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Document Type

Dissertation - University Access Only

Award Date


Degree Name

Doctor of Philosophy (PhD)


Electrical Engineering and Computer Science

First Advisor

Qiquan Qiao

Second Advisor

David Galipeau


Solar cells are promising alternatives to fossil fuels as they provide a clean source of renewable energy. Organic bulk heterojunction solar cells based on semiconducting polymers have potential as a low cost alternative to silicon solar cells due to reduced material costs, mechanical robustness and simple fabrication techniques. However, polymer solar cells have relatively low power conversion efficiency (~ 10%) compared to inorganic cells which limit their commercial applications. The nanoscale morphology dependent charge transport in polymer solar cells needs to be better understood so that the morphology can be optimized for enhanced photovoltaic efficiency. The goal of this dissertation was to control the morphology of bulk heterojunction solar cells by varying processing parameters such as thermal annealing, solvent additives and donor-acceptor ratio, and to develop a deeper insight into relationship between morphology and photovoltaic performance. Three different donor polymers were studied along with two fullerene derivatives as acceptors. The materials and solar cells were characterized by UVVisible absorption spectroscopy, current vs voltage measurements, EQE spectroscopy, transient photocurrent/photovoltage and photo-generated charge extraction spectroscopy. Intermixed amorphous polymer and fullerene domains were observed as limiting factors for photovoltaic conversion. The use of solvent additives enhanced photovoltaic performance by 25% to 85 % due to the formation of a morphology of networks with crystalline polymer domains and sharp interfaces which promoted efficient exciton dissociation and enhanced mobility. Future work should include studying the effects of solvent additives on polymer orientation to improve vertical charge transport.

Library of Congress Subject Headings

Solar cells
Cells -- Morphology
Charge transfer


Includes bibliographical references (pages 110-128)



Number of Pages



South Dakota State University


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