Solar Cells prepared with Spray-ILGAR Indium Sulfide buffer layers on Cu(In,Ga)Se 2 Absorbers
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Solar Cells prepared with Spray-ILGAR Indium Sulfide buffer layers on Cu(In,Ga)Se2 Absorbers Nicholas A. Allsop, Christian A. Kaufmann, Axel Neisser, Marin Rusu, Andreas Hänsel, Martha C. Lux-Steiner and Christian-H. Fischer. Department of Solar Energy, Hahn-Meitner-Institute Berlin, Germany D14109. ABSTRACT Indium sulfide buffer layers deposited by the Spray-Ion Layer Gas Reaction (SprayILGAR) technique have recently been used with Cu(In,Ga)(S,Se)2 absorbers giving cells with an efficiency equal to the cadmium sulfide references. In this paper we show the first results from cells prepared with Cu(In,Ga)Se2 absorbers (sulfur free). These cells reach an efficiency of 13.1% which remains slightly below the efficiency of the cadmium sulfide reference. However, temperature dependant current-voltage measurements reveal that the activation energy of the dominant recombination mechanism remains unchanged from the cadmium sulfide buffered cells indicating that recombination remains within the space charge region. INTRODUCTION Indium sulfide is a very promising candidate for the replacement of the standard chemical bath deposited cadmium sulfide (CBD-CdS) buffer in chalcopyrite solar cells. High efficiency cells have been reported with indium sulfide buffer layers prepared by Atomic Layer Deposition (ALD) [1], Physical Vapour Deposition (PVD) [2] and recently Spray-ILGAR [3]. Spray-ILGAR is based upon the Dip-ILGAR technique which has previously been used to prepare high efficiency cells with a zinc oxide buffer [4]. The technique is a cyclical process in which a precursor layer is deposited by spraying a metal salt solution onto a heated substrate and then converting the precursor film to a chalcogenide using a reactive gas. Recently, we have shown that Spray-ILGAR indium sulfide buffer layers prepared on Shell-Solar Cu(In,Ga)(S,Se)2 absorbers give an average efficiency within 0.1% of the CBD-CdS buffered reference cells, when 48 cells of each type were prepared from the same absorber plate [3]. The efficiency of these cells reached a certified value of 14.7%, however, it is reasonable to expect a different behaviour on sulfur free absorbers. This is because the Shell-Solar absorber has a high sulfur and low gallium content close to the surface [5] and therefore the interface properties and band alignment with the buffer may be significantly different when compared to a pure selenide absorber with a gallium containing surface. EXPERIMENTAL Cu(In,Ga)Se2 absorber layers were prepared on molybdenum coated glass substrates using a 3-step physical vapour deposition process with laser light scattering used as an in-situ process control [6]. The indium sulfide buffer layer was deposited by Spray-ILGAR. In this process a 25mM solution of indium chloride in ethanol is converted into a very fine spray using an ultrasonic source. The spray is then swept towards the heated absorber substrate using nitrogen as a carrier gas. After spraying for 1 minute the spray is switched off and the precursor layer which has been
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