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Competition between Exceptionally Long‐Range Alkyl Sidechain Ordering and Backbone Ordering in Semiconducting Polymers and Its Impact on Electronic and Optoelectronic Properties

DOI:10.1002/adfm.201806977
Authors:
Joshua Carpenter at North Carolina State University
Joshua Carpenter
Masoud Ghasemi at North Carolina State University
Masoud Ghasemi
Eliot Gann at National Institute of Standards and Technology
Eliot Gann
Indunil Angunawela
Indunil Angunawela
Indunil Angunawela
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Abstract and Figures

Intra‐ and intermolecular ordering greatly impacts the electronic and optoelectronic properties of semiconducting polymers. The interrelationship between ordering of alkyl sidechains and conjugated backbones has yet to be fully detailed, despite much prior effort. Here, the discovery of a highly ordered alkyl sidechain phase in six representative semiconducting polymers, determined from distinct spectroscopic and diffraction signatures, is reported. The sidechain ordering exhibits unusually large coherence lengths (≥70 nm), induces torsional/twisting backbone disorder, and results in a vertically multilayered nanostructure with ordered sidechain layers alternating with disordered backbone layers. Calorimetry and in situ variable temperature scattering measurements in a model system poly{4‐(5‐(4,8‐bis(3‐butylnonyl)‐6‐methylbenzo[1,2‐b:4,5‐b′]dithiophen‐2‐yl)thiophen‐2‐yl)‐2‐(2‐butyloctyl)‐5,6‐difluoro‐7‐(5‐methylthiophen‐2‐yl)‐2H‐benzo[d][1,2,3]triazole} (PBnDT‐FTAZ) clearly delineate this competition of ordering that prevents simultaneous long‐range order of both moieties. The long‐range sidechain ordering can be exploited as a transient state to fabricate PBnDT‐FTAZ films with an atypical edge‐on texture and 2.5× improved field‐effect transistor mobility. The observed influence of ordering between the moieties implies that improved molecular design can produce synergistic rather than destructive ordering effects. Given the large sidechain coherence lengths observed, such synergistic ordering should greatly improve the coherence length of backbone ordering and thereby improve electronic and optoelectronic properties such as charge transport and exciton diffusion lengths.
a) Structure of FTAZ. b–e) Conceptualization of the competition in ordering between sidechains and backbones in FTAZ. The long‐range 2D crystalline order of the sidechain layers in the drop‐cast samples induces torsional disorder in the backbone, preventing intermolecular π‐stacking, as shown at an angle in (b) and along the backbone direction in (c). When the highly ordered sidechain phase has melted, the backbones are no longer as torsionally constrained and can π‐stack more efficiently, though with a much shorter coherence length than exhibited by the sidechain crystals, as shown at an angle in (d) and along the backbone direction in (e).
a) Structure of FTAZ. b–e) Conceptualization of the competition in ordering between sidechains and backbones in FTAZ. The long‐range 2D crystalline order of the sidechain layers in the drop‐cast samples induces torsional disorder in the backbone, preventing intermolecular π‐stacking, as shown at an angle in (b) and along the backbone direction in (c). When the highly ordered sidechain phase has melted, the backbones are no longer as torsionally constrained and can π‐stack more efficiently, though with a much shorter coherence length than exhibited by the sidechain crystals, as shown at an angle in (d) and along the backbone direction in (e).
… 
Incident angle dependent TEY NEXAFS spectra of a) drop‐cast and b) spin‐cast FTAZ using p‐polarized X‐rays, normalized to 320 eV, where 90° is normal incidence. Insets show full measured energy range. d) Micro‐UV–vis spectra of an optically aggregated and nonaggregated region of the drop‐cast FTAZ film are shown with c) locations where the spectra were measured shown in a microscopy image.
Incident angle dependent TEY NEXAFS spectra of a) drop‐cast and b) spin‐cast FTAZ using p‐polarized X‐rays, normalized to 320 eV, where 90° is normal incidence. Insets show full measured energy range. d) Micro‐UV–vis spectra of an optically aggregated and nonaggregated region of the drop‐cast FTAZ film are shown with c) locations where the spectra were measured shown in a microscopy image.
… 
2D GIWAXS patterns of FTAZ films a) spin‐cast and b) drop‐cast from CB measured 7 days after film drying. c) In‐plane and out‐of‐plane azimuthally averaged GIWAXS intensity versus q profiles for both are shown. d) 2D GIWAXS pattern of drop‐cast FTAZ (measured at a second beamline after 40 days of storage at RT). e) Zoomed‐in portion of the out‐of‐plane low‐q region of the scattering pattern in (b), while f) zoomed‐in portion of the in‐plane high‐q region of (b). g) The zoomed‐in, in‐plane high‐q portion of (d). In (c), (e), (f), and (g), the green arrows highlight the location of in‐plane features in qxy and the pink arrows highlight the location of out‐of‐plane features in qz. The log intensity color scale (shown) was scaled for each pattern to make the features most visible. The scale has the same values between (b) and (f), but different between (b) and (e).
2D GIWAXS patterns of FTAZ films a) spin‐cast and b) drop‐cast from CB measured 7 days after film drying. c) In‐plane and out‐of‐plane azimuthally averaged GIWAXS intensity versus q profiles for both are shown. d) 2D GIWAXS pattern of drop‐cast FTAZ (measured at a second beamline after 40 days of storage at RT). e) Zoomed‐in portion of the out‐of‐plane low‐q region of the scattering pattern in (b), while f) zoomed‐in portion of the in‐plane high‐q region of (b). g) The zoomed‐in, in‐plane high‐q portion of (d). In (c), (e), (f), and (g), the green arrows highlight the location of in‐plane features in qxy and the pink arrows highlight the location of out‐of‐plane features in qz. The log intensity color scale (shown) was scaled for each pattern to make the features most visible. The scale has the same values between (b) and (f), but different between (b) and (e).
… 
a) 2D GIWAXS pattern of FTAZ drop‐cast from TCB at the first temperature step during in situ heating (35 °C). b) 2D GIWAXS pattern of FTAZ drop‐cast from TCB and after annealing and cooling to RT. Azimuthally averaged (±2°) GIWAXS intensity versus q profiles in the direction of the most intense sidechain scattering peak (at 1.5 Å⁻¹, 12° from in‐plane relative to the beam center) at varying temperature for the FTAZ film drop‐cast from TCB are shown at c) low q and d) high q. e) DSC thermograms of FTAZ drop‐cast from CB and TCB in the temperature range of the sidechain melting peak, plotted with the normalized sidechain (q ≈ 1.5 Å⁻¹, labeled high q) and layer spacing (q ≈ 0.15 Å⁻¹, labeled low q) peak areas at varying temperature as calculated from peak fits to the profiles in (c) and (d). The dashed vertical line at 47 °C indicates the temperature of the DSC melting peak of the TCB cast film.
a) 2D GIWAXS pattern of FTAZ drop‐cast from TCB at the first temperature step during in situ heating (35 °C). b) 2D GIWAXS pattern of FTAZ drop‐cast from TCB and after annealing and cooling to RT. Azimuthally averaged (±2°) GIWAXS intensity versus q profiles in the direction of the most intense sidechain scattering peak (at 1.5 Å⁻¹, 12° from in‐plane relative to the beam center) at varying temperature for the FTAZ film drop‐cast from TCB are shown at c) low q and d) high q. e) DSC thermograms of FTAZ drop‐cast from CB and TCB in the temperature range of the sidechain melting peak, plotted with the normalized sidechain (q ≈ 1.5 Å⁻¹, labeled high q) and layer spacing (q ≈ 0.15 Å⁻¹, labeled low q) peak areas at varying temperature as calculated from peak fits to the profiles in (c) and (d). The dashed vertical line at 47 °C indicates the temperature of the DSC melting peak of the TCB cast film.
… 
a) In‐plane GIWAXS intensity versus q profiles of FTAZ spin‐cast and drop‐cast from TCB, before and after annealing. b) Normalized area of the in‐plane π‐stacking peak for each sample as fit to the profiles. c) Typical OFET transfer curves (Vd = −100 V) in the saturation regime for the different film preparation conditions. d) Average mobilities of n > 30 devices for each condition extracted from transfer curves, with one standard deviation error.
+1
a) In‐plane GIWAXS intensity versus q profiles of FTAZ spin‐cast and drop‐cast from TCB, before and after annealing. b) Normalized area of the in‐plane π‐stacking peak for each sample as fit to the profiles. c) Typical OFET transfer curves (Vd = −100 V) in the saturation regime for the different film preparation conditions. d) Average mobilities of n > 30 devices for each condition extracted from transfer curves, with one standard deviation error.
… 
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FULL PAPER
www.afm-journal.de
© 2018 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
1806977 (1 of 13)
Competition between Exceptionally Long-Range
Alkyl Sidechain Ordering and Backbone Ordering in
Semiconducting Polymers and Its Impact on Electronic
and Optoelectronic Properties
Joshua H. Carpenter, Masoud Ghasemi, Eliot Gann, Indunil Angunawela, Samuel J. Stuard,
Jeromy James Rech, Earl Ritchie, Brendan T. O’Connor, Joanna Atkin, Wei You,
Dean M. DeLongchamp, and Harald Ade*
Intra- and intermolecular ordering greatly impacts the electronic and optoelectronic
properties of semiconducting polymers. The interrelationship between ordering
of alkyl sidechains and conjugated backbones has yet to be fully detailed, despite
much prior effort. Here, the discovery of a highly ordered alkyl sidechain phase
in six representative semiconducting polymers, determined from distinct spec-
troscopic and diffraction signatures, is reported. The sidechain ordering exhibits
unusually large coherence lengths (≥70 nm), induces torsional/twisting back-
bone disorder, and results in a vertically multilayered nanostructure with ordered
sidechain layers alternating with disordered backbone layers. Calorimetry and in
situ variable temperature scattering measurements in a model system poly{4-(5-
(4,8-bis(3-butylnonyl)-6-methylbenzo[1,2-b:4,5-b′]dithiophen-2-yl)thiophen-2-yl)-
2-(2-butyloctyl)-5,6-difluoro-7-(5-methylthiophen-2-yl)-2H-benzo[d][1,2,3]triazole}
(PBnDT-FTAZ) clearly delineate this competition of ordering that prevents simulta-
neous long-range order of both moieties. The long-range sidechain ordering can be
exploited as a transient state to fabricate PBnDT-FTAZ films with an atypical edge-
on texture and 2.5× improved field-effect transistor mobility. The observed influence
of ordering between the moieties implies that improved molecular design can pro-
duce synergistic rather than destructive ordering effects. Given the large sidechain
coherence lengths observed, such synergistic ordering should greatly improve the
coherence length of backbone ordering and thereby improve electronic and opto-
electronic properties such as charge transport and exciton diffusion lengths.
DOI: 10.1002/adfm.201806977
Dr. J. H. Carpenter, Dr. M. Ghasemi, I. Angunawela, S. J. Stuard,
Prof. H. Ade
Department of Physics and Organic and Carbon Electronics
Lab (ORaCEL)
North Carolina State University
Raleigh, NC 27695, USA
E-mail: harald_ade@ncsu.edu
Dr. E. Gann, Dr. D. M. DeLongchamp
Materials Science and Engineering Division
National Institute of Standards and Technology
100 Bureau Drive, Gaithersburg, MD 20899, USA
J. J. Rech, E. Ritchie, Prof. J. Atkin, Prof. W. You
Department of Chemistry
University of North Carolina at Chapel Hill
Chapel Hill, NC 27599-3290, USA
Prof. B. T. O’Connor
Department of Mechanical and Aerospace Engineering and ORaCEL
North Carolina State University
Raleigh, NC 27695, USA
insulating alkyl sidechains, cause complex
and competing phenomena in terms of
material phase behavior,[1,2] morphology
formation,[3–6] and local ordering,[7–12] that
inherently affect material performance.
Results in the literature for SCPs that are
generally more crystalline and have sim-
pler backbone structures, such as poly-
akylthiophenes (PATs), demonstrate that
ordering in the backbone and
π
-stacking
directions are not necessarily strongly cor-
related with ordering in the alkyl stacking
direction or ordering between alkyl side-
chains, especially in materials having
sufficiently high molecular weight.[13,14]
More broadly, it has been shown that
phase separation between backbone and
alkyl sidechain is a rather general trait
among SCPs and that ordering within
the alkyl nanodomains can occur even
when the nanodomains of the more rigid
backbones are amorphous, e.g., in regio-
random PATs.[15,16] Whether sidechains
can readily order independently of the
aromatic backbones and form separate
nanophases when monomer structures
and sidechain attachments are highly
asymmetric, as is often the case for amorphous donor–acceptor
(D–A) copolymers, is currently an open question.
Studies of ordering in SCPs have shown correlations between
backbone ordering and both charge transport and spectroscopic
Organic Electronics
The ORCID identification number(s) for the author(s) of this article
can be found under https://doi.org/10.1002/adfm.201806977.
1. Introduction
The two dissimilar constituents of semiconducting polymers
(SCPs), the semiconducting conjugated backbone and
Adv. Funct. Mater. 2019, 29, 1806977
... [13] Depending on the driving forces of the aggregation process, different directions of molecular ordering contribute to a different extent, enabling a higher quality of order in one direction at the cost of the quality of order in one or both other directions. [13,14] As transport properties are mostly influenced by backbone ordering, an aggregation mechanism leading to a high quality of backbone order is of interest. ...
... In this case, sidechain ordering and backbone ordering are both contributing, and the quality of the backbone order suffers in a more polar solvent. [13,14] Compared with the non-chlorinated solvents, CID treatment in CF leads to exceptionally high backbone ordering despite possessing a higher polarity (δ p = 3.1 MPa) than o-xylene and toluene. [36] One reason causing an increased planarization during the CID treatment could be an exceptionally high and sufficiently long-lived doping ratio achieved in CF. ...
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... In the ordered regions, transport may occur through extended electronic states whereas in the disordered regions charges are thought to move by hopping between localized sites 3,4 . The ability of high resolution transmission electron microscopy (HR-TEM) [5][6][7][8][9][10][11] and X-ray scattering methods [12][13][14] to reveal the detailed morphology of semiconducting polymers presents an opportunity to reveal how ordered and disordered regions impact charge transport. The challenge is to model how charge transport occurs between these two regions, which can guide the design of new polymers and processing routes to achieve higher carrier mobilities. ...
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Article
  • Dec 2016
The aggregation of π-conjugated materials significantly impacts on the photophysics and performance of optoelectronic devices. Nevertheless, little is known about the laws governing aggregate formation of π-conjugated materials from solution. In this perspective article, we compare, discuss and summarize how aggregates form for three different types of compounds, that is, homopolymers, donor-acceptor type polymers and low molecular weight compounds. To this end, we employ temperature dependent optical spectroscopy, which is a simple yet powerful tool to investigate aggregate formation. We show how optical spectra can be analysed to identify distinct conformational states. We find aggregate formation to proceed alike in all these compounds by a coil-to-globule like first order phase transition. Notably, the chain expands before it collapses into a highly ordered dense state. The role of side chains and the impact of changes in environmental polarization is addressed.
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About different packing states of alkyl groups in comb-like polymers with rigid backbone
Article
  • Oct 2016
We report a comparative X-ray diffraction study on three series of comb-like polymers with rigid backbones and layered morphologies [regio-regular poly(3-alkyl thiophenes), alkoxylated polyesters, alkoxylated polyphenylenevinylenes] highlighting the importance of the volume per methylene unit VCH2 in alkyl nanodomains for the overall packing state. We demonstrate that there is a large (≈30%) variation in the VCH2 values for different polymer series and packing states but no significant change in VCH2 depending on the length of the alkyl side groups. This calls into question commonly used structural models which are based only on tilting and interdigitation of ideally stretched alkyl side groups. We argue that a linear dependence of the layer spacings with side chain length can also be explained by a constant VCH2 value and unchanged main chain packing. The potential importance of side chain packing for the occurrence of different (liquid-) crystalline modifications in various polymer series and possible interrelations between main and side chain packings are discussed.
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Side-Chain-Induced Rigid Backbone Organization of Polymer Semiconductors through Semi-fluoroalkyl Side Chains
Article
  • Dec 2015
While high-mobility p-type conjugated polymers have been widely reported, high-mobility n-type conjugated polymers are still rare. In the present work, we designed semi-fluorinated alkyl side chains and introduced them into naphthalene diimide-based copolymers (PNDIF-T2 and PNDIF-TVT). We found that the strong self-organization of these side chains induced a high degree of order in the attached polymer backbones by forming a superstructure composed of "backbone crystals" and "side-chain crystals". This phenomenon was shown to greatly enhance the ordering along the backbone direction, and the resulting polymers thus exhibited unipolar n-channel transport in field-effect transistors with remarkably high electron mobility values of up to 6.50 cm(2) V(-1) s(-1) and with a high on-off current ratio of 10(5).
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