High performance photonic devices made with semiconducting polymers

High performance photonic devices fabricated from conjugated polymers have been demonstrated, including light-emitting diodes, lightemitting electrochemical cells, photovoltaic cells, photodiodes and optocouplers. Performance parameters have been improved to levels comparable to, or better than, their inorganic counterparts. A brief review is given with emphasis on recent progress in Santa Barbara.     

Temperature-dependent kinetics of printed polymer light-emitting electrochemical cells

The thermal activation energy for ion transport and related junction formation in a polymer light-emitting electrochemical cell (LEC) has been determined by measuring the transient behavior of light emission as a function of temperature from 257 K to 297 K. We find the initial time derivative of the light intensity to have an Arrhenius temperature dependence, with an activation energy of 1.6 eV. Our devices utilize a flourene-based polymer emitter which fluoresces at red/near-infrared wavelengths. The polymer matrix includes phenyl alkoxy-substituted PPV and an electrolyte/dopant system based on both solid polymer and ionic liquid dopants. Transient measurements of current and light emission have been used to capture the kinetics of both junction formation and junction decay and to measure variations in optical external quantum efficiency during device turn-on.     

Structure-phase morphology – Property relationship of a series of light-emitting alternating copolymers with distyrylbenzenes segments and oligo(ethylene oxide) spacers

A series of light-emitting alternating copolymers: DSB–TEO, DSB–SEO, DSB–PEO₁₅₀₀ and dimethoxy(DM)DSB–SEO, consisting of 1,4-distyrylbenzene (DSB) and different length of ionic conducting oligomer ethylene oxide (OEO) spacers, were synthesized. The thermal stabilities, electrochemical behavior and photoluminescent properties of these polymers were studied systematically. It was found that with increasing OEO chain length, the solubility, film-forming property and relative fluorescence quantum yield were improved accordingly. The introduction of two methoxy groups as side chains into the DSB segment to form DMDSB not only enhanced the film-forming property, but also improved the quantum efficiency of the polymer. Phase contrast microscopy, polarized optical microscopy and atomic force microscopy (AFM) were employed to investigate the phase morphology of the thin films prepared from the pure polymer or polymer/LiCF₃SO₃ blends. Results revealed that compared with pure polymer thin films, there was significant change in the phase morphology of the thin films prepared from polymer/LiCF₃SO₃ blends. The reason could be attributed to the contrary roles of dissolved and undissolved lithium salt played in the polymer matrix, which was discussed in detail in the paper. Phase morphology study of DMDSB–SEO revealed that, compared with DSB–SEO, there was much less crystallization of OEO segments and the solubility of lithium salt had increased greatly. Furthermore, AFM was used to investigate the dependence of surface morphology on the spin-coat processing conditions, such as the solvents, substrates and rotation rate. A light-emitting electrochemical cell device of the copolymer was fabricated, and the brightness and maximum external quantum efficiency of the device were investigated.     

Controlled syntheses of conducting polymer micro- and nano-structures for potential applications

The development of nanostructured polymers has opened up novel fundamental and applied frontiers, which has attracted tremendous interest in recent years. We have recently developed techniques for the micro-lithographic formation of conducting polymer patterns on the micrometer scale and for coating conducting polymers on individual aligned carbon nanotubes. The micro- and nano-fabrication facilitates the use of conducting polymers for various applications ranging from multicolor polymer light-emitting displays to biosensors. More recently, we have demonstrated the controlled syntheses of conducting polymer microcontainers through electrochemical generation of surfactant (i.e. β-napthalenesulfonic acid, β-NSA)-stabilized H₂ gas bubbles on the working electrode, followed by electrochemical polymerization of pyrrole around the wall of the “soap bubble” template. By pre-patterning the working electrode surface with non-conducting polymers through micro-contact printing (μCP) or plasma patterning, we have also produced conducting polymer microcontainers in a patterned fashion. Furthermore, potential applications of the patterned and non-patterned conducting polymer microcontainers have been demonstrated, for example, by immobilizing glucouse oxidase (GOX) onto the conducting polymer microcontainers for glucose sensing. This paper provides a brief summary of our work on micro- and nano- fabrication of conducting polymer for various potential applications.     

Poly(p-phenylenevinylene) derivatives with conjugated thiophene side chains: Synthesis,photophysics and photovoltaics

Three novel poly(p-phenylenevinylene) (PPV) derivatives with conjugated thiophene side chains, P1, P2 and P3, were designed and synthesized for application in polymer solar cells (PSCs). The effects of the conjugated side chains on the thermal, photophysical, electrochemical and photovoltaic properties of these polymers were investigated. The polymers exhibited good thermal stability and film-forming ability. The absorption spectra indicated that the short conjugated side chains have slight influence on the UV-region spectra of PPVs; whereas with increasing the length of conjugated side chains, the absorption of the UV-region red-shifted. The photoluminescence spectra reveal that complete exciton energy transfer occur from the conjugated side chains to the main chains of the polymers. The polymers emitted yellow-orange light with the emission maximum peaks in the region of 525–550 nm in chloroform solution and 611–616 nm in thin films. Cyclic voltammograms displayed that the band gaps were reduced effectively by the attachment of the conjugated thiophene side chains. The bulk heterojunction solar cells were fabricated based on the blend of the polymers and [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) in a 1:4 weight ratio. The maximum power conversion efficiency (PCE = 0.53%) was obtained by using P3 as the electron donor under the illumination of AM 1.5, 100 mW/cm².     

Poly (p-pyridine) - and poly (p-pyridyl vinylene) -based polymers: their photophysics and application to SCALE devices

Photophysics and light-emitting device applications of poly (p-pyridine) - and poly(p-pyridyl vinylene)-based polymers are presented. Extensive time-resolved (ps to ms) photoluminescence, stimulated emission and photoinduced absorption studies of solutions, powders and films demonstrate that the primary photoexcitation of these polymers is an intrachain singlet exciton. The presence of (n,π^*) states leads to enhanced intersystem crossing to triplet excitons for the powder form, while aggregate formation plays a key role in the films. Polarons are important at longer times. These polymers were used to fabricate ‘conventional’ polymer light-emitting diodes. In addition, these polymers were used to demonstrate a novel light-emitting structure, the symmetrically configured a.c. light-emitting (SCALE) device. These new devices have potential advantages in their use with high workfunction electrodes, such as gold, and also in their a.c. operation.     

Blue-green light-emission LECs based on block copolymers containing di(α-naphthalene vinylene)benzene chromophores and tri(ethylene oxide) spacers

The polymer light-emitting electrochemical cells (LECs) were fabricated and characterized with two novel block copolymers as the luminescent polymers. The block copolymers are composed of the rigid segments (chromophores), 1,4-di(α-2-naphthalene vinylene)benzene (DNVB) or 2,5-dimethyloxy-1,4-di(α-2-naphthalene vinylene)benzene (MDNVB) and the flexible segments (spacers), tri(ethylene oxide) (TEO). Efficient blue-green light emission of the LECs was demonstrated with onset voltage lower than 3 V and electroluminescence (EL) efficiency of ∼0.2 cd/A. The a.c. impedance data indicate that the operation mechanism of the LECs agrees with the electrochemical doping model.     

Diodes based on blends of molecular switches and conjugated polymers

Here we report polymer diodes based on a conjugated polymer host and a dispersed molecular switch. In this case, the molecular switch is a photochromic (PC) molecule that can be reversibly switched between low and high energy gap states, triggered by exposure to ultra-violet and visible light, respectively. While dispersed inside the conjugated polymer bulk and switched to its low energy gap state, the PC molecules act as traps for holes. Solid-state blends of this PC material and conjugated polymers have been demonstrated in diodes. The state of the PC molecule controls the current density versus voltage (JV) characteristics of the resulting diode. Both poly(2-methoxy-5(2′-ethylhexyloxy)-1,4-phenylenevinylene) (MEH-PPV) and poly(3-hexylthiophene-2,5-diyl) (P3HT) host materials have been studied. The two conjugated polymers resulted in differing JV switching characteristics. A more pronounced JV switch is observed with MEH-PPV than with P3HT. We postulate that the PC material, while switched to its low energy gap state, act as traps in both the conjugated polymers but at different trap depth energies.     

Electrochemistry of conductive polymers 44: A comparative study on electrochemically polymerized polythiophenes from thiophene, bithiophene, and terthiophene

Polythiophenes obtained by electrochemical polymerization of thiophene, 2,2′-bithiophene, 2,2′:5′,2″-terthiophene in 1.0 M LiClO₄ propylene carbonate solutions have been studied using various experimental techniques including UV–vis absorption spectroscopic, electrochemical quartz crystal microbalance, and current sensing atomic force microscopic (CS-AFM) experiments. The polythiophene films were obtained by repeated potentiodynamic voltage scans on gold electrodes. Clear differences were observed from the polymer films obtained from the monomers in their electrical, electrochemical, and optical properties due to various factors such as degradation of the polymer at high anodic potentials, conjugation lengths of polymers, incorporated oligomers, and solvated ion pairs in the polymer matrix. The poly(bithiophene) displayed the best conductivity, while the polythiophene showed almost no electrical conductivity.     

Integration of conducting polymer network in non-conductive polymer substrates

A new method for integration of conjugated, inherently conducting polymers into non-conductive polymer substrates has been developed. A layer of the conducting polymer is polymerised by chemical oxidation, e.g. using Fe(III) p-toluene sulfonate (ferri tosylate) followed by washing with a solvent which simultaneously removes residual and spent oxidant and at the same time dissolves the top layer of the polymer substrate. This results in an integration of the conducting polymer into the surface layers of the polymer substrate. Several combinations of conducting polymers and substrates have been tested, with particular focus on poly(3,4-ethylenedioxythiophene) (PEDOT) on PMMA substrates. The structural, electrical and mechanical properties of this system has been characterised by atomic force microscopy, conductance measurements, and tribological tests. Furthermore, measurement of conductivity and optical absorption during sequential reactive ion etching has allowed for analysis of the PEDOT distribution within the surface layer of the PMMA substrate. The surface resistance of the conducting polymer layer remains low while the surface layer at the same time adapts some of the mechanical properties of the substrate, resulting in a highly conducting surface with very good wear resistance.     

Polymer charge-transfer complexes for opto-electronic applications

The formation of charge-transfer (CT) complex to increase the conductivity has been the subject of intense research activity for the past decades. Those CT complexes have been used as organic semiconductors in field effect transistors (FETs), charge injection and transport materials in organic light-emitting diodes (OLEDs) and organic photovoltaic (OPV) cells. In this paper, a serials of new CT complexes with polymers as donor and TCNQ as acceptor were prepared. The polymers are polycarbazoles with various content of carbazole moiety in the back chain. The X-ray crystal structure of the model compound 4,4′-bis (N-carbazolyl)-1,1′-biphenyl(CBP)/TCNQ complex showed the formation of 2:1 stack structure (with 1:1 carbazole moiety: TCNQ ratio). The polycarbazole/TCNQ complexes form uniform films by spin-coating. Devices with the structure of ITO/polycarbazole:TCNQ complex/Mg:Ag were fabricated. The current–voltage characteristics showed that the devices exhibit much higher conductivity compared to their analogy ITO/polycarbazole/Mg:Ag structure devices. Devices with different polycarbazole:TCNQ ratios were fabricated and the current–voltage results showed that the conductivity increases as the ratio of polycarbazole:TCNQ increases. The conductivity reaches the maximum at the ratio of 1:1. These polymer complexes can be low-temperature processed on large area flexible substrates and are of potential use for low-cost printed electronics.     

Synthesis and characterization of new light-emitting copolymers containing 3,4-dialkoxythiophenes

We report the synthesis, optical and electrochemical properties of a new series of polyoxadiazoles (P1–P3) consisting of 3,4-dialkoxythiophene and 1,4-divinylbenzene units. The polymers are prepared using the precursor polyhydrazide route. The polymers have well defined structure and exhibit good thermal stability with the onset decomposition temperature in nitrogen at around 330 °C. The optical and charge-transporting properties of the polymers are investigated by UV–vis spectroscopy, fluorescence emission spectroscopy and cyclic voltammetry. The UV–vis absorption spectra of polymers in solution showed a maximum at around 380 nm. The polymers depicted bluish-green fluorescence in solutions and green fluorescence in thin films. Cyclic voltammetry studies reveal that these copolymers have low-lying LUMO energy levels ranging from 3.25 to 3.31 eV and HOMO energy levels ranging from 5.48 to 5.56 eV, which indicated that the polymers are expected to provide enhanced charge-transporting (electron transport/hole blocking) properties for the development of efficient polymer light-emitting diodes (PLEDs).     

Inkjet printed electrochemical organic electronics

A conventional desktop inkjet printer has been used as a combined deposition and patterning tool of electrochemical organic transistors on rough flexible carriers. The functionality of these devices rely upon redox reactions occurring at the interface between a conjugated polymer film and an electrolyte. Both the electrolyte and the conjugated polymer suspension (an aqueous dispersion of poly(3,4-ethylenedioxythiophene):poly(styrene sulphonic acid)) were additively patterned with the inkjet printer, making the electrochemical device all-inkjet printed. Basic implementations of the transistor in simple electrochemical logical circuitry have been produced. The printing technique can be anticipated to be used for the production of small series of devices based on the electrochemical technology discussed.     

Adjustable electroluminescence: blue-green to red organic light-emitting diodes based on novel poly-non-conjugated oligomers

Two new polymers, poly-9,10(1,3-bis(4-ethynylphenoxy)propane)anthracene and poly-1,2(tetra-2,5-thienylene-1,2-vinylene)dimethylsilyslethane, based on conjugated chromophores that are interconnected via non-conjugated spacers, were prepared and characterized in terms of their photo- and electroluminescence (PL and EL, respectively) properties in pure films and in solid solutions. The application of solid solutions of the two polymers in PVK:PBD (polyvinyl carbazole:2-(4-biphenyl)-5-(4-tert-buthyl phenyl)-1,3,4-oxadiazole) matrices as active layers in adjustable blue-green to red OLED is presented.     

Electrochemical methods coupled with impedance measurement for energy gap study: correlation between the energy states and charge transport properties

The energy states of poly(3-hexylthiophene) (P3HT) polymer films degraded using UV/ozone environment were studied by optical and electrochemical methods. Energy levels of highest occupied and lowest unoccupied molecular orbitals as well as localized states between them were estimated. Electrical properties of polymer films sandwiched between two metal electrodes were investigated for various trap energy distributions. Comparison of the energy states density evaluated by optical/electrochemical methods and estimated from the current-voltage characteristics was carried out to discuss suitability of space-charge limited electrical conduction models for semiconducting polymers.     

Versatile synthesis of various conjugated aromatic homo- and copolymers

In recent years, variously substituted derivatives of poly(1,4-phenylene vinylene)s have emerged as efficient candidates for the emissive layer in polymer light emitting diodes. The synthetic routes for these polymers divide between precursor routes and those leading to fully conjugated solvent-processible polymers. The Gilch dehydrohalogenation polycondensation has largely been used for the latter class. In this presentation, we describe a novel family of 2,3-disubstituted aromatic precursors, derived from catechol, and we report their efficient polymerisation as homo- and copolymers with, for example, silyl-substituted derivatives to give materials which are highly fluorescent and serve as interesting materials in polymer LEDs.     

Low band gap donor–acceptor–donor polymers for infra-red electroluminescence and transistors

We report on transistors and light-emitting diodes using a conjugated polymer consisting of alternated segments of fluorene units and low-band gap donor–acceptor–donor (D–A–D) units. The D–A–D segment includes two electron-donating thiophene rings combined with a thiadiazolo-quinoxaline unit, which is electron withdrawing to its nature. The resulting polymer is conjugated and has a band gap of around 1.27 eV. Here we present the corresponding electro- and photoluminescence spectra, which both peak at approximately 1 μm. Single layer light-emitting diodes demonstrated external quantum efficiencies from 0.03% to 0.05%. The polymer was employed as active material in thin film transistors, a field-effect mobility of 3 × 10⁻³ cm²/V s and current on/off ratio of 10⁴ were achieved at ambient atmosphere.     

Crosslinked structure of pristine poly(p-phenylene vinylene) thin-films synthesized by chemical vapor deposition polymerization

The pristine conjugated polymers including poly(p-phenylene vinylene) (PPV) are often insoluble in organic solvents. In fact, PPV thin films prepared by chemical vapor deposition polymerization in this study were insoluble in common organic solvents as well. Although linear pristine conjugated polymers with saturated bonds and/or aromatics are often insoluble, the insolubility of CVDP-prepared PPV has never been studied. This study showed that the PPV film intentionally brominated to reduce the rigidity of polymer by saturating the vinylene groups in PPV was still insoluble indicating the crosslinking reaction occurred during polymerization. Insolubility of the precursor polymer was further confirmed and the swelling of the crosslinked polymers in various solvents was studied by optical microscopy. The presence of the diradical form of the activated monomer in the gas phase is proposed to explain the crosslinked structure.     

Synthesis,spectroscopy and electrochemistry study on a novel di-silyl substituted poly(p-phenylenevinylene)

A novel di-silyl substituted poly(p-phenylenevinylene) (PPV) derivative is synthesized through the Gilch route. The polymer is solution processable and shows high thermal stability in air. UV–Vis spectra for the film and solution samples illustrate some different features, in which the absorption peak corresponding to the π–π* transition of the delocalized electrons along the polymer back bone is red shifted and broadened in the film state compared to the solution one. The fluorescent emission spectra are quite similar for the polymer in solution and in the solid states. Cyclic voltammetric investigation reveals a partially reversible n-doping process and an irreversible p-doping process. The cathodic current is much greater than that of the anodic one, which means the polymer has the tendency to be charged through n-doping rather than p-doping process. The improved affinity of the polymer to electrons may be owing to the low interface barrier between the polymer films and the electrode surface. The HOMO and LUMO energy levels have also been estimated from the data of oxidation and reduction onset potentials. All of the results indicate that the synthesized polymer is a promising green emissive material for the light-emitting device application.