Low-cost 3D nanocomposite solar cells obtained by electrodeposition of CuInSe2

Thin CuInSe₂ films have been prepared by electrodeposition from a single bath aqueous solution on both dense and nanoporous TiO₂. The films are deposited potentiostatically using a N₂-purged electrolyte at different potentials. Various deposition times and solution compositions have been employed. The effect of annealing in air and in argon at different temperatures and times is also investigated. Thin films and nanocomposites of TiO₂ and CuInSe₂ have been studied with electron microscopy, X-ray diffraction, Raman spectroscopy, and optical absorption spectroscopy. After a thermal anneal in argon at 350 °C for 30 min excellent CuInSe₂ is obtained. In particular the nominal crystal structure and the bandgap of 1.0 eV are found. Although pinholes are present occasionally, good samples with diode curves showing a rectification ratio of 24 at ±1 V are obtained. Upon irradiation with simulated solar light of 1000 W m⁻² a clear photoconductivity response is observed. Furthermore, also some photovoltaic energy conversion is found in TiO₂|CuInSe₂ nanocomposites.     

Studies of single-step electrodeposition of CuInSe2 thin films with sodium citrate as a complexing agent

The influences of various parameters in a single-step electrodeposition of CuInSe₂ from aqueous solution containing CuCl₂, InCl₃, and SeO₂, with sodium citrate as the complexing agent, are investigated. Co-deposition of CuInSe₂ from a room temperature, aqueous bath of these electrolytes is accomplished by the aid of sodium citrate. In this work the optimum potential for deposition of CuInSe₂ is found to be −0.5 V vs. Ag/AgCl, the deposition time is 800 s, the concentration ratio of CuCl₂, InCl₃, and SeO₂ is 9 mM:22 mM:22 mM in aqueous solution, and the annealing temperature is 225 °C. Under the optimum conditions, crystalline layers of CuInSe₂ having the chalcopyrite structure can be successfully synthesized. Cyclic voltammetry (CV), scanning electron microscope (SEM), X-ray diffractometer (XRD), and energy dispersive X-ray spectrometer (EDX) were used to examine the electrochemistry, morphologies, structures, and compositions of CuInSe₂ thin films deposited on ITO glass.     

Laser-induced thermoelectric voltage in normal state MgB2 thin films

Laser-induced voltage has been observed in c-axis oriented MgB₂ thin film at room temperature. The amplitude of the signal is approximately proportional to the film thickness. For the film with the thickness of 150 nm, a very fast response has been detected when the film was irradiated by a 308 nm pulsed laser of 20 ns duration. The rise time and full width at half-maximum of the signal are about 3 and 25 ns, respectively. The physical origin of the laser-induced voltage can be attributed to a transverse thermoelectricity due to the anisotropic thermopower in MgB₂.     

Influence of complexing agent on the electrodeposited Co-Pt-W magnetic thin films

Complexing agents are often used to improve the quality of electrodeposited alloys. Influence of different complexing agents with hydroxycarboxylic acid group on the electrodeposited Co-Pt-W thin films has been investigated. Cathodic polarization curves show that the polarization behaviors of electroplating bath with different complexing agents are very different. Surface morphology, phase composition and magnetic properties are observed by means of FESEM, XRD and vibrating sample magnetometer (VSM), respectively. It has been found out that, if citrate was used as complexing agent, the Co-Pt-W thin films were homogeneous and the granular crystals with the average grain size of 2 μm have been observed. Co-Pt-W thin films exhibited hexagonal close packed (hcp) lattice and strong perpendicular anisotropic magnetic behavior (H_c⊥ = 215.5 kA/m; H_c∥ = 55.4 kA/m). In the presence of gluconate, needle-like deposits were obtained and a strong face centered cubic (fcc(1 1 1)) texture was measured. The Co-Pt-W thin films showed isotropic magnetic behavior. In the case of tartate and malate, the coexistence of needle-like deposits and cellular deposits appeared. The XRD patterns showed that the mixed fcc and hcp phase formed. Perpendicular anisotropic magnetic behaviors of thin films, from malate or tartate baths, were not obvious.     

Temperature dependence of absorption edge anisotropy in 2H- MoSe2 layered semiconductors

The absorption-edge anisotropy of 2H- MoSe₂ was studied by photoconductivity (PC) measurements as a function of temperature in the range of 12-300 K. A significant shift towards lower energies has been observed in the PC spectra on the edge plane with respect to those corresponding to the van der Waals (VdW) plane. The parameters that describe the temperature dependence of the absorption edges are evaluated by the Bose-Einstein empirical expression. Effective phonon energy was estimated from the temperature dependence of the Urbach energy. The estimated effective phonon energy for the VdW and edge planes, respectively, can be correlated to the observed Raman active and modes.     

Photocarrier injection and the I-V characteristics of La0.8Sr0.2MnO3/SrTiO3:Nb heterojunctions

Oxide heterojunctions made of p-type La_0.8Sr_0.2MnO₃ (LSMO) and niobium-doped n-type SrTiO₃ (STO:Nb) have been fabricated by the pulsed laser deposition (PLD) technique and characterized under UV light irradiation by measuring the current-voltage, photovoltaic properties and the junction capacitance. It is shown that the heterojunctions work as an efficient UV photodiode, in which photogenerated holes in the STO:Nb substrate are injected to the LSMO film. The maximum surface hole density Q/e and external quantum efficiency γ are estimated to be 8.3×10¹² cm⁻² and 11% at room temperature, respectively. They are improved significantly in a p-i-n junction of LSMO/STO/STO:Nb, where Q/e and γ are 3.0×10¹³ cm⁻² and 27%, respectively.     

TbMnO3 epitaxial thin films by pulsed-laser deposition

Epitaxial TbMnO₃ films have been fabricated on SrTiO₃(001) and LaAlO₃(001) substrates by pulsed laser deposition (PLD), the structure and surface morphology of the films were characterized by X-ray diffraction with Cu K_α radiation and atomic force microscopy. The electrical transport and magnetic properties of the TbMnO₃ films and bulk were examined, the resistivity and the forbidden band width E_g change with epitaxial orientation, semiconductor transport properties are found in the films and bulk, the average of the E_g of the films on SrTiO₃ and on LaAlO₃ is equal to the E_g of the bulk. The two TMO films have different magnetization mode, the magnetization of the film on SrTiO₃ have an analogy to that of TbMnO₃ single crystal.     

Study on the mechanisms of photoinduced carriers separation and recombination for Fe-TiO2 photocatalysts

The iron(III)-ion doped TiO₂ (Fe³⁺-TiO₂) with different doping Fe³⁺ content were prepared via a sol-gel method. The as-prepared Fe³⁺-TiO₂ nanoparticles were investigated by means of surface photovoltage spectroscopy (SPS), field-induced surface photovoltage spectroscopy (FISPS), and the photoelectrochemical properties of Fe³⁺-TiO₂ catalysts with different Fe³⁺ content are performed by electrical impedance spectroscopy (EIS) as well as photocatalytic degradation of RhB are studied under illuminating. Based on the experiment results, the mechanism of photoinduced carriers separation and recombination of Fe³⁺-TiO₂ was revealed: that is, the Fe³⁺ captures the photoinduced electrons, inhibiting the recombination of photoinduced electron-hole pairs, this favors to the photocatalytic reaction at low doping concentration (Fe/Ti ≤ 0.03 mol%); while Fe³⁺ dopant content exceeds 0.03 mol%, Fe₂O₃ became the recombination centers of photoinduced electrons and holes because of that the interaction of Fe₂O₃ with TiO₂ leads to that the photoinduced electrons and holes of TiO₂ transfer to Fe₂O₃ and recombine quickly, which is unfavorable to the photocatalytic reaction.     

Properties of Sr2FeMoO6 thin films fabricated by the chemical solution deposition method

We fabricated the ordered Sr₂FeMoO₆ (SFMO) thin film with a double perovskite structure using the chemical solution deposition (CSD) method. The highly c-axis oriented SFMO thin film with a high degree of Fe/Mo ordering was successfully synthesized on an MgO (001) substrate by optimizing processing conditions. The precise preparation process control of the SFMO precursor solution leads to a typical magnetoresistance effect in a low magnetic field at room temperature.     

Thermally Activated Photoconduction and Alternating-Current Conduction in Se75Ge20Ag5 Chalcogenide Glass: Investigation of Meyer--Neldel Rule

We report on the observation of Meyer-Neldel rule in glassy Se75Ge20Ag5 alloy where AE is varied by two different methods. In the first approach, the intensity of light varies while measuring the photoconductivity in amorphous thin films of Se75Ge20Ag5 instead of changing composition of the glassy system. In the second approach, the variation of ac conductivity with temperature is found to be exponential and the activation energy is found to vary with frequency.     

Laser-induced voltage characteristics of Bi2Sr2Co2Oy thin films on LaAlO3 substrates

We report the laser-induced voltage (LIV) effects in c-axis oriented Bi₂Sr₂Co₂O_y thin films grown on (0 0 1) LaAlO₃ substrates with the title angle α of 0°, 3°, 5° and 10° by a simple chemical solution deposition method. A large open-circuit voltage with the sensitivity of 300 mV/mJ is observed for the film on 10° tilting LaAlO₃ under a 308 nm irradiation with the pulse duration of 25 ns. When the film surface is irradiated by a 355 nm pulsed laser of 25 ps duration, a fast response with the rise time of 700 ps and the full width at half maximum of 1.5 ns is achieved. In addition, the experimental results reveal that the amplitude of the voltage signal is approximately proportional to sin 2α and the signal polarity is reversed when the film is irradiated from the substrate side rather than the film side, which suggests the LIV effects in Bi₂Sr₂Co₂O_y thin films originate from the anisotropic Seebeck coefficient of this material.     

First-principles study of photoconductivity in BaTiO3 with oxygen vacancies

The photoconductivity of BaTiO_2.5 with oxygen vacancy has been studied by the linear muffin-tin orbital method in the atomic sphere approximation (LMTO-ASA). The ground-state structure of BaTiO_2.5 is obtained by minimization of the total energy. The partial densities of states show that the occupied states at the bottom of the conduction band have primarily Ti d orbital character. The photoconductivity shows that two novel features, in the low energy side, can be attributed to the intraband transition of free electronic carriers in the vicinity of the Fermi level and the interband transition of the Ti 3d(yz) related band states, to the Ti 3d(xy,xz) related band states, respectively. In addition, it is also found that the anisotropy of photoconductivity is enhanced because of the introduction of oxygen vacancy. The system can show the conductive behavior of electronic carriers, which is qualitatively in agreement with a recent experimental finding.     

Electrical and structural properties of Ce0.9CoFe3Sb12 thermoelectric thin films

In this work, we show experimental results for growth conditions of thermoelectric Ce_0.9CoFe₃Sb₁₂ thin films. An rf-magnetron sputtering system has been used to grow the films on single crystal substrates of sapphire (Al₂O₃), silicon (Si), and magnesium oxide (MgO) at different substrate temperatures between 250 and 450 °C. The films were thermoelectrically characterized with resistivity and thermopower measurements as functions of temperature. The results show linear behavior of resistivity with temperature, and thermopower growth with the temperature increase. Such behavior is typical for metallic materials. The structure and surface morphology of the samples were analyzed by X-ray diffraction pattern and atomic force microscopy (AFM), respectively.     

Effect of illumination on negative linear expansion of TlGaSe2 layered crystals

Effect of illumination on negative thermal expansion of TlGaSe₂ crystals has been investigated. Strong trasformation of negative linear expansion coefficient has been observed after the illuminations. It is supposed that two main mechanisms are responsible for observed effect: formation of photoelectret states and reverse piezoelectric effect. The electronic nature of negative linear expansion of TlGaSe₂ crystals has been proved for the first time.     

Parallel syntheses and thermoelectric properties of Ce-doped SrTiO3 thin films

Thermoelectric properties of single crystalline Ce_xSr_1−xTiO₃ films (0 ≤ x ≤ 0.5) have been studied by using combinatorial pulsed-laser deposition. Temperature gradient method was used for identifying an optimum growth temperature for SrTiO₃ homoepitaxial growth, at which both oxygen stoichiometry and persisting layer-by-layer growth mode could be accomplished. Electrical conductivity (σ) and Seebeck coefficient (S) were measured at room temperature for the composition-spread films grown at the optimized temperature and found to be considerably higher than those reported for bulk poly-crystalline compounds. Hall measurement revealed that carrier density linearly increased with increasing x, suggesting that a trivalent Ce ions substituted divalent Sr ions to supply electrons. A maximum power factor (S²σ) was obtained for the x = 0.2 film, being 7 and 14 μW/K² cm at 300 and 900 K, respectively.     

Pulsed laser deposition of NdNiO3 thin films

We report the structural and transport properties of NdNiO₃ thin films prepared via pulsed laser deposition over various substrates. The films were well textured and c-axis oriented with good crystalline properties. The electrical resistivity of the films undergoes a metal-insulator transition, depending on the deposition process. Well-defined first order metal-insulator phase transition (T_MI) was observed in the best quality films without high pressure processing. Various growth conditions such as substrate temperature, oxygen pressure and thickness were varied to see their influence on T_MI. Deposition temperature was found to have a great impact on the electrical and structural properties of these films. Further the films deposited on LaAlO₃ substrate were found to be highly oriented with uniform grain size as observed from X-ray diffraction and atomic force microscopy, whereas those on Si substrate were polycrystalline, dense and randomly oriented.     

Microstructure of epitaxial SrRuO3 thin films on MgO substrates

SrRuO₃ thin films have been grown on singular (1 0 0) MgO substrates using pulsed laser deposition (PLD) in 30 Pa oxygen ambient and at a temperature of 400-700 °C. Ex situ reflection high-energy electron diffraction (RHEED) as well as X-ray diffraction (XRD) θ/2θ scan indicated that the films deposited above 650 °C were well crystallized though they had a rough surface as shown by atom force microscopy (AFM). XRD Φ scans revealed that these films were composed of all three different types of orientation domains, which was further confirmed by the RHEED patterns. The heteroepitaxial relationship between SrRuO₃ and MgO was found to be [1 1 0] SRO//[1 0 0] MgO and 45°-rotated cube-on-cube [0 0 1] SRO//[1 0 0] MgO. These domain structures and surface morphology are similar to that of ever-reported SrRuO₃ thin films deposited on the (0 0 1) LaAlO₃ substrates, and different from those deposited on (0 0 1) SrTiO₃ substrates that have an atomically flat surface and are composed of only the [1 1 0]-type domains. The reason for this difference was ascribed to the effect of lattice mismatch across the film/substrate interface. The room temperature resistivity of SrRuO₃ films fabricated at 700 °C was 300 μΩ cm. Therefore, epitaxial SrRuO₃ films on MgO substrate could serve as a promising candidate of electrode materials for the fabrication of ferroelectric or dielectric films.     

Enhanced activity of mesoporous Nb2O5 for photocatalytic hydrogen production

The mesoporous Nb₂O₅ photocatalysts were synthesized via an evaporation-induced self-assembly (EISA) method. The mesoporous structure of the as-made samples was studied by small-angle X-ray diffraction, N₂ adsorption-desorption isothermal and transmission electron microscopy. The increase of the calcination temperature during the synthesis resulted in enhanced crystallization, but decreased mesoporosity of the samples. The later was found to have a crucial influence on the photocatalytic activity by bringing on decreased BET surface area and especially increscent pore wall thickness. The advantage of the mesoporous Nb₂O₅ was also proved by performing 20 times higher photocatalytic activity than a bulk Nb₂O₅ without any porosity. A model was given to describe the effect of mesoporosity on the transportation and recombination of carriers.     

Effect of substrate temperature on structure and electrical resistivity of laser-ablated IrO2 thin films

IrO₂ thin films were prepared on Si(1 0 0) substrates by laser ablation. The effect of substrate temperature (T_sub) on the structure (crystal orientation and surface morphology) and property (electrical resistivity) of the laser-ablated IrO₂ thin films was investigated. Well crystallized and single-phase IrO₂ thin films were obtained at T_sub = 573-773 K in an oxygen partial pressure of 20 Pa. The preferred orientation of the laser-ablated IrO₂ thin films changed from (2 0 0) to (1 1 0) and (1 0 1) depending on T_sub. With the increasing of T_sub, both the surface roughness and crystallite size increased. The room-temperature electrical resistivity of IrO₂ thin films decreased with increasing T_sub, showing a low value of (42 ± 6) × 10⁻⁸ Ω m at T_sub = 773 K.     

Significant modifications in the electrical properties of poly(methyl methacrylate) thin films upon dispersion of silver nanoparticles

Electrical conductivity of thin solid films of PMMA with dispersed silver nanoparticles, synthesized by a novel method, was studied in dark conditions by changing the applied voltage and temperature and also under photoexcitation (by a mercury lamp, 125 W) at room temperature. Anomalous hysteresis in current-temperature characteristics during heating and cooling cycles was observed. The hysteresis-like behaviour was explained on the basis of the movements of molecules associated with different parts of a PMMA matrix and diffusion of silver nanoparticles in the PMMA matrix. Dark current in the PMMA films with dispersed silver nanoparticles has been observed to be higher than the corresponding current in the PMMA films without silver nanoparticles due to the creation of conduction paths by the silver nanoparticles/nanoclusters. The photoresponse in the thin solid films of PMMA with dispersed silver nanoparticles was the reverse of that observed in thin solid films of PMMA without silver nanoparticles. A decrease in photocurrent under illumination of light was observed due to the destruction of conduction paths by the illumination of light.