Characterization and Optimization of The Tco/a-Si:H(,B) Interface for Solar Cells by In-Situ Ellipsometry and Sims/XPS Depth Profiling

Abstract

ABSTRACTThe chemical reactions at the surface of transparent conductive oxides (SnO2, ITO and ZnO) have been studied in silane and hydrogen plasmas by in-situ ellipsometry and by SIMS as well as XPS depth profiling. SIMS and XPS of the interface reveal an increasing amount of metallic phases upon lowering a-Si:H growth rates (controlled by plasma power), indicating that the ion and radical impact is more than compensated by protecting the surface by a rapidly growing a-Si:H film. Hence, optical transmission of TCO films as well as the efficiency of solar cells can be improved if the first few nanometers of the p-layer are grown at higher rates. Comparing a-Si:H deposition on top of different TCOs, reduction effects on ITO and SnO2 have been detected whereas ZnO appeared to be chemically stable. Therefore an additional shielding of the SnO2 surface by a thin ZnO layer has been investigated in greater detail. Small amounts of H are detected close to the ZnO surface by SIMS after hydrogen plasma treatment, but no significant changes occur to the optical and electrical properties. In-situ ellipsometry indicates that a ZnO layer as thin as 20 nm completely protects SnO2 from being reduced to metallic phases. This provides for shielding of textured TCOs, and hence rising solar cell efficiencies, too. Regarding light trapping efficiency we additionally investigated the smoothing of initial TCO texture when growing a-Si:H on top by combining atomic force microscopy and spectroscopie ellipsometry.