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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

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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.
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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-
(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
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
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
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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The properties of semiconducting polymers are strongly influenced by their aggregation behavior, that is, their aggregate fraction and backbone planarity. However, tuning these properties, particularly the backbone planarity, is challenging. This work introduces a novel solution treatment to precisely control the aggregation of semiconducting polymers, namely current-induced doping (CID). It utilizes spark discharges between two electrodes immersed in a polymer solution to create strong electrical currents resulting in temporary doping of the polymer. Rapid doping-induced aggregation occurs upon every treatment step for the semiconducting model-polymer poly(3-hexylthiophene). Therefore, the aggregate fraction in solution can be precisely tuned up to a maximum value determined by the solubility of the doped state. A qualitative model for the dependences of the achievable aggregate fraction on the CID treatment strength and various solution parameters is presented. Moreover, the CID treatment can yield an extraordinarily high quality of backbone order and planarization, expressed in UV-vis absorption spectroscopy and differential scanning calorimetry measurements. Depending on the selected parameters, an arbitrarily lower backbone order can be chosen using the CID treatment, allowing for maximum control of aggregation. This method may become an elegant pathway to finely tune aggregation and solid-state morphology for thin-films of semiconducting polymers.
... 11 As a result, signicant efforts have been dedicated to developing conjugated polymers with different side chains to improve the solubility and backbone modication for tuning the planarity and inter-chain stacking. [12][13][14][15] Among various conjugated polymer families, polythiophene derivatives show the most promising optical and electrical characteristics where the electrical conductivity is signicantly improved through structural modications, doping strategies, and by controlling the microstructure through process optimization. [16][17][18][19][20][21] The electrical conductivity of the most commonly used poly(3-hexylthiophene-2,5-diyl) (P3HT) doped with 2,3,5,6-tetrauoro-7,7,8,8-tetracyanoquinodimethane (F 4 TCNQ) is around 10 S cm −1 , 22 which increases to 100 S cm −1 when the P3HT structure is modied with ethylene glycol side chains. ...
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Absorption spectra of doped conjugated polymer films provide valuable information on the degree of crystallinity, doping efficiency, material composition, and film thickness. The absorption spectral features commonly observed in doped polymers are due to intra-, inter-chain excitons, exciton–phonon coupling, polarons, and bipolarons that are branched differently in films prepared at different process parameters and doping conditions. Thus, the spectral features of thin films can be used to monitor and tune process parameters. However, probing spectral information at a point does not provide complete information on the solution-processed films where film characteristics are significantly influenced by uncontrolled process parameters. Hyperspectral imaging (HSI) is a high throughput spectral diagnostic method that provides the spatial distribution of spectral features where the process-induced variations of thin film quality and their influence on final performance metrics can be effectively analysed. In this report, we present a methodology for diagnosing thin film characteristics using the HSI technique by implementing automated spectral feature extraction and visualisation. For this study, we used the well-established F4TCNQ-doped regio regular poly-3-hexyl thiophene (P3HT) film as a model system and show film quality parameters, such as variation in film thickness, homogeneity of materials composition, degree of crystallinity and polaron concentration. We also present a generic process flow for the rapid screening of thin film and process optimization using the HSI technique.
... 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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Charge transport in molecular solids, such as semiconducting polymers, is strongly affected by packing and structural order over several length scales. Conventional approaches to modeling these phenomena range from analytical models to numerical models using quantum mechanical calculations. While analytical approaches cannot account for detailed structural effects, numerical models are expensive for exhaustive (and statistically significant) analysis. Here, we report a computationally scalable methodology using graph theory to explore the influence of molecular ordering on charge mobility. This model accurately reproduces the analytical results for transport in nematic and isotropic systems, as well as experimental results of the dependence of the charge carrier mobility on orientation correlation length for polymers. We further model how defect distribution (correlated and uncorrelated) in semiconducting polymers can modify the mobility, predicting a critical defect density above which the mobility plummets. This work enables rapid (and computationally extensible) evaluation of charge mobility semiconducting polymer devices.
... In the study of CP films, incident angle dependent XANES is generally used to target the C K-edge to distinguish different components and determine the packing direction 297−300 as well as fine grained microstructure in the film. 28,301,302 In addition, a high energy resolution XANES library has been established for commonly used conjugated polymers. 303 Recently, in situ XANES on the S K-edge has been employed on the interconversion of lithium−sulfur compounds during the charging and discharging of lithium−sulfur batteries. ...
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Operando characterization plays an important role in revealing the structure–property relationships of organic mixed ionic/electronic conductors (OMIECs), enabling the direct observation of dynamic changes during device operation and thus guiding the development of new materials. This review focuses on the application of different operando characterization techniques in the study of OMIECs, highlighting the time-dependent and bias-dependent structure, composition, and morphology information extracted from these techniques. We first illustrate the needs, requirements, and challenges of operando characterization then provide an overview of relevant experimental techniques, including spectroscopy, scattering, microbalance, microprobe, and electron microscopy. We also compare different in silico methods and discuss the interplay of these computational methods with experimental techniques. Finally, we provide an outlook on the future development of operando for OMIEC-based devices and look toward multimodal operando techniques for more comprehensive and accurate description of OMIECs.
Side chains of conjugated polymers (CPs) have played a vital role in the regulation of solution processibility and semiconducting performance. Herein, carbosilane side chains, which manifest the advantages of low-cost and facile synthesis as compared to the common alkyl ones, were employed to regulate the aggregation behaviors and charge transport properties of isoindigo-based polymers. Three polymers, IIDSiC12, IIDSiC8, and IIDC12, with carbosilane side chains of varying bulkiness and alkyls as the control were synthesized via direct arylation polymerization. Anisotropic one-dimensional pre-aggregates of different aspect ratios were developed in solution, as disclosed by solution small-angle neutron scattering, which further determined the order of the microstructure in bar-coated films for organic thin-film transistors (OTFTs). It is noteworthy that the relatively large radius (20.5 Å) and moderate aspect ratio (∼20) of IIDSiC12 solution aggregates ensured a solid-state microstructure of the highest order, as evidenced by well-aligned fiber-like morphology and the largest range of packing order. As a result, OTFTs based on IIDSiC12 achieved the maximum saturation electron mobility up to 4.10 cm2 V-1 s-1 with a reliability factor of 81%, which is among the highest mobilities of n-type CPs. The results reported here may encourage the easier synthesis of high mobility CPs using the carbosilane side chain engineering.
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Largely soluble tris(pentafluorophenyl)borane (BCF) has recently emerged as a promising molecular dopant for preparing highly conductive organic thermoelectrics (TEs) using a one‐step solution‐mixing method. However, the unique doping mechanisms that include both undesirable Lewis acid doping with BCF and effective Brønsted acid doping with BCF–water complexes limit its widespread applications. Herein, the feasibility of modulating the two doping mechanisms by utilizing the competitive Lewis acid–base interactions of BCF with H2O or Lewis basic groups in conjugated polymers is demonstrated. The polymer without strong Lewis basic groups undergoes Brønsted acid doping, which efficiently forms delocalized free charge carriers and leads to superior TE power factors and figures of merit of 49.6 µW m⁻¹ K⁻² and 0.061, respectively. However, the polymer with strong Lewis basic groups undergoes both doping mechanisms competitively. BCF–polymer Lewis complexes not only generate free charge carriers inefficiently, but also hinder possible Brønsted acid doping and localize charge carriers, significantly lowering the TE properties. Nevertheless, the reduced TE properties can be dramatically improved by thermally annealing the predoped polymer films because Lewis acid doping can be substantially replaced by Brønsted acid doping owing to the different thermal stabilities between the BCF–polymer and BCF–water complexes.
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The stiff backbones of conjugated polymers can lead to a rich phase behavior that includes both crystalline and liquid crystalline phases, making measurements of the glass transition challenging. In this work, the glass transitions of regioregular poly(3-hexylthiophene-2,5-diyl) (RR P3HT), regiorandom (RRa) P3HT, and poly((9,9-bis(2-octyl)-fluorene-2,7-diyl)-alt-(4,7-di(thiophene-2-yl)-2,1,3-benzothiadiazole)-5′,5″-diyl) (PFTBT) are probed by linear viscoelastic measurements as a function of molecular weight. We find two glass transition temperatures (Tg’s) for both RR and RRa P3HT and one for PFTBT. The higher Tg, Tα, is associated with the backbone segmental motion and depends on the molecular weight, such that the Flory–Fox model yields Tα = 22 and 6 °C in the long chain limit for RR and RRa P3HT, respectively. For RR P3HT, a different molecular weight dependence of Tα is seen below Mn = 14 kg/mol, suggesting this is the typical molecular weight of intercrystal tie chains. The lower Tg (TαPE ≈ −100 °C) is associated with the side chains and is independent of molecular weight. RRa P3HT exhibits a lower Tα and higher TαPE than RR P3HT, possibly due to a different degree of nanophase separation between the side chains and the backbones. In contrast, PFTBT only exhibits one Tg above −120 °C, at 144 °C in the long chain limit.
A longer exciton pathway Organic semiconductors typically exhibit exciton diffusion lengths on the order of tens of nanometers. Jin et al. prepared nanofibers from block polymers consisting of emissive polyfluorene cores surrounded by coronas of polyethylene glycol and polythiophene (see the Perspective by Holmes). Excitons generated in the polyfluorene cannot enter the polyethylene glycol layer and so diffuse more than 200 nm. This distance can be tuned by varying the length of the polyethylene glycol—a feature that could potentially be exploited in the development of organic devices such as photovoltaics. Science , this issue p. 897 ; see also p. 854
Several recent reports have demonstrated that fluorinated analogues of donor/acceptor copolymers surpass nonfluorinated counterparts in terms of performance in electronic devices. Using a copolymer series consisting of fluorinated, partially fluorinated, and nonfluorinated benzotriazole, we confirm that the addition of fluorine substituents beneficially impacts charge transport in polymer semiconductors. Transistor measurements demonstrated a factor of 5 increase in carrier mobilities with the degree of fluorination of the backbone. Furthermore, grazing-incidence X-ray diffraction data indicates progressively closer packing between the conjugated cores and an overall greater amount of π-stacking in the fluorinated materials. It is likely that attractive interactions between the electron-rich donor and fluorinated electron-deficient acceptor units induce very tightly stacking crystallites, which reduce the energetic barrier for charge hopping. In addition, a change in crystallite orientation was observed from primarily edge-on without fluorine substituents to mostly face-on with fluorinated benzotriazole.
Carbon 1s Near Edge X-ray Absorption Fine Structure (NEXAFS) and UV-vis spectroscopy are used to examine differences between highly aggregated and poorly aggregated forms of the polymer poly(3-hexylthiophene) (P3HT), based on as-cast and annealed regio-random and regio-regular P3HT samples. UV-vis spectra show characteristic signatures of unaggregated P3HT in regio-random P3HT, and of H-aggregation in regio-regular P3HT samples. Distinct spectroscopic differences, including energy shifts, are observed in the NEXAFS spectra of aggregated P3HT relative to the unaggregated forms. These differences are reproduced with Transition – Potential Density Functional Theory (TP-DFT) calculations which explore aggregation and molecular confirmation. Differences in the NEXAFS spectra of P3HT are assigned to thiophene backbone twisting in the unaggregated forms of P3HT, and to various degrees of chain planarization in aggregated forms of P3HT that also correlate to the exciton bandwidth. This opens up the prospect of charactering conformation and related difficult to assess structural details with a new tool.
Despite rapid advances in the field of nonfullerene polymer solar cells (NF-PSCs), successful examples of random polymer-based NF-PSCs are limited. In this study, it is demonstrated that random donor polymers based on thieno[2′,3′:5′,6′]pyrido[3,4-g]thieno[3,2-c]isoquinoline-5,11(4H,10H)-dione (TPTI) containing two simple thiophene (T) and bithiophene (2T) electron-rich moieties (PTTI-Tx) can be promising materials for the fabrication of highly efficient NF-PSCs. With negligible influence on optical bandgaps and energy levels, the crystalline behavior of PTTI-Tx polymers was modulated by varying the T:2T ratio in the polymer backbone; this resulted in the formation of different microstructures upon blending with a nonfullerene m-ITIC acceptor in NF-PSCs. In particular, a PTPTI-T70:m-ITIC system enabled favorable small-scale phase separation with an increased population of face-on oriented crystallites, thereby boosting the processes of effective exciton dissociation and charge transport in the device. Consequently, the highest power conversion efficiency of 11.02% with an enhanced short-circuit current density of 17.12 mA cm−2 is achieved for the random polymer-based NF-PSCs thus far. These results indicate that random terpolymerization is a simple and practical approach for the optimization of a donor polymer toward highly efficient NF-PSCs.
Fluorinated conjugated polymers leading to enhanced photovoltaic device performance has been widely observed in a variety of donor-acceptor copolymers; however, almost all these polymers have fluorine substituents on the acceptor unit. Building upon our previously reported PBnDT-FTAZ, a fluorinated donor-acceptor conjugated polymer with impressive device performance, we set this study to explore the effect of adding the fluorine substituents onto the flanking thiophene units between the donor unit (BnDT) and the acceptor unit (TAZ). We developed new synthetic approaches to control the position of the fluorination (3’ or 4’) on the thiophene unit, and synthesized four additional PBnDT-TAZ polymers incorporating the fluorine-substituted-thiophene (FT) units, 3’-FT-HTAZ, 4’-FT-HTAZ, 3’-FT-FTAZ and 4’-FT-FTAZ. We discover that relocating the fluorine substituents from the acceptor to the flanking thiophene units have negligible impact on the device characteristics (short circuit current, open circuit voltage, and fill factor) when comparing 4’-FT-HTAZ with the original FTAZ. Combining these two fluorination approaches together, 4’-FT-FTAZ shows even higher device performance than FTAZ (7.7% vs. 6.6%) with active layers over 200 nm in thickness. Furthermore, high values of fill factor ~ 70% are all achieved for photovoltaic devices based on 3’-FT-HTAZ, 4’-FT-HTAZ or 4’-FT-FTAZ, ascribed to the observed high hole mobilities (over 1 × 10-3 cm2/Vs) in these devices. Our study offers a new approach to utilize the fluorinated thiophene units in developing new conjugated polymers to further improve the device performance of polymer solar cells.
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.
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.
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).