For every kg of conjugated polymer used to produce an organic photovoltaic (OPV) device, 20-80 kg of organic (usually halogenated) solvent is consumed. Currently, organic solvent waste is commonly disposed of by incineration, which undermines the renewable nature of solar energy. While conjugated polymers with charged pendant groups, conjugated polyelectrolytes (CPEs), can be processed from water, their amphiphilic nature drives aggregation, which precludes their use as the active layer in an OPV device. We have developed a new class of materials, conjugated polyions (CPIs), in which charges are placed both in the backbone and in the pendant groups, eliminating the tendency to aggregate. Figure 1 is a cartoon contrasting the two motifs of charged, conjugated polymers; PFSC is a CPI, PFS is a CPE.Due to the presence of closed-shell charges (i.e., charges not formed by redox) in the band structure of these polymers, their band-gaps can be tuned (from ~3.2-1.8 eV) by adjusting pH or by exposure to certain nucleophiles. Furthermore, they can be cast into smooth, homogenous films from aqueous solutions and environmentally benign solvents such as methanol. Analysis by AFM shows evidence of nano-structuring upon annealing, a property usually associated with enhanced pi-pi stacking. Although much work remains to incorporate them into OPV devices (including finding a compatible acceptor), we believe that they demonstrate the potential for truly sustainable solar cells, without a waste stream of halogenated solvents.

Water-processible conjugated polymers with dynamic band-gaps

247th National Meeting (2014)

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