Polyfluorene Backbone Research Articles

Lithium-Ion-Based Conjugated Polyelectrolyte as an Interface Material for Efficient and Stable Non-Fullerene Organic Solar Cells.

An eco-friendly n-type water/alcohol-soluble conjugated polyelectrolyte PFEO SO3 Li was synthesized and applied as a cathode interfacial layer in organic solar cells. The π-delocalized polyfluorene backbone has an intimate connection with the hydrophobic active layer, and the side chain with lithium ion may move toward the ZnO layer through the self-assembly property of conjugated polyelectrolytes. UV photoelectron spectroscopy indicated that modification with PFEO SO3 Li dramatically lowers the work function of indium-doped tin oxide (ITO)/ZnO and may form strong interfacial dipoles between ZnO and the active layer. Meanwhile, introduction of lithium ions as spectator cations may contribute to reduction of the intrinsic surface defects of ZnO. The green emission in the photoluminescence spectrum of ZnO disappeared after modification with PFEO SO3 Li. In addition, the roughness of ZnO barely changed after coating with PFEO SO3 Li, and the surface became more hydrophobic, which demonstrates that the thin conjugated polyelectrolyte layer exhibits good adhesion with both ZnO and the active layer. These phenomena indicate that the introduction of PFEO SO3 Li makes ITO/ZnO an efficient cathode. As a result, inverted organic solar cell devices with ZnO/PFEO SO3 Li double-interlayers exhibit high efficiencies of 11.7 and 10.6 % for PBDB-T:IT-M and PBDB-T:ITIC blend systems, respectively.

Managing intramolecular energy transfer in well-defined polyfluorenes grafting one/two orange emissive groups on central or terminal fluorene unit

Three sets of polyfluorenes grafting one/two orange emissive groups on central or terminal fluorene unit have been synthesized successfully through the catalyst-transfer polymerization. The molecular weights linearly increase with increase in the feed ratios of [monomer]/[catalyst] in a certain molecular weight range. The initiators with bis(2,6-diisopropylphenyl)imidazolin-2-ylidene) (IPr) as the ancillary ligand show better catalytic activity, leading to higher molecular weight of up to 126.3 kDa. All polymers in toluene have the similar absorption spectra to polyfluorene homopolymer, while the weak absorption bands of 420–500 nm are observed only in the polymers with low molecular weight. Photoluminescence spectra display a dominant blue emission from the polyfluorene backbone and a weak orange emission (508–661 nm) from the orange group. The clearly enhanced orange emission in the polymers with low molecular weight is mainly ascribed to the intramolecular Förster energy transfer from the polyfluorene segment to the orange group, which is further confirmed by the electroluminescent behavior of the polymers.

A bifunctional conjugated polyelectrolyte for the interfacial engineering of polymer solar cells

In this work a novel combination of side chain functionalities, alkyl-phosphonate (EP) and alkyl-ammonium bromide (NBr) groups, on a polyfluorene backbone (PF-NBr-EP) was studied as cathode interfacial material (CIM) in polymer-based solar cells. The devices were made with a conventional geometry, with PTB7:PC71 BM as active layer and aluminum as metal electrode. The CIM showed good solubility in ethanol and film forming ability onto the active layer so that its deposition could be finely tuned. The interface engineering imparted by this CIM was assessed and discussed through kelvin probe force microscopy (KPFM), impedance spectroscopy, charge recombination and electron transport characterizations. To discriminate between the interfacial modifications imparted by the interlayer and its solvent, we included in this study a surface ethanol treated device. In the optimized conditions an average power conversion efficiency of 7.24% was obtained, which is about 60% higher when compared to devices made with bare Al and 26% when compared to devices made with a standard calcium/aluminum cathode. Besides performances, some insights about the devices shelf life stability are also presented. A good persistency through aging was found for the cathode interfacial engineering capabilities of PF-NBr-EP.

Diblock conjugated polyelectrolyte electron transport layer modulating the morphology of the active layer for efficient nonfullerene organic solar cells

To acquire high-performance nonfullerene organic solar cells (OSCs), the effective approach is to explore new interfacial layer that not only can modulate the interfacial contact, but can also tune the morphology of the active layer. A diblock CPE PFEO-b-PTNBr with polyfluorene and polythiophene backbone, quaternary ammonium salt and ethylene oxy polar side chains, has been used to modify ZnO as ETL for PBDB-T:ITIC OSCs. The influences of the diblock CPE PFEO-b-PTNBr electron transport layer (ETL) on the morphology and crystalline properties of the PBDB-T:ITIC, exciton diffusion and transfer, and the device performances have been thoroughly investigated. Remarkably, the self-assembled diblock polymer PFEO-b-PTNBr can really induce the corresponding PBDB-T:ITIC active layer to obtain an ordered nano-fiber morphology. Consequently, in sharp comparison to the OSC utilizing ZnO as ETL, the performances of the device with the ZnO/PFEO-b-PTNBr ETL have been simultaneously enhanced, exhibiting a remarkably superior power conversion efficiency of 10.8%. These findings manifest that the diblock CPE PFEO-b-PTNBr is a promising ETL in nonfullerene OSCs.

Efficient blue light-emitting polymers containing fluorene[2,3-b]benzo[d]thiophene-S,S-dioxide unit

Abstract Blue light-emitting polymers were synthesized by Suzuki polycondensation via introducing 9,9-dioctylfluorene [2,3-b]benzo [d]thiophene-S,S-dioxide (BTOF) unit into the polyfluorene and polycarbazole backbone, respectively. All the polymers show high fluorescence quantum yield and thermal stability. The highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) energy levels of the polymers reduce slightly with increasing the content of BTOF in the polymers. The polymers exhibit stable electroluminescence spectra with the variation of applied current densities up to 300 mA cm−2. The peak luminous efficiencies (LEmax) of 3.7 and 3.1 cd A−1 (corresponding to a maximum external quantum efficiencies (EQEmax) of 3.5 and 2.5%) with the luminances of 10085 and 19662 cd m−2 were obtained at a turn on voltage of 3.0 and 3.2 V, respectively, based on PF-BTOF30 and PCz-BTOF20 with a single-layer configuration of ITO/PEDOT:PSS/polymer/CsF/Al. Compared to the homopolymers PF and PCz, the enhanced device efficiencies about 6–8 times and the improved device lifetime exceeded an order of magnitude of the copolymers implied that introducing BTOF unit into the polymer backbone could be a promising strategy for the development of highly efficient blue light-emitting polymers.

Effect of the introduction of an alcohol-soluble conjugated polyelectrolyte as cathode interlayer in solution-processed organic light-emitting diodes and photovoltaic devices

Interfacial engineering provides an important tool for optimizing the performances of optoelectronic devices. We show that poly[(2,7-(9,9′-dioctyl)fluorene)-alt-(2,7-(9,9′-bis(5″-trimethylammonium bromide)pentyl)fluorene)])], an alcohol-soluble π-conjugated polymer based on polyfluorene backbone and ammonium groups on the alkyl side chains, is capable of modifying the interface between the organic layer and the metal cathode in both organic solar cells and light-emitting diodes based on commercial materials and conventional architectures, improving their performances. The introduction of the cathode interlayer enhances the efficiency of a red-emitting phosphorescent OLED by 15% and decreases its turn-on voltage. The same polymer improves the power conversion efficiency of a PTB7/PC71BM solar cell by 55% and shows a beneficial effect in terms of device stability.

Green-emitting Polyfluorenes Containing Hexylthiophen-dibenzothiophene-S,S-dioxide Unit with Large Two-photon Absorption Cross Section

Green light-emitting polyfluorenes containing 3,7-bis(4-hexylthiophen-2-yl)dibenzo[b,d]thiophene 5,5-dioxide (DHTSO) unit were synthesized. All the resulted polymers show high thermal stability with the decomposition temperatures (Td) over 420 °C and the glass transition temperatures (Tg) over 75 °C. The polymers exhibit the enhanced highest occupied molecular orbital (HOMO) energy levels and the depressed lowest unoccupied molecular orbital (LUMO) energy levels with the increase of DHTSO unit in polymers. The photoluminescence (PL) spectra of the polymers show positive solvatochromism in solution with the variation of solution polarities, indicating remarkable intramolecular charge transfer (ICT) effect in the polymers containing DHTSO moiety. The fluorescence quantum yields (ΦPL) are in the range of 34%−67% for PF-DHTSOs in film. All polymers possess two photon absorption (TPA) properties, and the TPA cross sections (δ2) are enhanced with increasing DHTSO unit in polymers. The highest δ2 is 2392 GM for PF-DHTSO15 in chloroform solution upon 740 nm excitation. The device of PF-DHTSO15 shows green emission with the Commission Internationale de L’. Eclairage (CIE) coordinates of (0.26, 0.59), and the maximum luminous efficiency (LEmax) of 10.8 cd·A−1 with the configuration of ITO/PEDOT:PSS/EL/CsF/Al. These results indicate that introducing DHTSO unit into polyfluorene backbone could be a promising molecular design strategy for TPA and effective green-light emission.

Preparation, Single-Molecule Manipulation, and Energy Transfer Investigation of a Polyfluorene-graft-DNA polymer.

Conjugated polymers have been intensively studied due to their unique optical and electronic properties combined with their physical flexibility and scalable bottom up synthesis. Although the bulk qualities of conjugated polymers have been extensively utilized in research and industry, the ability to handle and manipulate conjugated polymers at the nanoscale lacks significantly behind. Here, the toolbox for controlled manipulation of conjugated polymers was expanded through the synthesis of a polyfluorene-DNA graft-type polymer (poly(F-DNA)). The polymer possesses the characteristics associated with the conjugated polyfluorene backbone, but the protruding single-stranded DNA provides the material with an exceptional addressability. This study demonstrates controlled single-molecule patterning of poly(F-DNA), as well as energy transfer between two different polymer-DNA conjugates. Finally, highly efficient DNA-directed quenching of polyfluorene fluorescence was shown.

Visualization of trap dilution in polyfluorene based light-emitting diodes

The effects of electron trapping and resulting trap-assisted recombination can be strongly suppressed by diluting a semiconducting polymer with a large band gap polymer such as polystyrene. Polyfluorene (PFO)-based light-emitting diodes (PLEDs) are an excellent model system to visualize this trap dilution effect. The blue emission from the pristine PFO backbone is accompanied by a broad, featureless green emission band originating from monomeric ketone defects that act as an electron trap. We demonstrate that the green emission from radiative trap-assisted recombination at these ketone defects is nearly eliminated upon dilution. The ratio between bimolecular blue emission and trap-assisted green emission as a function of dilution is shown to be in quantitative agreement with model predictions.

Aggregation-Induced Energy Transfer of Conjugated Polymer Materials for ATP Sensing.

Water-soluble conjugated polymers are attractive fluorescent materials for applications in chemical and biological sensing. The molecular wire effect of such polymers amplifies changes in the fluorescence signal, which can be used for detecting various analytes with high sensitivity. In this work, we report an efficient ratiometric fluorescent probe based on a water-soluble conjugated polymer that showed high sensitivity and selectivity toward adenosine 5'-triphosphate (ATP). The macromolecular probe consisted of a polyfluorene backbone doped with 5 mol % 1,4-dithienylbenzothiadiazole (DBT) modified by bis-imidazolium and oligo(ethylene glycol) moieties. Solutions of the polymer emitted purple fluorescence, which changed to red upon addition of ATP molecules. The addition of ATP caused the polymer to aggregate, which enhanced fluorescence resonance energy transfer efficiency from the fluorene segments to DBT units, leading to an increase in red emission. The ratio of the fluorescence at these different wavelengths (I655/I423) showed a strong dependence on the ATP concentration. PF-DBT-BIMEG also exhibited high selectivity for ATP sensing over other representative anions and discriminated it from adenosine 5'-diphosphate (ADP) and adenosine 5'-monophosphate (AMP). This can be explained by the much stronger electrostatic interactions between the polymer and ATP than the interactions between the polymer and ADP or AMP, as confirmed through molecular dynamics simulations.

Blue light-emitting polymers containing ortho-linking carbazole-based benzothiophene-S, S-dioxide derivative

The blue light-emitting polymers PF-CzBTOs and PCz-CzBTOs were synthesized by Suzuki polycondensation via introducing ortho-linking N-(2-decyltetradecyl) carbazole bis- [2,3-b,6,7-b]benzo[d]thiophene-S,S-dioxide (CzBTO) into the polyfluorene (PF) and polycarbazole (PCz) backbone, respectively. All the resulted copolymers showed high thermal stability with the glass transition temperatures (Tg) of over 130 °C. The fluorescence quantum yields (ФPL) are in the range of 41%–59% and 20%–31% for PF-CzBTOs and PCz-CzBTOs, respectively. The highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) energy levels of the copolymers are basically unchanged with the increase of CzBTO content in the copolymers. The PL spectra of copolymers showed only a faint broadening with the increase of CzBTO content in the copolymers, and with the increase of the solvent polarities, indicating no remarkable ICT effect in polymers containing ortho-linking CzBTO moiety. The copolymers showed a pure blue emission with a CIE coordinates of (0.15, 0.11) for the PF-CzBTO10, and a CIE coordinates of (0.17, 0.18) for PCz-CzBTO15. The results indicated that the strategy of introducing ortho-linking CzBTO unit into the polyfluorene or polycarbazole backbone can effectively restrain ICT effect and result in the pure blue emission.

π-Conjugated polymers with pendant coumarins: design, synthesis, characterization, and interactions with carbon nanotubes

A series of new fluorene-based π-conjugated polymers having coumarin derivatives as part of dendritic side chains were designed and prepared using the Suzuki–Miyaura cross-coupling reaction. A new coumarin derivative bearing a heptyl side chain for solubility was utilized to ensure solubility of the final polymers. It was found that fluorescence resonance energy transfer (FRET) from the coumrains to the polyfluorene backbone was efficient, especially for the polymers decorated with lower-generation dendrons. Each of the polymers was found to interact strongly with the surface of single-walled carbon nanotubes (SWNTs) in THF, and their ability to selectively disperse specific SWNT chiralities was investigated. Photoluminescence studies revealed that the strong polymer emission is efficiently quenched in the corresponding supramolecular complexes with SWNTs. This high quenching efficiency indicates that the coumarin–polymer FRET system can be supramolecularly bound to the surface of (SWNTs to produce an energy transfer system in which the energy absorbed by the donor coumarin chromophores is channeled to the SWNTs.

High-Performance Electron Injection Layers with a Wide Processing Window from an Amidoamine-Functionalized Polyfluorene.

In this work, we present organic light-emitting diodes (OLEDs) utilizing a novel amidoamine-functionalized polyfluorene (PFCON-C) as an electron injection layer (EIL). PFCON-C consists of a polyfluorene backbone to which multiple tertiary amine side chains are connected via an amide group. The influence of molecular characteristics on electronic performance and morphological properties was tested and compared to that of the widely used, literature known amino-functionalized polyfluorene (PFN) and polyethylenimine (PEI). PFCON-C reduces the turn-on voltage (VON) of poly(p-phenylene vinylene) (PPV)-based OLEDs from ∼5 to ∼3 V and increases the maximum power efficiency from <2 to >5 lm W(-1) compared to that of PFN. As a result of its semiconducting backbone, PFCON-C is significantly less sensitive to the processing parameters than PEI, and comparable power efficiencies are achieved for devices where thicknesses of PFCON-C are between 15 and 35 nm. Atomic force microscopy (AFM) measurements indicate that the presence of nonpolar side chains in the EIL material is important for its film-forming behavior, while Kelvin probe measurements suggest that the amount of amine groups in the side chains influences the work-function shift induced by the EIL material. These results are used to suggest strategies for the design of polymeric electron injection layers.

Diarylmaleimide fluorophores: intensely emissive low-band-gap guest for single white polymers with highly efficient electroluminescence

Intensely emissive diarylmaleimides PM and TM were introduced into the backbone of polyfluorene (PF) to obtain single white polymers, PFPM and PFTM. PFPM showed highly efficient and white electroluminescence for effective energy transfer from the PF host to the maleimide guest.

Blue light-emitting polymers containing fluorene-based benzothiophene-S,S-dioxide derivatives

Blue light-emitting polymers were synthesized via introducing a 9,9-dioctylfluorene bis[2,3-b;6,7-b]benzo-[d]thiophene-S,S-dioxide (FBTO) unit into the polyfluorene or polycarbazole backbone.

Electrosynthesis, characterization and optical sensing application of amino acid functionalized polyfluorene

An amino acid side chain functionalized polyfluorene derivative poly[N-(9-fluorenylmethoxycarbonyl)-glycine] (P9FG) was facilely electrosynthesized and characterized, and the structure, properties and optical sensing application of the obtained polymer were described and discussed. The electropolymerization occurred at C2 and C7 positions of fluorene units, and amino acid side chain groups were not cleaved from polyfluorene backbone in mixed electrolytes of boron trifluoride diethyl etherate and dichloromethane. Thermal analysis demonstrated good thermal stability of P9FG. Fluorescent spectra indicated that P9FG was a good blue light emitting material that could be employed as optical sensors. The soluble P9FG as a turn-off fluorescent sensor could realize the detection of Fe3+, Cu2+ and Cr2O72-, respectively. In addition, P9FG as a turn-off ultraviolet sensor could realize the detection of Cu2+ while as turn-on ultraviolet sensors could also realize the determination of Fe3+ and Cr2O72-, respectively. All results indicate that P9FG is a promising candidate for optical sensing.

Light Harvesting and Charge Separation in a π-Conjugated Antenna Polymer Bound to TiO2

This paper describes the photophysical and photoelectrochemical characterization of a light harvesting polychromophore array featuring a polyfluorene backbone with covalently attached Ru(II) polypyridyl complexes (PF-Ru-A), adsorbed on the surface of mesostructured TiO2 (PF-Ru-A//TiO2). The surface adsorbed polymer is characterized by transmission electron microscopy (TEM), scanning electron microscopy (SEM), and attenuated total reflectance-Fourier transform infrared (ATR-FTIR) spectroscopy, providing evidence for the morphology of the surface adsorbed polymer and the mode of binding. Photoexcitation of the Ru(II) complexes bound to the metal oxide surface (proximal) results in electron injection into the conduction band of TiO2, which is then followed by ultrafast hole transfer to the polymer to form oxidized polyfluorene (PF+). More interestingly, chromophores that are not directly bound to the TiO2 interface (distal) that are excited participate in site-to-site energy transfer processes that transport...

Amide-based diarylmaleimide derivatives and polymers: Highly selective and ratiometric fluorescence sensing for anions

A series of diarylmaleimide derivatives and polymers based on amide group were designed and synthesized to detect anions selectively and sensitively. Three bisindolylmaleimide derivatives (FAM-p, FAM-m and FAM-o) employing trifluoroacetamide as receptor exhibited strong fluorescent quenching upon fluoride and cyanide. Trifluoroacetamide receptor was replaced by acetamide to obtain other two derivatives (AM-p and AM-o) which showed higher selectivity than FAMs with sensing for only fluoride. The diarylmaleimide derivatives were introduced into the backbone of polyfluorene by Suzuki reaction to prepare polymer PFAM and PAM with same sensing selectivity for anions as FAM and AM, respectively. PFAM and PAM were found to be ratiometric fluorescence sensors for a fluorescence resonance energy transfer (FRET) from fluorene segments to diphenylmaleimide units. The experimental and DFT/TDDFT calculation results demonstrated that the interaction mechanism of FAMs with F− and CN− was hydrogen bond interaction and nucleophilic addition reaction, respectively.

Fluorene-based cathode interlayer polymers for high performance solution processed organic optoelectronic devices

A hydrophilic polyfluorene-based conjugated polyelectrolyte (CPE) Poly[9,9-bis(4′-(6″-(diethanolamino)hexyloxy) phenyl)fluorene], PPFN-OH (Scheme 1) has been synthesized and utilized as cathode interlayer for both polymer light emitting diodes (PLEDs) and solar cells (PSCs). For comparison, another CPE namely Poly[9,9-bis(6′-(diethanolamino)hexyl)fluorene] (PFN-OH) has also been investigated. They comprise the same polyfluorene backbone structures with, respectively, diethanolaminohexyl (PFN-OH) and diethanolaminohexoxyphenyl (PPFN-OH) substituents attached to the C9 carbon of the fluorene repeat unit. In comparison to reference devices with more reactive Ca/Al cathodes, utilizing these CPEs as interlayers allowed an Al cathode to be used for blue light emission PLEDs, yielding 51% and 92% enhancement of maximum luminous efficiency (LE) for PFN-OH and PPFN-OH, respectively. The PLEDs with PPFN-OH showed both higher maximum LE and maximum luminance (L) (LE=2.53cd/A at 6.2V, L=9917cd/m2 at 8.3V) than devices with PFN-OH (2.00cd/A at 4.1V, 3237cd/m2 at 7.2V). The PPFN-OH PLEDs also showed no significant roll-off in efficiency with increasing current density up to 400mA/cm2, indicating excellent electron injection ability and stability for this interlayer. The insertion of alkoxy-phenyl groups at the C9-position in PPFN-OH is clearly advantageous. This simple modification significantly improves the CPE cathode interlayer performance. Parallel investigations of the electron extraction properties of PPFN-OH in inverted architecture PSCs with PCDTBT:PC70BM bulk heterojunction active layers demonstrated a power conversion efficiency enhancement of ∼19% (from 4.99% to 5.95%) for indium tin oxide cathode devices compared with reference devices using Ca/Al cathodes. These results confirm PPFN-OH to be a promising interlayer material for high performance solution processed organic optoelectronic devices.

White polymer light emitting diode materials introducing dendritic quinoxaline derivative: Synthesis, optical and electroluminescent properties

Copolymers including dendritic-quinoxaline (DTQ1G, DTQ2G) (<0.1mol%) as dopant derivative have been synthesized on a polyfluorene (PF) backbone based on the Suzuki coupling reaction. The UV–vis spectra of polymers showed similar behaviors in the solution and on the film. However, PL spectra were similar to that of PF in solution, but the peak around 590nm increased as the amounts of DTQ1G and DTQ2G were increased in the casting film. In thin film, the intensities of the dendritic TQ monomers increased with the generation number of the dendrons. In case of PFDTQ2G03, the luminous efficiency and power efficiency were 0.66cd/A and 0.29lm/W, respectively with a maximum brightness of 3792cd/m2. The CIE coordinates of PFDTQ1G05 and PFDTQ2G10 were (0.33, 0.30) and (0.35, 0.34) close to the pure white coordinates of (0.33, 0.33).