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We report recent advances in the development of conjugated polymer device structures. We have fabricated vertical microcavity structures in which the optical path distance is tunable through control of the orientation of a liquid crystal layer, and demonstrate cavity tuning over 59 nm. We show that this structure allows convenient matching of cavity modes to the freespace emission spectrum from poly(r henylene vinylene), PPV. We have investigated the operation of electrically-excited polymer LEDs with short pulse operation, and have achieved current densities above 103 A/cm2 and peak brightnesses of up to 5 × 106 cd/m 2. We have measured the optical absorption due to the injected carriers under these drive conditions, and find an absorption band near 1.6 eV which we attribute to the presence of polarons. This allows measurement of the injected charge density, from which we deduce that carrier mobilities are strongly enhanced at high fields, reaching values of 4 x 10−2 cm2/Vsec for PPV diodes.
We have studied the transient electroluminescence (EL) from poly(phenylenevinylene), PPV, as a function of electric field under strong electric pulses up to fields of E≈ 109 V/m with emphasis on (a) the time delay, τ, between the electric pulse and the onset of the EL emission pulse and (b) the EL intensity as a function of the field. A monotonic decrease of τ with increasing E is explained by an increase of the carriers mobility according to Frenkel- Poole model. The EL intensity at high fields is proportional to E3 suggesting that the contacts at the polymer-metal electrode interfaces are practically ohmic. We demonstrate significantly improved brightness, peak power and lifetimes for polymer-based light emitting diodes working under such a pulsed mode.
Various copolymers of arylene vinylenes, having strong fluorescence, showed predominantly the emission in the multi-layer device using an electron-transporting material(ETM) such as tris(8-quinolinolato)aluminum(Alq3). The emission from Alq3 was suppressed due to the high hole-injection barrier from the copolymers to ETM in spite of low or no barriers of electron injection from ETM to the copolymers. We have successfully prepared highly hole-transporting polysilane having a triphenylamine group as a side chain(TPA-PS). The hole mobility as high as 10 cm3cm2/Vs is attributable to the intermolecular hopping process facilitated by the interaction between the polysilane backbone and the triphenylamine group. The polysilane is effectively used as a hole transporting material. The bilayer LED device consisting of TPA-PS and Akb3 showed high luminance (2000cd/m2) and high efficiency (4cd/A).
We report on the properties of white light emitting electroluminescence (EL) devices fabricated by combining blue and orange light emitting organic materials, parahexaphenyl (PHP) and a new isothianaphthen-S-oxide (ITSO) oligomer. The EL spectrum of this oligomer blend covers the range from 400 to 650 nm. When the concentration ratios of these materials are suitably adjusted a pure white light emission (CIE coordinates of x=0.333 and y= 0.338) and a high external EL quantum efficiency of 1.2% photons/electron can be obtained. The mechanism of the white light generation is ascribed to an internal self-absorption of part of the blue PHP light emission by ITSO and a subsequent photoluminescence emission process of ITSO. The results of photoinduced absorption, which is sensitive to charged species and triplet states of these guest-host systems, are discussed.
Enhanced performance has been observed for plastic molecular organic light emitting diodes (MOLEDs) consisting of two to four organic layers sequentially vacuum vapor deposited onto patterned indium-tin oxide (ITO) on polyester films. For all device structures studied, the performance of plastic diodes is comparable to or better than their analogs on glass substrates. At 100 A/m2, a luminous power efficiency of 4.4 lm/W and external quantum yield of 2.7% are measured for a device structure consisting of two hole transport layers, a doped emitting layer and an electron transport layer on a polyester substrate. The same device made on a silica substrate has a luminous power efficiency of 3.5 lm/W and external quantum yield of 2.3%. Electrical and optical performance for comparable device structures has been characterized by current-voltage-luminance measurements and electroluminescence spectra collected normal to the emitting surface. In addition, an integrating sphere was used to collect the total light emitted and to determine the optical output coupling on glass versus plastic substrates.
The tailoring of organic molecules and polymers has enabled the recent development of multifunctional materials such as photorefractive polymers and organic electroluminescent materials. This paper presents recent advances in both areas.
High-efficiency polymer light-emitting diodes (LEDs) are often fabricated using multilayered structures with separate carrier transport and light emission layers. Recently, we reported on the synthesis and electroluminescence (EL) characteristics of poly(2,6-quinoline vinylene) (PQV) and its potential for use as an electron transport layer in poly(phenylene vinylene) (PPV) LEDs. To take advantage of the high emission efficiency of PPV and electron accepting ability of PQV, a copolymer of PPV and PQV, poly(phenylene vinylene-co-quinoline vinylene) (PPVQV) was synthesized via the precursor polymer route and converted to the conjugated form by thermal elimination. When used as the emissive layer with Indium-Tin Oxide (ITO) and aluminum as positive and negative electrodes respectively, PPVQV emitted blue light at an onset electric field of 1.05x 106 V/cm and emission efficiency of 0.08%. Improved efficiencies of the order of 0.15% were obtained when blends of copolymer with PPV were used in conjunction with PPV in a multi-layered structure. Along with copolymer chemical characterization data, results from EL studies on single and multi-layered devices are discussed. We also report on a simple and costeffective chemical deposition of silver for the negative electrode in polymer LEDs.
We report the application of the blue light emitting conjugated polymer m-LPPP (methyl substituted laddertype poly(paraphenylene) ) in light emitting electrochemical cells. The active layer of the LEC consists of a blend of m-LPPP with the ionically conductive polymer PEO and LiCF3SO3 as ionic salt. Investigations of different concentrations of PEO and salt showed that the best LECs made of m-LPPP up to now where realised with an active layer consisting of a blend of m-LPPP:PEO:salt in the range of 20:10:3. In this case we are able to realise LECs with response times in the range of 30 μs. The I/U characteristics show low turn on voltages both for current and electroluminescence, but only in the case of ITO biased as a cathode. The initial electroluminescence spectra are quite the same as those for LEDs made of m-LPPP but turn into green after some time of operation.
We have investigated a new light emitting material, Ru(bpy)3 2+ polyester for fabricating electrochemically based solid state light emitting devices using the layer-by-layer sequential adsorption processing technique. By controlling the deposition conditions such as the pH of the Ru(bpy)32+ polyester and poly(acrylic acid) (PAA) solutions, we systematically altered the layer thickness and bilayer composition to obtain multilayers that contain different amounts of Ru(bpy)32+ polyester (from 46% to 70%). Differences in the Ru(bpy)32+ polyester composition, in turn, influence the device performance dramatically.
We present the fabrication and characterization of organic light emitting devices (OLEDs) using thin insulating layers for improved electron injection. The OLEDs are constructed with an ITO anode and an aluminum cathode. For the active layer we use either ladder-type Polyparaphenylene (m-LPPP) or Parahexaphenyl (PHP). A thin film of an insulating material is applied between the active layer and the cathode, in order to achieve a better tunnel injection due to a higher electric field at the interface. We compared different insulating materials with various thickness. The best results are obtained by using a LiF-layer with a thickness between 10 Å and 15 Å. Thereby the onset voltage decreases and the current density in the device increases significantly.
We present the use of polymer/polymer interfaces to control light-emitting polymer devices. Bilayer devices utilizing poly(9-vinyl carbazole) (PVK) as a hole transporting/electron blocking polymer together with a pyridine containing electron transporting layer show dramatically improved efficiency and brightness as compared to single layer devices. This is attributed to charge confinement and exciplex emission at the PVK/emitting polymer interface. The introduction of emeraldine base (EB) form of polyaniline (PAN) on both side of the emitting layer enables the device to work under both forward and reverse bias, as well as in AC modes. Interfaces play an important role in the operation of these devices. Furthermore, when the EB is replaced by sulfonated polyaniline (SPAN) on the cathode side and the emitting layer is properly modified to balance electron and hole transport, the device generates different colors of light, red under forward bias and green under reverse bias.
A new poly(phenylenevinylene) [PPV] derivative 8 with dialkoxy substituents at the 2,3- positions of the phenylene ring of the polymer backbone has been prepared. This exhibited significantly different properties compared with the typical 2,5-dialkoxy substituted PPV derivatives. The polymer is not only significantly blue-shifted in its optical and luminescence properties but also has high photoluminescence (PL) and electroluminescence (EL) efficiencies. Investigation of the model oligomer 11 offers insight into this interesting behavior.
We report the fabrication and characterization of alternating current light-emitting diodes (LEDs) with quinquethiophene as the emitting material. We have obtained equal electroluminescence intensity in both bias sections. From the frequency response of the LEDs, we have estimated the device response times and compared them with the response times obtained from the transient response of dc LEDs. Langmuir-Blodgett film deposition technique has been employed to control the thickness of the emitting layer on the molecular scale. We have shown that the response times originate from the accumulation rather than the transit of charge carriers. We have compared the photo- and electroluminescence spectra of QT LEDs.
The conjugated ladder-type poly(paraphenylene) is an attractive material for blue polymer light emitting devices (PLED). Blending the active layer with small amounts of a red emitting guest polymer, the emission shifts from blue to red with increasing guest concentration due to efficient excitation energy transfer. The results of photoluminescence detected magnetic resonance, electroluminescence detected magnetic resonance measurements and current detected magnetic resonance measurements on PLEDs based on 0.05w%/o - 2w%/o red emitting poly(perylene-co-diethynylbenzene) (PPDB) in the active layer of the PLED are presented and discussed.
Bifunctional 8,8'-dihydroxy-5,5'-biquinoline (bisquinoline) is reactively self-assembled with diethyl zinc to form a linear coordination polymer. The potential of this method to produce insoluble and intractable structures of controllable supramolecular architecture suitable for semiconducting applications has stimulated an in-depth investigation of the growth mechanism of these polymeric chelates. These films were characterized by FTIR, UV/VIS and photoluminescence spectroscopy. The film growth on glass or indium-tin oxide (ITO) coated substrates was monitored by UV/VIS spectroscopy and ellipsometry. FTIR spectroscopy indicates that the self-assembled films are polymeric in nature. Single layer light emitting diodes exhibited an orange electroluminescence, consistent with the corresponding photoluminescence spectrum
The fabrication and characterization of polyaniline (PANI) derivatives deposited on ITO coated glass is investigated as possible hole injection layers for MEH-PPV based polymer light emitting diode (PLED) devices. This involved multilayer ordering by the alternate polyelectrolyte adsorption of polyaniline and sulfonated poyaniline with an oppositely charged polyelectrolyte from solution. A combination of spectroscopic and microscopic techniques was utilized to determine the layer ordering, film structure, morphology, and homogeneity. The deposition process generally showed a linear behavior for all pairs as shown by ellipsometry and UV-vis spectroscopy. However, surface plasmon spectroscopy (SPS) and AFM revealed that thicker films are accompanied by increased surface roughness regardless of concentration. Comparison in performance was made between bare ITO and PANI or SPANI coated devices. Initial investigations of PLED performance showed significant improvements in lifetime and efficiency compared to bare ITO.
Double layer organic light emitting devices (OLED) are constructed by evaporating tris(8 -hydroxy) quinoline aluminum (Alq3) on a spin cast thin film of a methyl substituted ladder type poly -para -phenylene (m -LPPP). A thick layer of Mg:Ag is used as the cathode material. These organic materials are very suitable for application in OLEDs both, as transporting materials as well as active layers. Alq3 predominantly transports electrons while m - LPPP is a conjugated polymer having higher hole mobilities. Due to these transport properties the formation and radiative recombination of the excitons in ITO/m -LPPP/Alq3/Mg:Ag devices occur close to the m -LPPP/Alq3 interface. We compare the device performance of OLEDs with varying Alq3 layer thickness (0, 50, 150, 300, 500Å) and constant m -LPPP layer thickness (900Å). A difference in the device parameters and performance as a function of the Alq3 layer thickness is observed. We analyze these results with respect to the internal electric field distribution of the double layer devices derived from electroabsorption measurements.
We present a comprehensive study of the optical and electronic properties of a series of oligothiophenes in which one or two inter-ring torsional angles have been blocked by chemical bridging. These give us the possibility to investigate the role of the conformational mobility and of the coupling with the inter-ring torsion on the deactivation process of the singlet excited states. We find that both the radiative and the non-radiative deactivation channels are affected by the inter-ring bridging.
This paper reports the synthesis and characterizations of a new water-soluble poly(paraphenylene) (PPP) and its applications in preparing self-assembled multi-layer films. This new water-soluble conducting polymer was prepared through the sulfonation reaction of poly(pquarterphenylene- 2,2'-dicarboxylic acid). The incorporation of sulfonate groups has dramatically improved PPP's solubility in water at a wide pH range, whereas previous PPP is only slightly soluble in basic solutions. Dilute aqueous solutions of this polymer with acidic, neutral or basic pH emit brilliant blue light while irradiated with UV light. The sulfonated PPP emits from 350 nm to 455 nm with a maximum intensity at 380 nm. Self-assembled multilayers of this sulfonated PPP were constructed with a positively charged polymer poly(diallyl dimethyl ammonium chloride) and characterized with various surface analyses. Conductive (RuO2 and ITO), semiconductive (Si wafer), and non-conductive (SiO2) substrates were used in the preparation of self-assembled multilayers. Electrical, optical and structural properties of these novel self-assembled thin films will be discussed.
A series of azobenzene functionalized polymers has been synthesized by post polymerization azo coupling reaction. Photo-fabrication of surface relief gratings were studied on the polymer films. Epoxy based azo polymers were prepared by post azo coupling reaction to form polymers containing donor-acceptor type azo chromophores. The azo chromophores were designed to contain ionizable groups to impart self-assembling and photoprocessing capabilities to the polymers. The polymers containing 4-(4-(carboxylic acid)phenylazo)aniline chromophores can be directly photofabricated to form surface relief gratings with large surface modulations. Charge interactions had a strong influence on the details of the writing process. A new soluble polydiacetylene, post-functionalized with azobenzene groups was also prepared. Large amplitude surface gratings could be fabricated on this polydiacetylene film as well.