Low absorption semi-transparent cathode for micro-cavity top-emitting organic light emitting diodes

We present a low absorption semi-transparent cathode for top-emitting organic lighting diodes (TEOLEDs). To reduce the light loss by electrode absorption, transparent cathode materials with a high refractive index and low extinction coefficient characteristics at the visible light range are investigated for the highly efficient TEOLEDs. Molybdenum trioxide (MoO₃) and tungsten trioxide (WO₃) are considered as the transparent materials. Due to the low conductivity of MoO₃ and WO₃, a small amount of silver (Ag) is blended in order to enhance the electrical property as electrodes. Optimized MoO₃:Ag mixed film shows the reduced extinction coefficient by half compared to Mg:Ag. With a general organic material (refractive index∼1.8) as a capping layer, the average absorption, transmittance and omnidirectional reflectance of 18.2, 65.9, and 25.6% are observed, respectively, with a sheet resistance of 12.7 Ω/□. The current efficiencies of fabricated red, green and blue TEOLEDs with optimized MoO₃:Ag cathode are 32.4, 76.8, and 2.8 cd/A, respectively. The performance enhancement ratio is 9.8, 13.6, and 86.7% for red, green and blue devices compared with Mg:Ag cathode devices.