Solution-processed Polymer Light-emitting Diodes Research Articles

Molecular design strategy for through-space charge transfer blue polyimides with rigid non-conjugated backbone and the role of alicyclic imide linker

The exploration of blue polymeric emitters plays an important role in the application of solution-processed polymer light-emitting diodes (PLEDs) in full color display. But the development of highly efficient blue thermally activated delayed fluorescence (TADF) polymers is still extremely challenging. Here, a series of rigid non-conjugated polyimide (PI)-based blue thermally activated delayed fluorescence (TADF) polymers with through-space charge transfer effect were firstly reported based on our novel design strategy. Among them, TRZ-oC-ph (λem = 497 nm) with a typical face-to-face structure as TADF unit was introduced into a polyimide backbone using twisted alicyclic imide as Linker unit. The twisted Linker unit with different structures can effectively regulate the d-spacing between donor and acceptor in the TADF unit, thus obtaining a series of blue TADF polyimides with λem at 485 nm, and high thermal stability (Tg > 277 °C, Td > 477 °C). Remarkably, highly-efficient solution-processing blue polymer light emitting diodes (PLEDs) based on the above polyimides were successfully assembled, with a maximal external quantum efficiency of 23.2%. Such outstanding efficiency is amongst the state-of-the-art performance of non-conjugated PLEDs. It confirms the effectiveness of the structural design strategy, and provides useful and valuable guidance for the development of highly-efficient blue TADF polymer materials and PLEDs.

Metal Oxide Films as Charge Transport Layers for Solution-Processed Polymer Light-Emitting Diodes

Here, we discuss the use of metal oxide films as charge transport layers in polymer light-emitting diodes containing poly(9,9-dioctyl-9H-fluorene-2,7-diyl) (PFO) as emissive layer. A simple device architecture consisting of glass-ITO | poly(3,4-ethylenedioxythiophene)-poly(styrenesulfonate) (PEDOT:PSS) | poly(9-vinylcarbazole) (PVK) | PFO | Ca | Al was used as starting point. This device assembly allows the investigation of basic properties of polymeric emitting layers, but does not provide stable devices with high performances. Thus, a more complex, multilayered diode structure is needed. We pursuit that goal with focus on the use of low-cost, easily processed materials. At one side, solution-processed non-stoichiometric MoOx films were used to replace the PEDOT:PSS as hole transport layer. At the other interface, solution-processed ZnO films containing either the bare oxide or a ZnO/carbon dots composite were introduced as electron transport layer. By tunning the characteristics of the metal oxide films, the performance of the blue-emitting PFO-based polymer light-emitting diodes (PLEDs) was massively enhanced.

Controlling Molecular Conformation for Highly Efficient and Stable Deep-Blue Copolymer Light-Emitting Diodes.

We report a novel approach to achievedeep-blue, high-efficiency, and long-lived solution-processed polymer light-emitting diodes (PLEDs) via a simple molecular level conformation change of an emissive conjugated polymer.We introduce rigid β-phase segments into a 95% fluorene-5% arylamine copolymer emissive layer. The arylamine moieties at low density act as efficient exciton formation sites in PLEDs, whereas the conformational change alters the nature of the dominant luminescence from a broad, charge transfer like emission to a significantly blue-shifted and highly vibronically structured excitonic emission. As a consequence, we observe a significant improvement in the Commission International de L'Eclairage ( x, y) coordinates from (0.149, 0.175) to (0.145, 0.123) while maintaining high efficiency and improved stability. We achieve a peak luminous efficiency, η = 3.60 cd/A, and a luminous power efficiency, ηw = 2.44 lm/W, values that represent state-of-the-art performance for single copolymer deep-blue PLEDs. These values are 5-fold better than for otherwise-equivalent, β-phase poly(9,9-dioctylfluorene) PLEDs (0.70 cd/A and 0.38 lm/W). This report represents the first demonstration of the use of molecular conformation as a simple but effective method to control the optoelectronic properties of a fluorene copolymer; previous examples have been confined to homopolymers.

Crosslinkable triphenylamine-based hole-transporting polymers for solution-processed polymer light-emitting diodes

Abstract We report the synthesis and characterization of a thermally crosslinkable hole-transporting poly(indenofluorene-co-triphenylamine) copolymer (X-IFTPA) containing a vinyl-functionalized triphenylamine moiety. This copolymer exhibited an exothermal peak at 125 °C in the first heating/cooling cycle of a differential scanning calorimetry study, indicating that crosslinking occurred. The highest occupied molecular orbital energy level of this copolymer was estimated to be −5.30 eV, as determined by cyclic voltammetry measurement. Flat and uniform films were formed by spin casting X-IFTPA from chlorobenzene solution. Crosslinking can be finished by thermally annealing the film at 150 °C for 24 min, which can be confirmed by reflectance Fourier transform infrared spectroscopy and UV–Vis absorption spectra. Multi-layer polymer light-emitting diodes (PLEDs) incorporating X-IFTPA as the hole transport layer and a green-light-emitting copolymer as the emissive layer exhibited enhanced luminous efficiency relative to that of a control device without a hole transport layer. Thus, the developed cross-linkable copolymer X-IFTPA is a promising material for the construction of efficient PLEDs.

Hydroxyethyl cellulose doped with copper(II) phthalocyanine-tetrasulfonic acid tetrasodium salt as an effective dual functional hole-blocking layer for polymer light-emitting diodes

We report a doping method to improve the performance of solution-processed polymer light-emitting diodes (PLEDs). Doping 12 wt% copper(II) phthalocyanine-tetrasulfonated acid tetrasodium salt (TS-CuPc) into hydroxyethyl cellulose (HEC) as a dual functional hole-blocking layer (df-HBL) of multilayer PLED (glass/ITO/PEDOT:PSS/HY-PPV/TS-CuPc-doped HEC/LiF/Al) significantly enhanced maximum luminance, maximum current and power efficiency over that without the df-HBL (10,319 cd/m2, 2.98 cd/A and 1.24 lm/W) to (29,205 cd/m2, 13.27 cd/A and 9.56 lm/W). CV measurements reveal that HEC possesses a powerful hole-blocking capability. Topography and conductivity AFM images show that doping TS-CuPc increases the interfacial contact area and interfacial conductivity, which can overcome the insulating nature of HEC and thus further facilitate electron injection. Enhancements in device performance are attributed to the improved carrier balance and recombination in the presence of df-HBL, confirmed in electron-only and hole-only devices. Moreover, apparently raised open-circuit voltages provide further evidence that enhanced electron injection is indeed realized by the df-HBL. This study demonstrates an effective approach to develop highly efficient PLEDs.

Efficient near-infrared emitting tetradentate bis-cyclometalated platinum (IV) complexes for solution-processed polymer light-emitting diodes

Two novel tetradentate bis-cyclometalated platinum (IV) complexes of TPA-2q-PtCl2 and 3TPA-2q-PtCl2 with CˆN*NˆC cyclometalated ligands and two coordinated chlorine atoms were synthesized and characterized. The crystal structure of TPA-2q-PtCl2 shows a trans-form irregular octahedral geometry by measurement of single-crystal X-ray diffraction, where two CˆN coordination moieties are twisted and two chlorine atoms are located on both sides of platinum atom. A medium absorption band in a range of 450–474 nm is observed, which is assigned to the metal-to-ligand charge-transfer (MLCT) and intramolecular charge transfer (ILCT). And two obvious emission peaks at about 635 and 750 nm are displayed for both platinum (IV) complexes in 2-methyltetrahydrofuran solutions, which originate from the ILCT and MLCT characters to the lowest excited state. Using one of platinum (IV) complexes as a single dopant and a blend of poly(vinylcarbazole) as a host matrix, their single-emissive-layer polymer light-emitting diodes (PLEDs) exhibit an outstanding near-infrared emission with an electroluminescent (EL) peak of 750 nm. The better electroluminescent properties are obtained in the 3TPA-2q-PtCl2 doped devices with an external quantum efficiency of 0.85% and a radiance of 164 μW/cm−2. This is the first reported sample about tetradentate biscyclometalated platinum (IV) complexes used as organic NIR electroluminescent materials for promising application in PLEDs.

Dual phosphorescence emission of dinuclear platinum(ii) complex incorporating cyclometallating pyrenyl-dipyridine-based ligand and its application in near-infrared solution-processed polymer light-emitting diodes.

Two novel mono- and binuclear cyclometalated platinum(ii) complexes of (BuPyrDPy)Pt(dpm) and (BuPyrDPy)[Pt(dpm)]2 incorporating a pyrenyl-dipyridine-based cyclometalated ligand were synthesized and characterized, respectively. Single-crystal X-ray diffraction of the two materials revealed each complex's coordination mode; their photophysical, electrochemical as well as electroluminescent properties were also investigated. Both complexes exhibited good solubility and excellent thermal stability. (BuPyrDPy)[Pt(dpm)]2 presented dual phosphorescence emissive character at room-temperature and showed an increased quantum efficiency compared to that of (BuPyrDPy)Pt(dpm). Density functional theory (DFT) calculations were carried out to model their photophysical process, and found a significant contribution of the second Pt center to the LUMO plot, giving the T1 and T2 states considerable LMCT nature, which is quite rare in metallic complexes. A device with the structure of ITO/PEDOT (40 nm)/PVK : 30 wt% OXD-7 : 16 wt% (BuPyrDPy)[Pt(dpm)]2 (60 nm)/TPBI (30 nm)/Ba (4 nm)/Al (100 nm) showed a stable NIR emission peak at 695 nm accompanied by two shoulders at 599 nm and 762 nm, with a maximum external quantum efficiency (EQE) of 0.31% and a radiance of 26.9 mW cm-2, which are about 2 and 1.4 times higher than those of (BuPyrDPy)Pt(dpm)-doped devices. This study provides an efficient strategy to simultaneously design novel biluminescent materials and achieve NIR emission through adjusting the emissive triplet states by introducing a second metal into an asymmetric bimetallic system.

Benzotriazole-containing donor–acceptor–acceptor type cyclometalated iridium(III) complex for solution-processed near-infrared polymer light emitting diodes

A novel near-infrared-emitting cyclometalated iridium(III) complex of (CH3OTPA-BTz-Iq)2Irpic containing benzotriazole unit with a donor–acceptor–acceptor (D-A-A) chromophore was synthesized and characterized. The optophysical, electrochemical and electroluminescent characteristics were primarily studied. A near-infrared emission peaked at 716 nm with a shoulder at 790 nm was exhibited in the (CH3OTPA-BTz-Iq)2Irpic dichloromethane solution at 298 K. In its optimized solution-processed polymer light-emitting diodes, a near-infrared electroluminescent emission peaked at 723 nm with a shoulder at 780 nm was observed with a maximum external quantum efficiency of 0.41% at 8.14 mA cm−2. This work indicates that introducing appropriate D and A units to develop D-A-A structure is an efficient approach to construct near-infrared-emitting iridium(III) complex in the polymer light-emitting diodes with suppressive efficiency roll-off.

Polymer light-emitting diodes based on cationic iridium(III) complexes with a 1,10-phenanthroline derivative containing a bipolar carbazole–oxadiazole unit as the auxiliary ligand

A 1,10-phenanthroline derivative (co-phen) containing a bipolar carbazole–oxadiazole unit was synthesized and used as the auxiliary ligand in cationic iridium(III) complexes [(ppy)2Ir(co-phen)]PF6 (ppy: 2-phenylpyridine) and [(npy)2Ir(co-phen)]PF6 (npy: 2-(naphthalen-1-yl)pyridine). Two complexes have high thermal stability with the glass-transition temperatures (Tg) of 207°C and 241°C, and the same 5% weight-reduction temperatures (ΔT5%) of 402°C. Both of them were used as phosphorescent dopants in solution-processed polymer light-emitting diodes (PLEDs): ITO/PEDOT: PSS/PVK: PBD: complex (mass ratios 100: 40: x, x=1.0, 2.0, and 4.0)/CsF/Al. The maximum luminances of the PLEDs using [(ppy)2Ir(co-phen)]PF6 and [(npy)2Ir(co-phen)]PF6 were 12567cdm−2 and 11032cdm−2, the maximum luminance efficiencies were 17.3cdA−1 and 20.4cdA−1, the maximum power efficiencies were 9.8lmW−1 and 10.3lmW−1, and the maximum external quantum efficiencies were 9.3% and 11.4% respectively. The CIE color coordinates were around (0.37, 0.57) and (0.44, 0.54) respectively, corresponding to the yellow green region.

Improved electron injection from silver electrode for all solution-processed polymer light-emitting diodes with Cs2CO3:conjugated polyelectrolyte blended interfacial layer

This study demonstrates the incorporation of a Cs2CO3:conjugated polyelectrolyte blended interfacial layer between the emissive layer and a silver (Ag) cathode, for realizing all-solution processed polymer light-emitting diodes. For a device with poly(9,9-dioctylfluorene-co-benzothiadiazole) (F8BT) as the emissive layer, this approach improves the maximum luminance of approximately 80,000cd/m2 and maximum current efficiency of 10.6cd/A. It is clarified that the interfacial layer prevents Ag nanoparticles from penetrating into the emissive layer, resulting in yellow–green emission from F8BT. We also demonstrate the possibility of all-solution processed polymer light-emitting diodes utilizing solution-processed Cs2CO3:conjugated polyelectrolyte interfacial layer and Ag nano-ink.

An electrochemically deposited film as an interface layer to improve the performance of polymer light-emitting diodes

An insoluble electrochemically cross-linked thin film is successfully applied as an interface layer between PEDOT:PSS and the light emitting layer in solution-processed polymer light-emitting diodes for enhanced device performance.

P.119: High‐Performance Polymer Light‐Emitting Diodes with a Conjugated Polyelectrolyte

AbstractWe demonstrated red, yellow, green and blue polymer light‐emitting diodes (PLEDs) with high performances using a conjugated polyelectrolyte electron injection layer and high work function metal (Ag). PLEDs using a conjugated polyelectrolyte layer show improved performances in terms of the luminance and efficiency, as compared to the devices without this layer. Furthermore, it is providing a new possibility of all solution‐processed PLEDs with Ag electrodes.

Effect of Solvents, Their Mixture and Thermal Annealing on the Performance of Solution Processed Polymer Light-Emitting Diodes.

In this study, we first investigated changes seen in electrical and optical properties of a polymer light-emitting diode due to using different kinds of solvents and their mixture. Two-layer light emitting diodes with organic small molecules doped in a PVK polymer host were fabricated using (i) non-aromatic solvent chloroform with a high evaporation rate; (ii) aromatic solvent chlorobenzene with a low evaporation rate, and (iii) their mixture with different relative ratios. The effect of nano-scale layer thickness, surface roughness and internal nano-morphology on threshold voltage and the amount of electric current, the luminance and efficiency of a device were assessed. Results indicated the importance of majority charge carriers’ type in the selection of solvent and tuning its properties. Then, the effect of thermal annealing on electrical and optical properties of polymer light emitting diodes was investigated. During the device fabrication, pre-annealing in 80 and/or 120 °C and post-annealing in 120 °C were performed. The nano-scale effect of annealing on polymer-metal interface and electric current injection was described thoroughly. A comparison between threshold voltage, luminance and electric current efficiency of luminescence for different annealing processes was undertaken, so that the best electric current efficiency of luminescence achieved at 120 °C pre-annealing accompanied with 120 °C post-annealing.

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.

Crosslinkable hole-transporting materials for solution processed polymer light-emitting diodes

One of the most challenging tasks in fabricating multilayer polymer light-emitting diodes (PLEDs) by solution processes is to avoid the interfacial mixing between different layers because most of the commercially available emissive and charge-transporting materials are soluble in common organic solvents. To overcome this difficulty, extensive efforts have been invested in developing novel crosslinkable hole-transporting materials (HTMs). After thermo- or photo-crosslinking, all these crosslinked HTMs possess very good solvent resistance which greatly facilitates the subsequent processing of the emitting layer. By taking advantage of these HTMs, high efficiency red–green–blue (RGB)-emitting PLEDs, as well as white- and quantum dot based PLEDs, have been realized. This article provides a brief overview of the recent development of crosslinkable HTMs and their unique advantages in enhancing the performance of LEDs.