A low temperature, single step, pulsed d.c magnetron sputtering technique for copper indium gallium diselenide photovolt
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A low temperature, single step, pulsed d.c magnetron sputtering technique for copper indium gallium diselenide photovoltaic absorber layers Sreejith Karthikeyan1, Kushagra Nagaich1, Arthur E Hill2, Richard D Pilkington2, and Stephen A Campbell1 1
Department of Electrical and Computer Engineering, University of Minnesota, Minneapolis, MN -55414, USA 2 Materials and Physics Research Centre, University of Salford, Salford, M5 4WT, UK
ABSTRACT Pulsed d.c Magnetron Sputtering (PdcMS) has been investigated for the first time to study the deposition of copper indium gallium diselenide (CIGS) thin films for photovoltaic applications. Pulsing the d.c. in the mid frequency region enhances the ion intensity and enables long term arc-free operation for the deposition of high resistivity materials such as CIGS. It has the potential to produce films with good crystallinity, even at low substrate temperatures. However, the technique has not generally been applied to the absorber layers for photovoltaic applications. The growth of stoichiometric p-type CIGS with the desired electro-optical properties has always been a challenge, particularly over large areas, and has involved multiple steps often including a dangerous selenization process to compensate for selenium vacancies. The films deposited by PdcMS had a nearly ideal composition (Cu0.75In0.88Ga0.12Se2) as deposited at substrate temperatures ranging from no intentional heating to 400 0C. The films were found to be very dense and pin-hole free. The stoichiometry was independent of heating during the deposition, but the grain size increased with substrate temperature, reaching about ~ 150 nm at 400 oC. Hot probe analysis showed that the layers were p-type. The physical, structural and optical properties of these films were analyzed using SEM, EDX, XRD, and UV-VIS-NIR spectroscopy. The material characteristics suggest that these films can be used for solar cell applications. This novel ion enhanced single step low temperature deposition technique may have a critical role in flexible and tandem solar cell applications compared to other conventional techniques which require higher temperatures. INTRODUCTION Copper Indium Gallium diSelenide (CuInGaSe2 – CIGS) is a well-known absorber layer in the thin film solar cell industry. Recently researchers reported a laboratory scale efficiency of 20.3%[1]. Various commercial industries such as Shell Solar, Würth Solar, Nanosolar, Miasolé, and Ascent Solar make use of CIGS based solar cells. CIGS layers are extensively studied by researchers worldwide because of their solar cell application potential and their stability with respect to radiation damage. Various physical and chemical methods have been employed for the formation of this layer. These techniques typically require multi step processes in order to achieve the stoichiometry required for solar cell applications. The majority of these techniques produce films with low Se content. Unless the film is deposited in a very Se-rich ambient, post deposition selenization is usually require
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