A simple filter turns blue OLED lamp into rare white light

This ingenious method may make TV and smartphone screens use more xx organic light-emitting diodes.
A blue light-emitting OLED is displayed before (left) and after (right) adding a distributed Bragg reflector to generate white light. The reflector converts blue light into white light with two color temperatures.
Organic light-emitting diodes (OLEDs) have made great progress since the first working device was reported 30 years ago. OLED is highly praised for its advantages such as black, clear image reproduction and energy saving. Now it has a dominant position in the screen of mobile phones and LG TV. It is expected that OLED will take over the iPhone screen as early as next year.
Konstantinos Daskalakis, a postdoctoral researcher at the University of Alto, Finland, said that because OLEDs are relatively low in cost and easy to manufacture, we should consider using them to produce white light for general lighting.
Stimulating white light is the fatal weakness of OLED. Generally, in order to get white light, individual red, green and blue emitters will glow at the same time, thus producing white light. This makes white the most power-consuming color. It is reported that this requires 6 times the power required to generate black on Google pixels. Other methods to generate white light include doping chemicals in the emission layer, but this method makes it more difficult to manufacture equipment.
In a proof-of-concept experiment, Daskalakis and his mentor Paivi Torma converted the traditional blue-light OLED into the white-light OLED. The method is very simple, which is to place a set of distributed Bragg reflectors (DBR) made of alternating materials with high and low refractive index on the OLED.
In order to manufacture this device, Daskalakis first prepared OLED emitting blue light using standard vacuum evaporation technology. He directly covered each organic light emitting diode (OLED) with six alternating layers of silicon dioxide and tantalum oxide, and then sprayed a DBR.
The so-called DRB is usually used as a mirror to make optical cavities in devices. On the contrary, Daskalakis and Torma decided to use the so-called Bragg fiber mode resonant in the DBR and use the DBR as the converter. Bragg fiber mode can be tuned by changing the DBR layer thickness. Daskalakis said that these modes occur in the red, green and blue bands. When the blue light of OLED passes through the DBR, some high-energy blue photons will be converted into low-energy red and green photons, and then the mixture of red, green and blue photons will generate white light from the device.
In this way, the color temperature of light can be adjusted by changing the structure of the DBR stack. In a device, the thickness of silicon dioxide layer is 43 nm, and the thickness of tantalum oxide layer is 41 nm. The equipment produces a warm white sunlight with a temperature of 6007k; The other device has a 53-nanometer-thick silicon dioxide layer and a 42-nanometer-thick tantalum oxide layer, producing a cold white light at 4450K.
At the same time, the quantum efficiency of devices can be optimized by applying reflectors to different types of OLEDs. Compared with ordinary blue OLEDs, the quantum efficiency of converted white OLEDs is improved by 20%. Moreover, the modified white OLED can continue to work after two months, while the ordinary blue OLED stopped working the next day.
Torma hopes that this work will inspire other researchers to find more uses for DBR. "They are a bit neglected," "especially the Bragg model. People usually think it is better to have a very narrow model, but we find that these methods are actually very suitable for our purposes."
The two companies have applied for patents, and are making efforts to further characterize and optimize the design of the equipment, so that the technology can show its potential application in the field of lighting and consumer electronics.