Highly Transparent AZO/Ag/AZO Multilayer Front Contact for n-i-p Silicon Thin-Film Solar Cells
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Highly Transparent AZO/Ag/AZO Multilayer Front Contact for n-i-p Silicon Thin-Film Solar Cells Martin Theuring1, Martin Vehse1, Ibrahim Noureddine1, Karsten von Maydell1, Carsten Agert1 1 NEXT ENERGY - EWE Research Center for Energy Technology at Carl von Ossietzky University Oldenburg, Germany
ABSTRACT Oxide-metal-oxide structures are an alternative to single material transparent electrical contacts. Among other advantages, these multilayer systems provide good conductivity and transmittance, even when fabricated at room temperature. Low temperature processing is a requirement for silicon thin-film solar cells on various flexible substrates. The design and fabrication of oxide-metal-oxide structures based on ZnO:Al and Ag are investigated in this work. Further the integration of an optimized multilayer electrode into an amorphous silicon solar cell in substrate configuration was performed. Measurement results and possible loss mechanisms are discussed. INTRODUCTION Most state-of-the-art silicon thin-film (TF) solar cells are fabricated on heavy and rigid glass substrates. Hence, in most of these cells two of their major advantages, the low weight and their mechanical flexibility, are not fully utilized. For this, a replacement of the substrate materials is necessary. Plastic and metal foils meet the named requirements and both were already successfully used in TF solar cell fabrication [1,2]. Cells on opaque substrates are fabricated in the substrate configuration. Here one of the key issues is the transparent front electrode, which are generally realized with a layer of doped semiconducting oxide such as ZnO:Al (AZO) [3,4]. AZO contains abundant materials and matches the required electrical and optical properties for silicon TF solar cells. When sputtering is used to deposit AZO, the process is preferably carried out at high temperatures to obtain highly conductive and transparent layers [5]. This is a major problem for the described cell configuration, as plastic substrates as well as isolating layers on metal foils are heat sensitive materials. Also the silicon layers, on which the electrode is fabricated, may be negatively affected by such an invasive process. Here, we want to show one possibility to realize a transparent front electrode for silicon TF solar cells with a low temperature process. We base our findings on the work of various other groups. Among others, Liu et al. [6] and Sahu et al. [7] demonstrated that a metal film of a few nanometers, embedded in transparent conductive oxide (TCO) layers, significantly increases the lateral conductivity. The resulting possibility to reduce the TCO thickness yields major advantages such as: lower material usage and production time, an improvement of the optical transmittance [8] and a higher mechanical robustness in a bendable device [9]. Oxide-metaloxide (OMO) layers promise to be a good match for front contacts in flexible TF solar cells. We verify this statement by investigating the limits of AZO/Ag/AZO multilayer electrodes and the ability of OMO structures
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