Polyethylenimine-assisted growth of high-aspect-ratio nitrogen-doped ZnO (NZO) nanorod arrays and their effect on performance of dye-sensitized solar cells.

The realization of arrays of high-aspect-ratio nitrogen-doped ZnO (NZO) nanorod is critical to the development of high-quality nanostructure-based optoelectronic and electronic devices. In this study, we used a solution-based method to grow arrays of vertically aligned high-aspect-ratio NZO nanorods on ZnO seed layer covered fluorine-doped tin oxide substrates. We investigated whether the diameters and aspect ratios of the nanorods were affected by the addition of polyethylenimine (PEI) to the precursor solution used as well as by variations in the growth temperature and the concentration of the precursor solution. The performances of dye-sensitized solar cells (DSSCs) in which the synthesized high-aspect-ratio NZO nanorods were used as the photoanode material were also studied. That the dopant, nitrogen, was introduced into the ZnO lattice was confirmed using X-ray photoelectron spectroscopy and energy-dispersive X-ray spectroscopy. It was seen that after the addition of PEI, the NZO and ZnO nanorod arrays increased in length and their diameters became smaller (i.e., their aspect ratios increased). This resulted in an increase in the amount of dye absorbed by them, leading to improvements in the DSSCs based on the nanorods. The structural, morphological, optical, and photovoltaic characteristics of ZnO and NZO nanorod arrays synthesized using different precursor concentrations and growth temperatures (160-190 °C) were also examined. We also investigated the effect of the use of PEI on these characteristics. The power conversion efficiency (PCE) of DSSCs fabricated using the NZO nanorod arrays was found to be significantly higher than that of DSSCs based on the pure ZnO nanorod arrays. This increase in efficiency could be attributed to the combined effects of the increase in the charge-carrier concentration, change in morphology, and increase in the Fermi energy levels of the nanorods, which resulted because of N doping. A PCE of 5.0% was obtained for a DSSC based on a film of arrays of NZO nanorods having an aspect ratio of ∼47 and synthesized using PEI.