Materials Science Forum Vols. 743-744

Abstract: Nowadays, high performance p-type transparent conductive oxide (TCO) thin films have gained tremendous intersts, and the fact is that if p-type TCOs with high electrical conductivity and optical transmittance can be fabricated, transparent p-n junctions can be obtained and invisible electronics be realized, and hence the use area of TCOs will be highly broadened. A lot of work have been done on non-stoichiometric and doped versions of p-type TCOs in the last few years to improve the optical and electrical properties by various deposition techniques. ZnO-based thin films were thought to be the most promising candidate for p-type TCOs based on the fact that ZnO has advantages over the others, so in this paper the development of ZnO-based p-type TCOs has been discussed. Firstly, the reasons why p-type ZnO-based TCOs are difficult to synthesize were discussed, and then the general ways now used to produce p-type ZnO-based TCOs were summerized, including intrinsic p-type ZnO, doping of groupelements, codoping of and elements, doping of group elements, the origin of p-type conductivity and the feasibility of each way, and the state-of-the-art optical and electrical properties were presented. Finally, the specific shortcomings in producing high quality p-type TCOs were discussed. Based on the comparision, it is believed that the doping of groupelements in ZnO may be the most pronising way in realizing p-type TCO.

Abstract: In order to get solar grade silicon, large cold crucible has been used in an induction heat furnace. By controlling the relative location of the crucible and coils, directional solidification was realized. More than 200 kg multi-crystalline silicon ingot was produced in a batch with short work time. The removal rate of most metal impurities was high, typically higher than 99% for transition metals like iron. Non-metallic elements such as boron and phosphorus could not be removed efficiently because of larger equilibrium segregation coefficient. The concentration of phosphorus was one third of the feedstock due to the vaporization in the melting process. The distribution of impurities agreed with the solidification principle. Quartzes and carbon was not used, which ensured silicon prevent from the contamination. Cooperated with other methods, large scale of solar grade silicon was produced.

Abstract: Graphene thin films were fabricated via layer-by-layer (LbL) assembly of oppositely charged reduced graphene oxides (RGOs) and benign post-treatment. RGOs dispersions and thin films were characterized by means of transmission electron microscopy, UV-visible absorption spectrophotometer, Raman spectroscopy, and four-point probe. It was found that graphene thin films exhibited a significant increase in electrical conductivity after post-treatment. In addition, post-treatment and film thickness showed an effect on transient optical properties of graphene thin films. The transmittance (800 nm, pulse) of (RGO-PDDA⁺/RGO-O^-)₃₀ film decreased after post-treatment, and the transmittance (800 nm, pulse) of post-treated films decreased with increasing the number of LbL assembled bilayers.

Abstract: The ultra-fine silver powders were prepared by chemical reduction using nitric acid silver (AgNO₃) as raw material, ascorbic acid (C₆H₈O₆) as deoxidizer and adding polyvinylpyrrolidone (PVP) as dispersing agent. The influence factors, such as AgNO₃ solution concentration, ascorbic acid solution concentration, PVP dosage and reaction conditions such as temperature, PH etc. were studied. The morphology and particle size distribution were observed by Field Emission Scanning Electron Microscopy (FSEM) and laser particle analyzer. The composition was analyzed by Energy Dispersive Spectrometer (EDS).The crystal phase was tested by X-ray Diffraction (XRD). It indicates that the spherical ultra-fine silver powder is face-centered cubic structure, its diameter is about 1.0 μm with homogeneous particle size distribution and smooth surface. It was prepared under the conditions as following: dropwise added 60 g/L AgNO₃ solution into 40g/L ascorbic acid solution, with the presence of PVP, and pH=3,45,360rpm. It meets the requirements of paste for solar cells top electrode.

Abstract: Si/C multilayer thin films were prepared by magnetron sputtering and post-annealing in N₂ atmosphere at 1100 for 1h. X-ray diffraction (XRD), Raman scattering and high-resolution transmission electron microscopy (HRTEM) were applied to study the microstructures of the thin films. For the case of Si/C modulation ratio smaller than 1,interlayer diffusion is evident, which promotes the formation of α-SiC during thermal annealing. If the modulation ratio is larger than 1, the Si sublayers are partially crystallized, and the thicker the Si sublayers are, the crystallinity increases. To be excited, brick-shaped nc-Si is directly observed by HRTEM. The brick-shaped nc-Si appears to be more regular near the Si (100) substrate but with twin defects. The results are instructive in the application of solar cells.

Abstract: Cu-doped CdS thin film has been successfully deposited by ion-beam sputtering deposition. The structural, morphology, optical and electrical properties of as-deposited and annealed Cu-doped CdS thin films were investigated. The heavily Cu-doped CdS films annealed at 400 °C was demonstrated to be improved in structural, morphology, electrical and optical properties. X-ray diffraction (XRD) analysis indicated the formation of polycrystalline CdS film with the structure of hexagonal wurtzite phase. No distinct impurity of Cu and Cu-S phase was detected in Cu-doped CdS thin films. Atomic force microscopy (AFM) revealed that the grain size was increased after annealed. Optical transmission and absorption spectroscopy measurement revealed a high absorption and energy band gap was of about 2.40 eV. The CdS thin film was of p-type conductivity and the resistivity was found to be 1.28×10^-1Ωcm.

Abstract: TiO₂ nanotubes array was fabricated by anodization. Effect of reaction duration on the morphology of TiO2 nanotube arrays was studied detailedly. The structure and morphology of the prepared nanotubes array was characterized by X-ray diffraction and scanning electron microscopy, respectively. The fabricated TiO2 arrays were peeled off and adhered to FTO glass with adhesive (mixture of tetrabutyl titanate and polyethylene glycol), then they were sintered at 450 for photoanode of DSSC. The photovoltaic performance of the prepared sample as the DSSC anode was investigated. An open circuit voltage of 0.69V and a short circuit current density of 7.78mA/cm2 were obtained, and the fill factor and the convert efficiency were 0.517 and 2.78%, respectively.

Abstract: Flaky ZnO thin films were electrodeposited from an aqueous solution of zinc nitrate (Zn (NO₃)₂) as electrolyte on ITO glass substrate by cathodic reduction method The effect of electrolyte concentration on the structure, morphology and optical properties of ZnO thin films were studied by X-ray diffraction, scanning electron microscopy and UV-Vis spectrometer. The results show that the ZnO films are of hexagonal wurtzite structure; When the concentration of Zn (NO₃)₂ solution is 0.24 M, the sparse flaky ZnO thin films were prepared and the thickness and length of ZnO flaky are about 50 nm and 1~3 μm, respectively; When the electrolyte concentration is 0.16 M, hexagonal and triangular structures are observed and some lathy and conical ZnO structure start to appear; When the concentration of Zn (NO₃)₂ solution drops to 0.08 M, the matrix of films is still flaky ZnO, but porous structures among the flaky structures appear and the size of conical ZnO has changed, whose bottom diameter reaches 1~3 μm and length is 2~4 μm; UV-Vis absorption test results indicate the absorption peak and the bandgap of the prepared ZnO thin films are about 350 nm and 3.28 eV, respectively.

Abstract: In this paper, CdSe-TiO₂ nanotube array composite films were successfully prepared through a two-steps method. TiO₂ nanotube arrays were firstly prepared by anodic oxidation method, based on which the composite films of CdSe-TiO₂ nanotubes arrays were prepared by electrochemical deposition. The influence of the concentration of SeO₂ on the structure and photoelectric performance of the composite films were studied by X-ray diffraction (XRD), field emission scanning electron microscopy (FESEM) and photocurrent response tests. The results show that the cubic phase CdSe particles with the size of about 15~20 nm were uniformly distributed in highly ordered TiO₂ nanotubes and around the mouths of the nanotubes; With the increasing of concentration of SeO₂, the content of CdSe increases gradually while the photocurrent density of the composite films increased and decreased, The optimal photoelectric performance of composite films were obtained when the SeO₂ concentration was 4 mmol/L.

Abstract: During the process of Laser beam long-range energy transmission based on the photovoltaic principle, the temperature elevation of the photovoltaic cell under Laser irradiation would greatly affect its photoelectric conversion efficiency. In this work, the heat generation mechanism of thin-film photovoltaic cell under Laser irradiation was analyzed based on the energy conversion relationship between light, electricity and heat. The output voltage characteristic of a thin film GaInP/GaAs/Ge triple-junction photovoltaic cell under irradiation of dual-wavelength Laser beam was investigated, and the results show the dependence of the conversion efficiency on the temperature and the valve effect, which was caused by the long wavelength Laser and would sharply reduce the photovoltaic conversion efficiency of short wavelength Laser. The temperature field of the multi-layer structure subjected to Laser irradiation was analyzed by finite element method, and the results indicate that the temperature elevation pattern depends on the spatial distribution of the light absorption and can be adjusted via optimizing the photovoltaic cell structure.