Conjugated Polymer Molecules Research Articles

Probing an Isolated Conjugated Polymer Molecule with Radiation-Generated Spin-Correlated Polaron Pairs.

An understanding of the interplay between the spin and electronic degrees of freedom of polarons migrating along conjugated polymer molecules is required to further the development of organic electronics and spintronics. In this study, a novel experimental approach is proposed for studying spin-correlated polaron pairs (PPs) on an isolated molecule of a conjugated polymer. The polymer molecule of interest is immobilized in a nonluminescent poly(vinyl chloride) matrix, which is irradiated with X-rays to rapidly form secondary PPs on the conjugated polymer. The migration, recombination, and evolution of the spin state of the PPs can be monitored at nanosecond resolution by observing the recombination fluorescence under different magnetic fields. Examples supporting this concept are presented.

Electronic energy transfer in single conjugated polymer molecules revealed by phase-modulated pulse-pair controlled single molecule spectroscopy

Detailed understanding of the electronic energy transfer dynamics in conjugated polymer molecules and their conformation dependence is central for improving the photophysical properties as well as the performance of devices based on conjugated polymers. In this work, we demonstrate simultaneous measurement of the absorption and emission sites in single conjugated polymer poly[2,7-(9,9-dioctyluorene)-alt-4,7-bis(thiophen-2-yl)benzo-2,1,3-thiadiazole] (PFO-DBT) molecules based on polarization-resolved confocal fluorescence microscopy with excitation of phase-modulated ultrashort pulse pairs. The evolution of absorbing chromophores can be derived by modulating the relative phase between ultrashort pulse pairs and extracting modulation information in phase-dependent fluorescence. Meanwhile, the emitting chromophore can be measured by polarization-resolved emission. Simultaneous absorption and emission measurements give new insights into the evolution of energy transfer pathways in individual conjugated polymer molecules. The results suggest that the conformation of single conjugated polymer chains can be influenced by solvents. Single PFO-DBT conjugated molecules spin-cast from toluene solution have relatively fixed absorption and emission dipole moments. In contrast, single conjugated polymer molecules prepared with chloroform show multichromophore behavior that is responsible for distribution of absorption and emission in a single chain. The proposed scheme paves the way for further understanding of conformation dependent photophysical properties and the possible role of quantum effects in the energy transfer pathway in both natural and artificial light harvesting systems in the nanoscale.

“High-entropy polymers”: A new route of polymer mixing with suppressed phase separation

Alloys and ceramics of multi-component compositions and hence having high entropy at random solution states recently received focused attentions. The present work emulates the “high-entropy” concept by using a common solvent to prepare polymer blends from a number of polymers, which shows interesting unique phenomena and promising properties. By investigating the heterogeneous domain size (Λ) of thin solid films prepared by spin coating, we found that de-mixing, commonly observed in polymer blends, can be suppressed when the number of polymer species (n) in the blend increased. In binary blends (n=2), de-mixing occurred, manifesting a wide spread of Λ strongly influenced by the inter-segmental enthalpy and chain length. However, as n increased, the Λ distribution shrank sharply by a declining upper edge, indicating significant de-mixing suppression. The suppression is attributable to high mixing entropy and a kinetic steric effect blocking like-polymer aggregation during film formation. The de-mixing suppression was found effective in dispersing poly(9,9’-dioctylfluorene), a conjugated polymer (CP), in the optically inert n = 5 blend where the CP molecules remained well separated, as shown spectroscopically, for CP fractions up to 50 wt.%, manifesting excellent efficiency performance.

'High-Entropy Polymers': A New Mixing Route of Suppressed Phase Separation

Alloys and ceramics of multi-component compositions and hence having high entropy at random solution states recently received focused attentions. The present work emulates the “high-entropy” concept by using a common solvent to prepare polymer blends from a number of polymers, which shows interesting unique phenomena and promising properties. By investigating the heterogeneous domain size (Λ) of thin solid films prepared by spin coating, we found that de-mixing, commonly observed in polymer blends, can be suppressed when the number of polymer species (n) in the blend increased. In binary blends (n=2), de-mixing occurred, manifesting a wide spread of Λ strongly influenced by the inter-segmental enthalpy and chain length. However, as n increased, the Λ distribution shrank sharply by a declining upper edge, indicating significant de-mixing suppression. The suppression is attributable to high mixing entropy and a kinetic steric effect blocking like-polymer aggregation during film formation. The de-mixing suppression was found effective in dispersing poly(9,9’-dioctylfluorene), a conjugated polymer (CP), in the optically inert n=5 blend where the CP molecules remained well separated, as shown spectroscopically, for CP fractions up to 50 wt.%, manifesting excellent efficiency performance.

Strongly localized states in a single carbon nanotube with polymer molecules

A single-walled carbon nanotube (SWCNT) with conjugated polymer molecules is analyzed via optical spectroscopy. The presence of strongly localized excitonic states in the SWCNT is confirmed using time-integrated photoluminescence (PL). The PL spectrum exhibits extremely narrow width (~0.8 meV) which is attributed to the strong confinement of the states by polymer molecules. In addition, I observed that the excited states are gradually filled as a function of the excitation power, which supports the localized excitonic behavior. Only the ground excitonic state is observed at low excitation powers, but three additional PL peaks appear as the excitation power is increased. Especially, the power-dependent PL spectrum shows a blueshift and increased width, which can be elucidated in terms of quantum confined stark effect and the screening of induced electric fields. Overall, I demonstrate that the presence of polymer molecules induces several localized states in a single SWCNT.

Triplet Population Dynamics of Single Conjugated Polymer Molecules and Nanoscale Assemblies

The formation of long-lived triplet excited electronic states has important ramifications for conjugated organic materials used in optoelectronic devices. In the case of polymers, unravelling vario...

Accounting for π–π stacking interactions in the mesoscopic models of conjugated polymers

Accounting for π–π interactions between conjugated polymer molecules via dynamic bonds allows their self-assembly into a lamellar morphology with π–π stacks.

Real-time measurement of dynamic evolution of absorption and emission properties of chromophores in single conjugated polymer molecules

Conjugated polymers have been widely used in optical sensors, light-emitting diodes and solar cells, due to their attractive optical and semiconducting properties. It is widely accepted that the optical and electrical properties of conjugated polymer molecules depend on the conjugated segments, i.e., chromophores in conjugated polymer molecule. The study of the evolution of the absorption and emission properties of single conjugated polymer molecules is essential to provide complementary information for the influence of conformation of conjugated polymer on its energy transfer process, as well as on the performance of optoelectronic devices based on conjugated polymers. Although the extensive studies have been reported to elucidate the optical properties of conjugated polymers with single molecule spectroscopy, simultaneous revealing their absorption and emission properties and their real-time evolution are rarely reported. In this paper, we simultaneously measure the absorption and emission properties of chromophores in single Poly[2,7-(9,9-dioctylfluorene)-<i>alt</i>-4,7-bis(thiophen-2-yl)benzo-2,1,3-thiadiazole](PFO-DBT) conjugated polymer molecules and their real-time evolution by frequency-domain reconstructed defocused wide-field imaging. The emission dipole orientation of chromophore is achieved by applying defocused wide-field fluorescence imaging. The change of defocused patterns of individual polymer chain describes the angular distribution of emitted light and thus the emitting dipole orientation. Meanwhile, the absorption dipole orientation of chromophore in single conjugated PFO-DBT polymer molecule can be clarified in reconstructed frequency-domain imaging by modulating the relative phase of the pulse pairs and performing Fourier transform to the photoluminescence response. The population density of excited state of absorbing chromophore depends both on the relative phase between the ultrashort pulse pairs and on the orientation of absorption transition dipole moment of the chromophore. By extracting the frequency-domain information of fluorescence that is proportional to the population density of excited state, the evolution of absorption dipole orientation of chromophore can be derived. We distinguish three cases for the evolution of chromophores of single PFO-DBT conjugated polymer molecules: the absorption and emission chromophores both keep constant in single PFO-DBT conjugated polymer molecules; one of the dipole orientations of absorption and emission changes, while the other remains unchanged; both of them change simultaneously. The results may pave the way for the further understanding of the role of conformation in the energy transfer pathway in both natural and artificial light harvesting systems at nano- and micro-level.

High performance organic transistors and phototransistors based on diketopyrrolopyrrole-quaterthiophene copolymer thin films fabricated via low-concentration solution processing

Conjugated polymers have received considerable attentions over the past years due to their large-area potential applications via low-cost solution processing. Improving crystallinity of conjugated polymer molecules in solution-processed thin films is crucial for their efficient charge transport and thus high performance optoelectronic devices. Herein, with diketopyrrolopyrrole-quaterthiophene (PDQT) copolymer as an example, it is found that by simply reducing the solution concentration for spin-coating meanwhile with the assistance of post-annealing, significantly enhanced film crystallinity with formation of typical single crystalline domains is obtained, which benefits from the enough space for better molecular assembly especially at the semiconductor/dielectric interface. High performance polymer transistors and phototransistors were finally constructed based on the optimal low-concentration (2mg/mL) spin-coated PDQT films (∼12nm), which giving a high charge carrier mobility of 2.28cm2V−1s−1 and a photoresponse on/off ratio of 2.1×107 at VG=0V under white light irradiation of 6mW/cm2. The results suggest that the bright future of PDQT crystalline films for large-area flexible integrated optoelectronic devices and the application of effective low-concentration processing approach in solution-processed organic electronics with reduced material waste.

Absorption and Quantum Yield of Single Conjugated Polymer Poly[2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylenevinylene] (MEH-PPV) Molecules.

We simultaneously measured the absorption and emission of single conjugated polymer poly[2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylenevinylene] (MEH-PPV) molecules in a poly(methyl methacrylate) (PMMA) matrix using near-critical xenon to enhance the photothermal contrast for direct absorption measurements. We directly measured the number of monomers and the quantum yield of single conjugated polymer molecules. Simultaneous absorption and emission measurements provided new insight into the photophysics of single conjugated polymers under optical excitation: quenching in larger molecules is more efficient than in smaller ones. Photoinduced traps and defects formed under prolonged illumination lead to decrease of both polymer fluorescence and absorption signals with the latter declining slower.

The role of chain conformation in energy transfer properties of single conjugated polymer molecule

Study of the relationship between conformation and photophysics of individual -conjugated polymer chain is one of the most important problems in polymer nanoscience and nanotechnology, which will facilitate the application of conjugated polymer in a range of electronic devices such as organic field-effect transistors, light-emitting diodes, and solar cells. Single-molecule spectroscopy has emerged as a powerful tool to unravel structure and dynamic heterogeneities that are hidden in ensemble average. Identification of the emitting segments through fluorescence of single conjugated polymer molecules and their dependence on the conformation can help reveal the mechanism and the extent of energy transfer process in a single polymer chain. In this paper, the photophysical properties of individual poly[2, 7-(9, 9-dioctylfluorene)-alt-4, 7-bis(thiophen-2-yl) benzo-2, 1, 3-thiadiazole] (PFO-DBT) conjugated polymer molecules are measured based on the defocused wide-field microscopy of single molecules. The single PFO-DBT molecules are prepared on cleaned glass coverslips by spin-coating solution of poly[methyl methacrylate] (PMMA) containing 110-9 mol/L PFO-DBT molecules in chloroform and toluene, respectively. Defocused imaging of single conjugated polymer molecule is performed based on a wide-field fluorescence microscope system. The change of defocused patterns of individual polymer chain maps the angular distribution of emitted chromophore and thus the emitting dipole orientation. Fluorescence trajectory and corresponding emission dipole moments of single conjugated polymer molecules are analyzed to identify the emitting conjugated segments. It is found that single PFO-DBT conjugated polymer molecules prepared by chloroform solvent show extended conformation. The intrachain energy transfer is dominant in the single conjugated polymer molecules that take extended conformation, which leads to photophysical properties of multiple chromophores. In contrast, single PFO-DBT conjugated polymer molecules prepared by toluene solvent hold folded conformation, which exhibit emission from single chromophore due to efficient interchain energy transfer. The emitting chromophore is not constant in a single PFO-DBT conjugated polymer molecule with folded conformation. About 35% of the single conjugated molecules prepared with toluene show only one constant emitting chromophore before photobleaching. However, about 65% of single conjugated polymer molecules prepared with toluene show two or more sequencely emitting chromophores. It can be concluded that the energy transfer properties of single PFO-DBT conjugated polymer molecule is greatly dependent on the conformation, which can be reflected in its photophysical properties. The study on the influence of single conjugated polymer conformation on energy transfer efficiency can provide the reference for the preparation and performance of optoelectronic devices and molecular devices based on conjugated polymer.

Effect of a heavy heteroatom on triplet formation and interactions in single conjugated polymer molecules and aggregates.

Triplet formation and interactions with emissive singlet excitons are investigated in poly(3-hexylselenophene) (P3HS) using single molecule spectroscopy. P3HS is a heavy atom analog of the more commonly studied poly(3-hexylthiophene) (P3HT), a benchmark polymer for solar cells. P3HS tends to aggregate strongly which necessitates dilution to ultra-low levels within a solid inert host in order to resolve photophysical responses of single chains. Fluorescence excitation intensity modulation is performed on isolated P3HS chains using a sequence of rectangular pulses of varying intensities to probe the presence of spin-forbidden triplet excitons. Triplet population dynamics originating from singlet-triplet and triplet-triplet interactions appear as quenching of the initial fluorescence intensity to steady-state levels on characteristic time scales of ∼1-10 μs. Over 80% of all molecules studied display significant fluorescence intensity modulation (quenching depths >50%) indicative of efficient intersystem crossing and large triplet occupancies. Because triplets are highly localized and singlet-triplet and triplet-triplet annihilation rate constants are comparable to those of intersystem crossing, multiple triplets are present at any given time on single P3HS chains. Triplet lifetimes were estimated to be ∼4 μs (upper limit) determined from recovery to the ground electronic singlet state in the absence of light and, surprisingly, triplets vanish at the onset of P3HS aggregation. This result was unexpected since P3HS triplet formation takes place on time scales <30 ps making this process competitive with most accessible non-radiative deactivation pathways.

Solventless processing of conjugated polymers—A review

The molecular mobility of polymers in their solid or molten states allows their processing without the need for toxic, “non-friendly” solvents. In this work, the main features of solvent-free processing methods applied to conjugated polymers are reviewed taking into consideration that these materials are largely used in a broad range of (opto-)electronic applications, including organic field-effect transistors, polymer light-emitting diodes and polymer photovoltaic devices. This review addresses the main advantages of processing such materials in their solid state offering an alternative fabrication scheme for several opto-electronic devices. Moreover, the properties of polymer-based opto-electronic devices are largely dependent on the conformations and orientations of polymer molecules in the corresponding polymer thin film layers; especially the ordering of conjugated polymer molecules in the active layer has been a topic of intense research in organic electronics. Therefore, this review also highlights the control of conjugated polymer molecule ordering when using solvent free processing techniques. Finally, conclusions and perspectives of solventless processed conjugated polymer molecules in organic electronics are addressed.

ChemInform Abstract: Ordering of Conjugated Polymer Molecules: Recent Advances and Perspectives

AbstractReview: 94 refs.

Ordering of conjugated polymer molecules: recent advances and perspectives

Molecular order and orientation significantly influence the physical properties of organic materials and their corresponding device properties. Therefore ordering organic molecules especially conjugated polymer molecules in the active layer has been a hot topic in organic electronics for high performance organic electronics and remarkable progress has been demonstrated recently. The purpose of this minireview is to highlight the novel strategies for controlling conjugated polymer molecule orders in the solid state, including rational chemical design, optimizing solution-processing techniques and growth of high quality polymer crystalline micro/nanostructures. Moreover, the charge transport mechanisms in conjugated polymers and the relationship of oriented polymer molecules with their field-effect properties are also discussed. Finally, a concise conclusion and perspective of highly ordered conjugated polymer molecules in organic electronics are also addressed.

Molecular weight dependence of emission intensity and emitting sites distribution within single conjugated polymer molecules

We investigated exciton migration, trapping and emission processes occurring within a single conjugated polymer molecule by means of superresolution fluorescence localization microscopy. This methodology allowed us to locate the spatial distribution of emitting sites within single chains with nanometre precision. The study was done on individual poly[2-methoxy-5-(2'-ethyl-hexyloxy)-1,4-phenylene vinylene] (MEH-PPV) molecules with average molecular weights ranging from 215,000 to 1,440,000 and with narrow weight distributions. We found that the mean emission intensity increases proportionally to the polymer molecular weight. The localization experiments suggest that the emitting sites are distributed nearly uniformly within a single chain and that the sites are on average 10 nm apart, irrespective of the molecular weight of the polymer. Furthermore, spatial contours formed by all the combined emitting sites within one chain show elongated shapes, in agreement with a rod-like structure of MEH-PPV in a collapsed state.

Influence of ZnS quantum dots on optical and photovoltaic properties of poly(3-hexylthiophene)

The influence of ZnS quantum dots (QDs) on the photo-physical and electrical properties of poly(3-hexylthiophene) (P3HT) conjugated polymer, has been investigated. Composite material has been prepared by blending P3HT with low concentration of chemically synthesized ZnS nanocrystals. Absorption spectra of conjugated polymer exhibit a small blue shift of about 3 nm, with the incorporation of ZnS nanocrystals. We observed the charge transfer between conjugated polymer molecules and ZnS nanoparticles which is discussed with the energy levels of the materials. Current density–voltage measurement of the hybrid device showed a significant improvement in V oc in comparison to P3HT only device.

Factors Controlling Hole Injection in Single Conjugated Polymer Molecules

New insights on the molecular level details of the recently reported light-assisted injection of positive charge into single conjugated polymer chains are reported. Extensive new fluorescence-voltage single molecule spectroscopy (FV-SMS) measurements were performed on single chains of the archetypical conjugated polymer MEH-PPV embedded in a capacitor device to complement previous studies of the influence of the bias scan rate and optical excitation intensity. The use of a vacuum microscope allowed for the precise control of the device atmosphere, demonstrating the influence of triplet states in the MEH-PPV on the FV-SMS modulation. For identical device conditions, little variation was observed in the rate and yield of charging from molecule to molecule. Through the use of thicker supporting matrices and insulating polymer "blocking layers", it was determined that good electrical contact between the hole transport layers and the single molecules was necessary for charge injection. The results demonstrate the complexity of charge transfer processes at the interface of organic semiconductors and highlight the ability of single molecule methods to advance the understanding of such processes at the nanoscale.

Theoretical study on photon emission statistics from single conjugated polymer molecules excited by laser pulses

In recent experiments, the signal-to-noise ratio and the photon pair correlation are measured simultaneously by improving the Hanbury-Brown and Twiss method. We show that judgement on whether the background noise obeys Poisson statistics or not is possible from the data of such measurements. If the background is subject to Poisson statistics, the results of both measurements enable to estimate the photon pair correlation in the absence of noise from that measured under background noise. The method is useful to remove the effect of background noise from the photon correlation measured by exciting multichromophore systems. We also show that the normalized photon pair correlation is not necessarily zero in the absence of background noise when single photon is emitted from multiple excitons as a result of the competition among the pair annihilation, and the unimolecular nonradiative and radiative processes.

Mapping the emitting sites within a single conjugated polymer molecule

We map the location of emitting sites within a single conjugated polymer molecule poly[2-methoxy-5-(2'-ethyl-hexyloxy)-1,4-phenylene vinylene] with nanometre accuracy by means of a single-molecule imaging technique.