Polaron Delocalization in Donor–Acceptor Polymers and its Impact on Organic Electrochemical Transistor Performance

Article abstract of DOI:10.1002/anie.202014078

Donor–acceptor (D-A) polymers are promising materials for organic electrochemical transistors (OECTs), as they minimize detrimental faradaic side-reactions during OECT operation, yet their steady-state OECT performance still lags far behind their all-donor counterparts. We report three D-A polymers based on the diketopyrrolopyrrole unit that afford OECT performances similar to those of all-donor polymers, hence representing a significant improvement to the previously developed D-A copolymers. In addition to improved OECT performance, DFT simulations of the polymers and their respective hole polarons also reveal a positive correlation between hole polaron delocalization and steady-state OECT performance, providing new insights into the design of OECT materials. Importantly, we demonstrate how polaron delocalization can be tuned directly at the molecular level by selection of the building blocks comprising the polymers’ conjugated backbone, thus paving the way for the development of even higher performing OECT polymers.

Full text of DOI:10.1002/anie.202014078

A Journal of the Gesellschaft Deutscher Chemiker
Angewandte
Chemie
International Edition
www.angewandte.org
Accepted Article
Title: Polaron Delocalization in Donor-Acceptor Polymers and its
Impact on Organic Electrochemical Transistor Performance
Authors: Maximilian Moser, Achilleas Savva, Karl Thorley, Bryan D.
Paulsen, Tania Cecilia Hidalgo, David Ohayon, Hu Chen,
Alexander Giovannitti, Adam Marks, Nicola Gasparini,
Andrew Wadsworth, Jonathan Rivnay, Sahika Inal, and Iain
McCulloch
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To be cited as: Angew. Chem. Int. Ed. 10.1002/anie.202014078
Link to VoR: https://doi.org/10.1002/anie.202014078

10.1002/anie.202014078
Angewandte Chemie International Edition
Polaron Delocalization in Donor-Acceptor Polymers and its Impact on Organic
Electrochemical Transistor Performance
Maximilian Moser,* Achilleas Savva, Karl Thorley, Bryan D. Paulsen, Tania Cecilia Hidalgo,
David Ohayon, Hu Chen, Alexander Giovannitti, Adam Marks, Nicola Gasparini, Andrew
Wadsworth, Jonathan Rivnay, Sahika Inal and Iain McCulloch*
M. Moser, Dr. A. Wadsworth, Prof. I. McCulloch
University of Oxford, Department of Chemistry, Oxford, OX1 3TA UK
Corresponding authors: maximilian.moser@chem.ox.ac.uk and iain.mcculloch@kaust.edu.sa
Dr. A. Savva, T. C. Hidalgo, D. Ohayon, Prof. S. Inal
King Abdullah University of Science and Technology (KAUST), Biological and
Environmental Science and Engineering Division, Thuwal, 23955-6900, Saudi Arabia
Dr. K. Thorley
University of Kentucky, Department of Chemistry, Lexington (KY), 40506-0055, USA
Dr. B. D. Paulsen, Prof. J. Rivnay
Northwestern University, Department of Biomedical Engineering, Chicago (IL), 60208, USA
Dr. H. Chen, Prof. I. McCulloch
King Abdullah University of Science and Technology (KAUST), Physical Science and
Engineering Division, Thuwal, 23955-6900, Saudi Arabia
Dr. A. Giovannitti
Stanford University, TomKatCenter for Sustainable Energy, Stanford (CA), 94305-4125,
USA
Dr. A. Marks, Dr. N. Gasparini
Imperial College London, Department of Chemistry and Center for Plastic Electronics, London,
W12 0BZ, United Kingdom
Prof. J. Rivnay
Northwestern University, Simpson Querrey Institute, Chicago (IL), 60611, USA
Keywords
Donor-acceptor copolymers, organic electrochemical transistors, polaron delocalization,
polymers, semiconductors
1
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Abstract
Donor-acceptor (D-A) polymers are promising materials for organic electrochemical
transistors (OECTs), as they minimize detrimental faradaic side-reactions during OECT
operation, yet their steady-state OECT performance still lags far behind their all-donor
counterparts. Here, we report three D-A polymers based on the diketopyrrolopyrrole unit that
afford OECT performances similar to those of all-donor polymers, hence representing a
significant improvement to the previously developed D-A copolymers. In addition to improved
OECT performance, DFT simulations of the polymers and their respective hole polarons also
revealed a positive correlation between hole polaron delocalization and steady-state OECT
performance, providing new insights into the design of OECT materials. More importantly, we
demonstrate how polaron delocalization can be tuned directly at the molecular level by selection
of the building blocks comprising the polymers’ conjugated backbone, thus paving the way for
the development of even higher performing OECT polymers.
Introduction
Polymer semiconductors which can support “mixed” ionic and electronic conduction
have received considerable interest in the last decade due to their applicability in bioelectronic
settings.[1–4] In this context, organic electrochemical transistors (OECTs), have been
demonstrated to be a particularly successful device type, as highlighted by their use as neural
recording elements, biomolecule sensors, cellular sensors and constituent components for
neuromorphic applications.[5–13]
While advancements in device architecture have significantly contributed to the
progress in OECTs,[14–16] the development of novel and carefully tailored channel materials has
also been a crucial component for consistently reaching higher performances.[17–22] Amongst
the several classes of OECT channel materials developed, ethylene glycol (EG) functionalized
2
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organic semiconductors, have shown to be particularly promising candidates due to: (i) their
lack of any insulating polyelectrolyte phase increasing the density of electronic charges that can
be accumulated thus boosting their volumetric capacitance[21] (ii) their tendency to achieve
optimum OECT performances without the need of any solvent-additives and post-processing
treatments,[21,23] (iii) their excellent OECT operation, ranking them amongst the highest
performing channel materials reported to date,[18,20,22,24], (iv) their good enzyme compatibility
enabling for the direct detection of biologically relevant ions and molecules foregoing the need
of any mediators[10] and (v) their facile synthetic tunability at the molecular scale. Given the
promise of EG functionalized organic semiconductors, several different molecular engineering
strategies have been pursued to maximize their performance in OECTs.[25,26] Broadly speaking
these can be categorized into those focusing on the modification of the side chains[22,27–32] and
those altering the aromatic building blocks of the conjugated polymer backbone.[17–19,29,33,34]
The efficacy of these modifications is typically assessed by comparing the maximum
transconductance (gm) that can be achieved in devices, see equation 1.
푔푚 = 휇퐶∗ ∙ 푊푑 (푉푇퐻 − 푉퐺)
Equation (1)
퐿
Where μ is the electronic charge carrier mobility, C* the volumetric capacitance, W the channel
width, d the channel depth, L the channel length, VTh the threshold voltage and VG the gate
voltage.
As illustrated in Equation 1, the maximum gm that can be accomplished in devices is
dependent on both device geometry (Wd/L) and material dependent ([μC*]) terms.
Consequently, a fairer comparison of the performance of different channel materials is
accomplished by considering the [μC*] of the various semiconductors.[24] Currently, the highest
performing EG functionalized semiconductors for OECT applications are based on all-donor
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10.1002/anie.202014078
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backbones and can reach [μC*] up to ~500 F V-1 cm-1 s-1.[17,18,22] The performance of channel
materials not featuring an all-donor but instead a donor-acceptor (D-A) polymer backbone, e.g.
employing diketopyrrolopyrrole (DPP), isoindigo (IID), benzodithiophene (BDT), naphthalene
[17,19,29,34–36]
tetracarboxylic diimide (NDI) or tetrafluorophenylene as the acceptor unit,
is
however yet to match those of polythiophene based materials, with the reported [μC*] of
materials typically ~10 F V-1 cm-1 s-1.[24,33] This class of materials has however been
demonstrated to minimize noncapacitive faradaic side-reactions during OECT operation, thus
preventing the formation of reactive and hazardous side-products that in turn are detrimental to
OECT performance and safe operation.[34] As a result, there is a need to develop high-
performance D-A copolymers for OECTs, in addition to the elucidation of broad structure-
property relationships for the design of future high-performance D-A channel materials.
In this contribution we report the synthesis of three EG functionalized D-A copolymers
for OECT applications, all of which are based on the DPP unit, see Figure 1. Following initial
cyclic voltammetry (CV) measurements, we determined that each of the three polymers showed
stable and reversible electrochemical switching in both aqueous and organic media, thus
suggesting their suitability as channel materials for OECTs. Subsequent OECT fabrication and
testing demonstrated that each of the polymers affords good OECT performances, incurring
[μC*] between 50-350 F cm-1 V-1 s-1, which compares well with the best performing all-donor
based polymers.[17,18,24,28,31] Furthermore, we highlight how differences in the polymers’
microstructure and degree of hole polaron delocalization directly impact the polymers’ OECT
mobility and hence performance. Specifically, we find that matching the relative electron
density on the donor and acceptor moieties, i.e. reducing the energy level offset between the
donor and acceptor units in D-A copolymers, is beneficial in terms of spreading the hole polaron
over a larger proportion of the conjugated polymer backbone, which in turn boosts OECT
mobility. On the other hand, we note that the operational stability of the polymers depends
strongly on the overall electron density of the D-A polymer backbone, whereby the polymers
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with higher electron densities, i.e. lower ionization potentials (IP) afford the highest operational
stability. Consequently, we demonstrate how control over both the overall and relative energy
levels in channel materials for OECTs is of utmost importance in terms of tuning their
respective OECT steady-state performance and operational stability, thus providing valuable
and generic guidelines for the design of stable high-performance D-A copolymers for OECT
applications.
Figure 1. Chemical structures of p(gDPP-TT), p(gDPP-T2) and p(gDPP-MeOT2).
Results and Discussion
The thiophene flanked DPP core was synthesized according to literature procedures,
proceeding via
a
condensation reaction between diethyl succinate and 2-
thiophenecarbonitrile.[37] The DPP unit was subsequently functionalized with triethylene glycol
chains and brominated to afford the central DPP monomer used in each of the subsequent
polymerization reactions. Triethylene glycol chains were utilized as solubilizing groups in the
synthesized polymers, as the vast majority of the glycolated semiconducting p-type channel
materials reported to date employ these triethylene glycol units as side chains, thereby
minimizing the impact of the side chain on the OECT performance and allowing for a better
comparison between polymer backbones.[17,18,35,36,38] The dibrominated DPP monomer was then
subjected to Stille cross-coupling polymerization to afford the polymers p(gDPP-TT), p(gDPP-
T2) and p(gDPP-MeOT2) respectively, with their chemical structures illustrated in Figure 1. A
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more detailed account of the synthetic procedures can be found in the supporting information
(S.I.).
All polymers exhibited good solubility in chloroform, allowing for facile solution
processing. Similar as for previously reported all EG functionalized DPP-based D-A
copolymers, gel permeation chromatography (GPC) proved to be inconclusive affording
unrealistically high molecular weights, potentially due to polymer aggregation in solution.[34]
Although exact molecular weights could thus not determined for the polymers, the polymeric
nature of the materials was evidenced by their similar 1H NMR and UV-Vis spectra compared
to their alkylated equivalents, vide infra.[39,40]
UV-vis absorption spectroscopy was performed on as cast polymer films, see Figure S1
and Figure S2 in the S.I. Each polymer exhibited a maximum absorption wavelength (λmax,film
)
above 800 nm, whereby p(gDPP-TT) and p(gDPP-T2) also featured relatively well-resolved
vibronic features. The absorption profile of p(gDPP-MeOT2) on the other hand did not show
any such features, indicating a higher degree of molecular ordering of p(gDPP-TT) and
p(gDPP-T2) in the solid state. These results are in line with previous literature, in which the
dimethoxybithiophene unit was also reported to lead to a loss in vibrational fine structure.[34]
The optical gap (Eg,opt) for each polymer was calculated from the onset of absorption, incurring
values of 1.22 eV, 1.22 eV and 0.96 eV for p(gDPP-TT), p(gDPP-T2) and p(gDPP-MeOT2)
respectively. The bathochromically shifted λmax,film and the smaller Eg,opt of p(gDPP-MeOT2)
compared to p(gDPP-TT) and p(gDPP-T2) were attributed to the presence of the electron-
donating methoxy substituents on the bithiophene unit in p(gDPP-MeOT2), which induced a
reduced IP in the polymer.
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Table 1. Optoelectronic properties of the polymers under investigation.
λmax,film
[nm]
833
Eg,opt
[eV]
1.22
1.22
0.96
Eox,aq
Eox,org
IP
Polymer
[V]a
[V]b
[eV]c
5.11
5.03
4.56
p(gDPP-TT)
+0.46
+0.39
-0.14
+0.12
+0.04
-0.43
p(gDPP-T2)
812
p(gDPP-MeOT2)
916
a) Measured employing a 0.1 М NaCl(aq) solution as the supporting electrolyte; b) measured
employing a 0.1 М solution of tetrabutylammonium hexafluorophosphate in acetonitrile as the
supporting electrolyte; c) calculated from the Eox,org
.
Evaluation of the polymers’ electrochemical properties was performed by CV in both
organic and aqueous media. In both instances, a conventional three-electrode set up was
employed, with the working electrode consisting of a polymer coated indium tin oxide (ITO)
substrate, the counter electrode of a platinum wire and the reference electrode of an Ag/AgCl
wire couple. The onset of oxidation in aqueous (Eox,aq) and organic (Eox,org) media, as well as
the IP of the polymers are summarized in Table 1. The voltammograms corresponding to each
polymer using an aqueous electrolyte can be found in Figure 2a, while those recorded
employing an organic electrolyte in Figure S3 in the S.I. As shown in Figure 2a and Figure S3
the functionalization of each DPP derivative with the polar triethylene glycol side chains
enabled reversible electrochemical switching in both organic and aqueous media, suggesting
the suitability of these materials as channel materials for OECTs. As anticipated, p(gDPP-
MeOT2) showed the lowest onset of oxidation, followed by p(gDPP-T2) and p(gDPP-TT). The
IP calculated for p(gDPP-T2) and p(gDPP-TT) were also in good agreement with those
calculated for their alkylated counterparts,[39,40] with minor variations in the recorded IP values
attributed to differences in polymer-polymer and polymer-electrolyte interactions, which can
lead to altered microstructures and ion penetration, which in turn affect IP values.[18] The trends
in the onset of oxidation remained identical upon switching from an organic to an aqueous
electrolyte, whereby a relatively large shift in oxidation potential was noted. Similar findings
have been reported for alternative OECT channel materials; this was attributed to changes in
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both the size and charge density of the counter anion and differences in the solute-solvent
interactions between the polymer and the supporting electrolyte.[17,18,38,41–43]
Good agreement was found between the experimental IP and those calculated for
tetramers of the polymers by range separated density functional theory calculations (IP-tuned
ωB97XD/6-31G*). Geometries were optimized in the gas phase and showed similar dihedral
angles between common adjacent groups, see Table S1 and Figure S10 in the S.I. The
thiophene-thiophene link in p(gDPP-MeOT2) is planarized by S-O non-covalent interactions,
while the same dihedral in p(gDPP-T2) shows the typical 20° twist found between unsubstituted
thiophenes. IP energies were calculated as the difference in electronic energy of radical cation
and neutral states in an aqueous polarizable continuum, giving energies of 4.97, 5.00 and 4.67
eV for p(gDPP-TT), p(gDPP-T2) and p(gDPP-MeOT2) respectively. The similarity in IP
between p(gDPP-TT) and p(gDPP-T2) can be explained by visualization of the molecular
orbitals of the positive polaron species, see Figure S11 in the S.I. The orbital, which was
previously the highest occupied, but becomes unoccupied and with beta spin upon oxidation,
shows delocalization over the DPP group and the adjoining thiophene rings. However, the
equivalent orbital in p(gDPP-MeOT2) is localized on four thiophene rings instead due to the
more electron rich nature, leading to a shift in the IP.
The kinetics of the doping/dedoping process occurring during CV, when employing an
aqueous 0.1 М sodium chloride solution as the supporting electrolyte, were further evaluated by
monitoring the anodic peak current (Imax) against the scan rate (10-1250 mV s-1), whereby a
negligible charge transfer resistance between the electrode and the polymer film was assumed.
As shown in Figure S4, both p(gDPP-TT) and p(gDPP-T2) demonstrated a linear dependence
on the scan rate up to 250 mV s-1. At scan rates higher than 250 mV s-1 deviation from linearity
for both polymers was noted with the dependence of the peak current on the scan rate appearing
to assume a square root dependence on the scan rate, suggesting a diffusion limited current.[44,45]
On the other hand, p(gDPP-MeOT2) showed a linear dependency of the peak current on the
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scan rate across the entire probing regime (10-1250 mV s-1), suggesting that the dimethoxy
moiety in the polymer is responsible for overcoming these kinetic limitations at high scan rates,
giving this material a very rapid charge/discharge cycling properties. It is likely that the
decreased molecular order in p(gDPP-MeOT2), as suggested by thin film UV-Vis data and
grazing incidence wide-angle X-ray scattering measurements (discussed in more detail below),
was responsible for this behavior, since in glycolated materials ionic mobility is higher in less
ordered phases compared to more ordered or crystalline ones.[23,46] This property of p(gDPP-
MeOT2) is likely to be particularly beneficial in the context of recording short-lived biological
signals, such as neuronal action potentials.
The doping mechanism of the polymers was subsequently evaluated by means of
spectroelectrochemistry.[47] As shown in Figure 2b-d, p(gDPP-TT), p(gDPP-T2) and p(gDPP-
MeOT2) all showed a significant electrochromic response upon the application of a positive
bias to the polymer coated ITO electrode. For p(gDPP-TT) and p(gDPP-T2) both a suppression
of the π-π* absorption band around 800 nm with the concomitant appearance of a new
absorption feature at longer wavelengths was noted, eventually leading to the formation of an
isosbestic point, whereby the longer wavelength absorbing features were related to the
formation of polaronic/bipolaronic species of the polymer.[48,49] On the other hand, for p(gDPP-
MeOT2), due to detector limitations, it was only possible to observe a decrease in the π-π*
transition. Given, however, the similarity in the CV curves and the depression in the π-π*
transition recorded across the three polymers, we speculate that p(gDPP-MeOT2) is also able
to form analogous hole polarons upon applying a positive bias. The strong and reversible
electrochromic response of these materials in aqueous media was viewed as a good indicator
for the suitability of these materials as OECT channel materials, since both of these
measurements rely upon the reversible electrochemical doping of the evaluated materials.
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a
b
0.5
0.4
0.3
0.2
0.1
0.0
0.00 V
0.85 V
-0.20 V
p(gDPP-TT)
p(gDPP-T2)
p(gDPP-MeOT2)
0.6
0.4
0.2
0.0
-0.2
-0.4
-0.6
-0.2
0.0
0.2
0.4
0.6
0.8
1.0
400
600
800
1000
Potential vs Ag/AgCl (V)
Wavelength (nm)
c
d
0.4
0.5
0.4
0.3
0.2
0.1
0.0
0.00 V
-0.30 V
0.85 V
0.65 V
0.3
0.2
0.1
0.0
-0.20 V
-0.30 V
400
600
800
1000
400
600
800
1000
Wavelength (nm)
Wavelength (nm)
Figure 2. a) Cyclic voltammograms of p(gDPP-TT), p(gDPP-T2) and p(gDPP-MeOT2)
employing a 0.1 М NaCl(aq) electrolyte. Spectroelectrochemical behavior of b) p(gDPP-TT), c)
p(gDPP-T2) and d) p(gDPP-MeOT2) employing a 0.1 М NaCl(aq) electrolyte and working
electrode potential steps of 0.05 V.
Following the complete electrochemical evaluation of these materials, their thermal
properties were probed by thermogravimetric analysis (TGA) and differential scanning
calorimetry (DSC). The TGA and DSC traces recorded for the three semiconductors are found
in Figure S5. As can be seen, each material displayed excellent thermal stability up to 330 °C;
while the lack of any features in the DSC traces indicated no significant morphological changes
occurring when heating or cooling the samples. To further deepen our understanding of the
structural properties of the materials, grazing incidence wide-angle X-ray scattering (GIWAXS)
analysis was performed on spin-cast films of the polymers. As shown in Figure S6, all three
polymers displayed a strong in-plane (010) peak from π-π scattering and two clear orders of
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out-of-plane lamellar scattering peaks, (100) and (200). This confirms the presence of a
significant population of edge-on oriented crystallites. Peaks were fitted to extract peak
positions and calculate d-spacings, shown in Table 2. The lamellar d-spacings, calculated from
the (200) peak, were around 13 Å for all the polymers, implying either a high degree of torsion
across each EG unit as predicted by the gauche effect or interdigitation of the triethylene glycol
side chains. Similar π-π stacking distances around 3.55±0.03 Å were obtained for p(gDPP-TT),
p(gDPP-T2) and p(gDPP-MeOT2), which are comparable to the 3.5-3.6 Å sized π-π stacking
distances obtained for other high-performance EG functionalized organic semiconductors.[17,18]
The pronounced lamellar and π-π scattering peaks evidence a relatively high degree of order
for an oligoethylene glycol side chain conjugated polymer. This was manifested in coherence
lengths (Lc) in the π-π stacking direction, calculated from the FWHM of the (010) peak, of 10.2,
11.1, and 7.8 nm for p(gDPP-TT), p(gDPP-T2) and p(gDPP-MeOT2), respectively.[50]
Interestingly p(gDPP-TT) and p(gDPP-T2) both display a clear out of plane scattering
peak at ~1.2 Å-1. It is not an integer multiple of the lamellar peak and being that it is in the out-
of-plane direction it cannot be a backbone scattering peak from the main crystallite population.
It may be from scattering related to the triethylene glycol side chains, similar to alkyl analogs,
but as this is purely speculative the peak has not been indexed.[51,52] This peak is absent in the
methoxy substituted p(gDPP-MeOT2). The presence of methoxy groups quantitatively disrupts
the molecular packing, diminishing the π-π stacking coherence length, consistent with the thin-
film UV-vis spectra analysis that suggests a decreased molecular ordering relative to the other
polymers in the series.
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Table 2. GIWAXS analysis results of the polymers.
Lamellar stacking
π-π stacking
q(010) (Å-1) d (Å)
Polymer
Texture
q(200) (Å-1)
d (Å)
13.1
12.6
13.5
Lc (Å)
102.3
110.7
78.4
p(gDPP-TT)
p(gDPP-T2)
Edge-on
Edge-on
0.961
1.753
1.774
1.784
3.58
3.54
3.52
0.995
p(gDPP-MeOT2) Edge-on
0.933
OECTs were fabricated according to previously reported methods,[53] resulting in
channels with a 100 μm width, 10 μm length and similar thicknesses around 100 nm. In each
case, the organic semiconductor was deposited by spin-coating without the need for any solvent
additives or annealing treatments. Each device was operated employing a 0.1 М NaCl(aq)
supporting electrolyte and an Ag/AgCl pellet gate electrode. Full details of the OECT
fabrication and operating process can be found in the S.I. The output characteristics recorded
for each polymer can be found in Figure S7, while the transfer and transconductance curves
are reported in Figure 3a-b. The steady state p-type channel characteristics on the other hand
are summarized in Table 3.
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a
b
d
-1.2
p(gDPP-TT)
p(gDPP-T2)
p(gDPP-MeOT2)
p(gDPP-TT)
p(gDPP-T2)
p(gDPP-MeOT2)
6
4
2
0
-0.9
-0.6
-0.3
0.0
VD = -0.6 V
VD = -0.6 V
0.0
-0.2
-0.4
-0.6
-0.8
0.0
-0.2
-0.4
-0.6
-0.8
VG (V)
VG (V)
c
101
100
10-2
10-3
10-4
[mC*] = 125±22
[mC*] = 342±35
F cm-1 V-1 s-1
F cm-1 V-1 s-1
10-1
10-2
10-3
10-4
[mC*] = 57±5
F cm-1 V-1 s-1
PEDOT based
D-A polymers
p(gDPP-T2)
All-donor polymers
p(gDPP-TT)
p(gDPP-MeOT2)
p(gDPP-TT)
p(gDPP-T2)
p(gDPP-MeOT2)
10-6
10-5
10-4
101
102
C* (F cm-3)
103
(Wd/L)(VTh-VG) (cm V)
Figure 3. Representative a) transfer and b) transconductance curves recorded for a single
channel of p(gDPP-TT), p(gDPP-T2) and p(gDPP-MeOT2) respectively. In each case a VD
sweep rate of 0.60 V s-1 was employed. c) Extraction of [μC*] from the transconductance
against channel geometry and operation parameters plot. d) Comparison of the steady-state
OECT performance of p(gDPP-TT), p(gDPP-T2) and p(gDPP-MeOT2) against other p-type
OECT channel materials with reported μ and C* values. Data for PEDOT based materials taken
from references [24] and [54], data for all-donor polymers taken from references [24], [28] and
[31], data for D-A polymers taken from references [24] and [34].
Table 3. Summary of the steady-state characteristics recorded for OECTs fabricated with
p(gDPP-TT), p(gDPP-T2) and p(gDPP-MeOT2).
a
c
gm
C*b
μOECT
VTh
[μC*]d
d
On/
off
Polymer
(mS) (F cm-3) (cm2 V-1 s-1) (V) (F cm-1 V-1 s-1) (nm)
p(gDPP-TT)
p(gDPP-T2)
3.1±0.5
6.3±0.7
184
196
169
0.57±0.09 -0.54
1.55±0.17 -0.52
0.28±0.04 -0.26
125±22
342±35
57±5
123
90
105
105
104
p(gDPP-MeOT2) 1.4±0.2
82
a) Averaged values over all operational channels on the same device; b) values calculated from
EIS curves; c) calculated from the transistor saturation mobility using the respective C* values;
d) calculated from the slope of gm as a function of [(Wd/L)(VTh-VG)].
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As shown in Figure 3a-b, each of the polymers studied operates in accumulation mode
with the devices being off at a VG of 0 V and switching on upon the application of a negative
gate voltage. The average maximum transconductance recorded for p(gDPP-T2), p(gDPP-TT)
and p(gDPP-MeOT2) was 6.3±0.7 mS, 3.1±0.5 mS and 1.4±0.2 mS respectively and occurred
at a VG of -0.8 V, -0.8 V and -0.6 V respectively. The drain current recorded for p(gDPP-T2)
and p(gDPP-TT) became unstable at gate voltages lower than -0.8 V, thus meaning that the
reported maximum transconductance values are those affording the maximum stable
transconductance. The higher VG required for p(gDPP-T2) and p(gDPP-TT) to reach their
maximum transconductance were expected due to their higher IP, thus resulting in the devices
to turn ‘on’ at more negative VG. This trend was further reflected in the recorded VTh, which
followed the order of the polymers’ Eox,aq. A consequence of the polymers’ varying IP and VTh
were the on-off ratios and off currents recorded for OECTs based on the various polymers,
which have previously been employed to evaluate the propensity of polymers to undergo any
spontaneous oxygen reduction reaction (ORR).[34] Devices based on each polymer afforded on-
off ratios in the order of 104-105, see Figure S8, and were thus within the same range as those
reported for previously developed all-donor and D-A polymers.[17,18,31,33,34] The slightly better
on-off ratios obtained for p(gDPP-T2) and p(gDPP-TT) stemmed from their larger IP, >4.9 eV,
thereby thermodynamically disfavoring any spontaneous ORR.
To exclude any channel geometry effects, thus enabling for a fair and material dependent
comparison of the steady-state performance of each polymer, the polymers’ [μC*] were
measured, see Figure 3c.[24] The [μC*] recorded were 342±35 F cm-1 V-1 s-1, 125±22 F cm-1 V-
1
s-1 and 57±5 cm-1 V-1 s-1 for p(gDPP-T2), p(gDPP-TT) and p(gDPP-MeOT2) respectively.
As can be seen, p(gDPP-T2) incurred the highest steady-state OECT performance of the series,
followed by p(gDPP-TT) with a slightly lower one. p(gDPP-MeOT2) on the other hand incurred
the lowest steady-state OECT performance, roughly six-times lower than that of p(gDPP-T2).
A comparison of the polymers’ steady-state performance with those of previously published p-
14
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type OECT channel materials, with particular emphasis on previously reported all-donor and
D-A copolymers, is shown in Figure 3d. Figure 3d reveals that all of the three D-A polymers
developed in this study perform significantly better than the previously reported D-A
copolymers, affording similar steady-state performances to the all-donor polymers. Figure 3d
also indicates that p(gDPP-T2) is one of the currently highest performing OECT channel
materials.[21,22,24] Note that the steady-state performance of p(gDPP-T2) was obtained without
any additives or post-processing treatment, which is often required for other high-performing
channel materials.[21,23]
To better understand the origin of the performance difference across the channel
materials reported in this study, we decided to individually compare their C* and μ. C* was
obtained from electrochemical impedance spectroscopy (EIS) of polymer coated gold
electrodes while applying a gate voltage corresponding to the VG incurring the highest gm, see
Figure S9 in the S.I. For each of the polymers, a similar C* of ~180 F cm-3 was recorded,
indicating that each polymer can reach similar levels of bulk doping in aqueous electrolytes. In
comparison to the previously reported p-type active layer materials, the values obtained for C*
herein are amongst the highest.[24] It is also worthwhile to note that these C* values are around
50-400% higher than those of the commonly employed PEDOT derivatives.[24]
Given the similar C* achieved by the various polymers, the difference in their steady-
state performance was assumed to be a consequence of their differing charge carrier mobilities.
This hypothesis was confirmed by the mobility values reported in Table 3, with p(gDPP-T2)
showing the highest μ of 1.55±0.17 cm2 V-1 s-1, while p(gDPP-TT) and p(gDPP-MeOT2)
revealed lower values of 0.57±0.09 cm2 V-1 s-1 and 0.28±0.04 cm2 V-1 s-1 respectively. Given
the previous findings from thin-film UV-vis absorption spectroscopy and GIWAXS analysis
on the polymers, the higher charge carrier mobilities recorded for p(gDPP-T2) and p(gDPP-
TT) were attributed in part to their higher degree of molecular order. Further insights into the
higher μ recorded for p(gDPP-T2) and p(gDPP-TT) came from DFT calculations (tuned
15
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ωB97XD/6-31G*), beginning with a population analysis of the polymers’ molecular orbitals.
This approach has previously been used in the literature to explain differences in the redox
stability of glycolated semiconductors for OECT applications.[33,34] Key aspects of these
computational simulations are the necessity to first optimize the polymer geometry in the gas
phase, before optimizing the range separation parameter, ω, to satisfy Koopmans’ theorem.
This provides a realistic description of the charge delocalization and avoids the over-
delocalization errors of standard DFT functionals. The tetrameric polymer chains were then
subdivided into shorter fragments to determine the contribution of each fragment to the orbital
under consideration. The tetrameric polymer species were optimized in the gas phase at the
ωB97XD/6-31G* level. Polymer fragments were subsequently defined analogously to a
previous study on OECT channel materials, see Figure 4a, thus facilitating a comparison of the
DFT simulations across both sets of polymers.[34]
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a
b
p(gDPP-TT)
p(gDPP-T2)
p(gDPP-MeOT2)
0.25
0.20
0.15
0.10
0.05
0.00
1
2
3
4
5
6
7
8
1
2
3
4
5
6
7
8
1
2
3
4
5
6
7
8
Fragment
Fragment
Fragment
c
0.8
0.6
0.4
0.2
0.0
p(gDPP-TT)
p(gDPP-T2)
p(gDPP-MeOT2)
1
2
3
4
5
6
7
8
1
2
3
4
5
6
7
8
1
2
3
4
5
6
7
8
Fragment
Fragment
Fragment
Figure 4. a) Details regarding the fragmentation of the polymer chains into shorter segments;
DFT calculations of the b) HOMO orbital contribution of the various polymer fragments and c)
charge distribution over the various polymer fragments in the polymers’ radical cation form.
As can be seen from Figure 4b, p(gDPP-TT) and p(gDPP-T2) show similar orbital
distributions of their neutral polymer HOMO, with the HOMO having strong contributions over
five fragments (3-7). Conversely, p(gDPP-MeOT2) demonstrates a narrower HOMO
distribution, only extending over four fragments (3-6), suggesting a higher degree of
localization of the wavefunction. Moreover, whereas both p(gDPP-TT) and p(gDPP-T2)
display stronger contributions of the DPP-based fragments towards the HOMO, this is not the
case for p(gDPP-MeOT2), where the aryl (Ar) comonomer fragment dominates the orbital
contribution. Summation of the fragment orbital distributions further highlights this concept,
whereby the incorporation of the MeOT2 unit in p(gDPP-MeOT2) (60% Ar, 40% DPP) inverted
the orbital contribution relative to p(gDPP-TT) (40% Ar, 60% DPP) and p(gDPP-T2) (43% Ar,
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57% DPP). Similar findings were obtained for the vacant beta spin orbital of the radical cation,
see Figure S12 in the S.I. and were reaffirmed by Hirshfield charge analyses of the radical
cation form of the polymer, which has previously been used to model the polymers’ polaronic
species.[34] As follows from Figure 4c the charge in p(gDPP-TT) and p(gDPP-T2) was spread
to a greater extent and more evenly as compared to p(gDPP-MeOT2), where the positive charge
of the polymer resides almost entirely on a single fragment (4). More delocalized polarons may
contribute to higher charge mobility in two ways; firstly both intra- and inter-chain mobility is
expected to increase as the charge is spread out over more atoms, resulting in lower polaron
binding energy.[55] Secondly, the increased spread of charge provides a larger surface area for
charges to hop to adjacent polymer chains, increasing the interchain charge mobility. The above
analysis thus suggests a more delocalized nature of the polaron in p(gDPP-T2) and p(gDPP-
TT) compared to p(gDPP-MeOT2), which was attributed to the closer energy match between
the T2 and TT moieties with the DPP unit compared to MeOT2. This aspect is particularly
interesting to note for the molecular design of future conjugated polymers for OECT
applications featuring a D-A conjugated backbone, as it would suggest that donor and acceptor
units that are more closely matched in terms of their electron-densities and hence energy levels,
should partially be responsible for yielding materials with better charge transport properties.
This hypothesis could potentially also form part of the explanation why a higher μ of 0.28±0.04
cm2 V-1 s-1 was recorded for p(gDPP-MeOT2) compared to its pyridine-flanked DPP analogue,
p(gPyDPP-MeOT2), for which a μ of 0.030±0.007 cm2 V-1 s-1 was reported.[34] A fair
comparison can be made between these materials, especially given that the two polymers
featured very similar degrees of lamellar ordering and π-π stacking distances (both around 3.5
Å), as well as a lack of any significant vibronic fine-structure in their UV-vis thin film spectra.
We speculate that this design concept of reducing the energy level offset between the donor and
acceptor fragments may also hold true for n-type OECT polymers, thus explaining the higher
OECT mobility achieved by the all-acceptor polymer BBL over the naphthalene bisimide-co-
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bithiophene (NDI-T2) based D-A copolymers reported up-to-date.[19,27,56] Finally, this could
also partially explain why all-donor based glycolated polymers have thus far been very
successful p-type OECT channel materials, given the closer match in energy levels between
their comonomers.[17,18,22]
Having fully characterized the steady-state performance of the OECTs fabricated with
the various polymers, device operational stability was evaluated next. In this context, the
percentage retention of the initial current as a function of OECT switching cycles (ID/ID,0) is
typically employed as figure-of-merit.[18,22,34,35,56,57] Consequently, we chose to also evaluate
the operational stability of our materials by this metric. Specifically, this involved switching
the devices between their ‘on’ and ‘off’ state for 5 s each and measuring the ID/ID,0 over 100
min of continuous electrochemical cycling (~600 doping/dedoping cycles). To best compare
the operational stability of the polymers, we wanted to ensure that similar doping levels were
achieved within each polymer in their ‘on’ state.[58] Consequently, the VG applied for the various
polymers was -0.1 V smaller than their corresponding VTh. The operational stability of the
polymers is summarized in Figure S13. Figure S13 demonstrates that the polymers had
significantly different electrochemical cycling profiles, with p(gDPP-TT), p(gDPP-T2) and
p(gDPP-MeOT2) incurring 9%, 53% and 99% retention of the initial ‘on’ current after 100 min
of continuous electrochemical cycling. The relative operational stability of the polymers under
long-term cycling was ascribed to the different overall electron density on the polymer
backbones’, i.e. the polymers’ IPs, whereby decreasing the polymers’ IP resulted in
progressively more stable devices due to better stabilization of the resulting hole polaron, thus
corroborating previous literature findings.[33,34,58]
Conclusion
In summary, three polymers for accumulation mode p-type OECTs have been
developed, all of which were based on a common diketopyrrolopyrrole unit. As confirmed by
19
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CV, each of the synthesized polymers can undergo stable and reversible doping, hence
suggesting the suitability of the diketopyrrolopyrrole moiety for OECTs. This was further
confirmed by spectroelectrochemistry, which indicated a reversible suppression in the π-π*
transition and a concomitant formation of an absorption feature at longer wavelengths, which
was indicative of the reversible formation of charged species of the polymers.
GIWAXS analysis of the materials highlighted their semicrystalline nature, whereby
p(gDPP-T2) and p(gDPP-TT) were found to display a higher degree of order compared to
p(gDPP-MeOT2). These findings were further reflected in thin-film UV-Vis spectroscopy, with
p(gDPP-T2) and p(gDPP-TT) both showing vibrational fine-structures in their absorption
spectra, that were absent in the case of p(gDPP-MeOT2).
Subsequent OECT fabrication and analysis demonstrated good OECT performance for
each of the materials developed, incurring [µC*] values between 57-342 F cm-1 V-1 s-1, which
compare well with the highest performing ethylene glycol functionalized semiconductors
reported to date and represent a significant improvement to previously developed D-A
copolymers for OECTs. Out of the polymers developed, p(gDPP-T2) and p(gDPP-TT)
performed the highest, whereby their increased performance was attributed partially to their
higher degree of order in their solid state. Importantly, DFT simulations of the polymers
indicated p(gDPP-T2) and p(gDPP-TT) to also have significantly more delocalized hole
polarons compared to p(gDPP-MeOT2), which was ascribed to the closer energy level match
between the T2 and TT units with the thiophene-flanked DPP core, compared to the MeOT2
comonomer. Consequently, these findings highlight the significance of the degree of
delocalization of polarons in OECT channel materials. More importantly, we demonstrate how
this aspect can be tuned directly at the molecular level by selection of the building blocks
comprising the polymers’ conjugated backbone, thus paving the way for the development of
even higher performing D-A copolymers for OECT applications.
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Acknowledgements
The authors acknowledge generous funding from KAUST for financial support. The
research reported in this publication was sponsored by funding from King Abdullah University
of Science and Technology Office of Sponsored Research (OSR) under awards no. OSR-2018-
CARF/CCF-3079, no. OSR-2015-CRG4-2572 and OSR-4106 CPF2019. We acknowledge EC
FP7 Project SC2 (610115), EC H2020 (643791), and EPSRC Projects EP/G037515/1,
EP/M005143/1, and EP/L016702/1. B.P. and J.R. gratefully acknowledge support from the
National Science Foundation Grant No. NSF DMR-1751308. The authors would like to thank
Joseph Strzalka and Qingteng Zhang for beam line assistance. This research used resources of the
Advanced Photon Source, a U.S. Department of Energy (DOE) Office of Science User Facility
operated for the DOE Office of Science by Argonne National Laboratory under Contract No. DE-
AC02-06CH11357.
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10.1002/anie.202014078
Angewandte Chemie International Edition
Table of Contents
μ
h+
D-A Engineering for
Improved μOECT
μ
h+
Monomer selection in organic mixed ionic-electronic conduction polymers is shown to directly
impact their polaron delocalization and consequently their performance in (bio-)electronic
devices, thus outlining molecular design guidelines for the development of next generation
high-performance materials.
26
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