Effect of Er³⁺ and Yb³⁺ co-doping on the performance of a ZnO-based DSSC

Zinc-oxide (ZnO) nanoparticles (NPs) co-doped with different concentrations of rare-earth ions of erbium and ytterbium, (ZnO: Er³⁺, Yb³⁺) were synthesized for applications to ZnO-based dye sensitized solar cells (DSSC). The composite NPs used for the photoelectrode (PE) were synthesized using a simple co-precipitation technique. X-ray diffraction and scanning electron microscopy measurements on the prepared samples revealed a single phase wurzite ZnO powder with approximate sizes in the range from 15 to 20 nm. Photoluminescence (PL) measurements confirmed that the synthesized composite NPs had a good up-conversion (UPC) property. The prepared powders were directly used to make PEs for DSSCs. The photovoltaic efficiency of the DSSCs was enhanced compared to that of pure ZnO-based DSSCs. Particularly, the PE made up of ZnO: Er³⁺, Yb³⁺ NPs with 4 wt% of Er³⁺ and Yb³⁺ generates a short-circuit current density (J_sc) of 4.794 mA·cm ⁻² and an open circuit voltage (V_oc) of 0.602 V with an efficiency (η) of 1.58%. The result indicates a 48.4% J_sc improvement compared to a pure ZnO PE-based DSSC. The photocurrent improvement is due to an increase in the light-harvesting capacity of the PEs attained through the UPC property of ZnO: Er³⁺, Yb³⁺ NPs. As confirmed by PL and electrochemical impedance spectra (EIS), the use of ZnO: Er³⁺,Yb³⁺ NPs as PEs for DSSCs enhances charge concentration and transport as a result of n-type doping. However, all ZnO: Er³⁺, Yb³⁺ NP based PEs exhibited a lower V_oc as a result of a down shift in the Fermi energy, which affects the overall efficiency of the cell.