Lower toxic bismuth and antimony halide/sulfide photo-absorbers, such as A₃M₂X₉ and AMSX₂ (A = cation, M = Bi/Sb, S = sulfur, X = halogen), are promising candidates as the counterpart to lead halide perovskite for solar cell application. However, there are largely varied possible compositions and crystal structures (0–3 dimensions), which makes these materials largely overlooked. Therefore, an intuitive evaluation strategy is required to properly explore intrinsically superior photo-absorbers, which have moderate energy level and charge transport property.
Here, we present a comprehensive study of the energetics (bandgap, etc.) and optoelectronics (photoconductivity and charge transfer to charge transport layer) of the Bi- and Sb-based photo-absorbers, including 6 crystal structures with 44 compositions, by using time-resolved microwave conductivity (TRMC). TRMC can detect photo-excited charge carriers, played key role for optoelectronic study. As a result, we found a specifically efficient electron transfer from some Sb-based materials, such as SbSI, Sb₂S₃, and CsSbSI₂, to electron transport layer (TiO₂), resulted in 2.2~2.5 fold greater TRMC signal than well-known photo-absorber AgBiI₄ and CsAgBiBr₆. Whereas, inclusion of the thiophene component to the hole transport materials promoted efficient hole extraction. Based on the insights, we developed Sb₂S₃-containing SbSI solar cell by using novel 1-step method, yielded an improved power conversion efficiency of 2.91%. These compositional and structural effect on charge transport are possibly attributed to interfacial coordination at the hetero junction, suggesting further possibility of Sb-based photo-absorbers. Our study establishes a key design rule to accelerate the evolution of Bi- and Sb-based lead-free solar cells.

Dr. Ryosuke Nishikubo

Assistant Professor, Osaka university

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