Since antimony sulfide (Sb₂S₃) has much attraction for photovoltaics in terms of appropriate bandgap, high absorption coefficient, low toxicity, and abundance, it has been recently applied for organic-inorganic hybrid solar cells, which has been attained the power conversion efficiency (PCE) of 7.5%.[1] We also have prepared Sb₂S₃-based solar cells with TiO₂ or ZnO nanoparticles for electron transporting layer in addition to poly(3-hexylthiophene)-2,5-diyl (P3HT)/ (3, 4-ethylenedioxythiophene) : poly(styrene sulfonate) (PEDOT : PSS), zinc phthalocyanine (ZnPc), or MoO₃ for hole transporting layer and compared their photovoltaic performance. Among these hybrid solar cells, it was found that the combination with ZnPc and TiO₂ has the highest durability with keeping the relative PCE of 90% after the stability test under 1 sun at 63 °C at a relative humidity of 50% for 1,500 h.[2] For further improvement of the photovoltaic performance, we newly added La or Zn into　the precursor of Sb₂S₃ to prepare Sb₂S₃–metal composites and applied them for organic-inorganic hybrid solar cells.

Remarkable enhancement of the open-circuit voltage for glass-FTO/TiO₂/Sb₂S₃ + La/P3HT/poly[(3-carboxybutyl)thiophene-2,5-diyl] (P3CT)/Au was observed after the addition of La to Sb₂S₃ and 33% improvement of PCE was attained. Impedance analyses of the hybrid solar cells with or without La revealed that the resistance at the interface was effectively lowered through the addition of La though the bulk-resistance had not been changed. It was found that the added La was phase-separated and accumulated between the layers of TiO₂ and SbS₃ through the TEM-EDX observations of the cross-sectional views of the La-added device. In this context, La was not doped into Sb₂S₃ but inserted into the interface of TiO₂ and SbS₃.

On the contrary, addition of Zn into the Sb₂S₃ precursor resulted in the enlargement of the crystalline size of the Sb₂S₃ from 14 nm to 25 nm, which was estimated from XRD peaks. No phase-separation was confirmed by TEM observations of the cross-sectional views. Therefore, it was concluded that the Zn was doped into Sb₂S₃. By using the Zn-doped Sb₂S₃, remarkable increment of the current density was observed in addition to relatively high fill factor and 57% improvement of PCE. These results are ascribed to the effective lowering the bulk-resistance, which was confirmed by the impedance analyses.

[1] J. A. Chang, J. H. Rhee, S. H. Im, Y. H. Lee, H. J. Kim, S. I. Seok, M. K. Nazeeruddin, and M. Graetzel, Nano Lett., 2010, 10, 2609–2612.

[2] A. Hayakawa, M. Yukawa, and T. Sagawa, ECS Trans., ahead of print.