Design of a window layer for flexible Cu(In,Ga)Se 2 thin film solar cell devices
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Design of a window layer for flexible Cu(In,Ga)Se2 thin film solar cell devices Christian A. Kaufmann, Axel Neisser, Reiner Klenk, Roland Scheer and Hans-Werner Schock Hahn-Meitner-Institut Berlin, Glienicker Strasse 100, D-14109 Berlin, Germany ABSTRACT For laboratory scale devices the transfer of the Cu(In,Ga)Se2 (CIGSe) thin film technology from a rigid float glass substrate to flexible titanium foil is achieved. Here we will highlight the effect of the rough, flexible substrate on the electrical and optical characteristics of the transparent window layers of such devices. The roughness of the substrate may give rise to changes in sheet resistance of the transparent front contact and in the reflectivity of the completed device. However, the devices deposited onto the kind of titanium foil substrate used in this work are smooth enough not to affect the sheet resistance; nevertheless optical effects are enough to affect the process window for an antireflective coating. With the use of an antireflective coating, a maximum efficiency of 17.4% could be reached for 0.5 cm² area devices on glass. INTRODUCTION Using titanium foil instead of glass as a substrate for CIGSe thin film solar cells produces lightweight, robust and highly efficient flexible devices. Such flexible devices are an attractive option for use as power generators, in particular for mobile applications. One example is the use as power source for communication satellites. Due to their excellent radiation hardness and a favorable power to weight ratio, chalcopyrite solar cells on flexible substrates are one of the main future alternatives to existing space photovoltaic technologies. Highly effective CIGSe thin film solar cell devices can be produced on flexible substrates [1,2]. Table I lists the best device efficiencies that, within this work, could recently be achieved on float glass and on titanium foil; there remains no lag for the flexible thin film technology in comparison to devices that have been prepared on rigid glass substrates. For large area cells with a total area of 27.1 cm² the best efficiency till date is η = 15.0 %. The main component of these devices is the Cu(In,Ga)Se2 (CIGSe) thin film absorber layer deposited via a 3-step physical vapor deposition process that uses laser light scattering (LLS) as an in-situ process control [3]. It is known from LLS that the optical behavior of the flexible titanium foil substrate, in terms of scattering, differs due to its roughness from the much smoother glass substrate [4]. Figure 1 shows the two different light signals in comparison for a complete process with e(1) to e(3) denoting the end points of the process phases 1 (evaporation of In, Ga and Se), 2 (Cu indiffusion) and 3 (In, Ga enrichment) where s(1) marks the point of stoichiometry. Due to the increased scattering of the rough Ti substrate the scattered light signal does not increase during the In-Ga-Se deposition in the first deposition phase. Hence the question arises whether window Table I. Recent best total area efficiencies ac
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