Organic Electrophosphorescent Devices Research Articles

The Effect of Electron‐Withdrawing Groups on Electron Transporting Silane Derivatives with Wide Energy Gap for Green Electrophosphorescent Devices

Silane derivatives with wide energy gap (≈3.5 eV) containing different electron‐withdrawing groups of quinoline and naphthyridine are synthesized and used as the electron transporting materials. The different electron transporting and hole/exciton blocking properties of the silane derivatives are investigated via multilayered structure of organic electrophosphorescent devices by using fac‐tris(2‐phenylpyridine)iridium (Ir(ppy)3) as the phosphorescent emitter. 15.4% of maximum external quantum efficiency (EQE) corresponding to 56.2 cd A−1 of maximum current efficiency is obtained with a maximum power efficiency of 58.9 lm W−1 by employing di‐(4‐(1,8‐naphthyridin‐2‐yl)phenyl)diphenylsilane (DNPS) as the electron transporting material, combining with 2,9‐dimethyl‐4,7‐diphenyl‐1,10‐phenanthroline as the hole blocking layer, which is higher than the performance of conventional Alq3 device. When changing naphthyridine of DNPS to the electron‐withdrawing group of quinoline (di‐(4‐(isoquinolin‐4‐yl)phenyl)diphenylsilane), only 11.4% of maximum EQE with 41.4 cd A−1 of maximum current efficiency and 32.5 lm W−1 of a maximum power efficiency is obtained. These indicate that the electron transporting ability increases while the electron‐withdrawing group changes from quinoline to naphthyridine, which is also consistent with the calculated reorganization energy.

Numerical study of the influence of applied voltage on the current balance factor of single layer organic light-emitting diodes

Current balance factor (CBF) value, the ratio of the recombination current density and the total current density of a device, has an important function in fluorescence-based organic light-emitting diodes (OLEDs), as well as in the performance of the organic electrophosphorescent devices. This paper investigates the influence of the applied voltage of a device on the CBF value of single layer OLED based on the numerical model of a bipolar single layer OLED with organic layer trap free and without doping. Results show that the largest CBF value can be achieved when the electron injection barrier (ϕn) is equal to the hole injection barrier (ϕp) in the lower voltage region at any instance. The largest CBF in the higher voltage region can be achieved in the case of ϕn > ϕp under the condition of electron mobility (μ0n) > hole mobility (μ0p), whereas the result for the case of μ0n < μ0p, is opposite. The largest CBF when μ0n = μ0p can be achieved in the case of ϕn = ϕp in the entire region of the applied voltage. In addition, the CBF value of the device increases with increasing applied voltage. The results obtained in this paper can present an in-depth understanding of the OLED working mechanism and help in the future fabrication of high efficiency OLEDs.

Highly efficient deep-red organic electrophosphorescent devices with excellent operational stability using bis(indoloquinoxalinyl) derivatives as the host materials

Three new bis(indoloquinoxalinyl) derivatives, BIQF (6,6′-(4,4′-(9H-fluorene-9,9-diyl)bis(4,1-phenylene))bis(6H-indolo[2,3-b]quinoxaline)), BIQTP (9,9′-(1,1′:4′,1′′-terphenyl-4,4′′-diyl)bis(6H-indolo[2,3-b]quinoxaline), and BIQMCz (6,6′-(9-p-tolyl-9H-carbazole-3,6-diyl)bis(6H-indolo[2,3-b]quinoxaline)) linked by the fluorenyl, terphenyl and carbazolyl groups, respectively, were synthesized, characterized and used as the hosts for deep-red phosphorescent organic light-emitting diodes (PhOLEDs). The central linkers provide a way to reach higher molecular weights with little change of the energy gap of the molecules relative to the parent indoloquinoxaline molecule. These materials are bipolar and show high glass transition temperatures ∼200 °C and triplet energy gaps 2.25–2.37 eV. The electroluminescent (EL) devices based on these host materials and iridium deep-red emitters exhibit very high device efficiency with a EQEmax mostly above 20%. For example, the device utilizing (tmq)2Ir(acac) as the dopant and BIQMCz as the host shows a maximum current efficiency of 32.4 cd A−1 and a maximum power efficiency of 30.9 lm W−1. The operational lifetimes (T50) of these devices were also measured. The results show that the BIQMCz/Ir(piq)3-based device gave T50 of more than 20 000 h at an initial luminance of 500 cd m−2 with a current efficiency (CE) of 23.2 cd A−1 and Commission Internationale de l'Éclairage (CIE) coordinates of (0.67, 0.33).

Highly Efficient Electron‐Transporting/Injecting and Thermally Stable Naphthyridines for Organic Electrophosphorescent Devices

AbstractA series of 1,8‐naphthyridine derivatives is synthesized and their electron‐transporting/injecting (ET/EI) properties are investigated via a multilayered electrophosphorescent organic light‐emitting device (OLED) using fac‐tris(2‐phenylpyridine)iridium [Ir(ppy)3] as a green phosphorescent emitter doped into a 4,4′‐N,N′‐dicarbazolebiphenyl (CBP) host with 4,4′‐bis[N‐(1‐naphthyl)‐N‐phenylamino]biphenyl (a‐NPD) as the hole‐transporting layer, and poly(arylene ether sulfone) containing tetraphenylbenzidine (TPDPES) doped with tris(4‐bromophenyl)ammonium hexachloroantimonate (TBPAH) as the hole‐injecting layer. The turn‐on voltage of the device is 2.5 V using 2,7‐bis[3‐(2‐phenyl)‐1,8‐naphthyridinyl]‐9,9‐dimethylfluorene (DNPF), lower than that of 3.0 V for the device using a conventional ET material. The maximum current efficiency (CE) and power efficiency (PE) of the DNPF device are much higher than those of a conventional device. With the aid of a hole‐blocking (HB) and exciton‐blocking layer of bathocuproine (BCP), 13.2–13.7% of the maximum external quantum efficiency (EQE) and a maximum PE of 50.2–54.5 lm W−1 are obtained using the naphthyridine derivatives; these values are comparable with or even higher than the 13.6% for conventional ET material. The naphthyridine derivatives show high thermal stabilities, glass‐transition temperatures much higher than that of aluminum(III) bis(2‐methyl‐8‐quinolinato)‐4‐phenylphenolate (BAlq), and decomposition temperatures of 510–518 °C, comparable to or even higher than those of Alq3.

The effect of carrier injection and transport of hosts on organic electrophosphorescent device

Recently the organic electrophosphorescent host materials have attracted increasing attention. The selection principle of suited host materials is important to obtain high efficient electrophosphorescence. In this paper, we report the effect of carrier injection and transport properties of hosts to organic electrophosphorescent devices. A system, consisting of a red phosphorescent [Ru(4,7-Ph2-phen)3]2+ dopant and two blue-emitting hosts THPH and TCPC introducing carbazole substituent groups, was studied. Both the hosts showed similar electronic properties, forming film ability and energy transfer degree to dopant, however, EL efficiency with TCPC host significantly exceeds that with THPH host. The Cyclic Voltammogram curves and single carrier devices of hosts TCPC and THPH show that TCPC with carbazole groups possesses stronger hole injection and transport ability, and this induced TCPC is more suited host than THPH. This result demonstrates our previous study that the electrochemical properties of dopant [Ru(4,7-Ph2-phen)3]2+ molecule, which reflects the carrier trapping ability of guest, may be a basic design criterion for the selection of host.

Photoluminescence Characteristics of Organic Host Materials with Wide Energy Gaps for Organic Electrophosphorescent Devices

The photoluminescence, i.e., fluorescence and phosphorescence, characteristics of host materials with wide energy gaps suitable for phosphorescence-based organic light-emitting diodes (OLEDs) are summarized. In the vacuum deposited films, the host materials show phosphorescent emission in the blue to green region and triplet energy levels of ET = 2.6–3.1 eV because of the presence of steric hindrance. An OLED containing 1,2,3,4-tetraphenyl-5-(3-pyridyl)benzene as a host and iridium(III) bis[(4,6-difluorophenyl)-pyridinato-N,C2]picolinate as a blue emissive phosphorescent dopant exhibits an external electroluminescence efficiency of ∼12%.

Emission zone control in blue organic electrophosphorescent devices through chemical modification of host materials

We report blue organic light-emitting devices with iridium (III) bis[(4,6-difluorophenyl)-pyridinato-N,C2′] picolinate as an emitter doped into a series of phosphine oxide-based host materials that have significantly different charge transport properties: 4-(diphenylphosphoryl)-N,N-diphenylaniline (HM-A1), N-(4-diphenylphosphoryl phenyl) carbazole (PO12), 9-[6-(diphenylphosphoryl)pyridin-3-yl]-9H-carbazole (HM-A5), and 6-(diphenylphosphoryl)-N,N-diphenylpyridin-3-amine (HM-A6). Depending on the nature of the host material, the location of the emission zone can be moved within the emissive layer from the hole transport layer interface to the electron-transport layer interface. The charge transport properties of the materials were evaluated using single carrier devices.

Solution processable organic electro-phosphorescent iridium complex based on a benzothiazole derivative

We have synthesized a new solution processable iridium complex, di[2-(4′-octyloxyphenyl) benzothiazole]iridium(III)acetoacetone, [(OPBT) 2Ir(acac)], based on benzothiazole derivative for organic electro-phosphorescent devices. The synthesized molecule was identified by 1H NMR and 13C NMR, and readily soluble in common organic solvents such as chlorobenzene. The UV–visible absorption and photoluminescence properties of pristine [(OPBT) 2Ir(acac) thin film as well as poly( N-vinylcarbzole) (PVK) thin film doped with the iridium complex were studied. The maximum UV–visible absorption and photoluminescence (PL) spectra are found to be at 337 nm and 547 nm, respectively. We have fabricated phosphorescent organic light-emitting devices using the ITO/PEDOT:PSS (40 nm)/PVK:(OPBT) 2Ir(acac) (40 nm)/Balq (40 nm)/LiF (1 nm)/Al (80 nm) configuration with the iridium complex as a triplet emissive dopant in poly( N-vinylcarbazole) (PVK) host. The electroluminescence (EL) devices showed greenish yellow light emission with maximum peak at 551 nm. Especially, the maximum external quantum and current efficiency of 1 mol% doped device were 1.74% and 4.89 cd/A, respectively.

Tailoring the work function of indium tin oxide electrodes in electrophosphorescent organic light-emitting diodes

We investigate the use of organic surface modifiers based on phosphonic acid anchoring groups that react with the surface of indium tin oxide (ITO) in order to modify its work function, surface energy, and the charge injection in organic multilayer electrophosphorescent devices. The phosphonic acid surface modifiers, possessing different substituting groups, are found to tune the work function of ITO in the range of 4.40–5.40 eV. These surface modifiers have been tested as an interfacial layer between the ITO anode and hole transport layers (HTL) that are either processed from the vapor phase or from solution. The use of this interfacial layer with a solution-processible HTL results in high quantum and luminous efficiencies of 20.6% and 68 cd/A at 100 cd/m2 (17.5% and 60 cd/A at 1000 cd/m2). The enhanced performance of the devices incorporating phosphonic acid modifiers could be associated with an improved charge injection and a better compatibility with the hydrophobic nature of the organic layer. The performance of these devices is also compared to that of devices in which ITO is modified with other well-known techniques such as air plasma treatment or the use of a layer of poly(3,4-ethylenedioxythiophene)/poly(styrenesulfonate).

A morphologically stable host material for efficient phosphorescent green and red organic light emitting devices

A carbazole-based host material, 4,4′- N, N′-[di(3, 6-di( t-butyl)carbazole]biphenyl (ttbCBP), with four t-butyl steric spacers on peripheral carbazole moieties was effectively synthesized through Friedel–Crafts Alkylation. Owing to the presence of sterically hindered t-butyl groups, ttbCBP exhibits a high glass transition temperature (175 °C) and is morphologically stable. This host material also preserves the characteristics of wide band-gap of 3.2 eV and high triplet energy of 2.64 eV. In addition, ttbCBP has been shown to be effective hosts for green and red phosphorescent emitters in organic electrophosphorescent devices.

Novel fluorene/carbazole hybrids with steric bulk as host materials for blue organic electrophosphorescent devices

The problem of self-quenching in organic electrophosphorescence devices has been extensively studied and partially solved by using sterically hindered spacers in phosphorescent dopants. This paper attempts to address this problem by using sterically hindered host materials. Novel fluorene/carbazole hybrids with tert-butyl substitutions, namely 9,9-bis[4-(3,6-di- tert-butylcarbazol-9-yl)phenyl]fluorene (TBCPF) and 9,9-bis[4-(carbazol-9-yl)phenyl]-2,7-di- tert-butylfluorene (CPTBF), have been synthesized and characterized. The compounds exhibit not only high triplet energy (>2.8 eV), but also high glass transition temperature (>160 °C) and thermal stability. The substitution of inert tert-butyl groups to the carbazole/fluorene rings of these host molecules has a remarkable effect on the corresponding properties of the host materials, i.e. enhancing the thermal and electrochemical stability, weakening the intermolecular packing, and tuning the solid-state emission. Blue electrophosphorescent devices with enhanced performance were prepared by utilizing the sterically hindered host materials. The devices based on the four tert-butyl substituted material TBCPF exhibit unusual tolerance of high dopant concentration up to 20% and marked reduction of efficiency roll-off at higher current, indicating significant suppression of self-quenching effect in organic electrophosphorescent devices by the substitution of steric bulks.

Well Defined Carbazol-3,9-Diyl Based Oligomers as Host Materials for Organic Electro-Phosphorescent Devices

Well defined carbazol-3,9-diyl-based derivatives as host materials for electro-phosphorescent devices have been synthesized by the modified Ullmann reaction as a key step. The full characterization of their structure by mass spectrometry, infrared absorption and proton magnetic resonance spectroscopy is presented. The amorphous compounds with glass transition temperatures of 103–154°C possess high thermal stability with onset decomposition temperatures exceeding 360°C. Conjugation of the 3,9-linked systems is rather confined within one carbazole unit, retaining large triplet energies suitable for green and red electro-phosphorescence. Using the new host material, efficient green phosphorescent organic light emitting diodes was demonstrated.

High performance yellow light-emitting organic electrophosphorescent devices based on Re(I) complex

Yellow organic light-emitting devices (OLEDs) with (2,3-diphenylpyrazino [2,3-f] [1, 10] phenanthroline) Re(CO)3Br (Re-Dppp) doped CBP as emissive layer were studied. It is found experimentally that Re-Dppp based yellow OLEDs possess high electroluminescent efficiency and brightness: a maximum brightness of 6249 cd/m2 at 15 V and a highest current efficiency of 14.2 cd/A with a brightness of 180 cd/m2 occurring at 1.3 mA/cm2 are achieved, which indicate that Re(I) complexes have a potential application in commercialized PMOLEDs.

Hexaphenylbenzene Derivatives for Blue Organic Light-emitting Devices

Abstract Novel hexaphenylbenzene derivatives having high triplet energy levels were synthesized and used in blue organic electrophosphorescent devices. The device with a blue phosphorescent emitter, iridium(III) bis[(4,6-difluorophenyl)pyridinato-N,C2′]-picolinate, exhibited a high external quantum efficiency of 11% (24 cd/A) and a high power efficiency of 12 lm/W at 100 cd/m2.

Small Molecule-based Organic Electrophosphorescent Devices fabricated by Spin Coating

Organic electrophosphorescent devices using several small molecules as a host and Ir(ppy)3 as a green emitting guest have been fabricated by spin coating. We used new host materials in which a cyclohexanyl or adamatyl group was inserted between two phenyl rings of CBP (named Cy-Cz, Ad-cz). The device with Cy-Cz host material and a guest concentration of 24wt% showed an external quantum efficiency of 11% and a power efficiency of 27lm/W at 100cd/m2. These results suggest that small molecule-based organic electrophosphorescent devices fabricated by spin coating can be highly efficient.

Coexistence of dissociation and annihilation of excitons on charge carriers in organic phosphorescent emitters

We examined exciton quenching mechanisms in organic electrophosphorescent emitters. As an example we have performed detailed steady-state (dc) and alternating (ac) electric-field-modulated (photo)luminescence (EML) studies of the green phosphorescent iridium (III) complex, $[\mathrm{Ir}{(ppy)}_{3}]$, doped into a hole-transporting diamine derivative (TPD) blended with polycarbonate, varying electrode contacts from the hole-injecting indium-tin-oxide (ITO) and Au (dc EML) to the weakly electron-injecting Al on both sides (ac EML) of a sandwich type formed thin film samples. Analysis of ac EML results in terms of the three-dimensional (3D)-Onsager theory of geminate recombination shows that both the EML for the $\mathrm{Ir}{(ppy)}_{3}$ phosphorescence and TPD fluorescence components of the emission spectrum from the emitters is underlain by the dissociation of $\mathrm{Ir}{(ppy)}_{3}$ triplet and TPD singlet exciton precursors composed of an electron and a hole separated by a distance ${r}_{0}=2.1\ifmmode\pm\else\textpm\fi{}0.1\phantom{\rule{0.3em}{0ex}}\mathrm{nm}$. We find that for samples with the ITO/Au electrode combination, the dissociation is accompanied by exciton annihilation on injected charge. The annihilation proceeds by injected holes with the second order rate constant of ${\ensuremath{\gamma}}_{\mathrm{Sq}}=(2\ifmmode\pm\else\textpm\fi{}1)\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}11}\phantom{\rule{0.3em}{0ex}}{\mathrm{cm}}^{3}∕\mathrm{s}$ characterizing the diffusivity of TPD singlets, and ${\ensuremath{\gamma}}_{\mathrm{Tq}}=(1\ifmmode\pm\else\textpm\fi{}0.5)\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}14}\phantom{\rule{0.3em}{0ex}}{\mathrm{cm}}^{3}∕\mathrm{s}$ indicative of the relative diffusion motion of the $\mathrm{Ir}{(ppy)}_{3}$ triplet and a charge carrier. It is shown that the annihilation to dissociation efficiency has a minimum at an electric field of ${F}_{\mathrm{min}}\ensuremath{\cong}{10}^{6}\phantom{\rule{0.3em}{0ex}}\mathrm{V}∕\mathrm{cm}$ for the present system but the derived theoretical expression allows us to predict the evolution of ${F}_{\mathrm{min}}$ with various material and sample parameters, and by this to establish criteria to assist in the optimized design of quenching effects in organic electrophosphorescent devices.

Highly Efficient Organic Blue Electrophosphorescent Devices Based on 3,6‐Bis(triphenylsilyl)carbazole as the Host Material

Highly Efficient Organic Blue Electrophosphorescent Devices Based on 3,6‐Bis(triphenylsilyl)carbazole as the Host Material

Solution-processed Small Organic Electrophosphorescent Devices with Arylamine Polymer Buffer Layer

Solution-processed organic electrophosphorescent devices with arylamine polymer buffer layer have been investigated. Device characteristics were similar to the device using commonly used conducting polymer as buffer layer, and the uniform emission was obtained in the device. The lifetime of the device, defined as half-luminous at an initial luminance of 1,000 cd/m2, was 40 h.

Durability Test of Solution-Processed Organic Electrophosphorescent Devices with Small Organic Molecules

Durability tests of solution-processed organic electrophosphorescent devices (SP-PHOLEDs) with small organic molecules have been investigated. By using poly(ethylene dioxythiophene)/poly(styrene sulfonate) (PEDOT) as a buffer layer and indium zinc oxide (IZO) as an anode, device durability was markedly improved. The lifetime of the SP-PHOLED, defined as half-luminous at an initial luminance of 1,000 cd/m2, was 60 h.

11.4: Highly Efficient Blue Organic Electrophosphorescent Devices Based on 3,6-Bis(triphenylsilyl)Carbazole as the Host Material

High-efficiency blue phosphorescent OLEDs using a novel host material of 3,6-bis(triphenylsilyl)carbazole (CzSi) are reported. Devices with the CzSi host and the blue phosphorescent dopant FIrpic show a maximum external quantum efficiency of ∼16% (30.6 cd/A), a maximum power efficiency of 26.7 lm/W, and a maximum brightness of ∼59000 cd/m2.