High-contrast organic light-emitting diodes.

High-contrast organic light-emitting diodes (OLEDs) with low-reflection cathode structures were constructed. The cathode structure consisted of a semitransparent cathode layer, a transparent phase-changing layer, and a highly reflective metal overlayer. When samarium (Sm), tris (8-quinolinolato) aluminum (Alq[sub 3]), and Al were used as the cathode layer, phase-changing layer, and reflective layer, respectively, luminous reflectance of the resulting OLED was reduced from 81% to 2.7% as compared to a conventional OLED with a LiF/Al cathode. A further reduction to 0.9% was realized by replacing a multilayer of Alq[sub 3]/Sm/Alq[sub 3] for the single phase-change layer of Alq[sub 3]. In such a structure, the spectral reflectance was below 0.03 in the entire visible region and showed a weak dependence on incident angles of light. A contrast ratio of 250:1 can be achieved at a brightness of 100 cd/m2 under 140 lx of ambient-lighting. Device efficiencies of the high-contrast OLEDs were reduced, while their electrical characteristics were almost identical to that of the control device. © 2004 American Institute of Physics. [ABSTRACT FROM AUTHOR]