Polarons In Polymers Research Articles

The effect of interface hopping on inelastic scattering of oppositely charged polarons in polymers

The inelastic scattering of oppositely charge polarons in polymer heterojunctions is believed to be of fundamental importance for the light-emitting and transport properties of conjugated polymers. Based on the tight-binding SSH model, and by using a nonadiabatic molecular dynamic method, we investigate the effects of interface hopping on inelastic scattering of oppositely charged polarons in a polymer heterojunction. It is found that the scattering processes of the charge and lattice defect depend sensitively on the hopping integrals at the polymer/polymer interface when the interface potential barrier and applied electric field strength are constant. In particular, at an intermediate electric field, when the interface hopping integral of the polymer/polymer heterojunction material is increased beyond a critical value, two polarons can combine to become a lattice deformation in one of the two polymer chains, with the electron and the hole bound together, i.e., a self-trapped polaron—exciton. The yield of excitons then increases to a peak value. These results show that interface hopping is of fundamental importance and facilitates the formation of polaron—excitons.

Femtosecond Dynamics of Excitons and Hole-Polarons in Composite P3HT/PCBM Nanoparticles

The dynamics of charge separation in aqueous suspensions of regioregular P3HT nanoparticles containing PCBM were investigated for the first time using femtosecond transient absorption spectroscopy. This investigation is supported by the recently reported use of regioregular P3HT/PCBM nanoparticles as charge trapping and storage devices. In this study, the presence of excited-state and charge-separated species, including singlet excitons, polymer polarons and free charges, generated in rr-P3HT/PCBM nanoparticles was identified through visible pump and visible/near-infrared probe femtosecond transient absorption spectroscopy at a range of electron acceptor concentrations. The decrease of the singlet exciton lifetime by charge transfer to PCBM is well described by a one-dimensional diffusion model with a P3HT domain size of approximately 5 nm for 5-50 wt % PCBM. This model also indicates that bimolecular recombination is the dominant charge recombination mechanism at 20 wt % PCBM and above.

Synthesis and photo-induced charge separation of confined conjugation length phenylene vinylene-based polymers

We report the synthesis and photophysics of four confined chromophore phenylene vinylene (PPV)-based polymers where PPV-type trimer chromophores are linked by alkyl ether chains of increasing length (PT3–PT6). These confined chromophore polymers were blended in a 1 : 1 w/w ratio with the fullerene derivative PCBM and the photoinduced charge-separation properties compared to that of a fully conjugated polymer analogue (alt-co-MEH-PPV). Fluorescence from all polymers is quenched in the presence of PCBM and transient absorption measurements confirm the presence of polymer polarons arising from charge transfer from the polymer to the PCBM acceptor. The polaron decay kinetics of all the blend films were similar but the yields of polarons for the confined conjugation length polymers were lower than for alt-co-MEH-PPV:PCBM. These observations suggest that many of the initially formed radical ion pairs for the confined length polymers are unable to fully charge dissociate and undergo fast geminate recombination.

ESR and LESR X-band study of morphology and charge carrier interaction in blended P3HT–SWCNT and P3HT–PCBM–SWCNT solid thin films

An electron spin resonance (ESR) X-band study of regio-regular poly(3-hexylthiophene) (RR-P3HT) and RR-P3HT/PCBM ([6,6]-phenyl-C 61-butyric acid methyl ester) composites blended with a low concentration (∼0.1–1%) of single wall carbon nanotubes (SWCNT) is presented. The substantial line-width broadening of polaron and PCBM anion radical ESR spectra was registered after SWCNT incorporation. The possibility of an electron spin exchange interaction between SWCNT conductive electrons and polymer polarons in the RR-P3HT:SWCNT blend without light illumination and between SWCNT conductive electrons and polymer polarons and PCBM anion radical electrons in the RR-P3HT:PCBM:SWCNT blend under light illumination is discussed. In addition, the unusually significant RR-P3HT film morphology (dominant crystalline phase orientation) is substantially altered, in particular a rotation of up to 90° of the P3HT crystalline phase relative to the film plane, due to SWCNT annexation in the polymer, is registered by the ESR spectra external magnetic field angular dependences.

Charge generation dynamics in polymer/polymer solar cells studied by transient absorption spectroscopy

Currently, polymer/fullerene solar cells have significantly higher power conversion efficiency than polymer/polymer solar cells. In order to address the origin of this wide gap, we measured the transient absorption spectra of polymer/polymer blend films based on poly(3-hexylthiophene) (P3HT) and poly(9,9-dioctylfluorene-alt-benzothiadiazole) (F8BT), and analyzed the charge generation dynamics in comparison with that of a polymer/fullerene blend film based on P3HT and [6,6]-phenyl-C61 butyric acid methyl ether (PCBM). In P3HT/PCBM blend films, half of the polarons are generated immediately after pulsed laser excitation at the P3HT absorption band and the other half are generated with a rise constant of ∼10 ps. The prompt polaron formation is assigned to the charge generation from P3HT singlet excitons located near the P3HT/PCBM interface and the delayed polaron formation to the charge generation after the exciton migration to the P3HT/PCBM interface. In P3HT/F8BT, on the other hand, F8BT singlet excitons observed immediately after the laser excitation decayed in a picosecond and P3HT singlet excitons increased on the same time constant, suggesting efficient energy transfer from F8BT to P3HT. Polymer polarons were generated on a time scale of ∼40 ps, some of which were observed on a microsecond timescale but the yield was much smaller than that in P3HT/PCBM. We discuss the difference in charge generation dynamics in terms of kinetics and interfacial structures.

Magnetic Properties of Polypyrrole Doped with Iron

In this study the incorporation of small contents of iron species in polypyrrole was investigated by means of magnetization measurements. The incorporation was performed by the addition of FeCl3 to the solution during the chemical polymerization with a mass ratio of [monomer]/[Fe3+] = 10. Comparison was made between polypyrrole pellets with and without the addition of iron and showed an increase in the saturation magnetization and in the ferromagnetic properties of polypyrrole when iron is present in the material. This increase in the magnetic properties probably occurs due to an effective increase in local polarization of the polymer's polarons.

Charge Carrier Formation in Polythiophene/Fullerene Blend Films Studied by Transient Absorption Spectroscopy

We report herein a comparison of the photophysics of a series of polythiophenes with ionization potentials ranging from 4.8 to 5.6 eV as pristine films and when blended with 5 wt % 1-(3-methoxycarbonyl)propyl-1-phenyl-[6,6]C61 (PCBM). Three polymers are observed to give amorphous films, attributed to a nonplanar geometry of their backbone while the other five polymers, including poly(3-hexylthiophene), give more crystalline films. Optical excitation of the pristine films of the amorphous polymers is observed by transient absorption spectroscopy to give rise to polymer triplet formation. For the more crystalline pristine polymers, no triplet formation is observed, but rather a short-lived (approximately 100 ns), broad photoinduced absorption feature assigned to polymer polarons. For all polymers, the addition of 5 wt % PCBM resulted in 70-90% quenching of polymer photoluminescence (PL), indicative of efficient quenching of polythiophene excitons. Remarkably, despite this efficient exciton quenching, the yield of dissociated polymer+ and PCBM- polarons, assayed by the appearance of a long-lived, power-law decay phase assigned to bimolecular recombination of these polarons, was observed to vary by over 2 orders of magnitude depending upon the polymer employed. In addition to this power-law decay phase, the blend films exhibited short-lived decays assigned, for the amorphous polymers, to neutral triplet states generated by geminate recombination of bound radical pairs and, for the more crystalline polymers, to the direct observation of the geminate recombination of these bound radical pairs to ground. These observations are discussed in terms of a two-step kinetic model for charge generation in polythiophene/PCBM blend films analogous to that reported to explain the observation of exciplex-like emission in poly(p-phenylenevinylene)-based blend films. Remarkably, we find an excellent correlation between the free energy difference for charge separation (deltaG(CS)rel) and yield of the long-lived charge generation, with efficient charge generation requiring a much larger deltaG(CS)rel than that required to achieve efficient PL quenching. We suggest that this observation is consistent with a model where the excess thermal energy of the initially formed polaron pairs is necessary to overcome their Coulombic binding energy. This observation has important implications for synthetic strategies to optimize organic solar cell performance, as it implies that, at least devices based on polythiophene/PCBM blend films, a large deltaG(CS)rel (or LUMO level offset) is required to achieve efficient charge dissociation.

Spin-Flip of Polaron in Polymers with a Magnetic Impurity

Using a nonadiabatic evolution method, we investigate the spin-flipprocess of polaron in polymers with a magnetic impurity. Ourresults show that when the spin orientation of this impurity isfixed to be perpendicular to the spin of polaron (θ = π/2),the magnetic impurity causes a spin-flip process. The probabilityof the spin-flip increases with the increase of exchange integralJ up to about 0.35 eV and then decreases with the increase ofJ. In the case J is fixed while the spin orientation isadjustable, we find the probability of the spin-flip varies withthe impurity orientation and reaches a maximum value at θ = π/2.

Dynamics of polaron in polymers

In a polymer chain, an extra electron or hole will distort the chain to form a charged polaron. Under the influence of an applied electric field, the polaron will be accelerated for a short time and then move at a constant velocity while it will be dissociated for the field being stronger than a threshold. Within a dynamical non-adiabatic evolution method, we simulate the motion of polaron under the influence of the electric field which is present for different periods. (1) We find the lattice oscillation behind the polaron is a moving multi-breather excitation, which bears the incresed energy due to the electric field acting on the polaron, so that the polaron can move at a constant speed even in the presence of an electric field. (2) We present a full picture for the polaron scattering from an impurity, which may trap or repulse the charged polaron.

Inversed polarization of the single excited state of polaron

The single excited state of a polaron in polymers has been studied. It was found that its polarization is negative under an external electric field. The single excited state of a polaron has a similar behavior with that of a biexciton. The inversed polarization will disappear under a strong electric field and the single excited state of a polaron will dissociate into a polaron and a bipolaron.

Photoinduced Polaron-Splitting in Polymer

We simulate a relaxation process of a polaron in polymer after photoexcitation, and a new state is realized by means of proper excitation.The original lattice configuration of the polaron splits into twosymmetrical peaks, and consequently a double-well potential is formed,where the wavefunctions of electron localized in these two wells areentangled. Thus, this process provides a method to generate theSchrödinger cat state. According to the dynamical process of thelattice configuration, the relaxation time of splitting is about 150 fs.

ELECTRON INTERACTION AND THE POLARON IN POLYMERS

Currently, the extended Hubbard model, which is a good approximation only for narrow-band system, suggests that the width of polaron in polymer is heavily reduced by the electron-electron interaction. However, the polymers possess wide band, so the off-diagonal correlation cannot be neglected. We considered the effects of full electron-electron correlation on the width and the creation energy of polaron in polymer. In the case of strong screening, our results are greatly different from those of Hubbard model. This suggests that in polymer, the Hubbard model should be gone beyond.