C60 Composites Research Articles

Polyamide 12/ fullerene C60 composites: investigation on their mechanical and dielectric properties

The effect of fullerene С60 on mechanical properties of polymer nanocomposites based on PA12 was investigated. The nanocomposites were prepared by in situ polymerization. The Young’s modulus and tensile strength of the polymer nanocomposites are improved by about 20 % with loading of 0.02–0.08 wt.% the fullerene С60. Dielectric spectroscopy was used to investigate the influence of nanoparticles on the relaxation processes in polymer matrix. The segmental relaxation processes become faster with the fullerene С60 addition, an effect associated with a decrease of the glass transition temperature. In contrast, the secondary or γ relaxation process of PA12/ fullerene C60 nanocomposites was observed to slow down with addition of fullerene C60. These observations indicate that fullerene C60 as filler has an effect opposite to antiplasticizing that slow down the secondary relaxation (stiffening the material) in the glass state, while at the same time reducing the alpha relaxation time associated with cooperative segmental motions. It behaviour can be connected with inclusion complex formation between PA12 and fullerene C60.

Preparation of Composite Films of Conjugated Polymer and C60 by Electrophoretic Deposition and Their Photovoltaic Effect

Composite films of a polyfluorene derivative, poly[(9,9-dioctyl-2,7-divinylenefluorenylene)-alt-{2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylene}], and neat C60 with various C60 contents have been obtained by electrophoretic deposition from dilute suspensions. It has been observed that deposition from suspensions containing both components results in pinhole-free films that are applicable to photovoltaic devices, whereas inhomogeneous films are formed from the suspensions containing only one of the components. Much improvement in the power conversion efficiency was achieved in the photovoltaic devices using the polymer–C60 composites compared with the devices with a pristine polymer film. The applicability of electrophoretic deposition to the fabrication of photovoltaic devices with polymer–C60 composites has been successfully demonstrated, although the power conversion efficiency was only on the order of 10-3% at this stage.

Quadrimolecular recombination kinetics of photogenerated charge carriers in the composites of regioregular polythiophene derivatives and soluble fullerene

Light-induced electron spin resonance (LESR) measurements have been performed on the composites of regioregular polythiophene derivatives and [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) in order to study the recombination kinetics of photogenerated charge carriers. We adopt two regioregular polymers with different side chains; head-to-tail poly(3-hexylthiophene) (RR-P3HT) and head-to-head poly(3-dodecynylthiophene-2,5-diyl) [HH-P3(C≡CDec)Th]. In both systems, two LESR signals due to positive polarons on the polymer (g∼2.002) and fullerene radical anions (g∼2.000) have been observed. Quadrimolecular recombination (QR) kinetics, previously reported for RR-P3HT/C60 composites, where two positive polarons and two radical anions recombine simultaneously, has been confirmed in both systems by the observation of Iex0.25 dependence of the LESR intensity on the excitation light intensity (Iex) and the decay curve of the LESR intensity. This process implies the formation of doubly-charged states such as bipolarons or polaron pairs on the polymer to attract two radical anions. Temperature dependence of the QR rate constant, γ, in both systems has exhibited a crossover of the transport mechanism from low temperature tunneling to high temperature hopping process, as in the case of RR-P3HT/C60 composites. In the RR-P3HT/PCBM composites, γ has exhibited marked dependencies on the PCBM concentration or annealing, which may be related to the change of the crystallinity of the phase-separated polymer and fullerene domains as well as their interface structures, affecting the carrier mobilities or the trap states at the interface. Associated change of the molecular orientation of RR-P3HT crystalline domains with the lamellar structure has been further confirmed from the anisotropic LESR signals of the cast films on the substrates, exhibiting a qualitative agreement with the reported x-ray or optical analyses. In the HH-P3(C≡CDec)Th/PCBM composite, γ has been smaller than those in the RR-P3HT/PCBM composites, reflecting the difference of local structures due to the different molecular structure. Furthermore, the hyperfine-determined LESR linewidth of the positive polaron has exhibited a smaller value than those in the RR-P3HT composite, implying the larger extension of the polaron wave function on the polymer chain, which is consistent with the highly coplanar structure of this polymer.

AC CONDUCTIVITY AND DIELECTRIC PROPERTIES OF PMMA/FULLERENE COMPOSITES

Dielectric properties and electrical conductivity of different concentrations of Poly methylmethacrylate (PMMA)/fullerene C60 composites have been reported at room temperature in the frequency range from 1 kHz to 5 MHz. The frequency dependence of complex permittivity, ε*, and complex electrical modulus, M*, have been measured. Frequency dependence of dielectric constant, follows Cole–Cole dielectric relaxation equations, was investigated diagrammatically. Moreover, equations of Tsangaris et al. have been tested for our case to describe the dielectric behavior of particulate polymeric composite containing fillers which give a satisfactory agreement taking into account the variation of the aspect ratio with the volume fraction of fullerene C60 doped in the PMMA matrix.

Nonlinear optical properties of poly(N-vinylcarbazole)-C60 nanocomposites

The influence of the fullerene content and temperature on the IR absorption and luminescence spectra of poly(N-vinylcarbazole) (PVK)-C60 composites prepared by vacuum deposition has been studied. It has been demonstrated that the luminescence spectra of the composites produced by vacuum deposition on substrates differ significantly from the spectra of the samples prepared by pouring of a toluene solution of PVK on the same substrates and the spectra of the samples synthesized by deposition in an inert gas atmosphere. The explanation for this phenomenon has been proposed. It has been revealed that the sizes of the structures absorbing IR radiation increase with an increase in the C60 concentration in PVK-C60 layers. It has been experimentally proved that, under the influence of light with a wavelength of 514.5 nm, the PVK-C60 composites undergo photoinduced reversible changes, which lead to nontrivial concentration-dependent changes in the luminescence spectrum.

Synthesis of nanostructured metal (Fe, Al)-C60 composites

AbstractComposites of M-2.5 mol. % Fullerene C60 composites (where M= Fe or Al) are prepared by mechanical milling and Spark Plasma Sintering (SPS). The SPS technique has been used to consolidate the resulting powders and preserve the massive nanostructure. Results of X-Ray Diffraction and Raman Spectroscopy show that larger milling balls (9.6 mm in diameter) produce transformation of the fullerene phase during mechanical milling. Alternatively smaller milling balls (4.9 mm in diameter) allow retention of the fullerene phase. SEM shows homogeneous powders with different particle sizes depending on milling times. Sintering produces nanostructured composite materials with different reinforcing phases including C60 fullerenes, diamonds and metal carbides. The presence of each phase depends characteristically on the energy input during milling. Transmission Electron Microscopy (TEM) and Raman Spectroscopy show evidence of the spatial distribution and nature of phases. Diamonds and carbides can be identified for the sintered Fe containing composites with a relatively high volume fraction.

The influence of thermal annealing on the morphology and structural properties of a conjugated polymer in blends with an organic acceptor material

In this report, the influence of thermal annealing of thin P3HT films and P3HT:C60 composites were studied regarding their morphology and structural properties. Atomic force microscope measurements on P3HT films and P3HT:C60 composite disclose some variation in morphology during annealing due to the crystallization of C60. The as-prepared P3HT:C60 films have a higher surface roughness and larger cluster size compared to the as-prepared P3HT films. The thermal annealing effects on the optical microscopy indicate that the polymer shows improved capability to self-organize. Their structural properties were studied by X-ray diffraction analysis. It was found that the crystallinity of the investigated films is drastically increased upon annealing and a decrease in the grain sizes is observed.

Electrical Properties and Operating Mechanisms of Nonvolatile Organic Memory Devices Fabricated Utilizing Hybrid Poly(N-vinylcarbazole) and C60 Composites

Organic memory devices based on hybrid poly(N-vinylcarbazole) (PVK) and C60 composites were fabricated with a spin-coating method. Atomic force microscopy images showed that the surface of the PVK layer containing the C60 molecules was relatively smooth. Capacitance–voltage (C–V) measurements on the Al/C60 embedded in PVK layer/p-Si(100) device at room temperature showed a clockwise hysteresis with a large flatband voltage shift due to the existence of the C60 molecules in the PVK layer, indicative of charge storage in the embedded C60 molecules. The clockwise direction of the C–V hysteresis for the devices was attributed to carrier tunneling through the PVK layer emitting from the Al gate electrode, and the negative and positive flatband voltage shifts of the C–V curves originated from the holes and electrons captured in the C60 molecules, respectively. Possible operating mechanisms corresponding to writing and erasing processes for the Al/C60 embedded in PVK layer/p-Si (100) device are described on the basis of the C–V results.

Ambipolar charge transport in bulk heterojunction of poly(2-methoxy-5-(2′-ethylhexyloxy)-1,4-phenylenevinylene)∕C60 composite

Using the time-of-flight technique (TOF), the influence of C60 concentrations on ambipolar charge transport in poly(2-methoxy-5-(2′-ethylhexyloxy)-1,4-phenylenevinylene) (MEH-PPV)∕C60 composite has been investigated. Adding C60 to the blend increases both electron and hole mobilities, compared to the pristine polymer. In the pristine MEH-PPV, the mobility of electrons is below the measurable range. However, electron mobility (10−7 cm2 V−1 s−1) could be calculated when the weight ratio of C60: MEH-PPV increased to 1:18. The temperature and electric field dependence of mobility is analyzed in the disorder formalism. It is observed that above ∼290 K the slope of the electric field dependence of the electron mobility becomes negative. The occurrence of negative field dependence in composite samples is attributed to the large positional disorder (Σ=4).

Influence of the processing conditions on charge transfer and transport properties in poly(2-methoxy-5-(2′-ethylhexyloxy)1-4-phenylenevinylene):C60 composites for photovoltaics

Polymer-based photovoltaic devices have been elaborated by blending the conjugated polymer, poly(2-methoxy-5-(2′-ethylhexyloxy)1-4-phenylenevinylene) (MEH-PPV) with the buckminsterfullerene, C60. The solvent used has a main influence on the optical and electrical properties of the photoactive layers. This could be attributed to different dispersion abilities of C60 in the polymer layers shown by Atomic Force Microscopy and Scanning Electron Microscopy. Its effect on the transfer and transport properties has been studied. Thin film processing conditions have been modified by the preparation of blends of solutions of the polymer in Tetrahydrofuran and fullerene in Ortho-dichlorobenzene. The resulting spin casted layers show improved morphologies implying better dispersion of the fullerenes and increased short circuit currents. A decrease in the fullerene critical concentration to form percolation paths has been demonstrated. The improvement of the photovoltaic properties of the MEH-PPV/C60 composites has been attributed to the nanosized fullerene domains formed upon phase separation. We used optical spectroscopy to study the charge transfer efficiency and electrical measurements to investigate charge transport properties in MEH-PPV:C60.

Ultrafast Spectroscopic Study of Photoinduced Electron Transfer in an Oligo(thienylenevinylene):Fullerene Composite

AbstractPhotoinduced electron transfer and competing processes have been studied in composites of an oligo(thienylenevinylene) (OTV), comprised of ten dibuthoxyl‐thiophene units separated by vinylene units, and a C60 derivative, [6,6]‐phenyl‐C61 butyric acid methyl ester (PCBM), by using femtosecond transient absorption spectroscopy and sub‐nanosecond transient photoconductivity. We find that in OTV:PCBM the photoexcitations decay primarily via intrachain relaxation rather than photoinduced electron transfer from OTV to PCBM. The electron‐transfer process requires ca. 14 ps; larger by more than two orders of magnitude than the required time observed in conjugated‐polymer:C60 composites, and also larger than the 0.6 ps singlet‐state lifetime in OTV. These observations indicate that the quantum efficiency for photoinduced electron transfer in OTV:PCBM is less than 5 %.

A study of C60–poly(m-phenylenevinylene-co-2,5-dioctoxy-p-phenylenevinylene) nanocomposite

We present spectroscopic and conductivity studies of poly(m-phenylenevinylene-co-2,5-dioctoxy-p-phenylenevinylene) (PmPV)/C60 composite. The UV–Vis absorption spectroscopy, resonance Raman spectroscopy, near-field scanning optical and atomic force microscopy, and fluorescence spectroscopy confirm charge transfer and excitonic transitions between both materials in the composite morphology. Both UV–Vis and fluorescence spectra have shown the existence of an electron transfer from the polymer to C60, while broadening of the already existing peak at 624 cm−1 and the appearance of a new broad peak at 1850 cm−1 for the Raman spectra are due to exciton generation. The results from the conductivity measurements also show enhanced responsiveness with increasing C60 concentration. We observe consistent dark and photocurrent increase due to C60 encapsulation by PmPV, where the composite has much better charge transfer capacity and better responsiveness to exciton generation.

Influence of the Nanoscale Morphology on the Photovoltaic Properties of Fullerene/MEH‐PPV Composites

AbstractThe relation between morphology and photoelectric properties of PPV derivatives/fullerene composites forming bulk heterojunction solar cells has been investigated. The solvent used to spin cast the photoactive layers has a main influence on the quenching of the MEH‐PPV fluorescence, which could be attributed to different dispersion abilities of C60 in the polymer layer shown by AFM microscopy. Formation of large fullerene aggregates is observed at fullerene concentrations of the order of 10% leading to phase separation for composite layers processed in THF, whereas more dispersed distributions of fullerenes are observed in an aromatic solvent like ODCB which accounts for a more efficient luminescence quenching with increasing filler concentrations. However the improvement of the dissociation of photogenerated charge pairs is counterbalanced by a less efficient charge transport in the composite shown by lower short circuit currents probably due to unfavorable polymer chain arrangement in ODCB. Thin film processing conditions have been modified by the preparation of blends of solutions of the polymer in THF and fullerene in ODCB. The resulting spin casted layers show improved morphologies implying better dispersion of the fullerenes and increased short circuit currents. The improvement of the photovoltaic properties of the MEH‐PPV/C60 composites has been attributed to the nanosized fullerene domains formed upon phase separation.

Physicochemical properties of Water-Soluble Fullerene C60-Carbohydrate Composites

Water-soluble composites of fullerene C60 with carbohydrates (saccharose, fructose, and dextrans) prepared by the mechanochemical procedure were analyzed by electronic absorption and 13C NMR spectroscopy, thin-layer chromatography, and differential scanning calorimetry.

Long‐Lived Photoinduced Charge Separation in (Zinc, Lead) Phosphate Glass–C60 Composites

A group of (zinc, lead) phosphate glass–C60 composites that display long‐lived photoinduced electron transfer from the Pb2+ center to the fullerene upon 230–400 nm UV irradiation has been developed. Both the steady state photoinduced absorption and photoinduced electron spin resonance spectra of the irradiated system support this view. Analysis of the carrier dynamics suggests that the electrons migrate from the hole sites to the fullerene sites in a tunneling mode.

Composites of C60 based poly(phenylene vinylene) dyad and conjugated polymer for polymer light-emitting devices

A C60-based poly(phenylene vinylene) (PPV) dyad poly{(2,5-di-pentoxyl-phenylene)-4diylvinylene-3,6-[9-(1-azafulleroid-propyl)-carbazolenevinylene]} (PPV–AFCAR) and an emissive conjugated polymer poly{1,4[2-(3,7-dimethyloctyloxy)-3,5,6-trimethoxy]phenylene vinylene} (POMPV) were prepared to investigate the role of [60]fullerene in conjugated polymer composites. Multilayer organic light-emitting devices with the configuration of indium tin oxide/poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS)/blends/2-(4-biphenylyl)-5-(4-tert-butylphenyl)-1,3,4-oxadiazole(Bu-PBD)/LiF/Al were fabricated using PPV–AFCAR as a dopant and POMPV as an emissive material.

The Role of Interfaces in Photovoltaic Devices

Organic opto-electronic devices comprise one or more organic layers and the electrodes. The interfaces between these very different components play a crucial role to the performance of the devices. In donor–acceptor composites for photovoltaics, the electronic processes occurring at the interface will benefit from a particular interface morphology on the 10–100 nanometer level; this is demonstrated for composites of oligophenylenevinylene and C60. Phase separation on such a scale may be achieved naturally in diblock copolymers. The synthesis of an OPV–C60 diblock copolymer is described.

Electron Spin Resonance and Infrared Spectroscopy Study of the Polyvinylpyrrolidone-C60 Composite

Recently we have found from ESR spectra that the electron charge transfer take place in the polyvinylpyrrolidone-C60 composite. In this paper the polystyrene-C60, polyvinylchloride-C60, poly(ethylene glycol)-C60, and poly(ethylene oxide)-C60 composites are prepared and probed by the ESR technique for comparison. They show no signal which might be referred to the electron transfer. Along with this, we continue ESR study of the polyvinylpyrrolidone-C60 composite and also present IR spectra, which indicate the involvement of the carbonyl group of polyvinylpyrrolidone into the electron charge transfer process.

Optical Properties from Composites of C60 and FAU Zeolite

We have investigated the photophysical properties of C60 (guest)-FAU zeolite (host) composite, focusing mainly on the properties of a single crystallite using microscopic techniques (UV-Vis-IR absorption and photoluminescence). The composite C60-FAU is of great interest because it exhibits new optical properties different from that of either C60 powder or FAU zeolite.

Photoinduced charge transfer in conducting polymer C60 composites

We propose a simple model for photoinduced charge transfer in conducting polymer C60 composites. Our model consists of coupled Su–Schrieffer–Heeger models for a single polymer chain and a C60 molecule. We have included the coupling of the molecular vibrations to the π electrons but have neglected explicit electron–electron interaction. Using adiabatic and nonadiabatic molecular dynamics, we find photoinduced electron transfer takes place in 200–300 fs. This is consistent with an experimental upper bound of 300 fs. Nonadiabatic effects tend to average out pronounced oscillatory behavior in the adiabatic calculations.