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Polymer and Fullerene Zwitterions: From Synthesis to Solar Cells

This thesis describes the synthesis and applications of hydrophilic conjugated polymers and fullerenes containing dipole-inducing pendent groups. The pendent groups include tertiary amines, sulfobetaine (SB) zwitterions, quaternary ammoniums, and sulfonates, providing solubility in polar solvents. Particular emphasis is placed on zwitterions functalized structures. Suzuki-Miyaura (SM) and Horner-Wadsworth-Emmons (HWE) coupling reactions proved valuable for the preparation of the hydrophilic conjugated polymers, while the Prato reaction afforded the functional fulleropyrrolidines. Ultraviolet photoelectron spectroscopy (UPS) probed the interactions between the hydrophilic conjugated polymers and conductive metal substrates. In particular, UPS revealed that conjugated polymer zwitterions (CPZs) substantially reduce work function (Φ) of metals, represented by a negative interfacial dipole (Δ). Their solubility properties and interactions with metals make CPZs attractive for integration into solar cells, specifically at the interface between a photoactive layer and high Φ metal cathode. This thesis thus provides routes to improve polymer-based solar cell (PSC) technology through the implementation of novel hydrophilic semiconductors. Initial syntheses focused on the preparation of polythiophene with pendent SB groups, producing CPZs that were incorporated into PSCs as cathode modification layers. Tuning the electronic properties of CPZs with different polymer backbones further enhanced their effectiveness as interlayers in PSCs. Diketopyrrolopyrrole (DPP), iso-indigo (iIn) and naphthalene diimide (NDI) were functionalized with SB, followed by SM polymerization to provide the corresponding CPZs. Unprecedented power conversion efficiency (PCE) values (> 10%) were achieved for devices containing the NDI CPZs, and improved electron transport of the interlayers was found central to this efficiency enhancement. Fulleropyrrolidines functionalized with tertiary amines and SB groups represent an alternative, non-polymeric, class of materials studied as interfacial modifiers in PSCs. The intrinsic n-type properties of fullerene provide an ideal platform for such interlayers, and led to state-of-the-art devices with record PCE values, irrespective of the selection of conductive cathode (Al, Ag, Cu and Au), while eliminating the need for precise control over interlayer thickness. Finally, HWE coupling was investigated as a new approach to hydrophilic CPZs. The methodology presented afforded room temperature production of a variety of hydrophilic poly(arylene vinylene)s (PAVs) from water, including zwitterionic, cationic and anionic derivatives. The scope and limitations of the HWE reaction in water is discussed, along with the utility of the resulting PAVs in sensing and PSCs.
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