Conjugated Polymer Systems Research Articles

Efficient heterojunction solar cells based on the synergy between planarity and dipole moment in fluorinated-thienothiophenes-based donor-acceptor polymers

Polymers consisting of benzodithiophene, dithienopyrrole derivative donor monomers and thiophene derivative acceptor units are synthesized. The effects of F on optical performance for several Donor-Acceptor conjugated polymer systems are investigated. The introduction of F is conducive to increasing the dipole moment of the molecule for effective intramolecular charge transmission. The incorporation of F in acceptor unit improves molecular planarity simultaneously. It is found that dipole moment and molecular planarity play a synergistic role in photovoltaic performance. Under the premise of ensuring the planarity, the dipole moment is enhanced, the charge transfer within the molecule can also be strengthened, thus, the short-circuit current is greatly increased. The theoretical calculations show and explain the effect of the F on the dipole moment. Finally, a PCE of 4% is achieved with a large short-circuit current density of 14.36 mA cm−2. These results represent a synergy between planarity and dipole moment for improving the PCE.

Theoretical study on naphthobischalcogenadiazole conjugated polymer systems and C61 derivative as organic photovoltaic semiconductors

We investigated the charge-transfer reactions of solar cells including a quaterthiophene copolymer with naphtho-bis-thiadiazole (PNTz4T) and naphtho-bis-oxadiazole (PNOz4T) using constrained density functional theory (CDFT). According to our calculations, the high electron-transfer rate results in a highly efficient solar cell, and the stable charge-transfer state results in low energy loss. Our computations imply that the following three factors are crucial to improve the performance of semiconducting polymers: (i) large structural changes following charge-transfer, (ii) narrow band gap, and (iii) spatially delocalized lowest unoccupied molecular orbital (LUMO) of the ground state.

Proton radiation effects of conjugated polymer thin films

ABSTRACTPolymeric-thin-film-based electronic and optoelectronic materials and devices are attractive for potential space and certain radiation-related applications due to their inherent features such as lightweight, flexible, biocompatible, etc. Proton radiation is a major form of ionizing radiation in space, particularly in the so-called inner Van Allen belt region where most near-earth satellites are orbiting, yet very few literature and data are available on proton radiation effects on conjugated polymer systems. In this report, the proton radiation effects on potential electronic/optoelectronic properties of several conjugated polymers and their composites are briefly evaluated. Specifically, UV–VIS absorption spectra of several conjugated polymers and/or their composite thin films were measured and compared right before and after the proton radiations at different dosages. The results revealed that proton radiation has very little or insignificant impact up to 800 rad on the optoelectronic properties of poly-3-hexyl-thiophene (P3HT), P3HT:PC60BM blend, a light harvesting donor-bridge-acceptor (DBA) and a novel donor-bridge-fluorinated-acceptor (DBfA)-type block copolymer thin films.

Proton Radiation Studies on Conjugated Polymer Thin Films

ABSTRACTPolymeric thin film based electronic and optoelectronic materials and devices are attractive for potential space and certain radiation related applications due to their inherent features such as being light weight, flexible, biocompatible and environmental friendly, etc. Proton radiation is a major form of ionizing radiation in space, yet very few literature and data are available on proton radiation effects on conjugated polymer systems. In this study, UV-Vis absorption spectra of several conjugated polymers and/or their composite thin films were measured and compared right before and after a 200 MeV proton beam irradiation at different dosages, and the results revealed that proton radiation has very little or negligible impact up to 800 Rads on the optoelectronic properties of several polymers and their composite thin films.

Perspective: Optical spectroscopy in π-conjugated polymers and how it can be used to determine multiscale polymer structures.

Exciton delocalization in conjugated polymer systems is determined by polymer conformations and packing. Since exciton delocalization determines the photoluminescent vibronic progression, optical spectroscopy provides an indirect link to polymer multiscale structures. This perspective describes our current theoretical understanding of how exciton delocalization in π-conjugated polymers determines their optical spectroscopy and further shows how exciton delocalization is related to conformational and environmental disorder. If the multiscale structures in conjugated polymer systems are known, then using first-principles modeling of excitonic processes it is possible to predict a wide-range of spectroscopic observables. We propose a reverse-engineering protocol of using these experimental observables in combination with theoretical and computational modeling to determine the multiscale polymers structures, thus establishing quantitative structure-function predictions.

Nucleation, Growth, and Alignment of Poly(3-hexylthiophene) Nanofibers for High-Performance OFETs.

Conjugated semiconducting polymers have been the subject of intense study for over two decades with promising advances toward a printable electronics manufacturing ecosystem. These materials will deliver functional electronic devices that are lightweight, flexible, large-area, and cost-effective, with applications ranging from biomedical sensors to solar cells. Synthesis of novel molecules has led to significant improvements in charge carrier mobility, a defining electrical performance metric for many applications. However, the solution processing and thin film deposition of conjugated polymers must also be properly controlled to obtain reproducible device performance. This has led to an abundance of research on the process-structure-property relationships governing the microstructural evolution of the model semicrystalline poly(3-hexylthiophene) (P3HT) as applied to organic field effect transistor (OFET) fabrication. What followed was the production of an expansive body of work on the crystallization, self-assembly, and charge transport behavior of this semiflexible polymer whose strong π-π stacking interactions allow for highly creative methods of structural control, including the modulation of solvent and solution properties, flow-induced crystallization and alignment techniques, structural templating, and solid-state thermal and mechanical processing. This Account relates recent progress in the microstructural control of P3HT thin films through the nucleation, growth, and alignment of P3HT nanofibers. Solution-based nanofiber formation allows one to develop structural order prior to thin film deposition, mitigating the need for intricate deposition processes and enabling the use of batch and continuous chemical processing steps. Fiber growth is framed as a traditional crystallization problem, with the balance between nucleation and growth rates determining the fiber size and ultimately the distribution of grain boundaries in the solid state. Control of nucleation can be accomplished through a sonication-based seeding procedure, while growth can be modulated through supersaturation control via the tuning of solvent quality, the use of UV irradiation or through aging. These principles carry over to the flow-induced growth of P3HT nanofibers in a continuous microfluidic processing system, leading to thin films with significantly enhanced mobility. Further gains can be made by promoting long-range polymer chain alignment, achieved by depositing nanofibers through shear-based coating methods that promote high fiber packing density and alignment. All of these developments in processing were carried out on a standard OFET platform, enabling us to generalize quantitative structure-property relationships from structural data sources such as UV-vis, AFM, and GIWAXS. It is shown that a linear correlation exists between mobility and the in-plane orientational order of nanofibers, as extracted from AFM images using advanced computer vision software developed by our group. Herein, we discuss data-driven approaches to the determination of process-structure-property relationships, as well as the transferability of structural control strategies for P3HT to other conjugated polymer systems and applications.

Effect of Polymer–Fullerene Interaction on the Dielectric Properties of the Blend

It is commonly believed that large dielectric constants are required for efficient charge separation in polymer photovoltaic devices. However, many polymers used in high‐performance solar cells do not possess high dielectric constants. In this work, the effect of polymer–fullerene interactions on the dielectric environment of the active layer blend and the device performance for several donor–acceptor conjugated polymer systems is investigated. It is found that, while none of the high‐performing polymers studied has a dielectric constant value larger than 3, all polymer–fullerene blends have a significantly larger dielectric constant compared to their pristine constituents. Additionally, it is found that the blend dielectric constant reaches a maximum value in fully optimized devices. Using PTB7:PC71BM blends as an example, it is showed that, in addition to a small increase in the dielectric constant, devices fabricated using the optimum processing additive concentration exhibit almost 3X larger excited state polarizability. This large increase in excited state polarizability results in a substantial difference in short‐circuit current and ultimately device performance. The results show that the excited state polarizability critically depends on polymer–fullerene interactions, and can be a leading indicator of device performance for a given material system.

Supramolecular Conjugated Polymer Systems with Controlled Antibacterial Activity.

Infections of antibiotic-resistant pathogens have caused a series of public health crises across the world. According to the latest published reports, an antibiotic switch has been recognized as a potential strategy to control antibacterial activity for combating this serious drug resistance. Thus, it is anticipated that more effective antibiotic switches should be obtained by further exploring the developed strategies. Here, we report an improved pretreatment strategy using a surfactant (Triton X-100) for constructing an effective supramolecular antibiotic switch based on a poly(fluorene-co-phenylene) derivative (PFP) and cucurbit[7]uril (CB[7]), which can regulate the aggregation state of polymers before the supramolecular self-assembly process occurs. Triton X-100 can regulate the aggregation states of conjugated polymers, which is used to successfully realize the reversible control of bactericidal activity of PFP in the dark and under white light irradiation by supramolecular assembly/disassembly between PFP and CB[7]. Specialized antibiotic switches are significantly important to fight pathogenic infections and solve the drug resistance crisis in the future.

Multilevel Investigation of Charge Transport in Conjugated Polymers.

Conjugated polymers have attracted the world's attentions since their discovery due to their great promise for optoelectronic devices. However, the fundamental understanding of charge transport in conjugated polymers remains far from clear. The origin of this challenge is the natural disorder of polymers with complex molecular structures in the solid state. Moreover, an effective way to examine the intrinsic properties of conjugated polymers is absent. Optoelectronic devices are always based on spin-coated films. In films, polymers tend to form highly disordered structures at nanometer to micrometer length scales due to the high degree of conformational freedom of macromolecular chains and the irregular interchain entanglement, thus typically resulting in much lower charge transport properties than their intrinsic performance. Furthermore, a subtle change of processing conditions may dramatically affect the film formation-inducing large variations in the morphology, crystallinity, microstructure, molecular packing, and alignment, and finally varying the effective charge transport significantly and leading to great inconsistency over an order of magnitude even for devices based on the same polymer semiconductor. Meanwhile, the charge transport mechanism in conjugated polymers is still unclear and its investigation is challenging based on such complex microstructures of polymers in films. Therefore, how to objectively evaluate the charge transport and probe the charge transport mechanism of conjugated polymers has confronted the world for decades. In this Account, we present our recent progress on multilevel charge transport in conjugated polymers, from disordered films, uniaxially aligned thin films, and single crystalline micro- or nanowires to molecular scale, where a derivative of poly(para-phenylene ethynylene) with thioacetyl end groups (TA-PPE) is selected as the candidate for investigation, which could also be extended to other conjugated polymer systems. Our systematic investigations demonstrated that 3-4 orders higher charge transport properties could be achieved with the improvement of polymer chain order and confirmed efficient charge transport along the conjugated polymer backbones. Moreover, with downscaling to molecular scale, many novel phenomena were observed such as the largely quantized electronic structure for an 18 nm-long TA-PPE and the modulation of the redox center of tetrathiafulvalene (TTF) units on tunneling charge transport, which opens the door for conjugated polymers used in nanometer quantum devices. We hope the understanding of charge transport in PPE and its related conjugated polymer at multilevel scale in this Account will provide a new method to sketch the charge transport properties of conjugated polymers, and new insights into the combination of more conjugated polymer materials in the multilevel optoelectronic and other related functional devices, which will offer great promise for the next generation of electronic devices.

Effect of the Pauli exclusion principle on the singlet exciton yield in conjugated polymers

Optical devices fabricated using conjugated polymer systems give rise to singlet exciton yields which are high compared to the statistically predicted estimate of 25 % obtained using simple recombination schemes. In this study, we evaluate the singlet exciton yield in conjugated polymers systems by fitting to a model that incorporates the Pauli exclusion principle. The rate equations which describe the exciton dynamics include quantum dynamical components (both density and spin-dependent) which arise during the spin-allowed conversion of composite intra-molecular excitons into loosely bound charge-transfer (CT) electron-hole pairs. Accordingly, a crucial mechanism by which singlet excitons are increased at the expense of triplet excitons is incorporated in this work. Non-ideal triplet excitons which form at high densities, are rerouted via the Pauli exclusion mechanism to form loosely bound CT states which subsequently convert to singlet excitons. Our derived expression for the yield in singlet exciton incorporates the purity measure and provides a realistic description of the carrier dynamics at high exciton densities.

Elastomer–Polymer Semiconductor Blends for High-Performance Stretchable Charge Transport Networks

An inverse relationship between mechanical ductility and mobility/molecular ordering in conjugated polymer systems was determined definitively through systematic interrogation of poly(3-hexylthiophene) (P3HT) films with varied degrees of molecular ordering and associated charge transport performance. The dilemma, whereby molecular ordering required for efficient charge transport conclusively undermines the applicability of these materials for stretchable, flexible device applications, was resolved using a polymer blend approach. Specifically, the molecular interactions between dissimilar polymer materials advantageously induced semiconducting polymer ordering into efficient π–π stacked fibrillar networks. Changes in the molecular environment surrounding the conjugated polymer during the elastomer curing process further facilitated development of high mobility networked semiconductor pathways. A processed P3HT: poly(dimethylsiloxane) (PDMS) composite afforded a semiconducting film that exhibits superior du...

Synthesis of conducting azopolymers by electrochemical grafting of a diazonium salt at polypyrrole electrodes

In this paper we report a new strategy for obtaining conducting azopolymers having pendant azobenzene moieties via functionalization of polypyrrole by electrochemical reduction of 4-[(4′-nitrophenyl)azo]benzenediazonium tetrafluoroborate (NABDBF4−). The functionalization of polypyrrole films with 4′-nitroazobenzene (NAB) groups has been evaluated by cyclic voltammetry and by spectroscopic techniques such as XPS, Raman and UV–vis spectroscopy. Moreover, the third-order nonlinear optical response of NAB-functionalized polypyrrole has been investigated using the optical third-harmonic generation technique. All these investigations confirmed the presence at the polymeric films surface of the functional groups introduced by the electrochemical reduction of diazonium salt. The versatility of this methodology allows not only the functionalization of polypyrrole-type materials with chromophore groups, but can also be successfully applied to other conjugated polymer systems in order to create various photoresponsive materials.

Self-Assembly of Conjugated Polymers Into Ring Structures by Breath Figures

A simple method to fabricate rings of conjugated polymers is presented based on the breath figure technique. In humid atmosphere, the rapid evaporation of solvent induces the cooling of the solution surface of conjugated polymers. Water droplets condense onto the solution surface and undergo coalescence to form larger ones. After the solvent and water droplets evaporate completely, isolated rings are formed. Our results show that it is a general method to produce ring structure in different conjugated polymer systems. The influences of solvent properties, solution concentrations, etc. on the pattern formation are investigated. Compared with the conventional honeycomb structure obtained by breath figures, the key to the formation of ring structure is discussed. Furthermore, ring structure can be tuned from circular to elliptic by applying moist air flow with controlled direction onto the surface of the polymer solution. This approach offers a complimentary way to pattern conjugated polymers in a simple, cost-effective, and one-step matter.

Singlet–Singlet Exciton Recombination: Theoretical Insight into the Influence of High Density Regime of Excitons in Conjugated Polymers

Using a modified version of the Su-Schrieffer-Heeger (SSH) model combined with the extended Hubbard model (EHB), the recombination between a singlet exciton pair is investigated under the influence of an external electric field, electron-electron interactions, and temperature effects in the scope of a nonadiabatic evolution method. The excitons are positioned very close to each other in a way to mimic a high-density region in monomolecular conjugated polymer systems. Results show that there are mainly three possible channels resulting from singlet-singlet exciton recombination: (1) forming an excited negative polaron and an excited positive bipolaron, (2) forming two free and excited oppositely charged polarons, and (3) forming a biexciton. These results suggest that the recombination processes critically depends on the condition imposed to the system. The description of this dependence, as carried out in the present work, may provide guidance to improve the generation of free charge carriers in organic optoelectronic devices.

Incorporation of benzocarborane into conjugated polymer systems: synthesis, characterisation and optoelectronic properties

We present the novel 1,4-difunctionalisation of benzocarborane with organometallic groups suitable for cross-coupling and its subsequent insertion for the first time into conjugated polymer backbones. Copolymers with solubilised cyclopentadithiophene and diketopyrrolopyrrole derivatives were prepared by Stille polymerisation in good molecular weight. The physical, material and optoelectronic properties of the resulting polymers were investigated, demonstrating that benzocarborane acts similarly to a stabilised, electron-deficient cis-diene linker. We also report the first polymer field effect transistors incorporating a benzocarborane in the backbone.

Uniaxial macroscopic alignment of conjugated polymer systems by directional crystallization during blade coating

We present a single step, solution based method to deposit polymer films with uniaxial orientation and demonstrate its potential by fabricating polarization sensitive photovoltaic devices.

Impurity effects on polaron-exciton formation in conjugated polymers

Combining the one-dimensional tight-binding Su-Schrieffer-Heeger model and the extended Hubbard model, the collision of two oppositely charged polarons is investigated under the influence of impurity effects using a non-adiabatic evolution method. Results show that electron-electron interactions have direct influence on the charge distribution coupled to the polaron-exciton lattice defect. Additionally, the presence of an impurity in the collisional process reduces the critical electric field for the polaron-exciton formation. In the small electric field regime, the impurity effects open three channels and are of fundamental importance to favor the polaron-exciton creation. The results indicate that the scattering between polarons in the presence of impurities can throw a new light on the description of electroluminescence in conjugated polymer systems.

Charge Redistribution at GaAs/P3HT Heterointerfaces with Different Surface Polarity

The nature of charged photoexcitations at the interface of highly delocalized inorganic crystals and more localized conjugated polymer systems is of great fundamental interest for a number of hybrid photovoltaic applications. Here we study the interaction between mainstream compound semiconductor GaAs and conjugated polymer P3HT by means of density functional theory simulations. When considering both nonpolar GaAs(110) and polar GaAs(111)B surfaces, we find that polarity of the GaAs surface strongly affects the electronic orbitals and charge redistribution: electrons are efficiently transferred to GaAs substrates, implying the formation of hybrid delocalized states at the interface. Furthermore, P3HT can act as an “acceptor” for GaAs(111)B via hole transfer from GaAs valence band states. Overall the intrinsic surface dipole moment of GaAs surfaces is enhanced by the charge displacement induced by adsorbed P3HT. These theoretical predictions correlate well with energy alignments derived by ultraviolet photoelectron spectroscopy and provide a robust methodology for the design of polymer/III–V heterointerfaces that optimize photovoltaic performance.

Impurity effects and temperature influence on the exciton dissociation dynamics in conjugated polymers

We investigate temperature effects on exciton dissociation dynamics in conjugated polymer systems. Using a modified version of the tight-binding Su–Schrieffer–Heeger model, the dissociation is studied under the influence of impurity effects with a nonadiabatic evolution method. Our results show that temperature effects reduce the critical electric field for the exciton dissociation. In the small temperature regime, the exciton is not trapped by the impurity and it is observed to perform a random walk, a fact not observed in the absence of temperature. This letter might enlighten the description of electroluminescence yields and charge transport efficiency in organic based electronic devices.

Fluorescence Intensity Modulation in Photochromic Conjugated Polymer Systems

AbstractThe ability to modulate fluorescence intensity with a light signal enables a variety of applications based on fluorescence imaging. One approach to fluorescence photomodulation involves using a photochromic moiety that responds to a light signal in conjunction with a nearby fluorophore. We employ conjugated polymers based on poly(p‐phenylene vinylene) (PPV) as the fluorophore in photochrome‐fluorophore systems for fluorescence modulation. Advantages of using conjugated polymers for this purpose include their intrinsic energy migration processes that enable amplified fluorescence quenching as well as their processability. Here we present examples of PPV‐based photomodulation systems that employ photochromic dyes from three common photochromic families: azobenzenes, spironaphthoxazines, and diarylethenes. In all cases we observe reversible fluorescence quenching due to fluorescence resonance energy transfer to the photogenerated form of the photochrome. Examples of the photomodulation of photochromic PPV systems in organic solution, polymer films, and conjugated polymer nanoparticles are presented.