Optical Fiber/Nanowire Hybrid Structures for Efficient Three-Dimensional Dye-Sensitized Solar Cells†

Renewable and green energy are the technological drivers of the future economy. Solar cells (SCs) are one of the most important sustainable energy technologies that have the potential to meet the world s energy demands. Among the various approaches to SCs, the performance of dyesensitized solar cells (DSSCs) is largely influenced by the surface area of adsorbed light-harvesting molecules. Traditional DSSCs utilize a nanoparticle film for enhancing the SC conversion efficiency. Photons absorbed by the dye monolayer create excitons that are rapidly split at the surface of the nanoparticles. After splitting, electrons are injected into the nanoparticles and holes move towards the opposite electrode by means of a redox species in an electrolyte. The surface area of the nanoparticle film and the effectiveness of charge collection by the electrodes determine the photovoltaic efficiency of the cell. The latter property has been improved by using aligned ZnO nanowire (NW) arrays, which provide direct electrical pathways for rapid collection of carriers generated throughout the device, and a full-sun efficiency of 1.5% has been demonstrated. However, the design is still based on a two-dimensional (2D) planar substrate, which has a relatively low surface area that limits the dye loading capacity and restricts mobility and adaptability for remote operation. Moreover, the increasing surface area is limited by the requirement that the electron transport distance d remains significantly smaller than the electron diffusion length Ln in order to minimize recombination of electrons with holes or other species. For wire-based SCs, in which light is illuminated perpendicular to the wire, the shadow effect from the entangled wire shaped electrode may limit the enhancement in power efficiency. We report herein an innovative hybrid structure that integrates optical fibers and nanowire (NW) arrays as threedimensional (3D) dye-sensitized solar cells (DSSCs) that have a significantly enhanced energy conversion efficiency. The ZnO NWs grow normal to the optical fiber surface and enhance the surface area for the interaction of light with dye molecules. The light illuminates the fiber from one end along the axial direction, and its internal reflection within the fiber creates multiple opportunities for energy conversion at the interfaces. In comparison to the case of light illumination normal to the fiber axis from outside the device (2D case), the internal axial illumination enhances the energy conversion efficiency of a rectangular fiber-based hybrid structure by a factor of up to six for the same device. Furthermore, the absolute full-sun efficiency (AM 1.5 illumination, 100 mWcm ) is increased to 3.3%, which is 120% higher than the highest value reported for ZnO NWs grown on a flat substrate surface and 47% higher than that of ZnO NWs coated with a TiO2 film. This research demonstrates a new approach from 2D to 3D solar cells with advantages of high efficiency, expanded mobility, surface adaptability, and concealed/remote operation capability. The DSSC hybrid structure is an integrates optical fibers and ZnO NWs grown by a chemical approach on the fiber surfaces. The design principle is shown in Figure 1. The main structure consists of a bundle of quartz fibers arranged such