Jun-ichi Nomoto

High-Efficiency Oxide Solar Cells with ZnO/Cu2O Heterojunction Fabricated on Thermally Oxidized Cu2O Sheets

High conversion efficiencies were achieved in low cost n–p heterojunction oxide solar cells with an Al-doped ZnO (AZO)/non-doped ZnO (ZO)/Cu2O structure. This achievement was made possible by the formation of an n-ZO thin-film layer, prepared with an appropriate thickness by low damage deposition, on high quality Cu2O sheets produced by the thermal oxidization of copper sheets: n-ZO thin film optimal thickness ranges from 30 to 50 nm. Photovoltaic characteristics such as an open circuit voltage of 0.69 V, a fill factor of 0.55 and a conversion efficiency of 3.83% were attained under simulated AM1.5G solar illumination.

Resistivity characteristics of transparent conducting impurity-doped ZnO films for use in oxidizing environments at high temperatures

The behavior of resistivity in transparent conducting Al-doped ZnO (AZO) and Ga-doped ZnO (GZO) thin films, prepared with the same thickness under the same deposition conditions by dc magnetron sputtering, was investigated for use in oxidizing environments at high temperatures. These thin films with thicknesses in the range from 100 to 3000 nm were prepared on glass substrates at a temperature of 200 °C. The increase in resistivity of GZO thin films over long-term testing at 85% relative humidity and 85 °C was lower than that of AZO thin films, while, in contrast, that of AZO thin films over long-term testing at 90% relative humidity and 60 °C was lower than that exhibited by GZO. The moist-resistant tests showed that both AZO and GZO thin films prepared with a thickness above 500 nm were suitable for transparent electrode applications used in thin-film solar cells. The resistivity increase of GZO thin films after heat treatment for 30 min in air at a temperature up to 400 °C was lower than that of AZO th...

Impurity-doped ZnO Thin Films Prepared by Physical Deposition Methods Appropriate for Transparent Electrode Applications in Thin-film Solar Cells

This paper describes the development of transparent conducting impurity-doped ZnO thin films that would be appropriate for applications as transparent electrodes in thin-film solar cells. Transparent conducting Al-, B- and Ga-doped ZnO (AZO, BZO and GZO) thin films were prepared in a thickness range from 500 to 2000 nm on glass substrates at 200°C using various physical deposition methods: BZO films with vacuum arc plasma evaporation, AZO and GZO films with different types of magnetron sputtering depositions (MSDs) and all films with pulsed laser deposition (PLD). The suitability and stability of the electrical properties and, in addition, the suitability of the light scattering characteristics and surface texture formation were investigated in the prepared thin films. In particular, the suitability and stability evaluation was focused on the use of AZO, BZO and GZO thin films prepared by doping each impurity at an appropriate content to attain the lowest resistivity. The higher Hall mobility obtained in impurity-doped ZnO thin films with a resistivity on the order of 10−4 Ωcm was related more to the content, i.e., the obtained carrier concentration, rather than the kind of impurity doped into the films. The stability of resistivity of the BZO thin films in long-term moisture-resistance tests (in air at 85% relative humidity and 85°C) was found to be lower than that of the AZO and GZO thin films. The surface texture formation was carried out by wet-chemical etching (in a 0.1% HCl solution at 25°C) conducted either before or after being heat-treated either with rapid thermal annealing (RTA) or without RTA. The suitability of the light scattering characteristics and the surface texture formation obtainable by wet-chemical etching (for use in transparent electrode applications) was considerably dependent on the deposition method used as well as whether the wet-chemical etching was conducted with or without RTA. A significant improvement of both transmittance and haze value at wavelengths up to about 1200 nm in the near-infrared region was attained in surface-textured AZO films that were prepared by r.f. power superimposed d.c. MSD as well as etched after being heat treated with RTA at 500°C for 5 min in air. The obtained suitability and stability in impurity-doped ZnO thin films were related more to the content rather than the kind of impurity doped into the films as well as to the deposition method used.

Transparent conducting Si-codoped Al-doped ZnO thin films prepared by magnetron sputtering using Al-doped ZnO powder targets containing SiC

Transparent conducting Al-doped ZnO (AZO) thin films codoped with Si, or Si-codoped AZO (AZO:Si), were prepared by radio-frequency magnetron sputtering using a powder mixture of ZnO, Al2O3, and SiC as the target; the Si content (Si∕[Si+Zn] atomic ratio) was varied from 0to1at.%, but the Al content (Al∕[Al+Zn] atomic ratio) was held constant. To investigate the effect of carbon on the electrical properties of AZO:Si thin films prepared using the powder targets containing SiC, the authors also prepared thin films using a mixture of ZnO, Al2O3, and SiO2 or SiO powders as the target. They found that when AZO:Si thin films were deposited on glass substrates at about 200°C, both Al and Si doped into ZnO acted as effective donors and the atomic carbon originating from the sputtered target acted as a reducing agent. As a result, sufficient improvement was obtained in the spatial distribution of resistivity on the substrate surface in AZO:Si thin films prepared with a Si content (Si∕[Si+Zn] atomic ratio) of 0.75at.% using powder targets containing SiC. The improvement in resistivity distribution was mainly attributed to increases in both carrier concentration and Hall mobility at locations on the substrate corresponding to the target erosion region. In addition, the resistivity stability of AZO: Si thin films exposed to air for 30min at a high temperature was found to improve with increasing Si content.Transparent conducting Al-doped ZnO (AZO) thin films codoped with Si, or Si-codoped AZO (AZO:Si), were prepared by radio-frequency magnetron sputtering using a powder mixture of ZnO, Al2O3, and SiC as the target; the Si content (Si∕[Si+Zn] atomic ratio) was varied from 0to1at.%, but the Al content (Al∕[Al+Zn] atomic ratio) was held constant. To investigate the effect of carbon on the electrical properties of AZO:Si thin films prepared using the powder targets containing SiC, the authors also prepared thin films using a mixture of ZnO, Al2O3, and SiO2 or SiO powders as the target. They found that when AZO:Si thin films were deposited on glass substrates at about 200°C, both Al and Si doped into ZnO acted as effective donors and the atomic carbon originating from the sputtered target acted as a reducing agent. As a result, sufficient improvement was obtained in the spatial distribution of resistivity on the substrate surface in AZO:Si thin films prepared with a Si content (Si∕[Si+Zn] atomic ratio) of 0.75at...

Optical and electrical properties of transparent conducting B-doped ZnO thin films prepared by various deposition methodsa)

B-doped ZnO (BZO) thin films were prepared with various thicknesses up to about 500 nm on glass substrates at 200 °C by dc or rf magnetron sputtering deposition, pulsed laser deposition (PLD), and vacuum arc plasma evaporation (VAPE) methods. Resistivities of 4–6 × 10−4 Ω cm were obtained in BZO thin films prepared with a B content [B/(B + Zn) atomic ratio] around 1 at. % by PLD and VAPE methods: Hall mobilities above 40 cm2/Vs and carrier concentrations on the order of 1020 cm−3. All 500-nm-thick-BZO thin films prepared with a resistivity on the order of 10−3–10−4 Ω cm exhibited an averaged transmittance above 80% in the wavelength range of 400–1100 nm. The resistivity in BZO thin films prepared with a thickness below about 500 nm was found to increase over time with exposure to various high humidity environments. In heat-resistance tests, the resistivity stability of BZO thin films was found to be nearly equal to that of Ga-doped ZnO thin films, so these films were judged suitable for use as a transpare...

Transparent conducting impurity-doped ZnO thin films prepared using oxide targets sintered by millimeter-wave heating

The preparation of transparent conducting impurity-doped ZnO thin films by both pulsed laser deposition (PLD) and magnetron sputtering deposition (MSD) using impurity-doped ZnO targets sintered with a newly developed energy saving millimeter-wave (28GHz) heating technique is described. Al-doped ZnO (AZO) and V-co-doped AZO (AZO:V) targets were prepared by sintering with various impurity contents for 30min at a temperature of approximately 1250°C in an air or Ar gas atmosphere using the millimeter-wave heating technique. The resulting resistivity and its thickness dependence obtainable in thin films prepared by PLD using millimeter-wave-sintered AZO targets were comparable to those obtained in thin films prepared by PLD using conventional furnace-sintered AZO targets; a low resistivity on the order of 3×10−4Ωcm was obtained in AZO thin films prepared with an Al content [Al∕(Al+Zn) atomic ratio] of 3.2at.% and a thickness of 100nm. In addition, the resulting resistivity and its spatial distribution on the s...

Spatial resistivity distribution of transparent conducting impurity-doped ZnO thin films deposited on substrates by dc magnetron sputtering

In transparent conducting impurity-doped ZnO thin films prepared by a conventional dc magnetron sputtering deposition (dc-MSD), the key factors in the deposition conditions that are necessary for practical use in transparent electrode applications were investigated. It was found that impurity-doped ZnO targets with a resistivity higher than approximately 3 mΩ cm are unsuitable for practical use in the preparation of transparent conducting Al-doped ZnO and Ga-doped ZnO thin films by conventional dc-MSD. Improvements of both the resulting resistivity distribution and resistivity can be sufficiently obtained only by using targets with a resistivity lower than about 0.5 mΩ cm. Using a low oxygen content target having a lower resistivity was found to reduce both the amount of oxygen in the chamber and the amount of oxygen reaching the substrate surface. As a result, it was demonstrated that sintered impurity-doped ZnO targets optimized for the preparation of thin films with lower resistivity as well as more un...

Reduction of obtainable resistivity in transparent conducting impurity-doped ZnO thin films prepared with a very thin buffer layer by magnetron sputtering

Reductions of the obtainable resistivity as well as improvements of the crystallinity in transparent conducting impuritydoped ZnO thin films prepared on low-temperature glass substrates are demonstrated using a newly developed d.c. or r.f. superimposed d.c. magnetron sputtering (dc-MS or rf+dc-MS) deposition technique. The improvements of the obtainable lowest resistivity as well as the crystallinity in Al- and Ga-doped ZnO (AZO and GZO) thin films were achieved by inserting a very thin buffer layer that was deposited using the same d.c. MS apparatus with the same target used to deposit the AZO and GZO thin films. In addition, the insertion of the very thin buffer layer also improved the resulting resistivity distribution on the substrate surface in AZO and GZO thin films. The buffer layer between the thin film and the glass substrate was prepared by dc-MS or rf+dc-MS depositions using a target surface that was more strongly oxidized than usually used during depositions conventionally optimized to obtain lower resistivity; the resulting thin films could exhibit better crystallinity. A resistivity of approximately 3×10-4 Ωcm was obtained in 150-nm-thick-GZO and -AZO thin films prepared on glass substrates at 200oC.