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Producing White OLEDs in a Manner Similar to While LEDs
November 04, 2019
 
Nichia discovered that putting a yellow phosphor over a blue LED would deliver white light with sufficient wavelengths to support the LCD backlighting and the solid-state lighting market. So why not try something similar with OLEDs? In a proof-of-concept experiment, Konstantinos.
 
Figure 1: Blue OLEDs w/ Distributed
Source: Konstantinos Daskalakis 
​Daskalakis and his supervisor Paivi Torma converted conventional blue-emitting OLEDs to white-emitting ones simply by depositing a distributed Bragg reflector (DBR)—a stack of two alternating materials with high and low refractive indexes—on top of the OLEDs. The blue-emitting OLED has a distributed Bragg reflector deposited on top.  Daskalakis first prepared blue-emitting OLEDs with standard VTE techniques. Then, he sputtered a DBR comprising of six alternating layers of silicon dioxide and tantalum oxide directly on top of each OLED. DBRs are usually used as reflective mirrors to create optical cavities in devices. Instead, Daskalakis and Torma decided to use a DBR as a converter, leveraging so-called Bragg modes that resonate within a DBR.
 
The Bragg modes can be tuned by varying the thickness of the DBR's layers. Those modes are made to occur at red, green, and blue wavelengths, so as the OLED’s blue light passes through the DBR, some high-energy blue photons relax into lower-energy red and green photons, Daskalakis says. The mixture of red, green, and blue photons emerging from the device produces white light. 
 
With that approach, the light’s color temperature could be tuned by varying the structure of the DBR stack. In one device, the silicon dioxide layer was 43 nanometers (nm) thick and the tantalum oxide layer was 41 nm thick. That device produced a warm, daylight white color with the temperature 6007 K. Another device with 53-nm-thick silicon dioxide layers and 42-nm tantalum oxide layers generated a cool white light with a temperature of 4450 K. The device's quantum efficiency can be optimized separately by applying the reflector to different types of OLEDs. 

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