Jinsoo Song

Optical and electrical properties of Ga2O3-doped ZnO films prepared by r.f. sputtering

Abstract Optical and electrical properties of Ga2O3doped ZnO films prepared by r.f. sputtering have been investigated as functions of preparation conditions and dopant concentration in an attempt to develop transparent films with low electrical resistivity and with good stability at higher temperatures. The electrical resistivity of the sputtered films depends strongly on the r.f. power density, the argon gas pressure, the Ga2O3 concentration and the thickness of the films when the thickness is less than about 2500 A. The optical transparency depends on the thickness and dopant concentration and is almost independent of the sputtering conditions. Ga2O3-doped ZnO films become degenerate semiconductors when the carrier concentration exceeds about 1019 cm−3 and the optical band gap increases with increasing electron concentration owing to the increase in the Fermi level in the conduction band. Ga2O3-doped ZnO films 3000 A thick with an optical transmission higher than 85% and with electrical resistivity lower than 10−3 ohmcm can be produced by sputtering a ZnO target containing 5 wt.% Ga2O3 with an r.f. power density of 0.84 W cm−2 and an argon gas pressure of 5 m Torr.

Highly textured ZnO thin films doped with indium prepared by the pyrosol method

Indium doped ZnO thin films have been prepared on heated Corning 7059 glass by the pyrosol spray method. It was found that indium doping has an important role in grain growth at high substrate temperature. Indium also was used to improve the electrical properties, acting as an N type dopant, and we obtained highly conductive ZnO:In thin films with a resistivity of 3.0 × 10−3 Ω cm. At substrate temperatures from 425°C to 475°C, the deposited ZnO:In thin films have clear hexagonal crystallites and, therefore, a highly textured surface showing optical haze phenomena due to the crystallites. The haze ratio of ZnO:ln thin films can be controlled from 10% to 50% at the wavelength of 550 nm by varying the substrate temperature from 375°C to 475°C.

Maximum power point tracking with temperature compensation of photovoltaic for air conditioning system with fuzzy controller

Fuzzy control has received a great deal of attention due to its excellent behavior when applied to complex and nonlinear system. As fuzzy theory is close to human reasoning, fuzzy controllers are easy to design and cheap to produce. In this paper, we investigate the possibilities of P/sub max/ control using the fuzzy controller, and we also examine the optimal power converter using a step-up chopper circuit to operate the solar cell at an optimal voltage of compensated voltage (i.e. of determined transducer by temperature (-40/spl deg/C/spl sim/+100/spl deg/C)). The proposed step-up maximum power point tracking (MPPT) system is studied by simulation and is implemented by using a microcomputer which controls the duty ratio of an IGBT boost converter.

Characterization of CuInS 2 Thin Films Grown by Close-spaced Vapor Sulfurization of Co-sputtered Cu?In Alloy Precursors

Cu–In alloy films were prepared on bare or Mo-coated glass by co-sputtering from Cu and In targets at ambient temperature. The formation of CuInS2 films was accomplished by sulfurization within a graphite container under high S vapor pressure. X-ray diffraction (XRD) analysis of the alloy films showed predominant variation of the phases from In→CuIn2→Cu11In9 as the Cu content in the films increased. The sulfurized In-rich films formed the CuIn5S8 phase that steadily transformed into CuInS2 as film composition changed toward the Cu-rich region. SEM analysis showed different morphologies for the CuIn5S8 and CuInS2 films. Cu-rich films exhibited very dense crystal structures. EDX composition measurements on the films showed Cu/(Cu+In) varying from 0.21 to 0.64 and S/(Cu+In) from 0.80 to 1.36. Resistivities in the range of 2.36 to 1.7×108 Ωcm were obtained. Studies of the growth mechanism indicated formation of CuIn5S8 as the main secondary phase in both Cu-rich and In-rich films at low temperatures before conversion into CuInS2 at temperatures >400°C.

North-East Asian Super Grid: Renewable energy mix and economics

Further development of the North-East Asian energy system is at a crossroads due to severe limitations of the current conventional energy based system. For North-East Asia it is proposed that the excellent solar and wind resources of the Gobi desert could enable the transformation towards a 100% renewable energy system. An hourly resolved model describes an energy system for North-East Asia, subdivided into 14 regions interconnected by high voltage direct current (HVDC) transmission grids. Simulations are made for highly centralized, decentralized and countrywide grids scenarios. The results for total system levelized cost of electricity (LCOE) are 0.065 and 0.081€/(kW&h) for the centralized and decentralized approaches for 2030 assumptions. The presented results for 100% renewable resources-based energy systems are lower in LCOE by about 30–40% than recent findings in Europe for conventional alternatives. This research clearly indicates that a 100% renewable resourcesbased energy system is THE real policy option.

The influence of filament temperature on crystallographic properties of poly-Si films prepared by the hot-wire CVD method

Abstract This paper presents the deposition and characterization of polycrystalline silicon (poly-Si) films by the hot-wire chemical vapor deposition (HWCVD) method. The filament temperature was determined to be a critical parameter for the deposition of large-grained Si films. Poly-Si films with a moderate lateral grain-size of ∼1 μm and a vertical grain size approximately the same as the film thickness (approx. 3 μm) could be deposited on a glass substrate at a substrate temperature of less than 550°C by increasing the filament temperature to 2000°C. The surface of the films has a natural textured structure, which is believed to give some positive effects on solar cell performance. Some experimental results are presented in order to elucidate these improvements in crystalline properties of the Si films deposited at high filament temperature.

Characterization of Surface Morphology and Light Scattering of Transparent Conducting ZnO:Al Films as Front Electrode for Silicon Thin Film Solar Cells

Changes in the surface morphology and light scattering of textured Al doped ZnO thin films on glass substrates prepared by rf magnetron sputtering were investigated. As-deposited ZnO:Al films show a high transmittance of above 80% in the visible range and a low electrical resistivity of 4.5×10 Ω·cm. The surface morphology of textured ZnO:Al films are closely dependent on the deposition parameters of heater temperature, working pressure, and etching time in the etching process. The optimized surface morphology with a crater shape is obtained at a heater temperature of 350 C, working pressure of 0.5 mtorr, and etching time of 45 seconds. The optical properties of light transmittance, haze, and angular distribution function (ADF) are significantly affected by the resulting surface morphologies of textured films. The film surfaces, having uniformly size-distributed craters, represent good light scattering properties of high haze and ADF values. Compared with commercial Asahi U (SnO :F) substrates, the suitability of textured ZnO:Al films as front electrode material for amorphous silicon thin film solar cells is also estimated with respect to electrical and optical properties.

Hydrogen Plasma Resistive and Highly Textured SnO2:F/ZnO:In Bilayer Films Prepared by the Pyrosol Method

Indium doped ZnO (ZnO:In) thin films were prepared on corning 7059 glass by the pyrosol method and the hydrogen plasma durability of the films was investigated. We found that the ZnO:In films prepared at the higher substrate temperature of 425° C to 475° C have better hydrogen plasma durability because its denser surface with smaller grain boundary region kept energetic hydrogen ions from diffusing into the grain boundaries. Two kinds of SnO2:F/ZnO:In bilayer films were fabricated at the temperature of 425° C through a successive process by the pyrosol method: the SnO2:F/ZnO:In with thin (580 A) ZnO:In and the SnO2:F/ZnO:In with thick (2000 A) ZnO:In. The former has excellent durability owing to its thin (580 A) ZnO:In layer which acts as a barrier against hydrogen ions. It has a resistivity of 6.6×10-4 Ω cm and a transmittance of 83.8% at the wavelength of 550 nm. The latter is a bilayer film of the SnO2:F/ZnO:In with thick (2000 A) ZnO:In which acts not only as a barrier but as a light scattering layer due to the highly textured thick (2000 A) ZnO:In film.

Structural and electrical properties of polycrystalline silicon films deposited by hot-wire CVD

The polycrystalline silicon (poly-Si) films are deposited on low-temperature glass substrate by hot-wire CVD. The SiH 4 gas flow rate {F(SiH 4 )} is a critical parameter both for deposition rate and for crystalline properties. The poly-Si films deposited at low F(SiH 4 ) with moderate deposition rate (about 3 μm/h) have superior crystalline properties and crystalline volume fraction that exceeds 90%. The films also have natural texture structure on the surface that is strongly recommended in thin-film solar cells in order to obtain high current density by increasing incident light trapping. By increasing F(SiH 4 ) to 10 sccm, the high deposition rate (20 μm/h) is obtained, but crystalline properties deteriorated in these samples. However, these films have high potentials for thick (20-30 μm) solar cell applications due to the high deposition rate and enhanced grain growth; average grain sizes are larger than that of low-F(SiH 4 ) samples although small nano-sized grains still exist, by which crystalline volume fraction was <70%. The secondary ion mass spectroscopy and Fourier transform infrared spectroscopy analysis showed that the films have considerable amount of O and C within the films and this is originated from impurity penetration during storing the samples in atmosphere.

The role of p-type buffer layers between ZnO:Al and P a-SiC:H for improving fill factor and V oc of a-Si:H solar cells

This study addresses the role of p-type buffer layers between ZnO:Al (AZO) TCO and p a-SiC:H window layer. When AZO is used as a front TCO in conventional a-Si:H solar cells incorporating p a-SiC:H window, the fill factor (FF) significantly decreases in different with SnO2:F (FTO). Various buffer layers with different conductivity and crystalline properties are inserted between AZO and p a-SiC:H and solar cell performances are compared. The FF deterioration of AZO/p a-SiC:H cells is directly related to the interface potential barrier. This potential barrier can be controlled by tuning electron affinity and mobility gap as well as electrical conductivity of buffer layers. The p μc-Si:H buffer improves FF up to 0.72, which results from high conductivity of buffer layer. The p a-Si:H buffers also improves FF although they have similarly low conductivity with p a-SiC:H because of low electron affinity and/or mobility gap.