Geometric, electronic and optical properties of organic charge transfer systems: Photovoltaic blends and donor-acceptor co-crystals

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Ashokan, Ajith
Brédas, Jean-Luc
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Over the past two decades, π-conjugated organic molecules have found applications in the active layer of different types of organic electronic devices. To optimize and improve the performance of each of these devices, it is important to establish clear connections between chemical-structure, intermolecular packing and their impact on the electronic and charge transport properties in these systems. In this Thesis, we focus on two-component organic material systems – one acting as a π-electron donor (D) and the other as a π-electron acceptor (A) for applications in organic photovoltaics (OPV) and organic field-effect transistors (OFETs). On the OPV side (Chapters 3 & 4), initially, we investigate the solution temperature-dependent aggregation property of a few polymers in their pure phases, which has been recently established as a potential method for morphology control in high-performing OSC devices. We then explore the intermolecular packing properties in the binary blends of polymer and two small molecule acceptors, which in their binary as well as ternary combinations exhibit high power conversion efficiencies. We elucidate clear connections between the molecular scale features that impact the device parameters in both the binary blends. We also obtain useful trends to explain the linear evolution of device parameters in the ternary blends. On the OFET side (Chapters 5 & 6), our focus is on DA charge-transfer co-crystals, which possess potential applications as active layer components in OFET devices. Initially, we investigate the effect of packing on electronic properties of co-crystals based on F6TNAP acceptor and a series of donor molecules. Further, we focus on understanding the evolution in electronic, vibrational and charge-transport properties with sequential addition of alkyl chains on the donor and fluorine atoms on the acceptor on co-crystals based on BTBT-FmTCNQ (m=0, 2, 4) and di-CnBTBT-FmTCNQ (n=8, 12; m=0, 4) series. Finally, we explore the degree of charge-transfer in these systems using an approach based on Mulliken charges. While these results are limited to the systems under consideration, our simulations provide a reliable, molecular-level understanding to systematically improve the morphological characteristics that impact the device performances in organic electronic devices.
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