Understanding intermolecular interactions in organic heterojunction devices with the use of density functional theory

Ayoub, Sarah (2018) Understanding intermolecular interactions in organic heterojunction devices with the use of density functional theory. Doctoral (PhD) thesis, Memorial University of Newfoundland.

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Abstract

In the past two decades, the field of organic semiconductors has gathered immense attention and development due to their remarkable advantages in the applications in devices such as organic light emitting diodes (OLEDs) and organic solar cells (OSCs). The performance of these devices has significantly improved after the introduction of heterojunction structures which combine donor and acceptor type conjugated materials. Experimental studies suggest that, in addition to electronic and optical material properties, intermolecular interactions are critical for determining the efficiency of such devices. However, a detailed understanding of the impact of these intermolecular interactions is still lacking. In this thesis, I employ dispersion corrected density functional theory (D-DFT) methods to investigate the properties of these interfacial regions in the various promising (monomer/monomer and monomer/fullerene) combinations that are used in OLEDs and OSCs. I analyze binding energies and employ DFT (B3LYP) to obtain the electronic offsets of gas phase and interacting D-DFT monomers and fullerenes combinations. For the various pairings used in OSCs, I first assess the accuracy of D-DFT methods and then I investigate their properties and the effect of alkyl side chains on their interfacial interactions. My study shows that B97-D3 and B3LYP-D3 methods yield the most accurate electronic and absorption results. My results highlight useful (general) trends in electronic, structural, and intermolecular properties and side chains effect of these combinations that are well correlated with the experimentally determined efficiencies. In particular, I determine common factors that lead to achieving the best device performance for combinations of fluorene-based polymers in OLEDs and the highest experimental efficiency (over 10%) for combinations of quaterthiophene-based polymers and fullerenes in OSCs. For example, monomer/fullerene pairings that have some of the highest OSC efficiencies exhibit the lowest interfacial LUMO offset and largest ratio of open-circuit voltage (as determined by interfacial band gap) to monomer’s energy gap. For fluorene-based dimers used in OLEDs, I found monomers that have well-matched chain-lengths and HOMO-LUMO energy gaps exhibit the best device performance. I hope this thesis (which connects theory with the experimental data) will expedite the process of finding promising materials for organic heterojunction devices to improve their efficiencies.

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