As an abundant and non-toxic wide band gap semiconductor with a high electron mobility, ZnO in the form of nanowires has emerged as an important electron transporting material in a vast number of nanostructured solar cells. ZnO nanowires are grown by low-cost chemical deposition techniques and their integration into solar cells presents, in principle, significant advantages including efficient optical absorption through light trapping phenomena and enhanced charge carrier separation and collection. However, they also raise some significant issues related to the control of the interface properties and to the technological integration. The present review is intended to report a detailed analysis of the state-of-the-art of all types of nanostructured solar cells integrating ZnO nanowires, including extremely thin absorber solar cells, quantum dot solar cells, dye-sensitized solar cells, organic and hybrid solar cells, as well as halide perovskite-based solar cells.

A statistical analysis of the electrical properties of selective area grown O- and Zn-polar ZnO nanorods by chemical bath deposition is performed by four-point probe resistivity measurements in patterned metal contact and multiprobe scanning tunneling microscopy configurations. We show that ZnO nanorods with either polarity exhibit a bulklike electrical conduction in their core and are highly conductive. O-polar ZnO nanorods with a smaller mean electrical conductivity have a nonmetallic or metallic electrical conduction, depending on the nano-object considered, while the vast majority of Zn-polar ZnO nanorods with a larger mean electrical conductivity present a metallic electrical conduction. We reveal, from Raman scattering and spatially resolved 5 K cathodoluminescence measurements, that the resulting high carrier density of ZnO nanorods with O or Zn polarity is due to the massive incorporation of hydrogen in the form of interstitial hydrogen in bond-centered sites (HBC), substitutional hydrogen on the oxygen lattice site (HO), and multiple O–H bonds in a zinc vacancy (VZn–H n ). While HBC is largely incorporated in ZnO nanorods with either polarity, HO and (VZn–H n ) defect complexes appear as the dominant hydrogen-related species in O- and Zn-polar ZnO nanorods, respectively. These findings reveal that polarity greatly affects the electrical and optical properties of ZnO nanorods. They further cast a light on the dominant role of hydrogen when ZnO nanorods are grown by the widely used chemical bath deposition technique. This work should be considered for any strategy for thoroughly controlling their physical properties as a prerequisite for their efficient integration into nanoscale engineering devices.