Growth of Sn(O,S)2 buffer layers and its application to Cu(In,Ga)Se2 solar cells

Sn-based thin films as new buffer layer for Cd-free Cu(In,Ga)Se2 (CIGS) solar cells were developed. The Sn(O,S)2 films were formed on CIGS substrates by chemical bath deposition from an alkaline ammonia solution by reacting tin(IV) chloride with thiourea. Optimization of the growth process allowed the smooth and conformal coverage of the films on the CIGS substrates with a thickness of 20 nm that was a self-limited thickness in the chemical bath deposition process. XPS analysis revealed that the as-deposited films contained Sn–O, Sn–OH, and Sn–S bondings and the ratio of Sn–S bonding to Sn–O bonding was 0.3. The CIGS solar cell fabricated with a 20-nm thick Sn(O,S)2 buffer layer had the best efficiency of 11.5% without AR coating. The open circuit voltage, short circuit current, and fill factor were 0.55 V, 34.4 mA/cm2, and FF = 0.61, respectively. The open circuit voltage and fill factor were low compared to the conventional CIGS solar cell with a 50-nm thick CdS buffer due to too thin Sn(O,S)2 buffer layer.