The Key Role of Side Chain Linkage in Structure Formation and Mixed Conduction of Ethylene Glycol Substituted Polythiophenes

Abstract

Functionalizing conjugated polymers with polar ethylene glycol side chains enables enhanced swelling and facilitates ion transport in addi-tion to electronic transport in such systems. Here we investigate three polythiophene homopolymers (P3MEET, P3MEEMT and P3MEEET), having differently linked (without, methyl and ethyl spacer, respectively) diethylene glycol side chains. All the polymers were tested in organic electrochemical transistors (OECTs). They show drastic differences in the device performance. The highest µOECT C* product of 11.5 F/cmVs was obtained for ethyl spaced P3MEEET. How the injection and transport of ions is influenced by the side-chain linkage was studied with electrochemical impedance spectroscopy (EIS), which shows a dramatic increase in volumetric capacitance from 80± 9 up to 242±17 F/cm3 on going from P3MEET to P3MEEET. Thus, ethyl-spaced P3MEEET exhibits one of the highest reported volumetric capacitance values among p-type polymers. Moreover, P3MEEET exhibits in dry thin films an OFET hole mobility of 0.005 cm2/Vs, highest among the three, which is one order of magnitude higher than for P3MEEMT. The extracted hole mobility from OECT (in oxidized swollen state) and the hole mobility in solid state thin films (OFET) show contradictory trends for P3MEEMT and P3MEEET. In order to understand exactly the properties in the hydrated and dry states, the crystal structure of the polymers was investi-gated with WAXS and GIWAXS and the water uptake under applied potential was monitored using E-QCMD. The WAXS measurements reveal an amorphous state for P3MEET, whereas a semicrystalline state for P3MEEMT and P3MEEEET. On the other hand, E-QCMD confirms that P3MEEET swells ten times more than P3MEEMT in the oxidized state. Thus, the importance of the ethyl spacer towards crystallinity and mixed-conduction properties was clearly demonstrated, emphasizing the impact of side chain-linkage of diethylene glycol. This detailed study offers a better understanding how to design high performance organic mixed conductors