Acost-competitive solution for increasing the light extraction efficiency of organic light-emitting diodes (OLEDs) with efficient and stable color rendering index (CRI) for solid-state lighting (SSL) was developed and demonstrated. Solution processable quantum dot (QD) films were integrated into OLED indium tin oxide (ITO)-glass substrates to generate tunable white emission from blue-emitting OLED devices as well as out-couple light from the ITO film.
This QD light-enhancement substrate (QD-LES) technology simultaneously increased OLED light out-coupling, improved external quantum efficiencies, and provided > 80 CRI white light that is readily tunable and inherently stable for any diffuse lighting application.
The efficacy of a light source is given by the product of the electrical-to-optical conversion efficiency (ηe-o = Po/Pe) and the luminous efficacy of radiation (LER),
Efficacy [lm/W] = LER [lm/W(optical)] ηe-o [%],
where Po and Pe are the optical output power of the light source and the electrical input power (voltage times current). ηe-o is directly proportional to the internal quantum efficiency (IQE), extraction efficiency, ηe, and inversely proportional to the driving voltage. In light of the equation above, the research work directly addressed improving OLED lamp efficacy and reliability in the following ways:
- The IQE of the blue OLED can be optimized for blue light generation.
- ηe-o > 40% is possible by using a combination of internal and external out-coupling layers.
- Higher LER values > 350 lm/W are possible by leveraging the narrow band emission of green-yellow-red QDs in combination with bluer, broader OLED emission; current OLED LER values are limited to 350 lm/W due the inherently broad emission of OLEDs resulting in lost photons outside the visible spectra.
- High CRI due to customizable narrow QD emission profiles.
- Higher operational lifetimes are possible due to the single emitter architecture, reducing the color drift failure mode common in mixed-emitter and tandem OLED devices.
- Improved color stability as a function of drive current by using a single blue-emitting OLED.
- Simpler construction due to the single-emitter OLED architecture and solution-processable QD-LES.
Assuming an LER of 350 lm/W, an average blue phosphorescent OLED (PhOLED) wavelength of 490 nm, an operating voltage of 3.3 V, an IQE of 100%, and an extraction efficiency of 40%, the QD-LES-based PhOLED would have an efficacy ranging 100-110 lm/W (electrical-optical power conversion of approximately 31%). With further improvements in LER and extraction efficiency, OLED lamp efficacies can exceed 150 lm/W using the new approach. Lifetime improvements can be expected, since the stable downconverters will essentially eliminate color shift over time, causing lifetime to be limited solely by the blue OLED, not the color shift experienced by all mixed-emitter OLED systems.
This work was done by Sean Evans, James M. Perkins, Ph.D., Matthew Stevenson, Gagan Mahan, Seth Coe-Sullivan, Ph.D., and Peter Kazlas, Ph.D., of QD Vision, Inc. For more information, download a technical support package at www.greendesignbriefs.com.