Multi-Phase Sputtered TiO₂-Induced Current–Voltage Distortion in Sb₂Se₃ Solar Cells

Despite the recent success of CdS/Sb₂Se₃ heterojunction devices, cadmium toxicity, parasitic absorption from the relatively narrow CdS band gap (2.4 eV) and multiple reports of inter-diffusion at the interface forming Cd(S,Se) and Sb₂(S,Se)₃ phases, present significant limitations to this device architecture. Among the options for alternative partner layers in antimony chalcogenide solar cells, the wide band gap, non-toxic titanium dioxide (TiO₂) has demonstrated the most promise. It is generally accepted that the anatase phase of the polymorphic TiO₂ is preferred, although there is currently an absence of analysis with regard to phase influence on device performance. This work reports approaches to distinguish between TiO₂ phases using both surface and bulk characterization methods. A device fabricated with a radio frequency (RF) magnetron sputtered rutile-TiO₂ window layer (FTO/TiO₂/Sb₂Se₃/P3HT/Au) achieved an efficiency of 6.88% and near-record short–circuit current density (J_sc) of 32.44 mA cm⁻², which is comparable to established solution based TiO₂ fabrication methods that produced a highly anatase-TiO₂ partner layer and a 6.91% efficiency device. The sputtered method introduces reproducibility challenges via the enhancement of interfacial charge barriers in multi-phase TiO₂ films with a rutile surface and anatase bulk. This is shown to introduce severe S-shaped current–voltage (J–V) distortion and a drastic fill–factor (FF reduction in these devices.