Reactive DC Sputtered TiO₂ Electron Transport Layers for Cadmium‐Free Sb₂Se₃ Solar Cells

Abstract The evolution of Sb₂Se₃ heterojunction devices away from CdS electron transport layers (ETL) to wide bandgap metal oxide alternatives is a critical target in the development of this emerging photovoltaic material. Metal oxide ETL/Sb₂Se₃ device performance has historically been limited by relatively low fill factors, despite offering clear advantages with regards to photocurrent collection. In this study, TiO₂ ETLs are fabricated via direct current reactive sputtering and tested in complete Sb₂Se₃ devices. A strong correlation between TiO₂ ETL processing conditions and the Sb₂Se₃ solar cell device response under forward bias conditions is observed and optimized. Numerical device models support experimental evidence of a spike‐like conduction band offset, which can be mediated, provided a sufficiently high conductivity and low interfacial defect density can be achieved in the TiO₂ ETL. Ultimately, a SnO₂:F/TiO₂/Sb₂Se₃/P3HT/Au device with the reactively sputtered TiO₂ ETL delivers an 8.12% power conversion efficiency (η), the highest TiO₂/Sb₂Se₃ device reported to‐date. This is achieved by a substantial reduction in series resistance, driven by improved crystallinity of the reactively sputtered anatase‐TiO₂ ETL, whilst maintaining almost maximum current collection for this device architecture.