Fabrication Of Organic Light-emitting Devices Research Articles

The Root Causes of the Limited Electroluminescence Stability of Solution-Coated Versus Vacuum-Deposited Small-Molecule OLEDs: A Mini-Review.

Using solution-coating methods for the fabrication of organic light-emitting devices (OLEDs) offers a tremendous opportunity for enabling low-cost products and new applications. The electroluminescence (EL) stability of solution-coated (SOL) OLEDs, however, is significantly lower than that of vacuum-deposited (VAC) OLEDs, causing their operational lifetimes to be much shorter—an issue that continues to hamper their commercialization. The root causes of the lower EL stability of these devices remain unclear. This article briefly reviews and summarizes some of the work that has been done to-date for elucidating the root cause of the lower EL stability of SOL OLEDs, giving special attention to studies where side-by-side comparisons of SOL and VAC devices of the same materials have been conducted. Such comparisons allow for more-reliable conclusions about the specific effects of the solution-coating process on device stability to be made. The mini-review is intended to introduce the work done to-date on the causes of lower stability in SOL OLEDs and to stimulate further work for the purpose of closing the existing knowledge gap in this area and surmounting this long-standing challenge in the SOL OLED technology.

Synthesis of acridone-naphthylamine derivative and its thermally-activated delayed fluorescence studies for application in OLEDs

Acridone (acceptor) and naphthylamine (donor) based Donor-Acceptor-Donor (D-A-D) compound (1) was synthesised, characterised and its thermally-activated delayed fluorescence (TADF) properties were studied in detail. Compound 1 is fluorescent and emits in the green region (550 nm). The energy gap between the ground and the lowest excited singlet (S1) state is estimated to be 2.55 eV. The energy gap between the CT singlet and triplet states (∆EST) was found to be ~0.3 eV. Small ∆ES1-T1 is one of the important criteria for TADF to take place in a molecule and thus detailed photophysics has been studied. Transient lifetime measurements showed an increase in the fluorescence lifetime (τ) on purging with N2, as compared with that in air-saturated solution, indicating the involvement of the triplet state in emission. Emission at 550 nm was also observed with a delay of 100 µs which corresponded to the delayed fluorescence in 1. The lifetime of TADF was found to be 176 µs. Applications of TADF materials in organic light-emitting devices (OLEDs) has gotten attention as TADF materials utilise the triplet excitons which helps in increasing internal quantum efficiency of device. Air-saturated based on 1 were fabricated and their intensity was found to be nearly as high as 17,000 Cd/m2 at 25 mA/cm2 which was comparable to many of the known TADF emitters. Acridone-amine derivatives was synthesized and its photophysical properties were carried out to establish TADF. It was also used in fabrication of OLEDs.

An Indolocarbazole‐Based Thermally Activated Delayed Fluorescence Host for Solution‐Processed Phosphorescent Tandem Organic Light‐Emitting Devices Exhibiting Extremely Small Efficiency Roll‐Off

AbstractThe study reports the development of a solution‐processed phosphorescent tandem organic light‐emitting device (OLED) exhibiting extremely small efficiency roll‐off. The OLED comprises two light‐emitting units (LEUs) connected by an interconnecting unit and employs a thermally activated delayed fluorescence host material. One of the most difficult tasks in the fabrication of OLEDs is to form a multilayer structure without dissolving the underlayer during the coating of the upper layer. The developed host materials exhibit high tolerance to methanol. The upper‐layer adjacent to the light‐emitting layer consists of ZnO nanoparticles, which could be dispersed in methanol by improving the preparation method. This results in the successful fabrication of a solution‐processed phosphorescent tandem OLED comprising two LEUs. The maximum external quantum efficiency (EQE) of the tandem device is 22.8%, and the EQE is 21.9% even at a high luminance of 10 000 cd m−2. The suppression of efficiency roll‐off is among the best of those previously reported. Moreover, the operational stability of the tandem device is much higher compared with single‐LEU devices.

Bonding, Luminescence, Metallophilicity in Linear Au3 and Au2Ag Chains Stabilized by Rigid Diphosphanyl NHC Ligands.

The heterofunctional and rigid ligand N,N'-diphosphanyl-imidazol-2-ylidene (PCNHCP; P = P(t-Bu)2), through its phosphorus and two N-heterocyclic carbene (NHC) donors, stabilizes trinuclear chain complexes, with either Au3 or AgAu2 cores, and dinuclear Au2 complexes. The two oppositely situated PCNHCP (L) ligands that "sandwich" the metal chain can support linear and rigid structures, as found in the known tricationic Au(I) complex [Au3(μ3-PCNHCP,κP,κCNHC,κP)2](OTf)3 (OTf = CF3SO3; [Au3L2](OTf)3; Chem. Commun. 2014, 50, 103-105) now also obtained by transmetalation from [Ag3(μ3-PCNHCP,κP,κCNHC,κP)2](OTf)3 ([Ag3L2](OTf)3), or in the mixed-metal tricationic [Au2Ag(μ3-PCNHCP,κP,κCNHC,κP)2](OTf)3 ([Au2AgL2](OTf)3). The latter was obtained stepwise by the addition of AgOTf to the digold(I) complex [Au2(μ2-PCNHCP,κP,κCNHC)2](OTf)2 ([Au2L2](OTf)2). The latter contains two dangling P donors and displays fluxional behavior in solution, and the Au···Au separation of 2.8320(6) Å in the solid state is consistent with metallophilic interactions. In the solvento complex [Au3Cl2(tht)(μ3-PCNHCP,κP,κCNHC,κP)](OTf)·MeCN ([Au3Cl2(tht)L](OTf)·MeCN), which contains only one L and one tht ligand (tht = tetrahydrothiophene), the metal chain is bent (148.94(2)°), and the longer Au···Au separation (2.9710(4) Å) is in line with relaxation of the rigidity due to a more "open" structure. Similar features were observed in [Au3Cl2(SMe2)L](OTf)·2MeCN. A detailed study of the emission properties of [Au3L2](OTf)3, [Au3Cl2(tht)L](OTf)·MeCN, [Au2L2](OTf)2, and [Au2AgL2](OTf)3 was performed by means of steady state and time-resolved photophysical techniques. The complex [Au3L2](OTf)3 displays a bright (photoluminescence quantum yield = 80%) and narrow emission band centered at 446 nm with a relatively small Stokes' shift and long-lived excited-state lifetime on the microsecond timescale, both in solution and in the solid state. In line with the very narrow emission profile centered in the violet-blue region, fabrication of organic light-emitting devices (OLEDs) comprising the [Au3L2](OTf)3 complex demonstrated its usefulness as a deep-blue emitter in solution-processed OLEDs. Electrochemical and Raman spectroscopic studies were also performed on [Au3L2](OTf)3. Experimental results were rationalized by means of Wave-Function Theory (WFT) and Density Functional Theory (DFT). MP2 calculations gave a satisfactory description of the structures of the cationic complexes [Au3L2](3+) and [Au2L2](2+) and pointed to Au···Au interactions having an electrostatic component owing to the dissimilar charge distribution in the chain caused by the heterofunctional ligand. The nature of the emitting states and their geometric distortions relative to the ground states in [Au3L2](3+) and [Au2L2](2+) was studied by DFT, revealing contraction of the Au···Au distances and coordination geometry changes by association of the dangling P donor, respectively.

Application of CdSe/ZnS/CdS/ZnS Core–multishell Quantum Dots to Modern OLED Technology

Semiconductor quantum dots (QDs) obtained by means of colloidal synthesis are widely used in fabrication of organic light-emitting devices (OLEDs). We have fabricated QD–OLEDs with an active light-emitting layer formed by novel CdSe/ZnS/CdS/ZnS core–multishell QDs that have luminescence quantum yield of more than 90%. We have compared the performance of QD–OLEDs with different thickness of the active layer and demonstrated that the best performance is achieved at 45-55nm, probably due to field redistribution inside the device. Optimization of the thicknesses of transport layers of QD–OLED is likely to further improve the performance of such devices.

Dendrimers with tetraphenylsilane core and 9-phenylcarbazole-based dendrons: synthesis, photophysics, and electrochemical behavior

Three generations of novel dendrimers (G1, G2 and G3) with a tetraphenylsilane core and 9-phenylcarbazole-based dendrons have been successfully synthesized. These dendrimers possess excellent thermal stability with 5% weight-loss temperatures in the range of 574 to 622 °C. Compared to the lower generation of dendrimer, G3 exhibits a high glass transition at 271 °C, indicating that the high content of 9-phenylcarbazole results in a dendrimer with significantly enhanced rigidity. The contorted dendrons effectively inhibit the aggregation of conjugative chromophore groups, leading to a fluorescence quantum yield of up to 99.1%. Moreover, all the dendrimers have high energy band gaps, a characteristic which is especially desirable for host materials in the fabrication of organic light-emitting devices.

Efficiency enhancement of solution‐processed single‐layer blue‐phosphorescence organic light‐emitting devices having co‐host materials of polymer (PVK) and small‐molecule (SimCP2)

Abstract— A new type of single‐layer blue‐phosphorescence organic light‐emitting devices (OLEDs) containing poly(9‐vinylcarbazole) (PVK) and small‐molecule‐based amorphous ambipolar bis(3,5‐di(9H‐carbazol‐9‐yl)phenyl) diphenylsilane (SimCP2) as the co‐host material have been demonstrated. All active materials [PVK, SimCP2, Flrpic (blue‐phosphorescence dopant), and OXD‐7 (electron transport)] were mixed in a single layer for solution processing in the fabrication of OLEDs. The SimCP2 small‐molecule host has adequate high electron and hole‐carrier mobiltieis of ∼10−4 cm2/V‐sec and a sufficiently large triplet state energy of ∼2.70 eV in confining emission energy on FIrpic. Based on such an architecture for single‐layer devices, a maximum external quantum efficiency of 6.2%, luminous efficiency of 15.8 cd/A, luminous power efficiency of 11 lm/W, and Commision Internale de l'Eclairage (CIEx,y) coordinates of (0.14,0.32) were achieved. Compared with those having PVK as the single‐host material, the improvement in the device performance is attributed to the balance of hole and electron mobilities of the co‐host material, efficient triplet‐state energy confinement on FIrpic, and the high homogeneity of the thin‐film active layer. Flexible blue‐phosphorescence OLEDs based on solution‐processed SimCP2 host material (withou PVK) have been demonstrated as well.

Organic light-emitting devices based on solution-processible quinolato-complex supramolecules

This paper discusses a new type of supramolecular material tris{5- N-[3-(9H-carbazol-9-yl)propyl]- N-(4-methylphenyl)aminesulfonyl-8-hydroxyquinolato} aluminum(III), Al(SCarq) 3, which we synthesized using three 5- N-[3-(9H-carbazol-9-yl)propyl]- N-(4-methylphenyl)aminesulfonyl-8-hydroxyquinoline as bidentate ligands. The peak photoluminescence in the solid phase appears at 488 nm. In cyclic voltammetric measurement, two oxidation peaks, which were obtained at −5.6 and −5.9 eV, correspond to HOMO sites of carbazoyl and aluminum quinolates, respectively. In the investigation of solid morphological thin film, the flat surface was investigated using an atomic force microscope. The root mean square (rms) and mean roughness ( R a) were respectively measured to be 0.427 and 0.343 nm. For the fabrication of organic light-emitting devices (OLEDs) using spin-coating techniques, the turn-on voltage and maximum luminescence of the optimized electroluminescence device, glass/ITO (20 nm)/PEDOT:PSS (75 nm)/Al(SCarq) 3 (85 nm)/BCP (8 nm)/LiF (1 nm)/Al (200 nm), were respectively 9.6 V and 35.0 cd m −2. Due to the electroplex formation between the carbazole (electron-donor) and the aluminum quinolates (electron-acceptor) moieties under an applied DC bias, the chromaticity of electroluminescence shifted to green-yellow with 1931 CIE x, y (0.40, 0.47).

Phosphorescent organic light-emitting devices: working principle and iridium based emitter materials.

Even though organic light-emitting device (OLED) technology has evolved to a point where it is now an important competitor to liquid crystal displays (LCDs), further scientific efforts devoted to the design, engineering and fabrication of OLEDs are required for complete commercialization of this technology. Along these lines, the present work reviews the essentials of OLED technology putting special focus on the general working principle of single and multilayer OLEDs, fluorescent and phosphorescent emitter materials as well as transfer processes in host materials doped with phosphorescent dyes. Moreover, as a prototypical example of phosphorescent emitter materials, a brief discussion of homo- and heteroleptic iridium(III) complexes is enclosed concentrating on their synthesis, photophysical properties and approaches for realizing iridium based phosphorescent polymers.

Improved property in organic light-emitting diode utilizing two Al/Alq3 layers

We reported on the fabrication of organic light-emitting devices (OLEDs) utilizing the two Al/Alq3 layers and two electrodes. This novel green device with structure of Al(110nm)/tris(8-hydroxyquinoline) aluminum (Alq3)(65nm)/Al(110nm)/Alq3(50nm)/N,N′-dipheny1-N, N′-bis-(3-methy1phyeny1)-1, 1′-bipheny1-4, 4′-diamine (TPD)(60nm)/ITO(60nm)/Glass. TPD were used as holes transporting layer (HTL), and Alq3 was used as electron transporting layer (ETL), at the same time, Alq3 was also used as emitting layer (EL), Al and ITO were used as cathode and anode, respectively. The results showed that the device containing the two Al/Alq3 layers and two electrodes had a higher brightness and electroluminescent efficiency than the device without this layer. At current density of 14mA/cm2, the brightness of the device with the two Al/Alq3 layers reach 3693cd/m2, which is higher than the 2537cd/m2 of the Al/Alq3/TPD:Alq3/ITO/Glass device and the 1504.0cd/m2 of the Al/Alq3/TPD/ITO/Glass. Turn-on voltage of the device with two Al/Alq3 layers was 7V, which is lower than the others.

Fabrication of Organic Light-Emitting Devices with Indium–Tin-Oxide Anode Prepared by Spray Chemical Vapor Deposition

Organic light-emitting devices (OLEDs) were fabricated using an indium–tin-oxide (ITO) anode and a small molecular light-emitting material, tris(8-hydroxyquinolinato) aluminum (Alq3). The ITO anode (thickness, 120 nm) was prepared inexpensively by spray chemical vapor deposition using ethanol solution consisting of indium chloride and tin chloride onto a glass substrate at 270 °C, which is 80 °C lower than the temperature previously reported by the present authors. The work function and lowest resistivity of the as-deposited anode containing 6.6 at. % Sn were respectively 4.7 V and 3.7×10-4 Ω·cm. The luminance and turn-on threshold voltage of the OLED were respectively 6500 cd/m2 and 3.5 V. These values agreed with those of an OLED with the same layer structure but without the commercial ITO anode deposited by physical vapor deposition. The effects of tin concentration in the present ITO anode on the work function and device performance were also investigated.

Improvement in performance of organic light-emitting devices by inclusion of multi-wall carbon nanotubes

Poly(3,4-ethylene dioxythiophene):poly(styrene sulfonate) (PEDOT:PSS) was modified by different concentrations of multi-wall carbon nanotubes (MWNTs), and the nanocomposites of PEDOT:PSS and MWNTs were firstly used as hole-injection layer in fabrication of organic light-emitting devices (OLEDs) by using a double-layer structure with hole-injection layer of doped PEDOT:PSS and emitting/electron transport layer of tris(8-hydroxyquinolinato) aluminum (Alq 3). PEDOT:PSS solution doped with MWNTs was spin-coated on clean polyethylene terephthalate (PET) substrate with indium tin oxide (ITO). It was found that the electroluminescence (EL) intensity of the OLEDs were greatly improved by using PEDOT:PSS doped with MWNTs as hole-injection layer which might have resulted from the hole-injection ability improvement of the nanocomposites. Higher luminescence intensity and lower turn-on voltage were obtained by these devices and the luminance intensity obtained from the device with the hole-injection layer of PEDOT:PSS doped by 0.4 wt.% MWNTs was almost threefolds of that without doping.

Organic light-emitting devices with double-block layer

We report on the fabrication of organic light-emitting devices (OLEDs) using double-block layers on the electron transport layer and emitting layer. The current efficiency of the organic light-emitting diode is improved by 43% to 9.16cdA−1 as compared to the device with a single host of Alq3 as the electron transport layer. The maximum luminance is over 23750cdm−2 at the bias of 18V and the current of 338.3mAcm−2, which is 33% higher than the single host Alq3 device without block layer. Using a step-by-step procedure to smooth electron injection and transport, the energy levels introduced by the insertion layers are an effective method of improving the luminance characteristics.

Fabrication of organic light-emitting devices on flexible substrates using a combined roller imprinting and photolithography-patterning technique

A patterning method has been developed to fabricate patterned organic light-emitting devices (OLEDs) (60×80mm2) with a pixel size of 500×300μm2 on flexible polyethylene terephthalate substrates. The patterns of the pixel array were defined in crossed-strip style with indium tin oxide anode and patterned using a combination of roller-type imprinting lithography and photolithography (CRIP) followed by wet etching. Compared with conventional imprint lithography or photolithography, the CRIP technique has the advantages of better uniformity, less force, consuming less time, lower cost, and higher aspect ratio. The performance of the CRIP OLEDs was the following: the turn-on voltage at 1cd∕m2 was 7.5V and the maximum luminance was 13530cd∕m2 at 17.5V. The highest luminous efficiency reached 1.27lm∕W (3.23cd∕A) at a luminance of 4.8cd∕m2 and kept the values stable from 5to15V. The performance was comparable to that of devices patterned by conventional photolithography.

Electrochemical Depositon of End-Capped Triarylamine and CBP Dendrimers: Alternate Technique for the Fabrication of Organic Light-Emitting Devices

ABSTRACTCarbosilane dendrimers adorned with either triarylamine or carbazole units in their periphery exhibit novel electrochemical behavior in which the electrochemical deposition is controlled by dendrite generation. In addition, the deposited layers remained intact in the depositing solvent, methylene chloride, allowing a second layer to be deposited on top of the first layer. We have sought to establish the suitability of this electrochemical deposition technique for use in the construction of multi-layer OLEDs, which cannot be fabricated via conventional spin-coating with a polymeric precursor. Thus, the electrochemical deposition-based process could potentially offer an ideal combination of deposition control on the one hand and multi-layer fabrication on the other. We report herein the novel electrochemical deposition behavior of arylamine or carbazole end-capped carbosilane dendrimers of the type GnNPB or GnCBP (n = 1-4) and their use for the formation of multi-layer devices for OLEDs.

Organic light-emitting devices with energy-level aligned double-buffer layers

We report on the fabrication of organic light-emitting devices (OLEDs) using double-buffer layers on the indium tin oxide (ITO) anode. Both the luminance and the quantum efficiency of organic light-emitting devices have been improved by inserting double-buffer layers. Using a step-by-step procedure to smooth hole injection and transport, the energy levels introduced by the insertion layers are an effective method of improving the luminance characteristics.

High Tg Fluorene-based Hole-transporting Materials for Organic Light-emitting Diodes

Abstract A new series of fluorene-based hole-transporting materials, 9,9-bis[4-(di-p-tolylamino)phenyl]-2,7-bis(diphenylamino)fluorene (1), 9,9-bis[4-(di-p-tolylamino)phenyl]-2,7-bis(di-p-tolylamino)fluorene (2), 9,9-bis[4-(di-p-tolylamino)phenyl]-2,7-bis(2-naphthylphenylamino)fluorene (3), and 9,9-bis[4-(di-p-tolylamino)phenyl]-2,7-bis(9-carbazolyl)fluorene (4), was prepared and employed in the fabrication of organic light-emitting devices (OLEDs). These materials were designed to have bulky triarylamine moieties at the C-9 position, which provides them with excellent thermal stability and high glass-transition temperatures (Tg = 139–171 °C). 1 and 3 are comparable to 1,4-bis(1-naphthylphenylamino)biphenyl (NPB) in terms of device performance (maximum luminence and current efficiency) in a standard hole-transporting layer (HTL)/tris(8-hydroxyquinolino)aluminum (Alq3) double-layer device, but are superior to NPB in terms of higher glass-transition temperatures (Tg = 139 and 145 °C, respectively), while 2 shows higher luminance and current efficiency as well as Tg (140 °C) than those of NPB (100 °C).

Combinatorial Fabrication and Screening of Organic Light-Emitting Device Arrays

AbstractStudies of combinatorial fabrication and screening of organic light-emitting devices (OLEDs) are reviewed. These studies include screening of luminescent materials, electron and hole transport layers, lower-gap emitting guest dopants in small molecular emitters, and electronically doped polymeric anodes. The review focuses on screening of 2-dimensional (2-d) small molecular UV/violet arrays, 1-d blue-to-red arrays, and 1-d intense white OLED libraries, and briefly describes arrays fabricated to study Förster energy transfer in guest-host OLEDs. It demonstrates that combinatorial fabrication of OLEDs has become a powerful tool for screening OLED materials and configurations, and for studying their basic properties.

Organic light-emitting devices with laminated top contacts

We demonstrate the fabrication of organic light-emitting devices based on a ruthenium complex with indium tin oxide anodes and laminated Au cathodes. Light emission was uniform over the whole device area, indicating a high-quality mechanical and electrical contact. The devices showed no rectification, indicating that the laminated contact was ohmic and caused no damage to the ruthenium complex. Comparison with devices using evaporated Au cathodes confirmed the quality of the lamination process.

Fabrication of Organic Light‐Emitting Devices by Low‐Pressure Cold Welding

Low‐pressure cold welding has been used to fabricate 200 μm diameter organic light‐emitting devices (OLEDs) (see Figure). The contacts are made by transferring a metal film from a patterned, soft elastomeric stamp onto unpatterned organic and metal layers on a substrate. The process does not adversely affect the OLED performance when compared with those fabricated by shadow mask patterning.