Real time spectroscopic ellipsometry of Ag/ZnO and Al/ZnO interfaces for back-reflectors in thin film Si:H photovoltaics Lila Raj Dahal ⁎, Deepak Sainju, N.J. Podraza, S. Marsillac, R.W. Collins Center for Photovoltaics Innovation and Commercialization, University of Toledo, Toledo OH 43606, USA abstract article info Available online 10 December 2010 Keywords: Real time spectroscopic ellipsometry Interfaces Plasmons Metal films Zinc oxide Thin silicon solar cells Reflectors Real time spectroscopic ellipsometry (RTSE) has been applied to analyze the optical characteristics of Ag/ZnO and Al/ZnO interfaces used in back-reflector (BR) structures for thin film silicon photovoltaics. The structures explored here are relevant to the substrate/BR/Si:H(n-i-p) solar cell configuration and consist of opaque Ag or Al films having controllable thicknesses of microscopic surface roughness, followed by a ZnO layer up to ~3000 Å thick. The thicknesses of the final surface roughness layers on both Ag and Al have been varied by adjusting magnetron sputtering conditions in order to study the effects of metal film roughness on interface formation and interface optical properties. The primary interface loss mechanisms in reflection are found to be dissipation via absorption through localized plasmon modes for Ag/ZnO and through intraband and interband transitions intrinsic to metallic Al for Al/ZnO. Published by Elsevier B.V. 1. Introduction A widely-used back-reflector (BR) for thin film hydrogenated silicon (Si:H) solar cells in the substrate/BR/Si:H(n-i-p) configuration consists of opaque Ag or Al, followed by a ~1500–3000 Å thick layer of ZnO, both sputtered onto a low-cost substrate such as stainless steel or polymer foil [1,2]. Quantum efficiency enhancement in the solar cell over the near- infrared range (1.2 to 1.8 eV; 650 to 1000 nm), where the thin film Si:H absorption is weak, arises due to reflection of near-infrared wavelengths back into the cell for possible absorption during additional passes. Additionally, the metal/ZnO interface is designed to be macroscopically rough, with protrusions on the order of the wavelength (λ ~ 800 nm), so that the light rays are non-specularly scattered upon back-reflection which increases their optical path lengths. Such enhancement relies on a high reflectance of the metal/ZnO interface in the near-infrared spectral range; however, the incorporation of roughness at this interface can result in greater dissipative losses due to confined plasmons localized at the surfaces of roughness protrusions or due to propagating plasmons that require periodic roughness components for their excitation [3,4]. Using real time spectroscopic ellipsometry (RTSE) in this study, both the Ag/ZnO and Al/ZnO interfaces are characterized in detail as functions of the metal film surface roughness thickness on the microscopic scale, i.e., with protrusions on a scale much smaller than the wavelength. Although the microscopic interface roughness values studied here are smaller than those of optimized devices, trends observed as the roughness increases are of particular interest. This report focuses on the RTSE analysis and its outcome, whereas a previous paper has focused on the application of these RTSE results to improve BR performance in cells [2]. 2. Experimental details The Ag and Al films were deposited by rf magnetron sputtering onto smooth c-Si wafer substrates covered with thermal oxides (d ox ~ 400 Å). The target surface area is 20 cm 2 and the target-to- substrate distance is 7 cm. The deposition process leading to the smoothest Ag yields a microscopic roughness thickness of d s = 4 Å, as deduced by an effective medium theory analysis of RTSE data after a final Ag bulk layer thickness of d b ~ 1500 Å. The deposition process for the smoothest Al yields a microscopic roughness thickness of d s = 15 Å after a final Al bulk layer thickness of d b ~ 1000 Å. Both such films were obtained at room temperature using a low Ar sputtering gas pressure of 4 mTorr, an Ar flow of 10 sccm, and a metal target power of 50 W. For Ag deposition, an increase in the substrate temperature to 190 °C, yields an increase in the final microscopic surface roughness thickness as deduced by RTSE to d s = 105 Å, whereas for Al deposition, an increase to 85 °C yields an increase in this roughness to d s = 122 Å. The final roughness values from RTSE were compared with the root mean square (rms) values obtained from AFM measurements which probed 5 × 5 μm 2 areas of the metal films. Finally, ZnO was deposited on both Ag and Al surfaces at room temperature without a vacuum break using the same conditions under which the smoothest Ag and Al were prepared. RTSE was performed using a rotating-compensator multichannel instrument (J.A. Woollam Co.) that can provide spectra in (ψ, Δ) from 0.75 to 5.0 eV. Rotating-compensator multichannel instrument designs are described in previous publications [5,6]. To improve Thin Solid Films 519 (2011) 2682–2687 ⁎ Corresponding author. E-mail address: ldahal@rockets.utoledo.edu (L.R. Dahal). 0040-6090/$ – see front matter. Published by Elsevier B.V. doi:10.1016/j.tsf.2010.11.093 Contents lists available at ScienceDirect Thin Solid Films journal homepage: www.elsevier.com/locate/tsf