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Study of molybdenum back contact layer to achieve adherent and efficient CIGS2 absorber thin-film solar cells

Journal of Vacuum Science and Technology A: Vacuum, Surfaces and Films, ISSN: 0734-2101, Vol: 23, Issue: 4, Page: 1197-1201
2005
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Conference Paper Description

Molybdenum is used as back contact layer in I-III- VI2 compound thin-film solar cells. Mo film was sputter deposited on 125-mm-diameter Si wafer having 100 orientation using dc magnetron sputtering. Films with similar parameters were also deposited on 2.5 cm×10 cm soda-lime glass for studying the adhesion to the substrate and chemical reactivity of molybdenum with H2 S gas at 475 °C for 20 min. Mo being refractory material develops stresses. It is essential to deposit stress-free and relatively inert Mo films in order to achieve well adherent and highly efficient Cu In1-x Gax S2 absorber thin film solar cells on flexible metallic foil and glass substrates. Earlier studies have shown that films deposited at sputtering power of 300 W and 0.3× 10-3 Torr working argon pressure develop compressive stress, while the films deposited at 200 W and 5× 10-3 Torr pressure develop tensile stress. Four sets of experiments were carried out to achieve optimum deposition cycle to deposit stress-free Mo. In the first experiment, Mo thickness of 138 nm was deposited at 300 W power and 0.3× 10-3 Torr pressure. In the second experiment Mo thickness of 127 nm was deposited at power of 200 W and pressure of 5× 10-3 Torr. Two more experiments were carried out by using alternate layers to reduce the overall stress. In a third experiment, two high power cycles were sandwiched between three low power cycles with total film thickness of 330 nm. In a fourth experiment two low power cycles were sandwiched between three high power cycles resulting in effective thickness of 315 nm. This article describes the wafer bending analysis for stress measurement, x-ray diffraction for crystal quality, scanning electron microscopy for surface morphology and Auger electron spectroscopy for the extent of sulfur diffusion in Mo layer. © 2005 American Vacuum Society.

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