Solid-state polymer light-emitting electrochemical cells

Solid-state polymer light-emitting electrochemical cells have been developed using thin films of conjugated polymers blended with solid electrolytes. The cells contain three parts: the polymer blend films as the active medium, and two contact electrodes: indium-tin oxide and aluminum. When externally biased, the conjugated polymers are p-doped and n-doped on opposite sides of the polymer layer, and a dynamic light-emitting p-n junction is formed between the doped regions. The admixed solid electrolyte provides the dopant counterions and the ionic conductivity necessary for the doping. The p-n junction is dynamic and reversible, with an internal built-in potential close to the bandgap of the redox-active conjugated polymers. Orange, green and blue lights emitted from the p-n junction have been obtained with turn-on voltage less than 3 V and external quantum efficiency higher than 2% photons/electron. In addition, a two-color light-emitting electrochemical cell has also been fabricated based on a bilayer structure consisting of two different luminescent polymers. The light-emitting p-n junction can be switched from one layer to another under different bias conditions.     

Electrochemical properties of luminescent polymers and polymer light-emitting electrochemical cells

The electrochemical p-doping potentials (φ_p) and n-doping potentials (φ_n) of 10 soluble poly(1,4-phenylene vinylene) (PPV) derivatives and of the conjugated polymer blend in a light-emitting electrochemical cell (LEC) were measured by cyclic voltammetry. The energy levels corresponding to the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) of the polymers were determined from the onset potentials for n-doping (φ_n′) and p-doping (φ_p′), respectively. The electrochemical energy gap, E_g′=Δφ=φ_p′−φ_n′, agrees well with the optical energy gap (E_g). For copolymers of poly(2-methoxy, 5-(2′-ethyl-hexyloxy) paraphenylene vinylene) (MEH-PPV) and poly(2-butyl, 5-(2′-ethyl-hexyl) paraphenylene vinylene) (BuEH-PPV), the n-doping potentials are nearly independent of the ratio of MEH-PPV to BuEH-PPV. However, p-doping potentials depend strongly on the ratio, the higher the MEH-PPV fraction, the lower the p-doping potential of the copolymer. For cyano-PPV (CN-PPV), both p-doping and n-doping potentials were shifted by ca. 0.6 V (more electronegative) as a result of the electron withdrawing effect of cyano side group. The electrochemical p-doping and n-doping processes of the MEH-PPV polymer blend in the LEC device were confirmed from linear sweep voltammetry of the LEC; the doping onset potentials were the same as for an MEH-PPV film measured in a liquid electrolyte.     

Organic light-emitting diodes and organic light-emitting electrochemical cells based on silole–fluorene derivatives

Silole groups are known to present a high electron affinity. Initially, copolymerization of siloles with fluorene was aimed at improving electron injection into the polymer layer and so improving the electroluminescent properties of organic light-emitting diodes (OLED's) made from fluorene. But it also provides the ability to turn the light emission colour to the green part of the spectrum and to stop the well-known spectral shift degradation occurring in fluorene-based materials. In this paper we report the synthesis and the characterisation of 1,1-dimethyl-2,5-bis(fluoren-2-yl)-3,4-diphenylsilole 4, and of two soluble conjugated random copolymers derived from 9,9-ditetradecylfluorene and 1,1-dialkyl-2,5-diphenylsilole, where the alkyl group is either methyl 11a or n-hexyl 11b. Silole 4 crystallizes in the triclinic P-1 space group with a = 9.8771(8), b = 10.6240(10), c = 16.585(2) Å, α = 95.775(8), β = 97.025(7), and γ = 111.738(8)°. The results obtained with this molecule, operating in a single-layer OLED (luminance ≈450 Cd/m² at 12 V; η_max = 0.2 Cd/A), give evidences for the complementarity of the silole and the fluorenyl moieties in the improvement of the charge injection processes when compared with 1,1-dimethyl-2,3,4,5-tetraphenylsilole. The results obtained from organic light-emitting electrochemical cells (LEC's) made from silole–fluorene copolymers 11a, 11b and molten salts show an improvement of both the device lifetime and the spectral stability when compared with polyfluorene. To explain devices performances electrical characterisation data and atomic force microscope (AFM) imaging were combined.     

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.     

High-efficiency red polymer light-emitting diodes based on optimizing blend polymer host

High efficiency stable red light-emitting diodes have been realized employing red copolymer (PFO-DHTBT15) from 9,9-dioctylfluorene (DOF) and 4,7-di(3-hexylthien-2-yl)-2,1,3-benzothiadiazole (DHTBT) blending into green copolymer (PFO-BT15) from 9,9-dioctylfluorene and 2,1,3-benzothiadiazole (BT) as a novel fluorescent emitting layer. The external quantum and luminous efficiency of device from blend film (PFO-DHTBT15:PFO-BT15 = 10:90) reached 5.2% and 3.16 cd/A at the current density of 35 mA/cm², respectively. The corresponding Commission Internationale de l’Eclairage coordinates is (0.64, 0.36). In comparison with the devices from phenyl-substituted poly [p-phenylene vinylene] derivative (P-PPV) as host of PFO-DHTBT15, the device from PFO-DHTBT15/PFO-BT15 blend film shows higher luminous efficiency and better stability under high current density at the same blend weight ratio. The improved device performance is mainly attributed to the effective energy transfer from PFO-BT15 copolymer to PFO-DHTBT15 copolymer and better carrier confinement.     

Multilayer blue polymer light-emitting devices with spin-coated interlayers

Employment of multilayer heterostructures is a common approach to achieve efficiency and stable organic light emitting diodes (OLEDs). In this work, we report multilayer blue polymer light-emitting devices (PLEDs) by using spin-coated fluorene-triarylamine copolymers as interlayers between the conductive polymer poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT) and the emitting layer. A blue PLED with stepped hole injection profile yields an external quantum efficiency of 6.0% at a luminance of 9500 cd/m² at 5.5 V and an extrapolated lifetime of more than 18,000 h from 100 cd/m².     

Static and dynamic electrical investigations on AVB polymer light-emitting diode

We report investigations of current–voltage (I–V), capacitance–voltage (C–V) and impedance–frequency (Z–ω) measurements performed on ITO/1,4-bis-(9-anthrylvinyl)-benzene (AVB)/Al light-emitting diodes under different bias conditions. These measurements give information about the nature of the conduction mechanism in AVB. It was found that the current is space-charge limited with traps (SCLC) and conductivity exhibits a power low frequency dependence. Moreover, we give an equivalent circuit of the device designed as a parallel resistor R_p and capacitor C_p network in series with resistor R_s. Using experimental data, we deduce numerical values of R_p, C_p and R_s.     

Soluble polypyrrole as the transparent anode in polymer light-emitting diodes

Polypyrrole (PPy) films processed from solution have been used as semi-transparent anodes in polymer light-emitting diodes. An external quantum efficiency of 0.5% was achieved with poly (2-methoxy,5-(2′-ethyl-hexyloxy)-1,4-phenylene-vinylene) (MEH—PPV) as the luminescent polymer. The hole-injection potential barrier between PPy and MEH—PPV, as determined by Fowler—Nordheim analysis, is≈ 0.23 eV.     

Color stable white polymer light-emitting diodes with single emission layer

We report color stable white polymer light-emitting diodes achieved with single emission layers. The emitting layers are blending films of blue-, green- and red-emitting polymers based on color-stabilized backbones of poly(2,6-(4,4-bis(2-ethylhexyl)-4H-cyclopenta[def]phenanthrene)) (PCPP) and poly(5,5,10,10-tetrakis(2-ethylhexyl)-5,10-dihydroindeno[2,1-a]indene-2,7-diyl) (PININE). The white emission is realized with a combination of energy transfer and charge carrier trapping between the polymers in the devices. The devices exhibit a high color quality of white light with CIE coordinates of (0.31, 0.32) and excellent color stability as the driving voltage increases.     

Performance of a polymer light-emitting diode with enhanced charge carrier mobility

The device characteristics of a polymer light-emitting diode (PLED) based on a poly(p-phenylene vinylene) (PPV) derivative with an enhanced charge carrier mobility have been investigated. Improvement of the mobility, which has been obtained by a decrease of the energetic disorder in the polymer, is expected to increase the power efficiency of a PLED. However, it is demonstrated that an increased mobility leads to a decrease as well as to a slower rise of the quantum efficiency with voltage. This performance reduction is explained in terms of an increased quenching of the electroluminescence (EL) at the cathode.     

Fluorene and silafluorene conjugated copolymer: A new blue light-emitting polymer

A novel series of soluble blue light-emitting conjugated random and alternating copolymers derived from 9,9′-dioctylfluorene (FO) and 3,6-dimethoxyl-9,9-dimethyl-9-silafluorene (DMSiF) were successfully synthesized by Suzuki coupling polymerization. The feed ratios of FO to DMSiF were 90:10, 80:20, 75:25, and 50:50. Chemical structures and optoelectronic properties of the copolymers were characterized by NMR, UV absorption, photoluminescence, and cyclic voltammetry. The ¹H NMR spactra of the copolymers indicated that DMSiF content in the copolymers was slightly lower than its feed composition. The random copolymers exhibited PFO-segment-dominated UV absorption and PL spectra in THF solution, in comparison with significantly blue-shifted spectra of the alternating copolymer. The blue shift of the spectra became more remarkable in cast film and increased with the increment of DMSiF content. The changes of the UV absorption and PL spectra in solution and film were ascribed to the effect of methoxyl substituents which can hinder the chain rotation and influence the polymer comformation espercially in the solid film. The systematic invesetigations on the solubility, thermostability, electrochemical property, and photophysical property of the copolymers showed that silafluorene was an attractive building unit for optoelectronic materials.     

Broad band and white phosphorescent polymer light-emitting diodes in multilayer structure

Efficient phosphorescent polymer light-emitting diode with poly(vinylcarbazole) (PVK) doped by two or three iridium complexes in single and bilayer structures are studied. With (tris-(2-4(4-toltyl) phenylpyridine) (Ir(mppy)₃) as the green emitter and (1-phenylisoquinoline) (acetylacetonate) iridium(III) (Ir(piq)₂) as the red emitter the efficiency is as high as 23 cd/A with broad band emission from 500 nm to 720 nm. For white emission a second layer is added with blue emitter ((III) bis[(4,-6-di-fluorophenyl-pyridinato)N,C²] picolinate) (FIrpic) doped in PVK. White light containing three spectral peaks results with efficiency 8.1 cd/A. As the second blue layer is replaced by the fluorescent (poly(9,9-dioctylfluorene)) (PFO) white emission with high color rendering index 86 is achieved. The efficiency is 5.7 cd/A with peak luminance 8900 cd/m². For a given iridium complexes ratio the relative intensity of the green and red emission depends sensitively on the second blue layer.     

Polythiophene–polyoxyethylene copolymer in polyfluorene-based polymer blends for light-emitting devices

Polyfluorene (PFO) films doped with different amount of a polythiophene-co-polyoxyethylene (P1) polymer were studied with the aim to obtain white emission. The Förster resonant energy transfer (FRET) is the basic mechanism that takes place in the photoluminescence of these blends. The non-conducting poly(oxyethylene) behaves as a inert spacer leading to a larger donor–acceptor chromophores separation and then a better control of FRET efficiency. As result, the light-emitting device with the architecture Al/Ca/P1:PFO/PEDOT:PSS/ITO was found to be more efficient than either the devices using pure PFO or P1. Near white CIE coordinates of (0.26, 0.33) with a maximum luminance of 2500 cd/m² were found for 2 wt.% device.A maximum external quantum efficiency of 1% was obtained, hence, suggesting that the enhancement of the electronic and optoelectronic properties could be achieved by incorporating P1 into the PFO.     

Synthesis and electrophosphorescent performances of alkyl-substituted bicycloiridium complexes in polymer light-emitting diodes

In this paper, two complexes (mppy)₂Ir(tmd) and (bppy)₂Ir(tmd) were synthesized and doped into PVK. We find that the alkyl substitution of bicycloiridium complexes on acetylacetonate ligand has a great effect on optimizing the PLED performances. The best device performance is observed for the (bppy)₂Ir(tmd)-doped PVK–PBD (40 wt%) device with the concentration of 1 wt%. A maximal external quantum efficiency of QE_ext = 14.2% ph/el and the luminous efficiency of LE = 33 cd/A with a luminance of 2099 mA/cm² were achieved at a current density of 6.4 mA/cm².     

Electrodynamics applied to electrode potential and electrochemical kinetics

A key idea of the possibility of describing electrochemical systems and processes in terms of Maxwell’s electromagnetic theory is presented. This is an alternative to conventional Nernst-Tafel concept based on the Arrhenius hypothesis about electrolytic dissociation, which, despite of its minor heuristic value, is adhered to in many branches of chemistry for more than a century, thus, decelerating the progress. Fundamental laws of thermodynamics and electromagnetism enable us to use sufficiently elementary mathematical tools when quantitatively describing and predicting not only equilibrium states, but also transient processes with no use of model concepts.     

Effect of methanol treatment on performance of phosphorescent dye doped polymer light-emitting diodes

Methanol treatment was performed on the emissive layer (EML) of conjugated polymer hosted phosphorescent dye doped polymer light-emitting diode (PLED) with a device configuration of indium tin oxide (ITO)/poly(ethylene dioxythiophene):poly(styrene sulfonic acid) (PEDOT:PSS)/poly(N-vinylcarbazole) (PVK)/poly(9,9-dioctylfluorene) (PFO):30 wt% 2-(4-biphenylyl)-5-(4-tert-butylphenyl)-1,3,4-oxadiazole (PBD):2 wt% fac-tris(2-phenylpyridine) iridium (III) (Ir(ppy)₃)/Ba/Al. Improvement of the external quantum efficiency and luminous efficiency were observed. After methanol treatment, partial electron-transportant dopant PBD was selectively removed from the EML. Though PBD was doped in the EML to achieve more balanced charge transport, its partially removal resulted in further enhancement of PLED performances. The surface topography of EML shows the reduction of PBD aggregation which is detrimental to luminescence.     

Light-emitting electrochemical cells and light-emitting diodes based on ionic conductive poly(phenylene vinylene): a new chemical sensor system

Red-orange light-emitting electrochemical cells (LECs) based on poly[1,4-(2,5-bis(1,4,7,10-tetraoxaundecyl))phenylene vinylene] also named poly[2,5-bis (triethoxy-methoxy)-1,4-phenylene vinylene], (BTEM-PPV) are fabricated and characterized. BTEM-PPV combines good electronic conductivity with ionic conductivity in the oxidized and reduced state due to its conjugated backbone and oligo(ethylene oxide) attached as side chains. When applying this polymer in LECs one obtains devices of moderate brightness and with fast response times. This high performance is achieved without blending an additional ionic conductive polymer into the film. The response time of such devices driven with square waveform voltage pulses was determined to be 480 μs. The turn-on voltage for electroluminescence occurs at 2 V and at 3 V a brightness of about 35 cd/m² was obtained. Due to the covalent linkage of glyme like side chains to the PPV backbone, BTEM-PPV, complexed with metal ions, shows an ionochromic effect in the absorption spectra and also in the electroluminescence spectra, which can be a new approach to chemical sensors.     

Application of polyaniline (emeraldine base,EB) in polymer light-emitting devices

We report the fabrication of several multilayer light-emitting-diode (LED) devices based on a novel conjugated polymer, poly(2,5-dihexadecanoxy phenylene vinylene pyridyl vinylene) (PPV-PPy V), involving the use of polyaniline (emeraldine base, EB) as an insulating layer between the emissive polymer layer and the electrodes. In all the above devices with various configurations (‘3-layers’, ‘4-layers’-1’, ‘4-layers-2’ and ‘5-layers’), only the symmetrically configured a.c. light-emitting (SCALE) (‘5-layers’) device shows the capability of operating in both forward and reverse bias modes and in an a.c. mode. The SCALE devices have a typical turn-on voltage of about 4–6 V and work well under both forward and reverse bias modes. It is important to note that the total resistance (R= V/I+) of the four devices at any given applied potential decreases as the number of insulating polymer layers increases, suggesting that the nature of the electrode/polymer interface plays a critical role in determining the characteristics of the devices.     

Temperature-dependent anisotropic material modeling of the sheet metal component within the polymer injection forming process

In the course of this work, an extended material model for a carbon steel sheet metal has been developed based on Hill’48 yield criterion, considering temperature-dependent plastic anisotropy coefficients. This material model is applied on a polymer injection forming process in which the sheet metal heats up to a critical forming temperature through the contact with the plastic melt. At this temperature range blue brittleness occurs. The elastic properties, the yield stress as well as the plastic anisotropy coefficients of the sheet material become significantly different compared to those at room temperature. It should be emphasized that especially temperature-dependent anisotropy coefficients are not yet considered in most common material models. With the help of the presented modelling approach a more precise modelling of the temperature-dependent carbon steel material behavior can be realised.