Black Phase-Changing Cathodes for High-Contrast Organic Light-Emitting Diodes

High-contrast organic light-emitting diodes (HC-OLEDs) with black phase-changing (PC) cathodes have been designed and fabricated. The black PC-cathode consists of a front thin semitransparent metal layer, an organic PC layer, and a rear thick reflective metal layer. To achieve a targeted high contrast over the full visible spectrum, several organic molecules and their binary and ternary compounds were researched to meet optical design requirement. By combining a design in the complex refractive index and in the thickness of the PC layer, it is demonstrated that the optical reflectance of the HC-OLED can be reduced from ∼80% to below that of the glass substrate (∼5%) by using a ternary organic compound as the PC layer. This black PC-cathode produces extremely high optical contrast, and yet it does not alter the current–voltage characteristics of the device.