Light-emitting Polymer Films Research Articles

Synthesis, characterization, and stability of poly(ethylene-co-acrylic acid) films surface functionalized with fluorescent moieties

Light emission has been introduced onto poly(ethylene-co-acrylic acid) (PEAA) surfaces by covalently grafting two different fluorescent moieties. The facile surface modification method presented allows commercial polymers to be used for light emitting and sensor applications. Two fluorescent compounds, 4′-(octyloxy)-4-biphenyl carboxylic acid (OBC) and 2,7-bis (bromomethyl)-9,9-dihexyl-9H-fluorene (BMDF), were covalently grafted to the surface of PEAA films. Conversion of the carboxylic acid functional group present in PEAA to acid chloride was followed by reaction with ethylenediamine to provide primary amine groups on the film surface. The resultant amine groups then served as the grafting site for OBC and BMDF on the PEAA film (OBC-PEAA and BMDF-PEAA). For each reaction step, the chemical modifications were confirmed using FTIR spectroscopy and static water contact angles. OCB-PEAA and BMDF-PEAA films were evaluated using UV–Vis spectrophotometry to determine UV absorption wavelength ranges. The modified films were then irradiated at their respective maximum absorption wavelengths in a fluorescence spectrophotometer and emissions were identified in both the violet-blue and red regions of visible light. When irradiated at 254 nm, both PEAA-OBC and PEAA-BMDF fluoresce blue. Films were reanalyzed after 6 months and again after 5 years to assess long-term stability of this light emitting behavior. OBC and BMDF grafted to the PEAA films remained chemically stable and functioned effectively as light emitting polymer films for up to 5 years.

To inhibit coffee ring effect in inkjet printing of light-emitting polymer films by decreasing capillary force

Polymer thin film with uniform thickness and flat surface profile is the key point for polymer light emitting diodes (PLEDs) by inkjet printing. However, the coffee ring effect is usually observed due to the mismatch between the evaporation of the solvent and the decrease of solution volume, which promotes the formation of radial flow from the interior of the drop to the edge. In this paper, coffee ring effects of inkjet printed poly(spirobifluorene) films were proposed to be restrained by decreasing capillary force by adding co-solvent with high boiling point and high viscosity to the main solvent. The low evaporation rate of the co-solvent can reduce the driving force of the radial flow; meanwhile the high viscosity of the co-solvent can increase the resistance of the radial flow. Thus, polymer films with improve uniformity can be obtained due to the suppression of the radial flow. The device performance was greatly improved under the condition of proper film thickness and film uniformity and the maximum luminous efficiency of devices with inkjet printed poly(spirobifluorene) can reach 80% of the spin-coated devices.

Waste chimney oil to nanolights: A low cost chemosensor for tracer metal detection in practical field and its polymer composite for multidimensional activity

Proper waste disposal from household and restaurants is becoming an important and recurring waste-management concern. Herein, a method of upcycling of waste kitchen chimney oil has been adopted to prepare fluorescent multifunctional carbon quantum dots. These nanodots showed superior biocompatibility, excellent optical properties, water solubility and high yield. Preparation of C-dots from highly abundant carbon source of waste refusals is highly effective in commercial aspect as well as in reducing the immense environmental pollution. The C-dots showed quasi-spherical size obtained from high resolution transmission electron microscopy (HRTEM) having an abundance of 1–4 nm in size. The ease of water dispersibility of the nanodots is a mere reflection of their surface polarity which has been supported by Fourier transformed infrared spectroscopy (FTIR) and X-ray photoelectron spectroscopy (XPS). In the field of practical acceptability, the C-dots have been experimented to sense Fe3+ ion in a wide range of concentration (1 nM to 600 μM) with a detection limit of 0.18 nM which can be termed as ‘tracer metal chemosensor’. Moreover, the prepared carbon dots were also tested against inter-cellular Fe3+ ion sensing probe. Lastly, we also fabricate the biopolymer‑carbon dots composite for fluorescent marker ink and light emitting polymer film.

White-light emission of blue-luminescent graphene quantum dots by europium (III) complex incorporation

Traditionally, graphene quantum dots are prepared by fragmentation of graphene sheets into the nanoscale particles of controlled sizes followed by band gap adjustment by doping with electron-donating elements. Our novel synthetic aqueous arc discharge process has been developed to produce the blue-luminescent graphene quantum dots (bGQDs). The resulting bGQDs are ∼15 nm in diameter and the amount of oxygen-including functional groups can be controlled to 27.4% and 30.8% at 1 and 4 A of a current level, respectively. The presence of a band gap is confirmed by using scanning tunneling microscopy/spectroscopy (STM/STS). Additionally, we investigated the effect of oxygen doping levels on the band gap by photoluminescence (PL) behaviors and a density functional theory (DFT). The PL emission is red-shifted from 397 to 425 nm corresponding to the amount of oxygen-including functional groups in bGQDs and the DFT calculation confirms the decrease in a band gap from ∼2.0 to ∼1.7 eV due to electron donation from oxygen. In addition, our quantum dots have promising applications for practical use in optoelectronics devices. For example, tris-dibenzoylmethane mono-1,10-phenanthroline-europium (III) (EuIIIDP) is incorporated with bGQDs for white-light emission and is shown to be successfully fabricated into light-emitting polymer films.

Selective Photophysical Modification on Light-Emitting Polymer Films for Micro- and Nano-Patterning

Laser-induced cross-linking in polymeric semiconductors was utilized to achieve micro- and nano-structuring in thin films. Single- and two-photon cross-linking processes led to the reduction in both the refractive index and thickness of the polymer films. The resultant photonic structures combine the features of both relief- and phase-gratings. Selective cross-linking in polymer blend films based on different optical response of different molecular phases enabled “solidification” of the phase-separation scheme, providing a stable template for further photonic structuring. Dielectric and metallic structures are demonstrated for the fabrication methods using cross-linking in polymer films. Selective cross-linking enables direct patterning into polymer films without introducing additional fabrication procedures or additional materials. The diffraction processes of the emission of the patterned polymeric semiconductors may provide enhanced output coupling for light-emitting diodes or distributed feedback for lasers.

Surface plasmon coupled emission using conjugated light-emitting polymer films [Invited

We experimentally investigate surface plasmon coupled emission (SPCE) in the Kretschmann geometry using thin conjugated polymer films. The broad spectral emission from the polymer films usefully demonstrates how the spectral behaviour of SPCE follows clearly from the underlying dispersion of surface plasmon polaritons supported by the configuration. We pay particular attention to how the spectral bandwidth of the underlying emissive layer impacts on the overall performance. Overall the work highlights the many factors that may be considered when designing optimum optoelectronic devices based on metal-organic multilayers.

Control of emission outcoupling in liquid-crystalline fluorescent polymer films

We report the improvement of light outcoupling efficiency of thermally annealed thin liquid crystalline light-emitting polymer films. Unlike the generally observed phenomena from thermally annealed fluorescent polymer films, in the liquid-crystalline fluorescent polymer (LCFP) films, the transverse electric waveguide mode was suppressed due to the light scattering by liquid-crystalline domains in the film. In addition, the optical anisotropy of the annealed LCFP was increased. As a result, the light extraction of photoluminescence and electroluminescence emission in the annealed LCFPs was enhanced by ∼2 times because the emitted light loss by waveguide was minimized. This approach to improve light extraction by forming scattering domains inside the LCFP film gives an insight to improve the outcoupling efficiency of the EL devices in the viewpoint of the material design and film morphologies without employing external geometric device structures.

Microstructure and device performance of thin film light emitting polymers

In this paper, the performance of polymer light emitting diodes (PLEDs) was investigated and related to their thin film morphology. Two methods were used to cast the thin film, i.e. spin coating and dip coating. The film morphology was controlled by different film formation conditions, e.g. spin speed and withdrawal speed, as well as different solution properties, e.g. polymer molecular weight and solvent. Phase separation was inspected using both optical microscopy and an atomic force microscopy. The device performances of interest were characterised by measuring optical response, the Commission Internationale de l'Eclairage Chromaticity, electrical characteristics, and device lifetime. The study has established that a high efficiency PLED can be achieved by using a blend of electron transporting emitter and a hole transporting polymer. The device prepared from the blend shows a low turn-on voltage, a large current density, a high efficiency, and an acceptable lifetime. The result shows that light emitting polymer film morphology is crucial in determining the device performance. As solvent evaporation rate increases, phase separation reduces to a fine-scale. The device efficiency decreases with a reduction of degree of phase separation while the lifetime enhances. It is likely that there is an optimum degree of phase separation for device optimisation (high efficiency and long lifetime). In addition, it was established that both dip and spin coating were effective fabrication processes.

Study of mechanical properties of light-emitting polymer films by nano-indentation technique

This paper presents our recent study on characterizing mechanical properties of a series of light-emitting conjugated polymer films designed for Polymer Light Emitting Diode (PLED) applications. The nano-indentation technique is used for such characterization. The elastic modulus and hardness of the conjugated polymers are found to be significantly higher than other polymeric materials. The mechanical properties of the conjugated polymer films are correlated with their chemical structures. The structures of side chains in the polymers show significant effects on the measured mechanical properties.

44.3: Ink Jet Printed Organic Displays

AbstractWe have optimized the key parameters in inkjet printing to obtain flat uniform hole transporting and RGB light emitting polymer films. The performance of the resulting inkjet printed devices is comparable to that of spin‐coated devices. We will also present results on reliability of jetting performance, which is key to manufacturing. Finally, we will present the results of a 160×120 full color ink jet printed display.