Improving the Brightness and Daylight Contrast of Organic Light-Emitting Diodes

For most applications of displays based on organic light‐emitting diodes (LEDs), it is desirable to have good daylight contrast in combination with a high intensity of emitted light. The conventional approaches to enhance the daylight contrast, using a black cathode or circular polarizers, result in a significant loss of light emitted by the LED. A rather novel approach to enhance daylight contrast while keeping loss of emitted light to a minimum is the introduction of a chiral‐nematic film in the device. This approach leads to an increase in light efficiency by a factor of 1.8 (with respect to circular polarizers) with some loss in daylight contrast values within the reflection band of the chiral‐nematic film. Outside the reflection band, however, the contrast approaches infinity. Chiral‐nematic films in combination with classical circular polarizers are used as additional layers in organic light‐emitting diodes (see Figure) to improve the daylight contrast while preserving the emitted light. Increased light efficiency at the expense of some daylight contrast is achieved within the reflection band of the films. Outside the band, the daylight contrast achieved is infinite.