Managing parasitic absorption and interfacial structure in Sb2S3/CdS planar heterojunction for efficient solar cells

Cadmium sulfide (CdS) is a widely utilized electron transport material (ETM) in antimony sulfide (Sb2S3) solar cells due to its superior electron mobility and favorable band alignment with Sb2S3. However, the conventionally thick (ca. 60-70 nm) CdS film as ETM layer often leads to parasitic absorption due to its relatively narrower bandgap of CdS (~2.4 eV) to deteriorate the device absorption for photocurrent generation in solar cells. To mitigate this limitation, we implemented a strategy that integrates reducing the CdS film thickness down to 30 nm with a straightforward ultraviolet ozone (UVO) treatment for surface modification. Our results demonstrate that reducing the CdS thickness significantly minimizes the loss of short-wavelength light, thereby enhancing the device’s spectral response. Furthermore, the UVO treatment effectively passivates sulfur vacancy defects on CdS surface and elevates the valence band maximum of CdS, which facilitates in situ growth of [hk1]-oriented Sb2S3 film over the CdS layer and passivates the interfacial defects at the Sb2S3/CdS interface for extracting electrons. Benefiting from the managed parasitic absorption and interfacial structure by using UVO to treat the thin CdS film, the resulting Sb2S3 solar cell based on Sb2S3/CdS planar heterojunction achieves an efficiency of 6.10%, much higher than the counterpart device consisting of thick and non-treated CdS film; in particular, the photocurrent is increased by 40%, the fill factor is improved by 27%, and the open-circuit voltage reaches 0.79 V. This study proposes a simple and scalable method to harmonize optical management with interfacial electronic properties, providing a promising route toward achieving high-efficiency and low-cost Sb2S3 solar cells.