MULTIFUNCTIONAL FIBER SOLAR CELLS USING TIO 2 NANOTUBES, PbS QUANTUM DOTS, AND POLY(3-HEXYLTHIOPHENE) by Dibya Phuyal MS Electrical Engineering EE 230 – Fiber Optic Communications

OVERVIEW Motivations for the Fiber Optic Solar Cell Characterization of Device Description of Materials Expected Results and Applications Recent Developments and Future Work Questions

A large fraction of the solar spectrum, in particular in the intense 600–1,100 nm spectral range, is poorly absorbed. Reducing the physical thickness of the photovoltaic absorber layers while keeping their optical path length Test the efficacy of a 3D model instead of the planar solar cell. MOTIVATI ON Figure 1 - Comparison of Si Crystalline Film (2um) Absorbance and Solar Spectrum Figure 2 - Charge carriers generated far away (more than the diffusion length L d ) from the pn junction are not effectively collected, owing to bulk recombination

ADVANTAGES OF FIBER SOLAR CELLS Enhance the surface area for the interaction of light Internal reflection provides multiple opportunities for absorption. Design concept transforms the traditional SC from action at illuminated side surface (2D or projection area) to action inside the volume (3D) Utilizes chemical synthesis at low temperatures with environmentally friendly and biologically safe materials Could have a smaller size, greater mobility, more robust design, flexible shape, and potentially lower production cost Figure 3 – Fiber Optic Cables Source: fiberoptics.info ( Source: fiberoptics.info )

CHARACTERISTICS AND PROPERTIES Figure 4 – Diagram of Fiber Optic Cell with components of each layer 1 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. Must strongly absorb in both the ultraviolet (UV) and infrared (IR) regions of the electromagnetic spectrum. Coated with ITO (Indium Tin Oxide) that simultaneously acts as a conductive electrode and high refractive index material that allows light to escape. The conducting polymer Poly(3- hexylthiophene) (P3HT) is added to improve visible absorption and promote electron injection into the nanotubes.

BROAD SPECTRUM ABSORPTION PbS quantum dots (strong IR absorbance) TiO2 Nanotubes - Strong UV absorbance Conducting polymer Poly(3-hexylthiophene) (P3HT) - Improve Visible absorption and promote electron injection into the nanotubes. PbS and P3HT were also chosen for their band gap energies that favorably align with the TiO 2 Figure 5 – Bang Gap Energies of Materials Used

TITANIUM NANOTUBES Large Surface Area Direct Path for Electrical Conduction w/higher mobility Faster transport and fewer potential surface recombination sites An Ohmic Material Conductive TiO 2 (anatase) has a large band gap (3.2 eV - UV) Typical wall thicknesses and intertube spacing of 8−10 nm and pore diameters of about 30 nm Figure 6 – Closely Packed Array of TiO 2 Nanotubes

PBS QUANTUM DOTS (PbS QDs) are expected to improve photocatalytic activity of TiO 2 (35 times larger than bare TNTs) Multiple Exciton Generation (MEG) helps prevent electron-hole recombination Absorption range can be tuned by adjusting the particle size of the quantum dots Figure 7 -PbS QD synthesis process: thiolactic acid acts as a linker between Pb2+ ion sand TNTs. S2-ions then cause PbS QDs to form on the surface of the TNTs.

METHODS & MATERIALS Fibers are dip-coated in suspensions of ITO and TNTs and annealed after each coating. PbS quantum dots (QDs) are applied by a chemical bath with thiolactic acid concentration determining QD size Fibers are then dip-coated in P3HT solution Figure 6 – Chemical Synthesis Materials Used to Process QDs, TiO2 NT, and P3HT

APPLICATIO NS Flexibility of the fibers, combined with the ability to make arrays of any shape, makes it possible to incorporate fiber solar cells directly into the structure of buildings. The SC unit can be concealed and located where the sunlight is available away from the surface, thus making unique designs and surface- confined applications possible such as underground and in deep water. Fibers can be tuned to transit specific wavelengths for other purposes (lighting).

RECENT DEVELOPMENTS & FUTURE WORK Morphology still needs improvements. O 2 - sites act as recombination centers on TNT walls. Fabricated by anodization of Ti foil Rectangular Fiber Geometry to increase the energy conversion which has been recently shown to be 6 times that of cylindrical fiber and to reduce recombination Using a dye [Black Dye, [(C4H9)4N]3[Ru(Htcterpy)(NCS)3], covering the visible and near IR regions up to 900nm. Figure 8 - Incorporating Double Dye Layers Currently at

OTHER FIBER BASED APPLICATIONS Convert low-frequency vibration/friction energy into electricity using piezoelectric zinc oxide nanowires grown radially around textile fibers (V O ~ 45 mV) ZnO nanowires grown on fibers, it is possible to fabricate flexible, foldable, wearable and robust power sources in any shape Figure 9 - Entangling two fibers and brushing the nanowires rooted on them with respect to each other, mechanical energy is converted into electricity owing to a coupled piezoelectric–semiconductor process

PLASMONIC SOLAR CELL Enhanced incoupling of light into semiconductor thin films by scattering from plasmonic nanoparticles Strong local field enhancement around the metal nanoparticles to increase absorption in a surrounding semiconductor material. Shape and size of metal nanoparticles are key factors determining the incoupling efficiency. Figure 10 – Plasmon Solar Cell Designs

REFERENCES 1. Rice, Adam. Multifunctional Novel Fiber Solar Cells Using TiO 2 Nanotubes, PbS Quantum Dots, and Poly(3- hexylthiophene).Carnegie Mellon University, Atwater, A. H & Polman, Albert. Plasmonics for improved photovoltaic devices. Nature Materials Vol. 9, Weintraub, B., Wei, Y., & Wang, L. Z. Optical Fiber/Nanowire Hybrid Structures for Efficient Three- Dimensional Dye- Sensitized Solar Cells. Angew. Chem. Int. Ed. 2009, 48, 1 – O’Connor, B., Pipe, K. P., & Shtein, M. Fiber based organic photovoltaic devices. Applied Physics Letters Vol. 92, Ratanatawanate, C., et al. Fabrication of PbS Quantum Dot Doped TiO2 Nanotubes. ACS Nano Vol 2, Fa`brega, C., et al. On the photoconduction properties of low resistivity TiO2 nanotubes. Nanotechnology 21, 2010.

Thank you. Questions? FIBER OPTIC SOLAR CELL