alq₃, which was found to be 1 × 10^?9 m²/V s at room temperature. Doping of Cs₂CO₃ leads to increase in conductivity as well as mobility. Mobility has increased to 1 × 10^?7 m²/V s for 33% doping of Cs₂CO₃.     

MoO3 as p-type dopant for Alq3-based p–i–n homojunction organic light-emitting diodes

Using high-work-function material MoO₃ as a p-type dopant, efficient single-layer hybrid organic light-emitting diodes (OLEDs) with the p–i–n homojunction structure are investigated. When MoO₃ and Cs₂CO₃ are doped into the conventional emitting/electron-transport material tris-(8-hydroxyquinoline) aluminum (Alq₃), respectively, a significant increase in p- and n-type conductivities is observed compared to that of intrinsic Alq₃ thin films. With optimal doping, the hole and electron mobilities in Alq₃:MoO₃ and Alq₃:Cs₂CO₃ films was estimated to be 9.76 × 10⁻⁶ and 1.26 × 10⁻⁴ cm²/V s, respectively, which is about one order of magnitude higher than that of the undoped device. The p–i–n OLEDs outperform undoped (i–i–i) and single-dopant (p–i–i and i–i–n) OLEDs; they have the lowest turn-on voltage (4.3 V at 1 cd/m²), highest maximum luminance (5860 cd/m² at 11.4 V), and highest luminous efficiency (2.53 cd/A at 100 mA/cm²). These values are better than those for bilayer heterojunction OLEDs using the same emitting layer. The increase in conductivity can be attributed to the charge transfer process between the Alq₃ host and the dopant. Due to the change of carrier concentration in the Alq₃ films, the Fermi level of Alq₃ is close to the highest occupied molecular orbital (HOMO) or lowest unoccupied molecular orbital (LUMO) energy levels upon p- and n-type doping, respectively, and the carrier injection efficiency can thus be enhanced because of the lower carrier injection barrier. The carriers move closer to the center energy levels of the HOMO or LUMO distributions, which increases the hopping rate for charge transport and results in an increase of charge carrier mobility. The electrons are the majority charge carriers, and both the holes and electrons can be dramatically injected in high numbers and then efficiently recombined in the p–i–n OLEDs. As a result, the improved conductivity characteristics as well as the appropriate energy levels of the doped layers result in improved electroluminescent performance of the p–i–n homojunction OLEDs.     

Comparison of two cohost systems for doped red organic light-emitting devices in an effort to improve the efficiency and the lifetime

Blue-emitting 2-methyl-9,10-di(2-napthyl)anthracene (MADN) and yellow-emitting 5,6,11,12-tetraphenylnaphthacene (rubrene) were used as cohost materials together with tris(8-hydroxyquinolinato)aluminum (Alq₃) to form emission layers doped with the red dopant molecule 4-(dicyanomethylene)-2-t-butyl-6-(1,1,7,7-tetramethyljulolidyl-9-enyl)-4H-pyran (DCJTB). DCJTB-doped red organic light-emitting diodes based on both cohost systems showed remarkable improvements in terms of efficiency compared to DCJTB-doped Alq₃ single-host devices. With 2% DCJTB doping, the respective efficiencies of Alq₃ single-host, Alq₃ (60%)/rubrene (40%)-, and Alq₃ (20%)/MADN (80%)-cohost devices were 1.79, 4.44 and 5.42 cd/A at 20 mA/cm². Unlike Alq₃/rubrene-cohost devices, which experienced substantial current-induced quenching, Alq₃/MADN-cohost devices showed only a slight efficiency change at high current densities. At the luminance of 7680 cd/m², which was the benchmark for a practical passive-matrix OLED array with 64 scan-lines, an aperture ratio of 50%, and a polarizer transmittance of 50%, the power efficiency of the 2% DCJTB Alq₃/MADN-cohost device was 4.1 and 1.5 times better than that of Alq₃ single-host and Alq₃/rubrene-cohost devices, respectively. Moreover, the half-decay lifetime of the Alq₃/MADN-cohost device, measured as 14,000 h at an initial luminance of 1000 cd/m², was 4.4 and 1.9 times longer than the respective half-decay lifetimes of Alq₃ single-host and Alq₃/rubrene-cohost devices.     

Hole mobilities of thermally polymerized triaryldiamine derivatives and their application as hole-transport materials in organic light-emitting diodes (OLEDs)

This paper describes the synthesis of three triaryldiamine derivatives presenting two thermally polymerizable trifluorovinyl ether groups that can be polymerized through thermal curing to form perfluorocyclobutyl (PFCB) polymers. These PFCB polymers, studied using time-of-flight techniques for the first time, exhibited remarkable non-dispersive hole-transport properties, with values of μ_h of ca. 10^?4 cm² V^?1 s^?1. When we employed these thermally polymerized polymers as hole-transport layers (HTLs) in electroluminescence devices containing tris(8-hydroxyquinolate) aluminum (Alq₃) as the emission layer, we obtained high current densities (ca. 3400 mA cm^?2), impressive brightnesses (5 × 10⁴ cd m^?2), and high external quantum efficiencies (EQEs = 1.43%). These devices exhibited the same turn-on voltage, but higher EQEs, relative to those incorporating the vacuum-processed model compound N,N′-di(1-naphthyl)-N,N′-diphenylbenzidine (α-NPD) (EQE = 1.37%) as the HTL under the same device structure.     

Investigation of deep level defects in copper irradiated bipolar junction transistor

Commercial bipolar junction transistor (2N 2219A, npn) irradiated with 150 MeV Cu¹¹⁺-ions with fluence of the order 10¹² ions cm^?2, is studied for radiation induced gain degradation and deep level defects. I–V measurements are made to study the gain degradation as a function of ion fluence. The properties such as activation energy, trap concentration and capture cross-section of deep levels are studied by deep level transient spectroscopy (DLTS). Minority carrier trap levels with energies ranging from E_C ? 0.164 eV to E_C ? 0.695 eV are observed in the base–collector junction of the transistor. Majority carrier trap levels are also observed with energies ranging from E_V + 0.203 eV to E_V + 0.526 eV. The irradiated transistor is subjected to isothermal and isochronal annealing. The defects are seen to anneal above 350 °C. The defects generated in the base region of the transistor by displacement damage appear to be responsible for transistor gain degradation.     

Efficient blue organic light-emitting diodes using newly-developed pyrene-based electron transport materials

We synthesized new kinds of pyrene-based electron transport materials: 1,6-di(pyridin-3-yl)-3,8-di(naphthalen-1-yl)pyrene (N1PP) and 1,6-di(pyridin-3-yl)-3,8-di(naphthalen-2-yl)pyrene (N2PP). The external quantum efficiencies of the device with these electron transport materials increase by more than 50% at 1 mA cm^?2 compared with those of the device with representative Alq₃ as an electron transport material. The enhanced quantum efficiency is due to a balanced charge recombination in an emissive layer. Electron mobilities in N1PP and N2PP films are three times higher than that in Alq₃. Highly enhanced power efficiency is achieved due to a low electron injection barrier and a high electron mobility. We find that the luminance degradation in the blue OLEDs is correlated with the HOMO energy levels of electron transport materials.     

Electrochemical growth of tin(II) oxide films: Application in photocatalytic degradation of methylene blue

We have investigated the semiconducting and photoelectrochemical properties of SnO films grown potentiostatically on tin substrate. The oxide is characterized by X-ray diffraction, scanning electron microscopy and Raman spectroscopy. The anodic process corresponds to the formation of SnO·nH₂O pre-passive layer that is removed upon increasing potential due to surface etching at the metal/oxide interface. SnO films deposited for long durations (>50 mn) are uniform and well adhered; they thicken up to ~50 nm by diffusion-controlled process and the growth follows a direct logarithmic law. The thickness is determined by coulometry and the X-ray diffraction indicates the tetragonal SnO phase (SG: P4/mmm) with a crystallite size of 32 nm. The Mott–Schottky plot is characteristic of n type conductivity with an electrons density of 5.72×10¹⁸ cm⁻³, a flat band potential of −0.09 V_SCE and a depletion width of ~10 nm. The valence band, located at 5.91 eV below, vacuum is made up of hybridized O²⁻:2p Sn²⁺:5s while the conduction band (4.45 eV) derives from Sn²⁺:5p orbital. The electrochemical impedance spectroscopy (EIS) measured in the range (10⁻²–10⁵ Hz) shows the contribution of the bulk and grain boundaries. The energy band diagram predicts the photodegradation of methylene blue on SnO films. 67% of the initial concentration (10 mg L⁻¹) disappears after 3 h of exposure to visible light (9 mW cm⁻²) with a quantum yield of 0.072.     

Electrodeposited diamond-like carbon (DLC) films on n-Si(100) substrates for photovoltaic application

Synthesis of n-Si/diamond-like carbon (DLC) bi-layer films was realized by depositing DLC films on n-Si (100) substrate by a simple electrodeposition technique using acetonitrile (CH₃CN) as the electrolyte. The films were characterized by Field Emission Scanning Electron Microscopy (FESEM), X-ray photoelectron spectroscopy (XPS), Fourier Transformed Infrared spectroscopy (FTIR) and Raman studies. Solar cell characteristics were investigated critically. Open circuit voltage (V_oc) and short circuit current density (J_sc) obtained for the best n-Si (100)/DLC structure were 335 mV and 9.6 mA/cm², respectively, corresponding to a total conversion efficiency of η= 1.65%. Open circuit photovoltage decay measurement was carried out to determine the carrier lifetime. Photoinduced charge separation in the n-Si (100)/DLC structure could be associated with an increase in the dielectric constant and a decrease in the device resistance. The process is cheap, reproducible and scalable, involving significantly lesser process steps. This is likely to usher in a new hope to the current competitive scenario of photovoltaic (PV) technology.     

Characterization of scaled MANOS nonvolatile semiconductor memory (NVSM) devices

We have modeled and characterized scaled Metal–Al₂O₃–Nitride–Oxide–Silicon (MANOS) nonvolatile semiconductor memory (NVSM) devices. The MANOS NVSM transistors are fabricated with a high-K (K_A = 9) blocking insulator of ALD deposited Al₂O₃ (8 nm), a LPCVD silicon nitride film (8 nm) for charge-storage, and a thermally grown tunneling oxide (2.2 nm). A low voltage program (+8 V, 30 μs) and erase (?8 V, 100 ms) provides an initial memory window of 2.7 V and a 1.4 V window at 10 years for an extracted nitride trap density of 6 × 10¹⁸ traps/cm³ eV. The devices show excellent endurance with no memory window degradation to 10⁶ write/erase cycles. We have developed a pulse response model of write/erase operations for SONOS-type NVSMs. In this model, we consider the major charge transport mechanisms are band-to-band tunneling and/or trap-assisted tunneling. Electron injection from the inversion layer is treated as the dominant carrier injection for the write operation, while hole injection from the substrate and electron injection from the gate electrode are employed in the erase operation. Meanwhile, electron back tunneling is needed to explain the erase slope of the MANOS devices at low erase voltage operation. Using a numerical method, the pulse response of the threshold voltages is simulated in good agreement with experimental data. In addition, we apply this model to advanced commercial TANOS devices.     

Aggregation,crystallization, and resistance properties of poly(3-hexylthiophene-2,5-diyl) solid films gel-cast from CHCl3/p-xylene mixed solvents

In this study we found that the gelation time and crystallinity of P3HT solid films are adjustable when aging and casting from CHCl₃/p-xylene mixed solvents. After aging for 36 h in pure p-xylene, CHCl₃, or various mixtures of the two as cosolvents, we found that the solid P3HT film gel-cast from 20 vol% CHCl₃ had the highest degree of crystallinity of its main chain (ϕ_m = 0.54), highest melting point of its main chain (T_m = 232.7 °C), fastest gelation time (30 min), largest melting enthalpy of its main chain (ΔH_m = 19.81 J g⁻¹), and lowest resistance (R_P = 0.76 MΩ); the latter value was three orders and one order of magnitude lower than those of the films cast from pure CHCl₃ (ca. 110 MΩ) and pure p-xylene (ca. 4.4 MΩ), respectively. In differential scanning calorimetry scans, we attribute the presence of melting peaks near 75 °C to the solid-to-solid phase transition of the side chain crystallites of P3HT, thereby affecting the aggregation of the P3HT main chain and resulting in the changes in resistance, crystallinity, melting enthalpy, and melting point of the gel-cast P3HT solid films.     

Growth temperature dependence of epitaxial Gd2O3 films on Si(1 1 1)

This article reports on the epitaxy of crystalline high κ oxide Gd₂O₃ layers on Si(1 1 1) for CMOS gate application. Epitaxial Gd₂O₃ thin films have been grown by Molecular Beam Epitaxy (MBE) on Si(1 1 1) substrates between 650 and 750 °C. The structural and electrical properties were investigated depending on the growth temperature. The C–V measurements reveal that equivalent oxide thickness (EOT) equals 0.7 nm for the sample deposited at the optimal temperature of 700 °C with a relatively low leakage current of 3.6 × 10^?2 A/cm² at |V_g ? V_FB| = 1 V.     

Effect of high-temperature annealing on lanthanum aluminate thin films grown by ALD on Si(1 0 0)

Amorphous lanthanum aluminate thin films were deposited by atomic layer deposition on Si(1 0 0) using La(ⁱPrCp)₃, Al(CH₃)₃ and O₃ species. The effects of post-deposition rapid thermal annealing on the physical and electrical properties of the films were investigated. High-temperature annealing at 900 °C in N₂ atmosphere leads to the formation of amorphous La-aluminosilicate due to Si diffusion from the substrate. The annealed oxide exhibits a uniform composition through the film thickness, a large band gap of 7.0 ± 0.1 eV, and relatively high dielectric constant (κ) of 18 ± 1.     

Improved reliability of AlGaN/GaN-on-Si high electron mobility transistors (HEMTs) with high density silicon nitride passivation

We have systematically studied the effects of Si_xN_1 − x passivation density on the reliability of AlGaN/GaN high electron mobility transistors. Upon stressing, devices degrade in two stages, fast-mode degradation and followed by slow-mode degradation. Both degradations can be explained as different stages of pit formation at the gate-edge. Fast-mode degradation is caused by pre-existing oxygen at the Si_xN_1 − x/AlGaN interface. It is not significantly affected by the Si_xN_1 − x density. On the other hand, slow-mode degradation is associated with Si_xN_1 − x degradation. Si_xN_1 − x degrades through electric-field induced oxidation in discrete locations along the gate-edges. The size of these degraded locations ranged from 100 to 300 nm from the gate edge. There are about 16 degraded locations per 100 μm gate-width. In each degraded location, low density nano-globes are formed within the Si_xN_1 − x. Because of the low density of the degraded locations, oxygen can diffuse through these areas and oxidize the AlGaN/GaN to form pits. This slow-mode degradation can be minimized by using high density (ρ = 2.48 g/cm³) Si₃₆N₆₄ as the passivation layer. For slow-mode degradation, the median time to failure of devices with high density passivation is found to increase up to 2× as compared to the low density (ρ = 2.25 g/cm³) Si₄₃N₅₇ passivation. A model based on Johnson-Mehl-Avrami theory is proposed to explain the kinetics of pit formation.     

High-performance pentacene field-effect transistors using Al2O3 gate dielectrics prepared by atomic layer deposition (ALD)

High-performance pentacene field-effect transistors have been fabricated using Al₂O₃ as a gate dielectric material grown by atomic layer deposition (ALD). Hole mobility values of 1.5 ± 0.2 cm²/V s and 0.9 ± 0.1 cm²/V s were obtained when using heavily n-doped silicon (n⁺-Si) and ITO-coated glass as gate electrodes, respectively. These transistors were operated in enhancement mode with a zero turn-on voltage and exhibited a low threshold voltage (< −10 V) as well as a low sub-threshold slope (<1 V/decade) and an on/off current ratio larger than 10⁶. Atomic force microscopy (AFM) images of pentacene films on Al₂O₃ treated with octadecyltrichlorosilane (OTS) revealed well-ordered island formation, and X-ray diffraction patterns showed characteristics of a “thin film” phase. Low surface trap density and high capacitance density of Al₂O₃ gate insulators also contributed to the high performance of pentacene field-effect transistors.     

Effect of substrate temperature on indium tin oxide (ITO) thin films deposited by jet nebulizer spray pyrolysis and solar cell application

This study focused on the effect of substrate temperature (350 °C, 400 °C, and 450 °C) on morphological, optical, and electrical properties of indium tin oxide (ITO) films deposited onto porous silicon/sodalime glass substrates through jet nebulizer spray pyrolysis for use in heterojunction solar cells. X-ray diffraction analysis confirmed the formation of pure and single-phase In₂O₃ for all the deposited films whose crystallinity was enhanced with increasing substrate temperature, as shown by the increasing (222) peak intensity. Morphological observations were conducted using scanning electron microscopy to reveal the formation of continuous dense films composed of nanograins. The UV–vis spectra revealed that the transmittance increased with increasing substrate temperature, reaching a value of over 80% at 450 °C. The photoelectric performance of the solar cell was studied using the I–V curve by illuminating the cell at 100 mW/cm². A high efficiency (η) of 3.325% with I_sc and V_oc values of 14.8 mA/cm² and 0.60 V, respectively, was attained by the ITO solar cell annealed at 450 °C.     

Solution-processable and thermal-stable triphenylamine-based dendrimers with truxene cores as hole-transporting materials for organic light-emitting devices

Two solution processable π-conjugated triphenylamine-based dendrimers, Tr-TPA3 and Tr-TPA9 were served as hole-transporting materials (HTMs) for organic light-emitting devices (OLEDs). The two dendrimers exhibit similar absorption and emission behaviors in solutions and thin films, which demonstrate that these dendrimers can form amorphous states in their films. The dendrimers showed excellent solubility, which are soluble in common organic solvents such as chloroform, tetrahydrofuran, and 1,1,2,2-tetrachloroethane, high thermal stability with high glass-transition temperature (T_g) of 115 °C for Tr-TPA3 and 140 °C for Tr-TPA9, high the highest unoccupied molecular orbital (HOMO) energy level (?5.12 eV for Tr-TPA3 and ?4.95 eV for Tr-TPA9, respectively) and good film forming property. When we employed these dendrimers as hole transport layer (HTL) in tris-(8-hydroxyquinoline) aluminum (Alq₃)-emitting electroluminescence (EL) devices, the Tr-TPA9-based double-layer device exhibited the turn-on voltage of 2.5 V, the maximum luminance of about 11,058 cd m^?2 and the maximum current efficiency of 4.01 cd A^?1. The comparison of the properties between the EL devices with dendrimers as HTL and the EL device with 1,4-bis(1-naphthylphenylamino)biphenyl (NPB) as HTL indicated that this series of dendrimers can be good candidates for HTM in OLEDs.     

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.     

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 temperature TiO2 rutile phase thin film synthesis by chemical spray pyrolysis (CSP) of titanyl acetylacetonate

Rutile phase TiO₂ thin films have been synthesized using chemical spray pyrolysis of titanyl acetylacetonate TiAcAc in ethanol at 500 °C. The first part of the paper focuses on the thermal decomposition behavior of the precursor by simultaneous thermogravimetry and differential thermal analysis (TG/DTA) coupled with differential scanning calorimetry (DSC). The second part of the paper focuses on the evolution of TiO₂ thin films and their structural transformation with substrate temperature. XRD revealed amorphous TiO₂ thin film at low substrate temperatures (<350 °C) and on high substrate temperatures anatase (3.84 g/cm³) or rutile (4.25 g/cm³) crystalline structure was obtained. The lattice constant, grain size, microstrain and the dislocation density of the film were obtained from the peak width. FTIR spectra of both anatase and rutile TiO₂ revealed stretching vibration of the Ti–O bond for tetrahedral and octahedral surroundings of the titanium atom. Scanning electron micrograph showed the compactness of the rutile film.     

Organic alternating current electroluminescence device based on 4,4′-bis(N-phenyl-1-naphthylamino) biphenyl/1,4,5,8,9,11-hexaazatriphenylene charge generation unit

An organic alternating current electroluminescence (OACEL) device based on 4,4′-bis(N-phenyl-1-naphthylamino) biphenyl (NPB)/1,4,5,8,9,11-hexaazatriphenylene (HAT-CN)/tris(8-hydroxy-quin-olinato) aluminum (Alq₃) doped with cesium carbonate (Cs₂CO₃) internal charge generation unit is demonstrated. Maximum luminance of 299 cd/m² is observed for Alq₃ doped with 10-(2-Benzothiazolyl)-2,3,6,7-tetrahydro-1,1,7,7-tetramethyl-1H,5H, 11H-(1) benzopyropyrano (6,7-8-I,j)quinolizin-11-one (C545T) fluorescent emission layer when driven with a peak–peak voltage of 80 V at 120 kHz. The key charge-generation role of NPB/HAT-CN interface is studied experimentally. Furthermore, influence of evaporation sequence of this internal charge generation unit on OACEL performance is investigated. This work demonstrated that the undoped charge generation unit – NPB/HATCN, can also be a good candidate for charge generation unit of OACEL device.