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Journal ArticleDOI

Weak microcavity effects in organic light-emitting devices

15 Aug 1998-Physical Review B (American Physical Society)-Vol. 58, Iss: 7, pp 3730-3740
TL;DR: In this article, an integrated classical and quantum-mechanical theory of weak microcavity effects in layered media that treats both radiative and wave-guided modes is presented, with the transition probability into each mode given by Fermi's ''golden rule''.
Abstract: We present an integrated classical and quantum-mechanical theory of weak microcavity effects in layered media that treats both radiative and waveguided modes. The electromagnetic field of radiative modes is determined using classical field quantization, with the transition probability into each mode given by Fermi's ``golden rule.'' We apply this theory to model the dependence of the electroluminescence spectral intensity and polarization of organic light-emitting devices (OLED's) on emission angle, organic layer thickness, and applied voltage. Light propagation in the OLED layers and the substrate is described by both ray and wave optics. Theoretical predictions are compared to experimental observations on single heterostructure, and multiple layer stacked red-green-blue OLEDs. Analysis of the polarization, spectral shape, and intensity of the electroluminescence spectrum in the forward-scattered half plane accurately fits the experimental data. The theory predicts, and the experimental measurements confirm, that the in-plane emission from conventional OLED structures is strongly TM polarized, and can be redshifted by as much as 60 nm with respect to the peak emission in the normal direction. Measurements coupled to our analysis also indicate that the efficiency of generating singlet excitons in aluminum tris(8-hydroxyquinoline) $({\mathrm{Alq}}_{3})$-based OLED's is $5\ifmmode\pm\else\textpm\fi{}1%,$ with a \ensuremath{\sim}500-\AA{}-thick ${\mathrm{Alq}}_{3}$ layer corresponding to the highest external power efficiency.
Citations
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Journal ArticleDOI
TL;DR: In this paper, the authors demonstrate very high efficiency electrophosphorescence in organic light-emitting devices employing a phosphorescent molecule doped into a wide energy gap host, achieving a maximum external quantum efficiency of 19.0±1.0 and luminous power efficiency of 60±5 lm/W.
Abstract: We demonstrate very high efficiency electrophosphorescence in organic light-emitting devices employing a phosphorescent molecule doped into a wide energy gap host. Using bis(2-phenylpyridine)iridium(III) acetylacetonate [(ppy)2Ir(acac)] doped into 3-phenyl-4(1′-naphthyl)-5-phenyl-1,2,4-triazole, a maximum external quantum efficiency of (19.0±1.0)% and luminous power efficiency of (60±5) lm/W are achieved. The calculated internal quantum efficiency of (87±7)% is supported by the observed absence of thermally activated nonradiative loss in the photoluminescent efficiency of (ppy)2Ir(acac). Thus, very high external quantum efficiencies are due to the nearly 100% internal phosphorescence efficiency of (ppy)2Ir(acac) coupled with balanced hole and electron injection, and triplet exciton confinement within the light-emitting layer.

3,302 citations

Journal ArticleDOI
TL;DR: In this article, the advantages and disadvantages of several WOLED architectures in terms of efficiency and color quality are discussed, as well as their widespread acceptance as solid-state lighting sources.
Abstract: White organic light-emitting devices (WOLEDs) have advanced over the last twelve years to the extent that these devices are now being considered as efficient solid-state lighting sources. Initially, WOLEDs were targeted towards display applications for use primarily as liquid-crystal display backlights. Now, their power efficiencies have surpassed those of incandescent sources due to improvements in device architectures, synthesis of novel materials, and the incorporation of electrophosphorescent emitters. This review discusses the advantages and disadvantages of several WOLED architectures in terms of efficiency and color quality. Hindrances to their widespread acceptance as solid-state lighting sources are also noted.

1,962 citations

Journal ArticleDOI
01 Oct 2018-Nature
TL;DR: The formation of submicrometre-scale structure in perovskite light-emitting diodes can raise their external quantum efficiency beyond 20%, suggesting the possibility of both high efficiency and high brightness.
Abstract: Light-emitting diodes (LEDs), which convert electricity to light, are widely used in modern society—for example, in lighting, flat-panel displays, medical devices and many other situations. Generally, the efficiency of LEDs is limited by nonradiative recombination (whereby charge carriers recombine without releasing photons) and light trapping1–3. In planar LEDs, such as organic LEDs, around 70 to 80 per cent of the light generated from the emitters is trapped in the device4,5, leaving considerable opportunity for improvements in efficiency. Many methods, including the use of diffraction gratings, low-index grids and buckling patterns, have been used to extract the light trapped in LEDs6–9. However, these methods usually involve complicated fabrication processes and can distort the light-output spectrum and directionality6,7. Here we demonstrate efficient and high-brightness electroluminescence from solution-processed perovskites that spontaneously form submicrometre-scale structures, which can efficiently extract light from the device and retain wavelength- and viewing-angle-independent electroluminescence. These perovskites are formed simply by introducing amino-acid additives into the perovskite precursor solutions. Moreover, the additives can effectively passivate perovskite surface defects and reduce nonradiative recombination. Perovskite LEDs with a peak external quantum efficiency of 20.7 per cent (at a current density of 18 milliamperes per square centimetre) and an energy-conversion efficiency of 12 per cent (at a high current density of 100 milliamperes per square centimetre) can be achieved—values that approach those of the best-performing organic LEDs. The formation of submicrometre-scale structure in perovskite light-emitting diodes can raise their external quantum efficiency beyond 20%, suggesting the possibility of both high efficiency and high brightness.

1,404 citations

Journal ArticleDOI
TL;DR: In this article, the authors 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.
Abstract: 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.

1,088 citations

References
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Journal ArticleDOI
TL;DR: In this article, a double-layer structure of organic thin films was prepared by vapor deposition, and efficient injection of holes and electrons was provided from an indium-tinoxide anode and an alloyed Mg:Ag cathode.
Abstract: A novel electroluminescent device is constructed using organic materials as the emitting elements. The diode has a double‐layer structure of organic thin films, prepared by vapor deposition. Efficient injection of holes and electrons is provided from an indium‐tin‐oxide anode and an alloyed Mg:Ag cathode. Electron‐hole recombination and green electroluminescent emission are confined near the organic interface region. High external quantum efficiency (1% photon/electron), luminous efficiency (1.5 lm/W), and brightness (>1000 cd/m2) are achievable at a driving voltage below 10 V.

13,185 citations

Journal ArticleDOI
TL;DR: In this paper, a multilayer-doped EL was constructed using a hole-transport layer and a luminescent layer, and the electron-hole recombination and emission zones can be confined to about 50 A near the hole.
Abstract: Electroluminescent (EL)devices are constructed using multilayer organic thin films. The basic structure consists of a hole‐transport layer and a luminescent layer. The hole‐transport layer is an amorphous diamine film in which the only mobile carrier is the hole. The luminescent layer consists of a host material, 8‐hydroxyquinoline aluminum (Alq), which predominantly transports electrons. High radiance has been achieved at an operating voltage of less than 10 V. By doping the Alq layer with highly fluorescent molecules, the EL efficiency has been improved by about a factor of 2 in comparison with the undoped cell. Representative dopants are coumarins and DCMs. The ELquantum efficiency of the doped system is about 2.5%, photon/electron. The EL colors can be readily tuned from the blue‐green to orange‐red by a suitable choice of dopants as well as by changing the concentration of the dopant. In the doped system the electron‐hole recombination and emission zones can be confined to about 50 A near the hole‐transport interface. In the undoped Alq, the EL emission zone is considerably larger due to excitondiffusion. The multilayerdopedEL structure offers a simple means for the direct determination of excitondiffusion length.

3,009 citations

Journal ArticleDOI
TL;DR: In this paper, a relation between the normalized radiated powers radiated by magnetic and electric dipoles is established, and the authors show that the classical results for the radiated power yield the correct normalized spontaneous emission rates from an excited atomic state for electric and magnetic dipole transitions, respectively.
Abstract: Expressions for the total power radiated by magnetic and electric dipoles of arbitrary orientation located in a medium 1 at distance z0 from the interface to a homogeneous or planar stratified medium 2 are derived. A relation between the normalized powers radiated by magnetic and electric dipoles is established. For a homogeneous loss-free medium 2, curves of the normalized powers L(z0)/L∞ radiated by magnetic and electric dipoles versus the normalized distance z0/λ1 are presented for different values of the relative refractive index n = n2/n1 as the only parameter. The computer calculations are compared with analytical expressions derived for small and large distances. For n > 1, the contribution of the evanescent waves to the radiated power is calculated separately. We show that the classical results for the normalized radiated power yield the correct normalized spontaneous emission rates from an excited atomic state for electric and magnetic dipole transitions, respectively. We point out that the results for the electric dipole also give the change of the total power scattered by a small dielectric scattering particle when it is placed close to an interface.

469 citations

Journal ArticleDOI
TL;DR: In this article, the important changes produced on the electroluminescence characteristics of organic materials due to planar microcavity effects are examined in detail, and the design considerations for and device characteristics of a novel multiple emissive layer LED are also described.
Abstract: The important changes produced on the electroluminescence characteristics of organic materials due to planar microcavity effects are examined in detail. The photon density of states is redistributed such that only certain wavelengths, which correspond to allowed cavity modes, are emitted in a given direction. This enables us to realize color selectivity over a large wavelength (and color coordinate) range with broadband emitters such as 8‐hydroxyquinoline aluminum (Alq), and intensity enhancement in narrow band emitters. The intensity enhancement in Alq‐based cavity light emitting diodes (LEDs) is extensively evaluated both experimentally and theoretically. The design considerations for and device characteristics of a novel multiple emissive layer LED are also described.

309 citations

Journal ArticleDOI
Kristiaan Neyts1
TL;DR: In this paper, a theoretical expression for the radiation pattern of light-emitting devices based on thin-film technology is provided, discriminating for polarization, emission angle, absorption, and transmission; and numerical calculation of discrete modes, narrow modes, and evanescent waves near absorbing media is discussed.
Abstract: In light-emitting devices based on thin-film technology, light waves that are partially or totally reflected at interfaces between different materials interfere and influence the angular distribution of the emitted light. For an electrical dipole transition, the radiation pattern is equivalent to that of an electrical dipole antenna. New theoretical expressions are provided for the radiation, discriminating for polarization, emission angle, absorption, and transmission; and the numerical calculation of discrete modes, narrow modes, and evanescent waves near absorbing media is discussed.

299 citations