Dielectric-constant effects on the exciton dissociation and photovoltaic conversion efficiency of water-soluble green conducting polymers†

Abstract

While the best strategy to design semiconducting polymers for organic photovoltaics (OPVs) is still debated, several reports link the performance of some such polymers to a high dielectric constant, with polythiophenes at the forefront of those studies. The use of the dielectric constant as a figure of merit to design OPVs will represent a game-changing strategy towards high photoconversion efficiency (PCE) if it is rigorously proven by a theoretical model on a specific OPV polymer with tunable susceptibility. Water-soluble poly[2-(3-thienyl)-ethoxy-4-butyl-sulfonate] (PTEB) is the ideal platform to test such a hypothesis, due to its properties that are tunable in liquid environments and are highly dependent on the pH of the aqueous solutions used to disperse it, in addition to offering a unique avenue toward fully water-processed, environmentally green OPVs. Here, we characterize a set of PTEB samples, assemble bulk heterojunction (BHJ) OPVs out of them, and develop a mean-field theory model explaining why the tunability of the optoelectronic properties of PTEB persists in the solid state, which is assigned to the different polarizability of the terminations of PTEB pendant groups that are stable in basic and acidic environments. Dielectric constants decreasing from ε_r = 4.5 (in layers spun from solutions at pH = 4) to ε_r = 2.9 (at pH = 10) are measured and shown to affect the operation of PTEB OPVs with bathocuproine (BCP) as an acceptor, increasing their PCE from 0.44% up to 2.8% at the highest value of ε_r. Even if these figures are still relatively low over OPV polymers soluble in nonpolar solvents, they are the highest obtained to date for PTEB and, to the best of our knowledge, also for any other water-soluble polythiophenes, except for polythiophene particles previously processed in nonpolar solvents.