ZnO nanowire/nanoparticles composite films for the photoanodes of quantum dot-sensitized solar cells

A quantum dot-sensitized solar cell (QDSSC) is fabricated using a composite layer, consisting of ZnO nanowires and ZnO nanoparticles (CL-ZnO) as its semiconductor film. The diameter and the thickness of each ZnO nanowire (ZNW) are ∼100 nm and ∼6 μm, respectively. The structure and morphology of the films with ZnO nanoparticles (ZNPs), ZNWs, and CL-ZnO are characterized by X-ray diffraction (XRD) patterns and scanning electron microscopic (SEM) images. Cadmium sulfide quantum dots (CdS QDs) are deposited on the ZNWs, ZNPs, and CL-ZnO by a successive ionic layer adsorption and reaction (SILAR) cycle. The highest efficiency (η) for the QDSSC sensitized by CdS (CdS-QDSSC) is obtained in the case of CL-ZnO photoanode, and this is attributed to significant improvement in the short-circuit photocurrent density (J_SC) of the pertinent cell, which is 2.81- fold higher than that of the cell with ZNWs. Moreover, cadmium selenide quantum dots (CdSe QDs) are used as co-sensitizer along with the CdS QDs for the CL-ZnO semiconductor layer (film designated as CL-ZnO/CdS/CdSe), and the pertinent QDSSC showed much higher efficiency than the one obtained in the case of the cell using only CdS QDs as the sensitizer. Finally, a passivation layer of ZnS was deposited on the film of CL-ZnO/CdS/CdSe, and the cell with this film (CL-ZnO/CdS/CdSe/ZnS) showed an efficiency of 0.55%, the highest in this research. Electrochemical impedance spectra (EIS), UV-vis absorbance spectra, transmission electron microscopy (TEM), and incident photon-to-current conversion efficiency (IPCE) curves are used to substantiate the explanations.