Date on Master's Thesis/Doctoral Dissertation

12-2014

Document Type

Master's Thesis

Degree Name

M. Eng.

Department

Chemical Engineering

Committee Chair

Willing, Gerold A.

Committee Co-Chair (if applicable)

Fu, Xiao-An

Committee Member

Walsh, Kevin M.

Author's Keywords

Silver nanowires; Electrode; PEDOT PSS; Solar cell; Composite

Subject

Nanowires; Nanostructured materials; Dye-sensitized solar cells

Abstract

Electrically conductive polymers encompass an exciting field of research for applications in dye-sensitized solar cells (DSSC’s). DSSC’s possess several advantages over other types of solar cells. They offer the potential for high quantum efficiency, solar conversion efficiency approaching that of traditional silicon panels, rapid charge transfer kinetics for photo-excited electrons, mechanical flexibility, and cost efficient manufacturing processes. However, key drawbacks to their large scale production and performance lifetime lie in their reliance on costly indium tin oxide (ITO), fluorinated tin oxide (FTO), and platinum for electrode materials, and the mechanical fragility inherent to a liquid electrolyte layer component. Much of the current research in the field concerns identifying an effective solid material to replace the liquid electrolyte presently used in DSSC’s. Within this field of research, electrically conductive polymers (ECP’s) have attracted much interest. One such ECP, PEDOT:PSS [poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate)] has a combination of high flexibility, electrical conductivity, and optical transmittance, making it a viable candidate. Another subset of this research field involves qualifying a cheaper candidate for replacing the costly platinum electrode of DSSC’s. Metallic nanoparticles, whose characteristic dimension is on the order of one to hundreds of nanometers, possess unique physical properties emergent at their size scale. Particularly, nanowires of highly conductive metals such as silver deposited on the surface of ECP’s raises the possibility of an electrode made using simple bulk processing techniques and using drastically lower quantities of expensive materials for the electrode than typical for DSSC’s while offering the same level of conductivity. Furthermore, an interesting challenge lies in patterning the nanowire network using a simple bulk driving force. Achieving facile nanowire alignment, and thus anisotropic electrical conductivity, opens the door to a variety of applications extending beyond solar cell electrodes, such as flexible nanoscale circuitry. The present thesis describes and evaluates the physical properties of composite thin films of PEDOT:PSS with silver nanowire networks. A simple laboratory scale method for creating a randomly aligned silver nanowire network on PEDOT:PSS is first studied, revealing impressive conductivity increases on the order of 120 versus the bare PEDOT:PSS film. Sheet resistances of the composite films average 37 Ω/□ with average ultraviolet and visible light (UV-Vis) transmittances of 63%. UV-Vis transmittance correlates inversely with the surface concentration of nanowires as expected, with a power regression fit to the data (R2 = 0.96). Of note, no strong correlation is detected between sheet resistance and nanowire surface concentration within the range tested. Additionally, a method for aligning silver nanowires on PEDOT:PSS using a magnetic field is explored. Unfortunately, no significant anisotropy in conductivity is measured using the conditions outlined in these experiments, and explanations are discussed leading into recommendations for future work.

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