A review was presented to demonstrate a historical description of the synthesis of light-emitting conjugated polymers for applications in electroluminescent devices. Electroluminescence (EL) was first reported in poly(para-phenylene vinylene) (PPV) in 1990 and researchers continued to make significant efforts to develop conjugated materials as the active units in light-emitting devices (LED) to be used in display applications. Conjugated oligomers were used as luminescent materials and as models for conjugated polymers in the review. Oligomers were used to demonstrate a structure and property relationship to determine a key polymer property or to demonstrate a technique that was to be applied to polymers. The review focused on demonstrating the way polymer structures were made and the way their properties were controlled by intelligent and rational and synthetic design.

Synthesis of Light-Emitting Conjugated Polymers for Applications in Electroluminescent Devices

White organic light-emitting devices (WOLEDs) have advanced over the last twelve years to the extent that these devices are now being considered as efficient solid-state lighting sources. Initially, WOLEDs were targeted towards display applications for use primarily as liquid-crystal display backlights. Now, their power efficiencies have surpassed those of incandescent sources due to improvements in device architectures, synthesis of novel materials, and the incorporation of electrophosphorescent emitters. This review discusses the advantages and disadvantages of several WOLED architectures in terms of efficiency and color quality. Hindrances to their widespread acceptance as solid-state lighting sources are also noted.

White Organic Light‐Emitting Devices for Solid‐State Lighting

reported under AM1.5 (AM: air mass) illumination, this efficiency is not sufficient to meet realistic specifications for commercialization. The need to improve the light-to-electricity conversion efficiency requires the implementation of new materials and the exploration of new device architectures. Polymer-based photovoltaic cells are thin-film devices fabricated in the metal-insulator-metal configuration sketched in Figure 1a. The absorbing and charge-separating bulk-heterojunction layer with a thickness of approximately 100 nm is sandwiched between two charge-selective electrodes; a transparent bilayer electrode comprising poly(3,4-ethylenedioxylenethiophene):polystyrene sulfonic acid (PEDOT:PSS) on indium tin oxide (ITO) glass for collecting the holes and a lower-work-function metal (here, Al) for collecting the electrons. The work-function difference between the two electrodes provides a built-in potential that breaks the symmetry, thereby providing a driving force for the photogenerated electrons and holes toward their respective electrodes. Because of optical interference between the incident (from the ITO side) and back-reflected light, the intensity of the light is zero at the metallic (Al) electrode; Figure 1a shows a schematic representation of the spatial distribution of the squared optical electric-field strength. [9–11] Thus, a relatively large fraction of the active layer is in a dead-zone in which the photogeneration of carriers is significantly reduced. Moreover, this effect causes more electron–hole pairs to be produced near the ITO/PEDOT:PSS electrode, a distribution which is known to reduce the photovoltaic conversion efficiency. [12,13] This “optical interference effect” is especially important for thin-film structures where layer thicknesses are comparable to the absorption depth and the wavelength of the incident light, as is the case for photovoltaic cells fabricated from semiconducting polymers. In order to overcome these problems, one might simply increase the thickness of the active layer to absorb more light. Because of the low mobility of the charge carriers in the polymer:C60 composites, however, the increased internal resistance of thicker films will inevitably lead to a reduced fill factor. An alternative approach is to change the device architecture with the goal of spatially redistributing the light intensity inside the device by introducing an optical spacer between the active layer and the Al electrode as sketched in Figure 1a. [11] Although this revised architecture would appear to solve the problem, the prerequisites for an ideal optical spacer limit the choice of materials: the layer must be a good acceptor and an electron-transport material with a conduction band edge lower in energy than that of the lowest unoccupied molecular orbital (LUMO) of C60; the LUMO must be above (or close to) the Fermi energy of the collecting metal electrode; and it must be transparent to light with wavelengths within the solar spectrum.

New Architecture for High-Efficiency Polymer Photovoltaic Cells Using Solution-Based Titanium Oxide as an Optical Spacer

A comprehensive review of the literature on electron transport materials (ETMs) used to enhance the performance of organic light-emitting diodes (OLEDs) is presented. The structure−property−performance relationships of many classes of ETMs, both small-molecule- and polymer-based, that have been widely used to improve OLED performance through control of charge injection, transport, and recombination are highlighted. The molecular architecture, electronic structure (electron affinity and ionization potential), thin film processing, thermal stability, morphology, and electron mobility of diverse organic ETMs are discussed and related to their effectiveness in improving OLED performance (efficiency, brightness, and drive voltage). Some issues relating to the experimental procedures for the estimation of relevant material properties such as electron affinity and electron mobility are discussed. The design of multifunctional electroluminescent polymers whereby light emission and electron- and hole-transport pro...

Electron Transport Materials for Organic Light-Emitting Diodes

Polymers for flexible displays: From material selection to device applications

A new series of high brightness and luminance efficient poly(p-phenylenevinylene) (PPV)-based electroluminescent (EL) polymers, poly[2-{4-[5-(4-(3,7-dimethyloctyloxy)phenyl)-1,3,4-oxadiazole-2-yl]phenyloxy}-1,4-phenylenevinylene] (Oxa-PPV), poly[2-{2-((3,7-dimethyloctyl)oxy)phenoxy}-1,4-phenylenevinylene] (DMOP-PPV), and their corresponding random copolymers, poly{[2-{4-[5-(4-(3,7-dimethyloctyloxy)phenyl)-1,3,4-oxadiazole-2-yl]phenyloxy}-1,4-phenylenevinylene]-co-[2-{2-((3,7-dimethyloctyl)oxy) phenoxy}-1,4-phenylenevinylene]} (Oxa-PPV-co-DMOP-PPV), with an electron-deficient 1,3,4-oxadiazole unit on the side groups, were synthesized through the Gilch polymerization method. The newly designed and synthesized asymmetric molecular structures of Oxa-PPV, DMOP-PPV, and Oxa-PPV-co-DMOP-PPV were completely soluble in common organic solvents, and defect-free optical thin film was easily spin-coated onto the indium tin oxide (ITO) substrate. Oxa-PPV shows a high glass transition temperature (Tg), which might be an...

High-efficiency poly(p-phenylenevinylene)-based copolymers containing an oxadiazole pendant group for light-emitting diodes.

Flexible Organic Electroluminescent Devices Based on Fluorine-Containing Colorless Polyimide Substrates

Poly(fluorenevinylene) Derivative by Gilch Polymerization for Light-Emitting Diode Applications

9,9-Dialkylfluorenevinylene-based electroluminescence (EL) polymers, poly(9,9-di-n-octylfluorenyl-2,7-vinylene) poly(FV) and poly[(9,9-di-n-octylfluorenyl-2,7-vinylene)-co-{2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylenevinylene}] poly(FV-co-MEHPV), have been synthesized by the Gilch polymerization method. The structure and properties of those polymers were characterized using 1H, 13C NMR, UV−vis spectroscopy, elemental analysis, GPC, DSC, TGA, photoluminescence (PL), and EL spectroscopy. The 9,9-dialkylfluorenevinylene-based EL polymers were soluble in common organic solvents and easily spin-coated onto the indium−tin oxide (ITO)-coated glass substrates. The weight-average molecular weight (Mw) and polydispersity of poly(FV) and poly(FV-co-MEHPV) were in the range (22.2−43.2) × 104 and 1.9−3.0, respectively. Double-layer light-emitting displays (LEDs) with an ITO/PEDOT/polymer/Al configuration were fabricated, and the devices using poly(FV-co-MEHPV) showed better EL properties than those using pure poly(FV) ...

Synthesis and Electroluminescence Properties of Poly(9,9-di-n-octylfluorenyl-2,7-vinylene) Derivatives for Light-Emitting Display†

A new green electroluminescence polymer, CN-poly(dihexylfluorenevinylene) (CN-PDHFV), which denotes poly[(9,9-dihexyl-9H-fluorene-2,7-diyl)(1-cyanoethene-1,2-diyl)(9,9-dihexyl-9H-fluorene-2,7-diyl)(2-cyaoethene-1,2-diyl)], was synthesized by condensation polymerization utilizing the Knoevenagel reaction. The resulting polymer exhibits good solubility in common organic solvents such as chloroform, THF, and ODCB. The polymer is also easily cast on a glass plate to green film. The UV-vis spectrum of the polymer exhibits characteristically a broad absorption band at 440 nm. This polymer shows photoluminescence around λ max = 535 nm (exciting wavelength 410 nm) and green electroluminescence around λ max = 530 nm. The current-voltage-luminance (I-V-L) characteristics of the polymer show a turn-on voltage of 4.8 V and a brightness of 600 cd/m 2 at 5.8 V in the Al/polymer/PEDOT/ITO device. The highest efficiency was observed to be 0.85 lm/W at 5.6 V.

Kwanghee Lee

Papers

High-efficiency poly(p-phenylenevinylene)-based copolymers containing an oxadiazole pendant group for light-emitting diodes.

Flexible Organic Electroluminescent Devices Based on Fluorine-Containing Colorless Polyimide Substrates

Poly(fluorenevinylene) Derivative by Gilch Polymerization for Light-Emitting Diode Applications

Synthesis and Electroluminescence Properties of Poly(9,9-di-n-octylfluorenyl-2,7-vinylene) Derivatives for Light-Emitting Display†

Design, Synthesis, and Electroluminescent Property of CN-Poly(dihexylfluorenevinylene) for LEDs