High-efficiency metal-free organic-dye-sensitized solar cells with hierarchical ZnO photoelectrode†

Self-assembled ZnO secondary nanoparticles have been fabricated as an effective photoelectrode for dye-sensitized solar cells (DSCs). The hierarchical architecture, which manifested the significant lightscattering, can provide more photon harvesting. In addition, dye-molecule adsorption was sufficient due to enough internal surface area provided by the primary single nanocrystallites. Two indoline dyes, coded D149 and D205, were used as the sensitizers of ZnO DSCs with the optimal energy conversion efficiencies of 4.95% and 5.34%, respectively, under AM 1.5 full sunlight illumination (100 mW cm ). The enhancement of the open-circuit photovoltage (Voc) and the short-circuit photocurrent density (Jsc) for D205-sensitized ZnO DSCs was ascribed to the effective suppression of electron recombination by extending the alkyl chain on the terminal rhodanine moiety from ethyl to octyl. Further evidence is obtained from the electrochemical impedance spectroscopy (EIS) which exhibits a longer electron lifetime for D205-sensitized ZnO DSC in comparison with the D149-sensitized one.