Temperature effect on the physical properties of CuIn11S17 thin films for photovoltaic applications

CuIn₁₁S₁₇ compound was synthesized by horizontal Bridgman method using high-purity copper, indium and sulfur elements. CuIn₁₁S₁₇ thin films were prepared by high vacuum evaporation on glass substrates. The glass substrates were heated at 30, 100 and 200 °C. The structural properties of the powder and the films were investigated using X-ray diffraction (XRD). XRD analysis of thin films revealed that the sample deposited at a room temperature was amorphous in nature while those deposited on heated substrates were polycrystalline with a preferred orientation along the (311) plane of the spinel phase. Ultraviolet–visible (UV–vis) spectroscopy was used to study the optical properties of thin films. The results showed that CuIn₁₁S₁₇ thin films have high absorption coefficient α in the visible range (10⁵–10⁶ cm⁻¹). The band gap E_g of the films decrease from 2.30 to 1.98 eV with increasing the substrate temperature (T_s) from 30 to 200 °C. We exploited the models of Swanepoel, Wemple–DiDomenico and Spitzer–Fan for the analysis of the dispersion of the refractive index n and the determination of the optical constants of the films. Hot probe method showed that CuIn₁₁S₁₇ films deposited at T_s=30 °C and T_s=100 °C are p-type conductivity whereas the sample deposited at T_s=200 °C is highly compensated.     

Study of the correlation between structural and photoluminescence properties of CdSe thin films deposited by close-spaced vacuum sublimation

CdSe polycrystalline films were deposited by a close-spaced vacuum sublimation method at different substrate temperatures (T_s) using glass slides as substrates. At T_s≤673 K the films have a structure with strong dispersion of grain size (d) (from 0.1 to 0.3 μm). In this case the layer-by-layer mechanism determines the growth process of the layers. For T_s=873 K they have a columnar-like structure with a clear growth texture and the average grain size d=3–4 μm. The films obtained at T_s>473 K are n-type and only correspond to a single wurtzite phase. The crystallites are preferentially oriented with the (102) planes parallel to the substrate. At lower temperatures the films are bi-phase. The microstress level in CdSe films obtained at Т_s=873 K (0.5×10⁻³) is considerably smaller than for the films deposited at Т_s=773 K (4.0×10⁻³). Increase of the value of T_s improves the stoichiometry of CdSe films. Analysis of the low-temperature photoluminescence (PL) spectra let us determine the nature and energy of point and extended defects in the investigated films. It was shown that the films contain Na(Li) and P residual impurities. The results of the structural and PL measurements showed that the CdSe polycrystalline films are of fairly good crystal and optical quality for T_s=873 K and can be suitable for various applications.     

The effect of annealing on the physical properties of thermally evaporated CuIn2n+1S3n+2 thin films (n=0, 1, 2 and 3)

In this study, the annealing effect on structural, electrical and optical properties of CuIn_2n+1S_3n+2 thin films (n=0, 1, 2 and 3) are investigated. CuIn_2n+1S_3n+2 films were elaborated by vacuum thermal evaporation and annealed at 150 and 250 °C during 2 h in air atmosphere. XRD data analysis shows that CuInS₂ and CuIn₃S₅ (n=0 and 1) crystallize in the chalcopyrite structure according to a preferential direction (112), CuIn₅S₈ and CuIn₇S₁₁ (n=2 and 3) crystallize in the cubic spinel structure with a preferential direction (311). The optical characterization allowed us to determine the optical constants (refractive indexes 2.2–3.1, optical thicknesses 250–500 nm, coefficients of absorption 10⁵ cm^?1, coefficients of extinction <1, and the values of the optical transitions 1.80–2.22 eV) of the samples of all materials. We exploited the models of Cauchy, Wemple–DiDomenico and Spitzer–Fan for the analysis of the dispersion of the refractive index and the determination of the optical and dielectric constants.     

Synthesis and luminescent properties of ternary complexes EuXLa1−X(TTA)3Dipy

Rare-earth ternary complexes Eu_XLa_1?X(TTA)₃Dipy (X = 0, 0.1, 0.25, 0.5, 0.75, 0.9, 1.0) were synthesized. Characterization with DTA-TG, IR, elemental analysis and fluorescent spectra had also been carried out. It is found that the enhanced luminescence of Eu³⁺ ions by La³⁺ ions occurs in ternary complexes, and when X = 0.25, Eu_0.25La_0.75(TTA)₃Dipy has the highest luminescence efficiency and lifetime. It is proved by TG curve that the complexes are stable, and we monitored the spectra of Eu_XLa_1?X(TTA)₃Dipy[PVK:Eu_XLa_1?X(TTA)₃Dipy/BCP/AlQ/Al] at the different rate r min^?1. The results showed that the La³⁺ ion acts as an energy transfer bridge that helps energy transfer from PVK to Eu³⁺.     

First principles study of electronic and optical properties of the ternary SnSb4S7 using modified Becke-Johnson potential

The electronic and optical properties of SnSb₄S₇ compound are calculated by the full-potential linearized augmented plane-wave (FP-LAPW) method. The density of states (DOS) is carried out by the modified Becke-Johnson (mBJ) exchange potential approximation based on density functional theory (DFT). The compound SnSb₄S₇ has a monoclinic structure with the space group P2₁/m with lattice parameters of a=11.331 Å, b=3.865 Å and c=13.940 Å. The band gap is calculated to be 0.8 eV. The optical parameters, like dielectric constant, refractive index, reflectivity and energy loss function were also calculated and analyzed. The present work provides information about variation of the electronic and optical properties which reveals that SnSb₄S₇ is suitable for optoelectronic devices.     

Spirobifluorene/sulfone hybrid: Highly efficient solution-processable material for UV–violet electrofluorescence,blue and green phosphorescent OLEDs

A high efficient UV–violet emission type material bis[4-(9,9′-spirobifluorene-2-yl)phenyl] sulfone (SF-DPSO) has been synthesized by incorporating electron deficient sulfone and morphologically stable spirobifluorene into one molecule. The steric and bulky compound SF-DPSO exhibits an excellent solid state photoluminescence quantum yield (Φ_PL = 92%), high glass transition temperature (T_g = 211 °C) and high triplet energy (E_T = 2.85 eV). In addition, the uniform amorphous thin film could be formed by spin-coating from its solution. These promising physical properties of the material made it suitable for using as UV–violet emitter in non-doped device and appropriate host in phosphorescent OLEDs. With SF-DPSO as an emitter, the non-doped solution processed device achieved an efficient UV–violet emission with the EL peak around 400 nm. By using SF-DPSO as a host, solution processed blue and green phosphorescent organic light emitting diodes showed a high luminous efficiency of 13.7 and 30.2 cd A⁻¹, respectively.     

Porous network of samarium sulfide thin films for supercapacitive application

In the present letter, a novel aqueous chemical method is employed to prepare thin film of Sm₂S₃ material containing porous network of interconnected nanoparticles for supercapacitive application. The orthorhombic phase formation of Sm₂S₃ film is concluded from X–ray diffraction study. The chemical states of samarium and sulfur are determined using X–ray photoelectron spectroscopy study. The pseudocapacitive behavior of Sm₂S₃ showed a maximum specific capacitance of 248 F g⁻¹ in 1.5 M LiClO₄ electrolyte prepared in propylene carbonate electrolyte. The nature of charge and discharge curves confirmed pseudocapacitive behavior of film electrode. The highest power and energy densities of 15.6 kWh kg⁻¹ and 54.6 Wh kg⁻¹, respectively are obtained. An electrochemical stability of 94% is retained after 1500 cycles.     

Operational frequency degradation induced trapping in scaled GaN HEMTs

Cut-off frequency increase from 12.1 GHz to 26.4 GHz, 52.1 GHz and 91.4 GHz is observed when the 1 μm gate length GaN HEMT is laterally scaled down to L_G = 0.5 μm, L_G = 0.25 μm and L_G = 0.125 μm, respectively. The study is based on accurately calibrated transfer characteristics (I_D-V_GS) of the 1 μm gate length device using Silvaco TCAD. If the scaling is also performed horizontally, proportionally to the lateral (full scaling), the maximum drain current is reduced by 38.2% when the gate-to-channel separation scales from 33 nm to 8.25 nm. Degradation of the RF performance of a GaN HEMT due to the electric field induced acceptor traps experienced under a high electrical stress is found to be about 8% for 1 μm gate length device. The degradation of scaled HEMTs reduces to 3.5% and 7.3% for the 0.25 μm and 0.125 gate length devices, respectively. The traps at energy level of E_T = E_V + 0.9 eV (carbon) with concentrations of N_IT = 5 × 10¹⁶cm^− 3, N_IT = 5 × 10¹⁷cm^− 3 and N_IT = 5 × 10¹⁸cm^− 3 are located in the drain access region where highest electrical field is expected. The effect of traps on the cut-off frequency is reduced for devices with shorter gate lengths down to 0.125 μm.     

Defect characterization of Tl4GaIn3Se2S6 layered single crystals by photoluminescence

Photoluminescence (PL) spectra of Tl₄GaIn₃Se₂S₆ layered crystals grown by the Bridgman method have been studied in the energy region of 2.02–2.35 eV and in the temperature range of 16–45 K. A broad PL band centered at 2.20 eV was observed at T=16 K. Variations of emission band has been studied as a function of excitation laser intensity in the 0.1 to 149.9 mW cm⁻² range. Radiative transitions from shallow donor level located at 10 meV below the bottom of conduction band to moderately deep acceptor level located at 180 meV above the top of the valence band were suggested to be responsible for the observed PL band. An energy level diagram showing transitions in the band gap of the crystal was plotted taking into account the results of present work and previously reported paper on thermally stimulated current measurements carried out below room temperature. Analysis of the transmission and reflection measurements performed in the wavelength range of 400–1030 nm at room temperature revealed the presence of indirect transitions with 2.22 eV band gap energy.     

Effect of aqueous electrolyte on pseudocapacitive behavior of chemically synthesized La2S3 electrode

Lanthanum sulfide electrode (La₂S₃) is prepared by a low cost, simple and room temperature chemical route for energy storage. The surface morphology of La₂S₃ film is revealed through field emission scanning electron microscopy. For the energy storage purpose, the pseudocapacitive behavior of La₂S₃ electrode is studied in 1 M aqueous Na₂SO₄ and 1 M KOH electrolytes. La₂S₃ electrode achieved maximum specific capacitance of 358 F g⁻¹ at 5 mV s⁻¹ scan rate with 78% electrochemical cyclic stability over 1000 cycles in 1 M Na₂SO₄ electrolyte. The galvanostatic charge–discharge study demonstrated the energy density of 35 Wh kg⁻¹ at power density of 1.26 kW kg⁻¹. The electrochemical impedance study showed field assisted charge transfer process with relaxation time of 32 ms in 1 M Na₂SO₄ electrolyte ensuring fast redox reaction.     

Physical investigations on Sb2S3 sprayed thin film for optoelectronic applications

Sb₂S₃ thin films have been obtained at 250 °C on glass substrates using the spray pyrolysis techniques. The structural study by means of XRD analysis shows that Sb₂S₃ thin film crystallized in the orthorhombic phase. The discussion of some structural constants has been made by means of both XRD and AFM investigations. Moreover, the optical analysis via the transmittance and the reflectance measurements reveals that Sb₂S₃ sprayed thin film has a direct transition with the band gap energy E_g equal to 1.72 eV. The analysis in 300–2500 nm domain of the refractive index data through Wemple–DiDomenico model leads to the single oscillator energy (E₀=2.32 eV), and the dispersion energy (E_d=10.03 eV). The electrical study leads to the dc activation energy is of the order of 0.72 eV and the maximum barrier high is W_M=0.87 eV. From the power exponent variation in terms of the heated temperature, it is found that the mechanism of conduction matches well the correlated barrier hopping CBH model.     

Analysis of recursive convolutional codes and turbo codes as sources with memory

The paper proposes a general method to analyze discrete sources with memory. Besides the classical entropy, we define new information measures for discrete sources with memory, similar to the information quantities specific to discrete channels. On the base of this method, we show for the first time that, as result of convolutional and turbo encoding, sources with memory are obtained. We apply this information analysis method for the general case of a recursive convolutional encoder of rate R_CC = 1/n₀ and memory of order m, and for a turbo encoder of rate R_TC = 1/3, with two systematic recursive convolutional component encoders. Each component encoder has memory of order m, and is built based on the same primitive feedback polynomial. For the convolutional and turbo codes, the information quantities H(Y/S), H(S,Y), H(S/Y), H(Y), H(S) and I(S,Y) have been computed, where S and Y denote the set of states and the set of messages of the encoder, respectively. The analysis considered two cases: n₀ ≤ m + 1 and n₀ > m + 1. When n₀ = m + 1, the mutual information I(S,Y) is maximum and equal to m, as is the entropy of the set of states. For turbo codes, the quantity I(S,Y) also depends on the input bit and on its probability.     

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.     

Investigation on the thermal behavior of 0.15 μm gate-length In0.4Al0.6As/In0.4Ga0.6As MHEMT

In this study, we have successfully investigated the electrical performances of In_0.4Al_0.6As/In_0.4Ga_0.6As metamorphic high-electron-mobility transistor (MHEMT) at temperatures range from 275 K to 500 K comprehensively. By extracting the device S-parameters, the temperature dependent small signal model has been established. At room temperature, 0.15 μm T-gate device with double δ-doping design exhibits f_T and f_MAX values of 103 GHz and 204 GHz at V_ds = 1 V, an extrinsic transconductance of 678 mS/mm, and a current density of 578 mA/mm associated with a high breakdown voltage of ?13 V. Power measurements were evaluated at 40 GHz and the measured output power, linear power gain, and maximum power-added efficiency, were 7.12 dBm, 10.15 dB, and 23.1%, respectively. The activation energy (E_a) extracted from Arrhenius plots is = 0.34 eV at 150 ≦ T ≦ 350 K. The proposed device is promisingly suitable for millimeter-wave power application.     

Tuning the energy gap and charge balance property of bipolar host by molecular modification: Efficient blue electrophosphorescence devices based on solution-process

Three bipolar hosts composed of electron-accepting diphenylphosphine oxide and electron-donating carbazole/triphenylamine have been synthesized and characterized. With structural topology modification, the particular physical properties of the materials can be subtly optimized, such as the thermal stability, singlet–triplet energy gap and charge balance ability. Both DFT calculation and experiment results demonstrate that the introduced triphenylamine can effective minimize the HOMO–LUMO energy gap, while the carbazole units can prevent the excited energy loss and keep high triplet energy (E_T = 3.0 eV) due to the enhanced molecular rigidity. As a result, solution-processed blue PHOLEDs exhibited a high current efficiency of 25.2 cd A⁻¹ and a power efficiency of 11.5 lm W⁻¹, which implies that the unique molecular modulation is very cost-effective and competitive for the device performance improving.     

Functionalized terfluorene for solution-processed high efficiency blue fluorescence OLED and electrophosphorescent devices

A new multifunctional blue-emitting terfluorene derivative (TFDPA) featured with triphenylamine groups for hole-transportation and long alkyl chains for solution processability on the conjugation inert bridge centers was reported. TFDPA can give homogeneous thin film by solution process and exhibits high hole mobility (μ_h ≈ 10^?3 cm² V^?1 s^?1) and suitable HOMO for hole injection. Particularly, TFDPA performs efficient deep-blue emission with high quantum yield (～100% in solution, 43% in thin film) and suitable triplet energy (E_T = 2.28 eV), making solution-processed OLED devices of using TFDPA as blue emitter and as host for iridium-containing phosphorescent dopants feasible. The solution-processed nondoped blue OLED device gives saturated deep-blue electroluminescence [CIE = (0.17, 0.07)] with EQE of 2.7%. TFDPA-hosted electrophosphorescent devices performed with EQE of 6.5% for yellow [(Bt)₂Ir(acac)], 9.3% of orange [Ir(2–phq)₃], and 6.9% of red [(Mpq)₂Ir(acac)], respectively. In addition, with careful control on the doping concentration of [(Bt)₂Ir(acac)], a solution-processed fluorescence–phosphorescence hybrided two-color-based WOLED with EQE of 3.6% and CIE coordinate of (0.38, 0.33) was successfully achieved.     

Structural and magnetic properties of silver doped melt-textured YBCO prepared in a solar furnace

Magnetic measurements and structural investigation have been performed on melt-textured YBCO and AgYBCO HTS. The “sun” technique produces very dense YBCO (ρ=5.86 g cm⁻³) and AgYBCO [ρ=6.36 g cm⁻³; 10% b/w (by weight) silver. This technique renders samples with a large volume fraction of the Y₂BaCuO₅ (211) phase. The material is characterized by very high J_c values, as compared with bulk polycrystalline YBCO prepared by other methods. This feature is attributed to the enhanced amount of 211 particles which serve as pinning centers. Additional significant densification of the structure due to silver incorporation is obtained, and a reduction of the size of 211 inclusions is also observed. Silver doped samples show “butterfly”-like hysteresis loops at relatively high temperatures (T≥60K). This feature is probably associated with oxygen deficiency which arises from the slower oxygen diffusion into silver doped samples. J_c values enhancement was obtained in silver doped “sun” samples at high temperatures (T≥60K) and fields of 20–30 kOe. The temperature dependence of effective activation energy of pinning, U_eff, was measured for YBCO and AgYBCO materials. U_eff is higher in silver doped samples in the high temperature region T≥60K.     

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.     

An experimental investigation of the off-diagonal Seebeck effect on textured YBCO high-Tc superconductor

The off-diagonal Seebeck effect was investigated on a melt-textured YBa₂Cu₃O_7−δ (YBCO) high-T_c superconductor. It was found that the transverse voltage V_x was proportional to the longitudinal temperature difference Δ T_z, measured directly with a pair of differential thermocouples, for textured samples with their c-axes tilted with respect to the surface’s normal. This supported the idea that the off-diagonal thermoelectric effect accounts for the anomalously high laser-induced transverse voltage on the oriented YBCO superconducting thin films. The variation of the V_x against the sample’s thickness d, at a given Δ T_z, deviated from the inversely proportional relationship when the sample was too thin. The deviation was discussed qualitatively in terms of deteriorated surface layers. The data of V_x(d) was fitted to obtain a reasonably reliable ∣ S_c−S_ab∣ value of 12 μ V K⁻¹.     

Broadband nuclear magnetic resonance using DC SQUID amplifiers

We have constructed two pulsed NMR spectrometers in which the signal is coupled to the input coil of a low T_c DC SQUID using a superconducting flux transformer, yielding broadband response, with bandwidth determined by the SQUID electronics. A 50 kHz bandwidth commercial system has been used to observe free induction decay signals from platinum powder, bulk platinum, ³He gas and surface monolayers of ³He in the temperature range from 1.4 to 4.2 K and at frequencies from 5 to 40 kHz. The observed signal-to-noise ratio is as calculated with the noise dominated by flux noise in the SQUID in all samples but the bulk metal. A second system, which operates in flux-locked loop mode with bandwidth of 3.4 MHz using a SQUID with additional positive feedback, has been used to observe NMR signals from platinum powder at frequencies from 38 to 513 kHz and at a temperature of 4.2 K. The advantage of this technique in the study of systems with short T₂ at frequencies below 1 MHz is discussed. In addition we discuss the benefits of both broadband and tuned input circuits for NMR detection and we describe the performance of a spectrometer with a tuned input circuit which has been used to obtain signals at 1 MHz from platinum powder at 4.2 K and from ∼2 layers of ³He absorbed on a surface area of 0.11 m² at 1.7 K. The amplifier noise temperature is predicted to be 60 mK in the ³He experiment. This demonstrates the potential of the tuned set-up for measurements at low millikelvin temperatures on systems with low spin density and with T₂ greater than several hundred microseconds.