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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

Venkat Bommisetty


Photovoltaics is a direct way to harvest sunlight into electricity using solar cells made from both organic and inorganic semiconductor materials. Organic solar cells have lower efficiency but have the potential to become low cost photovoltaic technology. Lack of understanding of charge transport mechanisms arises due to material complexities such as variations in nanoscale morphology and high energetic disorder have been identified as reasons for low efficiency in bulk heterojunction solar cells. The goal of this dissertation was to connect solar cell terminal characteristics with nanoscale processes to predict optimum morphological organization and energetic environments for efficient device fabrication to reduce cost and commercialize the technology. A new stochastic model was developed that accounted for charge diffusivity, trapping and mutual distances for geminate and non-geminate charge recombination. Morphology domain size was calculated from energy filtered transmission electron microscope image. In the low energetic disorder regime, exciton transport kinetics depended on nanoscale organization of the morphology, average domain size and energetic disorder. However, at high energetic disorder, exciton diffusion probability was fairly independent of morphology and mesoscopic organization, suggesting that disorder alone was the dominant factor in determining the exciton dynamics. The dominant loss mechanism switched from charge recombination to exciton recombination after 25 nm domain size. Internal quantum efficiency first increased and then decreased with morphology domain size and was maximum for 15 nm domain size morphology. Solar cell efficiency first increased and then decreased with energetic disorder and its value was maximum for 30 meV. The simulation suggested that bulk heterojunction solar cell made with active layer morphology domain size of 10 to 20 nm range can improve solar cell efficiency. The simulation predicted that exciton and geminate recombinations are the potential loss mechanisms in bulk heterojunction solar cells as compared to bimolecular recombination and optimal value of energetic disorder can enhance device efficiency by improving hot exciton dissociation process. This work can be extended to other donor/acceptor material systems such as PCDTBT/PC 70BM.

Library of Congress Subject Headings

Solar cells
Heterojunctions Cells -- Morphology
Charge transfer


Includes bibliographical references (pages 110-128)



Number of Pages



South Dakota State University


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