Date of Award

Summer 7-9-2014

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Physics

First Advisor

Jeffrey L. Blackburn

Second Advisor

Daniel S. Dessau

Third Advisor

Jao van de Lagemaat

Abstract

Single-walled carbon nanotubes (SWCNTs) have a number of unique and remarkable properties, including high electrical conductivity and tunable optical absorption. Due to their optoelectronic properties, considerable interest has been expressed in incorporating them into organic photovoltaic devices. Several challenges, as well as opportunities, have arisen in recent years as SWCNTs have been investigated for photovoltaic applications. This dissertation covers both main roles for SWCNTs in solar cells: as charge collecting transparent electrodes and as charge generating light absorbers in the active layer.

Typical SWCNT synthesis methods produce both metallic and semiconducting species with a wide range of diameters, electron affinities, and ionization potentials. To overcome this polydispersity, I employ post-synthetic separation techniques to extract purely semiconducting species and even narrow chiral distributions. To incorporate SWCNTs as electrodes in solar cells, chemical doping can be used to improve both conductivity as well as transparency. While p-type doping has been more common and typically environmentally stable, I investigated n-type doping as a means for increasing flexibility in device architecture as well as for use in nanoelectronics. A full spectroscopic characterization of and comparison of transport properties between n- and p-type films is presented.

Thus far, SWCNTs have not produced highly efficient photovoltaic devices. In part, this has been due to a poor understanding of how multichiral distributions with widely varying band-gaps and energetic offsets affect photoinduced charge generation and separation in the active layer. By taking into account differences in electron affinity, exciton binding energy, thermodynamic driving force, and charge transfer reorganization energy, I systematically investigate how to improve exciton dissociation in two widely differing SWCNT samples using a large range of fullerene acceptors. Finally, using a similar experimental technique, I report on progress towards a fundamental understanding of nanotube mobility and how it is affected by nanotube length, diameter, and environment.

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