Luminescence properties of β-FeSi2 and its application to photonics

We have investigated the photoluminescence properties of β-FeSi₂ and have subsequently reported many studies on the enhancement of luminescence efficiency. We have further discussed a limit for the luminescence efficiency based on the weak confinement theory of excitons in β-FeSi₂ nanocrystals. Moreover, we have reported an application of β-FeSi₂ with a high refractive index to photonic crystals.     

Growth of β-Ga2O3 nanorods by ammoniating Ga2O3/V thin films on Si substrate

This paper reports that/3-Ga2O3 nanorods have been synthesized by ammoniating Ga2O3 films on a V middle layer deposited on Si（111） substrates. The synthesized nanorods were confirmed as monoclinic Ga2O3 by x-ray diffraction,Fourier transform infrared spectra. Scanning electron microscopy and transmission electron microscopy reveal that the grown β-Ga2O3 nanorods have a smooth and clean surface with diameters ranging from 100 nm to 200 nm and lengths typically up to 2μm. High resolution TEM and selected-area electron diffraction shows that the nanorods are pure monoclinic Ga2O3 single crystal. The photoluminescence spectrum indicates that the Ga2O3 nanorods have a good emission property. The growth mechanism is discussed briefly.     

Influence of surface recombination and interface states on the performance of β-FeSi2/c-Si heterojunction solar cells

To explore the origin of low conversion efficiency for novel β-FeSi₂/c-Si heterojunction solar cells, the effect of surface recombination and interface states on the cell performance has been investigated by numerical simulation. The present results show that surface recombination of β-FeSi₂ film plays an important role in limiting the cell property since the photovoltaic behavior of β-FeSi₂ is quite sensitive to surface recombination due to its especial characteristic of very high optical absorption coefficient. Surface quality of β-FeSi₂ film should be much improved for better cell performance. In addition, it is shown that interface states between β-FeSi₂ film and crystalline silicon are critical to device characterization. Interface states should be minimized to obtain higher conversion efficiency. If surface recombination and interface states can be best suppressed, potential conversion efficiency for the cell may be up to 28.12% at 300 K under illumination of AM 1.5, 100 mW/cm².     

The influence of SiNx substrate on crystallinity of μc-Si film used in thin film transistors

This paper found that the crystalline volume ratio （Xc） of μc-Si deposited on SiNx substrate is higher than that on 7059 glass. At the same silane concentration （SC） （for example, at SC=2%）, the Xc of μc-Si deposited on SiNx is more than 64%, but just 44% if deposited on Conning 7059. It considered that the ‘hills＇ on SiNx substrate would promote the crystalline growth of μc-Si thin film, which has been confirmed by atomic force microscope （AFM） observation. Comparing several thin film transistor （TFT） samples whose active-layer were deposited under various SC, this paper found that the appropriate SC for the μc-Si thin film used in TFT as active layer should be more than 2%, and Xc should be around 50%. Additionally, the stability comparison of μc-Si TFT and a-Si TFT is shown in this paper.     

Spectroscopic characterization of β-FeSi2 single crystals and homoepitaxial β-FeSi2 films by XPS and XAS

Chemical state analysis by a combination of X-ray photoelectron spectroscopy (XPS) and X-ray absorption spectroscopy (XAS) using synchrotron radiation is performed for β-FeSi₂ single crystals and homoepitaxial β-FeSi₂ films. The Si 2p XPS and Fe L-edge XAS spectra imply that the annealing at 1173 K to remove native oxide layers on the crystal induces the formation of FeSi in the surface. The formation of FeSi is also confirmed by Si K-edge XAS analysis. For the homoepitaxial β-FeSi₂ films grown on the crystals, the Si K-edge XAS spectra indicate that structurally homogeneous β-FeSi₂ films can be grown on the β-FeSi₂ single crystals when the substrate temperatures of 973 and 1073 K are applied for molecular beam epitaxy (MBE). Consequently, it is indicated that the combination of XPS and XAS using synchrotron radiation is a useful tool to clarify chemical states of β-FeSi₂ single crystals and homoepitaxial β-FeSi₂ films, which is important to reveal optimized growth conditions of homoepitaxial films.     

Band structure and valence-band offset of HfO2 thin film on?Si?substrate from photoemission spectroscopy

Synchrotron radiation photoemission spectroscopy and optical transmission spectrum measurements have been performed on an HfO₂ thin film grown on a Si(100) substrate to determine the band structure of the HfO₂/Si stack. The result shows a valence-band offset of 2.5 eV and a conduction-band offset of 2.2 eV for the HfO₂/Si interface. The Schottky barrier height between Au and HfO₂ is obtained from current density–voltage measurement. The characterization reveals that the dominant conduction mechanism in the region of low field under gate injection is Schottky emission. The energy-band diagram of an Au–HfO₂–Si MOS stack was obtained from these results.     

Electrochemical deposition of quaternary Cu2ZnSnS4 thin films as?potential solar cell material

Quaternary compound semiconductor Cu₂ZnSnS₄ (CZTS), which appears to be a promising candidate for the absorber of a thin film type solar cell, was grown on polycrystalline Ag substrates by electrochemical epitaxial method. The elements were deposited in the following sequence: S/Sn/S/Cu/S/Zn/S/Cu… , the order being one cycle of SnS, one cycle of ZnS and two cycles of CuS. Morphology of the deposit has been characterized by field emission scanning electron microscopy (FE-SEM) with an energy dispersive X-ray (EDX) analyzer. X-ray diffraction (XRD) studies showed a (112) preferred orientation for the deposit. X-ray photoelectron spectroscopy (XPS) of the deposit indicated an approximate ratio 2:1:1:4 of Cu, Zn, Sn, and S, the expected stoichiometry for the deposit, and similar results have been obtained from EDX data. Near IR absorption measurements of the deposit at room temperature indicated a direct band gap of 1.5 eV, and open-circuit potential (OCP) studies indicated a good p-type property, both of which were suitable for fabricating a thin film solar cell.     

Atomically controlled surfaces with step and terrace of β-Ga2O3 single crystal substrates for thin film growth

The surface of β-Ga₂O₃ (1 0 0) single crystal grown with floating zone method was treated by chemical-mechanical-polishing (CMP) for 30-120 min followed by annealing in oxygen atmosphere at temperature 600-1100 °C for 3-6 h. The evolution of the step arrangement was investigated with reflection high energy electron diffraction and atomic force microscopy. Atomically smooth surfaces with atomic step and terrace structure of β-Ga₂O₃ substrates were successfully obtained after just CMP treatment as well as CMP treatment and post annealing at 1100 °C for 3 h. The uniform step height was 0.57 nm, and smooth terrace width was 100 nm, where the misorientation angle was about 0.36°. The obtained atomically smooth surface provides a potential application for the high-quality epitaxial film growth.     

Study on the n-β-FeSi2/p-Si solar cells under different illuminated directions

The n-β-FeSi₂/p-Si heterojunction solar cells can be used under illumination of β-FeSi₂ side or Si side. In this work, the effects of illuminated direction on the photovoltaic properties of n-β-FeSi₂/p-Si heterojunction solar cells were analyzed by numerical methods. The calculated results show that the n-β-FeSi₂/p-Si heterojunction solar cell under illumination of β-FeSi₂ side has superior photovoltaic properties, which is consisting with the experimental reports. For the illumination of Si side, the photo-generated carriers in the back surface of Si substrate are far from the built-in electric field, resulting in the reduced conversion efficiency. The calculated results indicate that we should choose the illumination of β-FeSi₂ side for n-β-FeSi₂/p-Si heterojunction solar cell application.     

Electrodeposition of TiO2–RuO2–IrO2 coating on titanium substrate

TiO₂, RuO₂, and IrO₂ transition metal oxides have many applications in the field of applied electrochemistry. In this work, the mixed solid solutions of TiO₂–RuO₂–IrO₂ coatings have been electrodeposited from aqueous–unaqueous baths.     

X-ray photoelectron and X-ray absorption spectroscopic study on β-FeSi2 thin films fabricated by ion beam sputter deposition

A combination of X-ray photoelectron spectroscopy (XPS) and X-ray absorption spectroscopy (XAS) using synchrotron radiation is applied to clarify surface chemical states of β-FeSi₂ films fabricated by an ion-beam sputtering deposition method. The differences in the chemical states of the films fabricated at substrate temperatures of 873, 973 and 1173 K are investigated. For the film fabricated at 873 K, Si 2p XPS spectra indicate the formation of a relatively thicker SiO₂ layer. In addition, Fe L-edge XAS spectra exhibit the formation of FeSi_2−X by preferential oxidation of Si or the presence of unreacted Fe. The results for the film fabricated at 1173 K imply the existence of FeSi₂ with α and ? phases. In contrast, the results for the film fabricated at 973 K indicate the formation of relatively homogeneous β-FeSi₂. These imply that the relatively excellent crystal property of the film fabricated at 973 K is due to the formation of homogeneous β-FeSi₂. As a conclusion, the combination of XPS and XAS using synchrotron radiation is a powerful tool to elucidate the surface chemical states of thin films.     

The hybrid photocatalyst of TiO2–SiO2 thin film prepared from rice husk silica

The TiO₂–SiO₂ thin film was prepared by self-assembly method by mixing SiO₂ precursor with titanium precursor solution and aged to obtain a co-precipitation of silica and titanium crystals. Dip coating method was applied for thin film preparation on glass slide. The X-ray diffraction (XRD) of the self-assembly thin film had no characteristic property of SiO₂ and even anatase TiO₂ but indicated new crystal structure which was determined from the Fourier Transform Infrared Spectrophotometer (FTIR) as a hybridized Ti–O–Si bonding. The surface area and surface volume of the self-assembly sample were increased when SiO₂ was incorporated into the film. The self-assembly TiO₂–SiO₂ thin film exhibited the enhanced photocatalytic decolorization of methylene blue (MB) dye. The advantages of SiO₂ are; (1) to increase the adsorbability of the film and (2) to provide the hydroxyl radical to promote the photocatalytic reaction. The self-assembly thin film with the optimum molar ratio (SiO₂:TiO₂) as 20:80 gave the best performance for photocatalytic decolorization of MB dye with the overall efficiency of 81%.     

P/i interfacial engineering in semi-transparent silicon thin film solar cells for fabrication at a low temperature of 150 °C

In this study, we aimed to develop semitransparent solar cells (STSCs) using hydrogenated amorphous silicon (a-Si:H) at a low temperature of 150 °C to support the fabrication of flexible solar modules, applicable in building-integrated photovoltaics (BIPV). To compensate for the presumable loss of device performance at such a low processing temperature, careful control of the p/i interface is proposed. We fabricated buffer layers with hydrogen (H₂)/silane (SiH₄) gas flow ratios (R) ranging from 4 to 16 (R₄–R₁₆) to investigate their characteristics and incorporate them at the p/i interface by considering energy band matching. By employing this buffer, the power conversion efficiency (PCE) of a STSC was improved from 4.83% to 5.57% which is the best record in a-Si:H STSCs processed at a low temperature of 150 °C. This p/i interfacial buffer can support the realization of flexible a-Si:H-based BIPV systems using plastic- or polymer-based substrates.     

Optical properties of single-phase β-FeSi2 films fabricated by electron beam evaporation

Single-phase semiconducting iron disilicide (β-FeSi₂) films on silicon substrate were fabricated by electron beam evaporation (EBE) technique. For preventing the oxidation of Fe film, silicon/iron/silicon sandwich structure films with different thickness of silicon and iron were deposited and then annealed at different temperatures. X-ray diffraction (XRD), Raman and Fourier transform infrared spectroscopy (FTIR) measurements were carried out to study the phase distribution and crystal quality of the films. Single-phase β-FeSi₂ with high crystal quality was achieved after annealing at 800 °C for 5 h. An apparent direct bandgap E_g of approximately 0.85-0.88 eV was observed in the β-FeSi₂ films. It is considered that the silicon/iron/silicon sandwich structure is suited for formation of single-phase β-FeSi₂ with high crystal quality.     

Microcrystalline silicon oxide (μc-SiO:H) alloys as a contact layer for highly efficient Si thin film solar cell

We observe >6% efficiency enhancement in silicon thin film solar cell using a p-type microcrystalline silicon oxide (μc-SiO:H) contact layer between transparent conducting oxide (TCO) electrode and the hydrogenated amorphous silicon (a-Si:H) layer. The role of the above contact layer is to reduce the Schottky barrier effect as well as the hetero-junction barrier formation at the interface. Despite its nanometer scale thickness, the properties of the contact layer significantly affect the solar parameters. Based on our results, p-type doped μc-SiO:H can be an ideal material as a contact layer due to its good optical response without noticeable degradation in its electrical property.     

Quick preparation and thermal transport properties of nanostructured β-FeSi2 bulk material

This paper reports that the nanostructured β-FeSi2 bulk materials are prepared by a new synthesis process by combining melt spinning(MS) and subsequent spark plasma sintering(SPS).It investigates the influence of linear speed of the rolling copper wheel,injection pressure and SPS regime on microstructure and phase composition of the rapidly solidified ribbons after MS and bulk production respectively,and discusses the effects of the microstructure on thermal transport properties.There are two crystalline phases(α-Fe2Si5 and ε-FeSi) in the rapidly solidified ribbons;the crystal grains become smaller when the cooling rate increases(the 20 nm minimum crystal of ε-FeSi is obtained).Having been sintered for 1 min above 1123 K and annealed for 5 min at 923 K,the single-phase nanostructured βFeSi2 bulk materials with 200-500 nm grain size and 98% relative density are obtained.The microstructure of β-FeSi2 has great effect on thermal transport properties.With decreasing sintering temperature,the grain size decreases,the thermal conductivity of β-FeSi2 is reduced remarkably.The thermal conductivity of β-FeSi2 decreases notably(reduced 72% at room temperature) in comparison with the β-FeSi2 prepared by traditional casting method.     

Electroreflectance study of CuIn1−xGaxSe2 thin film solar cells

We have investigated the optical properties of CuIn_1−xGa_xSe₂ (CIGS) thin film solar cells using their electroreflectance (ER) at room temperature. The ER spectra exhibited one broad and two narrow signal regions. Using the photoluminescence (PL) and photocurrent (PC) spectra, the peaks in the low-energy region (1.02–1.35 eV) can be assigned to the CIGS thin film. The PC results implied that the peaks in the high-energy region (2.10–2.52 eV) can be assigned to the CdS band-gap energy. Using the applied bias voltage, the broad signals in the 1.35–2.09 eV region can be assigned to the Franz–Keldysh oscillation (FKO) due to the internal electric field. The ER spectra exhibited a distorted CdS signal for the CIGS thin film solar cell with low shunt resistance and efficiency.     

Thickness optimization of Mo films for Cu（InGa）Se2 solar cell applications

Mo thin films are deposited on soda lime glass (SLG) substrates using DC magnetron sputtering. The Mo film thicknesses are varied from 0.08 μm to 1.5 μm to gain a better understanding of the growth process of the film. The residual stresses and the structural properties of these films are investigated, with attention paid particularly to the film thickness dependence of these properties. Residual stress decreases and yields a typical tensile-to-compressive stress transition with the increase of film thickness at the first stages of film growth. The stress tends to be stable with the further increase of film thickness. Using the Mo film with an optimum thickness of 1 μm as the back contact, the Cu(InGa)Se2 solar cell can reach a conversion efficiency of 13.15%.     

β-FeSi2 as the bottom absorber of triple-junction thin-film solar cells: A numerical study

Using β-FeSi2 as the bottom absorber of triple-junction thin-film solar cells is investigated by a numerical method for widening the long-wave spectral response. The presented results show that the β-FeSi2 subcell can contribute 0.273 V of open-circuit voltage to the a-Si/μc-Si/β-FeSi2 triple-junction thin-film solar cell. The optimized absorber thicknesses for a- Si, μ-Si, and/3-FeSi2 subcells are 260 nm, 900 nm, and 40 nm, respectively. In addition, the temperature coefficient of the conversion efficiency of the a-Si/μc-Si//3-FeSi2 cell is -0.308 %/K, whose absolute value is only greater than that of the a-Si subcell. This result indicates that the a-Si/μc-Si/β-FeSi2 triple-junction solar cell has a good temperature coefficient. As a result, using β-FeSi2 as the bottom absorber can improve the thin-film solar cell performance, and the a-Si/μc-Si/β-FeSi2 triple-junction solar cell is a promising structure configuration for improving the solar cell efficiency.     

Highly transparent Zn1−xMgxO/ITO multilayer for window of thin film solar cells

Texture-etched Zn_1−xMg_xO films were fabricated and applied as front transparent electrodes for superstrate type thin film solar cells. The Zn_0.65Mg_0.35O film (x = 0.35) showed optical transparency better than commercially available Asahi VU and double-textured ZnO (WT-ZnO) substrates. To provide pertinent conductivity, ITO film was coated on the texture-etched Zn_0.65Mg_0.35O. By employing the Zn_0.65Mg_0.35O/ITO substrate instead of the SnO₂, we demonstrated an enhancement of quantum efficiency for amorphous silicon thin film solar cell devices, resulted in efficiency improvement from 8.92 to 9.56%. We also examined effectiveness of the Zn_0.65Mg_0.35O/ITO substrate for the microcrystalline silicon solar cells which delivered an efficiency of 9.73% with proper anti-reflection coating. Our experimental results suggest that the Zn_0.65Mg_0.35O/ITO multilayer front contact can be beneficial for reinforcing performances of silicon-based thin film solar cell devices.