Organic Electrophosphorescent Devices Research Articles

Energy transfer probability in organic electrophosphorescence device with dopant

Based on the energy transfer process from host to dopant in an organic electrophosphorescent (EP) device, the expression of energy transfer probability (η) between the host (TPD) and guest (Ir(ppy)3) EP systems was proposed. The results show that: (1) The rate of the triplet energy transfer (KHG and KGH) increases exponentially with increasing donor-acceptor molecular distance (R), whereas decreases as the intermolecular distance (RHH) increases from 0.8 to 2.4 nm. Furthermore, KGH changes more quickly than KHG. (2) The energy transfer probability (η) increases as R reduces and the RHH changes can be safely neglected for R 1.1nm, the transfer probability will be below zero. Here, the energy transfer principle may be less important and the high electroluminescence (EL) quantum efficiency of phosphorescent system will be attributed to the direct electron-hole recombination in phosphorescent molecules. (3) The η will increase when the Forster radius (R0) increases or Gibb's energy decreases.

High-efficiency top-emissive white-light-emitting organic electrophosphorescent devices

We demonstrate an efficient, top-emissive, white-light-emitting organic device (WOLED) employing the phosphorescent emitters, tris-(1-(4,6-difluorophenyl)pyridinato,N,C2′)iridium(III) and iridium(III) bis(2-phenyl quinolyl-N,C2′)acetylacetonate combining a sputtered Ni anode and an ITO cathode. The electron transport layer is doped with Li for efficient electron injection from the cathode, thereby also avoiding strong microcavity effects. The operating voltage is substantially reduced compared with a top-emissive device with an undoped electron transport layer. Peak external quantum and power efficiencies of 10.5±1.0% and 9.8±1.0lm∕W are achieved at current densities of 1.6 and 1.0mA∕cm2, respectively. The emission is characterized by Commission Internationale de L’Eclairage coordinates of (x=0.42, y=0.39). The top-emissive device, useful for generating full color images when combined with color filters or for display backlights, exhibits characteristics competitive with those of a bottom-emission WOLED using the same multilayer structure.

Photophysics of an electrophosphorescent platinum (II) porphyrin in solid films

We examine electronic processes in platinum (II) octaethyl porphyrin (PtOEP) embedded in an organic solid state matrix and in the form of vacuum-evaporated neat films in conjunction with potential applications of this compound to organic photovoltaic and electrophosphorescent devices. Absorption, photoexcitation, and luminescence spectra indicate the excitonic dimers to be dominant excited states, and their dissociation underlies the charge photogeneration process. Different charge separation distance (1.5 nm and 2.6 nm) in opposite charge carrier pairs preceding dissociation can be distinguished based on the fit of the three-dimensional Onsager theory of geminate recombination to electromodulated luminescence and photoconduction measurements. The near-positive electrode concentrated triplet dimer excitons, produced by strongly 370 nm absorbed light in neat PtOEP films, are efficiently quenched by electron transfer to the metal (Al), generating the positive charge with an efficiency eta+ exceeding 0.15 at high electric fields and dominating the measured photocurrent. Their dissociation efficiency in the bulk, eta- (negatively biased illuminated electrode), is more than one order of magnitude lower than eta+. The dissociation of singlet dimer states dominates the bulk photogeneration process induced by the weakly-absorbed light at 450 nm, with comparable eta+ and eta-. The "hot excited state" underlying the temperature-increasing emission at 540 nm has been attributed to the upper excitonic component Q+ of the first absorption band Q consistent with the exciton concept applied successfully to the interpretation of all dimer-underlain spectroscopic features of PtOEP samples studied.

Sprayed Organic Electrophosphorescent Devices with Small Organic Molecules

Sprayed organic electrophosphorescent devices with small organic molecules have been investigated. By heating the entire spray system, i.e., a spray gun and a glass substrate, a high device performance without leakage current has been realized. Using the green electrophosphorescent molecule fac tris(2-phenyl-pyridine)iridium Ir(ppy)3 as a doped material, device performance with a maximum luminance of 18,400 cd/m2, a maximum luminous power efficiency of 20.9 lm/W, and a maximum quantum efficiency of 16.9% could be obtained. By mixing blue and red electrophosphorescent molecules of iridium(III) bis(2-(4,6-difluorophenyl) pyridinato-N,C2') picolinate (FIrpic) and iridium(III) bis(2-(2'-benzothienyl) pyridinato-N,C3') (acetylacetonate) (Btp2Ir(acac)), white emission with Commission Internationale d'Eclairage chromaticity coordinates of (0.33, 0.34) is also achieved.

Electron-transporting materials for organic electroluminescent and electrophosphorescent devices

One of the requirements for efficient organic electroluminescent devices (OLEDs) is balanced charge injection from the two electrodes and efficient transport of both holes and electrons within the luminescent layer in the device structure. Many of the common luminescent conjugated polymers, e.g. derivatives of poly(phenylenevinylene) and poly(fluorene), are predominantly hole transporters (i.e. p-dopable). This article gives a brief overview of organic electroluminescence and electrophosphorescence and provides a more detailed consideration of ways in which electron transport in these systems has been enhanced by the incorporation of electron-deficient (i.e. n-dopable) small molecules and polymers into the devices, either as blends or by covalent attachment of sub-units to the luminophore or as an additional electron-transporting, hole-blocking (ETHB) layer adjacent to the cathode. The chemical structures of these systems are presented and their roles are assessed. Most of these ETHB molecules are electron-deficient aromatic nitrogen-containing heterocycles, e.g. derivatives of 1,3,4-oxadiazole, pyridine, pyrimidine, pyrazine, quinoline, etc. Non-aromatic thiophene-S,S-dioxide derivatives are also discussed. The article is written from an organic chemist's perspective.

Ultrahigh Energy Gap Hosts in Deep Blue Organic Electrophosphorescent Devices

Four ultrahigh energy gap organosilicon compounds [diphenyldi(o-tolyl)silane (UGH1), p-bis(triphenylsilyl)benzene (UGH2), m-bis(triphenylsilyl)benzene (UGH3), and 9,9‘-spirobisilaanthracene (UGH4)] were employed as host materials in the emissive layer of electrophosphorescent organic light-emitting diodes (OLEDs). The high singlet (∼4.5 eV) and triplet (∼3.5 eV) energies associated with these materials effectively suppress both the electron and energy transfer quenching pathways between the emissive dopant and the host material, leading to deep blue phosphorescent devices with high (∼10%) external quantum efficiencies. Furthermore, by direct charge injection from the adjacent hole and electron transport layers onto the phosphor doped into the UGH matrix, exciton formation occurs directly on the dopant, thereby eliminating exchange energy losses characteristic of guest−host energy transfer. We discuss the material design, and present device data for OLEDs employing UGHs. Among the four host materials, UGH2 and UGH3 have higher quantum efficiencies than UGH1 when used in OLEDs. Rapid device degradation was observed for the UGH4-based device due to electro- and/or photooxidation of the diphenylmethane moiety in UGH4. In addition to showing that UGH materials can be used to fabricate efficient blue OLEDs, we demonstrate that very high device efficiencies can be achieved in structures where the dopant transports both charge and excitons.

Organic electrophosphorescent devices with mixed hole transport material as emission layer

We report on the reduction of driving voltage in organic electrophosphorescent devices with mixed hole transport material as emission layer. The emission layer was mixed with N,N′-diphenyl-N,N′-(3-methylphenyl) -[1,1′-biphenyl]-4,4′-diamine (TPD) as a hole-transport material, 4,4′-bis(9-carbazolyl)-biphenyl (CBP) as a host material for a phosphorescent material, tris(2-phenylpyridine) iridium (Ir(ppy)3) as a phosphorescent material and 2,9-dimethyl-4,7-diphenyl-1,10-phenanthroline (bathocuproine: BCP) as an electron-transport material. The emission occurred from Ir(ppy)3 and was independent on the ratio of TPD and CBP. For triple-layered device with the TPD:CBP mixing ratio of 10:90, maximum luminance obtained was 104,000 cd/m2 at V=12 V, J=490 mA/cm2. Maximum efficiency was 20 lm/W at 3.1 mA/cm2.

Study on electrical characteristics of organic electrophosphorescent devices based on new Ir complex

The organic light-emitting devices (OLEDs) based on fluorescence have low efficiencies due to the requirement of spin-symmetry conservation. By using the phosphorescent material, the internal quantum efficiency can reach 100%, compared to 25% in the case of the fluorescent material. Thus, the phosphorescent OLEDs have recently been extensively studied and showed higher internal quantum efficiencies than the conventional OLEDs. In this study, an orange-red electrophosphorescent devices have been fabricated with a new Ir complex, synthesized by coordinating a new electrowithdrawing lignad to (ppy) 2 Iracac, as the emitting dopant. It has been investigated the electrical and optical characteristics of the devices fabricated with a structure of ITO/NPB/Ir complex doped in CBP/BCP/Alq 3/Li:Al/Al. The device with the Ir complex demonstrated an efficiency of 3.53 cd/A with an initial brightness of 100 cd/m 2. The CIE coordinates for devices held at a current density of 1 mA/cm 2 were (0.585, 0.410).

An Organic Electrophosphorescent Device Driven by All-Organic Thin-Film Transistor using Polymeric Gate Insulator

In this paper, we demonstrate that the organic electrophosphorescent device is driven by the organic thin film transistor with spin-coated photoacryl gate insulator. It was found that electrical output characteristics in our organic thin film transistors using the staggered-inverted top-contact structure showed the non-saturated slope in the saturation region and the sub-threshold nonlinearity in the triode region, where we obtained the maximum power luminance that was about 90 cd/m². Field effect mobility, threshold voltage, and on-off current ratio in 0.45 µm thick gate dielectric layer were 0.17 cm²/Vs, -7 V and 106, respectively. In order to form polyimide as a gate insulator, vapor deposition polymerization process was also introduced instead of spin-coating process, where polyimide film was co-deposited by high-vacuum thermal evaporation from 4,4'-oxydiphthalic anhydride (ODPA) and 4,4'-oxydianiline (ODA) and cured at 150°C for 1hr. It was also found that field effect mobility, threshold voltage, on-off current ratio, and sub-threshold slope with 0.45 µm thick gate dielectric films were 0.134 cm²/Vs, -7 V and 106 AA, and 1 V/decade, respectively.

Hot band emission and energy transfer in organic electrophosphorescent devices

The evolution of the electroluminescence spectra of platinum porphyrin doped polyfluorene light-emitting diodes is investigated after turn-off of the driving pulse. In the delayed spectra a new emission band is identified, which is of comparable intensity to the delayed polyfluorene emission. Through temperature dependent photoluminescence measurements an assignment of this emission is made to a hot level of the platinum porphyrin with a thermal activation energy of 0.42 eV. The evolution of the delayed emission spectra as well as the observation that the hot band at 540 nm overlaps the polyfluorene triplet level suggest that energy transfer can occur between the host and guest.

An organic electrophosphorescent device driven by all-organic thin-film transistor using photoacryl as a gate insulator

Abstract Organic electrophosphorescent devices have been intensively investigated for using in full-color flat-panel display. Since the quantum efficiency of electrophosphorescent device decreases rapidly as the luminance increases, it is desirable to operate the electrophosphorescent display with active matrix rather than passive matrix. Here we report the study of driving electrophosphorescent diode with all-organic TFT. We obtained the maximum power luminance that was obtained about 90 cd/m 2 . Turn-on voltage is approximately 10 V. Field effect mobility, threshold voltage, and on–off current ratio in 0.5-μm thick gate dielectric layer were 0.13 cm 2 /V s, −7 V, and 10 6 A/A. The structure of electrophosphorescent diode is ITO/TPD/BCP:Ir(ppy) 3 /BCP/Alq 3 /Li:Al/Al. In organic TFT, photoacryl is used as an insulator and pentacene as an active layer.

High-efficiency organic electrophophorescent devices through balance of charge injection

Balancing the charge injection plays an important role not only in fluorescence-based organic light-emitting devices, but also in the performance of the recently introduced organic electrophosphorescent devices. By achieving a better balance in the injected charges, it is possible to fabricate devices based on the pure red emitter 2, 3, 7, 8, 12, 13, 17, 18-octaethyl-21H, 23H-porphine, PtOEP, with external quantum efficiency close to 9%.

Efficient electrophosphorescence using a doped ambipolar conductive molecular organic thin film

We demonstrate a high efficiency organic electrophosphorescent device comprised of a 4,4′,4′′-tris(3-methylphenylphenylamino)triphenylamine (m-MTDATA) hole transport layer and a 4,4′-N,N′-dicarbazole-biphenyl (CBP) host doped with the metallorganic phosphor, fac-tris(2-phenylpyridine)iridium (Ir(ppy)3) as the green light-emitting layer. The device exhibits peak external quantum and power efficiencies of (12.0±0.6)% and (45±2) lm/W, respectively, corresponding to ∼60% internal quantum efficiency. A luminance of 1850 cd/m2 is observed at a current density of 10 mA/cm2. The device operating properties are controlled by electron injection into, and transport by the CBP layer along with hole injection from m-MTDATA directly into the Ir(ppy)3 highest occupied molecular level, leading to direct carrier recombination and exciton formation on the phosphor dopant. Ambipolar conduction properties of the Ir(ppy)3:CBP layer are established by analysis of triplet–triplet annihilation, exciton formation and the luminance–current–voltage characteristics.

Transient analysis of organic electrophosphorescence: I. Transient analysis of triplet energy transfer

We examine triplet-exciton dynamics in several phosphorescent organic guest-host systems. In this first of two papers, transient studies are used to understand triplet energy transfer between molecules and also to ascertain the relative importance under electrical injection of charge trapping and direct exciton formation on phosphorescent guest molecules. As an example, we study the distribution of triplet excitons as they diffuse through amorphous films of tris(8-hydroxyquinoline) aluminum $({\mathrm{Alq}}_{3}).$ We find that triplet transport in ${\mathrm{Alq}}_{3}$ is dispersive, and for high concentrations of triplets we find an average lifetime of $\ensuremath{\tau}=25\ifmmode\pm\else\textpm\fi{}15\ensuremath{\mu}\mathrm{s}$ and a diffusion coefficient of ${D}_{T}=(8\ifmmode\pm\else\textpm\fi{}5)\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}8}{\mathrm{cm}}^{2}/\mathrm{s}.$ The understanding of the formation and transport of triplets in a host material is extended in the following paper [Phys. Rev. B 62, 10 967 (2000)] to the study of nonlinearities in the electroluminescent decay of phosphorescent organic guest materials. Finally, we summarize the principle determinants of the efficiency of organic electrophosphorescent devices.

High-efficiency organic electrophosphorescent devices with tris(2-phenylpyridine)iridium doped into electron-transporting materials

We demonstrate high-efficiency organic light-emitting devices employing the green electrophosphorescent molecule, fac tris(2-phenylpyridine)iridium [Ir(ppy)3], doped into various electron-transport layer (ETL) hosts. Using 3-phenyl-4-(1′-naphthyl)-5-phenyl-1,2,4-triazole as the host, a maximum external quantum efficiency (ηext) of 15.4±0.2% and a luminous power efficiency of 40±2 Im/W are achieved. We show that very high internal quantum efficiencies (approaching 100%) are achieved for organic phosphors with low photoluminescence efficiencies due to fundamental differences in the relationship between electroluminescence from triplet and singlet excitons. Based on the performance characteristics of single and double heterostructures, we conclude that exciton formation in Ir(ppy)3 occurs within close proximity to the hole-transport layer/ETL:Ir(ppy)3 interface.