Interchain Diffusion Research Articles

Exciton dynamics in conjugated polymer systems

Exciton dynamics in π-conjugated polymers systems encompass multiple time and length scales. Ultrafast femtosecond processes are intrachain and involve a quantum mechanical correlation of the exciton and nuclear degrees of freedom. In contrast, post-picosecond processes involve the incoherent Förster transfer of excitons between polymer chains. Exciton dynamics is also strongly determined by the spatial and temporal disorder that is ubiquitous in conjugated polymers. Since excitons are delocalized over hundreds of atoms, a theoretical understanding of these processes is only realistically possible by employing suitably parametrized coarse-grained exciton-phonon models. Moreover, to correctly account for ultrafast processes, the exciton and phonon modes must be treated on the same quantum mechanical basis and the Ehrenfest approximation must be abandoned. This further implies that sophisticated numerical techniques must be employed to solve these models. This review describes our current theoretical understanding of exciton dynamics in conjugated polymer systems. We begin by describing the energetic and spatial distribution of excitons in disordered polymer systems, and define the crucial concept of a “chromophore” in conjugated polymers. We also discuss the role of exciton-nuclear coupling, emphasizing the distinction between “fast” and “slow” nuclear degrees of freedom in determining “self-trapping” and “self-localization” of exciton-polarons. Next, we discuss ultrafast intrachain exciton decoherence caused by exciton-phonon entanglement, which leads to fluorescence depolarization on the timescale of 10-fs. Interactions of the polymer with its environment causes the stochastic relaxation and localization of high-energy delocalized excitons onto chromophores. The coupling of excitons with torsional modes also leads to various dynamical processes. On sub-ps timescales it causes exciton-polaron formation (i.e., exciton localization and local polymer planarization). Conversely, on post-ps timescales stochastic torsional fluctuations cause exciton-polaron diffusion along the polymer chain and at higher temperatures to transient exciton delocalization via extended exciton states. We next describe a first-principles, Förster-type model of interchain exciton transfer and diffusion in the condensed phase, whose starting point is a realistic description of the donor and acceptor chromophores. Finally, we discuss condensed phase transient exciton delocalization in highly-ordered nanofibers. We survey experimental results and explain how they can be understood in terms of our theoretical description of exciton dynamics coupled to information on polymer multiscale structures. The review also contains a brief critique of computational methods to simulate exciton dynamics.

Picosecond time-resolved photon antibunching measures nanoscale exciton motion and the true number of chromophores

The particle-like nature of light becomes evident in the photon statistics of fluorescence from single quantum systems as photon antibunching. In multichromophoric systems, exciton diffusion and subsequent annihilation occurs. These processes also yield photon antibunching but cannot be interpreted reliably. Here we develop picosecond time-resolved antibunching to identify and decode such processes. We use this method to measure the true number of chromophores on well-defined multichromophoric DNA-origami structures, and precisely determine the distance-dependent rates of annihilation between excitons. Further, this allows us to measure exciton diffusion in mesoscopic H- and J-type conjugated-polymer aggregates. We distinguish between one-dimensional intra-chain and three-dimensional inter-chain exciton diffusion at different times after excitation and determine the disorder-dependent diffusion lengths. Our method provides a powerful lens through which excitons can be studied at the single-particle level, enabling the rational design of improved excitonic probes such as ultra-bright fluorescent nanoparticles and materials for optoelectronic devices.

First-Principles Calculation of Triplet Exciton Diffusion in Crystalline Poly(p-phenylene vinylene)

Understanding and controlling exciton transport is a strategic way to enhance the optoelectronic properties of high-performance organic devices. In this article we study triplet exciton migration in crystalline poly($p$-phenylene vinylene) polymer (PPV) using comprehensive electronic structure and quantum dynamical methods. We solve the coupled electron-nuclear dynamics for the triplet energy migrating between two neighboring Frenkel sites in J- and H-aggregate arrangements. From the two-site model we extract key parameters for use with a master-equation approach that allows us to treat nanosize systems where time-dependent Schr\"odinger equation becomes intractable. We calculate the transient exciton density evolution and determine the diffusion constants along the principal crystal axes of the PPV. The triplet diffusion is characterized by two distinctive components: fast intrachain, and slow interchain. At room temperature the interchain diffusion coefficients are found to be $D_a=0.89\cdot10^{-2}$ cm$^2$s$^{-1}$ and $D_b=1.49\cdot10^{-2}$ cm$^2$s$^{-1}$ along the respective $\bar{a}$- and $\bar{b}$-axes, and the intrachain is $D_c=3.03$ cm$^2$s$^{-1}$ along the fast $\bar{c}$-axis. The exceptionally high exciton mobility along the $\pi$-conjugated backbone facilitates rapid triplet migration over long distances. Our results can be utilized in the design of efficient energy conversion and light-emitting devices with desired solid-state properties.

μSR Study with Light Irradiation of P3HT:ZnO Nanoparticles as Active Material of Hybrid Solar Cells

Recently, hybrid solar cell that consists of a combination of organic and inorganic materials offers promise in increasing efficiency. Combination of conjugated polymer of Poly (3-hexyl thiophene)/P3HT which has the highest hole mobility with inorganic materials which has high electron mobility, good physical and chemical stability of inorganic nanocrystals, shows a better performance of hybrid solar cells. We have conducted a measurement to investigate the charge carrier transport in active material of hybrid solar cells by using muon spin relaxation (μSR) with light irradiation. A bulk sample used in this study is hybrid organic-inorganic material consisting of regio-regular P3HT and ZnO nanoparticles. Longitudinal-field μSR measurements were performed in magnetic field ranging from 0 to 395 mT at temperature of 10 K, 15 K and 25 K. Based on μSR measurement with light irradiation, we found that for those temperatures, longitudinal field dependence of λ1 was proportional with C-H0.5 curve indicating three-dimensional inter-chain diffusion. We assume that light irradiation initiates the production of exciton that increases the number of charge carrier in the sample and transports not only along the chain of polymer (one-dimensional) but also perpendicular to other chain of polymer (three-dimensional). With light irradiation, the crossover temperature from one-dimensional to three-dimensional of regio-regular P3HT and ZnO nanoparticles bulk sample was obtained at lower temperature of 10 K compared to previous result at 25 K without light irradiation.

Atomistic Description of Ionic Diffusion in PEO–LiTFSI: Effect of Temperature, Molecular Weight, and Ionic Concentration

Understanding the ionic diffusion mechanism in polymer electrolytes is critical to the development of advanced lithium-ion batteries. We report here molecular dynamics-based characterization of structures and diffusion in poly(ethylene oxide) (PEO) with lithium and bis(trifluoromethysulfonyl)imide (TFSI) ions imbedded into the PEO structure. We consider a range of temperatures (360–480 K), molecular weights (43, 22, 10, and 2 chains with 23, 45, 100, and 450 EO monomers, respectively), and ion concentrations (r = 0.02, 0.04, 0.06, and 0.08 Li:EO) for which there is experimental data. The found dependence of the diffusion coefficients on these variables is in good agreement with experimental measurements. We then analyze how the diffusion performance depends on details of the atomistic diffusion mechanism, the motion of the Li and TFSI along the polymer chains and hopping between them, the role of polymer motion, the temperature dependence of the intrachain and interchain diffusion contributions to the tot...

Molecular Dynamics Simulations of Ionic Diffusion in PEO-LiTFSI Polymer Electrolyte: Effect of Temperature, Molecular Weight, and Ionic Concentration

Polymer electrolytes are promising materials for development of the next generation of energy storage devices, including lithium-ion polymer batteries, due to their exceptional chemical and mechanical stability, high energy density and long lifetimes. Understanding the ionic diffusion mechanism in polymer electrolytes is critical to the development of advanced lithium-ion batteries. A molecular dynamics-based characterization of structures and diffusion in poly(ethylene oxide) (PEO) with lithium and bis(trifluoromethy-sulfonyl-imide) (TFSI) ions imbedded into the PEO structure have been performed across a range of temperatures, molecular weights and ion concentrations, with relative ionic diffusion coefficients shown to be in good agreement with experimental measurements. To determine details of the atomistic diffusion mechanism, the chain coordination of Li atoms, polymer motion, and temperature dependences of the intrachain and interchain diffusion contributions into the total ionic diffusion coefficients on ionic concentration and molecular weight were analyzed. We find that the most diffusive Li atoms exhibit frequent interchain hopping, whereas the least diffusive Li atoms rather oscillate or “shift” coordination between two or more polymer chains. These shifts may actually reduce the segmental motion of the PEO-LiTFSI polymer, which is important for the fast lithium-ion diffusion. Good agreement between experiment and theory validates the approach and methodology used in this study and its further applications for predicting the structure and ionic conductivity of new advanced polymer materials for a new generation of electrochemical devices. Acknowledgement This work was supported by Bosch Energy Research Network Grant No. 13.01.CC11.

Study of Charge Carrier Dynamics of P3HT:PCBM Blend for Active Material Solar Cell Using Muon Spin Relaxation

Poly (3-hexylthiohene) (P3HT) has attracted much attention due to their chemical and thermal stability as well as their potential to be used as an active material in solar cells. Recently, bulk-heterojunction BHJ solar cell has been intensively developed due to its potential for high power conversion efficiency and low cost solar cells. Many studies have been done to understand the effect of PCBM to the morphology of the P3HT:PCBM blend and to performance of its solar cells. However, the clearly explanation about the effect of PCBM to the charge carrier dynamics in P3HT is not well understood yet. We have studied charge carrier dynamics in active material of solar cell P3HT:PCBM along and perpendicular to the chain by means of longitudinal field (LF) muon-spin-relaxation (μSR) method to elucidate the intra and inter-chain charge carrier in polymer hybrid P3HT:PCBM. We found the charge carrier transport changes from intra-chain to inter-chain diffusion. One dimensional intra-chain diffusion is observed in the samples at low temperatures below 10K, while three dimensional inter-chain is observed at high temperatures above 15K. This temperature is lower than that of P3HT only.

μSR Study of Charge Carrier Motion in Active Layer P3HT:ZnO:PCBM Hybrid Solar Cells

The so-called hybrid (organic-inorganic) solar cell has been developed due to the combining advantage between organic material such as Poly(3-hexylthiohene)/P3HT and inorganic material such as zinc oxide (ZnO). By adding fullerene derivative (6,6-phenyl-C61-butyric acid methyl ester/PCBM) to the hybrid solar cell, the performance of solar cell is predicted to be increased. We have studied the charge carrier motion in blend of P3HT:ZnO:PCBM using LF-μSR with and without light irradiation to clarify the charge carrier motion in bulk ternary system of hybrid material. We found that charge carrier motion in P3HT:ZnO:PCBM changes from intra-chain to inter-chain diffusion. One-dimensional intra-chain diffusion is observed in the sample at low temperature below 15 K, while three-dimensional inter-chain is observed at high temperature above 25 K.

Computational and Experimental investigation of Nalipoite-Li2APO4 (A = Na, K) electrolytes for Li-ion batteries

ABSTRACTThe potential ionic conductors Li2APO4 (A = Na, K) are investigated combining experiments and first principles calculations at the Density Functional Theory level. A high ionic conductivity of 6.5 x10−6 and 1.5 x10−5 S cm−1 at 25 and 70°C, respectively, is found in Nalipoite-Li2NaPO4. For this mixed phosphate the energy barriers to Li motion are calculated. The lower energy barrier (0.7 eV) implies the inter-chain diffusion of Li in the b-c plane. We predict that ionic mobility is enhanced in the isostructural Li2KPO4, with the lowest calculated energy barrier being 0.4 eV.

Charge Carrier Dynamics of Active Material Solar Cell P3HT:ZnO Nanoparticles Studied by Muon Spin Relaxation (μSR)

Poly (3-hexylthiohene) (P3HT) has attracted much attention due to their chemical and thermal stability as well as their potential to be used as an active material in solar cells. Recently, hybrid solar cell has been developed due to the combining advantages of organic material (P3HT) and inorganic material such as nanoparticle zinc oxide (ZnO). ZnO is an inorganic material with high electron mobility and easy to prepare as electron acceptor to dissociate excitons formed in conjugated polymer as the active material of solar cells. ZnO also can be prepared as a nanoparticle that can resolve a problem of small exciton diffusion length of P3HT. However, the clearly explanation about the effect of ZnO to the charge carrier dynamics in P3HT is not fully understanding yet. We have studied charge carrier dynamics in active material of solar cell P3HT:ZnO along and perpendicular to the chain by means of longitudinal field (LF) muon-spin-relaxation (μSR) method to elucidate the intra and inter-chain charge carrier in polymer hybrid P3HT:ZnO. We found the charge carrier transport changes from intra-chain to inter-chain diffusion. One dimensional intra-chain diffusion is observed in the samples at low temperatures, while three dimensional inter-chain is observed at high temperatures.

Synthesis and properties of near IR induced self-healable polyurethane/graphene nanocomposites

A series of self-healable polyurethane (SHPU)/modified graphene (MG) nanocomposites were synthesized from poly(tetramethylene glycol) (PTMG) and 4,4′-methylene diphenyl diisocyanate (MDI) with minute amounts (0–1wt%) of MG which was chemically modified graphene oxide (GO) with phenyl isocyanate and reduced in the presence of phenylhydrazine.MG gave dual effects of reinforcing filler and light absorption medium. That is, SHPU/MG nanocomposites showed significantly enhanced Young’s modulus and near infrared (NIR) absorption along with increased glass transition temperature (Tg). However, break strength and break strain decreased at high GO contents (MG075, MG100) implying that MG disturbs chain orientations.The self-healing behavior of nanocomposites was done by intermolecular diffusion of polymer chains which was accelerated by thermal energy generated by NIR absorptions. The self-healing effect was most pronounced with 0.75wt% MG (MG075) where the elastic strain energy was even greater than the fresh material up to over 200% strain. Further addition of MG (MG100) induced more light absorption, but physically disturbed the interchain diffusion to reduce the self-healing efficiency.

μSR Study of Charge Carrier Diffusion in Regioregular Poly(3-Butylthiophene-2,5-Diyl)

Temperature-dependent of charge carrier diffusion in regioregular-poly(3-butylthiophene-2,5-diyl) (RR-P3BT) has been studied by Longitudinal Field (LF) muon-spin-relaxation (μSR) method for determining intrachain and interchain charge carrier hopping mechanisms. It is found that the depolarization rates at the low temperatures below 50 K indicate that the H–0.5 field-dependent characteristics of the charge transport is dominated by mobility along the polymer chain (intrachain diffusion). With an increase in temperature, the charge carriers move perpendicular to the polymer chain (interchain diffusion), as characterized by the field dependence of depolarization rates of C-H0.5 curve. The initial change in the carrier mobility appeared at around 50 K to 75 K.

Field and temperature dependent charge transport characteristics in regio-regular Poly(3-octylthiophene-2,5-diyl) studied by Muon Spin relaxation

Spin dynamic of regio-regular Poly(3-octylthiophene-2,5-diyl) has been investigated with longitudinal field (LF) muon-spin-relaxation (μSR) techniques. The LF dependent muon-spin depolarization rate indicates the occurrence of dimensional crossover from one-dimensional intra-chain spin diffusion to three-dimensional inter-chain spin diffusion at 50 K.

Intra- and inter-chain polaron diffusion in regio-random polythiophene studied by muon spin relaxation

Longitudinal field (LF) muon-spin-relaxation measurements have been performed for polythiophene based polymers of regio-random poly(3-hexylthiophene-2,5-diyl) to elucidate the intra- and inter-chain hopping mechanisms. The LF dependent muon-spin depolarization rate indicates the occurrence of pronounced shift in the relative dominance of charge transport mechanism from intra-chain diffusion to inter-chain diffusion at 50 K which is higher than the 25 K observed previously for the regio-regular case.

μSR study of electron radical dynamics in regio-regular polythiophene

We have carried out longitudinal field (LF) muon-spin-relaxation (μSR) measurements in polythiophene based polymers of Poly(3-hexylthiophene-2,5-diyl) with regio-regular structure to elucidate the intra- and inter-chain hopping mechanisms. The LF dependent muon-spin depolarization rate indicates the occurrence of dimensional crossover from 1 dimensional intra-chain spin diffusion to 3 dimensional inter-chain spin diffusion at 25 K.

Direct Observation of Ligand Dynamics in Cytochrome c

Horse heart cytochrome c (cyt c) has emerged as a paradigm for the study of protein folding, in large part because the covalently bound heme facilitates its characterization. The folding of reduced cyt c induced by photodissociation of CO from the CO-bound unfolded protein has been extensively studied. Following a nanosecond light pulse, four transitions with time constants of approximately 1-5, 50-100, 200-500, and 1000-10000 micros have been resolved. While originally thought to be associated with CO rebinding to two different partially folded states of cyt c, the two slower processes are now understood to reflect the bimolecular reassociation of CO followed by religation of His18, which by the base elimination mechanism is induced to dissociate after CO photolysis. Thus, it turns out that the two longer time constants do not report on protein folding but instead reflect the complexity of heme ligation. The two shorter time constants have been attributed to ligation at the heme center by Met65 or Met80 and His33 or His26 and have been used to estimate interchain diffusion rates of the protein. Here, to unambiguously determine the post-photodissociation steps involving CO, we have monitored the CO vibration following photodissociation with step-scan FT-IR spectroscopy. We have found that like the longer time scale processes, the 50-100 mus time scale process is associated not with protein dynamics but with CO ligand dynamics. The data clearly demonstrate that whatever the origins of the spectral changes, they clearly involve CO rebinding or changes in the environment of an already bound CO ligand. We speculate that the observed changes reflect His18 religation after fast geminate recombination of the CO. The data suggest that the associated time constant should not be used as a measure of interchain diffusion, and the results emphasize the importance of studying protein folding with probes that have inherently high structural resolution.

Charge transfer in poly(3-octylthiophene) modified by fullerene derivative

Magnetic, relaxation, and electronic dynamic parameters of paramagnetic centers stabilized in an initial and fullerene-modified poly(3-octylthiophene) (P3OT) were studied at the 3-cm (9.7 GHz) waveband EPR in wide (77–320 K) temperature region. Both the systems demonstrate sum spectrum of localized and mobile polarons, whose contribution depends on polymer treatment. Both spin susceptibility and spin–spin relaxation rate were shown to govern by dipole–dipole interaction of the mobile polarons. The spin–lattice and spin–spin relaxation times were determined separately by using the steady-state saturation method. It was shown that the treatment of the P3OT sample with fullerene molecules leads to the decrease in activation energy of spin susceptibility, dipole–dipole interaction, and interchain diffusion. This is accompanied by the acceleration of spin relaxation and increase in the system dimensionality.

Ultrafast dynamics of singlet-singlet and singlet-triplet exciton annihilation in poly(3-2′-methoxy-5′octylphenyl)thiophene films

Singlet-singlet (S-S) and singlet-triplet (S-T) exciton annihilation was studied in poly(3-2(')-methoxy-5(')octylphenyl)thiophene films. For the S-S exciton annihilation studies, transient absorption spectroscopy at excitation laser pulse fluences of 1.2x10(13)-4.4x10(14) photons/cm(2) and 2.5 kHz pulse repetition rate was applied. The obtained kinetics demonstrate a typical nonexponential character with intensity-dependent amplitudes and lifetimes. In time-resolved fluorescence experiments, low excitation pulse fluences of 1.6x10(9)-2.2x10(12) photons/cm(2) at high repetition rates of 0.4, 0.8, 4, and 81 MHz lead to S-T exciton annihilation as a result of triplet exciton accumulation. S-T annihilation kinetics results in monoexponential decay of the fluorescence kinetics and manifests itself as a decrease of the singlet exciton lifetime. The calculated time-independent S-S and S-T exciton annihilation rates strongly support the conclusion that the processes are controlled by the interchain diffusion of singlet excitons. Despite the low efficiency of S-T annihilation compared to that of S-S annihilation, it has a substantial effect on the singlet exciton lifetime due to a relatively long triplet lifetime (60 mu s). Thus, even optical excitation with low fluence at high pulse repetition rate creates a significant concentration of triplet states that efficiently quenches singlet excitons. (Less)

Muon-spin relaxation study of anisotropic charge carrier motion in polyphenylenevinylene-based polymers

Muon-spin relaxation (μSR) experiments on the conducting polymers poly(2, 3-dibutoxy-1, 4-phenylenevinylene) and poly(2, 5-bis(dimethyloctylsilyl)-1, 4-phenylene vinylene) probe thedynamics of the highly mobile polarons created by the muon-implantation processin which muonium reacts with the polymer forming a radical state. Thefluctuating spin density induced by the electronic spin defect rapidly diffusing upand down the chain leads to a characteristic relaxation, the temperature and fielddependences of which permit the extraction of intrachain and interchain diffusionrates. The intrachain diffusion rate decreases with temperature and can be fittedto a model of phonon-limited transport. The interchain diffusion rate increaseswith temperature and can be fitted to an activated temperature dependence.

Muon-spin relaxation study of charge carrier dynamics in the conducting polymer PPV

Muon-spin relaxation (μSR) experiments on the conducting polymer poly(2,3-dibutoxy-1,4-phenylene vinylene) (DB-PPV) probe the dynamics of the highly mobile polarons created by the muon-implantation process in which muonium reacts with the polymer forming a radical state The fluctuating spin density induced by the electronic spin defect rapidly diffusing up and down the chain leads to a characteristic relaxation, the temperature and field dependence of which permit the extraction of on-chain and inter-chain diffusion rates. These results provide information concerning the charge-transport mechanism in DB-PPV.