Transparent top-emitting organic light-emitting diodes using a Cs/AI/Ag/ITO semi-transparent cathode

A top-emitting organic light-emitting diode (TEOLED) was fabricated with a semi-transparent multilayer cathode composed of Cs/Al/Ag and an ITO capping layer. The optical properties of Cs (0.5 nm)/Al (2.0 nm)/Ag (20 nm) at a wavelength of 533 nm exhibited a transmittance of 50 %. The light-output of the TEOELD with the Cs/AI/Ag/ITO showed a maximum luminance of 51,000 cd/m ² at a current density of 2.32 A/m² (corresponding to the forward bias voltage of 11.2 V); the device structure consisted of glass/Ag (100 nm)/ITO (125 nm)/2-TNATA (30 nm)/NPB (15 nm)/Alq₃ (55 nm)/Cs (1 nm)/Al (2 nm)/Ag (20 nm)/ITO (100 nm). Also, the external quantum efficiency and the power efficiency at a luminance of 1000 cd/m² (corresponding to the forward bias voltage of 6.8 V) were 1.7 % and 2.0 lm/W, respectively. This TEOLED exhibited similar electrical properties, but a little lower light output, compared with a reference TEOLED of the same structure using Alq₃ instead of ITO as the capping layer. These characteristics are strongly governed by the thickness, the electrical properties, and optical properties of the semi-transparent multilayer cathode composed of Cs/Al/Ag and by the optical properties of the top capping layer.