Band engineered ternary solid solution CdS _xSe _1-x-sensitized mesoscopic TiO ₂ solar cells

The optical band gap of the light absorber and the alignment of its bands with the underlying wide band gap metal oxide are critical for efficient light harvesting and charge separation in semiconductor-sensitized solar cells (SSCs). In practice, these two requirements are however not always fulfilled concomitantly in SSCs. Favourable band alignment in CdSe-sensitized TiO ₂ requires utilization of quantum sized CdSe, which causes great losses in the harvesting of long wavelength photons due to quantum confinement effects. In the present study, ternary cadmium sulfoselenide (CdS _xSe _1-x), which has a tunable band gap between those of CdSe and CdS without reducing the dimension, was proposed as a sensitizer for TiO ₂. CdS _xSe _1-x was successfully synthesized by alternately depositing CdS and CdSe layers under ambient conditions. SSCs utilizing CdS _xSe _1-x-sensitized TiO ₂ yielded a power conversion efficiency of 4.05% under simulated AM1.5 100 mW cm ^-2 illumination, rivalling the well-studied cascaded CdS/CdSe electrodes when an aqueous polysulfide solution was used as the electrolyte and Cu ₂S as the counter electrode. The findings of the present study provide an alternative and viable approach for optimizing the energetics of semiconductor sensitizers for efficient charge separation, while also maintaining good light harvesting.