Skip to Main Content
 

Global Search Box

 
 
 
 

ETD Abstract Container

Abstract Header

Synthesis and Optoelectronic Properties of Electron Accepting Azadipyrromethene-based Compounds

Daddario, Cassie M

Abstract Details

2016, Doctor of Philosophy, Case Western Reserve University, Chemistry.

Bulk heterojunction organic solar cells have garnered interest as a replacement for silicon based solar cells. The organic solar cells can be solution processable, lightweight and flexible structures. The bulk heterojunction active layer contains a donor and acceptor material. Fullerenes and its derivatives have been the most utilized acceptor material, due to its strong electron affinity and six reversible reduction potentials. However, due to its poor absorption in the visible to near-IR region and its limited tuning of energy levels, research has shifted to non-fullerene acceptors. Azadipyrromethene (ADP) is a chromophore with two reversible reduction potentials and good electron affinity. ADP has various areas for substitution and chelation to red-shift absorption spectra and tune energy levels, creating potential electron acceptors for organic photovoltaics (OPVs).

ADP ligands substituted with thienylethynyl substituents either at the distal phenyl groups, H(CD1), or the proximal phenyl groups, H(CD2), were synthesized and characterized. The thienyl groups have a hexyl group at the third position to improve solubility in organic solvents and prevent homocoupling of the ethynylthiophene reactants. To further tune the optoelectronic properties, the substituted ADPs were coordinated with BF2+ and Zn(II). Absorption spectroscopy shows that the thienylethynyl substitutions red-shift the absorption spectra of the dyes 22 nm and 41 nm for distal and proximal substitution, respectively, with the larger shift when the substituents are added on the proximal phenyl groups. Cyclic voltammetry experiments show that the substitutions stabilize the anion and dianion. The reduction potentials for the modified ligands (or zinc chelates) were not affected by the placement of the thienylethynyl groups. Preliminary studies of blends of the new zinc(II) chelates with poly(3-hexylthiophene) (P3HT) in films showcase significant fluorescence quenching of P3HT and a broad absorption spectra suggesting that the zinc(II) chelates have potential as electron acceptors for organic solar cells.

Small internal reorganization energy is desirable for high-performance optoelectronic materials, as it facilitates both charge separation and charge transport. However, only a handful of n-type electron accepting materials are known to have small reorganization energies. DFT calculations were performed on a series of model molecules to predict reorganization energy, electron affinity, and energy levels. All compounds studied were most stable in their anionic state and had high electron affinity, indicating their potential as n-type electron accepting materials. The internal reorganization energies were relatively low due to the presence of substituents, such as electron withdrawing nitrile groups, or thienylethynyl and phenylethynyl substituents that extended conjugation. The homoleptic zinc(II) complexes had significantly lower reorganization energies than either the free ligands or the BF2+ chelates. The low reorganization energies of the zinc(II) complexes are explained by the large and rigid π conjugated system that extends across the two azadipyrromethene ligands via interligand π-π interactions. This work suggests that certain substitutions along with Zn(II) complexation are novel strategies for obtaining materials that combine low internal reorganization energy with high electron affinity for the development of novel n-type optoelectronic materials.

Addition of electron withdrawing nitrile substituents at the p-distal phenyls for BF2+ chelate, 8 and zinc(II) chelate, 9, along with cyanophenylethynyl substitution at the pyrrolic position of Zn(ADP)2 analog, 13, was studied (Schemes 4-1 and 4-2). The nitrile group effectively lowered the energy levels of azadipyrromethene compared to unsubstituted ADP. The zinc(II) chelates had similar reduction and oxidation potentials compared to fullerene derivative [6,6]-phenyl-C61-butyric acid methyl ester (PCBM), which is commonly used in BHJ solar cells. Substitution of nitrile groups at the p-distal phenyls of ADP, 8 and 9, gives a red-shift of 14 nm compared to unsubstituted ADP, 2 and 3 (Figure 4-4). Cyanophenylethynyl substitution in the pyrrolic position of ADP, 13, gives a blue-shift of 9 nm compared to non-cyanated phenylethynyl-ADP, 4 (Figure 4-4). Blend films of the zinc(II) complexes with P3HT give good visible to near-IR absorption. Additionally, the fluorescence of P3HT is quenched by the zinc complexes, indicating electron/energy transfer. These results suggest that with the nitrile substitution, the zinc(II) chelates may be a successful alternative to PCBM.

John Protasiewicz, Ph.D. (Committee Chair)
Anna Samia, Ph.D. (Committee Member)
Clemens Burda, Ph.D. (Committee Member)
David Schiraldi, Ph.D. (Committee Member)
Genevieve Sauve, Ph.D. (Advisor)
293 p.

Recommended Citations

Citations

  • Daddario, C. M. (2016). Synthesis and Optoelectronic Properties of Electron Accepting Azadipyrromethene-based Compounds [Doctoral dissertation, Case Western Reserve University]. OhioLINK Electronic Theses and Dissertations Center. http://rave.ohiolink.edu/etdc/view?acc_num=case1444768936

    APA Style (7th edition)

  • Daddario, Cassie. Synthesis and Optoelectronic Properties of Electron Accepting Azadipyrromethene-based Compounds. 2016. Case Western Reserve University, Doctoral dissertation. OhioLINK Electronic Theses and Dissertations Center, http://rave.ohiolink.edu/etdc/view?acc_num=case1444768936.

    MLA Style (8th edition)

  • Daddario, Cassie. "Synthesis and Optoelectronic Properties of Electron Accepting Azadipyrromethene-based Compounds." Doctoral dissertation, Case Western Reserve University, 2016. http://rave.ohiolink.edu/etdc/view?acc_num=case1444768936

    Chicago Manual of Style (17th edition)