Selenization of printed Cu-In-Se alloy nanopowder layers for fabrication of CuInSe2 thin film solar cells

One of the promising low cost and non-vacuum approaches for the fabrication of semiconductor CuInSe2 and Cu(In,Ga)(S,Se)2 thin film absorbers is the printing of precursor materials followed by a sintering/selenization process. The selenization process parameters such as temperature, duration, and selenium vapor pressure strongly influence the morphology and electronic properties of the absorber film. In this study, the effect of pre-annealing in an inert atmosphere and selenization on printed mechanically synthesized CuInSe0.5 alloy nanopowder precursor films was investigated. 1-2 μm thick CuInSe0.5 alloy nanopowder layers were deposited on a Mo-sputtered glass substrate by means of doctor blade coating of a nanopowder based precursor suspension. Pre-annealing was performed on a hot plate inside a nitrogen gas filled glove box. Selenization was performed in a home-made rapid thermal processing (RTP) furnace with two RTP heating zones for independent temperature control of the selenium source and the coated substrate. The temperature of the selenium source was fixed at 390-410 °C during the selenization to provide a constant supply of selenium vapor. A two-step process, i.e., a pre-annealing in nitrogen atmosphere at 400 °C for 30 min followed by selenization at 530 °C for 15 min was found to result in better densification and grain growth of the CuInSe2 phase, compared to a single step selenization at 530 °C for 15 min. The solar cell fabricated by the two-step process had an efficiency of 5.4% and a fill factor of 52%, while the device fabricated by the single step selenization had an efficiency of 1.1% and a fill factor of 31%.