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DISSERTATION_SRIJANA BHANDARI.pdf (22.44 MB)

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AN ELECTRONIC STRUCTURE APPROACH TO UNDERSTAND CHARGE TRANSFER AND TRANSPORT IN ORGANIC SEMICONDUCTING MATERIALS

Bhandari, Srijana
http://orcid.org/0000-0002-3846-3874
http://rave.ohiolink.edu/etdc/view?acc_num=kent1606836665551399

Abstract Details

2020, PHD, Kent State University, College of Arts and Sciences / Department of Chemistry and Biochemistry.
Effective design for improving the efficiency of optoelectronic devices requires an understanding of the role of the molecular environment in determining the properties of organic semiconducting (OSC) molecules. For example, the ionization potential (IP) and electron affinity (EA) of OSCs, which are central in electron transfer and transport processes, vary greatly between the gas and solid phases and play a crucial role in the resultant photophysical characteristics. However, widely used forms of DFT and time-dependent DFT (TD-DFT) fail to accurately describe the frontier orbital gap and charge transfer states, respectively, of such molecular systems. In this dissertation, we address these limitations by implementing an optimally tuned range-separated hybrid (OT-RSH) functional approach in DFT and TDDFT calculations. We extended this scheme to address the challenges of calculating the condensed phase properties in both ground and excited states by implementing screened range separated hybrid (SRSH) functional combined with the polarizable continuum model (PCM). In this scheme, long-range electrostatic interactions in solid-state OSC molecules are consistently screened by 1/ϵ factor (ϵ is the solid-state dielectric constant). Using RSH functional, we establish a molecular design approach based on fluorination of oligophenyl series to achieve enhanced hole mobilities and relatively high electron mobilities that are crucial in improving the device efficiency. Furthermore, by combining RSH with polarizable continuum model (pcm), we study the impact of octa-acid encapsulation on photophysical properties of coumarin dyes. We found that the reduced Stokes shift upon encapsulation is due to the capsule’s role in limiting the relaxation of the excited state as it accepts the charge and therefore decreases the intramolecular charge transfer within the coumarin. The electronic properties of the organic materials in the condensed phase are calculated using SRSH-PCM. Impressive is the success of the single molecules calculations to reproduce condensed phase properties. We benchmark fundamental ground state properties against measured values in the condensed phase. In addition, we demonstrated the success of this approach in accurately calculating the charge transfer energies of a donor-acceptor complex in an aqueous solution. Finally, we applied the SRSH-PCM scheme to understand the reported experimental trends of the photophysical properties in a series of tetrapyrrolic macrocycles with varied saturation levels. In a related field of study, there is an ongoing effort for the miniaturization of electronic devices pursued in nanotechnology, where molecular junctions pose the ultimate limit. Computational studies can assist the challenging conductance measurements by providing necessary insights into the electron transport mechanism at the molecular level and therefore contribute to device fabrication efforts, but this has been obstructed by the discrepancies between measured conductance and calculated values. Indeed, computational studies tend to overestimate the conductance through molecular junctions. We implement and apply RSH with the NEGF scheme to overcome the tendency of traditional DFT based calculations to overestimate the conductance through molecular bridges. We demonstrated this scheme indicates the onset of negative differential resistance (NDR) at a bias that agrees well with the measured NDR trends. We relate the onset of NDR to the Coulomb blockade at the drain contact. Finally, we successfully analyze the role of junction asymmetry in NDR.
Barry Dunietz, Ph.D. (Advisor)
Mietek Jaroniec, Ph.D. (Committee Member)
Robert Twieg, Ph.D. (Committee Member)
Bjorn Lussem, Ph.D. (Committee Member)
Eitan Geva, Ph.D. (Committee Member)
John Portman, Ph.D. (Committee Member)
265 p.
Chemistry; Physical Chemistry
Optoelectronic devices, Density functional theory, Time-dependent density functional theory, Range separated hybrid functionals, Negative differential resistance

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Citations

  • Bhandari, S. (2020). AN ELECTRONIC STRUCTURE APPROACH TO UNDERSTAND CHARGE TRANSFER AND TRANSPORT IN ORGANIC SEMICONDUCTING MATERIALS [Doctoral dissertation, Kent State University]. OhioLINK Electronic Theses and Dissertations Center. http://rave.ohiolink.edu/etdc/view?acc_num=kent1606836665551399

    APA Style (7th edition)

  • Bhandari, Srijana. AN ELECTRONIC STRUCTURE APPROACH TO UNDERSTAND CHARGE TRANSFER AND TRANSPORT IN ORGANIC SEMICONDUCTING MATERIALS. 2020. Kent State University, Doctoral dissertation. OhioLINK Electronic Theses and Dissertations Center, http://rave.ohiolink.edu/etdc/view?acc_num=kent1606836665551399.

    MLA Style (8th edition)

  • Bhandari, Srijana. "AN ELECTRONIC STRUCTURE APPROACH TO UNDERSTAND CHARGE TRANSFER AND TRANSPORT IN ORGANIC SEMICONDUCTING MATERIALS." Doctoral dissertation, Kent State University, 2020. http://rave.ohiolink.edu/etdc/view?acc_num=kent1606836665551399

    Chicago Manual of Style (17th edition)

kent1606836665551399
155
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This open access ETD is published by Kent State University and OhioLINK.