Cu(In, Ga)(Se, S) 2 Absorbers Formed by Rapid Thermal Processing of Elemental Precursors: Analysis of Thin Film Formatio
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Cu(In,Ga)(Se,S)2 Absorbers Formed by Rapid Thermal Processing of Elemental Precursors: Analysis of Thin Film Formation and Implementation of A Large Area Industrial Process Joerg Palm, V. Probst, W. Stetter, R. Toelle Shell Solar GmbH, RS/T-CIS, Otto Hahn Ring 6, D-81739 Munich, Germany ABSTRACT Large area Cu(In,Ga)(Se,S)2 thin films (CIGSSe) for solar modules are processed by rapid thermal annealing of stacked elemental layers. A pilot line for 60x90 cm2 absorbers and 30x30cm2 modules is now up and running. Modules with efficiencies at 12% are fabricated with excellent structural and electrical uniformity. For the optimization of electrical performance the selenization and sulfurization process is analyzed by x-ray fluorescence (XRFA), x-ray photoelectron spectroscopy (XPS), secondary ion mass spectroscopy (SIMS) and current voltage (I-V) measurements. Sequentially processed CIGSSe films show characteristic depth distribution profiles of gallium and sulfur. Based upon our previously published model obtained by In-Situ XRD analysis the Ga profile can be explained by the strongly inhibited formation of binary gallium-selenides. The sulfur incorporation is investigated by comparing XRFA data and SIMS profiles of samples from different sulfurization recipes. A characteristic dependence of S on the Cu/(In+Ga) ratio is observed. Two different mechanisms for sulfur incorporation are proposed. It will be shown that the accumulation of sulfur towards the back is predominantly due to the incorporation into an intermediate molybdenum sulfo-selenide layer. Device characterization shows that the Ga and S profiles lead to a favorable double band gap grading structure. INTRODUCTION Thin film solar cells based on chalcopyrite semiconductors have reached a high level of performance over the last few years both for laboratory scale cells and commercial products [1]. Since 1998 a first generation of CIS modules is commercialized by Siemens Solar (now Shell Solar). Cost reduction strategies in thin film technology follow the route of large area float glass processing, applying by far the lowest cost substrate for PV applications. We have developed a novel process for CIS formation over the past years based on a two step approach: deposition of precursor materials at room temperature is followed by rapid thermal processing (SEL-RTP = stacked elemental layer rapid thermal processing). By conducting the deposition process at room temperature, proven standard processes such as magnetron sputtering can be applied. The processing cycle times are strongly reduced allowing for inline concepts and high throughput. The feasibility of this concept has been demonstrated in the past on a laboratory scale by high efficiency mini modules with 14.7 % champion conversion efficiency [2]. The up-scaling from 10x10cm2 to 60x 90cm2 substrate size has been successfully accomplished in the past two years by demonstrating average efficiencies of 11% on 30x30cm2 modules with CIGSSe absorbers selenized and sulfurized on 60x90 cm2 substrates [3,4]. Now
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