OLED fabrication and characterization

OLED devices were fabricated on 25 mm × 25 mm glass substrates with half-patterned 150-nm-thick ITO layers (AMG, Korea) to study EQE over various values of S (Type A). For fabrication of devices utilizing the scattering-based light extraction to its full potential, 40 mm × 70 mm glass substrates were used instead to accommodate a large back reflector and to avoid interference from adjacent devices or electrodes for contacts (Type B). The substrates were cleaned with deionized water, deionized water with detergent, isopropyl alcohol, and acetone for 15 min, respectively, in an ultrasonic bath. Then, the substrates were treated with UV-plasma for 1 min. Immediately after UV treatment, the substrates were transported to a vacuum thermal evaporator, and all the layers in the OLEDs were deposited under a pressure less than 5×10⁻⁶ torr without breaking the vacuum. OLEDs under study had the following structure: ITO (150 nm)/TAPC (130 nm for Ir(dmppy-ph)₂tmd; 90 and 80 nm for Ir(ppy)₂acac and Ir(ppy)₃, respectively)/TCTA (10 nm)/TCTA: B3PYMPM: Ir(dmppy-ph)₂tmd (4 wt.%, 30 nm) for orange OLEDs; Ir(ppy)₂acac (8 wt.%, 30 nm) or Ir(ppy)₃ (8 wt.%, 30 nm) for green OLEDs /B3PYMPM (65 nm)/LiF (1 nm)/Al (100 nm). Here, TAPC, TCTA, Ir(dmppy-ph)₂tmd, Ir(ppy)₂acac, Ir(ppy)₃, and B3PyMPM refers to di-[4-(N,N-di-p-tolyl-amino)-phenyl]cyclohexane, 4,4′,4″-tris(carbazol-9-yl)triphenylamine, bis(2-(3,5-dimethylphenyl)-4-phenylpyridine) Ir(III) (2,2,6,6-tetramethylheptane-3,5-diketonate), bis(2-phenylpyridine) (acetylacetonate) iridium(III), tris(2-phenylpyridine)iridium(III), and 4,6-bis(3,5-di(pyridin-3-yl)phenyl)-2-methylpyrimidine, respectively. All organic materials except Ir(dmppy-ph)₂tmd were purchased from Nichem Fine Technology co. Ltd. The phosphorescent emitter, Ir(dmppy-ph)₂tmd, was provided from Y.-H.K. and K.-H.K. Area of all devices were approximately 8.0 mm² and were estimated carefully from dimensions measured with a digitized vernier calipers.

The scattering layers were optically coupled to the glass substrates at the opposite side of OLED devices using index matching fluid having a refractive index of 1.52 (IML 150, Norland Products Inc.). The current density, luminance, electroluminance (EL) spectra were measured using a programmable source meter (Keithley 2400), a calibrated Si photodiode, and a fiber optic spectrometer (EPP2000, StellarNet Inc.) in a nitrogen-filled glove box. The photodiode and spectrometer were mounted on a custom-made goniometer to measure the angular EL intensity and spectra. The EQE, power efficiency, and current efficiency were calculated by fully accounting for the measured angular distribution of the EL intensity and spectra. When a scattering layer was applied to an OLED, a 100-nm-thick aluminum back reflector (23 mm × 11 mm for Type A or 38 mm × 38 mm for Type B) prepared on an Si wafer for Type A case and 40 mm × 40 mm glass for type B case was placed underneath the OLED to minimize the amount of light that leaks through the area without a cathode due to the finite dimension of the device area (see discussion in the main text and Fig. 4b). This reflector was essential in reducing the gap between the experiment and simulation, the latter of which assumed an infinitely large device area.