Investigation on Nanorod TCO Light-trapping for a-Si:H Solar Cells in Superstrate Configuration
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Investigation on Nanorod TCO Light-trapping for a-Si:H Solar Cells in Superstrate Configuration Martin Vehse1, Stefan Geißendörfer1, Tobias Voss2, Jan-Peter Richters2, Benedikt Schumacher1, Karsten von Maydell1, and Carsten Agert1 1 NEXT ENERGY, EWE Research Center for Energy Technology at Carl von Ossietzky University, 26129 Oldenburg, Germany 2 University of Bremen, Institute of Solid State Physics, Semiconductor Optics Group, 28334 Bremen, Germany ABSTRACT Light trapping due to rough transparent conductive oxide (TCO) surfaces is a common and industrially applied technique in thin film silicon solar cells. In this study, we demonstrate a novel light trapping solution using electrochemically deposited, highly doped zinc oxide (ZnO) nanorod arrays which goes beyond standard light management concepts. The n-doped ZnO rods enable the application as front electrode in superstrate configuration. We explain our experimental results by multidimensional solar cell simulations and show how the nanorod array geometry influences the cell performance. The requirement is demonstrated to choose an appropriate average nanorod distance which strongly influences the electrical cell characteristics. The results clearly outline the potential of TCO nanorod technology for enhanced light trapping. INTRODUCTION One of the main goals in thin film silicon (TFS) solar cell development is to increase the optical absorption by light trapping in order to achieve higher energy conversion efficiency. In addition to increased efficiencies of hydrogenated amorphous silicon (a-Si:H) solar cells, the approach of light trapping enables the use of thinner absorber layers and so reducing the lightinduced degradation mechanism due to hydrogen bond reorganization at defects [1] (StaeblerWronski effect [2]). There are several concepts to improve the light trapping beyond the standard TCO texturing by chemical wet etching [3]. In literature, several methods are introduced to integrate nanostructures [4-7] or plasmonic scatter particles [8–11] in the TFS solar cell. A promising new concept is the incorporation of vertically aligned zinc oxide (ZnO) nanorods which have been demonstrated by other groups [5,12]. Going beyond the existing studies, which prove the potential of the nanorod concept, this work introduces a new approach in which highly n-doped ZnO nanorods replace the textured TCO for light trapping in the superstrate cell configuration. This novel concept of doped nanorods enables the use of nanorod arrays as conductive electrode material without additional metal layers [12] to realize a sufficientconductivity. The solution-based fabrication of ZnO nanorod arrays is a simple process which is basically not limited to small substrate sizes. Furthermore, it does not involve complicated or expensive process steps like reactive ion etching which is required for other nanorod array fabrication techniques [4]. A comparison of experimental results with opto-electrical multidimensional solar cell simulations is shown. In particular, we found the need for
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