Solar energy represents one of the most abundant and yet least harvested sources of renewable energy. In recent years, tremendous progress has been made in developing photovoltaics that can be potentially mass deployed. Of particular interest to cost-effective solar cells is to use novel device structures and materials processing for enabling acceptable efficiencies. In this regard, here, we report the direct growth of highly regular, single-crystalline nanopillar arrays of optically active semiconductors on aluminium substrates that are then configured as solar-cell modules. As an example, we demonstrate a photovoltaic structure that incorporates three-dimensional, single-crystalline n-CdS nanopillars, embedded in polycrystalline thin films of p-CdTe, to enable high absorption of light and efficient collection of the carriers. Through experiments and modelling, we demonstrate the potency of this approach for enabling highly versatile solar modules on both rigid and flexible substrates with enhanced carrier collection efficiency arising from the geometric configuration of the nanopillars.

Controlled and uniform assembly of "bottom-up" nanowire (NW) materials with high scalability presents one of the significant bottleneck challenges facing the integration of nanowires for electronic applications. Here, we demonstrate wafer-scale assembly of highly ordered, dense, and regular arrays of NWs with high uniformity and reproducibility through a simple contact printing process. The assembled NW pitch is shown to be readily modulated through the surface chemical treatment of the receiver substrate, with the highest density approaching approximately 8 NW/mum, approximately 95% directional alignment, and wafer-scale uniformity. Such fine control in the assembly is attained by applying a lubricant during the contact printing process which significantly minimizes the NW-NW mechanical interactions, therefore enabling well-controlled transfer of nanowires through surface chemical binding interactions. Furthermore, we demonstrate that our printing approach enables large-scale integration of NW arrays for various device structures on both rigid silicon and flexible plastic substrates, with a controlled semiconductor channel width ranging from a single NW ( approximately 10 nm) up to approximately 250 microm, consisting of a parallel array of over 1250 NWs and delivering over 1 mA of ON current.

Single-crystal ZnO nanowires are synthesized using a vapor trapping chemical vapor deposition method and configured as field-effect transistors. Electrical transport studies show n-type semiconducting behavior with a carrier concentration of ∼107cm−1 and an electron mobility of ∼17cm2∕Vs. The contact Schottky barrier between the Au/Ni electrode and nanowire is determined from the temperature dependence of the conductance. Thermionic emission is found to dominate the transport mechanism. The effect of oxygen adsorption on electron transport through the nanowires is investigated. The sensitivity to oxygen is demonstrated to be higher with smaller radii nanowires. Moreover, the oxygen detection sensitivity can be modulated by the gate voltage. These results indicate that ZnO holds high potential for nanoscale sensing applications.