Document Type

Dissertation - University Access Only

Award Date

2012

Degree Name

Doctor of Philosophy (PhD)

Department / School

Electrical Engineering and Computer Science

First Advisor

Qiquan Qiao

Abstract

Photovoltaics has attracted extensive interest as an environmentally friendly, clean, renewable and sustainable source. However, the initial cost of photovoltaic systems is high. One of the ways to reduce the cost of photovoltaic cells is to develop conjugated polymer based solar cells that have low-cost and simple fabrication procedures. Previous work on P3HT/PCBM solar cells showed efficiencies of 3~5%, which are still lower than traditional silicon solar cells. There is a strong need to understand the relationship between P3HT/PCBM blend morphology and solar cell performance since the combination of microscale and nanoscale morphology engineering of P3HT /PCBM system is still limited. The goal of this dissertation was to develop an understanding of the relationship between P3HT/PCBM film morphology and solar cell performance and use this information to increase cell efficiency to 4-5%. The objectives were to: develop an understanding of the effect of processing on morphology; understand the internal mechanism of morphology control; and increase P3HT/PCBM solar cell efficiency. In bulk heterojunctions, donor/acceptor networks provide bicontinuous pathways for electron and hole transport. Large microscale aggregates in P3HT/PCBM solar cells might affect P3HT crystallinity, decrease interfacial donor/acceptor area and PCBM/metal interfaces, create defect sites and increase resistance. P3HT films and P3HT/PCBM blend films and solar cells were fabricated with different solvents and thermal annealing conditions. Optical imaging, atomic force microscope (AFM) phase imaging, current-sensing AFM (CS-AFM) imaging, fluorescence, Raman and absorption spectra were used to characterize these films. Time-resolved femtosecond fluorescence spectroscopy was used to provide quantitative information on charge and energy transfer processes and the morphologies mediated by these treatments. The photovoltaic performance (J- V and incident photon-to-electron conversion efficiency plots) of the thermally annealed P3HT/PCBM solar cell showed that the efficiency impairment from microscale aggregates can be compensated for by nanoscale organization and packing order formed during heat treatment. Both microscale and nanoscale morphology contributed to increased photovoltaic performance of the 1,2-DCB based P3HT/PCBM solar cell. The morphology of P3HT/PCBM blend films can be controlled at microscale and nanoscale by thermal treatments and choice of solvents. The highest efficiency for P3HT/PCBM solar cells in this dissertation was ~3.5%, which is comparable to the literature. Future work should include developing solar cells with an efficiency of more than 10% using low bandgap polymers and tandem structures.

Library of Congress Subject Headings

Solar cells
Cells -- Morphology
Heterojunctions
Photovoltaic power generations

Publisher

South Dakota State University

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Rights Statement

In Copyright