Aligned polymer dispersed liquid crystal film for light enhancement of quantum dot backlight

Abstract

Quantum dots (QDs) have been used to make backlight, which provides a superior color gamut, for liquid crystal flat panel displays. In the backlight system, quantum dots, embedded in a polymer film and illuminated by blue light, emit red and green light with narrow bandwidths. There is, however, a problem with the system in that the quantum dots emit light in all directions, and most of the emitted light is in directions with large incident angles and cannot exit the film due to the total internal reflection at the film-air interface and is wasted. We propose to use an aligned polymer dispersed liquid crystal (APDLC) film to reduce the total internal reflection in the QD backlight and thus improve the light efficiency. A regular PDLC film, where the embedded liquid crystal droplets are randomly oriented, exhibits isotropic scattering and is not a good candidate for the enhancement of light efficiency of QD backlight. Through a two-step polymerization, we successfully developed an aligned polymer dispersed liquid crystal (APDLC) film where the liquid crystal droplets are permanently unidirectionally aligned in the film’s normal direction. It exhibits selective scattering: it scatters light with large incident angles but not light with small incident angles. When the APDLC film is laminated on the QD backlight film, a significant enhancement of the light efficiency of the QD backlight is achieved. The APDLC film can also be used to increase the light efficiency of other flat panel displays, such as organic light emitting diode (OLED) display and micro-light emitting diode (MLED) display.