Functionalizing conjugated polymers with polar ethylene glycol side chains enables enhanced swelling and facilitates ion transport in addition 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 µ OECT 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/cm 3 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 cm 2 /Vs, highest among the three, which is one order of magnitude higher than for P3MEEMT. The extracted hole mobility from OECT (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 investigated with WAXS and GIWAXS and the water uptake under applied potential was monitored using E-QCMD. These measurements reveal an amorphous state for P3MEET and 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 mixedconduction 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.

A new generation of polythiophene-based polyelectrolytes is reported to address fundamental issues in organic electrochemical transistors (OECTs). In such devices, the semiconductor must be able to transport and store ions and possess simultaneously a very high electronic mobility. For this, the ion-conducting 6-(thiophen-3-yl) hexane-1-sulfonate tetramethylammonium monomer (THS − TMA + ) is copolymerized with the hole-conducting 3-hexylthiophene (3HT) to obtain copolymers, PTHS − TMA + -co-P3HT 1−3 with a gradient architecture. The copolymers having up to 50 mol % 3HT content are easily oxidizable and are crystalline. Consequently, for the copolymers, a higher stability in water is achieved, thus reducing the amount of cross-linker needed to stabilize the film. Furthermore, OECTs using copolymers with 75 and 50 mol % of PTHS − TMA + content exhibit 2−3 orders of magnitude higher ON/OFF ratio and an extremely lower threshold voltage (−0.15 V) compared to PTHS − TMA + . Additionally, high volumetric capacitance (C* > 100 F/cm 3 ) is achieved, indicating that the ion transport is not hampered by the hydrophobic 3HT up to 50 mol %, for which a very high OECT hole mobility of 0.017 cm 2 /(V s) is also achieved. Thus, the concept of copolymerization to combine both ionic and electronic charge transport in an organic mixed conductor offers an elegant approach to obtain highperformance OECT materials.

A comprehensive investigation of four polydiketopyrrolopyrroles (PDPPs) with increasing ethylene glycol (EG) content and varying nature of comonomer is presented, and guidelines for the design of efficient mixed ion‐electron conductors (MIECs) are deduced. The studies in NaCl electrolyte‐gated organic electrochemical transistors (OECTs) reveal that a high amount of EG on the DPP moiety is essential for MIEC. The PDPP containing 52 wt% EG exhibits a high volumetric capacitance of 338 F cm−3 (at 0.8 V), a high hole mobility in aqueous medium (0.13 cm2 V−1 s−1), and a μC* product of 45 F cm−1 V−1 s−1. OECTs using this polymer retain 97% of the initial drain‐current after 1200 cycles (90 min of continuous operation). In a cell growth medium, the OECT‐performance is fully maintained as in the NaCl electrolyte. In vitro cytotoxicity and cell viability assays reveal the excellent cell compatibility of these novel systems, showing no toxicity after 24 h of culture. Due to the excellent OECT performance with a considerable cycling stability for 1200 cycles and an outstanding cell compatibility, these PDPPs render themselves viable for in vitro and in vivo bioelectronics.

Molecular orientation is crucial for improving the efficiency of organic electronic devices. In many ordered materials, orientation is achieved by directional crystallization on a substrate but usually not in semicrystalline conjugated polymers. Specifically, a full face-on molecular alignment of poly(3-hexylthiophene) (P3HT) on a substrate has not been realized so far. Here, it is found that P3HT crystallized on graphene exhibits a double-layered edge-on and face-on crystal orientation with edge-on crystals formed on the top surface. This finding is interpreted as a result of two competing interfacial interactions of P3HT chains with graphene and vacuum. By modifying the side-chain chemical composition in poly [3-(6-bromohexyl)]thiophene, the influence of the interface to vacuum can be reduced, resulting in full face-on orientation in films with a thickness of up to 26 nm. These results demonstrate that directed crystallization can be used to control the orientation of semicrystalline functional polymers in thin films given that the interaction with both interfaces is taken into account.