1. Sample Preparation. For the defect study, the purity of sample is highly important since impurities usually introduce unexpected defects and add uncertainties in the defect analysis. Here we applied vapor deposition for the Sb-rich and S-rich Sb2S3 film fabrication. The precursor materials used are Sb, S and Sb2S3 powders, they are only the component of the final product. The vapor deposition can thus generate high-purity samples under suitable reaction temperature, which was confirmed by multiple characterisations such as XRD, Raman scattering as well as XPS. Therefore, it provides excellent model system for the defect investigation.
2. Unique defect properties. Different from the theorectical predication, there are only maximum three kinds of defects measured in the Sb2S3 (Figure 1). The reason shall be the theorectical study considering all kinds of defect regardless the formation energy and energy level, while the experimental study only measures the deep-level defect with relatively low formation energy. Notably, an interesting finding in this study is that the existence of Sbi in the Sb-rich Sb2S3 film generates less detrimental effect on the carrier lifetime, which should be related to Q1D crystal structure where the space between (Sb4S6)n ribbons can afford impurities to certain degree. This characteristic suggest a route to tailor the properties of Sb2S3 by incorporation of extrinsic element without deteriorating the photovoltaic properties.
Figure 1 Schematic diagram of quasi-1-dimensional structural Sb2S3, sideview (a) and aeroview (b) of [Sb4S6] ribbons along c axis, DLTS signals of Sb-rich (c) and S-rich Sb2S3 films (d) at pulse voltage ranging from 0.1 to 0.5 V, synergized with an identical pulse-width optical pulse.
3. The significance to solar cell performance improvement. The Sb-rich Sb2S3 displays two kinds of deep level defect while the S-rich Sb2S3 only show one kind of deep-level defect, the latter is thus more promising for obtaining high power conversion efficiency in a solar cell device. It should be noted that the defect characteristic is associated with material preparation method, one should carefully consider the defect engineering with regard to specific materials processing.