Scientists at 3M have unveiled a color-conversion technology that doubles the efficiency of green LEDs, compared with currently-available green emitters. The St. Paul, Minnesota-based company says that the technology could lead to accelerated adoption of LEDs in demanding applications such as LCD display backlights and pico-projectors
Mainstream blue and green LED chips are manufactured using gallium nitride (GaN)-based materials, but the efficiency varies by color (wavelength). The performance of green LEDs lags behind other colors, so much so that it is often necessary to use 2 green LEDs in combination with one red and one blue to create a balanced RGB pixel.
High performance green LEDs will reduce the total number of LEDs required in a device, while improving color performance, energy efficiency, heat dissipation and reliability.
Rather than use a green GaN-based chip, the 3M technology approach is to bond a color-converting material to a high-efficiency GaN-based blue LED chip. The 3M material absorbs the blue light and re-emits it as green light.
Using blue LEDs (ThinGaN high current UX:3) supplied by Osram Opto Semiconductors, record green emission of 216 lumens was achieved, with 181 lm/W efficiency at a drive current of 350 mA/mm2.
“By leveraging 3M’s physics and materials science expertise, we developed a process, covered by strong intellectual property protection, that will produce any color LED with a high degree of precision, durability and color saturation,” said Steven Webster, VP of research and technology commercialization for 3M’s Display and Graphics Business. “We believe this breakthrough shows a path to solving several traditional LED performance limitations.”
Webster told LEDs Magazine that 3M has a long-standing expertise in this area, and that the company has developed a range of enhancement films for displays that make the backlight more efficient, enabling fewer LEDs to be used. The company has also used LED light sources in pico-projector products (see News). The converter material was developed after 3M identified a specific requirement in LED technology.
The converter material is a cadmium zinc selenide (CdZnSe) quantum well structure, grown by MBE, which can be engineered to enable conversion to almost any wavelength. Webster said that color-converted amber showed very good results as well, but green represented the biggest improvement compared with current technology.
However, Webster did not comment on the suitability of this technology for creating a white light source. Generally speaking, white light requires several wavelengths or a broad spectral output.
The color-converter material can be bonded at the chip- or wafer-level. Typically less than 1% of the blue light leaks through the material.
Webster expects that LED products using the converter material are “about one year away.” He said that 3M is already speaking with LED makers, and has no plans to make LEDs itself.
