Organic light‐emitting diodes with multiple photocrosslinkable hole‐transport layers

We report on photocrosslinkable hole‐transport polymers and their use as photodefinable hole‐transport layers in organic light‐emitting diodes. The polymers were obtained by copolymerization of bis(diarylamino)biphenyl‐based acrylate monomers with cinnamate‐functionalized acrylate moieties. Polymers with a range of redox potentials were obtained by varying the substitution patterns of the bis(diarylamino)biphenyl units. The 2 + 2 cycloaddition of the cinnamate moieties following UV irradiation renders the material insoluble. This allows for patterning of the polymer and simultaneously enables the fabrication of multilayer structures from solution. Hole mobilities were measured in these copolymers with the time‐of‐flight technique. Their performance as hole‐transport layers in light‐emitting diodes, with tris(8‐hydroxyquinolinato)aluminum as the emitter and electron‐transport layer, is evaluated. Electroluminescent devices with multiple hole‐transport layers having different ionization potentials were fabricated from solution, and the quantum efficiency of these devices was greater than that for devices based on a single hole‐transport layer. © 2003 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 41: 2726–2732, 2003     

Cascade hole transport in efficient green phosphorescent light‐emitting devices achieved by layer‐by‐layer solution deposition using photocrosslinkable‐conjugated polymers containing oxetane side‐chain moieties

A hole‐injection/transport bilayer structure on an indium tin oxide (ITO) layer was fabricated using two photocrosslinkable polymers with different molecular energy levels. Two photoreactive polymers were synthesized using 2,7‐(or 3,6‐)‐dibromo‐9‐(6‐((3‐methyloxetan‐3‐yl)methoxy)hexyl)‐9H‐carbazole) and 2,4‐dimethyl‐N,N‐bis(4‐ (4,4,5,5‐tetramethyl‐1,3,2‐dioxaborolan‐2‐yl)phenyl)aniline via a Suzuki coupling reaction. When the oxetane groups were photopolymerized in the presence of a cationic photoinitiator, the photocured film showed good solvent resistance and compatibility with a poly(N‐vinylcarbazole) (PVK)‐based emitting layer. Without the use of a conventional hole injection layer (HIL) of poly(3,4‐ethylenedioxythiophene)/(polystyrenesulfonate) (PEDOT:PSS), the resulting green light‐emitting device bearing PVK: 5‐4‐tert‐butylphenyl‐1,3,4‐oxadiazole (PBD):Ir(Cz‐ppy)3 exhibited a maximum external quantum efficiency of 9.69%; this corresponds to a luminous efficiency of 29.57 cd/A for the device K‐4 configuration ITO/POx‐I/POx‐II/PVK:PBD:Ir(Cz‐ppy)3/triazole/Alq3/LiF/Al. These values are much higher than those of PLEDs using conventional PEDOT:PSS as a single HIL. The significant improvement in device efficiency is the result of suppression of the hole injection/transport properties through double‐layered photocrosslinked‐conjugated polymers. © 2011 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2012     

Synthesis and characterization of thieno[3,2‐b]thiophene‐isoindigo‐based copolymers as electron donor and hole transport materials for bulk‐heterojunction polymer solar cells

A novel class of thieno[3,2‐b]thiophene (TT) and isoindigo based copolymers were synthesized and evaluated as electron donor and hole transport materials in bulk‐heterojunction polymer solar cells (BHJ PSCs). These π‐conjugated donor‐acceptor polymers were derived from fused TT and isoindigo structures bridged by thiophene units. The band‐gaps and the highest occupied molecular orbital (HOMO) levels of the polymers were tuned using different conjugating lengths of thiophene units on the main chains, providing band‐gaps from 1.55 to 1.91 eV and HOMO levels from ?5.34 to ?5.71 eV, respectively. The corresponding lowest unoccupied molecular orbital (LUMO) levels were appropriately adjusted with the isoindigo units. Conventional BHJ PSCs (ITO/PEDOT:PSS/active layer/interlayer/Al) with an active layer composed of the polymer and PC₇₁BM were fabricated for evaluation. Power conversion efficiency from a low of 1.25% to a high of 4.69% were achieved with the best performing device provided by the D?π?A polymer with a relatively board absorption spectrum, high absorption coefficient, and more uniform blend morphology. These results demonstrate the potential of this class of thieno[3,2‐b]thiophene‐isoindigo‐based polymers as efficient electron donor and hole transport polymers for BHJ PSCs. © 2012 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2013     

Silole‐containing polymers for high‐efficiency polymer solar cells

Silole‐containing conjugated polymers ( P1 and P2 ) carrying methyl and octyl substituents, respectively, on the silicon atom were synthesized by Suzuki polycondensation. They show strong absorption in the region of 300–700 nm with a band gap of about 1.9 eV. The two silole‐containing conjugated polymers were used to fabricate polymer solar cells by blending with PC₆₁BM and PC₇₁BM as the active layer. The best performance of photovoltaic devices based on P1 /PC₇₁BM active layer exhibited power conversion efficiency (PCE) of 2.72%, whereas that of the photovoltaic cells fabricated with P2 /PC₇₁BM exhibited PCE of 5.08%. 1,8‐Diiodooctane was used as an additive to adjust the morphology of the active layer during the device optimization. PCE of devices based on P2 /PC₇₁BM was further improved to 6.05% when a TiO_x layer was used as a hole‐blocking layer. © 2011 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2011     

New blue electroluminescent n‐type polyfluorene copolymers with a 1,3,4‐oxadiazole unit in the main chain

Novel polyfluorene copolymers alternately having an 1,3,4‐oxadiazole unit in the main chain were prepared by both one‐step and two‐step methods for polyoxadiazole synthesis. They displayed highly efficient blue photoluminescence, the properties of which were affected by the extent of conjugation and the changes in the electron density by a side chain. An electrochemical analysis of the polymers using cyclic voltammetry suggested that they could be used as electron‐transport/hole‐blocking materials as well as blue emission materials for polymer light‐emitting diodes. A simple double‐layer device consisting of poly(N‐vinylcarbazole) as a hole‐transport layer and poly[(9,9′‐didodecylfluorene‐2,7‐diyl)‐alt‐((1,4‐bis(1,3,4‐oxadiazole)‐2,5‐di(2‐ethylhexyloxy)phenylene)‐5,5′‐diyl)] as an emission layer exhibited narrow blue electroluminescence with a maximum at 430 nm. © 2004 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 42: 1058–1068, 2004     

Electroluminescent properties of a partially‐conjugated hyperbranched poly(p‐phenylene vinylene)

In this paper, the electroluminescent properties of a new partially‐conjugated hyperbranched poly (p‐phenylene vinylene) (HPPV) were studied. The single layer light‐emitting device with HPPV as the emitting layer emits blue‐green light at 496 nm, with a luminance of 160 cd/m² at 9 V, a turn‐on voltage of 4.3 V and an electroluminescent efficiency of 0.028 cd/A. By doping an electron‐transport material [2‐(4‐biphenylyl)‐5‐phenyl‐1,3,4‐oxadiazole, PBD] into the emitting layer and inserting a thin layer of tris(8‐hydroxy‐quinoline)aluminum (Alq₃) as electron transporting/hole blocking layer for the devices, the electroluminescent efficiency of 1.42 cd/A and luminance of 1700 cd/m² were achieved. The results demonstrate that the devices with the hyperbranched polymers as emitting material can achieve high efficiency through optimization of device structures. Copyright © 2006 John Wiley & Sons, Ltd.     

Synthesis and properties of nitrogen‐linked poly(2,7‐carbazole)s as hole‐transport material for organic light emitting diodes

A novel class of carbazole polymers, nitrogen‐linked poly(2,7‐carbazole)s, was synthesized by polycondensation between two bifunctional monomers using the palladium‐catalyzed amination reaction. The polymers were characterized by ¹H NMR, Infrared, Gel permeation chromatography, and MALDI‐TOF MS and it was revealed that the combination of the monomer structures is important for producing high molecular weight polymers. Thermal analysis indicated a good thermal stability with high glass transition temperatures, e.g., 138 °C for the higher molecular weight polymer P2 . To pursue the application possibilities of these polymers, their optical properties and energy levels were investigated by UV‐Vis absorption and fluorescence spectra as well as their electrochemical characteristics. Although the blue light emission was indeed observed for all polymers in solution, the quantum yields were very low and the solid films were not fluorescent. On the other hand, the HOMO levels of the polymers estimated from the onset potentials for the first oxidation in the solid thin films were relatively high in the range of ?5.12 to ?5.20 eV. Therefore, light emitting diodes employing these polymers as a hole‐transport layer and iridium(III) complex as a triplet emitter were fabricated. The device of the nitrogen‐linked poly(2,7‐carbazole) P3 with p,p′‐biphenyl spacer, which has a higher HOMO level and a higher molecular weight, showed a much better performance than the device of P2 with m‐phenylene spacer. © 2009 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 47: 3880–3891, 2009     

Synthesis and characterization of novel poly(p‐phenylenevinylene) derivatives containing phenothiazine‐5‐oxide and phenothiazine‐5, 5‐dioxide moieties

PPV‐based copolymers containing phenothiazine‐5‐oxide and phenothiazine‐5, 5‐dioxide moieties have been successfully synthesized by Wittig‐Horner reaction and characterized by means of UV‐vis, photoluminescence, electroluminescence spectra, and cyclic voltammetry. All of these copolymers can be dissolved in common organic solvents such as chloroform, tetrahydrofuran, and toluene. The PL maxima in the film state are located at 582, 556, and 552 nm for P1, P2, and P3, respectively. The HOMO and LUMO levels of P2 are found to be ?5.21 and ?2.68 eV, respectively; whereas those of P3 are found to be ?5.26 and ?2.71 eV, respectively. The cyclic voltammetry result indicates that the conversion of electron‐donating sulfide to electron‐withdrawing sulfoxide or sulfone group in polymers plays a dominating role in increasing its oxidation potential. Yellowish‐green light ranging from 568 to 540 nm was observed for the single layer device with the configuration of ITO/Polymer/Ca/Al. Double layer devices with Zn (BTZ)₂ as a hole blocking layer exhibited enhanced EL performance compared to the single layer devices. The maximum brightness of the double layer devices of P1, P2, and P3 is 278, 400, and 796 cd/m², respectively. The results of EL and electrochemical analyses revealed that they are promising candidate materials for organic, light‐emitting diodes with hole‐transporting ability. © 2007 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 45: 4291–4299, 2007     

Acyclic,Linear Oligo‐meta‐phenylenes as Multipotent Base Materials for Highly Efficient Single‐layer Organic Light‐emitting Devices

Oligo‐meta‐phenylenes have been designed and synthesized as multipotent base materials of single‐layer organic light‐emitting devices. Simple molecular structures of oligo‐meta‐phenylenes composed of linear phenylene arrays benefited from the wealth of modern reactions available for biaryl couplings and were concisely synthesized in a series. Structure‐performance relationship studies with the first seven congeners revealed key features important for the multipotent materials in single‐layer devices. As a result, highly efficient phosphorescent electroluminescence was made possible in a highly simplified device architecture comprising one‐region, single‐layer configurations. Detailed investigations with hole‐only devices disclosed that the hole mobility was effectively retarded by potent materials, which should facilitate hole/electron recombination for electroluminescence.     

Synthesis of novel nitrogen‐ and sulfur‐containing conjugated polymers used as hole‐transporting materials for organic light‐emitting diodes

Two series of highly soluble novel nitrogen‐ and sulfur‐containing conjugated polymers were synthesized via an acid‐induced self‐polycondensation of functional monomers with methyl sulfinyl and aromatic groups. The well‐defined structures of synthesized polymers were confirmed by their NMR and IR spectra. The highest occupied molecular orbital energy values for these materials, estimated by cyclic voltammetry, showed a broad range of values from about 5.0 to 5.2 eV used as hole‐transport layers (HTL) in two‐layer light‐emitting diodes ITO/HTL/Alq₃/Mg:Ag [ITO = indium tin oxide, and Alq₃ = tris(8‐quinolinato) aluminum]. The typical turn‐on voltage of these diodes was about 4–5 V. The maximum brightness of the device was about 3440 cd/m² at 20 V. The maximum efficiency was estimated to be 0.15 lm/W at 10 V. © 2002 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 40: 1321–1333, 2002     

Synthesis of bipolar charge transporting block copolymers and characterization for organic light‐emitting diode

A series of hole and electron transporting random and block copolymers consisting of triphenylamine moiety as a hole transporting unit and oxadiazole moiety as an electron transporting unit have been prepared via a nitroxide mediated radical polymerization. Oxadiazole monomers with t‐butyl or trifluoromethyl groups, 2 and 7, respectively, were used for copolymerization. Photoluminescent measurements of polymers revealed that the formation of the exciplex between triphenylamine and oxadiazole units tends to occur in the order of random copolymers, block copolymers, and polymer blends, implying phase‐separated morphologies in block or blend systems. The polymers were applied for OLED devices, and we found that the morphology in the polymer layer critically affected device performance. The block copolymer comprising hole and electron transporting units with the composition of 14/86 showed the highest external quantum efficiency over 10%. © 2010 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 48: 1461–1468, 2010     

Thermally crosslinkable hole‐transporting poly(fluorene‐co‐triphenylamine) for multilayer polymer light‐emitting diodes

This article reports the synthesis and characterization of a novel thermally crosslinkable hole‐transporting poly (fluorene‐co‐triphenylamine) (PFO‐TPA) by Suzuki coupling reaction, followed with its application in the fabrication of multilayer light‐emitting diodes by wet processes. The thermal, photophysical, and electrochemical properties of PFO‐TPA were investigated by differential scanning calorimeter, thermogravimetric analysis, optical spectroscopy, and cyclic voltammetry, respectively. Thermally crosslinked PFO‐TPA, through pendant styryl groups, demonstrates excellent thermal stability (T_d > 400 °C, T_g = 152 °C), solvent resistance, and film homogeneity. Its highest occupied molecular orbital level (?5.30 eV) lies between those of PEDOT:PSS (?5.0 ～ ?5.2 eV) and poly(9,9‐dioctylfluorene) (PFO: ?5.70 eV), forming a stepwise energy ladder to facilitate hole injection. Multilayer device with crosslinked PFO‐TPA as hole‐injection layer (HIL) (ITO/PEDOT:PSS/HIL/PFO/LiF/Ca/Al) was readily fabricated by successive spin‐coating processes, its maximum luminance efficiency (3.16 cd/A) were about six times higher than those without PFO‐TPA layer (0.50 cd/A). The result of hole‐only device also confirmed hole‐injection and hole‐transport abilities of crosslinked PFO‐TPA layer. Consequently, the device performance enhancement is attributed to more balanced charges injection in the presence of crosslinked PFO‐TPA layer. The thermally crosslinkable PFO‐TPA is a promising material for the fabrication of efficient multilayer polymer light‐emitting diodes because it is not only a hole‐transporting polymer but also thermally crosslinkable. © 2010 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2011     

Preparation of few‐layer two‐dimensional polymers by self‐assembly of bola‐amphiphilic small molecules

Two bola‐amphiphilic small molecules, based on the diphenylanthracene skeleton structure, namely, BASM‐1 and its functionalized small molecule BASM‐2 , were designed and synthesized. The self‐assembly behavior and mechanism of these two molecules in aqueous solution were studied. The supramolecular two‐dimensional (2D) layer and the covalent 2D polymers were, respectively, prepared by these two molecules. What is more, the transverse size of the covalent 2D polymer laminates increased with the extension of the polymerization time. Atomic force microscopy results showed that both free‐standing single‐layer 2D polymers and few layer laminates with two to three molecular layers were obtained. So our work provides a simple and efficient method for directly preparing independent both supramolecular 2D polymers and covalent 2D polymers in liquid phase which is of great significance. © 2019 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2019, 57, 1748–1755     

Optoelectronic Properties of Dicyanofluorene‐Based n‐Type Polymers

Three new donor–acceptor‐type copolymers ( P1 , P2 , P3 ) consisting of dicyanofluorene as acceptor and various donor moieties were designed and synthesized. Optoelectronic properties were studied in detail by means of UV‐visible absorption and fluorescence spectroscopy, cyclic voltammetry, space‐charge‐limited current (SCLC), flash‐photolysis time‐resolved microwave conductivity (FP‐TRMC), and density functional theory (DFT). All polymers showed strong absorption in the UV‐visible region and the absorption maximum undergoes redshift with an increasing number of thiophene units in the polymer backbone. SCLC analysis showed that the electron mobilities of the polymers in the bulk state were 1 to 2 orders higher than that of the corresponding hole mobilities, which indicated the n‐type nature of the materials. By using FP‐TRMC, the intrapolymer charge‐carrier mobility was assessed and compared with the interpolymer mobility obtained by SCLC. The polymers exhibited good electron‐accepting properties sufficiently high enough to oxidize the excited states of regioregular poly(3‐hexylthiophene) (P3HT (donor)), as evident from the FP‐TRMC analysis. The P3 polymer exhibited the highest FP‐TRMC transients in the pristine form as well as when blended with P3HT. Use of these polymers as n‐type materials in all‐polymer organic solar cells was also explored in combination with P3HT. In accordance with the TRMC results, P3 exhibited superior electron‐transport and photovoltaic properties to the other two polymers, which is explained by the distribution of the energy levels of the polymers by using DFT calculations.     

Synthesis and opto‐electrical properties of dendron‐containing poly(2,3‐diphenyl‐1,4‐phenylenevinylene) derivatives

A new series of poly(2,3‐diphenyl‐1,4‐phenylenevinylene) derivatives containing dendritic side groups were synthesized. Different generations of dendrons were integrated on the pendant phenyl ring to investigate their effect on optical and electrical properties of final polymers. Homopolymers can not be obtained via the Gilch polymerization because of sterically bulky dendrons. By controlling the feed ratio of different monomers during polymerization, dendron‐containing copolymers with high molecular weights were obtained. The UV–vis absorption and photoluminescent spectra of the thin films are pretty close; however, quantum efficiency is significantly enhanced with increasing the generation of dendrons. The electrochemical analysis reveals that hole‐injection is also improved by increasing dendritic generation. Double‐layer light‐emitting devices with the configuration of ITO/PEDOT:PSS/polymer/Ca/Al were fabricated. High generation dendrons bring benefit of improved device performance. © 2007 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 45: 3440–3450, 2007     

Conjugated polymers containing electron‐transporting,hole‐transporting,and light‐emitting units in the polymer main chain

Conjugated polymers containing electron‐transporting, hole‐transporting, and blue light‐emitting units were synthesized by Suzuki polycondensation. These copolymers exhibited excellent thermal and optical stability. Optical investigation indicated that the incorporation of the spirobifluorene units in the polymer main chain could markedly increase the effective conjugation length of polymers. Electrochemical studies showed that the incorporation of spirobifluorene unit could raise the electrochemical stability and improve the electron‐ and hole‐injecting abilities. The electroluminescent results also showed that the introducing of spirobifluorene units could significantly improve the device performance. © 2008 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 46: 1349–1356, 2008     

Diketopyrrolopyrrole‐based Conjugated Polymers Bearing Branched Oligo(Ethylene Glycol) Side Chains for Photovoltaic Devices

Conjugated polymers are essential for solution‐processable organic opto‐electronic devices. In contrast to the great efforts on developing new conjugated polymer backbones, research on developing side chains is rare. Herein, we report branched oligo(ethylene glycol) (OEG) as side chains of conjugated polymers. Compared with typical alkyl side chains, branched OEG side chains endowed the resulting conjugated polymers with a smaller π‐π stacking distance, higher hole mobility, smaller optical band gap, higher dielectric constant, and larger surface energy. Moreover, the conjugated polymers with branched OEG side chains exhibited outstanding photovoltaic performance in polymer solar cells. A power conversion efficiency of 5.37 % with near‐infrared photoresponse was demonstrated and the device performance could be insensitive to the active layer thickness.     

Reversible addition‐fragmentation chain transfer polymerization of methacrylates containing hole‐ or electron‐transporting groups

The controlled/living radical polymerization of 2‐(N‐carbazolyl)ethyl methacrylate (CzEMA) and 4‐(5‐(4‐tert‐butylphenyl‐1,3,4‐oxadiazol‐2‐yl)phenyl) methacrylate (t‐Bu‐OxaMA) via reversible addition‐fragmentation chain transfer polymerization has been studied. Functional polymers with hole‐ or electron‐transfer ability were synthesized with cumyl dithiobenzoate as a chain transfer agent (CTA) and AIBN as an initiator in a benzene solution. Good control of the polymerization was confirmed by the linear increase in the molecular weight (MW) with the conversion. The dependence of MW and polydispersity index (PDI) of the resulting polymers on the molar ratio of monomer to CTA, monomer concentration, and molar ratio of CTA to initiator has also been investigated. The MW and PDI of the resulting polymers were well controlled as being revealed by GPC measurements. The resulting polymers were further characterized by NMR, UV‐vis spectroscopy, and cyclic voltammetry. The polymers functionalized with carbazole group or 1,3,4‐oxadiazole group exhibited good thermal stability, with an onset decomposition temperature of about 305 and 323 °C, respectively, as determined by thermogravimetric analysis. © 2006 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 45: 242–252, 2007     

Donor–acceptor photovoltaic polymers based on 1,4‐dithienyl‐2,5‐dialkoxybenzene with intramolecular noncovalent interactions

Donor–acceptor (D–A) conjugated polymers bearing non‐covalent configurationally locked backbones have a high potential to be good photovoltaic materials. Since 1,4‐dithienyl‐2,5‐dialkoxybenzene ( TBT ) is a typical moiety possessing intramolecular S…O interactions and thus a restricted planar configuration, it was used in this work as an electron‐donating unit to combine with the following electron‐accepting units: 3‐fluorothieno[3,4‐b]thiophene ( TFT ), thieno‐[3,4‐c]pyrrole‐4,6‐dione ( TPD ), and diketopyrrolopyrrole ( DPP ) for the construction of such D–A conjugated polymers. Therefore, the so‐designed three polymers, PTBTTFT , PTBTTPD , and PTBTDPP , were synthesized and investigated on their basic optoelectronic properties in detail. Moreover, using [6,6]‐phenyl‐C₇₁‐butyric acid methyl ester (PC₇₁BM) as acceptor material, polymer solar cells (PSCs) were fabricated for studying photovoltaic performances of these polymers. It was found that the optimized PTBTTPD cell gave the best performance with a power conversion efficiency (PCE) of 4.49%, while that of PTBTTFT displayed the poorest one (PCE = 1.96%). The good photovoltaic behaviors of PTBTTPD come from its lowest‐lying energy level of the highest occupied molecular orbital (HOMO) among the three polymers, and good hole mobility and favorable morphology for its PC₇₁BM‐blended film. Although PTBTDPP displayed the widest absorption spectrum, the largest hole mobility, and regular chain packing structure when blended with PC₇₁BM, its unmatched HOMO energy level and disfavored blend film morphology finally limited its solar cell performance to a moderate level (PCE: 3.91%). © 2018 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2018 , 56, 689–698     

Blue photo‐ and electroluminescence based on poly(2‐benzoyl‐1,4‐phenylene) and poly(2,5‐dibenzoyl‐1,4‐phenylene)

Poly(2‐benzoyl‐1,4‐phenylene) (PBP) and poly(2,5‐dibenzoyl‐1,4‐phenylene) (PDBP) were synthesized by coupling polymerization using a Ni catalyst of 2,5‐dichlorobenzophenone and 2,5‐dichloro‐1,4‐dibenzophenone, respectively. These polymers are soluble in common organic solvents, have high thermal stability and show bright blue photoluminescence. Light‐emitting diodes fabricated with these polymers as the active layer emit blue electroluminescence with wavelength at 433 nm for PBP and 475 nm for PDBP. Copyright © 1999 John Wiley & Sons, Ltd.