Date of Award

Spring 1-1-2012

Document Type

Thesis

Degree Name

Master of Science (MS)

Department

Electrical, Computer & Energy Engineering

First Advisor

Won Park

Second Advisor

Justin Johnson

Third Advisor

Garret Moddel

Abstract

Organic thin film solar cells can be paired with nanostructured substrates to overcome the issue of narrow spectral absorption in a thin-film configuration. The nanostructured surface acts not only as an effective scattering back reflector to increase the light path within the absorbing thin film but also affords plasmonic activity. The interface between the metal and the absorbing chromophore supports surface plasmon modes. The associated strong electromagnetic field can potentially couple with excitations of the chromophore, altering its exciton dynamics. Such a plasmon-exciton coupling can lead to control over excitation processes, namely singlet fission. Singlet fission is a sharing of excited state energy between chromophores that may regulate instances of multi-exciton generation, allowing the solar cell efficiency to exceed the thermodynamical Shockley-Queisser limit.

The current investigation focuses on hybridization of the plasmon and molecular exciton. We coat an organic semiconductor, 3,4,9,10-perylenetetracarboxylic dianhydride (PTCDA), on the nanogratings consisting of lines of Ag on a substrate coated with a thick Ag backing. A dielectric spacer layer is included between the organic and the metal in some samples to eliminate any reaction between the two. The SP resonance of the grating is tuned through a PTCDA exciton line by sweeping the incident wave vector. Successful anticrossing between the plasmon and the exciton peaks would be observed in steady-state reflectance data as a function of angle. Though a detailed analysis of reflectance spectra has not been completed, the potential for plasmon-exciton hybdridization is demonstrated.

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