Contents I. Introduction
DYE-sensitized solar cells (DSCs) have been found a wide practical application as an alternative to p-n junction solar cells because of their low fabrication cost and relatively high efficiency [1][2] [3]. The highly porous nanoparticles are the most important material of choice for DSCs and so far have shown to exhibit the highest overall light conversion efficiency ∼11.4% with a maximum incidence photon-to-current efficiency (IPCE) up to ∼85% [4], [5]. The ZnO-based DSCs have recently been explored as an alternative to in DSCs because of their distinct advantages over the conventional electrodes in aspect of high electron mobility [6], simple fabrication of the nanostructure, and easy modification of the surface structure. The bandgap (∼3.37 eV) and electron affinity of ZnO films are nearly identical to TiO films [2], [7]. However, overall photoelectric conversion efficiency (η) of ZnO-based DSCs is still relatively low. In recent decades, many researchers have been paying attention to improve the efficiency of DSCs using nanostructured ZnO materials, because of their fast electron transfer, reduction of charge recombination rate, and collection of carriers through electrical transport pathway [8], [9]. Recently, many kinds of 1-D ZnO nanostructures, such as nanobelts [10], nanowires [11], nanorods, nanowiskers, nanocages, nanotubes, nanonails [12], [13], nanotetrapods [14], [15], nanohelixes [16], and nanodisks [17] have been synthesized by different approaches. Among these different ZnO nanostructures, ZnO nanonails, especially well-aligned ones have attracted much attention owing to the well-defined geometry, the ordered structure, and potential applications. The ZnO nanonails were prepared by the thermal evaporation process [18], [19], modified thermal evaporation process [20], thermal vapor transport and the condensation method [21], [22], a facile combustion oxidation method [23], nanoparticle-assisted pulse laser ablation deposition [24]. All these aforementioned methods have needed high temperature (400–1000 °C) and gas flow during deposition. Moreover, no researchers have found ZnO nanonails structures by the hydrothermal method. In this study, the hydrothermal process is made simple and inexpensive to avoid complex and expensive autoclaves system. Novel ZnO NMs have been first synthesized on sputter-deposited ZnO/FTO substrate by the uncovered hydrothermal process at low temperature of 150 °C. The prepared ZnO NMs are closer to ZnO nanonails structures. Comparing to ZnO nanonails, the cap of ZnO NMs is round shape and grown on the ZnO NRs separately. Moreover, ZnO NMs have been used as a photoelectrode for DSCs applications to improve the photovoltaic performances. The photovoltaic data of DSCs based on ZnO NMs have been compared with the ZnO NR (prepared at 85 °C) based DSCs. The structural, surface morphological properties of ZnO NMs/NRs photoelectrodes and their photovoltaic performance have been investigated and discussed.
