Yiru Sun

Management of singlet and triplet excitons for efficient white organic light-emitting devices

Lighting accounts for approximately 22 per cent of the electricity consumed in buildings in the United States, with 40 per cent of that amount consumed by inefficient (∼15 lm W-1) incandescent lamps. This has generated increased interest in the use of white electroluminescent organic light-emitting devices, owing to their potential for significantly improved efficiency over incandescent sources combined with low-cost, high-throughput manufacturability. The most impressive characteristics of such devices reported to date have been achieved in all-phosphor-doped devices, which have the potential for 100 per cent internal quantum efficiency: the phosphorescent molecules harness the triplet excitons that constitute three-quarters of the bound electron–hole pairs that form during charge injection, and which (unlike the remaining singlet excitons) would otherwise recombine non-radiatively. Here we introduce a different device concept that exploits a blue fluorescent molecule in exchange for a phosphorescent dopant, in combination with green and red phosphor dopants, to yield high power efficiency and stable colour balance, while maintaining the potential for unity internal quantum efficiency. Two distinct modes of energy transfer within this device serve to channel nearly all of the triplet energy to the phosphorescent dopants, retaining the singlet energy exclusively on the blue fluorescent dopant. Additionally, eliminating the exchange energy loss to the blue fluorophore allows for roughly 20 per cent increased power efficiency compared to a fully phosphorescent device. Our device challenges incandescent sources by exhibiting total external quantum and power efficiencies that peak at 18.7 ± 0.5 per cent and 37.6 ± 0.6 lm W-1, respectively, decreasing to 18.4 ± 0.5 per cent and 23.8 ± 0.5 lm W-1 at a high luminance of 500 cd m-2.

High-efficiency white organic light emitting devices with three separate phosphorescent emission layers

We demonstrate high-efficiency white organic light emitting devices (WOLEDs) employing three adjacent phosphorescent emission layers (3-EMLs). The metal-organic dopants for red, green, and blue emissions are each doped in separate hosts, allowing for separate optimization of the three dopant-host material combinations. This structure distributes the exciton generation region across the three hosts to form a stepped progression of highest occupied and lowest unoccupied molecular orbitals. The 3-EML WOLED has a color rendering index of 81 and peak forward-viewing external quantum (EQE) and power efficiencies (PE) of (16.6±0.8)% and 32±1lm∕W, respectively, corresponding to a total EQE=(28±1)% and a total PE=54±3lm∕W. When an n-doped electron transporting layer is used, the total PE peaks at 64±3lm∕W, and rolls off to 34±2lm∕W at 1000cd∕m2.

Organic light emitting devices with enhanced outcoupling via microlenses fabricated by imprint lithography

High efficiency white organic light emitting devices (WOLEDs) with optical outcoupling enhanced by hexagonal polymethylmethacrylate microlens arrays fabricated by imprint lithography on a glass substrate are demonstrated. Monte Carlo and finite difference time domain simulations of the emitted light are used to optimize the microlens design. The measured enhancement of light outcoupling and the angular dependence of the extracted light intensity are in agreement with the simulation. Using microlens arrays, we demonstrate a fluorescent/phosphorescent WOLED with a maximum external quantum efficiency of (14.3±0.3)% at 900cd∕m2 and power efficiency of 21.6±0.5lm∕W at 220cd∕m2. The electroluminescent spectra at viewing angles from normal to the substrate plane, to 60° off normal, remain almost unchanged, giving a color rendering index of 87.

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 u...

Stacked white organic light-emitting devices based on a combination of fluorescent and phosphorescent emitters

We demonstrate a white stacked organic light-emitting device (WSOLED) employing the blue fluorescent emitter, 4,4′-bis(9-ethyl-3-carbazovinylene)-1,1′-biphenyl, and the green and red phosphorescent emitters, fac-tris(2-phenylpyridinato-N,C2′) iridium (III) and iridium (III) bis(2-phenyl quinolyl-N,C2′) acetylacetonate, respectively. The charge generation region consists of a Li-doped electron transport layer and a highly transparent MoOx thin film. For a two-element white SOLED (2-WSOLED), the combination of red and green phosphors with a blue fluorophore yields maximum external quantum and power efficiencies of ηext=23%±2% at a current density of J=1mA∕cm2 and ηp=14±1lm∕W at J=0.17mA∕cm2, respectively. Due to the low optical and electrical losses of the charge generation layer, the efficiencies scale approximately linearly with the number of independent emissive elements in the WSOLED. Hence, for a 3-WSOLED, the total external and power efficiencies estimated for operation of the device in a light fixtur...

Photophysics of Pt-porphyrin electrophosphorescent devices emitting in the near infrared

The triplet annihilation dynamics of near infrared organic light-emitting devices are studied with peak electrophosphorescence at a wavelength of 772nm using a platinum-porphyrin derivative Pt(II)-tetraphenyltetrabenzoporphyrin as dopant. Both the photoluminescent decay transients of the thin films and the quantum efficiency versus current density characteristics of devices using tris(8-hydroxyquinoline) aluminum or 4,4′-bis(N-carbazolyl)biphenyl (CBP) as hosts are fitted by a model based on triplet-triplet annihilation. When the phosphor is codoped with Ir(III) bis(2-phenyl quinolyl-N,C2′) acetylacetonate in CBP, the quantum efficiency is enhanced, and the observed decrease of efficiency at high current densities is explained by field-induced charge pair dissociation. The external quantum efficiency has a maximum of (8.5±0.3)%, decreasing to (5.0±0.3)% at 1mA∕cm2.

White organic light-emitting device based on a compound fluorescent- phosphor-sensitized-fluorescent emission layer

The authors demonstrate a combination fluorescent and phosphor-sensitized-fluorescent white organic light-emitting device (WOLED), employing the conductive host material, 4,4′-bis(9-ethyl-3-carbazovinylene)-1,1′-biphenyl, doped with the phosphorescent green, and the fluorescent red and blue emitters, fac-tris(2-phenylpyridinato-N,C2′) iridium (III), 4-(dicyanomethylene)-2-t-butyl-6-(1,1,7,7-tetramethyljulolidyl-9-enyl)-4H-pyran, and 4,4′-bis (9-ethy-3-carbazolvinylene)-1-1′-biphenyl, respectively. Although two fluorescent dopants are employed along with only a single phosphor, this simple structure can, in principle, achieve 100% internal quantum efficiency. In the prototype, the phosphor-sensitized WOLED exhibits total external quantum and power efficiencies of ηext,tot=13.1±0.5% and ηp,tot=20.2±0.7lm∕W, respectively, at a luminance of 800cd∕m2 with Commission Internationale de L’Eclairage chromaticity coordinates of (x=0.38, y=0.42) and a color rendering index of 79.

Enhanced outcoupling from organic light-emitting diodes using aperiodic dielectric mirrors

Aperiodic dielectric stacks between the substrate and transparent anode in organic light-emitting diodes are used to improve the optical outcoupling efficiency. The authors demonstrate that a nine-layer SiO2∕SiNx aperiodic dielectric stack improves the brightness by 80% within a 60° viewing cone for a red-emitting organic light-emitting diode, while maintaining a Lambertian emission pattern. As the refractive index contrast between the two materials used in a two-component multilayer dielectric stack is increased, a brightness improvement of 170% in a 60° viewing cone is achievable while maintaining a Lambertian emission profile.

Direct patterning of organic light-emitting devices by organic-vapor jet printing

We demonstrate small molecular weight, fluorescent organic light-emitting devices directly patterned by organic-vapor jet printing (OVJP). In OVJP, a hot inert carrier gas picks up a molecular organic vapor that expands when passed through a nozzle, resulting in physisorption of the molecules onto a cooled substrate. A (1cm)2 flat organic semiconductor thin film is achieved by sweeping the nozzle over the substrate to print closely spaced lines. Using an array of nozzles, multiple parallel stripes are simultaneously printed at a growth rate of 3.4Acm2∕s. Organic heterostructure fluorescent-light-emitting devices consisting of a broad area hole transport layer, followed by a striped electron transport layer, both printed by OVJP, yield an external quantum efficiency of (0.84±0.03)%, which is comparable to that of similar devices deposited by vacuum thermal evaporation.

Taking a Visible Step Forward into the Non-Visible (Infrared) Region

This presentation focuses on our most recent work in the area of red to near-IR emitting OLEDs. The discussion will include descriptions of emitter design, device structures, external efficiencies and lifetimes of these devices.