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Synthesis of Azadipyrromethene-Based n-Type Organic Semiconductors, Structure-Property Studies and Their Applications towards Organic Photovoltaics

Jimenez, Jayvic Cristian
http://orcid.org/0000-0002-2342-5648
http://rave.ohiolink.edu/etdc/view?acc_num=case162627559992293

Abstract Details

2021, Doctor of Philosophy, Case Western Reserve University, Chemistry.
Organic semiconductors are poised to be prominent in electronic applications such as organic photovoltaics (OPVs) because they have potential for being cost-efficient and can be applied towards flexible substrates. Within the last decade, performance has increased significantly due to fundamental understanding in designing organic semiconductors, with several key structural features such as solubilizing groups, alternating acceptor-donor-acceptor (A-D-A) structures and molecular shape. Now, attention to new design considerations including the synthetic complexity (SC) of the final molecule have focused not only on the high performance, but the projected cost-efficiency in these types of materials. This thesis focuses on the design, synthesis and preliminary device applications of azadipyrromethenes (ADPs) as organic semiconductors. ADPs are attractive because they are electron deficient so they can be paired with electron rich donors such as poly-3-hexylthionphene (P3HT). Importantly, ADPs are relatively simple to synthesize which lead to low SC. Both planar and non-planar ADPs were synthesized and characterized. Their properties as well as their OPV performance are described herein. In order to increase performance of a successful complex, Zn(L2)2, fluoride groups were introduced on the distal phenyls (Zn(1F-L2)2), pyrrolic naphthyls (Zn(2F-L2)2), and in both positions simultaneously (Zn(3F-L2)2). These modifications led to higher performance in OPV. To understand how fluorination improves properties, the crystal structure and film morphology of neat and blend films were studied. Of the three molecules, we obtained the crystal structure of Zn(2F-L2)2 and Zn(3F-L2)2 and found them to have identical geometries and crystal packing. Having the distal –F increased the overlap integral of Vh+ from 1 meV to 6 meV and Ve- from 1 meV to 4 meV. We found that –F on the pyrrolic naphthyls increase crystallinity, guides intermolecular π-π stacking and influences surface orientation, while –F on the distal does not participate in π-π stacking but increases crystallinity and the overlap integrals of Zn(2F-L2)2. Both molecules can facilitate top-down charge transport which is ideal for OPV applications. Turning to planar systems, novel boron-chelated ADPs with various peripheral substitutions were synthesized and characterized. This series explored intramolecular BO chelation versus BF2+ chelation, as well as the use of hexyloxy solubilizing groups on the distal phenyl and pyrrolic phenylethynyls. Combinations of these functional group and chelation type allow for tuning the optical, electrochemical, and physical properties. Compound 6a resulted in λmax, abs over 800 nm with relative fluorescence quantum yields (QYs) ~6-8% owing to its planar geometry with the proximal phenyls covalently bonded to restrict free-rotation, whereas compound 8 shares similar chemical structure except for the restricted proximal phenyls. To assess their potential for OPV application, the new planar complexes were tested in OPVs using P3HT as the donor, and the neat and P3HT blended thin film properties were investigated. The blended films have strong absorption across the visible to nIR range except for 600-700 nm. Fluorescence quenching experiments revealed that a combination of phenylethynyls and BF2+ increases the electron transfer with P3HT. The overlap integral values of compounds 6a had Vh+ and Ve- as high as ~80 meV and 8 with Vh+ as high as ~60 meV and Ve- around 10 meV which are adequate for electronic applications. Preliminary OPV results show promise with 8 with 1.1% PCE; previous attempts using aza-BODIPY complexes had 0% PCE because of poor blend morphologies. These results suggest that solubilizing groups are key to optimize OPV performance. Realizing the importance of solubilizing groups to tune properties, we turned back to the non-planar Zn(WS3)2 complex and explored the role of solubilizing groups on properties. Complexes with solubilizing groups on the distal and proximal phenyls of non-planar Zn(WS3)2 were synthesized and characterized. The solubilizing groups were either hexyl or hexyloxy. Hexyloxy groups on the proximal phenyls red-shifts the absorbance to as high as 722 nm in solution and 889 nm in films due to increased HOMO energy levels. Crystal structure shows that all complexes had a distorted tetrahedral structure, though the solubilizing groups on the distal phenyls affected the dihedral angle with the hexyloxy group distorting the geometry as much as 48.0°. Analysis of film morphology revealed that hexyloxy solubilizing groups strongly increases the tendency to crystallize in thin films and guide molecular orientation, though the charge transfer directions do not favor OPVs. These might be better suited for transistor applications. Further work in OPVs is required to better understand the effect of the solubilizing groups on performance. These structure-property studies contribute to the development of ADP-based non-fullerene acceptors for OPV application, getting closer to obtaining materials with low SC and high performance. This research is thus key to make OPV commercially viable and wide-spread, enabling a greener future with less dependence on environmentally unsustainable sources.
Carlos Crespo-Hernandez (Committee Chair)
Genevieve Sauve (Advisor)
Thomas G. Gray (Committee Member)
Anna C. Samia (Committee Member)
Emily B. Pentzer (Committee Member)
425 p.
Chemistry

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Citations

  • Jimenez, J. C. (2021). Synthesis of Azadipyrromethene-Based n-Type Organic Semiconductors, Structure-Property Studies and Their Applications towards Organic Photovoltaics [Doctoral dissertation, Case Western Reserve University]. OhioLINK Electronic Theses and Dissertations Center. http://rave.ohiolink.edu/etdc/view?acc_num=case162627559992293

    APA Style (7th edition)

  • Jimenez, Jayvic Cristian. Synthesis of Azadipyrromethene-Based n-Type Organic Semiconductors, Structure-Property Studies and Their Applications towards Organic Photovoltaics. 2021. Case Western Reserve University, Doctoral dissertation. OhioLINK Electronic Theses and Dissertations Center, http://rave.ohiolink.edu/etdc/view?acc_num=case162627559992293.

    MLA Style (8th edition)

  • Jimenez, Jayvic Cristian. "Synthesis of Azadipyrromethene-Based n-Type Organic Semiconductors, Structure-Property Studies and Their Applications towards Organic Photovoltaics." Doctoral dissertation, Case Western Reserve University, 2021. http://rave.ohiolink.edu/etdc/view?acc_num=case162627559992293

    Chicago Manual of Style (17th edition)

case162627559992293
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This open access ETD is published by Case Western Reserve University School of Graduate Studies and OhioLINK.