Research Project Summary Information
Polarized Light Emitting Diodes(ST10289-1)
Troy Research Corporation
Liquid-crystal display (LCD) is a flat-panel display technology that forms images by passing polarized light through small cells (pixels), which change their ability to transmit this light by varying electrical charges on them. Today, LCD screens are used in all types of consumer electronics devices, including cell phones, digital cameras, portable media players, car-navigation systems, desktop monitors, laptop screens and televisions. Light sources are needed for backlighting the pixels. Cold-cathode fluorescent lamps are the common light-source technology used in backlighting currently, but LEDs may offer advantages including low cost, longer life, larger color ranges, immunity to vibration, low-operational voltage and precision control. Existing LEDs require more power than current cold-cathode fluorescent lamps, and more improvements in the efficacies of LEDs are required for them to complete with the cold-cathode fluorescent lamp. Conventional LEDs emit randomly-polarized light and as much as 62% of the light is wasted inside a backlight.
The Contractor shall produce a side-light emitting LED, which maximizes the polarized side-emitting light; develop an LED-encapsulation structure, which is shaped to maximize the output of light with the desired polarization; design a reflector cup to reflect the side emission with desired polarization towards the polarizer; and convert the side emission with undesired polarization into the emission with desired polarization. The Contractor shall set-up all the necessary equipment for pilot production of polarized LEDs, fabricate the polarized LEDs, and demonstrate the product manufacturing process to be commercially viable. The Contractor shall also develop the necessary tooling to commercialize the polarized LEDs and conduct outreach to identify and reach agreement with a manufacturer to assume the commercialization activities.
LEDs with improved optics and efficacies for LCD backlighting will decrease the power requirements for LCD screen technologies (including television and computer screens) and thereby reduce energy, reduce battery-backup requirements, and avoid CO2 and other global greenhouse- gas emissions associated with fossil-fuel combustion.
The Contractor reported in its final report that the work demonstrated that side-emitted light from 460 nm GaInN MZQ LEDs is highly polarized in the plane of the quantum wells. The intensity ratio of in-plane polarization to normal-to-plain polarization with a peak value as high as 7:1 was measured. The overall intensity ratio of the in-plane polarization to normal-to-plane polarization was measured to be 2.19:1. The non-rotationally symmetric encapsulate shape was shown to enhance the extraction of a particular linear polarization from an unpolarized source by both numerical ray-tracing simulations and experimental measurements, resulting in an overall theoretical enhancement of 8.3% when the refractive index is 1.5. The Contractor also demonstrated a reflector design tailored to take advantage of the inherently linearly polarized light. Measurements showed that reflected light traveling in close-to-vertical directions has an intensity ratio of desired-polarization-to-undesired-polarization of 2.5:1, and a combination of reflected and directly emitted light polarized with an average ratio of 1.9:1, with local polarization ratios of 3.5:1. An LED with back-side integrated wire-grid polarizer was fabricated successfully and the measurements had a ratio of 2:1. Although the results were promising, the Contractor did not complete the necessary tooling for commercialization and was not able to reach an agreement with a manufacturing partner to assume the commercialization activities.
Troy Research Corporation
17 Eaton Rd
Troy, NY 12180
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