Polymer Optoelectronic Devices Research Articles

Oligomeric Compatibilizers for Control of phase Separation in Conjugated Polymer Blend Films

Control over phase separation and morphology is critical to optimal function in polymer optoelectronic devices. Here, two fully conjugated oligomeric compatibilizers are introduced, and their effect on the phase separation of blends of poly(9,9′-dioctylfluorene-co-benzo-thiadiazole) (F8BT) with poly(9,9′-dioctylfluorene-co-bis-N,N′-(4,butylphenyl)bis-N,N′-phenyl-1,4-phenylenediamine) (PFB) are reported. AFM and STXM analysis demonstrate that the addition of compatibilizer altered the size and relative composition of phase-separated domains formed during spin-casting. Small structural differences between the two compatibilizers brought about significantly different morphological changes to the blends, suggesting that further development of compatibilizer structure could enable enhanced control toward desired blend film morphologies.

Phosphonate-functionalized polyfluorene and its application in organic optoelectronic devices

Conjugated polar polymers, in which the conjugated backbones are chemically anchored with functional polar side groups, can be processed with water/alcohol solvents, and thus multilayered device architectures can be easily realized via sequential solution processing of the toluene-soluble emissive polymer and alcohol-soluble electron-transporting polymer without intermixing. Regarding their use in organic optoelectronic devices, the success in achieving efficient charge injection and intimate contact between metal electrodes and organic semiconductors is very vital for enhancing the device performance. In this short review, it gives a brief review to neutral alcohol-soluble phosphonate-functionalized polyfluorene, mainly concerning the electronic structure at the phosphonate-functionalized polyfluorene/aluminum cathode interface and its successful application in multilayered polymer optoelectronic devices including polymer light-emitting diodes and polymer solar cells.

ChemInform Abstract: Water/Alcohol Soluble Conjugated Polymers as Highly Efficient Electron Transporting/Injection Layer in Optoelectronic Devices

AbstractReview: 135 refs.

Water/alcohol soluble conjugated polymers as highly efficient electron transporting/injection layer in optoelectronic devices

Water/alcohol soluble conjugated polymers (WSCPs) can be processed from water or other polar solvents, which offer good opportunities to avoid interfacial mixing upon fabrication of multilayer polymer optoelectronic devices by solution processing, and can dramatically improve charge injection from high work-function metal cathode resulting in greatly enhancement of the device performance. In this critical review, the authors provide a brief review of recent developments in this field, including the materials design, functional principles, and their unique applications as interface modification layer in solution-processable multilayer optoelectronic devices (135 references).

Efficiency enhancement in polymer optoelectronic devices by introducing titanium sub-oxide layer

We have demonstrated an efficiency enhancement in polymer light-emitting diodes (PLEDs) and polymer solar cells (PSCs) by introducing solution-based titanium oxide (TiO x ) layer between the polymer and Al electrode as an electron transport/injection layer. For the PLED case, the TiO x layer reduces the barrier height between the polymer and Al cathode, thereby facilitating the electron injection and enhancing the device performance. For the PSCs case, the TiO x layer plays a role as an optical spacer in PSCs leading an increase in absorption of photon in active layer by modifying the spatial distribution of the light intensity inside the device, thereby creating more photogenerated charge carriers in the bulk heterojunction layer.

Conjugated‐Polymer Blends for Optoelectronics

AbstractSolution‐processed polymer optoelectronic devices such as light‐emitting diodes and solar cells have many advantages for large‐area manufacture, and show increasing levels of performance. Here, we review recent progress in using blends of two conjugated polymers for optoelectronic devices. The blending of two or more polymers allows tuning of device performance, and for photovoltaics presents an attractive way to combine donor and acceptor materials with a morphology controlled by polymer phase separation. We discuss recent advances in imaging the microstructure of conjugated polymer blends, and we demonstrate how the blend structure leads to performance advantages in both LEDs and photovoltaic devices.

Insight into the Raman shifts and optical absorption changes upon annealing polymer/fullerene solar cells

Raman shifts and optical absorption spectra of bulk heterojunction films were measured to elucidate the origin of the optimum annealing parameters. A series of device optimization studies revealed 413K to be the optimum annealing temperature, leading to a power conversion efficiency of 2.95%. The highest power conversion efficiency coincides with the highest peak in the UV-visible absorption and the lowest full width at half maximum of the CC symmetric stretching signal in the Raman spectra. The changes observed in the vibronic shifts could be useful in obtaining information about the optimal performance and processing conditions for polymer optoelectronic devices.

Influence of photo-induced degradation on the optoelectronic properties of regioregular poly(3-hexylthiophene)

Recently, head-to-tail regioregular poly(3-hexylthiophene) (P3HT) has been widely used as an active material in fabricating polymer optoelectronic devices. This study employs UV-vis absorption, photoluminescence (PL), X-ray diffraction (XRD) and the space-charge limited current (SCLC) model to elucidate the effect of light illumination on the optic and optoelectronic properties of P3HT. The degraded performance of P3HT, such as low absorbance, PL emission, and charge-carrier mobility is caused mostly by a reduction in the degree of conjugated structure and the lower crystallinity.

Imaging the Interfaces of Conjugated Polymer Optoelectronic Devices

A focused ion beam is used to extract cross-sectional samples from multilayer conjugated polymer films and optoelectronic devices. Subsequent TEM, AFM and SIMS studies yield insight into polymer-polymer and polymer-electrode interfaces and how these interfaces can be controlled by processing conditions, such as choice of solvent and thermal annealing.

High‐Efficiency Electrophosphorescent Copolymers Containing Charged Iridium Complexes in the Side Chains

A convenient approach to novel charged Ir polymers for optoelectronic devices to achieve red emission was developed. 2-(Pyridin-2-yl)benzimidazole units grafted into the side chains of macroligands (PFCz and PFP) served as ligands for the formation of charged Ir complex pendants with 1-phenylisoquinoline (1-piq). The charged Ir polymers (PFPIrPiq and PFCzIrPiq) showed exclusive Ir(1-piq)(2){N-[2-(pyridin-2-yl)benzimidazole]hexyl}(+)BF(4)(-) (IrPiq) emission, with the peak at 595 nm. The best device performances were obtained from PFCzIrPiq4 with the device configuration of ITO/PEDOT:PSS/PFCzIrPiq4+PBD (30 wt %)/TPBI/Ba/Al (PBD: 5-(4-tert-butylphenyl)-2-(biphenyl-4-yl)-1,3,4-oxadiazole; TPBI: 1,3,5-tris-(2-N-phenylbenzimidazolyl)benzene). A maximum external quantum efficiency (EQE) of 7.3 % and a luminous efficiency (LE) of 6.9 cd A(-1) with a luminance of 138 cd m(-2) were achieved at a current density of 1.9 mA cm(-2). The efficiencies remained as high as EQE=3.4 % and LE=3.3 cd A(-1) with a luminance of 3770 cd m(-2) at a current density of 115 mA cm(-2). The single-layer devices based on charged Ir polymers also showed high efficiency with the high work-function metal Ag as cathode. The maximum external quantum efficiencies of the devices were 0.64 % and 0.66 % for PFPIrPiq2 and PFPIrPiq10, respectively. A possible mechanism of an electrochemical cell associated with its electrochemical redox pathway for single-layer devices has been proposed. The results showed that the charged Ir polymers are promising candidate materials for polymer optoelectronic devices.

Multicomponent semiconducting polymer systems with low crystallization-induced percolation threshold

Blends and other multicomponent systems are used in various polymer applications to meet multiple requirements that cannot be fulfilled by a single material. In polymer optoelectronic devices it is often desirable to combine the semiconducting properties of the conjugated species with the excellent mechanical properties of certain commodity polymers. Here we investigate bicomponent blends comprising semicrystalline regioregular poly(3-hexylthiophene) and selected semicrystalline commodity polymers, and show that, owing to a highly favourable, crystallization-induced phase segregation of the two components, during which the semiconductor is predominantly expelled to the surfaces of cast films, we can obtain vertically stratified structures in a one-step process. Incorporating these as active layers in polymer field-effect transistors, we find that the concentration of the semiconductor can be reduced to values as low as 3 wt% without any degradation in device performance. This is in stark contrast to blends containing an amorphous insulating polymer, for which significant reduction in electrical performance was reported. Crystalline-crystalline/semiconducting-insulating multicomponent systems offer expanded flexibility for realizing high-performance semiconducting architectures at drastically reduced materials cost with improved mechanical properties and environmental stability, without the need to design all performance requirements into the active semiconducting polymer itself.

High‐Conductivity Poly(3,4‐ethylenedioxythiophene):Poly(styrene sulfonate) Film and Its Application in Polymer Optoelectronic Devices

AbstractThe conductivity of a poly(3,4‐ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS) film can be enhanced by more than two orders of magnitude by adding a compound with two or more polar groups, such as ethylene glycol, meso‐erythritol (1,2,3,4‐tetrahydroxybutane), or 2‐nitroenthanol, to an aqueous solution of PEDOT:PSS. The mechanism for this conductivity enhancement is studied, and a new mechanism proposed. Raman spectroscopy indicates an effect of the liquid additive on the chemical structure of the PEDOT chains, which suggests a conformational change of PEDOT chains in the film. Both coil and linear conformations or an expanded‐coil conformation of the PEDOT chains may be present in the untreated PEDOT:PSS film, and the linear or expanded‐coil conformations may become dominant in the treated PEDOT:PSS film. This conformational change results in the enhancement of charge‐carrier mobility in the film and leads to an enhanced conductivity. The high‐conductivity PEDOT:PSS film is ideal as an electrode for polymer optoelectronic devices. Polymer light‐emitting diodes and photovoltaic cells fabricated using such high‐conductivity PEDOT:PSS films as the anode exhibit a high performance, close to that obtained using indium tin oxide as the anode.

Polymer Optoelectronic Devices with High‐Conductivity Poly(3,4‐Ethylenedioxythiophene) Anodes

The conductivity of poly(3,4‐ethylenedioxythiophene):polystyrenesulfonate (PEDOT:PSS) film can be enhanced by more than two orders in magnitude by adding a compound with two or more polar groups, such as ethylene glycol (EG), meso‐erythritol (IUPAC name: 1,2,3,4‐tetrahydroxybutane), or 2‐nitroethanol, into the PEDOT:PSS aqueous solution. The mechanism of the increase in conductivity for PEDOT:PSS has been studied using Raman spectroscopy and atomic force microscope (AFM). Here we propose that the change in conductivity is due to the conformational change of PEDOT chains in the film. In untreated PEDOT:PSS films, coil, linear, or expanded‐coil conformations of the PEDOT chains may be present. In treated PEDOT:PSS films, the linear or expanded‐coil conformations may becomes the dominant form for PEDOT chains. This conformational change results in the enhancement of charge‐carrier mobility in the film and leads to enhanced conductivity. The high‐conductivity PEDOT:PSS film is ideal as the electrode for polymer optoelectronic devices. In this article, we report on the fabrication of polymer light‐emitting diodes (PLEDs) and photovoltaic cells (PVs) made using a highly conductive form of PEDOT:PSS as anode, and we demonstrate its performance relative to that of similar device using indium‐tin oxide (ITO) as the anode.

Femtosecond laser irradiation of polymethylmethacrylate for refractive index gratings

Polymethylmethacrylate (PMMA) or Perspex is an inexpensive polymer widely used for making the cores of communications grade polymer optical fibres (POFs) and as a substrate for polymer optoelectronic devices and integrated waveguides. Periodic refractive index structures have been written in undoped PMMA using multiple pulses of 40 fs duration from a 1 kHz Ti:sapphire femtosecond laser operating at the fundamental (800 nm). A refractive index change (Δn) of 5 ± 0.5 × 10−4 was observed before the onset of striations. Optimization of writing conditions for refractive index modification of POF fibres or bulk undoped PMMA will enable structures such as Bragg gratings, long-period gratings, mode couplers, microlens arrays, and zone plates to be written.

Self Assembly of Polymer Structures Induced by Electric Field

Amethod has been developed for fabricating polymer microstructures based on electric field induced self assembly and pattern formation. A dielectric fluid placed in between two conductive plates experience a force in an applied electric field gradient across the plates, which can induce a diffusive surface instability and self construction of the fluid surface. This process is exploited for the fabrication of self assembled polymer microstructures as well as replicated patterns through the use of pre-patterned plates or electrodes. FEM simulation is used to decide the minimum wavelength and electric gradient distribution of polymer structures. A variety of structures in the micron and nanometer scales including bio-fluidic MEMS, polymer optoelectronic devices can be fabricated using this method.

Self Assembly of Polymer Structures Induced by Electric Field

Amethod has been developed for fabricating polymer microstructures based on electric field induced self assembly and pattern formation. A dielectric fluid placed in between two conductive plates experience a force in an applied electric field gradient across the plates, which can induce a diffusive surface instability and self construction of the fluid surface. This process is exploited for the fabrication of self assembled polymer microstructures as well as replicated patterns through the use of pre-patterned plates or electrodes. FEM simulation is used to decide the minimum wavelength and electric gradient distribution of polymer structures. A variety of structures in the micron and nanometer scales including bio-fluidic MEMS, polymer optoelectronic devices can be fabricated using this method.

Raman scattering as a probe of morphology in conjugated polymer thin films

Polarized confocal Raman mapping is used to characterize the degree of molecular alignment in spin coated conjugated polymer thin films. To this end, the polarization selection rules of a specific Raman active mode are employed to infer indirectly the local molecular alignment. The film can be monitored and characterized with a diffraction limited spatial resolution fixed by the microscope optics. In addition, we demonstrate examples of characterization of molecular alignment in highly oriented samples by monitoring the angular dependence of the polarized Raman signal. Finally, we show the use of temperature dependent Raman scattering to monitor thermal phase transitions in bulk conjugated polymer samples. These procedures can be used as in situ characterization methods in a wide variety of experimental situations and offer a sensitive probe of processing protocols used in the fabrication of polymer optoelectronic devices.

Two-photon photopolymerization and diagnosis of three-dimensional microstructures containing fluorescent dyes

We report in this letter two-photon-absorption photopolymerization using fluorescent dye-doped resin, by which complex fluorescence-active three-dimensional (3D) microstructures were readily produced. Structures were imaged by means of two-photon confocal techniques. The realization of dye-doping photopolymerization would be an important step towards production of photonic-crystal-based high-efficiency polymer optoelectronic devices. Furthermore, we found that the doped dye acted as label molecules to stain solidified structures, which provided an effective two-photon fluorescence technique for internal microdiagnosis of polymerized 3D microstructures.

Electric Field Induced Self Assembly and Template Patterning of Polymer Microstructures

ABSTRACTWe have developed a method for fabricating polymer microstructures based on electric field induced self assembly and pattern formation. A dielectric fluid placed in between to conductive plates experiences a force in an applied electric field gradient across the plates, which can induce a diffusive surface instability and self construction of the fluid surface. This process is exploited for the fabrication of self assembled polymer microstructures as well as replicated patterns through the use of pre-patterned plates or electrodes. We have used silicon wafers and transparent ITO (Indium-Tin Oxide) coated quartz substrates to fabricate the capacitor structures. The bottom silicon plate is spin coated with a 100-200 nm thick polystyrene film. The ITO substrate was placed over the polymer surface at a distance to leave a thin air gap using spacers. For directed pattern transfer, patterned ITO substrates were used. The capacitor setup was heated above the glass transition temperature of the polymer and a voltage was applied across the plates (50-150 Volts), which induces electric fields on the order of 107–108 V/m. The capacitor structure was quenched to observe the structures using optical microscopy and atomic force microscopy. The method described can be used to fabricate a variety of structures in the micron and nanometer scales including bio-fluidic MEMS, polymer optoelectronic devices and patterned templates for nanolithography.

Surface and bulk phenomena in conjugated polymers devices

We report a number of investigations into a variety of surface and bulk phenomena which are particularly relevant to the design of polymer optoelectronic devices, and in particular to light-emitting diodes (LEDs). First, we consider the surface properties of the most common anode, a thin layer of indium tin oxide (ITO), as modified by chemical and physical treatments. We show how the treatments affect the electrode workfunction, surface energy, morphology, sheet resistance and therefore, the LEDs efficiency and lifetime. We will also consider the effect of a conducting polymer hole-transport layer, such as poly(styrene sulfonate) doped poly(3,4-ethylene dioxythiophene) (PEDOT), incorporated between the ITO and the active layer. Secondly, we turn our attention to the optimisation of the photoluminescence (PL) efficiency, and report examples of how the polymer microstructure and possibly the engineering of disorder in a class of poly(p-phenylene vinylene)s and poly- or oligo-thiophenes can determine a significant increase of the PL efficiencies in the solid state.