An effective Li-containing interfacial-treating strategy for performance enhancement of air-processed CZTSSe solar cells

Abstract

Though Li-doping/alloying has powerful effects on tailoring the band-gap and altering grain boundaries of the Cu2ZnSn(S,Se)4 (CZTSSe) absorbers to achieve an enhanced CZTSSe device performance, a simple and effective method is meaningful for overcoming the surviving drawbacks of massive Li losses in in-situ doping processes of adding lithium salt in precursor solution. Herein, we demonstrate an effective approach to significant increase the open-circuit voltage (VOC) and efficiency of the CZTSSe solar cells by simply coating a LiOH layer on the air-processed Cu2ZnSnS4 (CZTS) precursor film before selenization. It is found that the VOC could be adjusted by altering the coated LiOH concentration, and a 412 mV VOC could be obtained for 1 mg/mL LiOH, which is 30 mV higher than that of non-treated CZTSSe solar cells, and a 1.43 times higher efficiency of 7.8% is achieved. Characterizations prove that such LiOH coating approach can make Li enter the CZTSSe absorber and has the trend of locally accumulated at the grain boundaries, promote the crystallinity, lower the optical band-gap, improve the carrier mobility and decrease the transfer resistance, thus leading to the enhanced short-circuit current and fill factor for high device efficiencies. This study might imply a new kind of potential method for element doping and thus improving the CZTSSe device performance.