Flexible dye-sensitized solar cells with one-dimensional ZnO nanorods as electron collection centers in photoanodes

A ZnO composite film with an electron-collection center of ZnO nanorods (ZnNRs) embedded in a ZnO nanoparticle (ZnNP) film was fabricated on flexible photoanodes of back-illuminated dye-sensitized solar cells (DSSCs). Different lengths of the ZnNRs in ZnNP films were compared by considering their electron-collection and -scattering abilities. The morphologies and structures of the ZnO films were observed by examining their X-ray diffraction (XRD) patterns and scanning electron micrographs. The highest light-to-electricity conversion efficiency (η) of 2.17% was obtained for DSSCs, the ZnO composite film of which had about the same thickness of ZnNPs as the length of ZnNRs on its photoanode. At a thickness of only 7 μm of ZnNPs, the highest η was achieved with the composite film, leading to a 66.9% enhancement in efficiency compared to that obtained with only ZnNPs (1.30%). When the ZnNR length was shorter than the thickness of the ZnNP film, electrons in the ZnNPs film were not effectively transferred. On the other hand, if the ZnNRs were too long and stuck out of the ZnNP film, the incident light was scattered, resulting in less light utilization. These explanations were substantiated by electrochemical impedance spectra, laser-induced photo-voltage transients, and incident photon-to-electron conversion efficiency curves.

Various lengths of ZnO nanorods (ZnNR) are fabricated in ZnO nanoparticle (ZnNP) films with a given thickness on Ti foils as flexible photoanodes of DSSCs. The best cell performance is obtained when the thickness of the ZnNP film is the same as the length of the embedded ZnNRs. The ZnNRs provide 1-D electron transfer pathway to reduce electron collection distance and enhance charge collection efficiency, but would not scatter incident light from the top of the ZnNP layer for the case with the most suitable length of ZnNRs embedded in ZnNPs films.Figure optionsDownload as PowerPoint slideHighlights
► ZnO nanoparticles (ZnNP) and ZnO nanorods (ZnNR) composite structure was fabricated on flexible photoanodes for dye-sensitized solar cells (DSSCs).
► The influence of ZnNR length in ZnNR/ZnNP composite films on the electron-collection and scattering effect was discussed.
► The DSSC with ZnO composite films shows higher efficiency (2.17%) than that of the cell with only ZnNP (1.30%).
► The laser-induced photo-voltage transient was applied to analyze the electron lifetime in the ZnO films.