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Structure-Property Studies of Substituted Azadipyrromethene-Based Dyes and High Dielectric Constant Polymers for Organic Electronic Applications

Pejic, Sandra
http://orcid.org/0000-0002-4492-2424
http://rave.ohiolink.edu/etdc/view?acc_num=case1527949734211196

Abstract Details

2018, Doctor of Philosophy, Case Western Reserve University, Chemistry.
Organic solar cells (OSCs) that use carbon-based organic semiconductors are attractive alternatives to inorganic solar cells due to their unique properties such as easy processing, flexibility, and scalability. However, OSCs have not attained power conversion efficiencies (PCEs) as high as silicon solar cells. A fundamental understanding of the relationship between an organic semiconductor’s chemical structure and its solar cell properties is needed to design new small molecules and polymers, and to optimize processing conditions. In this dissertation, zinc(II) azadipyrromethene (Zn(ADP)2)-based complexes and functionalized poly(3-hexylthiophene) (P3HT) were investigated as potential organic semiconductors for organic electronics. The Sauve group demonstrated that the zinc(II) azadipyrromethene complex with phenylethynyl pyrrolic substituents (Zn(WS3)2) is a promising electron acceptor for OSCs. While this acceptor worked well with P3HT as the donor, it did not work well with other state-of-the-art electron donors. That is because most donors were optimized to phenyl-C61-butyric acid methyl ester’s (PCBM) energy levels, which are lower than those of Zn(WS3)2. To deepen the energy levels of Zn(WS3)2, fluorine atoms were installed in different positions (Zn(L1-L4)2). In this dissertation, the organic photovoltaic (OPV) device performances of these fluorinated complexes were evaluated in blends with P3HT. PCEs for three out of the four complexes increased compared to Zn(WS3)2, and the highest PCE of 3.7% was obtained with fluorine in the pyrrolic position. Charge carrier mobilities showed all fluorinated acceptors (except for one) had higher mobilites and recombination studies revealed that fluorine suppressed bimolecular recombination. Interestingly, the electrochemical properties and resulting estimated energy levels of the fluorinated complexes were not as deepened as anticipated. Therefore, another popular electron withdrawing group, nitrile, was tested. Nitrile groups were added to the distal position on Zn(ADP)2, making Zn(DCN)2, and to the pyrrolic position on Zn(WS3)2, making Zn(PyCN)2. These indeed had deeper estimated energy levels, with LUMO energy levels similar to PCBM. In this dissertation, the effects of nitrile positioning on device performance was studied. When blended with P3HT, both acceptors gave much lower PCEs than their unsubstituted parent compounds. Electron mobility was estimated by space charge limited current (SCLC) in both neat and blended films. In neat films, the electron mobility of Zn(DCN)2 was good but that of Zn(PyCN)2 was very low. In blended films, electron mobility was low, partly explaining the low performance. Atomic Force Microscopy (AFM) images revealed slight aggregation and pinholes for each acceptor, respectively. Further processing optimization may therefore improve performance. The effect of replacing the triple bond in the pyrrolic phenylethynyl groups of Zn(WS3)2 with a double bond (Zn(WS6)2) was studied. The UV-vis absorption in film indicates that the double bond extends conjugation of ADP just like the triple bond does. Cyclic voltammetry indicates that Zn(WS6)2 is easier to oxidize and harder to reduce than Zn(WS3)2. OPV and mobility results indicate that pyrrolic groups with a double bond, phenylethenyl lowers electron mobility and lower PCEs. AFM reveals nodular-like feature rather than the required long interconnected pathways, suggesting phenylethenyl is hindering P3HT from p-stacking, which is needed for good performance. Using a double bond instead of a triple bond was therefore detrimental to the electron accepting properties and electron transport properties of the complex. Finally, three new polythiophene-based polymers (P1-P3) with high dielectric constants were studied. UV-visible absorption spectra in films suggest that the sulfone groups on P2 and P3 have a negative effect on morphology while P1 with sulfoxide groups still maintains good p-stacking. OPV bilayer devices were attempted but with poor results. Charge carrier mobilities were estimated through space charge limited current (SCLC) and OFETs. Due to the presence of pinholes, the results directly obtained are inconclusive. By carefully mapping the pinholes, a new film thickness was estimated for the SCLC method, and estimated mobilities slightly lower than that of P3HT were obtained. Further work is required to elucidate the charge transport properties and OPV performance of these promising polymers.
Genevieve Sauve, Ph.D. (Advisor)
Anna C. Samia, Ph.D. (Committee Chair)
Carlos E. Crespo-Hernandez, Ph.D. (Committee Member)
Emily B. Pentzer, Ph.D. (Committee Member)
Ina T. Martin , Ph.D. (Committee Member)
219 p.
Chemistry
Organic photovoltaics; OPVs; Organic solar cells; OSCs; Semiconductors; Zinc azadipyrromethene; Dielectric constant; Charge carrier mobility

Recommended Citations

Citations

  • Pejic, S. (2018). Structure-Property Studies of Substituted Azadipyrromethene-Based Dyes and High Dielectric Constant Polymers for Organic Electronic Applications [Doctoral dissertation, Case Western Reserve University]. OhioLINK Electronic Theses and Dissertations Center. http://rave.ohiolink.edu/etdc/view?acc_num=case1527949734211196

    APA Style (7th edition)

  • Pejic, Sandra. Structure-Property Studies of Substituted Azadipyrromethene-Based Dyes and High Dielectric Constant Polymers for Organic Electronic Applications. 2018. Case Western Reserve University, Doctoral dissertation. OhioLINK Electronic Theses and Dissertations Center, http://rave.ohiolink.edu/etdc/view?acc_num=case1527949734211196.

    MLA Style (8th edition)

  • Pejic, Sandra. "Structure-Property Studies of Substituted Azadipyrromethene-Based Dyes and High Dielectric Constant Polymers for Organic Electronic Applications." Doctoral dissertation, Case Western Reserve University, 2018. http://rave.ohiolink.edu/etdc/view?acc_num=case1527949734211196

    Chicago Manual of Style (17th edition)

case1527949734211196
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This open access ETD is published by Case Western Reserve University School of Graduate Studies and OhioLINK.