Light management in ultra-thin conjugated polymer layers using plasmonic nanostructures
Creators: name: Yu, Binxing
Publisher: name: No Publisher Supplied
Year: 2016

description: Conjugated polymer-based optoelectronic devices have been the focus of a growing body of investigations due to their potential for superb tuneability, high flexibility, low cost and low embodied energy. However, hurdles such as the relatively low efficiency and short operational lifetime are preventing conjugated polymer-based optoelectronic devices from being widely applied in consumer products. One of the main reasons for the low efficiency are the competing requirements for both electrically-thin conjugated polymer active layers (for efficient charge carrier transport) and optically-thick active layers (for efficient solar absorption). As a result, optimization of the device efficiency while maintaining a thin active layer has been an important pursuit for researchers in the field of organic optoelectronics. In that regard, the need for ever-thinner active layers in optoelectronic devices requires effective light trapping at deeply-sub-wavelength scales.   This thesis work focuses on the use of plasmonic nanostructures to improve light trapping in ultra-thin conjugated polymer films. First, we provide a way of applying  plasmonic nanorod arrays onto sub-50 nm polythiophene films on metallic substrates to show significant absorption enhancement (&gt;10 at the polythiophene band edge) and spectral broadening (250 nm increase) relative to polythiophene/metallic films without plasmonic nanorod arrays. Full-field electromagnetic simulations identify horizontal/longitudinal monopole antenna modes and gap modes, with the latter being the primary contributors to polythiophene absorption enhancement.   To further investigate the gap modes and their influence on the ultra-thin conjugated polymer active layer, a sphere-on-plane (SOP) system consisting of a gold nanoparticle on a conjugated polymer thin-film on a metallic substrate is fabricated and studied both theoretically and experimentally. Four different electromagnetic coupling modes are observed: a horizontal image dipole coupling mode, a vertical image dipole coupling mode and horizontal and vertical coupling modes between a localized surface plasmon resonance (LSPR) and a surface plasmon polariton (SPP). Relatively broadband spectral tuning of the modes can be achieved by modification of the thickness of either the absorptive spacer or the underlying metal film. Strong field confinement at longer wavelengths in the polythiophene spacer region, due to the vertical image dipole coupling mode and a LSPR-SPP coupling mode, is also observed in simulations and contributes to absorption enhancement. Furthermore, we find absorption enhancement in the polythiophene spacer increases with decreasing thickness, indicating the increased light trapping ability of the gold nanoparticles for ultra-thin active layers.   Dark-field optical images of SOP systems also reveal the existence of “red” particles for which the signal of the horizontal image dipole coupling mode is quenched. Subsequent defocused imaging and correlated AFM height analysis confirm this is attributed to partial-embedding of gold nanoparticles into the polythiophene spacer and leads to higher scattered light intensities at longer wavelengths. This work demonstrates that light trapping in sub-50-nm-thick semiconductor layers is possible using a “sphere-on-plane” system and offers insight into how coupling modes can be manipulated in this system.   In addition to plasmonic light trapping, energy/electron transfer in blends of organic semiconductors with cascading bandgap energies is investigated to broaden the absorption bandwidth in organic thin-films. Unary, binary, and ternary solutions of the following organic semiconductors: poly(9,9-dioctylfluorenyl-2,7-diyl) (PFO), poly(3-hexylthiophene) (P3HT), and 2,3,9,10,16,17,23,24-octakis(octyloxy)-29H,31H-phthalocyanine (OctPc), were used to prepare sub-55 nm thick unary-phase and blended thin-films. Spectroscopic analysis shows that absorption bandwidth full-width-at-half-maximum (FWHM) values increase from between 60 and 160 nm for the individual materials to greater than 450 nm for the composite thin-film ternary blend. Resonant energy or charge transfer is observed with efficiencies between 90% and 100% for the various blends. Grazing-incidence, wide-angle X-ray scattering data indicate that P3HT and OctPc exhibit the poorest blending. This correlates with the lowest donor photoluminescence quenching efficiency due to the extended separation of the P3HT chains from OctPc molecules, which is confirmed by pump-probe transient absorption spectra. It is notable that addition of a relatively small fraction of PFO disrupts OctPc crystallinity and enables improved energy/charge transfer between P3HT and OctPc.