ICMCTF2014 Session C3: Advances in Electrode Materials for Modern Device Applications

A novel hierarchical aluminum-doped zinc oxide thin films were deposited on low-cost flexible polymer substrates at room temperature using chemical bath deposition. The effects of reaction temperatures, precursor and time on film properties were investigated. And the morphological structure of ZnO:Al thin films, was studied using modern methods such as: X-ray photoelectron spectroscopy, X-ray diffraction (XRD) and atomic force microscopy. A simple growth mechanism had been proposed to interpret the formation of the hierarchical ZnO:Al films. Consequently, a low resistivity of ~ 8.7 × 10^-4 Ω cm with a high normalized transparency index of > 0.9 for a 150 ± 10 nm thick room-temperature deposited film was obtained, representing one of the best results obtained to date. Moreover, the efficiency of the thin-film solar cell with this ZnO:Al top electrode reached 8%.

DC high voltage thermionic and photoemission electron guns require cathode electrodes that do not exhibit field emission. Field emission is the unregulated release of electrons from the cathode electrode surface when the cathode is biased at high voltage. Field emission is problematic for a number of reasons: it degrades the vacuum level via electron stimulated desorption and leads to shortened operating lifetime of the gun, it creates unwanted x-rays that are harmful to nearby personnel, and high levels of field emission can damage electron gun components.

Cathode electrodes for electron guns are typically manufactured from hard metals like stainless steel, titanium-alloy, or molybdenum. Once fabricated at the machine shop, the relatively rough surface of the electrode must be mechanically polished by hand using silicon carbide paper and diamond grit, to obtain a smooth surface free of microscopic protrusions which cause field emission. However, the polishing process for electrodes manufactured from hard metals is time consuming and labor intensive.

ZnO is highly useful for various applications such as short-wavelength optoelectronics and transparent conductive electrodes. In a previous paper, we reported a new growth method for preparing ZnO films by reacting alkyl-zinc and high-temperature H₂O generated by a catalytic reaction [1]. The resulting ZnO films grown on a-plane sapphire (a-Al₂O₃) substrates exhibited excellent optical and electronic properties [2]. In this study, we investigated the influence of the N₂O gas supply during the growth on the properties of the ZnO films. The structure of the CVD apparatus and the growth procedure used in this study are the same as those shown in a previous paper [1], except for the addition of N₂O gas to the reaction zone. The ZnO films were directly grown on a-Al₂O₃ substrates at 773K for 60 min without any buffer layer. The thickness of the ZnO films characterized was 6–8 μm. Although the N₂O gas supply pressure varied from 3.2×10^-3 Pa to 9.7×10^-2 Pa, all films showed an n-type character. The electron mobility of N₂O doped (3.2×10^-3 Pa) film at RT (290 K) was 234 cm²/Vs, while that of non-doped ZnO film was 207 cm²/Vs. The mobility increases significantly with decreasing temperature to 100 K for all films. The mobility of the N₂O doped film (234 cm²/Vs at RT) increases to 1100 cm²/Vs at 100 K. Electron concentrations at RT of all films were 4.3-5.9×10¹⁶ cm^-3 and those of the N₂O doped ZnO films were smaller than that of the non-doped film from RT to 80 K. The FWHM value of the ZnO (0002) ω-rocking curve of the N₂O doped (3.2×10^-3 Pa) film was 142 arcsec, while that of the non-doped ZnO film was larger than 194 arcsec. The FWHM value of PL peak (3.36 eV) at 10K for the N₂O doped film was 0.7 meV, while that of the PL peak (3.36 eV) for the non-doped film was 0.9 meV. A significant correlation between the electron mobility and the FWHM value of the ZnO (0002) ω-rocking has been observed [2]. The reduction in the fluctuation of the crystal orientation along the c-axis appears to have yielded the excellent electrical and optical properties in the N₂O doped ZnO films. Although a part of the donor impurities are compensated by the nitrogen acceptor, the intrinsic donor impurities caused by the defects are considered to be reduced by the doping of nitrogen in view of the large electron mobility for N₂O doped ZnO films.

Acknowledgements: This work was supported in part by Tokyo Electron Ltd. and a Grant-in-Aid for Scientific Research (No. 24360014) from the Japan Society for the Promotion of Science.

[1] K. Yasui et al., MRS Symp. Proc., 1315 (2011) 21.

Sol-gel method has shown several advantages for oxide synthesis, such as lower cost production, coating large areas, lower processing temperatures and ease insertion of doping materials, thus, it has also been an attractive method in solar cells manufacturing. In this work, aluminum-doped titanium dioxide (AlTiO₂) modifying layers were synthesized by sol-gel method onto fluorine doped tin oxide (FTO) electrodes, in order to improve the efficiency of organic photovoltaic devices (OPVs). Before device fabrication, the structural, compositional, optical and electrical properties of the transparent oxide films were determined and compared with those obtained from undoped TiO₂ layers produced under the same conditions. The optimum Al-dopant concentration was determined by DC conductivity measurements on FTO/AlTiO₂ /Al devices. The performance of OPVs was investigated based on the FTO/AlTiO₂ (30nm)/CuPc (20nm)/C₆₀ (40nm)/Alq₃ (20nm)/Al device under dark and standard illumination conditions (1Sun, M1.5G). AlTiO₂ modifying layer improves the direct current under illumination and increases significantly the open-circuit voltage, V_oc, resulting in a value of 750 mV, while the devices based on FTO and TiO₂ presented a V_oc of about 250 mV. These results indicate the Al-doping of TiO₂ thin films induces a strong built-in potential modification. It is known that Al induces strong dipole interaction on interfaces poly(thiophene)/Al, increasing, considerably, the Voc. In the AlTiO₂/CuPc interface, the enhancement of built-in potential can result from dipole phenomena induced by Al-dopant. All the results will be presented and discussed in the work.

The TCO thin film is an important layer which influence on Rs (series resistance) of a-Si:H PV(photovoltaic) modules. So the stability test of physical/chemical properties of TCO (transparent conducting oxide) under weather environment stress is required. Flexible tandem a-Si:H PV modules were produced. ZnO thin film was used for TCO. To verify the stability of environment stress, reliability test based on MIL-STD-883G was conducted. Efficiency was decreased after damp heat (85℃, 85%RH) stress. This study is about degradation mechanism by analyzing reaction mechanism due to the moisture of ZnO thin film which TCO material was used in flexible a-Si:H PV modules. For fault isolation, cell defect by using electrical parameters of PV and EL(electroluminescence) from I-V characteristic was analyzed. Among electrical parameters, Rs showed dramatic increase but Isc and Voc showed a little change. There was no hot spot or dead cell in cell. This result can tell that the resistance of ZnO was increased by moisture so it increased Rs of a-Si:H PV. The changing mechanism of physical/chemical properties of ZnO by Moisture was analyzed. ZnO properties were analyzed with efficiency degradation rate of a-Si:H PV. Efficiency degradation sample was made by virgin, -5%, -10%, -20% and -50% level. Resistivity, surface profile and change of physical/chemical structure are analyzed by XRD, XPS, Raman, AFM, TEM etc. ZnO degradation mechanism was analyzed through the analyzing result

The progress of nanotechnology requires a fundamental research on the growth modes which may include different interaction parameters. Indeed, the experimental observations would be more robust by theoretical understanding of the growth process and hence to predict and control the microstructures to be obtained. This work is an attempt to understand the growth mechanism of AgCl obtained through an electrochemical process at room temperature. In effect, nanowires of Agcl are obtained under low potentials in AC mode and the microscopic investigation by SEM reveals clearly a new morphology of the surface which may be described as a random nanosheets. We think that the most suitable theory in this case is based on the STRANSKI KRASTANOV method. Consequently, the way is open to determine new mechanical and physical properties.