Polarons In Poly Research Articles

The optical signatures of molecular-doping induced polarons in poly(3-hexylthiophene-2,5-diyl): individual polymer chainsversusaggregates

For molecularly doped poly(3-hexyl-thiophene) solvated individual chains can be unambiguously differentiated from aggregated ones by diagnostic polaron absorption.

High Intrachain Order Promotes Triplet Formation from Recombination of Long-Lived Polarons in Poly(3-hexylthiophene) J-Aggregate Nanofibers

Photoluminescence (PL) of single poly(3-hexylthiophene) (P3HT) J-aggregate nanofibers (NFs) exhibits strong quenching under intensity-modulated pulsed excitation. Initial PL intensities (I(0)) decay to steady-state levels (ISS) typically within ∼ 1-10 μs, and large quenching depths (I(0)/I(SS) >2) are observed for ∼ 70% of these NFs. Similar studies of polymorphic, H-aggregate type P3HT NFs show much smaller PL quenching depths (I(0)/I(SS) ≤ 1.2). P3HT chains in J-type NF π-stacks possess high intrachain order, which has been shown previously to promote the formation of long-lived, delocalized polarons. We propose that these species recombine nongeminately to triplets on time scales of >1 ns. The identity of triplets as the dominant PL quenchers was confirmed by subjecting NFs to oxygen, resulting in an instantaneous loss of triplet PL quenching (I(0)/I(SS) ∼ 1). The lower intrachain order in H-type NFs, similar to P3HT thin-film aggregates, localizes excitons and polarons, leading to efficient geminate recombination that suppresses triplet formation at longer time scales. Our results demonstrate the promise of self-assembly strategies to control intrachain ordering within multichromophoric polymeric aggregate assemblies to tune exciton coupling and interconversion processes between different spin states.

Spectroelectrochemical characterization of anionic and cationic polarons in poly(3-hexylthiophene)/fullerene blend. Effects of morphology and interface

To investigate the effect of film morphology on the electronic structures of anionic and cationic polarons of poly(3-hexylthiophene), P3HT•− and P3HT•+, we have characterized the subgap optical absorption of these polaronic species by means of spectroelectrochemistry. For the pristine films of regiorandom (RRa) P3HT or its blend with [6,6]-phenyl-C61-butyric acid methyl ester (PC61BM), both the ground-state bleaching and the subgap absorption of RRa-P3HT•− resemble closely to those of RRa-P3HT•+. For the solvent vapor annealed neat film of regioregular (RR) P3HT, RR-P3HT•− shows distinctly different absorption spectra from RR-P3HT•+, and the difference is substantially enhanced by mixing up with PC61BM. The spectral differences are ascribed to the effects of the boundaries among the polymer phases and the polymer–fullerene interfaces. With respect to the neat RR-P3HT film, the RR-P3HT/PC61BM blend exhibits a 0.05–0.06eV downshift of the SOMO levels for both types of polarons, implying the anisotropic hole and electron conductivities of crystalline RR-P3HT phases, which may play important roles in the transport and photogeneration of free charge carriers.

Visualizations of charge transfer for the model of poly(3,4-alkylenedioxythiophene)s in neutral and various oxidation states

The charge transfer mechanisms of the poly(3,4-alkylenedioxythiophene)s model in neutral and various oxidation states are theoretically investigated, which is stimulated by the recent experimental reports [C. Lin, et al., J. Am. Chem. Soc. 2011, 133, 11339.]. Due to the difference of charge distribution on different units at various oxidation states, the bond lengths of inter-units are significantly different. The properties of excitons and polarons in poly(3,4-alkylenedioxythiophene)s model in neutral and various oxidation states were revealed by quantum chemical calculations. In particular, simultaneous use of different correlation plots in atomic site representation and transition densities in real space representation reveals the electronic structure and spatial localization of the elementary excitations. Theoretical results state that the charge transfer mechanisms for neutral and oxidation states are significantly different. The theoretical results promote deeper understanding for the structural, optical and electronic properties of poly(3,4-alkylenedioxythiophene)s model, and will offer crucial hints for the rational design of novel transparent conductive poly(3,4-alkylenedioxythiophene)s and relative polymers.

Theoretical Study of Polaron Formation in Poly(G)−Poly(C) Cations

Polaron formation in poly(G)-poly(C) cations is investigated with density functional theory (DFT) and molecular mechanics (MM) employing a two-layer ONIOM method. In these calculations, the high layer, composed of all complementary base pairs, is treated by a DFT method, while the low layer, which includes the sugar-phosphate backbone, counterions and water molecules, is described by the AMBER force field. The high layer is the model system in which the charge transfer takes place. According to our calculations, three or four guanines move in a paddle-like fashion when an electron is removed from the neutral model system. In the cation model system, about 80% of the charge is delocalized onto the guanine residues, and the remaining charge is delocalized onto the cytosine residues. This happens because guanine has a lower ionization potential (IP) than cytosine. The counterions and water molecules in the low layer are important in the geometry optimization. The optimized geometry of the model system is closer to the standard B-form structure when counterions and water molecules are included than when they are omitted. Comparison of the optimized neutral and cationic model systems reveals a polaron in poly(G)-poly(C) cations extending from the first to the third guanine. It is demonstrated that the position of counterions and the number of surrounding water molecules can affect polaron formation.

Determination of free polaron lifetime in organic bulk heterojunction solar cells by transient time analysis

A transient response technique that is widely used to measure the minority carrier lifetime in inorganic semiconductors is proposed to measure the lifetime of free polarons in a polymer:fullerene bulk heterojunction (BHJ) solar cell. A numerical model that can be used to describe the transient behavior of BHJ devices has been developed. Using the proposed method, the lifetime of free polarons in poly (3-hexylthiophene) and [6, 6]-phenyl C61-butyric acid methyl ester blend film is estimated to be in the range of 300–400 ns.

Feature of Polaronic Charge Carriers in Polysilanes: Experimental and Theoretical Approach

Formation and transport of polarons in poly[methyl(phenyl)silylene] is discussed. Analytical theory based on the effective medium approach is used to explain the electric field dependence of charge carrier mobility. The obtained value of the polaron binding energy is about 0.29 eV. This result is in agreement with quantum chemical calculations based on the B3LYP method. Charge carrier transport can be described by a small polaron movement in the presence of an energetic disorder. Polaron binding energy represents significant part of the activation energy of the charge mobility.

Electron spin resonance and electron nuclear double resonance of photogenerated polarons in polyfluorene and its fullerene composite

Electron spin resonance (ESR) and electron-nuclear double resonance (ENDOR) of photogenerated polarons in poly(9,9-dioctylfluorene) (PFO) and its composite with fullerene $({\text{C}}_{60})$ using variable photoexcitation energy up to 4.1 eV are reported. For PFO, a light-induced ESR (LESR) signal $(g=2.003)$ is observed below 60 K, and its transient response and excitation spectrum indicate that the observed spins are photogenerated polarons on PFO. For the ${\text{PFO-C}}_{60}$ composite, two LESR signals of photogenerated positive polarons on PFO $({g}_{1}=2.003)$ and radical anions on ${\text{C}}_{60}$ $({g}_{2}=1.999)$, respectively, are observed below 120 K, which are caused by photoinduced electron transfer from PFO to ${\text{C}}_{60}$. A remarkable enhancement of the LESR signals in the excitation spectrum at $\ensuremath{\sim}2.8\text{ }\text{eV}$ is observed compared with the case of pure PFO. The bimolecular-recombination kinetics of photogenerated charge carriers in the composite are confirmed by the dependence of the LESR on excitation-light intensity and by the decay dynamics. Light-induced ENDOR (LENDOR) signals are clearly observed for excitation around 2.8 eV owing to the highly efficient photoinduced electron transfer in the composite. Broad LENDOR shifts directly reflect the spin-density distribution of the polarons in PFO. We have determined its maximum shift using LENDOR-induced ESR, and have evaluated the maximum spin density on the carbon site coupled to the proton as 0.032. This value is consistent with the theoretical result obtained by Pariser-Parr-Pople (PPP) model, where the spatial extent of the polarons is calculated as $\ensuremath{\sim}3$ monomer units of PFO. The calculated LESR spectra of PFO based on the PPP model are consistent with the experimental spectra, which confirm the above spatial extension of the polaron in PFO.

Interactions of polarons in poly(3-dodecythiophene) at low temperature

Zero-field cooled-field-cooled (ZFC-FC) technique was used to investigate the effect of low temperature (4.6 K) on the interactions of polarons in weakly FeCl 3-doped poly(3-dodecylthiophene (PDDT). It was found that the magnetic properties of the system changed with the length of time system spent at low temperature: the Currie–Weiss paramagnetism gradually changed over to “antiferromagnetism” and ultimately to diamagnetism. This effect is connected with the thermochromic properties of self-organized PDDT in primary (10–15 nm) or secondary (30–50 nm) induced aggregates, in which cooling under the glass transition temperature T g (∼240 K) causes extension of the effective conjugation length of coplanar polymer chains. Changes in magnetic properties are related to the inter- and intra-chain interactions of polaron states within both the primary and secondary induced aggregates.

Hole polarons in poly(G)-poly(C) and poly(A)-poly(T) DNA molecules

The polaron might play an important role in the process of charge migration through duplex DNA stack. In the present work, we investigate properties of hole polarons in DNA molecules containing identical base pairs, such as poly(G)-poly(C) and poly(A)-poly(T), An extended tight-binding model (extended Su-Schrieffer-Heeger model), which involves the effect of an electric field in the direction of DNA stack, will be introduced. The transfer integral and electron-phonon coupling parameters in this model are obtained according to ab initio calculation for different base pair dimers. Calculations reveal that the polaron in poly(A)-poly(T) has a wider shape and a higher mobility under a specific electric field than that in poly(G)-poly(C) DNA stack.

Energy evolution of polaron in poly (p-phenylene) in the presence of high electric field

namics of polaron in poly (p-phenylene) (PPP) in the presence of high electric field is studied by using an extended tight-binding Model. Due to the strong electron-phonon coupling in PPP chain, the lattice potential energy and the electronic energy oscillate out of phase; the doped electron is still confined in a limited area of the lattice even if the applied field is as strong as 0.5V/nm.

ESR study of light-generated polarons in poly(p-phenylene vinylene) derivatives

Light-induced ESR (LESR) has been employed to study light-generated polarons in two dialkoxy derivatives of poly(p-phenylene vinylene) (PPV), MEH-PPV and CN-PPV, where cyano groups are attached at the vinylic carbon sites in the latter. LESR action spectra of both polymers show a clear threshold energy around 3eV, which compares well with the photocurrent action spectra reported for PPV derivatives. A clear reduction of LESR linewidth is observed in CN-PPV compared with MEH-PPV, providing direct evidence that the spin density of polaron predominantly resides on the vinylic carbons. This feature can be reproduced by the linewidth analysis based on the theoretical spin distribution of a polaron with the extension of approximately 4 phenyl rings.

Electroabsorption and photoconductivity studies of the Frenkel excitons and negative polarons in poly(2,5-pyridinediyl)

We present electroabsorption and photocurrent measurements on disordered and oriented films of the conjugated polymer poly(2,5-pyridinediyl) (PPY). The electroabsorption spectrum is dominated by a Stark-red-shift of the first allowed optical transition at 3.2 eV and by the emergence of a normally one-photon forbidden state at 3.7 eV. The spectrum was successfully fitted to a linear combination of the linear absorption spectrum and its first and second derivative revealing the transition at 3.2 eV to be due to Frenkel excitons. Polarized EA spectroscopy on oriented films allow us to separate the parallel and perpendicular components (with respect to the polymer chain axis) of the first allowed optical transition. Analysis of the spectra provide an indication for the existence of a small number of delocalized Wannier-like excitons photogenerated by light polarized perpendicular to the polymer chains. Photoconductivity measurements have been carried out on ITO/PPY/Au (semitransparent) sandwich cells using the four possible directions of the applied electric field and illumination directions. Analysis of the results shows that the photocurrent originates from the dissociation of excitons, which are photogenerated inside the polymer near the negative electrode and evidence is given for the electron transporting properties of PPY.

Spin distributions and excitation spectra of optically generated polarons in poly( p-phenylenevinylene) derivatives

Light-induced electron spin resonance (LESR) measurements have been performed on two dialkoxy derivatives (MEH-PPV and CN-PPV) of poly( p-phenylenevinylene) where cyano groups are attached at the vinylic carbon sites in CN-PPV. The transient response and excitation spectra of the LESR signals indicate that the observed spins are optically generated polarons. A clear reduction of ESR linewidth has been observed in CN-PPV compared with MEH-PPV, providing direct evidence that the polaron spin density resides predominantly on the vinylic carbons. Linewidth analysis based on a theoretical model for the spin distribution yields an extension of the polaron over approximately four phenyl rings.

Dynamics of Long-lived Polarons in Poly( p-phenylene)-Type Ladder Polymers (LPPP)

The X-band spin 1/2 photoluminescence-detected magnetic resonance (PLDMR) of LPPP films and solutions featuring a high degree of intrachain order and high PL quantum yields (24% in films and 84% in solutions) is described. Excitation at 458 run yields a PL-enhancing polaron resonance at g~2.00 of width ΔH 1/2~8G and 4G in films and frozen solutions, respectively. The resonances are assigned to enhanced nonradiative recombination of long-lived polaron pairs, which are singlet exciton quenching centers. Their behavior is consistent with two distinct polaron pair lifetimes or a very broad distribution of lifetimes which are ~10 times longer in solutions than in films. The distinct lifetimes which are ~40 and ~670μs in films and ~370 and ~3300μs in solutions, are discussed in relation to longer-lived intrachain interconjugation segment pairs and shorter-lived interchain pairs. The spectral dependence of the resonance demonstrates that the emission from charge transfer excitons in aggregates of the films is much more spin-dependent than that from single polymer chains of the frozen solutions.

Electron Spin Resonance Spectra of Photogenerated Polarons in Poly(Paraphenylene Vinylene)

Light-induced ESR (LESR) measurements have been performed on undoped poly(parapheny- lene vinylene) (PPV) down to 4 K. The ESR signal increases significantly for the excitation energy above 3.1–3.2 eV, as in the case of the excitation spectra of photocarriers reported in PPV derivatives. The anisotropic light-induced ESR spectra in oriented samples are well reproduced by the spectra calculated using a theoretical polaron spin distribution in the case of finite electron-electron interaction. These spectral features indicate that the observed spins are photogenerated polarons.

Photogeneration of polaron pairs in conducting polymers.

It is usually assumed that when photogeneration in a conducting polymer results in an electron and hole on separate chains they form negative and positive polarons that can move independently of each other. We show, on the basis of the small carrier yield per photon seen in picosecond photoconductivity, the different behavior of photoinduced absorption (PA) in dilute solution and thin films, and the spectral distribution of the PA in thin films, that photogenerated positive and negative polarons in poly(p-phenylene vinylene), polythiophene, and polyacetylene are, for the most part, bound in pairs by their Coulomb attraction. We also show that PA data give evidence for a gap of 2.8 eV, and thus an exciton binding energy of 0.4 eV, in poly(p-phenylene vinylene).

ELECTRON NUCLEAR DOUBLE RESONANCE SPECTROSCOPY OF SOLITONS AND POLARONS IN CONJUGATED POLYMERS

Novel elementary excitations such as solitons and polarons in conjugated polymers carry unpaired electrons. In this case electron nuclear double resonance (ENDOR) of protons on the conjugated chain provides a unique method to observe their wavefunctions. This information has two significant meanings. Firstly, it gives the spatial extension of solitons and polarons, the quantity that is essential in examining the reality of these self-localized excitations. Secondly, it directly probes the degree of electron correlation. In the case of finite electron correlation, negative spin sites appear in the spin-density distribution. The unique and detailed determination of the wavefunctions containing such information can be made only after the careful studies of oriented polymers because of the non-single-crystalline nature of these materials. In this article I review our ENDOR studies on solitons in polyacetylene and polarons in poly(paraphenylene vinylene), performed using stretch-oriented samples. All the analyses are based on the properties of π-electrons which show characteristic anisotropy in oriented samples. The observed spin distributions are compared with theoretical results as well as shown as the basis for further analyses of conjugated polymers.

Polarons and their stability in poly(phenylenevinylene)

To study properties of polarons in poly(phenylenevinylene) (PPV) we set up a tight-binding Hamiltonian along the lines of the SSH Hamiltonian for polyacetylene. The parameters were determined by fitting the bandgap, valence bandwidth and vinylene geometry to better than 1%. We used the resulting Hamiltonian to calculate the geometry of the polaron and the distance of the polaron level from the closest band edge as a function of chain length. To describe interchain coupling we took into account the actual separation of each atom from the closest atom on the neighboring chain which, unlike polyacetylene, is different for each atom in the PPV monomer. It was found that interchain coupling does not destabilize the polaron in PPV.

Intrachain photogeneration of polarons in poly(3-methylthiophene)

We report detailed measurements of the excitation profile of the absorption spectrum due to photoexcited polarons and bipolarons in the non-degenerate ground state conjugated polymer poly(3-methylthiophene). Two new sharp peaks at 2.10 and 2.25 eV are observed. The separation of the two peaks, which corresponds to an intrachain vibration, is taken as evidence for a channel of photogeneration of polarons via an intrachain process in poly(3-methylthiophene).