Hierarchical urchin-like CoSe₂/CoSeO₃ electro-catalysts for dye-sensitized solar cells: Up to 19% PCE under dim light illumination

A hierarchical urchin-like structure of CoSe₂/CoSeO₃ (denoted as CoSe₂/CoSeO₃-UL) is synthesized by a one-step hydrothermal method and investigated as the electro-catalyst for the counter electrode (CE) of dye-sensitized solar cells (DSSCs). The evolution of the CoSe₂/CoSeO₃ nanostructures as a function of the synthesis time has been identified by scanning electron microscopy. The growth mechanism is proposed and the desired CoSe₂/CoSeO₃ hierarchical UL structure is noticed at 4 h. CoSe₂/CoSeO₃-UL consists of nanoparticles and hexagonal prisms for providing a high surface area for catalytic reactions and a one dimensional charge transport route, respectively. The results show that DSSCs using the CoSe₂/CoSeO₃-UL CE exhibit a higher power conversion efficiency (η) of 9.29 ± 0.24% than that of the cell using a Pt CE (8.33 ± 0.07%). Under various dim light conditions (i.e., a T5 lamp), the DSSC with the CoSe₂/CoSeO₃-UL CE shows outstanding η's of 19.88 ± 0.10%, 18.24 ± 0.06%, and 16.00 ± 0.13% at 7000 lux (2.21 mW cm^-2), 6000 lux (1.89 mW cm^-2) and 4800 lux (1.55 mW cm^-2), respectively. This study demonstrates that CoSe₂/CoSeO₃-UL has great potential to replace Pt in DSSCs, and the application of DSSCs can be extended from outdoor to indoor conditions.