We report a low-temperature aqueous solution growth of uniformly aligned ZnO nanorod arrays on flexible substrates. The substrate is Indium Tin Oxide (ITO) film coated on polyethylene terephthalate (PET). Solutions with five different concentrations of the precursors with equimolar Zinc Nitrate and Hexamethylenetetramine (HMT) in distilled water were prepared to systematically study the effect of precursor solution concentration on the structural and optical properties of ZnO nanorods. It was concluded that the precursor concentration have great influence on the morphology, crystal quality, and optical property of ZnO nanorods. The diameter, density, and orientation of the nanorods are dependent on the precursor solution concentration. X-ray diffraction and micro-Raman spectroscopy showed that the ZnO nanorods with the highest concentration of 50？mM were highly aligned and have the highest level of surface coverage. It was also found that the diameter and length of the nanorods increases upon increasing precursor solution concentration. This is the first systematic investigation of studying the effect of precursor solution concentration on the quality of ZnO nanorods grown on ITO/PET substrates by low-temperature solution method. We believe that our work will contribute to the realization of flexible organic-inorganic hybrid solar cell based on ZnO nanorods and conjugated polymer. 1. Introduction Zinc oxide (ZnO) is a semiconductor with a wide direct band gap of 3.37？eV and a large exciton binding energy of 60？meV, which makes the material useful for optoelectronic application [1, 2]. Nanostructures of ZnO such as ZnO nanorods and nanowires have received increased attention due to their excellent electrical and optical properties [3]. Due to the high surface-to-volume ratio provided by the one-dimensional (1D) nanostructure, ZnO nanorod arrays are considered suitable to the application for hybrid photovoltaic devices [4–7]. In the past few years, ZnO nanorods have been synthesized via various physical and chemical methods including vapor phase synthesis [8–10], metalorganic chemical vapor deposition (MOCVD) [11–13], and solution-based synthesis [14–17]. Among these routes, solution-based method has the advantages of simplicity, low costs, low growth temperature, and easy coating of large surfaces Intensive research has been focused on the solution growth process of ZnO nanorods on ITO-coated glass substrates. Guo et al. [18] have systematically studied the effect of processing conditions such as pretreatment of the substrates, growth temperature,
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