Organic white-light-emitting devices based on balanced exciton-recombination-zone split using a carrier blocking layer

An organic white-light-emitting device is demonstrated using direct mixing of two colors from two different emissive layers separated by a carrier blocking layer. Direct mixing is achieved by balancing the exciton-recombination-zone split across the blocking layer. Depending on the applied voltage, the color emitted from the device is found to change toward blue color via white electroluminescence after starting from yellow color. The emission color could be controlled by the applied voltage and the thickness of the electron blocking layer. The white light that has the Commission Internationale d'Eclairage (CIE) chromaticity coordinate of (0.31, 0.33) is obtained at an applied voltage of 11 V for white organic light emitting device (OLED) with 4 run thickness of alpha-NPD. At the applied voltage, the luminance is measured to be 220 cd m(-2) with the power efficiency of 0.763 lm W-1. The maximum luminescence is 2230 cd m(-2); the power efficiency is 0.388 lm W-1 at the applied voltage of 16 V. (C) 2002 Elsevier Science B.V. All rights reserved.