ZnSnO as buffer layer in Cd-free kesterite solar cells: efficiency enhancement through thermal treatment and light soaking

Cu2ZnSn(S,Se)4 (CZTSSe) is a promising light absorber, being composed only of abundant and sustainable elements and showing excellent optoelectronic properties; the reported record efficiency is 15.1 %. [1] However, CZTSSe solar cells still suffer from a substantial VOC deficit. One reason that has hindered the VOC so far is related to CdS, the most used n-type buffer layer. CdS shows lattice mismatch and a non-optimal band alignment with CZTSSe and absorbs in the blue region of the light spectrum; besides, cadmium is very toxic [2]. ZnxSn1-xO (ZTO) is a wide band-gap and sustainable material suitable for CdS substitution in CZTSSe solar cells [3]. In this work, SCAPS 1-D software was initially used to model the experimental CZTSSe/CdS junction to analyse the defectivity of the CZTSSe layer. Subsequently, CdS was replaced with ZTO, and the CZTSSe/ZTO junction was also modelled using the same defect parameters previously found for the CZTSSe layer. The simulation determined ZTO’s optimal composition and thickness, suggesting an efficiency range of around 7 %, considering the parameters of a control device with an efficiency of about 4 %. Subsequently, ZTO thin films were grown through atomic layer deposition (ALD) and sputtering techniques on a film of CZTSSe grown by spin coating. The sputtered champion device showed initially a modest η = 2.94 %, but, after a thermal treatment of the device (10 minutes at 260°C) and six hours of light soaking, it reached η = 7.14 %, widely outmatching the CZTSSe/CdS reference device (η = 4.12 %). ALD-grown samples of ZTO have been characterised to unveil the reasons behind the effect of thermal treatment and light soaking. The thermal treatment enhances the crystallinity degree of ZTO thin film and promoted intermixing at the CZTSSe/ZTO interface, while light soaking progressively decreases ZTO resistivity up to about 0.1 Ω∙cm. Modelling of CZTSSe/ZTO junction and determination of mobility and carrier density of ZTO are currently ongoing; preliminary results suggest a reduced defectivity at CZTSSe/ZTO interface than at CZTSSe/CdS one and a carriers’ mobility increase after light soaking. In conclusion, this work confirms the suitability of ZTO as a buffer layer in Cd-free kesterite solar cells and reveals an efficiency enhancement of CZTSSe/ZTO junctions under operative conditions. [1] M. A. Green et al., Progress in Photovoltaics: Research and Applications, vol. 33, no. 1, pp. 3–15, Jan. 2025. [2] G. Tseberlidis, et al., Sustainable Materials and Technologies, vol. 41, p. e01003, 2024. [3] N. Ahmad et al., Advanced Science, vol. 10, no. 26, p. 2302869, Sep. 2023.