Spin-coating processing of oriented Sb2S3 layer for PV applications: effect of the precursors and the device architecture

Sb2S3 chalcogenide stands out as low toxic, promising material for photovoltaic applications because of its unique optoelectronic properties such as a suitable band gap (1.4-1.8 eV), a high absorption coefficient (105 cm-1), appreciable thermal and chemical stability, and a high tolerance to defects.
In the first part of this study, Sb(Ac)3 and SbCl3 are compared as antimony precursor for the formation of the Sb2S3 photoactive film. The Sb(Ac)3/thiourea (TU) precursor solution allows the formation of films with higher coverage rate and uniformity than films obtained from SbCl3/TU. In terms of PV efficiencies, Sb(Ac)3/TU and SbCl3/TU based layers respectively lead to 4.9 % and 4.8% efficiencies. Indeed, the band gap of the Sb2S3 layer obtained from Sb(Ac)3/TU (1.75 eV) is less favorable than from SbCl3/TU (1.65 eV). In addition, the [hk1] crystalline orientation of Sb2S3 is more favorable for efficient charge transfer in the devices and is more present in the SbCl3/TU films.
In the second part, the incorporation of a mesoporous TiO2 network is considered to improve the transport of charge at the Sb2S3/TiO2 electron transport layer interface and hence enhance the efficiency of the devices. However, the PV efficiencies are significantly lower in the case of the mesoporous architecture, which is mainly attributed to a [hk0] misorientation of the crystals in the mesoporous architecture leading to poor charge transfer.
By studying the impact of antimony precursor and the nature of the TiO2 electron transport underlayer (dense or mesoporous) on the properties of the Sb2S3 photoactive film, we highlight that a combination of three factors is crucial to boost device efficiencies: uniformity/coverage rate, adequate bandgap, and more importantly crystalline orientation.