Self-Assembly of Small Molecules for Organic Photovoltaic Applications

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Organic photovoltaic (OPV) solar cells aim to provide efficient, flexible and lightweight photovoltaics (PV) with simple processing and low-cost. Advances in device optimization, structural and molecular design, as well as mechanistic understanding have helped increase device efficiency and performance. Within the framework of active layer optimization, systematically improving bulk heterojunction (BHJ) morphology could improve the power conversion efficiency of OPVs. However, most strategies aimed at improving morphology focus on annealing methods or the use of solvent additives. Rational approaches in supramolecular self-assembly can potentially offer additional control over the morphology of BHJ active layers and lead to improved power conversion efficiencies. In Chapter 2, the author explores the effect of molecular shape on the assembly of electron donating small molecules, and its ensuing effect on OPV performance. Two tripodal ‘star-shaped’ donor molecules with diketopyrrolopyrrole (DPP) side chains were used to generate solution-processed BHJ OPVs. It was found that the tripod molecules neither aggregate in solution nor form crystalline domains in thin films when a branched alkyl solubilizing group is used. On the other hand, linear alkyl chains promote the formation of one-dimensional (1D) nanowires and crystalline domains as well. This work demonstrated that the one-dimensional assembly of donor molecules enhances the performance of the corresponding solution-processed OPVs by 50%. This is attributed to the reduction of trap states in the 1D nanowires, resulting in a significant increase in the fill factor of the devices. In Chapter 3, experiments are described in which the electron donor is a hairpin-shaped molecule containing a trans-1,2-diamidocyclohexane core and two DPP conjugated segments, and a fullerene derivative as the electron acceptor. Self-assembly of the donor molecule is driven by the synergistic interaction between hydrogen bonds and π–π stacking, and under proper conditions this results in the formation of long supramolecular nanowires that enhance charge transport and device efficiencies. The efficiencies of these devices were found to be 54% greater than those built with analogous donor small molecules containing only the conjugated structure. In addition, it was shown that a stepwise cooling process with minimal solution stirring is necessary to create robust wires that are not disrupted upon addition of acceptor molecules. This resulted in a 400-fold increase in device efficiency compared to those formed by simple mixing of donor and acceptor molecules. To further improve OPV device efficiency using hydrogen bonding in small molecule electron donors, the use of hydrogen bonding in linear molecules with DPP was investigated and this work is described in chapter 4. Two asymmetric derivatives were compared: one containing an amide bond, capable of forming hydrogen bonds, and a second one contained an ester bond in the same position. Although both molecules have very similar optoelectronic properties, films of the ester revealed greater crystallinity and π–π stacking, as characterized by grazing incidence x-ray diffraction. In great contrast, active layers formed with the amide derivative formed short fiber-like supramolecular aggregates with much smaller domain sizes and less order as characterized by atomic force microscopy and X-ray diffraction. Interestingly, devices fabricated with the amide-fullerene donor-acceptor combination have greater short circuit currents, leading to devices that are 50% more efficient than those built with the ester derivative. We concluded that the effective competition of hydrogen bonding over extensive π–π stacking results in morphologies that lead to higher photovoltaic efficiencies. Taken together, the findings in this work demonstrate improved OPV device efficiency by use of supramolecular self-assembly and the control of shape of donor molecules

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  • 02/19/2018
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