Synthesis by the Gilch Method of Blue-Light-Emitting Poly(<i>p</i>-phenylenevinylene) Derivatives Bearing Highly Phenylated Pendants

Two novel poly(p-phenylenevinylene) (PPV) derivatives bearing highly phenylated side groups with different chemical structures were prepared by Gilch dehydrohalogenation polyaddition. The intermediate substituted 1,4-bis(bromomethyl)benzenes (monomers 6 and 11) were synthesized by a convenient synthetic route utilizing pyrylium salts. The polymers were amorphous and showed limited solubility in THF, chloroform, 1,2-dichloroethane, 1,1,2,2-tetrachloroethane, and 1,2-dichlorobenzene. They displayed relatively high Tg values (158−176 °C) and good thermal stability, being stable up to approximately 270−350 °C in N2 or air and affording anaerobic char yields of 53−56% at 800 °C. The bulky side groups caused a significant steric effect on the PPV backbone and reduced the chromophore length. The solutions of polymers in THF emitted blue light with a maximum around 455 nm. This emission maximum is blue-shifted in comparison with that of other previously synthesized PPVs that emit green to red light. At low temperature, the photoluminescence (PL) spectra of polymers in THF became broader and were extended to longer wavelengths in comparison with the corresponding room-temperature spectra. The PL spectra of thin films of polymers were red-shifted with respect to those of solutions and showed a maximum at 476 nm

Synthesis by Heck Coupling of Soluble, Blue-Light-Emitting Fully Conjugated Poly(<i>p</i>-phenylenevinylene)s with Highly Phenylated Side Groups

Two new fully conjugated poly(p-phenylenevinylene)s (PPVs) were prepared by Heck coupling between p-divinylbenzene and two dibromides. The latter were synthesized by a six-step synthetic route using pyrylium salts. The polymers carried on the same phenyl ring two side oligophenyls per each repeat unit with various chemical structures. These polymers were amorphous and showed an excellent solubility being soluble in common organic solvents (THF, chloroform, methylene chloride, 1,2-dichloroethane). No weight loss was observed approximately up to 320−370 °C in N2 or air, and the anaerobic char yield was around 80% at 800 °C. The Tg values ranged from 106 to 122 °C, and they could be controlled by the chemical structure of the side substituents. The polymers behaved as strongly blue-light-emitting materials in THF solution with PL maximum at 453 or 461 nm. These values are among the bluest emission peaks that have been reported for a fully conjugated PPV. The bulky side oligophenyls caused significant steric hindrance which interrupted the conjugation in the PPV backbone. At low temperature, the PL maximum in THF was red-shifted by 2−10 nm in comparison with the respective room temperature spectra. The red shift was greater for the polymer that carried longer side oligophenyls. The bulky side oligophenyls hindered chain interactions, and the polymer displayed little tendency for aggregate formation. The PL quantum yields in THF were 0.24−0.31

Synthesis and Photophysical Characteristics of 2,7-Fluorenevinylene-Based Trimers and Their Electroluminescence

Three new 2,7-fluorenevinylene-based trimers were synthesized and characterized. The synthesis was carried out by the Heck coupling reaction of 9,9-dihexyl-2,7-divinylfluorene with 2-(4-bromophenyl)-5-phenyl-1,3,4-oxadiazole, N,N-diphenyl-4-bromoaniline, or 3-bromopyrene to afford the trimers OXD, TPA, and PYR, respectively. All the trimers were readily soluble in common organic solvents such as tetrahydrofuran, dichloromethane, chloroform, and toluene. Their glass transition temperatures ranged from 33 to 60 °C. The UV−vis spectra showed an absorption maximum at λa,max = 379−417 nm with optical band gap of Eg = 2.47−2.66 eV. In solution, they emitted strong blue-green photoluminescence (PL) with PL maximum at λf,max = 455−565 nm and fluorescence quantum yield of Φf = 0.65−0.74. On the other hand, in their spin-coated films, the PL efficiencies significantly decreased due to the presence of concentration quenching. All samples showed nanosecond transient lifetime containing two components, suggesting excimer formation. The organic light-emitting diodes (OLEDs) with OXD and TPA showed green emission with electroluminescence (EL) quantum efficiencies of ηEL ∼ 10-2%, while very weak EL efficiency of ηEL ∼ 10-5% was observed with PYR. The highest occupied molecular orbital (HOMO) levels of the films were found to be 5.05−5.75 eV

Novel Low Band Gap Small Molecule and Phenylenevinylene Copolymer with Cyanovinylene 4-Nitrophenyl Segments: Synthesis and Application for Efficient Bulk Heterojunction Solar Cells

A novel star-shaped small monomer SM containing a 1,3,5-triazine core and arms with terminal cyanovinylene 4-nitrophenyls was synthesized. Moreover, an alternating p-phenylenevinylene copolymer P containing thiophene with cyanovinylene 4-nitrophenyl side segments was synthesized by Heck coupling. Both SM and P showed broad absorption spectra with long-wavelength maximum at 630−648 nm, which for P is attributable to an intramolecular charge transfer. The optical band gap was 1.57 eV for SM and 1.70 eV for P. Both SM and P were blended with PCBM to study the donor−acceptor interactions on the blend film morphology and device characteristics of organic bulk heterojunction solar cells. A combination of characterization techniques including X-ray diffraction and optical topographical images were used to investigate the film morphology. The HOMO and LUMO levels of both SM and P are well-aligned with those of the PCBM acceptor, allowing efficient electron transfer and suitable open circuit voltage, leading to overall power conversion efficiencies (PCEs) of 2.53 and 1.43% for SM:PCBM and P:PCBM-based devices, respectively. The thermal annealing leads to suitable phase separation due to the increase in crystallinity of donor material and material distribution so that highly effective bulk heterojunction morphologies are obtained which further increases the PCE up to 3.82% and 2.37% for SM:PCBM and P:PCBM-based devices, respectively. These results are preliminary based on the illumination without using a solar simulator

Effect of Solvent and Subsequent Thermal Annealing on the Performance of Phenylenevinylene Copolymer:PCBM Solar Cells

The morphology of the photoactive layer used in the bulk heterojunction photovoltaic devices is crucial for efficient charge generation and their collection at the electrodes. We investigated the solvent vapor annealing and thermal annealing effect of an alternating phenylenevinylene copolymer P:PCBM blend on its morphology and optical properties. The UV−visible absorption spectroscopy shows that both solvent and thermal annealing can result in self-assembling of copolymer P to form an ordered structure, leading to enhanced absorption in the red region and hole transport enhancement. By combining the solvent and thermal annealing of the devices, the power conversion efficiency is improved. This feature was attributed to the fact that the PCBM molecules begin to diffuse into aggregates and together with the ordered copolymer P phase form bicontinuous pathways in the entire layer for efficient charge separation and transport. Furthermore, the measured photocurrent also suggests that the space charges no longer limit the values of the short circuit current (Jsc) and fill factor (FF) for solvent-treated and thermally annealed devices. These results indicate that the higher Jsc and PCE for the solvent-treated and thermally annealed devices can be attributed to the phase separation of active layers, which leads to a balanced carrier mobility. The overall PCE of the device based on the combination of solvent annealing and thermal annealing is about 3.7 %

Effect of the Incorporation of a Low-Band-Gap Small Molecule in a Conjugated Vinylene Copolymer: PCBM Blend for Organic Photovoltaic Devices

The effect of the incorporation of a low-band-gap small-molecule BTD-TNP on the photovoltaic properties of vinylene copolymer P:PCBM bulk heterojunction solar cells has been investigated. The introduction of this small molecule increases both the short-circuit photocurrent and the overall power conversion efficiency of the photovoltaic device. The incident photon-to-current efficiency (IPCE) of the device based on P:PCBM:BTD-TNP shows two distinct bands, which correspond to the absorption bands of P:PCBM and BTD-TNP. Furthermore, it was found that the IPCE of the device has also been enhanced even at the wavelengths corresponding to the absorption band of P:PCBM, when the thermally annealed blend was used in the device. This indicates that the excitons that are generated in copolymer P are dissociated into charge carriers more effectively in the presence of the BTD-TNP small molecule at the copolymer P:PCBM interface by energy transfer from P to the small molecule. Therefore, we conclude that the BTD-TNP small molecule acts as light-harvesting photosensitizer and also provides a path for the generated exciton in copolymer P toward the P:PCBM interface for efficient charge separation. The overall power conversion efficiency for the P:PCBM:BTD-TNP photovoltaic device is about 1.27%, which has been further enhanced up to 2.6%, when a thermally annealed blend layer is used

Novel <i>p</i>-Phenylenevinylene Compounds Containing Thiophene or Anthracene Moieties and Cyano−Vinylene Bonds for Photovoltaic Applications

Two novel soluble compounds T and A that contain a central dihexyloxy-p-phenylenevinylene unit, intermediate moieties of thiophene or anthracene, respectively, and terminal cyano−vinylene nitrophenyls were synthesized and characterized. They showed moderate thermal stability and relatively low glass transition temperatures. These compounds displayed similar optical properties. Their absorption was broad and extended up to about 750 nm with the longer-wavelength maximum around 640 nm and an optical band gap of ∼1.70 eV. From the current−voltage characteristics of the devices using both compounds T and A, it was concluded that both compounds behave as p-type organic semiconductors with hole mobility on the order of 10−5 cm2/(V s). The power conversion efficiency (PCE) of the devices based on these compounds was 0.019% and 0.013% for compounds A and T, respectively. When compounds A and T were blended with [6,6]-phenyl-C61-butyric acid methyl ester (PCBM), the PCE dramatically increased up to 1.66% and 1.36% for devices with A:PCBM and T:PCBM, respectively. The efficiencies of the devices were further enhanced upon thermal annealing up to 2.49% and 2.33% for devices based on A:PCBM and T:PCBM, respectively