Session U29: Organic Light Emitting Diodes

High Efficiency Organic Light-Emitting Devices Having Charge Generation Layers

A new type of organic LEDs having charge generation layers (CGLs) were developed. By applying voltage, holes and electrons are generated at CGL and injected to adjacent organic layers to recombine with the carriers with opposite polarity. Thus, current efficiency can be greatly improved. An extremely high current efficiency of 130 cd/A, which is equivalent to more than 30 % external quantum efficiency, was observed from a device. One of the devices exhibited the lifetime of over 1,000,000 hours at the initial luminance of 100 cd/m2.   

White polymer LED and its integration with polymer transistor

Bright white emission with peak luminance near 10,000 cd/m$^{2}$ is achieved in multi-layer homojunction polymer light-emitting diode (PLED) fabricated by multiple spin coating. The homojunction has the advantages of exciton confinement, carrier balance, and reduced cathode quenching. In order to be applied in an all-polymer active matrix display, multi-layer PLED is integrated with polymer transistor to form a polymer active pixel without the patterning of any polymer layer. The key idea is to replace the conventional conductive hole-transport layer (HTL) for the PLED by a semiconductor, which can then be shared with the transistor in the integrated structure. In this integration both the semiconductor layer and the emissive layer can be spin-coated in large area covering the whole active matrix. We use high mobility polymer polythiophene for the HTL and the transistor. Peak luminance of 3000 cd/m$^{2}$ for white emission on P3HT is reached. A 200 $\mu $m$\times \mu $m polymer active pixel free of patterning of any organic layer is demonstrated.   

High Performance White Organic Light-Emitting Diodes

White organic light-emitting diodes (OLEDs) are being considered as potential solid-state lighting sources. Some of the challenges toward that goal include low-cost fabrication of white OLEDs with high brightness and efficiencies using simple device architectures. White OLEDs were fabricated from multilayers or blends based on poly(9,9-dioctylfluorene) (PFO) and poly(2-methoxy-5(2'-ethyl-hexyloxy)-1,4-phenylenevinylene) (MEH-PPV). Insertion of a non-emissive polymer buffer layer between MEH-PPV and PFO allowed regulation of energy transfer between the two emissive polymers, affording efficient white OLEDs. Bright white light with a brightness of 1446 cd/m$^{2}$, an external quantum efficiency (EQE) of 0.94{\%}, and a device efficiency of 1.1 cd/A was observed. The polymer blend OLEDs gave white light with CIE coordinates of (0.33, 0.34), a luminance of 4000 cd/m$^{2}$, an EQE of 3.1{\%} and a luminous efficiency of 3.7 cd/A. The emission color and the performance of the blend devices were highly dependent on the composition and the morphology of the blends.   

Highly Efficient Blue Electroluminescence from n-Type Conjugated Oligoquinolines

Achievement of blue electroluminescence (EL) with high efficiency, color purity and stability remains a challenge for full-color organic light-emitting diode (OLED) based displays. A series of n-type, thermally robust (glass transition temperature T$_{g} \quad >$ 130 \r{ }C) oligoquinolines based on the 6,6'-bis(4-phenylquinoline) core has been synthesized and used as emissive and electron transport materials for blue OLEDs. Simple bilayer diodes gave stable blue EL with CIE coordinates at (0.15, 0.16), maximum luminance of 4000 cd/m$^{2}$ and luminous efficiency of 7.9 cd/A (at 945 cd/m$^{2})$. These results represent one of the best blue OLED performances reported to date from non-doped, fluorescent organic emitters. The high T$_{g}$s render the amorphous oligoquinoline films very stable with excellent EL spectral stability. These results demonstrate that oligoquinolines are promising blue emitters and electron transport materials for developing high-efficiency OLEDs with a simple architecture.   

Electroluminescent devices from ionic transition metal complexes

During the last fifteen years dramatic advances have been achieved in the performance of organic light emitting diodes (OLEDs), and these devices can now be found in several consumer electronic products. A recent trend in OLEDs involves the use of ionic transition metal complexes as the electroluminescent layer. The mechanism of operation of OLEDs based on these materials is determined by a complex interplay between ionic and electronic charge. As a result of this interplay, efficient devices can be fabricated using air-stable electrodes. Moreover, large-area lighting panels that operate straight from the outlet without any additional circuitry can be fabricated. Materials issues that need to be addressed for these devices to succeed in applications will be discussed.   

A Time-Dependent Density Functional Theory Study of One-and Two-Photon Absorption: Stilbene- and Fluorene-Based Donor-Acceptor Chromophores

In our ongoing theoretical studies to predict the photophysical properties of optical materials, we report the results for one-photon, and two-photon absorption (OPA and TPA) spectra, for a series of compounds, in which electron donating and accepting groups are attached to a core having a delocalized electron structure, such as stilbene or fluorene. Linear response time-dependent density functional theory, with hybrid exchange-correlation functionals, was applied in all calculations. We find that the calculated excitation energies are generally in good agreement with experiment, particularly when compared to measurements carried out in a nonpolar solvent. Predicted TPA cross-sections, applying the two-state approximation, are also in relatively good agreement with experiment; however, a lack of systematic experimental data on solvent effects limits a detailed comparison as yet.   

Optically Detected Magnetic Resonance (ODMR) Studies Critical to the Determination of the Yield of Singlet Excitons in Fluorescence-Based OLEDs

Recent ODMR studies, including (1) photoluminescence (PL)-detected magnetic resonance (PLDMR) of small $\pi $-conjugated molecules, (2) electroluminescence (EL)- and electrically-detected magnetic resonance (ELDMR and EDMR, respectively) studies of small molecular OLEDs, (3) double modulation-PLDMR studies of $\pi $-conjugated polymers, and (4) joint PLDMR and thermally stimulated luminescence (TSL) studies of $\pi $-conjugated polymers are reviewed. The results of each of these studies are inconsistent with the model in which the positive spin 1/2 (polaron) resonance is due to enhanced delayed PL from nongeminate polaron recombination (``the delayed PL model''). Since the delayed PL model is the basis for the previous ODMR studies which predicted the yield of singlet excitons (SEs) in OLEDs, the recent ODMR studies reopen this issue. It is shown that all of the ODMR results obtained to date are consistent with ``the quenching model,'' in which the population of polarons and triplet excitons (TEs) is reduced by magnetic resonance conditions, and leads to reduced quenching of SEs by polarons and TEs. A detailed quantitative model confirms that the mechanism which causes the reduction in the polaron and TE population is the enhanced annihilation of TEs by polarons, whose populations are much larger than that of SEs under normal excitation conditions. *Operated by Iowa State University for the US Department of Energy under Contract No. W-7405-Eng-82.   

Quenching of Photoluminescence and Electroluminescence in OLEDs by Exciton-Charge and Exciton-Dopant Interactions

Electronic dopants such as the strong acceptor F4-TCNQ are used for p-type doping of hole transport layers (HTL) in organic light-emitting diodes (OLEDs). These molecules are found to quench the electroluminescence (EL) if they diffuse into the emissive layer. We have observed EL quenching in OLEDs with a HTL doped with F4-TCNQ. To separate the effects of exciton-dopant quenching from exciton-polaron quenching we have intentionally doped the emissive layer of Alq3 with three acceptors (A) of different electron affinity: F4-TCNQ, TCNQ and C60. We have also taken photoluminescence spectra of Alq3 films doped with identical concentrations of the three acceptors in order to separate the effects of these dopants on electroluminescence and photoluminescence. These results are presented, and channels for energy and charge transfer between excitons and both neutral and charged dopant molecules are described.   

Chain Conformations and Photoluminescence in Poly(di-$n$-octylfluorene)

The diverse steady-state spectroscopic properties of poly(di-$n$-octylfluorene) are addressed from a molecular-level perspective. Modeling of representative oligomers support the experimental observation of at least three distinguishable classes of conformational isomers with differing chain torsion angles. One class appears to be populated by a single compact structural isomer and this appears to conform to the so called $\beta$ phase. A rigorous Franck-Condon analysis of the photoluminescence in conjunction with Frenkel-type exciton band structure calculations are performed. These results accurately reproduce all major spectral features of the photoabsorption and those of singlet exciton emission.   

Enhanced triplet formation in polyfluorene blends

Formation of triplet excitons may be an important loss mechanism in organic light-emitting diodes (LEDs) and photovoltaics. Here we use photoinduced absorption spectroscopy to study the generation of triplet excitons after photoexcitation of a blend of the fluorene-based conjugated polymers poly(9,9'-dioctylfluorene-co-benzothiadiazole) (F8BT) and poly(9,9'-dioctylfluorene-co-bis-N,N'-(4-butylphenyl)-bis-N,N'-phenyl-1,4-phenylene-diamine) (PFB). The triplet generation rate is found to be $\sim $10 times higher in F8BT:PFB than in F8BT alone. We attribute this effect to increased intersystem crossing in the charge-separated states formed at the polymer/polymer heterojunctions in the blend. Applying an electric field dissociates these states and thus reduces the rate of triplet state formation. We will discuss the implications of this result for the operation of polymer blend LEDs and photovoltaics.   

Aggregation can enhance the O/PLED efficiency

In general, the aggregation effects are expected to quench the luminescence. Here, we show in two instances that the intermolecular interaction can enhance the O/PLEDs internal quantum efficiency. At first instance, for the organic LEDs, the siloles molecules exhibit exotic emission behavior, namely, non-luminescent in solution form, but highly luminescent in aggregation. After a detailed theoretical calculation on the non-adiabatic decay rate, we find that it is the aggregation inhibit the radiationless route, thus allowing the radiative decay in solid state. In terms of PLEDs, we take into account both the electronic correlation and electron-phonon coupling, and we find that the interchain coupling effects can actually allow PLEDs to have much higher internal quantum efficiency than the 25{\%} spin statistical limit.   

Field-induced switch from heterojunction to bulk charge recombination in bilayer light-emitting diodes

Optoelectronic devices made from semiconductor polymers often employ partially phase-separated binary polymer blends with distributed heterojunctions in the polymer film. We investigate the photo- and electroluminescence from bilayers of electron- and hole-transporting polyfluorene derivatives at different device temperatures. For low driving voltages (below 2.4$\times$10$^{5}$\,V/cm$^{2}$ at room temperature), we give direct evidence for barrier-free charge capture at the heterojunction. In this mechanism, charge capture produces an interfacial excited state (exciplex) directly and bulk exciton electroluminescence is only achieved through endothermic transfer (activation energy 200\,meV) from the exciplex. For high driving voltages (above 8.3$\times$10$^{5}$\,V/cm$^{2}$ at 43\,K), however, we find that charges are injected over the heterojunction barriers and subsequent charge capture occurs in the polymer bulk. Furthermore, if bulk excitons migrate to another heterojunction site within their lifetime they are re-trapped at the interface and again form exciplex states or dissociate completely. We demonstrate that in polymer blend light-emitting diodes this can reduce the exciton population by more than 70\% and strongly influences the emission spectrum. We then analyze exciton re-trapping in detail using time-resolved photoluminescence spectroscopy on blends with different morphologies and find that for nm-scale phases exciton emission is completely suppressed.   

Physics of Electroluminescent Devices Based on Ionic Transition Metal Complexes

A recent trend in OLEDs involves the use of ionic transition metal complexes as the electroluminescent layer. The mechanism of operation of OLEDs based on these materials is determined by a complex interplay between ionic and electronic charge. We carry out forward time integration of rate equations to solve the bipolar current problem in the presence of ionic charge. We discuss the transient behavior of the current and radiance, as well as steady-state parameters (electric field and charge distribution, recombination profile) as a function of the mobilities of electronic and ionic carriers.