Sergey Lamansky

Very high-efficiency green organic light-emitting devices based on electrophosphorescence

We describe the performance of an organic light-emitting device employing the green electrophosphorescent material, fac tris(2-phenylpyridine) iridium [Ir(ppy)3] doped into a 4,4′-N,N′-dicarbazole-biphenyl host. These devices exhibit peak external quantum and power efficiencies of 8.0% (28 cd/A) and 31 lm/W, respectively. At 100 cd/m2, the external quantum and power efficiencies are 7.5% (26 cd/A) and 19 lm/W at an operating voltage of 4.3 V. This performance can be explained by efficient transfer of both singlet and triplet excited states in the host to Ir(ppy)3, leading to a high internal efficiency. In addition, the short phosphorescent decay time of Ir(ppy)3 (<1 μs) reduces saturation of the phosphor at high drive currents, yielding a peak luminance of 100 000 cd/m2.

High-efficiency red electrophosphorescence devices

We demonstrate high-efficiency red electrophosphorescent organic light-emitting devices employing bis(2-(2′-benzo[4,5-a]thienyl)pyridinato-N,C3′) iridium(acetylacetonate) [Btp2Ir(acac)] as a red phosphor. A maximum external quantum efficiency of ηext=(7.0±0.5)% and power efficiency of ηp=(4.6±0.5) lm/W are achieved at a current density of J=0.01 mA/cm2. At a higher current density of J=100 mA/cm2, ηext=(2.5±0.3)% and ηp=(0.56±0.05) lm/W are obtained. The electroluminescent spectrum has a maximum at a wavelength of λmax=616 nm with additional intensity peaks at λsub=670 and 745 nm. The Commission Internationale de L’Eclairage coordinates of (x=0.68, y=0.32) are close to meeting video display standards. The short phosphorescence lifetime (∼4 μs) of Btp2Ir(acac) leads to a significant improvement in ηext at high currents as compared to the previously reported red phosphor, 2,3,7,8,12,13,17,18-octaethyl-12H, 23H-prophine platinum (II) PtOEP with a lifetime of ∼50 μs.

Cyclometalated Ir complexes in polymer organic light-emitting devices

Several new iridium based cyclometalated complexes were investigated as phosphorescent dopants for molecularly doped polymeric organic light-emitting diodes. Specifically, the complexes used in this study were iridium (III) bis(2-phenylpyridinato-N,C2′) (acetylacetonate) [ppy], iridium (III) bis(7,8-benzoquinolinato-N,C3′) (acetylacetonate) [bzq], iridium (III) bis(2-phenylbenzothiazolato-N,C2′) (acetylacetonate) [bt], iridium (III) bis(2-(2′-naphthyl)benzothiazolato-N,C2′) (acetylacetonate) [bsn] and iridium (III) bis(2-(2′-benzo[4,5-a]thienyl)pyridinato-N,C3′) (acetylacetonate) [btp]. Single layer devices of doped polyvinylcarbazole: 2-(4-biphenyl)-5-(4-tert-butylphenyl)-1,3,4-oxadiazole give maximum external quantum efficiencies that varied from 3.5% for the ppy dopant to 0.4% for the btp dopant. Several different device heterostructure architectures were explored, and the best quantum efficiency of the devices reached 4.2% for the heterostructures.

Molecularly doped polymer light emitting diodes utilizing phosphorescent Pt(II) and Ir(III) dopants

Abstract The use of molecular phosphorescent dyes in polymer-based organic light emitting diodes (OLED) of different architectures was investigated by incorporating several phosphorescent dopants into poly( N -vinylcarbazole) (PVK)-based single layer and single heterostructure light emitting diodes (LEDs). In particular, cis -bis[2-(2-thienyl)pyridine-N,C 3 ] platinum(II) (Pt(thpy) 2 ) and platinum(II) 2,8,12,17-tetraethyl-3,7,13,18-tetramethyl porphyrin (PtOX), and an Ir(III) compound, fac -tris[2-(4 ′ ,5 ′ -difluorophenyl)pyridine-C ′2 ,N] iridium(III) (FIrppy) were used. The maximum external quantum efficiency of phosphorescent devices exceeds 0.6% for the two Pt dopants and reaches ≈1.8% for FIrppy. An overall increase in LED efficiency compared to similar devices based on fluorescence is attributed to the fact that phosphorescent dopants allow both singlet and triplet excitons to be involved in emission. In addition to finding an energetically suitable dopant, such parameters as dopant concentration and organic layer thickness influence the performance of the LEDs. Introduction of an electron injecting layer of tris(8-hydroxyquinoline) aluminum(III) causes an increase of quantum efficiency of up to 1.8–2.8%. The second order quenching process present in these OLEDs, which is prevalent at high current densities, is most likely not due to T–T annihilation of excitons trapped at dopant sites in these OLEDs. T–T annihilation in the PVK matrix or trapped charge-triplet annihilation are more likely explanations for the decrease.

High-efficiency organic electrophosphorescent devices

We have fabricated saturated red, orange, yellow and green OLEDs, utilizing phosphorescent dopants. Using phosphorescence based emitters we have eliminated the inherent 25% upper limit on emission observed for traditional fluorescence based systems. The quantum efficiencies of these devices are quite good, with measured external efficiencies > 15% and > 40 lum/W (green) in the best devices. The phosphorescent dopants in these devices are heavy metal containing molecules (i.e. Pt, and Ir), prepared as both metalloporphyrins and organometallic complexes. The high level of spin orbit coupling in these metal complexes gives efficient emission from triplet states. In addition to emission from the heavy metal dopant, it is possible to transfer the exciton energy to a fluorescent dye, by Forster energy transfer. The heavy metal dopant in this case acts as a sensitizer, utilizing both singlet and triplet excitons to efficiently pump a fluorescent dye. We discuss the important parameters in designing electrophosphorescent OLEDs as well as their strengths and limitations. Accelerated aging studies, on packaged devices, have shown that phosphorescence based OLEDs can have very long device lifetimes.