Double-layer Light-emitting Diodes Research Articles

Trifluoromethyl Substituted Derivatives of Pyrazoles as Materials for Photovoltaic and Electroluminescent Applications

New 6-CF3-1H-pyrazolo[3,4-b]quinolines with a methyl and/or phenyl group attached to the pyrazole core (Molx (x = 1, 2, 3, 4)) were synthesized and characterized in terms of their optoelectronic applications: photovoltaic and electroluminescence. The fluorescence emissions of the investigated phenyl-decorated pyrazoloquinolines is caused by the photoinduced charge transfer p process occurring between the phenyl substituent and the pyrazoloquinoline core, while 1,3-dimethyl-6-CF3-1H-pyrazolo[3,4-b]quinoline exhibits an π,π*-type emission. The number of phenyls and their substitution positions modulate both emission properties and HOMO energy levels. Next, the bulk heterojunction BHJ solar cells based on 1H-pyrazolo[3,4-b] quinoline derivatives with architecture ITO/PEDOT:PSS/PDT + Molx/Al were fabricated. The organic active layer was a blend of Molx and poly(3-decylthiophene-2,5-diyl). The complex refractive index and the layer thickness of the organic solar cells were determined using a spectroscopic ellipsometer Woollam M2000 (J.A. Woollam Co., Inc., Lincoln, NE, USA) and CompleteEASE software. For solar devices with the best value of power efficiency of approximately 0.38%, the thickness of the active layer (Mol3 + PDT) was 111 nm, with a short-circuit current density of JSC = 32.81 μA/cm2 and an open–circuit voltage of VOC = 0.78 V. Finally, we demonstrated double-layer light-emitting diodes with an organic active layer (Molx + PVK) and an electron transporting material layer, ETM (2-[3,5-bis(4-phenyl-2-quinolyl)phenyl]-4-phenylquinoline (Tris-Q). Bright bluish-green light originating from the active layer was observed in the double-layer device, ITO/PEDOT:PSS/active layer/ETM/Ca/A. The active layer was a mixture of PV-doped 1H-pyrazolo[3, 4-b]quinoline dyes. An OLED device was constructed by employing Molx as an emitter, which gave a deep bluish-green emission with the spectra range of 481–506 nm. The best value of the maximum brightness at approximately 1436.0 cd/m2 was achieved for a diode based on Mol3 (1-phenyl-3-phenyl-6-CF3-1H-pyrazolo[3,4-b]quinoline) and [R1 = Ph, R3 = Ph and R6 = CF3]. The current efficiency was up to 1.26 cd/A at 506 nm with a CIE of 0.007, 0.692.

The Relation between the Electrical, Chemical, and Morphological Properties of Indium−Tin Oxide Layers and Double-Layer Light-Emitting Diode Performance

The relation between the electrical, chemical, and morphological properties of indium−tin oxide (ITO) thin films and organic light-emitting diode (OLED) performance is studied. We report on chemical (HCl, piranha solutions), thermal (vacuum annealing), physical (oxygen plasma, UV ozone), and combined treatments on ITO layers. The effects of these different treatments have been studied using the four-point probe resistivity measurement method, contact angle measurement, X-ray diffraction, surface profilometry, and UV−vis−IR transmittance. Double-layer OLEDs with treated ITO as the anode and poly(9,9-dihexyl-9H-fluorene-2,7-diyl) and 8-hydroxyquinoline aluminum salt as the hole transporter and emitting material, respectively, have been realized. The electrical and optical properties of OLEDs have been extensively investigated, and it is shown that UV ozone−HCl combined treatment yields the highest hole injection efficiency and luminance and the lowest drive voltage. For each OLED with treated ITO, the anode potential barrier height decrease is estimated using Fowler−Nordheim and Schottky−Richardson modeling of the electrical conduction.

Enhanced electroluminescence in emissive polymer/CdSe double-layer films

We demonstrate that the phase aggregation process can be generally applied to the fabrication of polymer/CdSe double-layer light-emitting diodes (LEDs). The current and luminance characteristics of devices with aggregated CdSe nanoparticles are much better than those of devices with well-dispersed CdSe nanoparticles. It has been found that during the spinning process, aggregated nanoparticles are separated from the polymer and become attached to the polymer film. Aggregated nanoparticles between the emitting layer and the Al cathode can act as a buffer layer, enabling polymer/CdSe-based LEDs to exhibit better luminescence efficiency than that of neat polymer-based LED.

Synthesis and Color Tuning of Poly(p-phenylenevinylene) Containing Terphenyl Units for Light Emitting Diodes

New PPV based conjugated polymers, containing terphenyl units, were prepared as the electroluminescent (EL) layer in light-emitting diodes (LEDs). The prepared polymers, poly[2,5-bis(4-(2-etylhexyloxy)phenyl)-1,4-phenylenevinylenej (BEHP-PPV), poly[2-(2-ethylhexyloxy)-5-(4-(4-(2-etylhexyloxy)phenyl)phenyl)-1,4-phenylenevinylene] (EEPP-PPV) and poly[2-(2-ethylhexyloxy)-5-(9,9-bis(2-etylhexyl)fluorenyl)-1,4-phenylenevinylene] (EHF-PPV), were soluble in common organic solvents and used as the EL layer in double layer light-emitting diodes (LEDs) (ITO/PEDOT/polymer/Al). The polymers were prepared by the Gilch reaction. The number-average molecular weight (M n ), weight-average molecular weight (M w ), and the polydispersities (PDI) of these polymers were in the range of 9000-58000, 27000-231000, 2.9-3.9, respectively. These polymers have quite good thermal stability with decomposition starting above 320-350. The polymers show photoluminescence (PL) with maximum peaks at around 526-562 nm (exciting wavelength, 410 nm) and blue EL with maximum peaks at around λ m a x = 526-552 nm. The current-voltage-luminance (I-V-L) characteristics of polymers show turn-on voltages of 5 V. Even though both of EEPP-PPV and BEHP-PPV have the same terphenyl group in the repeating unit, EEPP-PPV with directly substituted alkoxy group in the back bone has longer effective conjugation length than BEHP-PPV, and exhibits red shift in the PL spectra. Both of EEPP-PPV and EHE-PPV have ter-phenyl units and directly substituted alkoxy group in back bone. EHF-PPV with fluorenyl unit attached to the PPV backbone has shorter effective conjugation length than EEPP-PPV with biphenyl unit, and exhibits blue shift in the PL spectra.

Syntheses and properties of electroluminescent polyfluorene-based conjugated polymers, containing oxadiazole and carbazole units as pendants, for LEDs

New polyfluorenes (PF)-based conjugated copolymers, containing oxadiazole and carbazole units as pendants, were prepared as the electroluminescent (EL) layer in light-emitting diodes (LEDs) to show that most of them have higher maximum brightness and EL efficiency as compared to poly(2,7-(9,9-bis(2-ethylhexyl)fluorene)) (PF2/6). The prepared polymers, poly[(9-(6-( N-carbazolyl)-hexyl)-9-hexyl)-fluorene-2,7-diyl]- co-[(9-hexyl-9-(6-(4-(5-phenyl-1,3,4-oxadiazolyl)-phenoxy)-hexyl)-fluorene-2,7-diyl)] (Oxd-PF- co-Cz-PF), were soluble in common organic solvents and used as the EL layer in double layer light-emitting diodes (LEDs) (ITO/PEDOT/polymer/Al). All polymers show photoluminescence around λ max=430 nm (exciting wavelength, 370 nm) and blue EL around λ max=426 nm. The current–voltage–luminance ( I– V– L) characteristics of the polymers show turn-on voltages of 3.5–5.5 V which are lower than that of PF2/6. The maximum brightness and EL efficiency of the device with the configuration of ITO/PEDOT/polymer/Al were 3000 cd/m 2 at 10 V and 2.13 cd/A at 10.6 mA/cm 2, respectively, which are all higher than those of PF2/6.

Blue polymer light-emitting diodes with organic/inorganic hybrid composite as hole transporting layer

Abstract Polymer double-layer light-emitting diodes were fabricated with inorganic/organic hybrid composite as hole transporting layer. Two conjugated polymers, poly(9,9-dibutyl)fluorene (PDF) and a dendritic polyfluorene derivative, poly((9,9-dibutyl-2,7-diiodo-9H-fluorene)trisphenylamine) (PDFA) were introduced as light-emitting layer. The former organic double-layer devices showed luminance as high as 833 lm/m2, and the power efficiency reached 0.065 lm/W. The latter showed maximum luminance of 1100 lm/m2 and the quantum efficiency reached 0.104 lm/W. The values were higher than those single-layer devices without inorganic/organic hybrid hole transporting layer (HHTL). The improvement in the electroluminescent properties of these devices was attributed to that the introduction of HHTL enhanced hole injection and balanced the combination of hole and electron injected from anode (ITO) and cathode (Mg:Ag).

Synthesis, optical and electrochemical properties of luminescent polymers containing 1,2-diphenylmaleimide and thiophene segments

In an attempt to balance energy barriers of hole and electron injection we prepared and characterized homopolymer containing electron-transporting 1,2-diphenylmaleimide chromophores ( P1) and copolymers consisting of 1,2-diphenylmaleimide and hole-transporting 2,5-thiophene moieties ( P2, P3) via dehalogenation polycondensation. The copolymers are amorphous materials with decomposition temperature greater than 450 °C. Absorption and fluorescence spectra were employed to investigate their optical properties both in solution and film state. Photoluminescence maxima of P1, P2 and P3 films are 564, 559 and 558 nm, respectively. The HOMO and LUMO energy levels have been estimated from their cyclic voltammograms. The HOMO levels of P1, P2, and P3 were readily raised with increasing thiophene content (−5.99, −5.59, and −5.43 eV, respectively), whereas their LUMO levels were very similar (−3.61 to −3.65 eV). Double-layer light-emitting diodes (Al/PEDOT:PSS/ P1– P3/ITO) were fabricated to evaluate their optoelectronic characteristics. Incorporation of thiophene units successfully reduced the turn-on electric field from 11.0×10 5 to 2.9×10 5 V/cm, but it decreased the luminescent efficiency and the maximum brightness.

Raman spectral changes of PEDOT–PSS in polymer light-emitting diodes upon operation

The micro-Raman spectra of a double-layer light-emitting diode fabricated with poly(3,4-ethylenedioxythiophene)-poly(4-styrenesulfonate) (PEDOT–PSS) as a hole-injection layer and poly(2,7-(9,9-dioctylfluorene)- alt-benzothiadiazole) (F8BT) blended with poly(9,9-dioctylfluorene) (PF8) as an emissive layer have been measured with excitation at 633 nm. After the operation of the device, the intensities of the bands due to PEDOT have increased. These increases in Raman intensity have been attributed to the reduction (dedoping) of PEDOT chains during operation on the basis of the results of electrochemical oxidation and reduction of a PEDOT–PSS film. The reduction of PEDOT chains is most likely one of the intrinsic factors of the degradation of polymer light-emitting diodes.

Novel Poly(p-phenylenevinylene)s Derivatives with CF3-Phenyl Substituent for Light-Emitting Diodes

New PPV derivatives which contain electron-withdrawing trifluoromethyl (CF 3 ) group, poly[2-(2-ethylhexyl-oxy)-5-(4-trifluoro methylphenyl)-1,4-phenylenevinylene] (EHCF 3 P-PPV), and poly[2-(2-ethylhexyloxy)-5-(3,5-bis(trifluoromethyl)-phenyl)-l,4-phenylenevinylene] (EHBCF 3 P-PPV), have been synthesized by GILCH polymerization. As the result of the introduction of the electron-withdrawing CF 3 group to the phenyl substituent, the LUMO and HOMO energy levels of EHCF 3 P-PPV (2.8, 5.1 eV) and EHBCF 3 P-PPV (3.0, 5.3 eV) were lower than those of known poly[2-(2-ethylhexyloxy)-5-phenyl-1,4-phenylenevinylene] (EHP-PPV) (2.6, 4.9 eV). These polymers have been used as the electroluminescent (EL) layers in double layer light-emitting diodes (LEDs) (ITO/PEDOT/polymer/Al). EHCF 3 P-PPV, and EHBCF 3 P-PPV show maximum photoluminescence (PL) peaks at λ m a x = 550, 539 nm, and maximum EL peak at λ m a x = 545, 540 nm, respectively. The current-voltage-luminance (I-V-L) characteristics of the polymers show that turn-on voltages of EHCF 3 P-PPV and EHBCF 3 P-PPV are around 4.0 and 3.5 V, respectively.

Synthesis and luminescence properties of novel phenyl-substituted poly( p-phenylene vinylene) derivatives

Three novel phenyl-substituted poly( p-phenylene vinylene) (PPV) derivatives, BDP-PPV, BDMP-PPV, and BDP-co-BDMP-PPV (1:1), have been successfully synthesized according to the Gilch route. The structures and properties of the monomers and the resulting polymers were characterized by using 1H, 13C NMR, elemental analysis, GPC, TGA, UV–vis absorption spectroscopy, photoluminescence (PL) and electroluminescence (EL) spectroscopy. The EL polymers possess excellent solubility in common solvents, good thermal stability with 5% weight loss temperature of more than 370 °C, and higher photostability. The number-average molecular weight ( M n) and polydispersity index (PDI) of the polymers were in the range of (1.52–2.53) × 10 5 and 1.22–1.36, respectively. Double-layer light-emitting diodes (LEDs) with the configuration of ITO/polymer/Alq 3/Al devices revealed that the copolymer, BDP-co-BDMP-PPV (1:1), possessed the highest luminescence properties, which exhibited maximum luminance of up to 3850 cd/m 2 at 23.5 V, and maximum luminescence efficiency of up to 2.68 cd/A.

Luminescent poly(1,4-phenylene vinylene-4,4′-biphenylene vinylene)s containing oligo(ethylene oxide) or hexyloxy side groups

Two soluble poly(1,4-phenylene vinylene-4,4′-biphenylene vinylene)s ( P1, P2) were synthesized through the Wittig condensation, in which P1 and P2 contain flexible oligo(ethylene oxide) and hexyloxy side groups, respectively. Furthermore, a blend of P1 with lithium perchlorate ( P1– LP) was obtained to investigate the effect of doping. The polymers show good solubility in common organic solvents and are thermally stable ( T d>315 °C). The optical properties of the polymers, both in solution and as cast films, were investigated by absorption and photoluminescence (PL) spectra, whereas the electrochemical characteristics were obtained using cyclic voltammetric measurement. The LUMO levels of P1, P2, and P1– LP are −3.18, −3.21 and −3.79 eV and the band-gaps are 1.95, 2.18, and 1.27 eV, respectively. Double-layer light-emitting diodes (ITO/PEDOT/Polymer/Al) were successfully fabricated to investigate their electroluminescent (EL) properties. The PL and EL spectral maxima of P1, P2, and P1– LP locate at 507–509 nm and 511–516 nm, indicating that they are yellowish-green emitting materials. The turn-on electric field was 4.2×10 5, 5.6×10 5, and 3.4×10 5 V/cm and maximum luminance was 175, 268, and 620 cd/m 2 for P1, P2, and P1– LP, respectively.

Synthesis and Luminescent Properties of a New Twisted Copolymer Containing Phenylenevinylene and Spirobifluorene

Abstract A new twisted polymer, poly[9,9′-spirobifluoren-2,2′-ylene-vinylene-alt-2,5-di(n-buthoxy)-1,4-phenylene-vinylene] (PSBF-PV), which consists of phenylenevinylene chromophores linked through 9,9′-spirobifuorene, was synthesized through the Horner–Emons condensation. PSBF-PV showed strong green emission in solution and film. The single-layer light-emitting diode (LED) exhibited weak emission, and an improved performance with the maximum external quantum efficiency of 0.25% and luminance maxima of 1168 cd/m2 was achieved using Alq3 as electron injection layer in double layer LED for PSBF-PV.

Novel Electroluminescent Polymers with Fluoro Groups in Vinylene Units

New electroluminescent polymers with fluoro groups in vinylene units, poly(p-phenylenedifluorovinylene) (PPDFV) and poly(2-dimethyloctylsilyl-p-phenylenedifluorovinylene) (DMOS−PPDFV), have been synthesized by GILCH polymerization. These polymers have been used as the electroluminescent (EL) layers in double-layer light-emitting diodes (LEDs) (ITO/PEDOT/polymer/Al). PPDFV shows photoluminescence around λmax = 580 nm (exciting wavelength, 410 nm) and yellow electroluminescence around λmax = 565 nm. DMOS−PPDFV shows PL around λmax = 495 nm and green EL around λmax = 540 nm. The current−voltage−luminance (I−V−L) characteristics of the polymers show turn-on voltages of 3.0 V approximately. Two fluoro groups were introduced on every vinylene units of poly(p-phenylenevinylene) (PPV) and poly(2-dimethyloctylsilyl-1,4-phenylenevinylene) (DMOS−PPV) to give PPDFV and DMOS−PPDFV in an attempt to increase the electron affinity of the parent polymer. It was found that the efficiency of PLED of DMOS−PPDFV was about 7 times higher than that of DMOS−PPV.

Color tuning and efficiency enhancement through interfacial energy transfer and thickness control in light-emitting diodes

We report a possible interfacial energy transfer phenomenon in double-layer light-emitting diodes (LEDs) consisting of a poly(p-phenylene vinylene) (PPV) hole-transport layer and an emitting polymer layer containing either of two blue emitting polymers (I and II). The polymers in film form have almost the same photoluminescence (PL) spectra though they have different band gaps as determined from their absorption and PL spectra. This is due to intermolecular∕intramolecular energy transfer in polymer II films since the latter structure contains two differing oligomeric chromophores regularly interspersed by long nonconjugated blocks. In double-layer LEDs, PPV functions not only as a hole-transport layer which improves hole injection but also as a chromophore participating in the tuning of the LED emission color through interfacial energy transfer. The significant spectral overlap between the absorption spectrum of PPV and the emission spectra of I or II supports this energy transfer. The presence of PPV can modify the LED color from blue to blue-green or green and also contributes to a high external quantum efficiency. Poly(3,4-ethylenedioxythiophene)∕poly(styrene sulfonate) was also used as a hole-transport layer and its LEDs were compared to PPV-containing devices. Thicker films of the conjugated polymer chromophores produced red shifts in both PL and electroluminescence (EL) spectra which was attributed to molecular packing and self-absorption effects. At higher applied voltage, these LEDs showed blue shifts, and the emission corresponding to a higher energy transition was enhanced at the expense of emissions from the lower energy transitions. Chromophore heating is also held responsible for some of the blueshifts in EL. The thicker LEDs showed lower efficiency because of self-absorption.

Synthesis and properties of new orange red light-emitting hyperbranched and linear polymers derived from 3,5-dicyano-2,4,6-tristyrylpyridine

Two new orange red light-emitting hyperbranched and linear polymers, poly(pyridine phenylene)s P1 and P2, were prepared by the Heck coupling reaction. In particular, an A2 + B3 approach was developed to synthesize conjugated hyperbranched polymer P2 via one-pot polycondensation. The polymers were characterized by NMR, Fourier transform infrared, ultraviolet–visible, and elemental analysis. They showed excellent solubility in common solvents such as tetrahydrofuran, CH2Cl2, CHCl3, and N,N-dimethylformamide and had high molecular weights (up to 6.1 × 105 and 5.8 × 105). Cyclic voltammetry studies revealed that P2 had a low-lying lowest unoccupied molecular orbital energy level of −3.22 eV and a highest occupied molecular orbital energy level of −5.43 eV. The thin film of P2 emitted strong orange-red photoluminescence at 595 nm. A double-layer light-emitting diode fabricated with the configuration of indium tin oxide/P2/tris(8-hydroxy-quinoline)aluminum/Al emitted orange-red light at 599 nm, with a brightness of 662 cd/m2 at 7 V and a turn-on voltage of 4.0 V; its external quantum efficiency was calculated to be 0.19% at 130.61 mA/cm2. This indicated that this new electroluminescent polymer (P2) based on 3,5-dicyano-2,4,6-tristyrylpyridine could possibly be used as an orange-red emitter in polymer light-emitting displays. © 2004 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 43: 493–504, 2005

New Series of Blue-Emitting and Electron-Transporting Copolymers Based on Cyanostilbene

A novel series of conjugated copolymers having fluorene and binaphthyl moieties in the main chain and based on cyanostilbene were synthesized in good yields by a palladium-catalyzed Suzuki coupling reaction, a new approach different from the traditional Knoevenagel condensation polymerization. Through controllable modification of the main chain structures, not only were the thermal, electronic, and optical properties of the polymers tuned, but also the structure−property relationships were studied. All these polymers possess excellent thermal stability with glass-transition temperatures of 60−159 °C and onset decomposition temperatures of 411−417 °C. Cyclic voltammetry studies reveal that these copolymers have low-lying LUMO energy levels ranging from −2.92 to −3.08 eV and low-lying HOMO energy levels ranging from −6.01 to −6.13 eV, and they may be promising candidates for electron-transporting or hole-blocking materials in light-emitting diodes (LEDs). The polymers in thin films emit strong blue luminance around 457−489 nm with narrow bandwidth upon photoexcitation. The single-layer LED fabricated with a copolymer F−CN composed of fluorene and cyanostilbene units using an air-stable aluminum electrode emits blue light with an external quantum efficiency of 0.006%. A double-layer LED, fabricated using a blend of poly(N-vinylcarbazole) and F−CN as emitting layer and tris(8-hydroxyquinolinato)aluminum as electron transporting layer, shows an external quantum efficiency of 0.2%. Preliminary electroluminescent results show that these polymers are new candidates for blue emissive materials in polymeric LEDs.

BLUE ELECTROLUMINESCENCE FROM A PROCESSABLE DERIVATIVE OF PPV BASED COPOLYMER WITH TRI(ETHYLENE OXIDE) SIGMENTS IN THE BACKBONE

A new light-emitting copolymer [poly(distyrylbenzene-block-tri(ethylene oxide)) (DSB-TEO)], containing alternating rigid, conjugated distyrylbenzene as light-emitting units and flexible tri(ethylene oxide) as ionic conductive units, was synthesized via Wittig reaction. The polymer has fairly good solubility in chloroform and excellent film-forming ability. Single-layer and double-layer light-emitting diodes with indium tin oxide anodes and calcium cathodes were fabricated, and the photoluminescence (PL) and electroluminescence (EL) properties were investigated. Single layer light emitting diode with DSB-TEO as the active layer(ITO/PEDOT:PSS/DSB-TEO/Ca) showed bright blue emission with luminance over 450 cd/m 2 and efficiency of 0.1 cd/A. Inserting a thin layer of Alq3 between the copolymer and the cathode improved the device performance, luminance over 1000 cd/m 2 and efficiency of 0.5 cd/A could be achieved. The enhanced emission in the longer wave part due to Alq 3 contribution is negligible. This polymer may also be a good candidate material for light emitting electrochemical cells (LECs).

Efficient emission from a europium complex containing dendron-substituted diketone ligands

A new europium (Eu) complex with dendron-substituted diketone ligands was synthesized and found to exhibit a photoluminescence efficiency of 45%. Double-layer light-emitting diodes based on polymer matrices doped with the Eu complex were fabricated. An electroluminescence external quantum efficiency of 0.80% was achieved when a copolymer containing side-chain carbazole and 1,3,4-oxadiazole groups was used as the matrix. The results are analyzed in the context of Förster energy transfer.

Comparison of models of electroluminescence in organic double-layer light-emitting diodes

A theoretical model of leakage and barrier-limited recombination of charge carriers at the interface separating two disordered organic materials in organic light-emitting diodes is formulated. Spatial disorder of molecular materials is reflected in the model by differentiating the hopping distances associated with jumps leading to recombination from those leading to leakage. The former and the latter are determined by the mean intersite distance and by the shortest hopping distance, respectively. It is shown that the effect of the difference between the two hopping distances on the current and recombination efficiencies depends on a barrier height and electric field strength at the organic–organic interface. The results of the models with barrier-limited recombination and Langevin recombination are compared. It is shown that both models yield comparable results for the steady-state recombination efficiency and current, if at least one of the interfacial energy barriers is small enough and the leakage is modeled in the same way. Thus broad applicability of the Langevin model is shown, when a suggested leakage model is used. The importance of microscopic correlations in the relative positions of electrons and holes at the organic–organic interface is discussed.

Effect of Cyano Substituents on Electron Affinity and Electron-Transporting Properties of Conjugated Polymers

A series of cyano-containing distyrylbenzenes were synthesized as the model compounds to systematically study the effect of cyano substituents on the redox behaviors of conjugated molecules. By introducing the electron-withdrawing functional groups (cyano and dicyanovinyl) onto the phenylene ring, both electron affinity and electrochemical stability of the resulting distyrylbenzenes are greatly enhanced. The results enabled us to design and synthesize a new class of highly electron affinitive, fluorene-based copolymers with these cyano-containing chromophores as comonomers. The effects of acceptor strength and side chain on electron-transporting properties of these polymers were also investigated. By properly adjusting copolymer compositions, a combined high electron affinity and transport was achieved in a statistic copolymer, poly(fluorenebenzothiadiazole−cyanophenylenevinylene) (PFB−CNPV). An external quantum efficiency up to 0.88% and brightness as high as 4730 cd/m2 were achieved in a double-layer light-emitting diode (LED) using PFB−CNPV as the emitting layer.