Exciton regeneration dynamics in model donor-acceptor polymer heterojunctions

We present a theoretical investigation on various semiconducting materials that exhibit photovoltaic and photoluminecent properties. Our focus is on the relaxation dynamics that occur upon photoexcitation of a couple of type II donor-acceptor heterojunction systems. In addition to the diabatic approach our two-band exciton model employs to study the phonon-assisted relaxations, we adopt the Marcus-Hush semiclassical method to incorporate lattice reorganization. This enables us to look at the state-to-state interconversions from the relaxed excited-state configurations in model polymer blends of poly(9,9-dioctylfluorene-co-N-(4-butylphenyl)diphenylamine) (TFB) with poly(9,9-dioctylfluorene-co-benzothiadiazole) (F8BT) and poly(9,9-dioctylfluorene-co-bis-N,N-(4-butylphenyl)-bis-N,N-phenyl-1,4-phenylenediamine) (PFB) with F8BT. Our results stress the significance of vibrational relaxation in the state-to-state relaxation. Furthermore, while a tightly bound charge-transfer state (exciplex) remains the lowest excited state, we show that the regeneration of the optically active lowest excitonic state in TFB:F8BT is possible via the existence of a steady state.     

Exciton dissociation at donor-acceptor polymer heterojunctions: quantum nonadiabatic dynamics and effective-mode analysis

The quantum-dynamical mechanism of photoinduced subpicosecond exciton dissociation and the concomitant formation of a charge-separated state at a semiconducting polymer heterojunction is elucidated. The analysis is based upon a two-state vibronic coupling Hamiltonian including an explicit 24-mode representation of a phonon bath comprising high-frequency (C==C stretch) and low-frequency (torsional) modes. The initial relaxation behavior is characterized by coherent oscillations, along with the decay through an extended nonadiabatic coupling region. This region is located in the vicinity of a conical intersection hypersurface. A central ingredient of the analysis is a novel effective mode representation, which highlights the role of the low-frequency modes in the nonadiabatic dynamics. Quantum dynamical simulations were carried out using the multiconfiguration time-dependent Hartree method.     

Exciton dissociation dynamics in model donor-acceptor polymer heterojunctions. I. Energetics and spectra

In this paper we consider the essential electronic excited states in parallel chains of semiconducting polymers that are currently being explored for photovoltaic and light-emitting diode applications. In particular, we focus upon various type II donor-acceptor heterojunctions and explore the relation between the exciton binding energy to the band offset in determining the device characteristic of a particular type II heterojunction material. As a general rule, when the exciton binding energy is greater than the band offset at the heterojunction, the exciton will remain the lowest-energy excited state and the junction will make an efficient light-emitting diode. On the other hand, if the offset is greater than the exciton binding energy, either the electron or hole can be transferred from one chain to the other. Here we use a two-band exciton to predict the vibronic absorption and emission spectra of model polymer heterojunctions. Our results underscore the role of vibrational relaxation and suggest that intersystem crossings may play some part in the formation of charge-transfer states following photoexcitation in certain cases.     

Exciton mobility and trapping in a MALDI matrix

Energy transfer (ET) from excited matrix to fluorescent traps is used to probe the mobility of excitations in the matrix-assisted laser desorption/ionization (MALDI) matrix material 2,5-dihydroxybenzoic acid. The dependence of host and guest fluorescence on excitation density (laser intensity) and trap concentration gives clear evidence for long-range energy transport in this matrix. This conclusion is further supported by time-resolved emission data showing a 2 ns delay between matrix and trap emission. Rate equation and random walker models give good agreement with the data, allowing determination of hopping, collision, and trapping parameters. Long-range energy transfer contributes to the pooling reactions which can lead to primary ions in MALDI. The results validate the pooling aspect of the prior quantitative MALDI ionization model (J. Mass Spectrom. 2002, 37, 867-877). It is shown that exciton trapping can decrease MALDI ion yield, even at low trap concentration.     

Exciton dynamics in alternating polyfluorene/fullerene blends

Exciton dynamics in alternating copolymer/fullerene solar cell blends have been investigated using femtosecond transient absorption spectroscopy. The acceptor concentrations have been varied over a wide range. Experimental data, kinetic modeling and simulations, all indicate that the efficiency of exciton conversion to charges is 100% even at acceptor concentrations as low as 20 wt%. The reported dependence of solar cell efficiency on fullerene concentration may thus arise from other factors. However, there exists an acceptor concentration threshold (5 wt%) below which a substantial fraction of the excitations remain unquenched. The results, we believe are very relevant to optimization of performance efficiency by clever manipulation of morphology. We have also observed exciton–exciton energy transfer in these blends at low acceptor concentrations.     

Transesterification in polymer blends including polycarbonate at high temperatures

Miscibility of polycarbonate (PC)/poly (arylate) (PAr) was studied by differential scanning calorimetry (DSC), phase contrast microscopy with digital image analysis (DIA) system, FT-IR spectroscopy, and cloud points for the PC/PAr/methylene chloride ternary systems. PC and PAr were immiscible over the whole composition range from the thermodynamical viewpoint. However, PC/PAr is homogenized by transesterification between PC and PAr at high temperatures. There is observed a competition between phase separation due to thermodynamical factor and phase dissolution due to transesterification. The pattern of the phase separation was spinodal decomposition type and the structure was successfully analysed by DIA.     

Interfaces in polymer blends

We investigate the structure and thermodynamics of interfaces in dense polymer blends using Monte Carlo (MC) simulations and self‐consistent field (SCF) calculations. For structurally symmetric blends we find quantitative agreement between the MC simulations and the SCF calculations for excess quantities of the interface (e.g., interfacial tension or enrichment of copolymers at the interface). However, a quantitative comparison between profiles across the interface in the MC simulations and the SCF calculations has to take due account of capillary waves. While the profiles in the SCF calculations correspond to intrinsic profiles of a perfectly flat interface the local interfacial position fluctuates in the MC simulations. We test this concept by extensive Monte Carlo simulations and study the cross‐over between “intrinsic” fluctuations which build up the local profile and capillary waves on long (lateral) length scales. Properties of structurally asymmetric blends are exemplified by investigating polymers of different stiffness. At high incompatibilities the interfacial width is not much larger than the persistence length of the stiffer component. In this limit we find deviations from the predictions of the Gaussian chain model: while the Gaussian chain model yields an increase of the interfacial width upon increasing the persistence length, no such increase is found in the MC simulations. Using a partial enumeration technique, however, we can account for the details of the chain architecture on all length scales in the SCF calculations and achieve good agreement with the MC simulations. In blends containing diblock copolymers we investigate the enrichment of copolymers at the interface and the concomitant reduction of the interfacial tension. At weak segregation the addition of copolymers leads to compatibilization. At high incompatibilities, the homopolymer‐rich phase can accommodate only a small fraction of copolymer before the copolymer forms a lamellar phase. The analysis of interfacial fluctuations yields an estimate for the bending rigidity of the interface. The latter quantity is important for the formation of a polymeric microemulsion at intermediate segregation and the consequences for the phase diagram are discussed.     

Exciton transfer integrals between polymer chains

The line-dipole approximation for the evaluation of the exciton transfer integral J between conjugated polymer chains is rigorously justified. Using this approximation, as well as the plane-wave approximation for the exciton center-of-mass wave function, it is shown analytically that J approximately L when the chain lengths are smaller than the separation between them, or J approximately L-1 when the chain lengths are larger than their separation, where L is the chain length. Scaling relations are also obtained numerically for the more realistic standing wave approximation for the exciton center-of-mass wave function, where it is found that for chain lengths larger than their separation J approximately L-1.8 or J approximately L-2, for parallel or collinear chains, respectively. These results have important implications for the photophysics of conjugated polymers and self-assembled molecular systems, as the Davydov splitting in aggregates and the Forster transfer rate for exciton migration decrease with chain lengths larger than their separation. This latter result has obvious deleterious consequences for the performance of polymer photovoltaic devices.     

Structured polymer blends

Various morphologies can be realized via processing of incompatible polymer blends such as droplets or fibers in a matrix and stratified or cocontinuous structures as is shown for the model system polyethylene/polystyrene The structures induced are usually intrinsically unstable. Modelling of extrusion processes and continuous mixers yields expressions for the shear rate and shear stress but also for the limited residence time and the number of reorientations. These results could be combined with detailed knowledge of respectively distributive and dispersive mixing processes to predict the development of various morphologies as a function of time. Control of morphology is of utmost importance. In the case of droplets in a matrix, usually encountered in toughening of glassy polymers, the use of compatibilizers and/or reactions at the interphases is utilized. However, in designing specific morphologies i.e. structured polymer blends, fixation of intermediate morphologies before final processing is a prerequisite. Some preliminary results will be presented.     

Interfacial tension in polymer blends

Theoretical models of the interfacial tension coefficient in polymer blends, v₁₂, were evaluated. A new working relation was derived that makes it possible to compute v₁₂ from the chemical structure of two polymers. The calculations involve determination of the dispersive, polar and hydrogen-bonding parts of the solubility parameter from the tabulated group and bond contributions. The computed values of v₁₂ for 46 blends were found to follow the experimental ones with a reasonable scatter of ± 36%. Next, the experimental methods of v₁₂-measurements were critically examined. Although many have been developed for low viscosity Newtonian fluids, most are irrelevant to industrial polymeric systems. For the present studies two were selected. Values of v₁₂ were measured using the so-called “capillary breakup method,” and a newly developed method based on the retraction rate of deformed drop.     

Colloidal aggregation in polymer blends

We consider here a low-density assembly of colloidal particles immersed in a critical polymer mixture of two chemically incompatible polymers. We assume that, close to the critical point of the free mixture, the colloids prefer to be surrounded by one polymer (critical adsorption). As result, one is assisted to a reversible colloidal aggregation in the nonpreferred phase, due the existence of a long-range attractive Casimir force between particles. This aggregation is a phase transition driving the colloidal system from dilute to dense phases, as the usual gas-liquid transition. We are interested in a quantitative investigation of the phase diagram of the immersed colloids. We suppose that the positions of particles are disordered, and the disorder is quenched and follows a Gaussian distribution. To apprehend the problem, use is made of the standard phi(4) theory, where the field phi represents the composition fluctuation (order parameter), combined with the standard cumulant method. First, we derive the expression of the effective free energy of colloids and show that this is of Flory-Huggins type. Second, we find that the interaction parameter u between colloids is simply a linear combination of the isotherm compressibility and specific heat of the free mixture. Third, with the help of the derived effective free energy, we determine the complete shape of the phase diagram (binodal and spinodal) in the (Psi,u) plane, with Psi as the volume fraction of immersed colloids. The continuous "gas-liquid" transition occurs at some critical point K of coordinates (Psi(c) = 0.5,u(c) = 2). Finally, we emphasize that the present work is a natural extension of that, relative to simple liquid mixtures incorporating colloids.     

Phonon-driven exciton dissociation at donor-acceptor polymer heterojunctions: direct versus bridge-mediated vibronic coupling pathways

We present a molecular-level, quantum dynamical analysis of phonon-driven exciton dissociation at polymer heterojunctions, using a linear vibronic coupling model parametrized for 3 electronic states and 24 vibrational modes. Quantum dynamical simulations were carried out using the multiconfiguration time-dependent Hartree method. In this study, which significantly extends the two-state model of Tamura et al. (Tamura, H.; Bittner, E. R.; Burghardt, I. J. Chem. Phys. 2007, 126, 021103), we focus on the role of bridge states, which can mediate the decay of the photogenerated exciton and possibly interfere with the direct transition toward an interfacial charge-separated state. Both the direct and bridge-mediated pathways are found to depend critically on the dynamical interplay of high-frequency C=C stretch modes and low-frequency ring-torsional modes. The dynamical mechanism is interpreted in terms of a hierarchical electron-phonon model, leading to the identification of generalized reaction coordinates for the nonadiabatic process. Variation of the vibronic coupling model parameters in a realistic range provides evidence that the direct exciton decay pathway is not dynamically robust, and bridge-mediated pathways can become dominant. The ultrafast, coherent dynamics is of pronounced nonequilibrium character and cannot be modeled by conventional kinetic equations. The predicted femtosecond to picosecond decay times are consistent with time-resolved spectroscopic observations.     

Exciton diffusion and relaxation in methyl-substituted polyparaphenylene polymer films

Exciton diffusion in ladder-type methyl-substituted polyparaphenylene film and solution was investigated by means of femtosecond pump-probe spectroscopy using a combined approach, analyzing exciton-exciton annihilation, and transient absorption depolarization properties. We show that the different views on the exciton dynamics offered by anisotropy decay and annihilation are required in order to obtain a correct picture of the energy transfer dynamics. Comparison of the exciton diffusion coefficient and exciton diffusion radius obtained for polymer film with the two techniques reveals that there is substantial short-range order in the film. Also in isolated chains there is considerable amount of order, as revealed from only partial anisotropy decay, which shows that only a small fraction of the excitons move to differently oriented polymer segments. It is further concluded that interchain energy transfer is faster than intrachain transfer, mainly as a result of shorter interchain distances between chromophoric units.     

Intrinsically conducting polymer blends

Polyaniline (PANI) doped with different dopants (HCl, dodecyl benzene sulfonic acid, (+)‐Camphor‐10 sulfonic acid, dinonyl naphthalene disulfonic acid) was synthesized by chemical oxidation method. The FTIR studies indicated that the back bone structure of doped PANI was similar. Thermal stability was evaluated in nitrogen atmosphere by dynamic thermogravimetry and PANI‐HCl sample showed minimum weight loss below 400°C. The electrical conductivity of PANI was not affected by the structure of dopants. The microwave absorption studies of several polymers blends containing PANI‐HCl and/or carbon black were also carried out by using wave guide technique.     

Thermodynamics of polymer blends

The general principles of thermodynamic equilibrium in binary liquid systems are reviewed briefly, and extended to quasi-binary mixtures of polydisperse polymers. Molecular models allowing actual phase behaviour to be discussed in terms of molecular parameters are exposed to data on the system polystyrene/polyvinylmethylether. Disparity in size and share between the repeating units must be introduced to obtain reasonable agreement between theory and experiment. The neccessary introduction of the molar-mass distribution detracts from this agreement which makes clear that other aspects exist that must be taken into account. For example, cross association between repeating units has a marked effect on phase behaviour. Blends are subject to two kinds of thermodynamic aging which lead either to considerable mutual solubility in supposedly immiscible blends, or to metastable equilibria transforming into states of lower Gibbs energy. In both cases physical proerties of the blend will change with time.     

Use of compatibilizers in polymer blends

Among the different ways of recycling plastic wastes, one of the techniques used consists in processing these products without any preliminary separation of the different plastic families. The bad performances of the obtained materials lead to the use of emulsifiers. The work described in this study concerns the synthesis of emulsifiers prepared by chemical modification of polymers with ozone. This reaction produces the formation of peroxides which are then used to initiate the grafting of comonomers. According to this method, we obtain graft copolymers usable as emulsifiers in the elaboration of polymer blends. Those graft copolymers prepared according to this method improve mechanical performances of polymer blends.     

Formation of crazes in polymer blends

High-voltage electron microscopy was used to study the micromechanical processes of deformation directly on thin deformed samples of rubber-modified, high-impact polymers. In these polymers the microprocesses are closely connected with the initiation and formation of crazes. Craze formation with its effects on the fracture toughness are discussed in dependence on several important morphological factors, particularly on the rubber volume content, particle diameter, and particle diameter distribution.