有机小分子掺杂的聚合物共混发光二极管

报道了用可溶性发光材料聚（２,５－二丁氧基苯）做发光材料,分别与母体聚合物聚乙烯基咔唑（ＰＶＫ）和聚甲基丙烯酸甲脂（ＰＭＭＡ）共混,并掺杂电子传输材料叔丁基联苯基苯基口恶二唑和空穴传输材料二胺衍生物作发光层,用铟锡氧化物和铝分别作正负电极,制作了两种蓝紫光有机／聚合物单层发光器件。通过比较两种器件的器件特性,发现以ＰＭＭＡ做母体的器件比用ＰＶＫ做母体的器件有更好的稳定性,器件开启电压为１０Ｖ左右,发光峰值波长均位于４２４ｎｍ,电致发光效率可达２．９％,比用ＰＶＫ做母体的器件效率高一倍多。     

Synthesis, optical and electroluminescent properties of novel polyfluorene/carbazole-based conjugated polyelectrolytes and their precursors

A series of novel aminoalkyl-substituted fluorene/carbazole-based main chain copolymers with benzothiadiazole (BTDZ) of different contents: poly[3,6-(N-(2-ethylhexyl)carbazole)-(9,9-bis(3'-(N,N-dimethyl-amino)propyl)-2,7-fluorene)-4,7-(2,1,3-benzothiadiazole)] (PCzN-BTDZ) were synthesized by Suzuki coupling reaction. Through a postpolymerization treatment on the precursor polymer, a corresponding quaternized ammonium polyelectrolyte derivatives: poly[3,6-(N-(2-ethylhexyl)carbazole)-(9,9-bis(3'-((N,N-dimethyl)-N-ethylammonium)propyl)-2,7-fluorene)-4,7-(2,l, 3-benzothiadiazole)] dibromide (PCzNBr-BTDZ) were obtained. It was found that devices from such polymers with high work-function metal cathode such as Al showed similar device performance to that by using low work-function cathode such as Ba, indicating the excellent electron injection ability of these polymers. The efficient energy transfer from fluorene-carbazole segment to the narrow band gap BTDZ site for both the neutral and the quaternized copolymers was also observed. The addition of BTDZ into the polymer main chain can also improve polymer LED (PLED) device performance. When poly(3,4-ethylenedioxythiophene) (PEDOT)/poly(vinylcarbazole) (PVK) was used as an anode buffer, the external quantum efficiency of the copolymer PCzN-BTDZ1 was 0.99%, which was much higher than the copolymer PCzN without the incorporation of BTDZ in the same device configuration.     

High triplet,bipolar polymeric hosts for highly efficient solution-processed blue phosphorescent polymer light-emitting diodes

Two polymeric hosts PCzTPP and PCzTPPO with twisted geometrical configurations for blue phosphorescent polymer light-emitting diodes (PhPLEDs) were designed and synthesized by incorporating electron-accepting carbazole units with electron-donating TPP/TPPO groups. This molecular design endows PCzTPP and PCzTPPO with high glass transition temperatures of 204 °C and 215 °C, high triplet energies of 2.72 eV and bipolar features. In addition, the HOMO and LUMO of these polymers matched well with the HOMO of the hole-transport layer and the Fermi level of cathode compared with PVK, which facilitated the injection of holes and electrons. PCzTPP- and PCzTPPO-based single-emissive-layer blue PhPLEDs were fabricated with simplified device configuration by solution process using FIrpic as a dopant. These devices exhibited lower turn on voltages (<8 V) than PVK-based devices (12 V). The maximum luminances of PCzTPP- and PCzTPPO-based devices were twofold and threefold that of PVK-based devices, and the maximum current efficiencies were nearly threefold and ninefold, respectively. Moreover, PCzTPPO-based solution processed blue PhPLEDs with improved configuration showed maximum current efficiency and external quantum efficiency of 14.5 cd/A and 6.6%, respectively.     

Enhanced green electrophosphorescence by using polyfluorene host via interfacial energy transfer from polyvinylcarbazole

For green-emitting Ir complex-doped polymer devices, high efficiencies have been achieved only when non-conjugated poly(N-vinylcarbazole) (PVK) was used as the host polymer. It is commonly believed that conjugated polyfluorenes (PFO) are not suitable for the use as host of green phosphorescent complexes due to the presence of the low-lying triplet state. In this paper we reported that despite very inefficient PL phosphorescent emission of Ir(Bu-PPy)₃ in the PFO films, strong green electroluminescence (EL) and high device efficiencies have been observed for devices from such blend films when multilayer device with PVK was used as anode buffer. We analyze the PL spectra in steady state and transient response and found that the energy transfer via PVK interlayer plays important role in the efficient EL performance. This fact clearly indicates the efficient electrophosphorescence might be achieved even if the triplet energy of phosphorescence dye is higher than that of the host. These findings could significantly broaden our selection in polymer hosts for phosphorescent devices.     

Efficient red-light-emitting diodes based on novel amino-alkyl containing electrophosphorescent polyfluorenes with Al or Au as cathode

Novel amino-alkyl containing polyfluorenes with Ir complex in pendant chain were synthesized by Suzuki polycondensation and efficient red-light-emitting devices based on the polymers that were fabricated using high work-function metal (Al or Au) as cathode. It was found that the prepared polyfluorenes exhibit good device performances, and the best device efficiency was achieved based on PFN-Irpiq1 that showed a maximal quantum efficiency (QE_max) of 3.7% and 1.6% for Al and Au as cathode, respectively, which was comparable to that of 5.5% for low work-function metals Ba as cathode. The efficiencies of devices from the aforementioned polymers showed a reduced roll-off upon the increase of current density, which is beneficial for the highly efficient phosphorescent dye/host-doped systems. The obtained high efficiencies with air-stable metals as cathode reflect that the copolymers have dual functions of being efficient light-emitting and electron injection ability. The results indicated that the incorporation of Ir complex into amino-alkyl containing polyfluorenes side chain is a potential approach to achieve efficient light-emitting devices with air-stable high work-function metal as the cathode.     

Amino N‐Oxide Functionalized Conjugated Polymers and their Amino‐Functionalized Precursors: New Cathode Interlayers for High‐Performance Optoelectronic Devices

A series of amino N‐oxide functionalized polyfluorene homopolymers and copolymers (PNOs) are synthesized by oxidizing their amino functionalized precursor polymers (PNs) with hydrogen peroxide. Excellent solubility in polar solvents and good electron injection from high work‐function metals make PNOs good candidates for interfacial modification of solution processed multilayer polymer light‐emitting diodes (PLEDs) and polymer solar cells (PSCs). Both PNOs and PNs are used as cathode interlayers in PLEDs and PSCs. It is found that the resulting devices show much better performance than devices based on a bare Al cathode. The effect of side chain and main chain variations on the device performance is investigated. PNOs/Al cathode devices exhibit better performance than PNs/Al cathode devices. Moreover, devices incorporating polymers with para‐linkage of pyridinyl moieties exhibit better performance than those using polymers with meta‐linked counterparts. With a poly[(2,7‐(9,9‐bis(6‐(N,N‐diethylamino)‐hexyl N‐oxide)fluorene))‐alt‐(2,5‐pyridinyl)] (PF6NO25Py) cathode interlayer, the resulting device exhibits a luminance efficiency of 16.9 cd A^?1 and a power conversion efficiency of 6.9% for PLEDs and PSCs, respectively. These results indicate that PNOs are promising new cathode interlayers for modifying a range of optoelectronic devices.     

Effect of multi-walled carbon nanotubes on electron injection and charge generation in AC field-induced polymer electroluminescence

We investigate the use of multi-walled carbon nanotubes (MWNTs) dispersed in an emissive layer of poly (N-vinylcarbazole) (PVK):fac-tris(2-phenylpyri-dine)iridium(III) [Ir(ppy)₃] in alternating current (AC) field-induced polymer electroluminescence (FIPEL) devices. A symmetric device structure, with the polymer/MWNT composite between two dielectric layers, was used to study the effect of MWNTs on charge generation within the active layer. An asymmetric device structure, using one dielectric layer, was used to study band alignment effects of carbon nanotubes in charge injection from a contact. The presence of MWNTs within the emissive layer facilitates effective internal charge generation in the symmetric devices, as would be expected if they acted as a charge source. However, electron injection under AC-driven fields also increases in the asymmetric devices, suggesting a modification to band alignment. Increase in light emission of five times is achieved in composite devices compared to devices with the pure polymer. From the trends in behavior with nanotube loading, we suggest that the nanotubes effectively doped the polymer, modifying energy level alignment in the device and increasing field-induced polarization currents. The combined effects of electron injection and charge generation may pave the way for widespread use of MWNTs in high-performance FIPEL devices.     

有机/聚合物异质结电致发光器件的光电性能

器件结构是影响有机发光器件(OLED)性能的重要因素之一.采用8-hydroxyquinoline-aluminum(AlQ)作为发光层(EML)和电子传输层(ETL),polyvinylcarbazole (PVK)作为空穴传输层(HTL),制备了具有有机小分子/聚合物异质结结构的OLED器件,通过其电压-电流-发光亮度(V-J-B)特性测试,研究了HTL的引入及其膜厚对器件性能的影响.实验结果表明,HTL的引入有效地改善了OLED的光电性能,同时HTL膜厚对器件性能具有显著影响,当HTL膜厚为20 nm时,所制备的OLED器件具有最小的驱动电压和启亮电压、最大的发光亮度和发光效率.

Abstract：

The device construction plays an important role in improving the optoelectronic performance of organic electroluminescence devices (OLEDs). Heterojunction OLEDs with a configuration of glass/ITO/PVK/AlQ/Mg/Al were fabricated by using 8-hydroxyquinoline-aluminum (AlQ) as the emission layer (EML) and electron transport layer (ETL) and polyvinylcarbazole (PVK) as the hole transport layer (HTL). The effect of the HTL thickness on the performance of OLEDs was investigated with respect to the driving voltage, turn-on voltage, electroluminescence brightness and efficiency of the devices. Experimental results demonstrate that the optical and electrical properies of OLEDs are closely related to the HTL thickness. The device fabricated with the HTL thickness of 20 nm possesses the best photoelectric properties such as the minimum driving voltage and turn-on voltage, and the maximum electroluminescence brightness and efficiency.     

Ethoxylated polyethylenimine as an efficient electron injection layer for conventional and inverted polymer light emitting diodes

We report inverted light emitting devices using ethoxylated polyethylenimine (PEIE) as a single electron injection layer for indium tin oxide cathode, which possess comparable efficiency to those using ZnO/PEIE double electron injection layers. Implementation of a PEIE layer between light emitting polymer layer and aluminum has been shown to significantly enhance device efficiency as well. Improvement of device efficiency can be attributed to increased electron injection due to the reduced work function of PEIE modified cathode as well as the hole blocking effect of PEIE layer. Furthermore, PEIE serves as an efficient electron injector for a range of light emitting polymers with wide distribution of energy levels.     

Highly efficient blue and all-phosphorescent white polymer light-emitting devices based on polyfluorene host

We report efficient blue electrophosphorescent polymer light emitting devices with polyfluorene (PFO) as the host and iridium bis[2-(4,6-difluorophenyl)-pyridinato-N,C²] picolinate (FIrpic) as the dopant. Despite the low-lying triplet energy level of the polyfluorene polymer host, phosphorescent quenching can be suppressed by using poly(N-vinylcarbazole) (PVK) as anode buffer layer, resulting in a high luminous efficiency of 26.4 cd A^?1, which is one of the best results in the literature based on conjugated polymer reported to date. The reduced phosphorescent quenching is found to be associated with the exciton formation and charge carrier recombination within the PVK layer and the PVK/PFO interface due to the accumulation of holes. As compared with the devices based on non-conjugated host polymer PVK, the devices based on PFO showed a lower turn-on voltage (3.6 V vs. 4.4 V) and higher power efficiency (17 lm W^?1 vs. 8.3 lm W^?1) due to the higher mobility of PFO. When doubly doped with a newly synthesized yellow-emitting metallophosphor, white polymer light-emitting devices with superior device performance (a peak device efficiency of 40.9 cd A^?1, a CIE coordinates of (0.32, 0.48), and a power efficiency of 31.4 lm W^?1) was achieved. These findings can broaden our selection in polymer hosts for highly efficient phosphorescent blue emitting devices and can find potential applications in full color displays and solid-state lighting applications in the future.     

Improved Efficiency of Single‐Layer Polymer Light‐Emitting Devices with Poly(vinylcarbazole) Doubly Doped with Phosphorescent and Fluorescent Dyes as the Emitting Layer

We demonstrate novel organic light‐emitting diode (LED) materials that contain a green phosphorescent dye (dmbpy)Re(CO)₃Cl (dmbpy = 4,4′‐dimethyl‐2,2′‐bipyridine), and a red fluorescent dye 4‐dicyanomethylene‐6‐(p‐dimethylaminostyryl)‐2‐methyl‐4H‐pyran (DCM) as dopants and polyvinylcarbazole (PVK) as the host. The photoluminescence (PL) and electroluminescence (EL) properties of these complex materials were studied. The energy transfer efficiency from PVK host to DCM is increased by the (dmbpy)Re(CO)₃Cl co‐dopant, which has an emission energy between that of PVK and DCM. The (dmbpy)Re(CO)₃Cl, which emits a long‐lived phosphorescence, is used as an energy coupler, providing the possibility to harvest both singlet and triplet energy in the devices. The pure red emission from DCM was observed from PL and EL spectra of (dmbpy)Re(CO)₃‐Cl(> 2.0 wt.‐%):DCM(> 0.5 wt. %) doped PVK films, demonstrating an efficient energy transfer from PVK and (dmbpy)Re(CO)₃‐Cl to DCM. By optimizing the concentration of DCM and (dmbpy)Re(CO)₃Cl in PVK, a maximum EL quantum efficiency of 0.42 cd A^–1 at a current density of 9.5 mA cm^–2 was obtained. The EL quantum efficiency of the doubly doped device is significantly enhanced in comparison with both a DCM‐only doped PVK device and a DCM‐doped PVK device with the green fluorescent dye Alq₃ as co‐dopant. The improvement in the operating characteristics of the phosphorescent and fluorescent dye doubly doped device is attributed to efficient energy transfer in the system, in which both triplet and singlet excitons are used for resultant emission in the polymer device.     

Solution-processed quantum dot light-emitting diodes with PANI:PSS hole-transport interlayers

For solution-processed quantum dot light-emitting devices (QD-LEDs), poly(3,4-ethylenedioxythiophene) polystyrene sulfonate/poly(N-vinylcarbozole) (PEDOT:PSS/PVK) bilayers have been widely used as the hole injection/transport layer. The high work function of the hole transport layer is crucial for high electroluminescence efficiency with balanced electron/hole charge injection. Herein, we report improvement of the performance of QD-LEDs by inserting a polyaniline (PANI)-poly (p-styrenesulfonic acid) (PSS) (PANI:PSS) hole-transport layer between the PVK and PEDOT:PSS layers. The insertion of the PANI:PSS layer significantly shifted the electronic energy levels of the PVK layers to lower values, which reduced the energy barrier of holes traveling to the QD layer by 0.22 eV. The QD-LEDs with PANI:PSS interlayer exhibited superior electric and electroluminescent characteristics. The hole-only devices with PANI:PSS interlayer also presented high hole injection and transport capability. Ultraviolet photoelectron spectroscopy (UPS) was used to investigate the electronic energy level alignment of the QD-LEDs with/without the PANI:PSS interlayer. The device performance results of QD-LEDs and hole-only devices indicated enhanced electric and electroluminescent characteristics for the PANI:PSS-inserted QD-LEDs with high hole conduction capability, in agreement with UPS findings.     

Highly efficient all-solution processed blue quantum dot light-emitting diodes based on balanced charge injection achieved by double hole transport layers

Solution-processed blue quantum dot light-emitting diodes (QLEDs) suffer from low device efficiency, whereas the balance of electron and hole injection is critical for obtaining high efficiency. Herein, synergistical double hole transport layers (D-HTLs) are employed, which use poly(9-vinylcarbazole) (PVK) stacked on poly[(9,9-dioctylfluorenyl-2,7-diyl)-alt-(4,4'-(N-(4-butylphenyl) (TFB). The fabrication of D-HTLs is achieved by using dimethyl formamide (DMF) as the solvent for PVK, with which the underlying TFB layer almost remains unwashed and undamaged during the spin-coating process of PVK layer. TFB/PVK D-HTLs form the stepwise energy level for hole injection, which reduces the hole injection barrier and favors the carrier balance in the emission layer (EML). The optimized blue QLED with TFB/PVK D-HTLs shows a maximum external quantum efficiency (EQE) of 13.7%, which is 3-fold enhancement compared to that of the control device with single TFB HTL. The enhancement of the QLED performance can be attributed to the improvement of surface morphology and charge injection balance for the stepwise D-HTLs based QLEDs. This work manifests the positive effect on performance boost by selecting appropriate solvents towards stepwise D-HTLs formation and paves the way to fabricate highly efficient all-solution processed light emitting diodes.     

Polymer Light-Emitting Electrochemical Cells for High-Efficiency Low-Voltage Electroluminescent Devices

Organic light-emitting devices exhibiting high power conversion efficiency and long operating lifetime may potentially be achieved with the polymer light-emitting electrochemical cell (LEC) configuration. An LEC device typically uses a thin layer of conjugated polymer sandwiched between two contact electrodes. The polymer layer contains an ionically conductive species that are essential in the formation of a light-emitting p-i-n junction. LEC devices are characterized with balanced electron and hole injections, high current density at relatively low bias voltages (2-4 V), and high electroluminescent power efficiency. We will describe the working mechanism of the LECs and review the recent developments in LEC materials, device fabrication and performance. Among the important developments are planar (surface-typed) LECs, bilayer LECs that emit different colors at forward and reverse biases, frozen p-i-n junction LECs that functions like diodes, and phosphorescent LECs. Extensive efforts have been made to improve the LEC performance by controlling the blend morphology, including the use of bipolar surfactant additives and new electrolytes, the synthesis of conjugated polymers with ion-transporting main chain segments or side groups and polyelectrolyte. Degradation mechanisms that limit the lifetime of the LECs will also be discussed     

Flash‐Memory Effect for Polyfluorenes with On‐Chain Iridium(III) Complexes

Polyfluorenes containing Ir(III ) complexes in the main chain are demonstrated to have promising application in a polymer memory device. A flash‐memory device is shown whereby a polymer solution is spin‐coated as the active layer and is sandwiched between an aluminum electrode and an indium tin oxide electrode. This device exhibits very good memory performance, such as low reading, writing, and erasing voltages and a high ON/OFF current ratio of more than 10⁵. Both ON and OFF states are stable under a constant voltage stress of ?1.0 V and survive up to 10⁸ read cycles at a read voltage of ?1.0 V. Charge transfer and traps in polymers are probably responsible for the conductance‐switching behavior and the memory effect. The fluorene moieties act as an electron donor and Ir(III ) complex units as the electron acceptor. Furthermore, through the modification of ligand structures of Ir(III ) complex units, the resulting polymers also exhibit excellent memory behavior. Alteration of ligands can change the threshold voltage of the device. Hence, conjugated polymers containing Ir(III ) complexes, which have been successfully applied in light‐emitting devices, show very promising application in polymer memory devices.     

蓝光聚合物共混材料的发光特性

研究了不同组成PVK和P6共混材料薄膜的光吸收特性和光致发光特性．用不同比例PVK∶P6共混材料为发光层和Alq3为电子传输层合成双层结构的蓝光器件,测量器件的电致发光谱、电流-电压特性和亮度-电压特性．结果表明,共混材料的光致发光强度比PVK和P6都有明显的增强,且随PVK浓度的增加而增强;器件电致发光强度随PVK浓度的增加而增强;不同PVK掺杂浓度对电致发光器件的开启特性没有明显影响,发光亮度随PVK掺杂浓度的增加而增大．     

Effect of different processing methods for the hole transporting layer on the performance of double layer organic light-emitting devices

The hole transporting layer （HTL） of organic light-emitting device （OLED） was processed by vacuum deposition and spin coating method, respectively, where N,N＇-biphenyl-N, N＇-bis（3-methylphenyl）- 1, l＇-biphenyl-4,4＇ -diamine （TPD） and poly （vinylcarbazole） （PVK） acted as the hole-transport materials. Tris-（8-hydroxyquinoline）- aluminum （Alq3） was utilized as both the light-emitting layer and the electron transporting layer. The basic structure of the device cell was： indium-tin-oxide （1TO）/PVK ： TPD/Alq3/Mg：Ag. The electroluminescent （EL） characteristics of devices were characterized. The results showed that the peak of EL spectra was located at 530 nm, which conformed to the characterizing spectrum of Alq3. Compared with using vacuum deposition method, the green emission with a maximum luminance up to 26135 cd/m2 could be achieved at a drive voltage of 15 V by selecting proper solvent using spin-coating technique, and its maximum lumi nance efficiency was 2.56 lm/W at a drive voltage of 5.5 V.     

Electron injection and interfacial trap passivation in solution-processed organic light-emitting diodes using a polymer zwitterion interlayer

Here we describe the use of a polymer zwitterion as a solution-processable material that serves as the key component of the electron injection layer (EIL) in solution processed organic light-emitting diodes (OLEDs). Poly(sulfobetaine methacrylate) (PSBMA) was employed in both regular and inverted device configurations as a work-function modifier for Al and ZnO cathodes, respectively. For both architectures, PSBMA significantly improved the OLED performance when compared to reference devices without EIL in terms of turn-on voltage and luminance. In inverted devices, PSBMA showed a passivation effect on ZnO surface trap states, producing better performing and more stable devices.     

Improved efficiency in semiconducting polymer light-emitting diodes

We report visible light emission from metal-polymer diodes made from semiconducting polymers, with indium-tin oxide as the “ohmic” contact, and a variety of metals as the barrier metal. Our results, which confirm the discovery by the Cambridge group [Nature347, 539 (1990)], demonstrate that light-emitting diodes can be fabricated by casting the polymer film on indium-tin oxide from solution with no subsequent polymer processing or heat treatment required. Electrical characterization reveals diode behavior with rectification ratios greater than 10⁵ at sufficiently high voltages. Use of an electrode material with low work function leads to more than an order of magnitude improvement in the room-temperature efficiency of the devices. For example, the most efficient devices made with calcium as the rectifying contact display efficiencies of 0.01 photons per electron.     

Solution-processed blue phosphorescent OLEDs with carbazole-based polymeric host materials

A new carbazole-based polymer PEPEK varying from the previously reported PEPK by the length of the spacer between the polymer backbone and the pendent carbazole moiety was investigated as polymeric host for solution-processed devices. Interestingly, if the two polymers are structurally close since the length of the alkyl chain only differs from one carbon atom, the previously reported PEPK gave higher performances than the newly synthesized PEPEK when tested as host for the wide bandgap triplet emitter FIrpic. To optimize electroluminescence performances, two device configurations were examined. On doping the emissive layer of phosphorescent organic light-emitting devices (OLEDs) at 16 wt% with FIrpic, best PEPK-based OLEDs gave an efficacy of 15.14 cd/A whereas PEPEK-based devices furnished an efficiency of 12.17 cd/A in the same conditions. To determine the origin of this unexpected behavior, the new polymer PEPEK was characterized by UV–visible absorption and luminescence spectroscopy as well as cyclic voltammetry. Thermal properties of PEPEK were also examined and compared to those of PEPK.