Abstract: Silicon-based devices currently dominate the solar cell market with efficiencies around 15-20% for commercial modules. These devices are not more widespread due to expensive processing. Another type of solar cell, the excitonic cell, which includes polymer blend, polymer-inorganic hybrid and dye-sensitized solar cells, is a low-cost alternative for efficient large-scale solar energy conversion. One-dimensional ZnO nanowire arrays show great promise as a material for these solar cells, but a controllable, scalable and low-cost synthesis is needed for them to have a technological advantage. In this dissertation, an environmentally benign synthesis for growing ZnO nanowire arrays from a zinc salt is presented. Arrays of wires with diameters in the nanoscale regime can be grown in an aqueous solution of zinc nitrate and hexamethylenetetramine from a substrate seeded with ZnO nanoparticles. The synthesis produces high-density arrays of single-crystalline nanowires on a variety of substrates. With the addition of poly(ethylenimine), the lengths of the wires can be increased to 25 μm with aspect ratios over 125. ZnO nanowire arrays can be grown vertically from a surface by nucleating the wires from textured ZnO nanocrystals that have their c axes normal to the substrate. Using the ZnO nanowire arrays as templates to form arrays of coaxial core-shell nanowires can vary the surface material and the properties of these wires. By coating the nanowires with a conformal oxide shell using atomic layer deposition, we have produced ZnO-Al₂O₃ and ZnO-TiO ₂ core-shell arrays. ZnO-ZnS core-shell arrays can be formed in an aqueous sodium sulfide solution by displacing the ZnO with polycrystalline ZnS. We conclude this research by using these ZnO and ZnO-TiO₂ nanowire arrays in dye-sensitized and polymer-inorganic solar cells. ZnO nanowire dye-sensitized cells show a possible improvement in the charge collection efficiency over traditional nanoparticle cells with an overall power conversion efficiency of 1.5% under 100 mW cm-2 AM 1.5 simulated sunlight. Coating the arrays with a thin shell of TiO₂ results in a substantial improvement in overall conversion efficiency, up to 2.25%. Vertical ZnO-TiO₂ core-shell nanorod arrays encased in the hole-conducting polymer poly-3-hexylthiophene show a fivefold increase in cell efficiency relative to devices without shells. We find that the core-shell cells must be exposed to air to reproducibly attain efficiencies higher than 0.05%. Cells stored in air for one month are 0.29% efficient. Solution-grown ZnO nanowire arrays have opened up the field of nanowire-based solar cells, and are promising contenders for alternative, low-cost photovoltaics.