Combinatorial study of NaF addition in CIGSe films for high efficiency solar cells
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AbstractWe report on a sodium fluoride (NaF) thickness variation study for the H2Se batch furnace selenization of sputtered Cu(In,Ga) films in a wide range of Cu(In,Ga) film compositions to form Cu(In,Ga)Se2 (CIGSe) films and solar cells. Literature review indicates lack of consensus on the mechanisms involved in Na altering CIGSe film properties. In this work, for sputtered and batch-selenized CIGSe, NaF addition results in reduced gallium content and an increase in grain size for the top portion of the CIGSe film, as observed by scanning electron microscopy and secondary ion mass spectrometry. The addition of up to 20nm of NaF resulted in an improvement in all relevant device parameters: open-circuit voltage, short-circuit current, and fill factor. The best results were found for 15nm NaF addition, resulting in solar cells with 16.0% active-area efficiency (without anti-reflective coating) at open-circuit voltage (VOC) of 674mV. © 2013 John Wiley & Sons, Ltd.