Low-temperature wet chemical deposition of ultra-thin ZnS/ZnO bilayers on plastic substrates for applications of photovo
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1260-T05-08
Low-temperature wet chemical deposition of ultra-thin ZnS/ZnO bilayers on plastic substrates for applications of photovoltaic devices Rong-Fuh Louh1, William Wu1, Jean Liu1 and Irene Tsai1 Dept. of Materials science and Engineering, Feng Chia University, Taichung, Taiwan 40724 ABSTRACT The ultra-thin II-VI semiconductor ZnS/ZnO bilayers (< 50 nm thickness for each layer) can be easily formed on the plastic substrates at 70~80oC for 20 min. By low temperature wet chemical synthesis techniques, namely chemical bath deposition (CBD) and successive ionic layer adsorption and reaction (SILAR). The specific microstructure of such ZnS/ZnO bilayers including film thickness, particle size and morphology, is also modified and obtained in accordance with processing parameters. Along with thin film quality and morphology, the transmittance and reflectance of ZnS/ZnO layers can be measured by field emission SEM and UV-Vis spectroscopy. Besides the bilayer of ZnS (~35 nm thick)/ZnO (~50 nm thick) film with uniform thickness was successfully deposited on the optical grade PET substrates, a welldistributed layer of ZnO nanoparticles with ~100 nm size on the top of ZnS (35 nm thick) film was also attempted. The average transmittance of these bilayer samples can reach greater 85%. Our future goal is to employ such ZnS/ZnO bilayer structure on potential organic substrates to be associated with flexible photovoltaic devices to meet desired cost-effectiveness requirements. Keyword: low temperature wet chemical deposition, chemical bath deposition (CBD), successive ionic layer adsorption and reaction (SILAR), ZnS/ZnO bilayers, plastic substrates, photovoltaics. INTRODUCTION In recent years, choice of suitable flexible substrates is rather prominent at advent of booming uses of flexible electronics. The polyethylene terephthalate (PET) substrates, which pose several attractive features such as light-weight, impact resistance, portability, rollability, wearability and compatibility for roll-to-roll, low-cost manufacture functionality, would meet the basic requirements for the trend of flexible electronic devices. Zinc oxide (ZnO) and zinc sulfide (ZnS) semiconductors are widely applied in the field of display and photovoltaic devices. The ZnO films possess hexagonal wurtzite structure with an energy gap (Eg) of 3.3 eV and high transmittance in the visible range with sharp UV cut-off edge. The chalcogenide films made from II-VI semiconductors such as CdS with Eg = 2.26-2.5 eV and ZnS with Eg = 3.5 eV are used as buffer layers in the CIGS thin film solar cells. [1, 2, 3, 4] Moreover, the fluorescent properties are associated with ZnO/ZnS thin film nanocomposite structure. In this study, a low-temperature wet chemical deposition route to produce ultra-thin ZnS/ZnO bilayers on plastic substrates was explored to meet requirements of applications of photovoltaic devices such as ZnO/ZnS/ITO structure of the dye-sensitized solar cell (DSSC) or AZO/ZnO/ZnS/CIGS structure of the copper indium gallium diselenide (CIGS) [5, 6, 7], as show
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