ZnO:Al films deposited by in-line reactive AC magnetron sputtering for a-Si:H thin film solar cells V. Sittinger a, * , F. Ruske a , W. Werner a , B. Szyszka a , B. Rech b , J. Hu ¨ pkes b , G. Scho ¨pe b , H. Stiebig b a Fraunhofer Institute for Surface Engineering and Thin Films IST, Bienroder Weg 54e, 38108 Braunschweig, Germany b Institute of Photovoltaics IPV, FZ-Ju ¨ lich, D-52425 Ju ¨ lich, Germany Available online 9 September 2005 Abstract Throughout the last years strong efforts have been made to use aluminium doped zinc oxide (ZnO:Al) films on glass as substrates for amorphous or amorphous/microcrystalline silicon solar cells. The material promises better performance at low cost especially because ZnO:Al can be roughened in order to enhance the light scattering into the cell. Best optical and electrical properties are usually achieved by RF sputtering of ceramic targets. For this process deposition rates are low and the costs are comparatively high. Reactive sputtering from metallic Zn/Al compound targets offers higher rates and a comparable high film quality in respect to transmission and conductivity. In the presented work the process has been optimised to lead to high quality films as shown by reproducible cell efficiencies of around 9% initial for single junction amorphous silicon solar cells on commercial glass substrates. The crucial point for achieving high efficiencies is to know the dependency of the surface structure after the roughening step, which is usually performed in a wet etch, on the deposition parameters like oxygen partial pressure, aluminium content of the targets and temperature. The most important insights are discussed and the process of optimisation is presented. D 2005 Elsevier B.V. All rights reserved. Keywords: Aluminium doped zinc oxide films; Solar cells; Sputter deposition 1. Introduction Within the scope of an increased importance of renewable energy sources photovoltaics has experienced an increased scientific and economic interest. Solar cells based on silicon wafer technology are being produced in large amounts and account for over 90% of the solar cell market [1]. In expectation of technological progress it is commonly accepted that the costs for solar energy will be dominated by the silicon material cost. The so-called second generation solar cells therefore make use of thin film active layers where light conversion takes place in layers only a few microns thick. Thin film solar cells based on amorphous or microcrystalline silicon (a-Si:H, Ac-Si:H) are examples for this type of solar cell. A good overview on the potential of this cell type can be found in [2], more aspects are found in [3]. High quality solar cells strongly rely on the quality of transparent and conductive oxide (TCO) films used as front contacts. This is especially true for superstrate (Fig. 1) cells where the solar cell is deposited top to bottom onto a glass substrate. The requirements which these films have to meet are numerous: (a) light loss in the front layers has to be prevented by minimisation of optical absorption, (b) the resistivity has to be as low as possible in order to keep the required film thickness for a good current transport small (lower deposition time, lower cost, less optical losses) and (c) the material cost has to be low. The minimisation of optical losses implies a matching of optical properties of the TCO material to the absorbing layer. While low optical absorption in the visible range is a common optimisation goal for all researchers of TCO films the transmission window has to be extended to the band gap of the absorbing layer in the case of solar cells. For a-Si:H with a bandgap of around 1.7 eV and Ac-Si:H (1.1 eV) this means that the plasma wavelength has to be increased to values of 750 and 1150 nm, respectively. According to Drude’s theory the optical properties in the near infrared (NIR) are closely linked to the properties of free carriers in the material and the plasma 0040-6090/$ - see front matter D 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2005.08.177 * Corresponding author. Tel.: +49 531 2155 641; fax: +49 531 2155 900. E-mail address: sittinger@ist.fraunhofer.de (V. Sittinger). Thin Solid Films 496 (2006) 16 – 25 www.elsevier.com/locate/tsf