Protein Nanofibrils Balance Colours in Organic White-Light-Emitting Diodes

In this review we discuss our efforts in using protein nanowires (amyloid fibrils) as structural templates for use in organic electronics applications, mainly focusing on organic light-emitting diodes (OLEDs). We discuss different ways of functionalising amyloid fibrils. In one method, the amyloid fibril is used to organise luminescent polymers. We also discuss an alternative preparative method, resulting in amyloid-like materials functionalised with phosphorescent organometallic complexes. We discuss the incorporation of such materials in organic electronics devices, such as OLEDs. When amyloid fibrils are integrated into the OLED active layer, consisting of an electroluminescent blue-emitting polyfluorene, the efficiency of the device increases by a factor of 10. Furthermore, when amyloid fibrils incorporating phosphorescent metal complexes are used, the phosphorescent guest functions more efficiently than in the corresponding case where naked metal complexes are used. By preparing amyloid fibrils incorporating green- and red-emitting phosphorescent complexes, and combining these with blue-emitting polyfluorene, we can fabricate devices for white-light emission. The origin of the effects of the biomaterial on device performance is discussed.     

Moisture permeability through multilayered barrier films as applied to flexible OLED display

Strict protection of organic light‐emitting diodes (OLEDs) and other optoelectronic materials from direct contact with ambient moisture and oxygen is one of the major challenges in the development of flexible OLED displays and other flexible electronic devices. This problem is typically addressed by the use of polymeric substrates with multilayered barrier coatings comprising alternating organic/inorganic layers. The multilayered barrier approach is critically examined using a numerical model based on a defect‐dominated diffusion process combined with experiments involving face‐to‐face lamination of two barrier films. The modeling results identify two regimes, corresponding to two distinct permeation mechanisms, and provide scaling relationships and general design criteria for multilayered barrier coatings. The results suggest that the most significant gain in barrier performance can be realized when the thickness of the organic/adhesive layer(s) in the multilayered structure is less than the average pinhole (defect) size in the inorganic barrier layer(s). © 2007 Wiley Periodicals, Inc. J Appl Polym Sci, 2007     

Optimization of opto‐electronic property and device efficiency of polyfluorenes by tuning structure and morphology

Polyfluorene‐based oligomers and polymers (PFs) have been studied intensively as active materials for organic optoelectronic devices. In this review, the optimization of the opto‐electronic property and device efficiency of polyfluorenes in the field of light‐emitting diodes (LEDs) and photovoltaic cells (PVs) by tuning structure and morphology are summarized in terms of two typical modification techniques: copolymerization and blending. The relationships between molecular structures, thin film morphologies, opto‐electronic properties and device efficiencies are discussed, and some recent progress in LEDs and PVs is simultaneously reviewed. After the introduction, the basic knowledge of molecular structures and properties of polyfluorene homopolymers is presented as a background for a better understanding of their great potential for opto‐electronic applications. Immediately after this, three different opinions on the origin of low‐energy emission band at 520–540 nm in polyfluorene‐based LEDs are addressed. Rod–coil block copolymers and alternative copolymers are focused on in the next section, which are a vivid embodiment of controlling supramolecular structures and tailoring molecular structures, respectively. In particular, various supramolecular architectures induced by altering coil blocks are carefully discussed. Recent work that shows great improvement in opto‐electronic properties or device performance by blending or doping is also addressed. Additionally, the progress of understanding concerning the mechanisms of exciton dynamics is briefly referred to. Copyright © 2006 Society of Chemical Industry     

Conjugated polyelectrolytes: A new class of semiconducting material for organic electronic devices

This feature article presents a short review of the recent developments in the synthesis of conjugated polyelectrolytes (CPEs) along with their applications in organic optoelectronic devices with particular focus on the molecular structures of CPEs with ionic functionality, synthetic approaches, and their utilization as an interfacial layer. The orthogonal solubility of the CPEs allows the simple preparation of multilayer organic devices by solution casting on top of a nonpolar organic photoactive layer without disturbing the interfaces, showing their effectiveness in tuning the electronic structures at the interfaces for improving the charge carrier transport and resulting device properties. These achievements highlight the dynamic nature of CPEs and their applicability to a wide range of optoelectronic devices.     

An alternating low band-gap polyfluorene for optoelectronic devices

An alternating polyfluorene (APFO) with low band-gap segments named APFO-Green1 has been designed and synthesized for use in optoelectronic devices. The low band-gap segment consists of an electron acceptor (A), fenced by electron donors (D). This D-A-D configuration leads to a partial charge transfer in the polymer backbone, and thereby a low band-gap (1.3 eV). Results obtained from characterization of APFO-Green1 include light absorption and emission at extended wavelengths as well as high hole mobility. Furthermore, blends of the polymer with different fullerene derivatives exhibit unusually high photovoltaic performance at long wavelengths, making this type of conjugated polymers promising for plastic solar cell applications.     

Synthesis and properties of novel electrophosphorescent conjugated polyfluorenes based on aminoalkyl-fluorene and bipyridine with rhenium(I) complexes

Novel bipyridine-based aminoalkyl-polyfluorene and their corresponding rhenium complex-contained copolymers with different rhenium complex contents in polymer backbone were synthesized by Suzuki polymerization and post-polymerization, respectively. The aminoalkyl-polyfluorenes have good solubility in organic solvents and also are able to dissolve in methanol by adding a few drops of acetic acid. The solubility in polar solvents such as alcohol provides a great advantage in fabrication of multi-layer PLEDs. However, the rhenium complex-based copolymers become insoluble in common organic solvents; instead have a good solubility in polar solvents, such as methanol and DMF. The optoelectronic and electroluminescent properties of these copolymers (bipyridine- and rhenium-based copolymers) were investigated. The electroluminescent properties of these copolymers were found to have similar device performances by using high work function metal (Al) and low work function metal (Ba) as cathodes. We also found that these rhenium copolymers can be used as the electron transport layer in PLED. After inserting a thin rhenium copolymer layer between emission layer (P-PPV or PFO-DBT15) and cathode, a great improvement in EL performance could be achieved.     

Optically and electrically excited emissions from organic semiconducting oligomer crystals

This paper reviews optically and electrically excited emissions from organic semiconducting oligomers in the form of their single crystals. Of the semiconducting oligomers, the review focuses on thiophene/phenylene co‐oligomers (TPCOs). The topics cover crystal growth and laser oscillation along with related spectrally narrowed emissions of TPCO crystals. Aside from the strong excitation with a laser beam, weak excitation using a mercury lamp produces optical fringes superimposed on broadband emission spectra. The laser oscillation spectra accompanied by longitudinal multimode and optical fringes observed from the weak excitation have the same origin. This enables us to determine optical constants (i.e. refractive indices) and their dispersion of the crystals and provides sufficient information for the construction of optoelectronic devices based on the organic crystals. As a typical example, the review outlines the improved device constitution and performance as well as device operation methods with light‐emitting field‐effect transistors (LEFETs), because the device configuration is suited for cutting‐edge devices including lasers. Finally the review presents device performance of LEFETs having a diffraction grating. These devices are suited for exploring the possibility of constructing a next‐generation current‐injected laser device. © 2016 The Authors. Polymer International published by John Wiley & Sons Ltd on behalf of Society of Chemical Industry.     

Controlling Polymer Solubility: Polyfluorenes with Branched Semiperfluorinated Side Chains for Polymer Light-Emitting Diodes

A series of novel polyfluorenes, soluble exclusively in perfluorinated solvents, were prepared. The new materials were studied with regard to orthogonal processing of organic electronic materials. The desired solubility was achieved by introducing semifluorinated side chains to the fluorene monomers. Since the use of long perfluoroalkyl chains (R^F) is restricted due to public health concerns, a synthetic route for polyfluorenes with short R^F chains branched by aromatic units has been developed. The photophysical behavior of the resulting polymers was investigated in solution and thin films by UV/Vis absorption and photoluminescence spectroscopy. The photoluminescence quantum yields were found to be in the range of those of alkylated polyfluorenes. The electroluminescent properties were studied in single-layer polymer light-emitting diodes, with the new polymers as active materials, which exhibited similar characteristics to previously published single-layer devices with polyfluorenes containing long R^F. The wetting properties of different polyfluorene films containing fluorinated, polar, polyethylene glycol, or nonpolar alkyl groups were investigated by contact angle measurements.     

BODIPY-based polymeric dyes as emerging horizon materials for biological sensing and organic electronic applications

The design, synthesis, and characterization of BODIPY (4,4′-difluoro-4-bora-3a,4a-diaza-s-indacene)-based small molecules has undergone tremendous progress in the past two decades. BODIPYs and their dipyrrin precursors have been recently reviewed with emphasis placed on their syntheses, reactions and applications including biochemical labelling, fluorescent switches, chemosensors, and electroluminescent devices. Compared to other dye classes, these systems offer unique attractions such as excellent thermal/photochemical stability, intense absorption/emission profiles, negligible triplet-state formation, and small Stokes shifts. Their optoelectronic and semiconductor properties can be finely tuned via facile synthetic modifications on the dipyrromethene core, while their optical characteristics are relatively insensitive to medium polarity and the pH of the environment. On the contrary, the optoelectronic and photophysical properties of BODIPY-based polymeric semiconductors the newest developed family in BODIPY-based organic materials (the first BODIPY based conjugated polymers were published in 2008) are still not well understood. For fully resolving these open questions, it will be shown in this review that the optoelectronic and photophysical properties of the BODIPY-based conjugated polymers are depending on various parameters including: (i) the positions where the BODIPY core will be attached into the polymer backbone (α-, β-, meso positions, or fluorine substitution) and (ii) the number of methyl substituents on 1, 2, 3, 5, 6, and 7 positions and finally (iii) if BODIPY functions as electron rich or electron deficient building block in “donor-acceptor” (D-A) conjugated polymers. This is a very significant advance in the emerging field of BODIPY chemistry since it opens the path for further optimization of the recent and new developed BODIPY-based polymeric semiconductors with predetermined optoelectronic and photophysical properties by providing new design rules to organic and material chemists as well as physical (bio)chemistry and device (bio)engineering scientists.     

Novel luminescent polymers containing backbone triphenylamine groups and pendant quinoxaline groups

Novel, conjugated polyfluorene derivatives that comprised an electron-donating triphenylamine group in the backbone and pendant, electron-accepting quinoxaline moieties, were synthesized via the Suzuki coupling reaction and their UV–vis absorption, fluorescence emission and electrochemical properties were investigated. The copolymers were readily soluble in common organic solvents and displayed good film-forming ability and excellent thermal stability. Electroluminescence devices, comprising indium tin oxide/poly(3,4-ethylenedioxythiophene) doped with poly(styrenesulfonic acid)/emitting polymer/Ba/Al, in which the polymers were employed as emissive layers, exhibited superior performance compared to that of corresponding, poly(9,9-dioctylfluorene) and poly(9,9-dioctylfluorene-co-4,4′-triphenylamine) based devices, indicating that the polymers offer promise as emissive materials in polymeric light-emitting diodes.     

Photocrosslinkable hyperbranched polyfluorenes containing oxadiazole: synthesis,photophysics and electroluminescence

BACKGROUND: Crosslinkable hyperbranched polymers have great potential for use in organic light‐emitting devices and photovoltaics. With this in mind, two crosslinkable hyperbranched polyfluorenes, P5 and P6, containing oxadiazole as the core, and linear polyfluorene P0 were synthesized via palladium‐catalyzed Suzuki coupling. A device was also made. RESULTS: The resulting polymers were characterized using NMR, gel permeation chromatography, differential scanning calorimetry and thermogravimetric analysis. The spectral behavior of the polymers was also investigated. Compared with uncrosslinked P5 and P6, the photoluminescence spectra of crosslinked P5 and P6 were little influenced by the crosslinking. After annealing for 3 h at 200 °C in air, the photoluminescence of P6 (with a greater content of branched units) showed excellent stability compared with that of P5 and P0. The results for the device revealed that photocrosslinking improves the stability of the electroluminescence. CONCLUSION: The crosslinkable polymers obtained can be used in multilayer devices. Copyright © 2008 Society of Chemical Industry     

Recent Advances in Hybrid Optoelectronics

Polymer/organic optoelectronic devices have drawn the attention of both the academic and industrial research communities due to the potential for a low-cost, large-area, solution-processable technology alternative to conventional inorganic optoelectronics. Issues related to the stability and degradation of the organic/polymer-based optoelectronics are hampering the progress in the field. The use of poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT : PSS) as the anode and reactive metals as the cathode, as well as the degradation of organic semiconductors in ambient atmosphere, are some of the stability issues to be addressed. To resolve these issues, in the past decade, there has been a growing interest in research of hybrid optoelectronic devices which employ metal oxides as air-stable charge injecting/extracting layers that sandwich the photo-responsive organic active layer and protect it from the ambient oxygen and moisture and prevent photo-oxidation by absorbing UV light. Herein, we review the recent advances made in hybrid optoelectronics and discuss the tremendous potential of these devices.     

New bead type and high symmetrical diallyl-POSS based emissive conjugated polyfluorene

A new bead type and diallyl-POSS based polyfluorene (P2) with high symmetrical structure was synthesized via Heck coupling reaction between oligomeric alkynyl fluorene (P1) and diallyl polyhedral oligomeric silsesquioxanes (diallyl-POSS). The molecular weight and the conjugated length of P1 and P2 were well controlled to acquire good solubility and excellent optical property. The bead-type POSS based polymer was characterized by gel permeation chromatography (GPC), FT-IR, ¹H NMR and photoluminescence (PL) spectra. High Resolution Transmission Electron Microscopy (HRTEM) micrographs showed that diallyl-POSS were uniformly nano-dispersed in the polymer matrix. Compared with P1, the POSS based polyfluorene P2 exhibited not only a higher thermal stability, but also an improved photophysical property in solution and solid states. The incorporation of diallyl-POSS resulted effectively in inhibiting the strong stacking/dipole–dipole interaction between fluorescent groups in the polyfluorene. The experimental results indicate that the bi-functional POSS based light-emitting polymers with high symmetrical structure can have great potential in optical materials and devices, such as OPV or PLED, etc.     

Theoretical investigation on the white-light emission from a single-polymer system with simultaneous blue and orange emission

White organic light-emitting devices (WOLEDs) have attracted considerable attention because of their good potential for various lighting applications. Among these devices, WOLEDs based on polymers (WPLEDs) are of particular interest. We report here a theoretical investigation of the white-light emission from a single-polymer system with simultaneous blue (polyfluorene as a blue host) and orange (2,1,3-benzothiadiazole-based derivative as an orange dopant) emission. A variety of theoretical methods are used and evaluated to calculate electronic and optical properties of polyfluorene and 2,1,3-benzothiadiazole-based derivatives. Simulated electronic and optical properties are found to agree well with available experimental measurements. The influence of the “CH”/N heterosubstitution on the electronic and optical properties of the 2,1,3-benzothiadiazole-based derivative is considered. Furthermore, we find that the electronic and optical properties of “CH”/N substitution derivatives can be tuned by symmetrically adding suitable electron-donating groups on N,N-disubstituted amino groups, implying good candidates as orange dopants in WPLEDs with polyfluorene as a blue-light-emitting host. Solvent (dichloromethane) effects on the electronic and optical properties of 2,1,3-benzothiadiazole-based derivatives have been investigated. In addition, low reorganization energy values of holes for designed 2,1,3-benzothiadiazole-based derivatives within the framework of the charge hopping model suggest them to be good hole transfer materials.     

Electrospray deposition of polymer thin films for organic light-emitting diodes

Electrospray process was developed for organic layer deposition onto polymer organic light-emitting diode [PLED] devices in this work. An electrospray can be used to produce nanometer-scale thin films by electric repulsion of microscale fine droplets. PLED devices made by an electrospray process were compared with spin-coated ones. The PLED device fabricated by the electrospray process showed maximum current efficiency of 24 cd/A, which was comparable with that of the spin-coating process. The electrospray process required a higher concentration of hole and electron transport materials in the inks than spin-coating processes to achieve PLED maximum performance. Photoluminescence [PL] at 407 nm was observed using electrosprayed poly(N-vinyl carbazole) films, whereas a peak at 410 nm was observed with the spin-coated ones. Similar difference in peak position was observed between aromatic and nonaromatic solvents in the spin-coating process. PLED devices made by the electrospray process showed lower current density than that of spin-coated ones. The PL peak shift and reduced current of electrosprayed films can therefore be attributed to the conformation of the polymer.     

D-A-based polyfluorene derivatives end-capped with cyclometalated iridium complexes by unconjugated linkage: Structure-property relationships

A series of donor(D)-acceptor(A)-based polyfluorene derivatives, which contain carrier-transporting units of carbazole and oxadiazole as the substitutes of the C-9 position of ?uorene and are end-capped with the red-emitting iridium bi(phenylisoquilonato)(picolinato) unit by unconjugated linkage, were synthesized and characterized. The molar ratios between the donor of carbazole and the acceptor of oxadiazole moieties were found to significantly influence photoluminescent efficiency, electrochemical and electroluminescent properties of these D-A-based polyfluorene derivatives. While the ratio increased to 3:7, the PFCz3OXD7Ir showed the best device performance in the polymer light-emitting device with a configuration of ITO/PEDOT/polymers/LiF/Al. A turn-on voltage of 6.0 V, a maximum current efficiency of 0.59 cd/A and the highest luminance of 917 cd/m² were presented.     

Enhanced electroluminescence from a conjugated polymer/ionomer blend

Poly [2-methoxy-5-(2′-ethylhexyloxy)-1,4-phenylene vinylene] (MEH-PPV) and poly (sodium 4-styrenesulfonate) ionomer (PSS-ionomer) blends were used as emitting layers in polymer light-emitting diodes (LEDs). The polymers were blended in various ratios, and their luminescent properties were compared with those of a MEH-PPV/polystyrene (PS) blend system. The MEH-PPV/PSS-ionomer and MEH-PPV/PS devices showed a much higher luminescent efficiency when compared with pure polymer devices, due to the dilution effect. The MEH-PPV/PSS-ionomer blend device achieved a more enhanced luminescent efficiency as compared with that of the MEH-PPV/PS device, due to polar groups in the PSS-ionomer, which may enhance the electron injection from the cathode to the emitting molecules. In addition, the turn-on voltage of the MEH-PPV/PSS-ionomer based on LEDs was also dramatically decreased.     

Designing π-conjugated polymers for organic electronics

Conjugated polymers have attracted an increasing amount of attention in recent years for various organic electronic devices because of their potential advantages over inorganic and small-molecule organic semiconductors. Chemists can design and synthesize a variety of conjugated polymers with different architectures and functional moieties to meet the requirements of these organic devices. This review concentrates on five conjugated polymer systems with 1D and 2D topological structures, and on one polymer designing approach. This includes (i) conjugated polyphenylenes (polyfluorenes, polycarbazoles, and various stepladder polymers), (ii) other polycyclic aromatic hydrocarbons (PAHs) as substructures of conjugated polymers, (iii) thiophene and fused thiophene containing conjugated polymers, (iv) conjugated macrocycles, (v) graphene nanoribbons, and finally (vi) a design approach, the alternating donor–acceptor (D–A) copolymers. By summarizing the performances of the different classes of conjugated polymers in devices such as organic light-emitting diodes (OLEDs), organic field-effect transistors (OFETs), and polymer solar cells (PSCs), the correlation of polymer structure and device property, as well as the remaining challenges, will be highlighted for each class separately. Finally, we summarize the current progress for conjugated polymers and propose future research opportunities to improve their performance in this exciting research field.     

Synthesis and Opto-electrical Properties of Stellar Polyfluorene Derivatives Containing Polyhedral Oligomeric Silsesquioxanes as the Center Core

Three polyfluorene derivatives (P1–P3) emitting blue, green and red light were prepared via a Suzuki coupling method. Polyhedral oligomeric silsesquioxanes (POSS) were integrated into the center core of the derivatives to create stellar light-emitting polymers (POSS-P1–POSS-P3). The glass-transition and decomposition temperatures were raised after introducing POSS moieties. The starlike polymers showed a reduced emissive band at 540 nm when annealing at 200 °C. The result implies that integrated POSS at the center core suppresses the keto effect generally occurring for polyfluorenes. Double-layer light-emitting devices of indium tin oxide/poly(ethylenedioxythiophene)/polymer/Ca/Al were fabricated to evaluate polymer potential. Compared with pristine polymers, the maximum brightness and the current efficiency of devices using POSS-containing polymers as active layers were enhanced. A blending method using P1 or POSS-P1 as the host matrix further improved device performance.     

On the phase behaviour of organic semiconductors

The physical organisation, from the molecular to the macroscale, of functional organic matter such as polymer semiconductors can profoundly affect the properties and features of the resulting architectures and their consequent performance when used as active layers in organic optoelectronic devices, including organic thin‐film field‐effect transistors, organic light‐emitting diodes or organic photovoltaic cells. Here, we present a survey on the principles of structure development from the liquid phase of this interesting and broad class of materials with focus on how to manipulate their phase transformations and solid‐state order to tailor and manipulate the final ‘morphology’ towards technological and practical applications. Copyright © 2012 Society of Chemical Industry