Materials For Polymer Light-emitting Diodes Research Articles

Synthesis and Electron Transporting Properties of Diblock Copolymers Consisting of Polyfluorene and Polystyrene.

Poly(9,9-di-n-octylfluorene) (PFO) is a promising material for polymer light-emitting diodes (PLEDs) due to its advantageous properties. To enhance its electron transporting capabilities, diblock polymers were synthesized by attaching polystyrene (PSt) chains of varying lengths to one end of the PFO molecule. In a comparative study with PFO homopolymer, the diblock polymers maintained similar thermal properties, absorption spectra, and photoluminescent stability, while exhibiting slightly deeper lowest unoccupied molecular orbital (LUMO) levels and higher crystallinity. Notably, diblock polymers with shorter polystyrene blocks demonstrated higher electron mobility than the PFO homopolymer and diblock polymers with excessively long polystyrene blocks. These findings suggest that the optimal chain length of the polystyrene block is crucial for maximizing electron mobility, thus offering valuable insights for designing high-performance PLED materials.

3PTZ and 3PXZ small molecular hole-transporting materials in polymer light-emitting diodes

The fabricated polymer light-emitting diodes (PLEDs) with new small molecular hole transport layers, 1,3,5-tri(10H-phenothiazin-10-yl) benzene (3PTZ) and 1,3,5-tri(10H-phenoxazin-10-yl) benzene (3PXZ).

Fluorene pendant-functionalization of poly(N-vinylcarbazole) as deep-blue fluorescent and host materials for polymer light-emitting diodes

π-Electron coupling of pendant conjugated segment in π-stacked semiconducting polymers always causes the formation of defect trapped sites and further quenched high-band excitons, which is harmful to the performance and stability of deep-blue polymer light-emitting diodes (PLEDs). Herein, considerate of “defect” carbazole (Cz) electromers in poly(N-vinylcarbazole) (PVK), a series of fluorene units are introduced into pendant segments (PVCz-DMeF, PVCz-FMeNPh and PVCz-DFMeNPh) to suppress the strong π-electron coupling of pendant Cz units and enhance radiative transition toward fabricating sable PLEDs. Compared to PVCz-FMeNPh and PVCz-DFMeNPh, PVCz-DMeF spin-coated films show a relatively efficient deep-blue emission, completely similar to its single pendant chromophore, confirmed an extremely weak charge-transfer and electron coupling between adjacent pendant segments. Therefore, PLEDs based on PVCz-DMeF present stable and deep-blue emission with a high color purity (0.17, 0.08), associated with extremely weak defect emission at 600∼700 nm (induced by carbazole electromers). Finally, PLEDs based on PVCz-DMeF/F8BT blended films (1:1) also present the high maximum luminance (Lmax) of 6261 cd/m2 and current efficiency (CEmax) of 2.03 cd/A, confirmed slightly trapped sites formation. Therefore, precisely control the arrangement and packing model of pendant units in π-stacked polymer is an essential prerequisite for building efficient and stable emitter for optoelectronic devices.

Layer‐by‐Layer Assembly of Multilayer Thin Films for Organic Optoelectronic Devices

AbstractOrganic optoelectronic devices based on conjugated polymers have attracted widespread attention and practical applications have been achieved. Various solution‐processing protocols for depositing conjugated polymeric thin films of organic optoelectronic devices have been developed. As a low‐cost solution‐processing technique, the layer‐by‐layer (LbL) assembly method can be used to fabricate multilayer thin films with a high‐quality surface and controlled thickness. Recent remarkable progress in LbL‐assembled multilayer films for the interface engineering of organic optoelectronic devices has demonstrated their great potential for practical applications in the near future. Here, the application of LbL assembly to the construction of multilayer films is discussed for use in polymer optoelectronic devices, especially polymer light‐emitting diodes (PLEDs) and polymer solar cells (PSCs). A brief introduction to LbL‐assembled multilayer films for the emissive/active layers in PLEDs/PSCs is provided and the limitations of LbL materials are discussed. The comprehensive application of LbL‐assembly techniques and LbL materials in the interface engineering of PLEDs and PSCs is also presented with a supplementary discussion of the working mechanism of LbL materials in PLEDs and PSCs. Finally, perspectives on the LbL‐assembly technique and challenges to its application in future organic optoelectronic devices are presented.

Alcohol-soluble conjugated poly(fluorene-co-thiophene) as an electron-transport material for polymer light-emitting diodes

ABSTRACTA series of alternating polyfluorene copolymers containing fluorene (P1-OF), bithiophene (P2-BT), and thiophene (P3-TH) moieties have been synthesized via a palladium-catalyzed Suzuki coupling reaction. Among them, the P3-TH polymer highly contains hydroxyl groups which provides enough solubility in polar solvents such as alcohols and dimethylformamide. Multilayer polymer light-emitting diodes have been fabricated using a semiconducting polymer, F8BT, as an emissive layer and the methanol-soluble P3-TH as an electron-transport layer. By the device configuration of ITO/PEDOT/TFB/F8BT/P3-TH/LiF/Al, it shows a luminescence efficiency of 1.48 cd/A and a turn-on voltage of 4.6 V. The emission with maxima at 448 nm and 495 nm which fell in the blue region with the CIE coordinates (x = 0.18, y = 0.24).

Effect of o-Carborane on the Optoelectronic and Device-Level Properties of Poly(fluorene)s.

Carboranes have been previously noted to distinctively affect the luminescent properties of semiconducting polymers when incorporated into the conjugated backbone. In this report, we use carborane-based poly(fluorene) derivatives as active materials for polymer light-emitting diodes and transistors. Optoelectronic analysis unequivocally shows that carborane does not participate in the π-conjugated network, yet their presence causes major red-shifting in device electroluminescence as well as in thin film photoluminescence. In field effect transistors, they also improve charge carrier mobility by an order of magnitude despite disrupting π-conjugation. This use of carborane-containing conjugated polymers in active devices holds promise as new responsive materials in electronic polymer applications.

A novel crosslinkable electron injection/transporting material for solution processed polymer light-emitting diodes

A novel crosslinkable water/alcohol soluble conjugated polymer PFN-C containing oxetane groups and aminoalkyl groups in the side chains has been developed and used as highly efficient electron injection and transporting material for polymer light-emitting diodes (PLEDs). The unique solubility in polar solvents and crosslinkable ability of PFN-C render it a good candidate for solution processed multilayer PLEDs. It was found that PFN-C can greatly enhance the electron injection from high work-function metal cathode, due to its pendant amino groups. As a result, PLEDs with PFN-C/Al cathode exhibited comparable device performance to the devices with Ba/Al cathode. The resulting green light-emitting device showed promising performance with a maximum luminance efficiency of 13.53 cd A−1.

Synthesis and Characterization of Fluorene Based Electron Transporting Materials for Polymer Light-Emitting Diodes

We synthesized fluorene based alternating copolymers comprised of several electron withdrawing groups via Suzuki coupling polymerization. Physical properties were characterized using the UV-vis spectroscopy, photoluminescence (PL) spectroscopy. The absorption peaks of copolymers were red-shifted compared to that of corresponding fluorene homopolymer.

Synthesis and characterization of red-emitting diketopyrrolopyrrole- alt-phenylenevinylene polymers

Two novel diketopyrrolopyrrole (DPP) and p-phenylenevinylene alternating copolymers, poly(1,4-(2,5-dicyano)-phenylenevinylene- alt-2,5-dioctyl-3,6-bis(4-vinylenephenyl)pyrrolo[3,4-c]pyrrole-1,4-dione) (P1) and poly(1,4-(2,5-diethoxy)-phenylenevinylene- alt-2,5-dioctyl-3,6-bis(4-vinylenephenyl)pyrrolo[3,4-c]pyrrole-1,4-dione) (P2), were synthesized through Wittig reaction in good yields, and were characterized by FTIR, NMR. Two polymers possessed moderate molecular weights and polydispersities, well-defined structures, and were readily soluble in common organic solvents. In particular, P1 and P2 exhibited excellent thermal stability with T d = 393 and 398 °C at 5% weight loss, respectively. Both P1 and P2 in solution and in thin films exhibited strong red photoluminescence. Both electroluminescence devices using ITO/PEDOT/polymer/Ba/Al configuration with P1 and P2 as the emitting layer showed nearly pure red emission with the emission peaks at 646 nm for P1 and 648 nm for P2, and exhibited low turn-on voltages of 4.5 and 3.4 V, respectively. The results show that P1 and P2 are promising candidates for red emissive materials in polymer light-emitting diodes.

Differences between nanoscale structural and electrical properties of AZO:N and AZO used in polymer light‐emitting diodes

Conducting atomic force microscopy and scanning surface potential microscopy were adopted to investigate the nanoscale surface electrical properties of N-doped aluminum zinc oxide (AZO:N) films that were prepared by pulsed laser deposition (PLD) at various substrate temperatures. Experimental results demonstrated that when the substrate temperature is 150 degrees C and the N(2)O background pressure is 150 mTorr, the N-dopant concentration on the surface is optimal. In addition, the root-mean-square roughness value of the film surface, the low contact current (<400 nA) conducting region as a percentage of the total area, and the mean work function value are 1.43 nm, 96.9%, and 4.88 eV, respectively, all of which are better than those of the optimal AZO film made by PLD. This result indicates that N-doped AZO films are better for use as window materials in polymer light-emitting diodes.

Nanoscale surface electrical properties of aluminum zinc oxide thin films investigated by scanning probe microscopy

Conducting atomic force microscopy and scanning surface potential microscopy were adopted to study the nanoscale surface electrical properties of aluminum zinc oxide (AZO) films that were prepared by pulsed laser deposition (PLD) at various substrate temperatures for use as anode materials in polymer light-emitting diodes (PLEDs). Experimental results indicate that when substrate temperatures exceed 100°C, the local conductivity and work function are positively correlated with the concentrations of Al dopant and O2− on AZO surface. When the substrate temperature is approximately 150°C, the percentage coverage of conducting regions of the AZO surface and the mean work function are 90.20% and 4.85eV, respectively. Additionally, both microcosmic uniformities meet the standard applied to PLEDs. This low-temperature condition for PLD significantly reduces the yield rate of impurities when AZO vacuum evaporation is performed on a plastic substrate, supporting various applications of AZO films.

Nanoscale surface electrical properties of zinc oxide films investigated by conducting atomic force microscopy

In this study, conducting atomic force microscopy was employed to investigate the nanoscale surface electrical properties of zinc oxide (ZnO) films prepared by pulsed laser deposition (PLD) at different substrate temperatures for use as anode materials in polymer light-emitting diodes. The results show that the surface conductivity distribution of ZnO is related to its surface structure. At substrate temperatures of 150-200 degrees C, the conducting regions may cover over 90% of the ZnO thin-film surface, thus providing the best local conductivity. Moreover, heating at substrate temperatures of above 250 degrees C can effectively make the conductivity on the ZnO surface uniform. In particular, at substrate temperatures of around 300 degrees C, the conducting regions where currents are between 1 and 2 muA may cover as much as 83% of the surface, and furthermore, the transmission ratio in the visible range is higher than 80%. This is a rather ideal production temperature for the PLD for ZnO films.

Synthesis, Photophysics, and Electroluminescence of Poly(dibenzofluorene)s

AbstractSummary: A series of soluble poly(dibenzofluorene) derivatives that contain dibenzo[a,g]fluorene, dibenzo[a,i]fluorene, and dibenzo[c,g]fluorene repeat units in the main chain have been synthesized, characterized, and explored as emissive materials in polymer light emitting diodes (PLEDs). These polymers possess higher glass transition temperatures (108–133 °C) than that of poly(2,7‐(9,9‐dialkyl)fluorene) (PFO). The photophysical and electrochemical properties of these polymers are affected by the steric hindrance effect. These polymers emit blue light in dilute solution (378–400 nm) and in the solid state (426–447 nm). As emissive materials in PLEDs, blue electroluminescence with a brightness of up to 3 130 cd · m−2 is obtained from single‐layer diodes of P2 with aluminum/barium in air.The photophysical and electrochemical properties of these polymers are affected by the size effect.magnified imageThe photophysical and electrochemical properties of these polymers are affected by the size effect.

Synthesis, Photophysics, and Electroluminescence of New Quinoxaline-Containing Poly(p-phenylenevinylene)s

Four new soluble poly(p-phenylenevinylene) (PPV) derivatives containing one or two quinoxaline moieties per repeat unit, either in the main chain or as pendants to the main chain, were synthesized, characterized, and explored as emissive and electron transport materials in polymer light-emitting diodes (LEDs). Polymers containing one quinoxaline moiety per repeat unit (QXPV1 and QXPV3) showed low melting transitions (<100 °C), whereas those with two quinoxaline moieties per repeat unit (QXPV2 and QXPV4) had relatively high glass transition temperatures (>140 °C). The polymers emit blue to green light (404−536 nm) in dilute solution and blue-green to yellow light (470−563 nm) in the solid state. The photoluminescence emission was well-described by single-exponential decay with lifetimes ranging from 200 ps to 2.2 ns in both dilute solution and thin film, indicating lack of intermolecular emissive species in the solid state. PPV derivatives with quinoxaline moieties in the main chain (QXPV1 and QXPV2) showed facile reversible electrochemical reductions with electron affinities of 2.63−2.75 eV. As emissive materials in LEDs, greenish-yellow electroluminescence with a brightness of up to 450 cd/m2 was obtained from single-layer diodes of QXPV1 with aluminum cathode in air.

Synthesis and Characterization of New Red-Emitting Polyfluorene Derivatives Containing Electron-Deficient 2-Pyran-4-ylidene−Malononitrile Moieties

A novel series of red-light-emitting copolymers derived from fluorene and 2-pyran-4-ylidene−malononitrile (PM) have been synthesized through a palladium-catalyzed Suzuki coupling reaction. The polymers were characterized by FT-IR, NMR, and elemental analysis. All these polymers are completely soluble in common organic solvents, such as THF, CH2Cl2, CHCl3, and toluene, and they have good thermal stability with onset decomposition temperature (Td) of 406−407 °C and glass-transition temperature (Tg) of 73−186 °C. Cyclic voltanmetry studies reveal that these copolymers have low-lying LUMO energy levels ranging from −3.53 to −3.57 eV and HOMO energy levels ranging from −5.77 to −5.79 eV, which indicated that they may be promising candidates for electron-transporting or hole-blocking materials in light-emitting diodes. These polymers in thin films can emit strong red photoluminescence (PL) around 641−662 nm with the corresponding additional peaks in the range 704−712 nm upon photoexcitation. Double-layer LEDs fabricated with the configuration of ITO/PEDOT/polymer/Ba/Al can emit red light with external quantum efficiencies of 0.21−0.38%. Preliminary electroluminescent (EL) results show that these polymers are novel promising candidates for red emissive materials in polymer light-emitting diodes.

Regioregular Head-to-Tail Poly(4-alkylquinoline)s: Synthesis, Characterization, Self-Organization, Photophysics, and Electroluminescence of New n-Type Conjugated Polymers

Five poly(4-alkylquinoline-2,6-diyl)s (P4AQs), having 100% head-to-tail regioregularity, were synthesized, characterized, and explored as emissive and electron transport materials in polymer light-emitting diodes. The organic solvent (chloroform, tetrahydrofuran, methylene chloride, etc.) soluble poly(4-alkylquinoline)s had reversible electrochemical reduction with an electron affinity (2.6 eV) that was independent of alkyl chain length. X-ray diffraction studies showed that solution-cast films of regioregular P4AQs have self-organized lamellar structures with an interchain π-stacking distance of 3.9−4.1 A and an interlayer d-spacing that varied linearly from 13.4 A for butyl to 22.3 A for tridecyl side chains. The new poly(4-alkylquinoline)s emit blue light at 423−426 nm in dilute chloroform solution and yellow light at 542−557 nm in the solid state. Yellow electroluminescence with a luminance of up to 86 cd/m2 and an external quantum efficiency of up to 0.6% was observed for the P4AQs as emissive materi...

New n-type organic semiconductors: synthesis, single crystal structures, cyclic voltammetry, photophysics, electron transport, and electroluminescence of a series of diphenylanthrazolines.

The synthesis, properties, and electroluminescent device applications of a series of five new diphenylanthrazoline molecules 1a-1e are reported. Compounds 1b, 1c, and 1d crystallized in the monoclinic system with the space groups P2(1)/c, C2/c, and P2(1)/c, respectively, revealing highly planar molecules. Diphenylanthrazolines 1a-1e have a formal reduction potential in the range -1.39 to -1.58 V (versus SCE) and estimated electron affinities (LUMO levels) of 2.90-3.10 eV. Compounds 1a-1e emit blue light with fluorescence quantum yields of 58-76% in dilute solution, whereas they emit yellow-green light as thin films. The diphenylanthrazoline molecules as the emissive layers in light-emitting diodes gave yellow light with a maximum brightness of 133 cd/m(2) and an external quantum efficiency of up to 0.07% in ambient air. Bilayer light-emitting diodes using compounds 1a-1e as the electron-transport layer and poly(2-methoxy-5-(2'-ethyl-hexyloxy)-1,4-phenylene vinylene) as the emissive layer had a maximum external efficiency of 3.1% and 2.0 lm/W and a brightness of up to 965 cd/m(2) in ambient air. These results represent enhancements of up to 50 times in external quantum efficiency and 17 times in brightness when using 1a-1e as the electron-transport materials in polymer light-emitting diodes. These results demonstrate that the new diphenylanthrazolines are promising n-type semiconductors for organic electronics.

Solution-processed anodes from layer-structure materials for high-efficiency polymer light-emitting diodes.

The development of low-cost, large-area electronic applications requires the deposition of active materials in simple and inexpensive techniques at room temperature, properties usually associated with polymer films. In this study, we demonstrate the integration of solution-processed inorganic films in light-emitting diodes. The layered transition metal dichalcogenide (LTMDC) films are deposited through Li intercalation and exfoliation in aqueous solution and partially oxidized in an oxygen plasma generator. The chemical composition and thickness of the LTMDC and corresponding transition metal oxide (TMO) films are investigated by X-ray photoelectron spectroscopy. The morphology and topography of the films are studied by atomic force microscopy. X-ray powder diffraction is used to determine the orientation of the LTMDC film. Finally, the LTMDC and their corresponding oxides are utilized as hole-injecting and electron-blocking materials in polymer light-emitting diodes with the general structure ITO/LTMDC/TMO/polyfluorene/Ca/Al. Efficient hole injection and electron blocking by the inorganic layers result in outstanding device performance and high efficiency.

Polybenzobisazoles Are Efficient Electron Transport Materials for Improving the Performance and Stability of Polymer Light-Emitting Diodes

A series of seven polybenzobisazoles were investigated as electron transport materials in arylene vinylene polymer-based electroluminescent devices. A large enhancement in performance and stability was observed in poly(p-phenylene vinylene) and poly(2-methoxy-5(2‘-ethyl-hexyloxy)-1,4-phenylene vinylene) light-emitting diodes by using polybenzobisthiazoles and poly(p-phenylene benzobisoxazole) as electron-transport materials. Devices using polybenzobisazole electron transport layers and aluminum cathodes had a turn-on voltage as low as 2.8 V, a luminance of up to 1400 cd/m2, and an external quantum efficiency of up to 2.5%. These polymer devices and their performances were stable under repeated testing over a period of 9−10 months storage in air. The superior performance of the polybenzobisazole thin films as electron-transport and hole-blocking materials in polymer light-emitting diodes is due to their high glass-transition temperature, environmental resistance, and photochemical/electrochemical stability...

Use of a Single Anionic Conductor as a Hole-Injecting Material for Polymer Light-Emitting Diodes

A single anionic conductor (SAC) was employed to improve the hole injection properties in an Al anode/SAC/poly[2-methoxy-5-(2′-(ethylhexyloxy)-1,4-phenylenevinylene) emissive layer/indium tin oxide cathode device. The SAC possesses an excellent hole-injection property, because of ionic space charge accumulation near the anode by the good chain segmental motion of the soft block in the polymer chains of the SAC. Thus, a balanced injection of an electron-hole pair can be achieved to greatly improve the quantum efficiency.