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The Key Role of Side Chain Linkage in Structure Formation and Mixed Conduction of Ethylene Glycol Substituted Polythiophenes
journal contribution
posted on 2020-03-05, 15:06 authored by Philip Schmode, Achilleas Savva, Robert Kahl, David Ohayon, Florian Meichsner, Oleksandr Dolynchuk, Thomas Thurn-Albrecht, Sahika Inal, Mukundan ThelakkatFunctionalizing
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 spacer and with 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/cm·V·s 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, 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 organic
field-effect transistor (OFET) hole mobility of 0.005 cm2/V·s, highest among the three, which is one order of magnitude
higher than that 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
wide-angle X-ray scattering (WAXS) and grazing incidence WAXS, and
the water uptake under applied potential was monitored using electrochemical
quartz crystal microbalance with dissipation monitoring (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 10 times more than P3MEEMT in the oxidized state.
Thus, the importance of the ethyl spacer toward 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 of how to design high-performance
organic mixed conductors.
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diethylene glycol side chainselectrochemical impedance spectroscopyP 3MEET P 3MEEMTethyl-spaced P 3MEEET exhibitsEthylene Glycol Substituted Polythiophenes FunctionalizingSide Chain Linkage3MEEEETWAXSE-QCMDP 3MEEEThole mobilityP 3MEEMTP 3MEEET exhibitselectrochemical quartz crystal microbalanceethylene glycol side chainsethyl spacerμ OECT CpolymerP 3MEETethyl-spaced P 3MEEETP 3MEEET swells 10 timesOFETside chain linkage
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