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Document Type

Open Access Dissertation

Degree Name

Doctor of Philosophy (PhD)

Degree Program

Chemistry

Year Degree Awarded

Spring 2015

First Advisor

Sankaran Thayumanavan

Second Advisor

Paul Lahti

Third Advisor

Ricardo Metz

Fourth Advisor

Kenneth Carter

Subject Categories

Organic Chemistry | Physical Chemistry

Abstract

Organic photovoltaics (OPVs) are desirable for the harvesting of solar energy. They provide distinct advantages over their inorganic counterparts, especially the high absorption coefficients of organic materials and their ability to be processed using inexpensive solution methods. This allows for potential development of lightweight and flexible devices for portable electronics. One of the drawbacks of organic photovoltaics is the low power conversion efficiency of the devices. Efforts to improve the efficiency often take place through molecular design of the electron rich donor material to improve light absorption of the active layer. However, significantly less effort has been put into modifying the acceptor.

BODIPY-dye based materials are well known for their photostability, strong visible absorption and tunable optical and electrochemical properties. Incorporation of these materials into an A-D-A architecture, in which the meso coupled BODIPY moiety acts as the acceptor cap is particularly advantageous. This instills predictable absorption properties, low lying LUMO level and consistent n-type characteristics, making these materials suitable for use as light harvesting electron acceptors in OPV devices. Using these materials in bulk heterojunction OPV devices with poly(3-hexylthiophene) (P3HT) as the donor material yielded efficiencies up to 1.5%. This efficiency was reached after mild optimization using solvent additives and thermal annealing, methods similar to those used with the most commonly employed acceptor, PCBM.

Analysis of the fabricated devices showed the BODIPY molecules contribute to the photogenerated current in the Vis-NIR region. In an effort to improve BODIPY absorption associated with the donor-acceptor intramolecular charge transfer (ICT) interaction, the linker between the donor core and BODIPY cap was systemically modified to increase the delocalization of the frontier molecular orbitals (m-phenylene < p-phenylene < fluoro-p-phenylene < 3-hexylthiophene < directly linked < ethynyl). This variation not only improved the red edge absorption as the delocalization increased, it also provided a method to tune the LUMO energy level, a valuable tool in the design of electron acceptors. The ethynyl linked, BDP-CC-DTP, yielded the best electron mobility (~10-3 cm2 V-1 s-1), likely due to the low lying LUMO (-4.06 eV) and its ability to form nanoscale aggregates to promote long range electron transport. Given the improved red edge absorption and higher electron mobility of BDP-CC-DTP compared to BDP-Th-DTP, which showed respectable efficiencies when used as an acceptor in BHJ OPV devices, we plan to investigate the use of this semiconductor in organic solar cells.

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