Noble metal nanoparticle-containing photoactive nanofibres for photovoltaic applications

Thesis

Achieving greater light-harvesting in thin photoactive layers remains critical to alleviating the absorption-extraction trade-off presented by organic photovoltaics (OPVs) - a flexible, lightweight, and low-cost alternative to silicon.

Herein, plasmonic nanoparticles are introduced to electrospinning solutions containing the electron donor polymer poly(3-hexylthiophene) (P3HT) and auxiliary polymer poly(ethylene oxide) (PEO), inducing viscosity/conductivity modulation and enabling the fabrication of record-low diameters of P3HT nanofibres with diameters of 55 nm, 30% lower than prior reports. Viscosity reduction is a time-dependent process allowing thinner fibres to be electrospun as the solution ages.

Electrospinning offers an impressive degree of chain alignment wherein anisotropic features orient along the fibre axis. For dissolved polymer, the chains are extended along the fibre. In crystalline regions, diffraction studies reveal the backbones are orthogonal to the axis. Crystallites assembled in solution before electrospinning grow in the π − π direction, and this anisotropy results in the observed backbone orientation.

Upon nanoparticle-induced diameter reductions, polymer alignment increases, providing optimal overlap of the transition dipole moment with the electric field vector of light, promoting photon-harvesting. Champion nanowebs provide up to 60% additional exciton generation versus a conventionally spin-coated thin-film.

A novel plasmon-active nanofibrous active layer is prepared by infiltration of the nanoweb with an electron acceptor. Photoluminescence quenching indicates that fibres offer competitive exciton dissociation levels of 95 – 97 %, whilst ultra-fast transient absorption confirms quenching translates into enhanced charge carrier populations with longer life-times. Compounded with absorption enhancements, ∼135% greater polaron populations were estimated 1 ns after photo-excitation in the presence of AgNPs prepared in-situ.

Proof-of-principle OPVs present efficiency approaching that of the bulk heterojunction, whilst AgNPs raise open-circuit voltage. Further work is required to achieve high-quality acceptor infiltration for effective carrier extraction. As the nanoparticle - PEO interaction dictates electrospinning, this platform is easily adapted to other contemporary semiconducting polymers.

Authors: Schofield, RM

• DOI: 10.5287/ora-kejdgaz1d
• Type of award: DPhil
• Level of award: Doctoral
• Awarding institution: University of Oxford
• Language: English
• Keywords: diameter reduction; plasmonic nanoparticles; electrospinning; organic semiconductors; nanofibres; nanostructure; photovoltaics
• Subjects: Nanostructured materials; Electrospinning; Organic electronics; Organic photovoltaic cells