Controlled Synthesis of Water-Soluble Conjugated Polyelectrolytes Leading to Excellent Hole Transport Mobility

Conjugated polyelectrolytes (CPE) find widespread applications due to their solubility in aqueous systems or highly polar solvents. However, ion reorganization under applied fields and low charge carrier mobility limit their use as active layers in electronic devices. Here, we present a novel controlled synthetic route for CPEs based on polythiophene carrying sulfonate side groups. We prepared three different polymers with varying molecular weights and narrow polydispersity. For the CPE with the highest molecular weight, we observed the formation of small aggregates in aqueous solution which was confirmed by UV–vis absorption and fluorescence spectroscopy. In the UV–vis spectrum, vibrational bands are observed, which are maintained in the thin film. These absorption bands are similar to those of crystalline poly(3-hexylthiophene). The fluorescence signal is almost completely quenched for these aggregates. Adding other polar solvents such as DMSO results in the dissolution of the aggregates indicated by the decrease of the vibrational bands in UV–vis and the increase of the fluorescence signal. This polymer further exhibits a remarkably high hole transport mobility of (1.2 ± 0.5) × 10–2 cm2/(V s) as determined by the space charge limited current method. The underlying transport mechanism was studied by current (J)–voltage (V) measurements and impedance spectroscopy. The former shows a quadratic dependence of J vs V and a fast response within microseconds characteristic for a classical semiconductor, while the latter shows no sign of any ion motion. In contrast to other reported CPEs, the regioregular chain conformation and the narrow molecular weight distribution here promote the formation of aggregates which improve the electronic charge transport throughout the bulk. Additionally, the presence of sterically demanding counterions suppress the ion motion and reorganization, resulting in a water-soluble semiconducting material with high hole transport mobility.