The benefit of organic semiconductors over traditional silicon-based semiconductors is the ability of roll-to-roll processing. However, for every kilogram of material that is used to manufacture an organic photovoltaic (OPV) device, 20-80 kilogram of organic (typically halogenated) solvent is consumed. These organic solvents are often petroleum-based and waste solvents are currently disposed by incineration, thereby largely undermining the renewable nature of photovoltaics. Although conjugated polymers can be rendered water- or alcohol-soluble by installing ionic pendant groups, these so-called conjugated polyelectrolytes (CPEs) are amphiphilic and tend to form aggregates in solution. Since aggregation states in solution nucleate and drive the morphology of films, the use of CPEs in solid state devices is limited to interlayers of just a few nanometers where they act as work-function modifying layers that are typically not continuous.
We have developed a new class of materials, conjugated polyions (CPIs), which are polymers with intrinsically water-soluble (cationic) backbones flanked with anionic pendant groups. CPIs are fully conjugated and Zwitterionic, so they do not form aggregates in water or alcohol. We demonstrate that CPIs can be processed from aqueous solutions (or environmentally benign solvents such as ethanol) into smooth, homogenous films that are qualitatively far superior to films of CPEs. Furthermore, the closed-shell cationic charges (i.e., charges that are not formed by redox) in the band structure of these polymers allow the band-gaps to be tuned within a large window of ∼3.2-1.8 eV post-polymerization, by simply adjusting pH or by treatment with certain nucleophiles. We are actively investigating the scope of this new class of materials, and although a lot of work remains to incorporate CPIs into OPV devices, we believe that these materials form a versatile platform for truly renewable organic photovoltaics.