Influence of Deposition Parameters on Surface Texturing of ZnO:Al Films Prepared by In-line RF Magnetron Sputtering
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1210-Q03-19
Influence of Deposition Parameters on Surface Texturing of ZnO:Al Films Prepared by In-Line RF Magnetron Sputtering Jun-Sik Cho, Young-Jin Kim, Jeong Chul Lee, Sang-Hyun Park and Kyung Hoon Yoon Photovoltaic Research Center, Korea Institute of Energy Research, 71-2 Jang-dong, Yuseong-gu, Daejeon, 305-343, Korea ABSTRACT A systematic study on the effect of sputtering deposition parameters on material properties of Al doped ZnO (ZnO:Al) films prepared by an in-line rf magnetron sputtering and on surface morphology of the films after wet etching process was carried out. For application to silicon thin film solar cells as a front electrode, the as-deposited films were surface-textured by a dilute HCl solution to improve the light scattering properties such as haze and angle resolved distribution of scattered light on the film surfaces. The microstructure of as-deposited films is affected significantly by the working pressure and film compactness decreases with increasing working pressure from 1.5 mTorr to 10 mTorr. High quality ZnO:Al films with electrical resistivity of 4.25 × 10-4 Ω⋅cm and optical transmittance of 80% in a visible range are obtained at low working pressure of 1.5 mTorr and substrate temperature of 100℃. Crater-like surface morphologies are observed on the textured ZnO:Al films after wet etching. The size and shape of craters are closely dependent on the microstructure and film compactness of as-deposited films. Haze values of the textured ZnO:Al films are improved in a whole wavelength of 300 – 1100 nm compared to commercial SnO2:F films (Asahi U type) and incident light on the textured films is scattered effectively with 30° angle. INTRODUCTION Magnetron sputtered ZnO films have been widely used as a transparent conducting electrode in a variety of thin film devices such as flat panel displays, antireflection coatings, heat mirrors and solar cells because of their high visible transparency and low electrical resistivity [1-3]. Recently, the ZnO films are intensively investigated as an alternative front TCO electrode for micromorph tandem silicon thin film solar cells in which hydrogenated amorphous silicon (aSi:H) and microcrystalline silicon (µc-Si:H) are used as an absorber layer of top and bottom cell, respectively [4]. Besides good electrical conductivity and optical transmittance, for application to this type of solar cell, a rough surface morphology, or so-called “textured surface” is required necessarily to provide an efficient light trapping through scattering at the surface because the
optical absorption of µc-Si:H is not sufficient in very thin (~2 µm) layer [5]. Therefore, the improved light scattering at the front TCO surfaces in a long wavelength region above 600 nm in which the µc-Si:H bottom cell operates, is one of the most important requirements for achieving high conversion efficiency of solar cells. There are many reports on the textured ZnO:Al films prepared by sputtering and wet etching process, but most of them were deposited in static mode, in which the substrate
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