pH Dependent Stability of Organic Electrochemical Transistors Made from Carboxylic Acid Functionalized Polythiophenes

Organic mixed ionic/electronic conductors (OMIECs) have drawn interest for their potential applications in diverse new technologies ranging from energy storage to biosensing. Of particular interest as sensors are organic electrochemical transistors (OECTs). Recent work has shown carboxylic acid functionalized polythiophenes are effective OECT materials. We investigate the pH dependent properties of these OECTs. We find that at low pH, the OECTs are stable to electrochemical cycling. However, at higher pH these devices display dramatic instability (~20% decay per cycle). We investigate the mechanism of this degradation via optical spectroscopy (UV-vis and FTIR). Further, we investigate the difference in charge compensation mechanism in the different electrolytes by quartz crystal microbalance (QCM). We find that at low pH oxidizing the film results in mass gain, similar to other p-type materials. However, at higher pH, oxidation of the film results in mass loss, suggesting an entirely different charge compensation mechanism.