EPJ Photovoltaics 5, 55205 (2014) www.epj-pv.org DOI: 10.1051/epjpv/2014004 EPJ Photovoltaic s EPJ Photovoltaics O A Thin metal layer as transparent electrode in n-i-p amorphous silicon solar cells Martin Theuring a , Stefan Geissend¨orfer, Martin Vehse, Karsten von Maydell, and Carsten Agert NEXT ENERGY – EWE Research Centre for Energy Technology at Carl von Ossietzky University, Carl-von-Ossietzky-Straße 15, 26129 Oldenburg, Germany Received: 30 July 2013 / Received in final form: 21 January 2014 / Accepted: 10 February 2014 Published online: 7 July 2014 c  Theuring et al., published by EDP Sciences, 2014 Abstract In this paper, transparent electrodes, based on a thin silver film and a capping layer, are inves- tigated. Low deposition temperature, flexibility and low material costs are the advantages of this type of electrode. Their applicability in structured n-i-p amorphous silicon solar cells is demonstrated in simulation and experiment. The influence of the individual layer thicknesses on the solar cell performance is discussed and approaches for further improvements are given. For the silver film/capping layer electrode, a higher solar cell efficiency could be achieved compared to a reference ZnO:Al front contact. 1 Introduction Transparent electrodes are used in many electro- optical devices such as displays and solar cells. The main requirements of these electrodes are a low electrical re- sistance and a high optical transmittance. However, both properties are interdependent, e.g. by the amount of free charge carriers, and improving one usually comes at the expense of the other. For an optimum trade-off between both properties, various different technologies can be ap- plied. The most common approach is the use of de- generate metal oxide semiconductors (TCO: transparent conductive oxide) such as indium tin oxide (ITO). Fur- thermore, graphene, carbon nanotubes and various types of metal nanostructures are currently under investigation. However, in terms of fabrication those are more complex technologies (review papers on transparent electrodes can be found e.g. from Granqvist [1] and Ellmer [2]). Solar cells are large area mass products. To be price competitive to other energy technologies, high priced ma- terials and costly fabrication steps have to be avoided. Hence, ITO and sophisticated nanostructures are not suit- able for the transparent electrode in these devices. Fur- ther cost reductions and new applications can be created with roll-to-roll processing and flexible devices. Yet, this leads to additional requirements for the individual layers in the cell, such as bendability. Temperature-sensitive sub- strates or absorber layers also limit the possible processing techniques. However, a transparent electrode is required in thin film solar cells for charge carrier collection. In silicon thin a e-mail: martin.theuring@next-energy.de film solar cells highly doped zinc oxides (e.g. AZO: alu- minum doped zinc oxide) or tin oxides are often applied for this purpose [3]. To achieve a sufficient conductivity for solar cell applications, thick layers (up to several μm) are needed, which are less flexible. Furthermore, for sput- tered AZO an optimized performance will be obtained if the material is deposited on a heated substrate [4]. But a high temperature process can degenerate the underlying layers by decomposition or cracking due to differences in their temperature coefficients. Amongst others (see [5]), Sahu et al. [6] have shown that the incorporation of a few nanometers of silver in thin TCO layers strongly reduces the sheet resistance, while still providing high optical transmittance in the visible part of the spectrum. Higher mechanical stability and a simple low temperature fabrication process are the advan- tages of this design [7]. The objective of this paper is to demonstrate the inte- gration of thin metal layers as a transparent electrode in n-i-p amorphous silicon (a-Si) solar cells. In this work, we focus on thin metal layers adjacent to the p-layer of the cell. As an anti-reflection (AR) and protection layer, sput- tered AZO is compared to an AZO/SiO x multilayer. We analyze the influence of the individual layers’ thicknesses and deduce design rules for an optimized performance. 2 Experimental details As substrates for the solar cells, we use glass slides with a layer of chemically wet etched, structured AZO. The lat- ter was coated with a 30 nm layer of aluminum followed This is an Open Access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/4.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.