The Influence of Cd and Ba Incorporation on Microstructure and Photovoltaic Properties in Cu₂ZnSnS₄ Thin Film

Cd-doping and Ba-doping Cu2ZnSnS4 (CZTS) thin films solar cells were fabricated and compared. Precursors of Cu2ZnSnS4, Cu2(CdZn)SnS4 (CCZTS) and Cu2(BaZn)SnS4 (CBZTS) absorber layer were prepared by the spin coating method and annealed in a powdered sulfur atmosphere. The theoretical Cd or Ba composition was 0.3 according to the element ratio in the solution. The microstructure, morphology, photovoltaic performance of Cd/Ba doping CZTS were analyzed. Ba and Cd oxidation states can be found in the doping samples by XPS. The preferred (112) diffraction peak in CCZTS shifted to low angle apparently due to the increase of the lattice constant with the larger Cd2+ cations. The actual Ba-doping amount was low because Ba alkaline-earth metal easily reacts with H2O in the solution method. Raman spectra showed that the major peaks of CCZTS appeared a red-shift because of Cd-substitution for Zn, while CBZST displayed a blue-shift which could be related to the trigonal structure of CBTS. The stacks structure ZnO/ITO/CdS/CZTS/Mo with Cd/Ba doping solar cells were investigated. Through Cd incorporation the short-circuit current improved from 10.64 to 14.86 mA/cm−2 which is relate to the reduction of the grain boundary defects states, thus achieved conversion efficiency of 3.65%. CBZTS solar cells with Ba incorporation demonstrated the worst performance with inferior short circuit current and open circuit voltage, which originated from optical absorption losses and rough CBZTS/CdS interface with a great deal of recombination centers.