Mechanism of carrier generation and recombination in conjugated polymers

Transient excited-state absorption measurements in the spectral region spanning the infrared active vibrational active (IRAV) modes in prototypical luminescent polymers, poly(phenylene vinylene) (PPV), and poly[2-methoxy-5-(2-ethyl-hexyloxy)-(phenylene vinylene)] (MEH-PPV), reveal charge carrier generation within 100 fs after photoexcitation. The photocarrier quantum efficiency in MEH-PPV is φ₀≈0.1 in zero applied electric field. There is no correlation between the temporal behavior of the photoinduced IRAV signals and the exciton lifetime. Thus, carriers are photoexcited directly and not generated via a secondary process from exciton annihilation. Our data indicate that the carrier recombination rate is sensitive to the strength of the interchain interaction.     

White emission from a ternary polymer blend by incomplete cascade energy transfer

White light emission was obtained from a light-emitting diode (LED) prepared from a ternary polymer blend (19:1:1 by weight) consisting of poly(9-vinylcarbazole) (PVK), poly(9,9′-dihexlyfluorene-2,7-divinylene-m-phenylenevinylene-stat-p-phenylenevinylene) (CPDHFPV), and poly[2-methoxy-5-(2′-ethyl-hexyloxy)-1,4-phenylenevinylene] (MEH-PPV), where the order of bandgap energy is PVK>CPDHFPV>MEH-PPV. The major component PVK acted not only as the matrix or diluent but also as the excitation energy donor to help the blend generate white light with high efficiency. Good miscibility between PVK and CPDHFPV facilitated the Förster-type excitation energy transfer from PVK to CPDHFPV enhancing the quantum efficiency. However, poor miscibility between CPDHFPV and MEH-PPV resulted in partial energy transfer between the polymers causing the blend to emit two colors simultaneously. Consequently, the incomplete cascade energy transfer in the blend generated a pure white color near CIE coordinate (0.33, 0.33) and the emissive color of this system showed a low sensitivity to the drive voltage.     

Synthesis,spectroscopy and electrochemistry study on a novel di-silyl substituted poly(p-phenylenevinylene)

A novel di-silyl substituted poly(p-phenylenevinylene) (PPV) derivative is synthesized through the Gilch route. The polymer is solution processable and shows high thermal stability in air. UV–Vis spectra for the film and solution samples illustrate some different features, in which the absorption peak corresponding to the π–π* transition of the delocalized electrons along the polymer back bone is red shifted and broadened in the film state compared to the solution one. The fluorescent emission spectra are quite similar for the polymer in solution and in the solid states. Cyclic voltammetric investigation reveals a partially reversible n-doping process and an irreversible p-doping process. The cathodic current is much greater than that of the anodic one, which means the polymer has the tendency to be charged through n-doping rather than p-doping process. The improved affinity of the polymer to electrons may be owing to the low interface barrier between the polymer films and the electrode surface. The HOMO and LUMO energy levels have also been estimated from the data of oxidation and reduction onset potentials. All of the results indicate that the synthesized polymer is a promising green emissive material for the light-emitting device application.     

Conjugated copolymers of 2-methoxy-5-2′-ethyl-hexyloxy-1,4-phenylenevinylene and 2,5-dicyano-1,4-phenylenevinylene as materials for polymer light-emitting diodes

A new series of electroluminescent polymers has been synthesized by incorporating the segments of 2,5-dicyano-1,4-phenylenevinylene (DCN-PV) into the backbone of poly(2-methoxy-5-(2′-ethyl-hexyloxy)-p-phenylene vinylene) (MEH-PPV). The copolymers have similar optical properties with MEH-PPV. The oxidation potentials of the copolymers are slightly larger than that of MEH-PPV. The copolymers exhibit “pre-reduction” processes, beginning at about −0.9 V (vs. SCE), before the main reduction processes. The “pre-reduction” feature is thought to be helpful in improving electron injection, and thus to be helpful in improving EL performance. The improved EL performance of the copolymers in comparison with MEH-PPV is demonstrated by a single-layer polymer light-emitting diode (PLED) device employing one of the copolymers as the emissive material.     

Electrochemical synthesis and redox processes of poly(paraphenylene vinylene): An in situ FTIR-attenuated total reflectance and electrochemical quartz crystal microbalance study

The electrochemical synthesis and the redox reactions of poly(paraphenylene vinylene) (PPV) have been studied by in situ Fourier transform infrared (FTIR) spectroscopy using the attenuated total reflection method and by the electrochemical quartz crystal microbalance, EQCM, technique. The polymer material has been synthesized by electrochemical reduction of α,α,α′,α′-tetrabromo-p-xylene on a Pt-electrode in dimethylformamide or acetonitrile using tetraethylammoniumtetrafluoroborate or tetraethylammoniumtosylate as electrolyte salts. The IR spectra of the doped conducting state show a very large broad absorption band starting at 1700 cm⁻¹ and extending to 5000 cm⁻¹. Additional changes in the spectra can be seen in the region between 1700 and 700 cm⁻¹ where new absorption bands appear. The intensity for these infrared active vibrations (IRAV) increase with increasing n-doping level. The reversible charging–discharging reaction is characterized by the growth in intensity of the IRAV during charging and diminish during discharging of the PPV film. Results from FTIR spectroscopy obtained from the charging–discharging reaction are consistent with the results obtained from the EQCM experiments, which show movement of counter cations during n-doping.     

Thermal treatment under reverse bias: Effective tool for polymer/fullerene bulk heterojunction solar cells

The effect of post-thermal treatment under reverse bias on the performance of bulk heterojunction photovoltaic cells based on poly[2-methoxy-5-(20-ethylhexyloxy)-p-phenylene vinylene] (MEH-PPV) and fullerene (C60) composites is investigated. The experimental results show that treated devices deliver increased short-circuit current density (J_sc) and conversion efficiency (η_p) values. Especially, for the device annealed under the bias of −6 V, the short-circuit current density (J_sc) and the conversion efficiency (η_p) are enhanced by a factor of 6 and 5.4, respectively. It is concluded that the enhancement of charge mobility induced by polymer chains modified orientation along the direction of electric field leads to a significant increase in the photovoltaic parameters.     

Efficient blue light-emitting diodes from a soluble poly (para-phenylene) internal field emission measurement of the energy gap in semiconducting polymers

Polymer light-emitting diodes with poly (2-decyloxy-1,4-phenylene), DO-PPP, as the luminescent polymer emit blue light with external quantum efficieny of ∼3%. Using internal field emission (Fowler-Nordheim tunneling) in single carrier devices, the top of the π band and the bottom of the π^* band have been determined for DO-PPP, and for poly (2-methoxy-5-(2'-ethyl-hexyloxy)-1,4phenylene vinylene), MEH-PPV. For DO-PPP, the single particle π-π^* energy gap is 3.4–3.6 eV; for MEH-PPV, the π-π^* energy gap is 2.2–2.3 eV. Comparison with optical absorption (the optical gap) implies that the exciton binding energy is small; for both polymers, E_exc ≤ 0.2 eV. Such small residual values are probably not significant in the presence of disorder broadening which is also 0.1–0.2 eV and in the presence of lattice relaxation with self-localization energies of approximately 0.2 eV.     

Photo- and electroluminescence of poly(2-methoxy,5-(2′-ethylhexyloxy)-p-phenylene vinylene) Langmuir-Blodgett films

Langmuir-Blodgett (LB) layers of poly(2-methoxy,5-(2′-ethylhexyloxy)-p-phenylene vinylene) (MEH-PPV) have been built up on quartz and indium-tin oxide (ITO) substrates. The optical properties of the films were characterized using time-correlated single-photon counting, and absorption and steady-state fluorescence spectroscopy. Electroluminescence (orange-yellow light) from ITO/MEH-PPV LB/A1 devices under forward bias was observed. The dependence of the electrical and electroluminescent characteristics on LB film thickness is reported. The quantum efficiency was 7 × 10⁻³% for a device containing 64 LB layers.     

Light-emitting electrochemical cells based on poly(p-phenylene vinylene) copolymers with ion-transporting side groups

We present the electro-optical characteristics of LECs based on two copolymers derived from poly(p-phenylene vinylene), DB-alt-BTEM-PPV and BDMOS-co-BTEM-PPV, having ion-transporting side groups. We find typical LEC behaviour upon addition of lithium triflate. Addition of poly(ethylene oxide), PEO, improves the EL efficiency, but the response time increases. We discuss these observations on the basis of PEO-induced phase separation.     

Energy transfer in molecularly doped conjugated polymers

We present a detailed investigation of energy transfer of optical excitations in a molecularly doped conjugated polymer. We have incorporated dye molecules into a polymer blend system consisting of poly (phenylphenylene vinylene) (PPPV) and polycarbonate. Using time-resolved luminescence spectroscopy we observe efficient picosecond energy transfer from the PPPV chromophores to the dye 4-dicyanomethylene-2-methyl-6-p-dimethylaminostyryl-4H-pyran (DCM) molecules. Our results are interpreted in terms of Förster-type energy transfer.     

Fluorescence studies of electrospun MEH-PPV/PEO nanofibers

We report a study of the fluorescence properties of the conjugated polymer poly [2-methoxy-5-(2′-ethyl hexyloxy)-p-phenylene vinylene] (MEH-PPV) and polyethylene oxide (PEO) nanofibers. MEH-PPV/PEO nanofibers with different compositions have been fabricated by the electrospinning technique. The fluorescence spectra of the nanofibers show that the emission shoulder at ∼630 nm blue-shifts ∼45 nm, whereas the main emission peak around 590 nm blue-shifts ∼15 nm with decreasing concentration of MEH-PPV in the nanofiber. In addition, confocal microscopic studies of a single MEH-PPV/PEO electrospun nanofiber indicate that the fluorescence spectra of the nanofiber do not show any polarization dependence. The results are discussed in terms of the aggregation of MEH-PPV in an inert matrix.     

Single-photon emission from a single nanoparticle consisting of a single conjugated polymer chain

Single-photon behavior in the emission from a single nanoparticle consisting of a single poly[2-methoxy-5-(2′-ethylhexyloxy)-p-phenylene vinylene] (MEH-PPV, molecular weight: 2,600,000 amu) chain has been investigated. Photon correlation measurements of a number of single nanoparticles revealed that the probability of single-photon emission from the single nanoparticles clearly increases, compared with that of single chains embedded in host polymer matrices, poly(methyl methacrylate) (PMMA) and polystyrene (PS). The result suggested that even a single MEH-PPV chain with high molecular weight, which does not exhibit single-photon emission in PMMA and PS, is forced to exhibit single-photon emission by adopting “compact” chain conformation like nanoparticle. Efficient exciton migration and exciton annihilation processes in the compact chain conformation most probably result in the efficient single-photon emission from the single nanoparticle. The present results indicate that multi-quantum systems consisting of a large number of chromophores, such as the MEH-PPV, can be made to behave as single-photon sources by appropriately controlling their size.     

Synthesis and characterization of a partial-conjugated hyperbranched poly(p-phenylene vinylene) (HPPV)

A novel partial-conjugated HPPV has been designed and easily synthesized from A₂ + B₃ monomers, employing distrylbenzene, an oligomer of poly(p-phenylene vinylene) with high fluorescence quantum yield, as one of starting materials. HPPV was characterized by FT-IR, H-NMR, GPC and elemental analysis. Its properties, such as thermal, electrochemical and photoluminescent, etc., have been investigated systematically. HPPV exhibits good solubility and processability, a high glass transition temperature (T_g) and high fluorescence quantum yield in chloroform solution (much higher than fully conjugated hyperbranched and linear PPV).     

Synthesis and electrochemical characterization of a new polymer constituted of alternating carbazole and oxadiazole moieties

We report herein the synthesis and electrochemical characterization of a novel copolymer containing an electron rich carbazole moiety and an electron deficient oxadiazole unit—poly[N-(2′-ethylhexyl)-carbazole-3,6-diyl-1″,3″,4″-oxadiazole-2″,5″-diyl] (PCO). PCO can be dissolved in THF, CHCl₃, xylene, and DMSO. The structure of the polymer is confirmed by FT-IR, NMR and elemental analysis. The optical and electronic properties of the polymer are investigated by UV–Vis absorption spectroscopy and photoluminescence spectroscopy as well as cyclic voltammetry. The results show that the polymer films emit greenish-blue light (λ_max 485 nm) upon UV excitation. Both p-doping and n-doping processes are observed in cyclic voltammetric investigation. A comparison between the properties of polycarbazole and polycarbazole–oxadiazole has been presented.     

Trapped polarons,free polarons and triplet excitons in π-conjugated polymer films and light-emitting diodes

Recent optically detected magnetic resonance studies of poly (p-phenylene vinylenes) (PPVs) and poly(3-alkyl thiophenes) (P3ATs) suggest that excitation at visible wavelengths λ_ex largely generates trapped polarons and polaron pairs, but UV excitation and carrier injection in LEDs generate free polarons. The photoluminescence (PL)-enhancing polaron resonance at long λ_ex is attributed to non-radiative recombination of trapped polaron pairs, which reduces their population and consequently the rate at which they non-radiatively quench singlet excitons. Interchain coupling, some defects induced by structural disorder, and sites adjacent to C₆₀^- counterions apparently enhance the generation of these trapped polarons as well as intersystem crossing from the singlet to the triplet manifold. At short λ_ex a PL-quenching polaron resonance emerges, and is assigned to fusion of photogenerated free polarons to bipolarons, which also quench singlet excitons. These interpretations imply that the l¹B_u exciton binding energy, defined as its dissociation energy to free polarons, is the difference between the absorption edge and the excitation energy at which the PL-quenching polaron resonance emerges. This difference is ∼0.8 eV in poly(2,5-dioctoxy-p-phenylene vinylene) and poly (3-hexylthiophene).     

Theoretical study of torsion and its effect on the structural and electronic properties of cyano-substituted poly (p-phenylene vinylene) and its derivatives

We report the results of a theoretical study on torsion and its effect on the structural and electronic properties of cyano-substituted poly (p-phenylene vinylene) and its dimethoxy and dimethyl ring-substituted derivatives. Geometries, inter-ring torsion potentials, and potentials associated with torsion of the ring substituents are calculated using the semiempirical quantum-chemical Austin model 1 method. The evolution of the bandgap, ionization potential, and electron affinity is evaluated using the pseudopotential valence effective Hamiltonian method.     

Two-dimensional ordered array of poly(2-methoxy,5-(n-hexadecyloxy)-p-phenylene vinylene) in monolayer by Langmuir–Blodgett technique

Two-dimensional ordered array of conjugated main chains has been prepared by spreading a chloroform solution of poly (2-methoxy,5-(n-hexadecyloxy)-p-phenylene vinylene) (MH-PPV) onto the air–water interface and being subsequently transferred onto substrate. The layer-by-layer structure obtained by Langmuir–Blodgett technique exhibits that the main chains are aligned in the LB dipping direction.     

Molecular scale organized poly(MDMO-p-phenylene vinylene)–heteropolyacid composites

In this study, the interaction of phosphomolybdic acid (H₃PMo₁₂O₄₀) with poly[2-metoxy-5-(3′,7′-dimethyloctyloxy)-p-phenylene vinylene] (MDMO-PPV) has been investigated by the methods of optical absorbance and luminescence spectroscopy, electrochemical, EPR and chemical analysis. It is shown that the interaction between MDMO-PPV and H₃PMo₁₂O₄₀ leads to the formation of charge-transfer complex, which involves one heteropolyanion per four monomer units of the polymer chain. We have demonstrated that this interaction causes reversible changes in the electronic structure of the polymer as well as the appearance of an additional line in a long-wavelength region of optical absorbance spectra, a sharp decrease of the efficiency of luminescence and the emergence of pronounced EPR signal.     

Electrosynthesis of 2,7-linked polycarbazole derivatives to realize low-bandgap electroactive polymers

Poly(2,7-carbazole) derivatives have been synthesized because they have more extended conjugation lengths and lower energy bandgaps than poly(3,6-carbazole)s; however, few studies regarding electrochemical synthesis of poly(2,7-carbazole)s have been reported. Here, a series of N-alkylated-2,7-di(2-furyl)carbazoles, N-butyl-2,7-di(2-thienyl)carbazole, and N-butyl-2,7-di(2-(3,4-ethylenedioxthienyl))carbazole, were synthesized to obtain electroactive films of poly(2,7-carbazole) derivatives by electrochemical polymerization. Cyclic voltammetry revealed that these monomers have excellent polymerization activity, due to their low oxidation potentials (<0.57 V vs. ferrocene (Fc/Fc⁺)), and the corresponding polymers exhibit good redox properties. The energy bandgaps of the polymers obtained from optical absorption spectra range from 2.1 to 2.3 eV. Among the polymers, poly[N-butyl-2,7-di(2-(3,4-ethylenedioxthienyl))carbazole] shows the lowest bandgap energy of 2.1 eV, which is lower than that of previously reported poly(3,6-carbazole) analogues (2.4 eV). This polymer exhibits a significant color change from red in the oxidized state to blue in the reduced state during an electrochemical redox process. The electrochemical and optical properties of the monomers are dependent on external heteroaromatic rings attached to the 2 and 7 positions of the carbazole unit.     

I–V characteristics of dark and illuminated PPV-PCBM blends solar cells

The effect of temperature and composition on the I–V characteristics of blends of (poly(2-methoxy-5-(2′-ethyl-hexyloxy)-l,4-phenylene vinylene) (MEH-PPV), and (6,6)-phenyl-C61-butyric acid methyl ester (PCBM) sandwiched between a transparent indium tin oxide (ITO) electrode and an Al backside contact (solar cell structure) are investigated. The observed dark I–V curves agree well with the trap-controlled space charge limited transport theory. The illuminated I–V curves do not obey the theory of space charge limited currents. The short-circuit currents in the illuminated samples depend strongly on the composition. The current is maximum for the 80% PCBM concentration in the blend. The open circuit voltage also depends on the composition. It is maximum for 0% PCBM concentration and decreases monotonically with PCBM concentration. This might lead to the conclusion that the open circuit voltage is due to internal bulk heterojunctions (BHs). However, we have pointed out that this conclusion, arrived at by earlier workers also, is not necessarily valid. The superposition principle is not valid but the discrepancy is small.