Pulsed Laser Processing of Electrodeposited CuInSe 2 Photovoltaic Absorber Thin Films
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Pulsed Laser Processing of Electrodeposited CuInSe2 Photovoltaic Absorber Thin Films A. Bhatia1, P. J. Dale2, M. M. Nowell1, 3, M. A. Scarpulla1 1 Materials Science and Engineering, University of Utah, Salt Lake City, Utah, USA 2 Laboratoire Photovoltaique, Universite du Luxembourg, Belvaux, Luxembourg 3 EDAX-TSL, Draper, Utah, USA
ABSTRACT CuInSe2 (CIS) is commercially processed using energy intensive vacuum processes such as sputtering and thermal evaporation followed by thermal annealing. In order to reduce the cost of fabricating CIS photovoltaic absorber layers we need fast and cheap processing methods. We have investigated the use of non-vacuum electrochemical deposition (ED) followed by ultra violet pulsed laser annealing (UV-PLA). We report here on the results of ns pulsed KrF irradiation of ED CIS films and ED CIS films which were first annealed in a Se atmosphere.
INTRODUCTION CuInSe2 (CIS), a derivative of the tetragonal chalcopyrite structure, is a leading photovoltaic technology as it offers a number of desirable properties: its direct band gap (≈1 eV), very high optical absorption coefficient (α≈105 /cm), low surface recombination velocity, and radiation resistance to both electrons and protons [1]. CIS is natively defect doped and as a result is always p-type. This has necessitated that it be used in heterojunction cells with the high band gap n-type CdS (≈2.5 eV) used as window layer and ZnO: Al as top contact. Commercially CIS films are deposited using energy-intensive thermal evaporation or sputtering of the metals in vacuum and subsequent annealing in chalcogen vapor. In our research we have deposited CIS films using electrochemical deposition (ED) followed by ultra violetpulsed laser annealing (UV-PLA) which may be viable techniques to reduce the cost of CIS absorber layers. In electrochemical deposition ions in solution are assembled as atoms on the surface of the back contact to form a semiconductor thin film via current flow. Compared to vacuum deposition, ED has the advantages of requiring only a small amount of energy, allowing fast deposition rates up to ~20 µm/hr [2], and having high material utilization. UV-PLA involves utilizing the heat generated upon irradiating a sample with a laser to affect changes in the sample. Since laser pulses heat the sample on a local scale there can be negligible increase in the temperature of the substrate. This is beneficial compared to thermal annealing which imposes constraints on the substrate type(s) and annealing temperatures that can be used as both the film and substrate are heated. Thus the lower thermal budget and faster processing times afforded by UV-PLA compared even to rapid thermal or conventional annealing may translate into cheaper and faster solar cell production and potentially superior material properties. Wang et al. [3] irradiated vacuum-deposited CIGS films with multiple shots from a pulsed KrF laser (248 nm, 25 ns) and reported improvements in grain size, carrier lifetime, mobility and resistivity coupled with increas
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