Increasing the Contrast Ratio of Organic Light-Emitting Diode by Organic–Metal Light-Absorbing Layer in Black Cathode

Abstract

In this study, we fabricated an organic–metal light-absorbing layer to form a black cathode to improve the display contrast ratio (CR) of organic light-emitting diodes (OLEDs). The proposed black cathode consists of a thin Al layer (10 nm), an organic–metal light-absorbing layer (100 nm) and a thick Al layer (100 nm). The best performance of the organic–metal light-absorbing layer is obtained using a mixture of 25% copper phthalocyanine (CuPc), 25% 4-(dicyanomethylene)-2-tert-butyl-6-(1,1,7,7-tetramethyljulolidyl-9-enyl)-4H-pyran (DCJTB), and 50% Al. By using this black cathode, the best performance of OLEDs is obtained at an average reflectance of 11.3% and a reflectance variation of 3.3% over the visible spectrum. Moreover, the CR (at 250 cd/m2) is 10.8 under ambient illuminance of 250 lx and 1.23 under a sunny sky of 23,450 lx. The black cathode can realize an increase in conductivity as well as a decrease in the reflection of ambient light, and can also improve both the device performance and the CR. The proposed black cathode has great potential for use outdoors and in high-contrast OLED displays.