ZNO FILMS WITH TAILORED MATERIAL PROPERTIES FOR HIGHLY EFFICIENT THIN-FILM SILICON SOLAR MODULES M. Berginski 1 , B. Rech 1 , J. Hüpkes 1 , G. Schöpe 1 , M.N. van den Donker 1 , W. Reetz 1 , T. Kilper 1 , M. Wuttig 2 1 Institute of Photovoltaics (IPV), Forschungszentrum Jülich GmbH, D-52425 Jülich, Germany Phone: +49 2461 613242, fax: +49 2461 613735, e-mail: M.Berginski@fz-juelich.de 2 Institute for Physics of New Materials – Department of Physics, RWTH, Aachen, Germany ABSTRACT: This study addresses the balance between electrical and optical properties of magnetron sputtered aluminum doped zinc oxide (ZnO:Al) films for the application as front contact in silicon thin-film solar cells and modules. We gradually decreased the amount of carrier concentration in the ZnO:Al films without any effect on surface topography. By reducing carrier concentration resistivity rose while parasitic free carrier absorption of the transparent conductive oxide in the near infrared decreased. The relationship between carrier concentration and short circuit current density of microcrystalline silicon (μc-Si:H) single junction thin-film solar cells has been studied experimentally. These data have been used to estimate optimal carrier concentration of the front TCO of single and multi junction modules. Besides, current enhancement by tailoring the charge carrier density, the open- circuit voltage was enhanced by in situ controlled SiH4 flow profiling during μc-Si:H deposition. In first promising results in triple junction a-Si:H / μc-Si:H / μc-Si:H modules (64 cm 2 ) we achieved an initial aperture area efficiency of 11.1%. Keywords: TCO Transparent Conductive Oxides, Light Trapping, Optical Losses 1 INTRODUCTION The transparent front contact is a crucial factor in thin-film silicon photovoltaics. Property-wise, it has to be optically transparent, electrically conductive and morphologically textured for efficient light trapping at the same parametric choice of thin-film growth. Transparent conductive oxides like ZnO:Al, SnO2:F and In2O3:Sn are commonly applied for this purpose. It is an experimentally challenging task to tune separately the transparency, surface morphology, and conductivity. In this contribution we investigate the case of ZnO:Al as front contacts for highly efficient thin-film silicon solar modules. Silicon thin-film solar cells are promising candidates for photovoltaic power generation in future. One approach employs hydrogenated amorphous (a-Si:H) and microcrystalline silicon (μc-Si:H) as active layers in single or multi junction cells [1]-[3]. An efficient light trapping scheme is particularly important for this case of silicon based thin-film solar cells, because of the intrinsically low absorbance of silicon, especially in long wavelength range [4],[5]. Light trapping is achieved by combining surface-textured transparent conductive oxide (TCO) films as front contacts with highly reflective back contacts. In previous work at the IPV texture-etched ZnO:Al was developed as front contact material with effective light scattering. Initially smooth sputter deposited ZnO:Al films can be surface textured by post-deposition wet-chemical etching [6]-[8]. Kluth et al. related the influence of pressure and substrate temperature during radio frequency (rf) sputter deposition of ZnO:Al at a fixed target alumina concentration of 2 wt.% to structural properties and post-etching surface topography in a modified Thornton model [8]. Depending on sputter parameters, crater-like surface topography with typical lateral length dimensions of 1 to 2 μm and depths of about 200 to 400 nm develops in a self-organized fashion. Additionally, in case of reactive sputter deposition the working point plays an important role for the surface topography that develops during wet- chemical etching [9]. Regarding conductivity and transparency, the front contact has to meet counteracting properties: a high conductivity requires high charge carrier mobility and concentration, whereas the transparency in red and near infrared (NIR) wavelength range requires low charge carrier concentration. Note, that the NIR absorption of TCO becomes more pronounced by light trapping effect: due to multiple passes of scattered light within the solar cell device, absorption in intrinsic silicon layers is enhanced, but parasitic light absorption in the photovoltaically non-active layers (highly doped p- and n-type silicon films, front TCO and back reflector) increases as well. In the NIR the absorption for a single passage of light through a 1 μm thick layer of μc-Si:H is lower than that of high quality TCOs. It is an experimental challenge to vary the ZnO:Al conductivity and transmission, while maintaining a surface topography equally efficient for light trapping. Preceding work by Agashe et al. studied the influence of the target alumina concentration on the electrical, structural and optical properties of sputter deposited ZnO:Al films [10]. They succeeded in lowering parasitic absorption using sputter targets with low amount of alumina and optimized deposition conditions, while low resistivity was maintained. The post-etching surface topography was not investigated in that study. Berginski et al. studied the influence of doping level and substrate temperature on the post-etching surface texture of ZnO:Al films and their light trapping ability in silicon thin-film solar cells [11]. Depending on the target alumina concentration and substrate temperature, three different post-etching surface topography types have been identified. Best light trapping in solar cells has been found in case of surfaces that are equally filled with craters of approximately same size (diameters about 1 to 3 μm and depths about 150 to 400 nm). A challenging task in the optimization of front