Effect of sodium incorporation on CuIn(S,Se)2 thin films fabricated by aqueous spray deposition and their photovoltaic properties

Abstract

Selenium-rich CuIn(S,Se)2 (CISSe) thin films on a Mo-coated glass substrate were fabricated by spray pyrolysis of an aqueous precursor solution containing Cu(NO3)2, In(NO3)3, and methylthiourea followed by selenization at 560 °C. Addition of low-concentration NaNO3 to the precursor solution (Na/(Cu + In) = 0.2) effectively improved the crystallinity of the Cu-poor CuInS2 precursor film as confirmed by XRD measurement. Furthermore, final CISSe film obtained from this precursor film had an improved grain growth with bigger grain sizes than that of the CISSe film from the precursor film without the addition of Na. As a result, solar cell based on that CISSe film had a conversion efficiency of 2.66 %, significantly improved from only 0.28 %. An increase of the CISSe film thickness from 900 nm to 2.2 μm led to further improvement of the conversion efficiency to 4.35 %.