Exciplex emission and energy transfer in white light-emitting organic electroluminescent device

We report a white light organic electroluminescent (EL) device achieved through exciplex formation between organic materials as a green color light source. Exciplex formation at 500 nm originated from poly(N-vinylcarbazole) (PVK) and 2,5-bis(5-tert-bytyl-2-benzoxazolyl)thiophene (BBOT), exciplex formation at 550 nm between from N,N′-diphenyl-N,N′-bis(3-metylphenyl)-1,1′-biphenyl-4,4′-diamine (TPD) and BBOT was assumed to correspond to the complex formation covering the green color region. In the mixed emitting materials, the PL decay lifetimes of BBOT and poly(3-hexylthiophene) (P3HT) were increased as much as 35–68 and 1.5–19 ns, respectively. This indicates that the energy transfer occurred from PVK to BBOT and P3HT, enhancing the quantum efficiency of these materials. We achieved white light emission with brightness as great as 12.3 μW/cm² at 20 V, corresponding to CIE coordinates of x=0.29 and y=0.353. The EL spectrum of the device was changed with a function of the applied voltage.     

Time-resolved Förster energy transfer in polymer blends

Sub-picosecond spectroscopy and pump–probe experiments show Förster energy transfer in blends from larger gap (blue or green-emitting) host polymers poly(2,3-diphenyl-5-hexyl-1,4-phenylenevinylene) (DP6-PPV) or poly[2-(meta-2′-ethylhexoxyphenyl)-1,4-phenylenevinylene) (m-EHOP-PPV) to the smaller gap, red-emitting guest polymer poly(2,5-bis(2′-ethylhexoxy)-1,4-phenylenevinylene) (BEH-PPV). The dynamics of the stimulated emission (SE) and photoinduced absorption (PA) of the blends indicate that 10–20 ps are required for complete energy transfer. Quantitative measurements of energy transfer rates give a Förster interaction range of 3–4 nm, 1.4 times longer than the theoretical values as calculated from the spectral overlap. We attribute this difference to delocalization of the excited state. Insufficient spectral overlap between the emission of the host and absorption of the guest is shown to be the cause for the absence of energy transfer in a blend with poly(2,5-bis(cholestanoxy)-1,4-phenylenevinylene) (BCHA-PPV) as the guest polymer.     

Photo-, and electroluminescence studies of 2,5-bis[2′-(4″-(6-hexoxy benzyl))-1′-ethenyl]-3, 4-dibutyl thiophenes

We have synthesized a thiophene-based compound 2,5-bis[2′-(4″-(6-hexoxy benzyl))-1′-ethenyl]-3, 4-dibutyl thiophene (HBDT) and a copolymer poly(2,5-bis(2′-(4″-(6-hexoxy benzyl))-1′-ethenyl)-3,4-dibutyl thiophene-1,6-diisocyanatohexane) (HBDT-PU) consisting of alternating HBDT and urethane spacer units. Absorption and photoluminescence (PL) spectra of the compounds coincide indicating that emission in HBDT-PU occurs from the thiophene containing unit. PL is emitted in the blue–green region with a maximum at 460 nm. Concentration quenching occurs in pure materials: PL efficiency is strongly enhanced when the compounds are dispersed in a polymer matrix like poly(N-vinyl carbozole) (PVK) or poly(methyl metacrylate) (PMMA). Light emitting devices were fabricated using a PVK:2-(4-biphenylyl)-5-(4-tert-butylphenyl)-1,3,4-oxadiazole (PBD) blend doped with HBDT as the emitter material. Efficient energy transfer from PVK:PBD to HBDT molecules takes place in blended films. The electroluminescence (EL) spectra coincided with the PL spectra of HBDT indicating that EL emission comes solely from HBDT molecules. The influence of the doping concentration on the EL efficiency was found to coincide with the concentration dependence of the PL quantum yield.     

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.     

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.     

Conjugated copolymers of 2-methoxy-5-2′-ethyl-hexyloxy-1,4-phenylenevinylene and 2,5-dicyano-1,4-phenylenevinylene as materials for polymer light-emitting diodes

A new series of electroluminescent polymers has been synthesized by incorporating the segments of 2,5-dicyano-1,4-phenylenevinylene (DCN-PV) into the backbone of poly(2-methoxy-5-(2′-ethyl-hexyloxy)-p-phenylene vinylene) (MEH-PPV). The copolymers have similar optical properties with MEH-PPV. The oxidation potentials of the copolymers are slightly larger than that of MEH-PPV. The copolymers exhibit “pre-reduction” processes, beginning at about −0.9 V (vs. SCE), before the main reduction processes. The “pre-reduction” feature is thought to be helpful in improving electron injection, and thus to be helpful in improving EL performance. The improved EL performance of the copolymers in comparison with MEH-PPV is demonstrated by a single-layer polymer light-emitting diode (PLED) device employing one of the copolymers as the emissive material.     

Energy transfer from poly(2-carboxyphenylene-1,4-diyl) to poly(p-phenylene vinylene) in bilayer films and its effects on luminescent properties

Luminescent properties of poly(p-phenylenevinylene) (PPV) in bilayer films were found significantly enhanced by energy transfer from poly(2-carboxyphenylene-1,4-diyl) (PCPD) to PPV prepared by alternatively spin-coating the poly(xylylene tetrahydrothiophenium chloride) (PXT, dubbed as PPV-precursor) aqueous solution and PCPD pyridine solution followed by heat treatments. The energy transfer process, as verified by the photoluminescent excitation (PLE) spectroscopy and the analysis of time-resolved photoluminescence (PL) decays, was attributed to the chemical interlocking between two polymers in the interfacial region. Accordingly, the efficiency of energy transfer from PCPD to PPV was calculated. Consequently, the (glass)PPV/PCPD configuration exhibited higher energy transfer efficiency than the (glass)PCPD/PPV, resulting in higher PL and electroluminescent (EL) quantum efficiencies. Besides, the improved EL emission of indium-tin-oxide (ITO)/PPV/PCPD/Al device was attributed to the energy barrier in the PPV/PCPD interfacial region, which trapped the majority carriers, holes, to form the excitons in situ, that decayed radiatively.     

Enhanced efficiency of polymer:fullerene bulk-heterojunction solar cells with the insertion of thin CdS layer near the Al electrode

The insertion layer of cadmium sulfide (CdS) between polymer–fullerene blend and Al electrode is used to enhance the short-circuit current (I_sc) and the power conversion efficiency (PCE). The solar cells based on the blend of poly[2-methoxy-5-(2′-ethylhexyloxy)-1,4-phenylenevinylene] (MEH-PPV) and C₆₀ with the function layer of CdS (∼10 nm) shows the open-circuit voltage (V_oc) of ∼0.7 V, short-circuit current (I_sc) of ∼4.6 mA/cm², filling factor (FF) of ∼0.28, and the power conversion efficiency (PCE) of ∼5.3% under monochromatic light (532 nm) photoexcitation of about 16.7 mW/cm². Compared to cells without the CdS layer, the power conversion efficiency increases about an order of magnitude. The thickness of CdS layer was varied from 10 to 40 nm using e-beam deposition, and we obtained optimum current density–voltage characteristics for 10 nm thick CdS layer.     

Improvement of efficiency of the single-layer polymer light-emitting diodes: the exciton confinement in the emitting layer by conjugated 1,3,4-oxadiazole

Two luminescent polymers, poly[(2-methoxy-(5-(2-(4-oxyphenyl)-5-phenyl-1,3,4-oxadiazole)-hexyloxy))-1,4-phenylenevinylene-alt-2,5-didodecyloxy-1,4-phenylenevinylene] (P1) and poly[(2-methoxy-(5-(2-(4-oxyphenyl)-5-(4-biphenyl)-1,3,4-oxadiazole)-hexyloxy))-1,4-phenylenevinylene-alt-2,5-didodecyloxy-1,4-phenylenevinylene] (P2) was synthesized by the Heck coupling reaction. Hole blocking-electron transporting pendants, conjugated 1,3,4-oxadiazole (OXD) derivatives, is attached on the main chain via linear 1,6-hexamethylenedioxy chain. The band gap of both P1 and P2 were 2.12 eV and the maximum of photoluminescence (PL) of P1 and P2 appeared at 576 and 573 nm, respectively. Moreover, the maximum of electroluminescence (EL) of single layer device based on P1 and P2 showed at 583 and 580 nm, respectivley. These values are very close to those of poly(2-methoxy-5-ethylhexyloxy-1,4-pnenylenevinylene) (MEH-PPV). Relative PL quantum yield of P1 and P2 film were 1.9 and 2.0 times higher than that of MEH-PPV. In the PL and EL spectra, emission from CNST (1,2-diphenyl-2′-cyanoethene) pendants was not observed. These indicate that the energy transfer from OXD pendants to main chain takes place completely. In addition, OXD pendants did not affect the EL and PL maximum of the main chain of MEH-PPV. A single-layer EL device based on P1 and P2 had an efficiency of 0.1 cd/A at 300 mA/cm² and 0.17 cd/A at 323 mA/cm², respectively, which were significantly higher than that of MEH-PPV measured under the same conditions. From the energy levels figured out from optical and electrochemical data strongly supports that the OXD pendants have good hole blocking property and promote electron-hole (exciton) recombination process.     

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.     

Amplified spontaneous emission from an MEH-PPV film in cylindrical geometry

We report amplified spontaneous emission (ASE) from a cylindrical capillary structure comprised of a poly(2-methoxy-5-(2′-ethyl-hexyloxy)-1,4-phenylenevinylene) (MEH-PPV) film deposited on the inside wall of a glass capillary tube. When pumped with a stripe-shaped pulsed laser beam focused onto the capillary edge and parallel to its axis, a collimated and circular-shaped emission emerges from the capillary. The threshold pump power for ASE is about 6 kW/cm². In the capillary configuration, the emission is unpolarized with spectral linewidth of 8 nm, centered at 638 nm. The glass capillary seems to play an important role in focusing the excitation beam, in waveguiding the emitted radiation, and in protecting the polymer from photo-oxidation.     

Photo- and electroluminescence of poly(2-methoxy,5-(2′-ethylhexyloxy)-p-phenylene vinylene) Langmuir-Blodgett films

Langmuir-Blodgett (LB) layers of poly(2-methoxy,5-(2′-ethylhexyloxy)-p-phenylene vinylene) (MEH-PPV) have been built up on quartz and indium-tin oxide (ITO) substrates. The optical properties of the films were characterized using time-correlated single-photon counting, and absorption and steady-state fluorescence spectroscopy. Electroluminescence (orange-yellow light) from ITO/MEH-PPV LB/A1 devices under forward bias was observed. The dependence of the electrical and electroluminescent characteristics on LB film thickness is reported. The quantum efficiency was 7 × 10⁻³% for a device containing 64 LB layers.     

Vinyl polymer containing 1,4-distyrylbenzene chromophores: Synthesis,optical, electrochemical properties and its blend with PVK and Ir(ppy)3

Vinyl polymer PDSB containing pendant 1,4-distyrylbenzene derivatives was prepared from its precursor poly(4-vinylbenzyl chloride) (PVBC: M_w = 10,400, PDI = 1.12), which had been synthesized by the controlled radical polymerization (RAFT). PDSB is readily soluble in common organic solvents and basically amorphous material with 5% weight-loss temperature higher than 390 °C. The HOMO level of PDSB, estimated from cyclic voltammetric data, is −5.15 eV, which is higher than −5.8 eV of PVK and thus effective in reducing the hole injection barrier between the PEDOT:PSS and emitting layer. The blend films of fluorescent isolated blue emitter (PDSB) and phosphorescent green system [PVK/Ir(ppy)₃] were prepared to tune color emission. The blend films [PVK:PDSB:Ir(ppy)₃] show composition-dependent behavior in PL spectral properties and EL characteristics. The PL spectra of the blend films [PVK:PDSB:Ir(ppy)₃] show a major emission at 515 nm and a minor peak at 454 nm, which are attributed to Ir(ppy)₃ and PDSB, respectively. The C.I.E. 1931 coordinates of the blend LED devices shift from (0.29, 0.61) for PVK:PDSB = 10:0 to (0.25, 0.35) for PVK:PDSB = 0:10 with the increase of PDSB content.     

Electroluminescence of (styrene-co-acrylic acid) ionomer/conjugated MEH-PPV blends

This work reports the electroluminescence of poly[2-methoxy-5(2′-ethylhexyloxy)-p-phenylenevinylene] (MEH-PPV) and poly(styrene-co-acrylic acid-co-1-pyrenylmethyl methacrylate) (SAA) blends in ratios from 0 to 100 wt.% in mass of MEH-PPV. The styrene-co-acrylic copolymer was synthesized with 3 mol% of acrylic acid units to simultaneously enhance blend miscibility and charge transport in MEH-PPV. The morphology was studied using epifluorescence microscopy and scanning electron microscopy in which the pyrenyl-labeled copolymer is used to enhance microscopic contrast. Device performances were compared: those using MEH-PPV have a turn-on voltage of 3.5 V, luminance of 500 cd/m² and current density of 430 mA/cm² at 5 V, while MEH-PPV blended with 50 wt.% styrene–acrylic copolymer showed a turn-on voltage of 2.5 V, a luminance of 2300 cd/A and a current density of 640 mA/cm² at 5 V.     

Enhancement of efficiency in luminescent polymer by incorporation of conjugated 1,3,4-oxadiazole side chains as hole-blocker/electron-transporter

A novel luminescent polymer poly(2-methoxy-5-{6′-[2″-(4‴-oxyphenyl)-5″-phenyl-1″,3″,4″-oxadiazole]-hexyloxy}-1,4-phenylenevinylene-alt-2,5-bis-dodecyloxy-1,4-phenylenevinylene) (MPPOXA), was synthesized by the Wittig reaction. Electron withdrawing pendant, 2-(4-oxyphenyl)-5-phenyl-1,3,4-oxadiazole (OXD), is separated from the main chain via linear 1,6-hexamethylene-dioxy chain. The band gap figured out from the UV-Vis spectrum and photoluminescence (PL) maximum of the polymer are 2.08 eV and 585 nm, respectively. These values are similar to those of MEH-PPV (2.12 eV and 580 nm). The maximum of electroluminescence (EL) of the device based on single layer structure (ITO/MPPOXA/Al) appeared at 586 nm, which is similar to that of MEH-PPV (583 nm). In PL and EL spectra, emission from OXD pendants was not observed. Single layer EL device based on MPPOXA have an external quantum efficiency of 0.01% at 2.3 mA/mm², which is significantly higher than that of MEH-PPV (0.0002% at 2.4 mA/mm²) measured under the same conditions. The HOMO and LUMO energy levels of the polymer main chain figured out from the cyclic voltammogram and the UV-Vis spectrum are −4.96 and −2.88 eV, respectively, which are similar to those of MEH-PPV (−4.98, −2.86 eV). The estimated HOMO and LUMO energy levels of the pendant were −6.17 and −2.47 eV, respectively. LUMO energy level is significant lower than those of the main chain. These results suggest that OXD units do not affect the emission maximum of the main chain comparison with MEH-PPV. The pendants block the injected holes from the anode and enhance electron-transporting property.     

Triplet state spectroscopy of conjugated polymers studied by pulse radiolysis

Using the technique of pulse radiolysis we have elucidated the energies and kinetics of triplet states in soluble luminescent conjugated polymers. Using poly(2-methoxy,5-(2′-ethyl-hexoxy)-p-phenylenevinylene) MEH-PPV as an example we explain this technique and show how it can be used to study the triplet states in conjugated polymers. Triplet energy transfer is used to determine $\text{1}{}^1\text{A}{}_{\mathrm{g}}\text{–1}{}^3\text{B}{}_{\mathrm{u}}$ energy gaps and the kinetics of triplet–triplet absorption yields triplet lifetimes. In the case of MEH-PPV, at concentrations up to 50 mg/l, the triplet decay rate shows no change, indicating self-quenching of triplets is not significant. However, if very high electron beam doses are used, high intra chain triplet concentrations can be generated. In this high concentration regime triplet–triplet annihilation becomes effective, as determined by the onset of delayed fluorescence.     

Aggregation quenching in thin films of MEH-PPV studied by near-field scanning optical microscopy and spectroscopy

Aggregates in thin films of conjugated polymers form excimer states and significantly reduce the photo- and electroluminescence efficiency in devices produced from these materials. We have studied the aggregate formation in thin films of poly(2-methoxy,5-(2′-ethyl-hexyloxy)-p-phenylene-vinylene) (MEH-PPV) by near-field scanning optical microscopy and spectroscopy. Local photoluminescence spectroscopy and photo-bleaching experiments have been used to show that thin films of MEH-PPV are homogeneously aggregated and do not form aggregated domains.     

The electroluminescent and photodiode device made of a polymer blend

Organic thin film diodes made by a polymer blend of poly[2-methoxy,5-(2'-ethyl-hexoxy)-1.4-phenylenevinylene] (MEH-PPV) and poly[1.3-propanedioxy-1.4-phenylene-1,2-ethenylene-(2.5-bis(trimethylsilyl)-1,4-phenylene)-1,2-ethenylene-1,4-phenylene] (called the B-polymer) are investigated. The device of sandwich configuration indium-tin oxide (ITO)/polymer-blend/A1 emits orange light under forward bias at + 10 V and the same device acts as a photodiode under reverse bias. To investigate the photodiode characteristics, the 516 nm wavelength with 9.5 mW/cm² intensity of light is illuminated through the A1 contact side of the device. The I-V characteristic measurement shows the short circuit current and the open circuit voltage of −1.22 × 10⁻⁹ A/cm² and 0.8 V, respectively. The ratio of the photocurrent to the dark current is about 4 × 10² at − 2.5 V reverse bias. The maximum d.c. sensitivity is 1.35 × 10 −⁵ A/W at 4× 7 V reverse bias voltage with 16 mW/cm² intensity of the incident light. The results indicate the possibility of making photosensors using this device.     

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.     

Doped polymeric cathodes for PPV/Al based LEDs

The effect of Li-doping in poly(para-phenylenevinylene) (PPV) based light emitting devices has been studied. In a standard structure with an indium tin oxide (ITO) anode, poly(3,4-ethylenedioxythiophene) doped with poly(4-styrenesulfonate) (PEDOT–PSS)-layer and an active PPV-layer, the effects of a thin (around 1 Å) Li-layer and a thin layer, (50 Å), of a large bandgap polymer, poly(2,5-diheptyl-1,4-phenylene-alt-1,4-naphthylene) (P14NHP) between the PPV and the aluminum cathode have been studied in terms of IV-characteristics and efficiency. The Li-atoms dope the interfacial layer of the PPV as seen by photoelectron spectroscopy. A thin layer of Li improves the charge balance by decreasing the energy barrier for injection of electrons for the Al/Li/PPV/PEDOT–PSS/ITO device. The efficient electron injection originates from a Fermi level alignment between the doped polymer and the aluminum cathode, which reduces the energy barrier. A thin layer of the large bandgap polymer P14NHP, between the PPV and Al contact, increases the light output and efficiency by blocking the holes. In addition, it may also reduce the light quenching by moving the region of recombination away from the Al-contact. The addition of a Li-layer on top of P14NHP leads to an increase of the quantum efficiency, because of better electron injection.