In this work we report highly efficient organic light emitting diode (OLED) based on dielectric/metal/dielectric (DMD) structured transparent anode. This anode consists of MoO3 as a hole-injection buffer layer, semitransparent metal (Au or Ag) and HfO2 as a high refractive index dielectric material. The optical properties of HfO2 were studied and a HfO2 thin film was spin-coated on the glass substrate. Based on morphology and ellipsometry measurements, the appropriate spin-coating condition and annealing temperature were selected. Further optical calculation shows that the highest micro-cavity effect at 520 nm occurs around the thickness of materials of 15 and 66 nm for Au and HfO2 respectively.Combining all of the measurements above, green OLED device on the glass substrate was fabricated with the structure of HfO2 (x nm) /Au (15 nm) /MoO3/di-[4-(N,N-ditolyl-amino)-phenyl]cyclohexane[TAPC] ( 50 nm) /4,4’-Bis(carbazol-9-yl)biphenyl [CBP] :[acetylacetonatobis(2-phenylpyridine)]iridium [Ir(ppy)2](8%, 30 nm)/[1,3,5-tris(1-phenyl-1H-benzimidazol-2-yl)]benzene[TPBi] (40 nm)/LiF (1 nm)/Al (100 nm). The thickness of HfO2 was varied from 60 to 90 nm to check the correlation between optical simulation and experimental results. Also the conventional green OLED device based on ITO anode was fabricated as a reference. The measured peak efficiencies of the devices with DMD electrode showed enhancement more over than 70% with external quantum efficiency (EQE) (20,53 %),luminous current efficiency (LCE) (77,82 cd/A),power efficiency (PE) (43,27 lm/W) versus to EQE(11,78 %),LCE (42,47 cd/A),PE (21,78 lm/W) for ITO based anode at 1000 cd/m2. The device with Ag in the DMD structurewas improved even more due to a better optical transmittance of silver in the optical region and EQE reached to 22.43% at 1000 cd/m2.
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Improvement of outcoupling efficiency in organic light-emitting diodes using multilayer electrode
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