We have developed various organic-inorganic hybrid solar cells using Sb₂S₃-based composites with π-electronic molecules [1,2]. For examples, utilization of zinc phthalocyanine (ZnPC) for the hybrid solar cell composed of glass/indium-tin-oxide (ITO)/TiO₂/Sb₂S₃/ZnPC/Au achieved long durability with keeping the relative power conversion efficiency (PCE) of 90% after the stability test under 1 sun at 63 °C at a relative humidity of 50% for 1,500 h [1]. Using La-decorated Sb₂S₃ for hybrid solar cell composed of glass/F-doped SnO₂ (FTO)/TiO₂/La-decorated Sb₂S₃/poly[(3-hexylthiophene)-2,5-diyl] (P3HT)/poly[3-(3-carboxypropyl)thiophene-2,5-diyl] (P3CT)/Au resulted in 33% improvement of PCE as compared to that of the non-decorated Sb₂S₃ because of lowering the resistance at the interface, which was confirmed by the impedance analyses [2]. While doping of Zn into Sb₂S₃ attained 57% enhancement of PCE. This result is ascribed to the effective lowering the bulk-resistance [2]. In this context, we newly inserted SrTiO₃ layer between TiO₂ and Sb₂S₃ interface and found the enhancement of the open circuit voltage (V_OC) from 0.61 V to 0.74 V. Temperature dependence of V_OC for SrTiO₃-inserted device indicated the maximum voltage was 1.7 V, which had been estimated by extrapolation of the line to 0 K. For further improvement of the photovoltaic performance, we modified the thin-film making process for Sb₂S₃ layer from the previously reported procedures [1,2]. After the spin-coating of the precursor solution of SbCl₃-thiourea complex in DMF onto the SrTiO₃ layer, heat treatment at 220 °C under vacuum was newly performed. The thickness, surface roughness, and sea-island structure were optimized by their uv-vis measurement, AFM, TEM observations, and current density-voltage (J-V) characteristics in terms of transmittance, reflectivity, and short circuit current density (J_SC) by changing the spin-coating speed, duration of heating, and so on. Through the optimization, remarkable enhancement of J_SC from 10.7 mA cm^-2 to 13.6 mA cm^-2 and improvement of PCE from 4.7% to 6.1% were achieved.

[1] A. Hayakawa, M. Yukawa, and T. Sagawa, ECS J. Solid-State Technology, 6(4), Q35-Q38 (2017).

[2] M. Yukawa, A. Hayakawa, and T. Sagawa, ECS Trans., 77(11), 653-659 (2017).