Band gap engineering by cationic substitution in Sn(Zr1−xTix)Se3 alloy for bottom sub-cell application in solar cells

Abstract

Next-generation solar cells employ multiple junctions to push power conversion efficiency beyond the Shockley–Queisser limit. As the tandem devices based on c-Si and wide band gap absorbers are showing impressive performances, the introduction of a third junction can boost the efficiency even higher. For a three-junction solar cell with c-Si as the middle one, the optimum band gaps for the top and bottom sub-cells are 1.7 and 0.7 eV, respectively. While there are numerous wide band gap compounds being explored and studied for the top sub-cells, there is a lack of suitable materials for bottom sub-cells. In this work, we explore a novel Sn(Zr_1−xTi_x)Se₃ alloy system and evaluate its fundamental optoelectronic properties. Using a solid state reaction, we synthesized Sn(Zr_1−xTi_x)Se₃ materials with various Ti/Zr ratios in both powder and single crystal forms. Structural, optical, and electrical properties were measured as a function of the Ti/Zr ratio. We found that Sn(Zr_1−xTi_x)Se₃ crystallizes in the needle-like phase and is stable up to x = 0.44. With an increase in the Ti/Zr ratio, the absorption edge of Sn(Zr_1−xTi_x)Se₃ shifted towards lower energy and the lowest band gap achieved at x = 0.42 was 0.78 ± 0.01 eV. Electrical measurements revealed that the resistivity of Sn(Zr_1−xTi_x)Se₃ with respect to the Ti/Zr ratio was non-linear and had three distinct regions. Finally, we determined that Sn(Zr_1−xTi_x)Se₃ with Ti-rich composition is a direct semiconductor, has a very high absorption coefficient and the band gap is located near the optimal region for the bottom sub-cell absorber, making it a promising material candidate for multijunction solar cell applications.