C. J. Lee

Transparent conducting metal electrode for top emission organic light-emitting devices: Ca–Ag double layer

We have fabricated a transparent conducting double-layer metal electrode for top emission organic light-emitting devices which consists of thin layers of Ca and Ag metals of different thicknesses, deposited by the vacuum evaporation technique. The process is clean and does not damage the underlaying organic layers. High optical transparency over 70%, low reflectivity (14%) in the visible region, and low electrical sheet resistance (12 ohms/square) in Ca(10 nm)–Ag(10 nm) structures are reported. This transparent conducting Ca–Ag metal electrode opens a practical way to fabricate top-emitting organic displays without generating damage-induced states.

Highly efficient top-emitting white organic electroluminescent devices

We have developed highly efficient white top-emitting organic light-emitting devices with broad emission by modifying both the anode and cathode. To alleviate the undesirable microcavity effect and obtain “broad” white emission, a CFx-coated Ag anode and an index-matching layer (SnO2) capped on a thin Ca∕Ag cathode with a maximum transparency of 80% were employed. A top-emitting broad white-light device, based on the dual-layer architecture of light blue and yellow emitters with one of the highest EL efficiencies of 22.2cd∕A (9.6lm∕W) at 20mA∕cm2 and 7.3V with Commission Internationale d’Eclairage coordinates of (x=0.31,y=0.47), has been demonstrated.

Green top-emitting organic light emitting device with transparent Ba/Ag bilayer cathode

Using a vacuum thermal technique, semitransparent Ba∕Ag bilayer cathode has been fabricated for the top-emitting organic light emitting devices. In this work, optical transparency over 60% in the visible region and low sheet resistance of about 15Ω∕sq in the Ba (10nm)∕Ag (8nm) structure are reported. The surface and compositional morphologies of the cathode play a crucial role in determining the optical properties. Top-emitting organic light emitting device using this cathode has been fabricated and studied.

Transient electrophosphorescence in red top-emitting organic light-emitting devices

We have fabricated a red electrophosphorescent top-emitting organic light-emitting device using the phosphorescent bis[2-(2′-benzothienyl)-pyridinato-N,C3′]iridium(acetylacetonate) doped in 4,4′-N,N′-dicarbazole-biphenyl host and Alq3 electron injection layer. The emission spectrum shows a strong peak at 620nm accompanied with a small peak at 675nm in the red region. Time evolution of electrophosphorescence reveals a decay time of 703μs at a voltage pulse of 5V in a device with an emitting area of 20mm2. Rise and delay times vary from 450to14μs and 73to3μs, respectively, as the voltage amplitude increases from 4.5to10V. These results are compared with the red emitting device without an electron injection layer.

Red electrophosphorescent top emission organic light-emitting device with Ca∕Ag semitransparent cathode

Using a Ca (10nm)∕Ag (10nm) semitransparent cathode and efficient electron transport and/or buffer layer, red electrophosphorescent top emission organic light-emitting device has been fabricated. A low turn-on voltage of 4.6V and clean electrophosphorence peaks at 616 and 674nm are reported. X-ray photoelectron spectroscopy depth profiling measurements reveal the presence of Ca(OH)2 in Ca∕Ag bilayer cathode and the skin depth over 15nm in the visible region is determined in this structure. Results show that the compositional modification of the Ca∕Ag cathode enhances its optical transparency and final performance of the electrophosphorescent top emission device.