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

Thursday, May 1, 2014 1:30 PM in Room Golden West

Thursday Afternoon

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1:30 PM C3-1 A Novel Hierarchical Aluminum-doped Zinc Oxide Thin Film for Flexible Thin-film Solar Cells
Xuan Huang, Feng-Yan Zhang (Xiamen University, China)

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%.

1:50 PM C3-2 Evaluation of TiN-Coated Aluminum Electrodes for DC High Voltage Electron Guns
MdAbdullahA. Mamun (Old Dominion University, US); Eric Forman (Thomas Jefferson National Accelerator Facility, US); Rhys Taus (Loyola Marymount University, US); Matthew Poelker (Thomas Jefferson National Accelerator Facility, US); AbdelmageedA. Elmustafa (Old Dominion University, US)

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.

In this research, the field emission characteristic of electrodes manufactured from soft aluminum metal were studied using a high voltage field emission test stand, before and after coating the surface of the electrodes with titanium-nitride (TiN). Electrodes manufactured from aluminum are relatively easy to polish, requiring only hours to obtain a mirror-like finish, rather than days. The electrodes coated with TiN performed markedly better than the uncoated electrodes and the best electrode exhibited no measurable field emission (<10 pA) at a field strength of > 22.5 MV/m. In sharp contrast, the uncoated electrodes exhibited significant field emission at field strength of just ~ 5 MV/m. The electrodes were evaluated using scanning electron microscopy with x-ray microanalysis (SEM/EDS) to investigate morphological and structural variations associated with the polishing and coating processes, and due to the high voltage processing which included gas conditioning with helium and krypton. Small test coupons were studied using an atomic force microscope and nanoindenter, to evaluate surface morphology, hardness and modulus of the TiN thin films.
2:10 PM C3-3 Effects of N2O Addition During the Growth of ZnO Films using High-temperature H2O Generated by a Catalytic Reaction
Tomoki Nakamura, Yuuki Ohashi, Naoya Yamaguchi, Eichi Nagatomi, Takahiro Kato, Yasuhiro Tamayama, Kanji Yasui (Nagaoka University of Technology, Japan)

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.

[2] N. Yamaguchi et al., Thin Solid Films, (2013) in press.
2:50 PM C3-5 Enhancement of Open-circuit Voltage on Organic Photovoltaic Devices by Al-doped TiO2 Modifying Layer Produced by Sol-gel Method
Rogerio Valaski, Caroline Arantes, CarlosAlberto Achete (Inmetro, Brazil); Marco Cremona (PUC-RIO, Brazil)

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, Voc, resulting in a value of 750 mV, while the devices based on FTO and TiO2 presented a Voc 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.

3:10 PM C3-6 Thermochromics and Electrochromics for Energy Efficient Fenestration
Claes Granqvist (Uppsala University, Angstrom Laboratory, Sweden)
Thermochromic and electrochromic materials, and devices based on these, can be used in energy efficient fenestration and lead to significant drops of the energy that is used for heating and cooling of buildings. Thermochromics allows temperature-dependent control of the inflow of visible light and solar energy, and electrochromics can give a similar functionality that is electrically controlled. This paper summarizes recent work, mainly from the author's laboratory, on thermochromic thin films and nanoparticles as well as on electrochromic devices embodying thin films of tungsten oxide and nickel oxide. The presentation covers materials, devices, operating principles, and prospects for the mentioned technologies.
3:50 PM C3-8 Degradation Mechanism of ZnO thin Film for TCO of Flexible a-Si:H PV Module due to Moisture
Jae-Seong Jeong (Korea Electronics Technology Institute, Republic of Korea)

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

4:10 PM C3-9 Growth Mechanism of Silver Chloride Nano Wires by Electrodeposition Route
Akila Derardja, Salah Seghir Mechouar (LaMSM, University of Batna, Algeria)

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.

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