Low Voltage White Phosphorescent OLED Achievements
Universal Display Corporation (UDC) and its research partners at Princeton University and the University of Southern California have succeeded in developing a white phosphorescent OLED (PHOLED™) that achieved a record efficiency of 20 lumens per watt. This achievement is the result of the team's collaborative efforts to increase the efficiency of PHOLED lighting by focusing on two critical factors: lowering the drive voltages and increasing the amount of light extracted.
Maximizing Quantum Efficiency Through Phosphorescence
Phosphorescence is widely recognized as an enabling technology for OLEDs to have the required efficiency (>100 lumens per watt) to become a low cost solution for the next generation of solid-state lighting. In conventional fluorescent OLEDs, only a small fraction of the generated excitons are in the singlet state, with the majority in the triplet state. In these fluorescent OLEDs, light emission occurs as a result of the radiative decay of singlet excitons, and the internal quantum efficiency is limited to approximately 25%.
UDC and its partners have developed an innovative, highly efficient device architecture employing phosphorescent dopants. Phosphorescent dopants contain a heavy metal atom that facilitates the mixing of singlet and triplet states, allowing singlet to triplet energy transfer through intersystem crossing. This leads to highly efficient devices where 100% of the excitons can potentially produce optical emission, in contrast to only approximately 25% in conventional fluorescent devices. The high conductivity hole and electron transport system is achieved by selecting p- and n-type dopants along with appropriate organic buffer layers, resulting in a p-i-n type device.
The team has successfully developed a white PHOLED that achieved 20 lumens per watt operating at 6.3 V, having a luminance of 800 cd/m2, and having a CIE (0.39, 0.40). Figure 2 shows red, green, and blue pens illuminated by this high efficiency white light source. Next, the team will vertically stack two to three highly efficient white PHOLEDs to increase the luminance and power density of lighting panels.