Ho Bin Im

Effects of oxidation conditions on the properties of tantalum oxide films on silicon substrates

Abstract Tantalum oxides have been prepared by thermal oxidation of sputtered tantalum films on silicon and silicon dioxide substrates at temperature between 450°C and 700°C. The composition of such oxide is tantalum pentoxide (Ta2O5) and the amount of silicon diffused into the tantalum oxide from the silicon substrate increases as the oxidation temperature increases. The temperature dependence of the dielectric constant of Ta2O5 on silicon can be explained in terms of its silicon concentration. CV curves of Al/Ta2O5/Si capacitors reveal the presence of donor-type interface states whose density decreases as the oxidation temperature increases. The flat band voltage of Al/Ta2O5/SiO2/Si capacitors become more negative as the oxidation or annealing temperature increases regardless of the annealing ambient.

Effects of Thickness and Sintering Conditions of CdS Films on the Photovoltaic Properties of CdS / CdTe Solar Cells

Sintered CdS films with various thicknesses and electronic properties have been prepared by changing the coated thickness and sintering conditions. All-polycrystalline CdS/CdTe solar cells have been fabricated by coating a CdTe slurry on the sintered CdS films and sintering in an attempt to optimize the thickness and conditions of CdS film whose role is to be the window as well as the front contact for the CdS/CdTe solar cell. Optical transmission of the sintered CdS film shows a maximum value for 25 m thick films when the films were sintered at 650C for lh in nitrogen whereas the maximum optical transmission is observed in 15 thick film when the films were sintered at 600C. The highest optical transmission is observed in 12 m thick CdS film which was sintered at 560C. Electrical resistivity of the sintered CdS films was less than l -cm. Solar efficiency of 11.2% was observed in all-polycrystalline CdS/CdTe solar cell that was fabricated on the 12 m thick CdS film.

Structural, electrical and optical properties of In-doped CdS thin films prepared by vacuum coevaporation

Abstract In-doped CdS films of 1 gmm thickness for a window layer of solar cells have been prepared by vacuum coevaporation of CdS and In on glass substrates at 150 °C. The In concentration in CdS films was varied from 1018 to 1021 cm−3. Structural, electrical and optical properties of CdS films have been investigated by X-ray diffraction, scanning electron microscopy, electrical resistivity measurement, the Hall effect and optical transmittance spectra. As the In concentration increased the preferred orientation of the films change from the (002) plane to the (110) plane. Also, the grain size became smaller and the grain shape changed. The electrical conductivity, carrier concentration and Hall mobility increased with increasing In concentration and then decreased with further increase in In concentration. CdS films became degenerate semiconductors as electron concentration exceeded 2 × 1018cm−3 and the optical band gap increased with increasing electron concentration due to the increase of the Fermi level in the conduction band. The optimum In concentration turned out to be 3 × 1020cm−3, which showed the lowest resistivity of 5 × 10−3ωcm and the largest optical band gap of 2.6 eV.

Effects of cell width on the photovoltaic properties of sintered Cd1-xZnxS/CdTe solar cells

Abstract Photovoltaic properties of sintered all-polycrystalline CdS/CdTe and Cd0.94Zn0.06S/CdTe solar cells have been investigated by varying cell width to maximize solar cell module efficiency. Short-circuit current density, fill factor and efficiency of the solar cells degrade with increasing cell width. Series resistance of the solar cells increases with the cell width resulting in the degradation of solar cell efficiency. Sintered Cd1−xZnxS/CdTe solar cells with a cell width of 4 mm showed the maximum module efficiency. Sintered CdS/CdTe and Cd0.94Zn0.06S/CdTe solar cells with a cell width of 4 mm showed efficiency of 10.0% and 11.0% under solar illumination with an intensity of 85 mW cm−2. The estimates values of the module efficiency were 6.7% and 7.3%, respectively. The reduction in efficiency for cell width greater than 4 mm is attributed to the resistance of the Cd1−xZnxS layer.

Structural and optical properties of microcrystalline silicon films deposited by plasma enhanced chemical vapour deposition

Hydrogenated microcrystalline (Μc) silicon films were prepared by plasma enhanced chemical vapour deposition using an Ar-diluted SiH4 gas at various deposition conditions. The substrate temperature and RF power were varied from 150 to 400 ‡C and from 10 to 120 W, respectively. Structure and microstructure were examined by X-ray diffraction, Raman spectroscopy, and scanning electron microscopy. Hydrogen bonding and optical properties were investigated by FTIR spectra and UV transmission spectra. The crystal fraction of the films increased as the deposition temperature decreased and RF power increased. More definite columnar morphology was developed with increasing crystal fraction. The existence of Μc-Si above a critical RF power (>30 W) suggests that SiH2 radical in plasma plays an important role for the formation of columnar morphology and Μc-Si. IR absorption analysis showed that the SiH2/SiH bonding ratio in the silicon films increased as the crystal fraction increased. The UV absorption coefficient of the films became smaller as the deposition temperature and RF power increased.

Recrystallization of LPCVD amorphous Si films using F+ implantation

Abstract The recrystallization behavior of low pressure chemical vapour deposition amorphous Si films by implanting 19F+ with various energies and doses and by annealing at 600 °C has been investigated by X-ray diffraction, transmission electron microscopy, and Raman spectroscopy. The electrical properties of these films have been studied by fabricating n-channel (n-ch) thin film transistors with these films. The grain size of 150 nm thick Si films increased from about 0.3 μm to about 2.5 μm with increasing F+ dose. The grain size enhancement was more effective, when the projection range of the fluorine ion was placed at the Si/SiO2 interface. This enhancement was attributed to the retardation in the nucleation rate owing to the higher degree of disorder at the Si/SiO2 interface by F+ implantation. A heavy dose of fluorine ions caused the increase in the threshold of n-ch thin film transistor, suggesting fluorine is a hole generator owing to its large electronegativity. The field effect mobility increased as the grain size increased. The maximum field effect mobility obtained without hydrogen passivation was about 23 cm2 V−1 s−1 when the dose was 2 x 1015 cm−2 and the projection range was targeted at the Si/SiO2 interface. It was found that the field effect mobility values mainlyd epended on the grain size and that the grain boundary passivation by fluorine atom seemed ineffective

Effect of hydrogen in the selenizing atmosphere on the properties of CuInSe2 thin films

Abstract CuInSe 2 thin films have been prepared by selenizing Cu/In metal layers both at 450 °C for 1 h and at 570 °C for 10 min with pure selenium as a selenium vapour source. The H 2 /N 2 volume fraction was varied during the selenization and the effect of H 2 in the selenizing atmosphere on the properties of the CuInSe 2 films was investigated by analyzing the morphological, structural and compositional changes of the CuInSe 2 films. All the selenized CuInSe 2 thin films crystalized in a chalcopyrite structure and the gain size of the CuInSe 2 films 1 μm thick ranged from 1 to 3 μm. The films had a more (112) preferred orientation and both the a axis and c axis lattice constants increased with increasing amount of hydrogen in the selenizing atmosphere up to 15 vol.%. Also, the resistivity and its activation energy increased significantly as the hydrogen volume percentage in the selenizing atmosphere increased. The compositional analysis showed that the Cu/In ratios decreased with increasing hydrogen volume percentage. The results indicated that the role of hydrogen in the selenizing atmosphere was to reduce the indium loss during selenization, causing increases in the (112) texture, the a axis and c axis lattice constants and the electrical resistivity.

Electrical and optical properties of vacuum-evaporated CdS films

Abstract1 Μm CdS films for the window layer of CdS/CulnSe2 solar cells have been prepared by vacuum evaporation at various deposition conditions. Deposition rates were 0.73 and 3.3 nms−1, and substrate temperature ranged from 50 to 225 ‡C. The effect of the deposition conditions on the properties of CdS films was investigated by measuring electrical resistivity, optical transmittance and reflectance.The resistivity of the evaporated CdS films strongly decreased as substrate temperature decreased and the films with high deposition rate showed lower resistivity compared to the films with low deposition rate. Interestingly, the combination of high deposition rate and very low substrate temperature resulted in an increase of resistivity. The optical transmittance of CdS films increased as substrate temperature decreased and then decreased with further decrease in substrate temperature. The transmittance strongly depended on deposition rate at low substrate temperature (<100‡C), while it was independent of deposition rate at high substrate temperature (>100‡C). In particular, high transmittance can be extended to lower substrate temperature by reducing deposition rate. Low optical reflectance can be obtained by lowering substrate temperature. The results indicate that CdS films of low resistivity and high transmittance can be produced by vacuum evaporation at low substrate temperature and low deposition rate.

Effects of morphologies of (Cd+Te) powders on the properties of sintered CdS/CdTe solar cells

Polycrystalline CdS/CdTe solar cells have been prepared by coating and sintering a CdS slurry and a (Cd+Te) slurry. CdS layers were first formed on borosilicate glass substrates at 600°C in nitrogen and then CdTe layers were formed on the sintered CdS layers at 625°C in nitrogen. The (Cd+Te) slurry contained (Cd+Te) powders mixed in a ball mill for 12–220 h instead of more expensive CdTe powders. The shape of cadmium particles changed from spherical to plate-like and the diameter of the plate-shaped particles became smaller as the ball-milling time increased. In addition, a compound CdTe started to form during a long milling time. The sintered CdTe layers were more compact as the diameter of plate-shaped cadmium particles decreased. However, cracks developed in the sintered CdTe layer when the diameter was small (∼ 2 μm). The efficiency of sintered CdS/CdTe solar cells increased with decreasing particle diameter and then decreased with further decrease in particle diameter. The highest efficiency of 12.1% was achieved using a mixture of (Cd+Te) powders which had plate-shaped cadmium particles with a diameter of 5 μm. The results suggest that high-efficiency sintered CdS/CdTe solar cells can be fabricated by using CdTe slurry from the mixture of (Cd+Te) powders with an inexpensive ball-milling process.

Photovoltaic Properties of Sintered CdS / CdTe Solar Cells with an Indium‐Tin‐Oxide Electrode

Indium-tin-oxide (ITO) electrodes were applied to sintered all-polycrystalline CdS/CdTe solar cells for large area cell fabrication. The CdS and CdTe layers were prepared by sequentially sintering (CdS+CdCl 2 ) slurry at 600 o C and (Cd+Te) slurry at 625 o C. The length of the cells was fixed at 10 mm and the width was varied from 3 to 9 mm. While the application of ITO did not change the junction properties after a high temperature annealing process, it significantly reduced the series resistance of the cells. As the cell width increased from 3 to 9 mm, the efficiency of CdS/CdTe solar cells with ITO electrode decreased slightly from 10.5 to 9.4%, while that of cells without ITO decreased from 10.4 to 8.4% under the illumination of 50 mW.cm -2