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Open Access Dissertation
Doctor of Philosophy (PhD)
Year Degree Awarded
Paul M. Lahti
Michael D. Barnes
All organic photovoltaics (OPVs) undergo four major processes to convert sunlight in electrical energy. The first process is the absorbance of sunlight. Due to the limit of available acceptor molecules, the burden of light absorbance weighs heavily on the donor material. This thesis focuses heavily on the development of dyes consisting of donor-acceptor dyads and triads for improved light harvesting in OPVs.
Squaraine dyes show impressive light harvesting properties with absorbances in the UV to near IR region with extinction coefficients on the order of 105 M--‐1 cm--‐1. Unfortunately, improved light harvesting is not enough to insure optimized OPVs. Energy level tuning to increase VOC and insure efficient exciton dissociation is also required. Functionalizing squaraine dyes with electron donating or electron withdrawing groups allow for the systematic tuning of the HOMO energy levels. This tenability allows for the concurrent optimization of bandgap and VOC.
Cyanine dyes have been explored for small molecule OPVs due to their impressive absorbance properties. The absorbance of ketocyanine dyes can be tuned by manipulating the strength of the acceptor moiety. Stronger acceptors are better able to stabilize the negative charge in the charge separated state of the dye. This stabilization allows for a greater contribution from the cyanine structure of the dye, thus red shifting the absorbance. Stronger acceptors also increase the communication between the two amine functionalities as demonstrated by cyclic voltammetry.
Block copolymers show impressive morphological control through the tuning of the molecular weight of the blocks as well as the compatibility of the functional groups. This allows for the access of morphologies with small, well ordered, and continuous domains thought to be beneficial in the active layer of OPVs. Unfortunately, block copolymers often show inferior light harvesting compared to their conjugated polymer counterparts. Donor-acceptor systems are explored as sensitizers for block copolymer OPVs. Small molecules without twists or bends or acetylene linkers were found to be most effective for lowering the bandgap and aligning the energy levels.
Della Pelle, Andrea, "Design and Syntheses of Donor-Acceptor Dyads and Triads for Improved Light Harvesting in Organic Photovoltaics" (2014). Doctoral Dissertations. 36.