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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.
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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
... 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. ...
Full-text available
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.
Molecular packing and texture of semiconducting polymers are often critical to the performance of devices using these materials. Although frameworks exist to quantify the ordering, interpretations are often just qualitative, resulting in imprecise use of terminology. Here, we reemphasize the significance of quantifying molecular ordering in terms of degree of crystallinity (volume fractions that are ordered) and quality of ordering and their relation to the size scale of an ordered region. We are motivated in part by our own imprecise and inconsistent use of terminology in the past, as well as the need to have a primer or tutorial reference to teach new group members. We strive to develop and use consistent terminology with regards to crystallinity, semicrystallinity, paracrystallinity, and related characteristics. To account for vastly different quality of ordering along different directions, we classify paracrystals into 2D and 3D paracrystals and use paracrystallite to describe the spatial extent of molecular ordering in 1-10 nm. We show that a deeper understanding of molecular ordering can be achieved by combining grazing-incidence wide-angle X-ray scattering and differential scanning calorimetry, even though not all aspects of these measurements are consistent, and some classification appears to be method dependent. We classify a broad range of representative polymers under common processing conditions into five categories based on the quantitative analysis of the paracrystalline disorder parameter (g) and thermal transitions. A small database is presented for 13 representative conjugated and insulating polymers ranging from amorphous to semi-paracrystalline. Finally, we outline the challenges to rationally design more perfect polymer crystals and propose a new molecular design approach that envisions conceptual molecular grafting that is akin to strained and unstrained hetero-epitaxy in classic (compound) semiconductors thin film growth.
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The thermomechanical behavior of polymer semiconductors plays an important role in the processing, morphology, and stability of organic electronic devices. However, donor–acceptor‐based copolymers exhibit complex thermal relaxation behavior that is not well understood. This study uses dynamic mechanical analysis (DMA) to probe thermal relaxations of a systematic set of polymers based around the benzodithiophene (BDT) moiety. The loss tangent curves are resolved by fitting Gaussian functions to assign and distinguish different relaxations. Three prominent transitions are observed that correspond to: i) localized relaxations driven primarily by the side chains (γ ), ii) relaxations along the polymer backbone (β ), and iii) relaxations associated with aggregates (α ). The side chains are found to play a clear role in dictating Tγ, and that mixing the side chain chemistry of the monomer to include alkyl and oligo(ethylene glycol) moieties results in splitting the γ ‐relaxation. The β relaxations are shown to be associated with backbone elements along with the monomer. In addition, through processing, it is shown that the α‐relaxation is due to aggregate formation. Finally, it is demonstrated that the thermal relaxation behavior correlates well with the stress–strain behavior of the polymers, including hysteresis and permanent set in cyclically stretched films. The molecular origin of the complex relaxation behavior in donor‐acceptor copolymers is described. Through studying a systematic set of polymers, thermal relaxations can be ascribed to side chains, features along the backbone, and aggregates. The results also indicate the lack of a glass transition. It is then demonstrated that these relaxations provide a basis for designing polymer semiconductors for stretchable applications.
A bulk heterojunction in organic solar cells is where charge separation and recombination occur. Molecular orientation at the interface is one of the key factors that dictate solar cell efficiency. Although X-ray scattering-based methods can determine donor/acceptor domain orientations between an anisotropic phase and an isotropic fullerene-based phase, the rise of non-fullerene solar cells presents a new challenge in delineating local molecular directions at the interface between two anisotropic donor/acceptor domains. Here, we determine interfacial molecular orientations of three high-efficiency small molecule solar cells (ZR1:Y6, B1:BO-4Cl, and BTR:BO-4Cl) using polarization-selective transient absorption spectroscopy. The polarization anisotropy of charge separation dynamics indicates an angle of ∼90° between ZR1 and Y6 molecules at the interface, an angle close to 0° between B1 and BO-4Cl, and random orientations between BTR and BO-4Cl. These observations provide complementary information to X-ray scattering measurements and highlight polarization-selective transient absorption spectroscopy as a tool to probe interfacial structure and dynamics of key photophysical steps in energy conversion.
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Donor–acceptor (D–A) type semiconducting polymers have shown great potential for the application of deformable and stretchable electronics in recent decades. However, due to their heterogeneous structure with rigid backbones and long solubilizing side chains, the fundamental understanding of their molecular picture upon mechanical deformation still lacks investigation. Here, the molecular orientation of diketopyrrolopyrrole (DPP)‐based D–A polymer thin films is probed under tensile deformation via both experimental measurements and molecular modeling. The detailed morphological analysis demonstrates highly aligned polymer crystallites upon deformation, while the degree of backbone alignment is limited within the crystalline domain. Besides, the aromatic ring on polymer backbones rotates parallel to the strain direction despite the relatively low overall chain anisotropy. The effect of side‐chain length on the DPP chain alignment is observed to be less noticeable. These observations are distinct from traditional linear‐chain semicrystalline polymers like polyethylene due to distinct characteristics of backbone/side‐chain combination and the crystallographic characteristics in DPP polymers. Furthermore, a stable and isotropic charge carrier mobility is obtained from fabricated organic field‐effect transistors. This study deconvolutes the alignment of different components within the thin‐film microstructure and highlights that crystallite rotation and chain slippage are the primary deformation mechanisms for semiconducting polymers.
Conjugated polymers are rapidly emerging as an attractive class of semiconductors for next-generation electronics thanks to their low-cost, high-throughput solution processability, mechanical flexibility, stretchability, self-healing properties, and ability to interface and communicate with biological systems. Accordingly, the last four decades has seen a surge of studies that have provided seminal contributions to the thorough understanding of conjugated polymers. One of the key factors that dictates the electronic performance of conjugated polymers is their assembly and crystallization behavior, which has remained intriguing and challenging to study. The complex solution processing environment and rapid kinetics strongly couple with the conjugated polymer assembly process, further complicating a full mechanistic picture. In this perspective, we summarize the charge transport mechanism, fundamentals of conjugated polymer assembly, and solution printing. We further discuss central strategies that have been developed to control and enhance their multiscale assembly during solution printing. Finally, we hope that our perspective will stimulate more studies on how processing can control morphology and charge transport of conjugated polymers and applications of these concepts to other advanced functional materials.
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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.
Easily processed materials with the ability to transport excitons over length scales of more than 100 nanometers are highly desirable for a range of light-harvesting and optoelectronic devices. We describe the preparation of organic semiconducting nanofibers comprising a crystalline poly(di-n-hexylfluorene) core and a solvated, segmented corona consisting of polyethylene glycol in the center and polythiophene at the ends. These nanofibers exhibit exciton transfer from the core to the lower-energy polythiophene coronas in the end blocks, which occurs in the direction of the interchain π-π stacking with very long diffusion lengths (>200 nanometers) and a large diffusion coefficient (0.5 square centimeters per second). This is made possible by the uniform exciton energetic landscape created by the well-ordered, crystalline nanofiber core.
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).