Electrical characteristics and efficiency of organic light-emitting diodes depending on hole-injection layer

In a device structure of ITO/hole-injection layer/N,N′-biphenyl-N,N′-bis-(1-naphenyl)-[1,1′-biphthyl]4,4′-diamine(NPB)/tris(8-hydroxyquinoline)aluminum(Alq₃)/Al, we investigated the effect of the hole-injection layer on the electrical characteristics and external quantum efficiency of organic light-emitting diodes. Thermal evaporation was performed to make a thickness of NPB layer with a rate of 0.5–1.0 Å/s at a base pressure of 5 × 10⁻⁶ Torr. We measured current–voltage characteristics and external quantum efficiency with a thickness variation of the hole-injection layer. CuPc and PVK buffer layers improve the performance of the device in several aspects, such as good mechanical junction, reducing the operating voltage, and energy band adjustment. Compared with devices without a hole-injection layer, we found that the optimal thickness of NPB was 20 nm in the device structure of ITO/NPB/Alq₃/Al. By using a CuPc or PVK buffer layer, the external quantum efficiencies of the devices were improved by 28.9% and 51.3%, respectively.     

Photo-induced isomerization of poly 4′-(6-acryloxy) hexyloxy-4-methoxyazobenzene ultra-thin solid film prepared by Langmuir–Blodgett technique

The photo-induced response of an ultra thin polymeric film of poly 4′-(6-acryloxy) hexyloxy-4-methoxyazobenzene (P5) is investigated. A monolayer of P5 at a gas–water interface possesses a mean molecular area of 28.0 Å²/monomer-repeat. Multilayer films of P5 were prepared by horizontal deposition at a surface pressure of 25 mN/m². The uniformity of the transfer process is shown by UV–vis absorption spectra where a linear relationship between the absorption maxima and the number of transferred layer was observed. The average layer thickness of the transferred film determined by XRD measurements is 34.0 Å. This is longer than the length of the azobenzene side group. The transferred film shows a blue shift of the π–π¹ transition from 357 nm for the P5 in solution to 340 nm for the P5 in the film. This suggests the formation of H-aggregate with a head-to-head arrangement of the dipole within the film. The optical property of the transferred film is changed by the irradiation of the film with the UV light at 385 nm. An irreversible change in its molecular packing in the film is seen in the shift of the UV–vis absorption maxima and the change in morphology as observed by AFM. The film morphology changes from being a smooth film into an island-like surface when exposed to the UV irradiation. The layer structure in the film is destroyed. A mass transport is observed during the cis–trans thermal back isomerization process. This suggests that movement of the P5 took place in both the trans–cis isomerization process and the cis–trans back isomerization process. The first movement leads to a molecular expansion while the second, to a molecular contraction.     

Structural and photoluminescence investigation on the hot-wire assisted plasma enhanced chemical vapor deposition growth silicon nanowires

High density of silicon nanowires (SiNWs) were synthesized by a hot-wire assisted plasma enhanced chemical vapor deposition technique. The structural and optical properties of the as-grown SiNWs prepared at different rf power of 40 and 80 W were analyzed in this study. The SiNWs prepared at rf power of 40 W exhibited highly crystalline structure with a high crystal volume fraction, X_C of ～82% and are surrounded by a thin layer of SiO_x. The NWs show high absorption in the high energy region (E>1.8 eV) and strong photoluminescence at 1.73 to 2.05 eV (red–orange region) with a weak shoulder at 1.65 to 1.73 eV (near IR region). An increase in rf power to 80 W reduced the X_C to ～65% and led to the formation of nanocrystalline Si structures with a crystallite size of <4 nm within the SiNWs. These NWs are covered by a mixture of uncatalyzed amorphous Si layer. The SiNWs prepared at 80 W exhibited a high optical absorption ability above 99% in the broadband range between 220 and ～1500 nm and red emission between 1.65 and 1.95 eV. The interesting light absorption and photoluminescence properties from both SiNWs are discussed in the text.     

Investigation of reduced graphene oxide effects on ultra-violet detection of ZnO thin film

The reduced graphene oxide (rGO) incorporated ZnO thin films were fabricated by dip-coating method. The Raman and FT-IR spectra of 0.075 wt% incorporated composite film showed reduction of GO in composite film. The transmittanceProd. Type: FTP spectra have shown that rGO incorporation increase the visible light absorption of ZnO thin film while the calculated band gaps of samples were decreased from 3.28 to 3.25 eV by increasing the rGO content. The linear trend of I–V curve suggests an ohmic contact between ZnO and rGO. Besides, it was found that by increasing the rGO content, the electrical resistivity was decreased from 4.32×10² Ω cm for pure ZnO film to 2.4×10¹ Ω cm for 0.225 wt% rGO incorporated composite film. The composite photodetectors not only possessed a desirable UV photosensitivity, but also the response time of optimum sample containing 0.075 wt% rGO was reduced to about one-half of pure ZnO thin film. Also, the calculated signal to noise (SNR) showed that highly conductive rGO in composite thin films facilitate the carrier transportation by removing the trapping centers. The mechanism of photoresponsivity improvement of composite thin films was proposed by carrier transportation process.     

The annealing effect on structural,optical and photoelectrical properties of CuInS2/In2S3 films

Successive Ionic Layer Adsorption and Reaction (SILAR) technique was used to deposit the CuInS₂/In₂S₃ multilayer thin film structure at room temperature. The as-deposited film was annealed at 100, 200, 300, 400 and 500 °C for 30 min in nitrogen atmosphere and the annealing effect on structural, optical and photoelectrical properties of the film was investigated. X-ray diffraction (XRD) and optical absorption spectroscopy were used for structural and optical studies. Current–Voltage (I–V) measurements were performed in dark environment and under 15, 30 and 50 mW/cm² light intensity to investigate the photosensitivity of the structure. Also, the electrical resistivity of the film was determined in the temperature range of 300–470 K. It was found that annealing temperature drastically affects the structural, optical and photoelectrical properties of the CuInS₂/In₂S₃ films.     

Optical properties of Cr/Fe(t)/MgO (0 0 1) thin films

In this work we report on the optical properties of single-crystalline iron thin films. For this, Cr-capped Fe films with thickness, t, in the range 30–300 Å were prepared on MgO (0 0 1) by DC magnetron sputtering, and then studied by optical absorption technique within the range from 1.0 to 3.6 eV. All measurements were carried out at room temperature using a fiber optics spectrophotometer. The intensity of the transmitted light decreases with increasing film thickness. The optical constants of the films are deduced from a model that considers the transmission of light by two absorbing films on an absorbing substrate. The absorption coefficient of the Fe films is also calculated from the transmission data. The absorption spectra show the following characteristics: (i) two large absorption peaks centered at about 1.20 and 2.65 eV; and (ii) a sharp step near 1.40 eV. These structures are associated with conventional interband transitions of the iron film.     

Terahertz pulse driven Josephson junctions

The voltage response of a Josephson junction to a pulsed terahertz current is evaluated in the limit of a negligible junction capacitance (overdamped limit). The time-dependent superconductor phase difference across the junction is calculated in the framework of the standard resistive shunted junction model by using a perturbative method. The pulsed current bias affects the time average value of the voltage across the junction and current steps are induced in the current–voltage characteristics for voltage values depending on the pulse repetition rate. The current step height is proportional to the square of the pulse time width (τ) to the period (T) ratio. A fast response detector for pulsed Terahertz radiation is proposed, with an expected responsivity of the order of 0.1 V/W and an equivalent noise power of about 3 × 10^?10 W/Hz^1/2.     

Optical absorption and dispersion characterizations of CuGaSe2 thin films prepared by flash evaporation technique

Thin films of CuGaSe₂ have been prepared by flash evaporation technique. The optical properties of the prepared films were investigated using spectrophotometric measurements of the transmittance and reflectance at normal incidence of light in the wavelength range from 400 to 2500 nm. The optical constants as refractive index, n, and absorption index, k, were calculated and found to be independent of film thickness in the range of the film thickness 132–423 nm. The analysis of the photon energy against the absorption coefficient showed three direct optical transitions (one of them is allowed while the others are forbidden). This direct transition was ascribed to the crystal field and spin orbital splitting of the upper most valence band. The crystal field and spin orbital splitting of CuGaSe₂ were found to be ? 0.15 eV and 0.45 eV, respectively. The dispersion of the refractive index is discussed in terms of the single oscillator Wemple–DiDomenico (WD) model. The single oscillator energy (E_o), the dispersion energy (E_d), the high frequency dielectric constant (ε_∞), the lattice dielectric constant (ε_L) and the ratio of free charge carrier concentration to the effective mass (N / m*) were estimated. The capacitance–voltage measurements of CuGaSe₂/p-Si heterojunction showed that the diode is abrupt junction diode. The carrier concentration and the built-in voltage were estimated. The current–voltage characteristics of the device under illumination were investigated and photovoltaic properties of the device were evaluated.     

Electronic properties of the polycrystalline tin dioxide interface with conjugated organic layers

The results on the electronic structure of the unoccupied electronic states of the polycrystalline SnO₂ in the energy range from 5 eV to 25 eV above the Fermi level are presented. The modification of the electronic structure and of the surface potential upon deposition of the ultrathin films of copper phthalocyanine (CuPc) and of perylene tetracarboxylic acid dianhydride (PTCDA) film onto the SnO₂ surface were studied using the very low energy electron diffraction (VLEED) method and the total current spectroscopy (TCS) measurement scheme. A substantial attenuation of the TCS signal coming from the SnO₂ surface was observed upon formation of a 1.5–2 nm thick organic deposit layer while no new spectral features from the deposit were distinguishable. It was observed that the electronic structure typical for the organic films was formed within the organic deposit thickness range from 2 nm to 7 nm. The interfacial charge transfer was characterized by the formation of the polarization layer up to 5 nm thick in the organic films. The PTCDA deposition on SnO₂ was accompanied by the negative charge transfer onto the organic layer and to the 0.65 eV increase the surface work function. At the CuPc/SnO₂ interface, the negative charge was transferred to the SnO₂ surface and the overall surface work function decreased by 0.15 eV.     

Structural origin of perpendicular magnetic anisotropy in Ni–W thin films

The magnetic properties and microstructure of electrodeposited Ni–W thin films (0–11.7 at% W in composition) were studied. The film structures were divided into three regions: an FCC nanocrystalline phase (0–2 at% W), a transition region from FCC nanocrystalline to amorphous phase (2–7 at% W), and an amorphous phase (>7 at% W). In the transition region, (4–5 at% W) films with perpendicular magnetic anisotropy (PMA) were found. The saturation magnetization, magnetic anisotropy field, perpendicular magnetic anisotropy and perpendicular coercivity for a typical Ni–W film (4.5 at% W) were 420 kA/m, 451 kA/m, 230 kJ/m and 113 kA/m, respectively. The microstructure of Ni–W films with PMA is composed of isolated columnar crystalline grains (27–36 nm) with the FCC phase surrounded by the Ni–W amorphous phase. The appearance of the interface between the magnetic core of Ni crystalline grains and the Ni–W non-magnetic boundary layer seems to be the driving mechanism for the appearance of PMA. The origin of PMA in Ni–W films is mainly attributed to the magnetoelastic anisotropy associated with in-plane internal stress and positive magnetostriction. The secondary source of PMA is believed to be the magnetocrystalline anisotropy of 〈1 1 1〉 columnar grains and its shape magnetic anisotropy. It is concluded that Ni–W electrodeposited films (4–5 at% W) may be applicable for perpendicular magnetic recording media.     

Microstructures and nano mechanical properties of the metal tungsten film

The W film was prepared on 1045 steel by magnetron sputtering, with the thickness of 2 μm, its surface and cross-section morphologies were investigated with SEM, and the phase structure was analyzed with XRD. X-ray stress determinator was utilized to measure its residual stress, and the nano-hardness and elastic modulus of the film were surveyed by nano-indentation tester. The results show that the surface of W film is very compact and smooth; the particles arranged regularly, the granularity of the thin film is about 1 μm. The microcracks, cavities and desquamation were not found in the film and interface, and the bonding between the W film and substrate is well. The XRD results showed that the W film had a body-centered cubic structure, the lattice constant: a = 0.316 nm, the growth preferred orientations are (1 1 0) and (2 2 0). The compressive stress (−169 MPa) was found on the surface. The average nano-hardness and elastic modulus of W film are 15.22 GPa, 176.64 GPa, respectively, and the mechanical properties of W film are well.     

Admittance of metal–insulator–semiconductor structures based on graded-gap HgCdTe grown by molecular-beam epitaxy on GaAs substrates

Metal–insulator–semiconductor structures based on n-Hg_1−xCd_xTe (x = 0.19–0.25) were grown by molecular-beam epitaxy on the GaAs (0 1 3) substrates. Near-surface graded-gap layers with high CdTe content were formed on both sides of the epitaxial HgCdTe. Admittance of these structures was studied experimentally in a wide temperature range (8–150) K. It is shown that an increase in the composition of the working layer and a decrease in temperature lead to a decrease in the frequency of transition to high-frequency behavior of the capacitance–voltage characteristics. The differential resistance of space charge region in the strong inversion increases with the composition of the working layer and for x = 0.22 and 0.25, the differential resistance is limited by the Shockley-Read generation. The values of the differential resistance of space charge region at different frequencies and temperatures were found.     

Measurement of optical and surface properties of PLZT thin films

Lanthanum-modified lead zirconate titanate (Pb_0.93La_0.07(Zr_0.3Ti_0.7)_0.93O₃, PLZT7/30/70) thin films with and without a seeding layer of PbTiO₃ (PT) were successfully deposited on indium-doped tin oxide (ITO) coated glass substrate via spin coating in conjunction with a sol–gel process, and a top transparent conducting thin film of SnO₂ was also prepared in the same way. The thicknesses of PLZT and PT layers are 0.5 μm and 24 nm, respectively. The retardance of PLZT film was measured by a new heterodyne interferometer and enhanced by application of a seeding layer of PT. The Pockels linear electro-optical coefficient of PLZT film with a PT layer was determined to be 3.17 × 10^?9 m/V when the refractive index is considered as 2.505, which is one order larger than 1.4 × 10^?10 m/V for PLZT12/40/60 doped with Dy reported in the literature. The root-mean-square (rms) roughness of PLZT thin film with a PT layer (R_rms = 6.867 nm) was larger than that of PLZT film (R_rms = 0.799 nm). From the comparisons, the average transmittance of PLZT film with a PT seeding layer was 77.01%, which was a little smaller than that of PLZT film (around 80.75%). Experimental results imply that the PT seeding layer plays a key role in the increase of retardance value, leading to a higher Pockels coefficient.     

Using high haze (> 90%) light-trapping film to enhance the efficiency of a-Si:H solar cells

The high haze light-trapping (LT) film offers enhanced scattering of light and is applied to a-Si:H solar cells. UV glue was spin coated on glass, and then the LT pattern was imprinted. Finally, a UV lamp was used to cure the UV glue on the glass. The LT film effectively increased the Haze ratio of glass and decreased the reflectance of a-Si:H solar cells. Therefore, the photon path length was increased to obtain maximum absorption by the absorber layer. High Haze LT film is able to enhance short circuit current density and efficiency of the device, as partial composite film generates broader scattering light, thereby causing shorter wave length light to be absorbed by the P layer so that the short circuit current density decreases. In case of lab-made a-Si:H thin film solar cells with v-shaped LT films, superior optoelectronic performances have been found (V_oc = 0.74 V, J_sc = 15.62 mA/cm², F.F. = 70%, and η = 8.09%). We observed ~ 35% enhancement of the short-circuit current density and ~ 31% enhancement of the conversion efficiency.     

Influence of γ-irradiation on the optical parameters of 4-tricyanovinyl-N,N-diethylaniline thin films

Thin films of 4-tricyanovinyl-N,N-diethylaniline (TCVA) were prepared by thermal evaporation technique. The spectral and the optical parameters have been investigated by using the spectrophotometric measurements of both transmittance and reflectance at normal incidence of light in the wavelength range 200–2500 nm. The effect of γ-irradiation on the optical parameters was investigated. It was observed that the increase in γ-irradiation dose caused an increase in the value of absorption index and a shift in the spectrum towards higher wavelengths. Therefore, the value of the optical band gap has decreased from 1.45 eV for as-deposited film to 1.39 eV for film exposed to γ-ray dose of 150 kGy and Urbach tail increased. On the other hand, the dispersion parameters of TCVA films were increased with the increase of the irradiation dose.     

Persistence in photoconductivity and optical property of nanostructured copper (II) phthalocyanine thin films

The optical and photoconductive properties of nanostructured copper (II) phthalocyanine (CuPc) surface cells have been studied at room temperature (30 °C). The change in the conductivity of the surface type sample cells under illumination of light was studied as a function of duration and wavelength of photoexcitation. The photoconductivity spectrum of a film of CuPc surface cell does not agree with the absorption spectrum. Particularly for the photoexciting wavelengths in the region 400–500 nm, the photoconductivity shows an appreciable value (or comparatively higher value), although the absorbance in this region is minimum or very less. The change in current under illumination of light and after turning it off has indicated that photoinduced effects persist for a longer period of time even after turning off the illumination of light of various wavelengths and duration. An increase in persistence in photocurrent was noticed with the repeated exposure within a short interval of time. Such studies are important for searching new types of electrically conducting systems (semiconductors) in the form of nanostructured thin films for use as photoactive materials in optoelectronic and memory devices.     

Photo-curable epoxy functionalized cyclotetrasiloxane as a gate dielectric for organic thin film transistors

We synthesized a new photo-curable organic/inorganic hybrid material, cyclotetrasiloxane (CTS) derivative containing cyclohexene-1,2-epoxide functional groups (CTS-EPOXY), and its characteristics are compared with a prototypical organic gate insulator of poly(4-vinylphenol) (PVP) in the organic thin film transistors (OTFTs) using pentacene as an active p-type organic semiconductor. Compared with PVP, CTS-EPOXY shows better insulating characteristics and surface smoothness. A metal/insulator/metal (MIM) device with the 300-nm-thick CTS-EPOXY film shows more than two orders of magnitude lower current (less than 40 nA/cm² over the voltage range up to 60 V) compared with PVP. In addition, the pentacene TFT with CTS-EPOXY as a gate dielectric layer shows slightly higher field-effect mobility of μ_FET = 0.20 cm²/V s compared to that with PVP.     

Structural and optical properties of chromotrope 2R thin films of different thicknesses

Chromotrope 2R (CHR) films of different thicknesses have been prepared using spin coater. The material has been characterized using FT-IR, DTA and X-ray diffraction. The XRD of the material in powder and thin film forms showed polycrystalline structure with triclinic phase. Preferred orientation at the (1 1 4) plane is observed for the deposited films. Initial indexing of the XRD pattern was performed using “Crystalfire” computer program. Miller indices, h k l, values for each diffraction line in X-ray diffraction (XRD) spectrum were calculated and indexed for the first time. The DTA thermograms of CHR powder have been recorded in the temperature range 25–350 °C with different heating rates. The spectra of the infra-red absorption allow characterization of vibration modes for the powder and thin film. The effect of film thickness on the optical properties has been studied in the UV-visible-NIR regions. The films show high transmittance exceeding 0.90 in the NIR region λ > 800 nm. The intensity of the absorption peaks for λ < 800 nm are enhanced as the film thickness increase. The absorption bands are attributed to the (π–π*) and (n–π*) molecular transitions. The optical properties have been analyzed according to the single-oscillator model and the dispersion energy parameters as well as the free charge carrier concentration have been determined. The optical energy gap as well as the oscillator strength and electric dipole strength have been calculated.     

Epitaxial growth of Ce2Y2O7 buffer layers for YBa2Cu3O7−δ coated conductors using reel-to-reel DC reactive sputtering

Biaxially textured Ce₂Y₂O₇ (CYO) films were deposited on Ni–5at.%W (Ni–5W) tapes by a DC reactive sputtering technique in a reel-to-reel system. Subsequent YBa₂Cu₃O_7?δ (YBCO) films were prepared using pulsed laser deposition leading to a simplified coated conductor architecture of YBCO/CYO/Ni–5W. X-ray diffraction measurements revealed an epitaxial growth of the CYO buffer layer with a texture spread down to 2.2° and 4.7° for the out-of-plane and in-plane alignment, respectively. Microstructural investigations showed a dense, smooth and crack-free surface morphology for CYO film up to a thickness of 350 nm, implying an effective suppression of cracks due to the incorporation of Y in CeO₂. The superconducting transition temperature T_c of about 90 K with a narrow transition of 0.8 K and the inductively measured critical current density J_c of about 0.7 MA/cm² indicate the potential of the single CYO buffer layer.     

Resistance oscillations in junctions of superconductor–magnetic system

Resistance oscillations as a function of magnetic field were observed in superconductor–magnetic tunnel junctions of Nb–Fe–FeO_x–SiO₂–Au–Nb. Junctions involving superconductor–magnetic layer superconductor system are exciting because for certain regime of ferromagnetic layer thickness, a Josephson coupling with an intrinsic phase difference of π might be stabilized. For fabrication of the tunnel junctions the thin films were deposited by RF/DC magnetron sputtering. Using photolithography and reactive ion etching, square junctions of size varying from 50 μm to 250 μm were defined. I–V characteristics and R vs. H characteristics were studied at 4.2 K. When the magnetic field is applied parallel to the junction plane, measurements of the junction resistance as a function of magnetic field at a fixed temperature show resistance peaks whenever the total magnetic flux through the junction equals an integral multiple of flux quantum. The penetration depth of the superconducting electrodes was estimated from the positions of the resistance peaks.