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Among the vast number of floras, Canna lily Red (CR) and Canna lily Yellow (CY) flower extracts were used as photosensitizers for natural dye-sensitized solar cells (NDSSC) with ZnO photoanode material. This study incorporates a blocking or compact layer (CL) below the nanoporous ZnO semiconducting metal oxide to reduce the device recombination rate. A noticeable increment of JSC after adding a compact layer is observed for both the dyes; that is for CR dye from 0.43 to 0.47 mA/cm2, and for CY dye, 0.38–0.44 mA/cm2 is obtained with a consistent high VOC of 0.54–0.60 V. Moreover, the device charge collection efficiency has been enhanced from ∼93 to ∼97% for CR dye and from ∼89 to ∼96% for CY dye-loaded films after adding a compact layer. The enhancement in JSC, VOC, and fill factor (FF) is credited to increased electron lifetime, increased diffusion length, reduced series resistance (Rs), and exclusively enhanced recombination resistance among the FTO/electrolyte interface due to the compact layer. Despite visible defect states in the ZnO photoanode, a consistent photovoltage of 0.54–0.6 V significantly manifests enhanced photovoltage production of Canna dyes in the device. The effect of compact layers on photovoltaic parameters is well established through the EIS study.

An anatase TiO2 material with hierarchically structured spheres consisting of ultrathin nanosheets with 100% of the [001] facet exposed was employed to fabricate dye-sensitized solar cells (DSCs). Investigation of the electron transport and back reaction of the DSCs by electrochemical impedance spectroscopy showed that the spheres had a threefold lower electron recombination rate compared to the conventional TiO2 nanoparticles. In contrast, the effective electron diffusion coefficient, Dn, was not sensitive to the variation of the TiO2 morphology. The TiO2 spheres showed the same Dn as that of the nanoparticles. The influence of TiCl4 post-treatment on the conduction band of the TiO2 spheres and on the kinetics of electron transport and back reactions was also investigated. It was found that the TiCl4 post-treatment caused a downward shift of the TiO2 conduction band edge by 30 meV. Meanwhile, a fourfold increase of the effective electron lifetime of the DSC was also observed after TiCl4 treatment. The synergistic effect of the variation of the TiO2 conduction band and the electron recombination determined the open-circuit voltage of the DSC.