Computational modeling of a Sb2S3 /CdxSb2-x(S1-ySey)3 monolithic tandem photocell structure

Abstract

This paper focuses on the numerical modeling of the quaternary alloy Cd_xSb_2-x(S_1-ySe_y)₃ as a lower cell absorber in an Sb₂S₃-based tandem system. To this end, the governing laws of the evolution of the band gap and the electron affinity of the alloy are established and solved with reference to the defined substitution proportions. The results show that the band gap of the alloy decreases from 1.88 eV to 0.93 eV with increasing x and y proportions. In a planar heterojunction of FTO/(ZnO/TiO₂)/Absorber/Spiro-OMeTAD/Au configuration, the Cd_xSb_2-x(S_1-ySe_y)₃ alloy, used as the absorber, exhibits a maximum efficiency of 8.81 %, which surpasses the 5.08 % efficiency of the Sb₂S₃ absorber. For specific band parameters and proportions (Eg = 1.22 eV, χ = 4.25 eV, x = 0.04 and y = 0.9), Cd_0.04Sb_1.96(S_0.1Se_0.9)₃ is used as the lower solar cell absorber in a tandem structure based on Sb₂S₃ as the upper cell. After optimization of the absorber thickness, the upper and lower cells demonstrated an efficiency of 8.42 % and 13.74 %. A preliminary simulation of the Sb₂S₃/Cd_0.04Sb_1.96(S_0.1Se_0.9)₃ tandem structure indicated an efficiency of 17.98 %. Following the matching of the current between the upper and lower cells for a thickness of 0.4 μm of each absorber, the Sb₂S₃/Cd_0.04Sb_1.96(S_0.1Se_0.9)₃ tandem solar cell demonstrated an efficiency of 16.19 %. This difference in performance is explained by the lower cell operating in saturation. These results illustrate the prospective applicability of Cd_xSb_2-x(S_1-ySe_y)₃ as an absorber material in both single and multi-junction solar cells.