Color-tunable white OLEDs raise luminous efficacy
Rather than stack multiple monochrome OLED layers vertically (compromising on individual colour efficiencies), they opted for alternating strips of yellow and blue p-i-n OLEDs, aligned side by side and as narrow as 20μm. A key improvement in the lateral colour mixing is the use of fine orthogonal photolithography techniques, which they say don’t suffer the resolution and substrate size limitations of fine metal masks typically used for patterning. Because they are able to align the OLED strips completely adjacent, with no gaps between them, the overall OLED emits across the full panel’s surface, without any dark areas between units hence improving overall luminance.
Publishing their results in Nature’s Light: Science & Applications journal in the paper “Adjustable white-light emission from a photo-structured micro-OLED array“, the researchers experimented with several micro-array dimensions, alternating strips of efficient fluorescent blue and phosphorescent yellow monochrome devices with different width ratios, varying from 50/50, 80/80, 80/20 and 100/30μm for the yellow/blue subunits.
What’s more, the photolithographic patterning technique they developed allowed them to precisely position top electrodes on the substrate, enabling each of the subunits to have separate drivers VY and VB for yellow and blue subunits, hence allowing them to drive a colour mix across the whole spectrum from blue to yellow (between the CIE color coordinates (0.14, 0.18) for the blue emitter and (0.5, 0.5) for the yellow emitter) at any given brightness level.
In effect, they were able to tune the white light from warm (CIE 0.45,0.41) to cold (CIE 0.33,0.33).
Next on their agenda, the researchers hope to further improve the light quality and luminous efficacy of their devices by enriching its emission spectrum through the addition of a green emitter. Additional light extraction techniques could also boost overall device efficiency.
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