OLEDs: Light extraction and deep blue emission
Is Version Of
OLEDs have emerged in the market over the last decade mainly in display applications due to their unique and attractive properties, such as lightweight, high brightness with strong color contrast, thin and flexible designs as well as potentially of low cost. OLEDs for lighting applications are under extensive research and development. Though promising for solid-state lighting, there are critical aspects that need to be addressed to increase their efficiency, which will lead to commercialization.
One major issue is the ~20% OLEDs’ limited forward outcoupling efficiency. This is due to several loss processes. More specifically, light generated in the emissive layer is internally trapped/ waveguided in the substrate or lost to surface plasmon excitation at the metal cathode. Another issue is the less efficient and unstable blue, especially deep blue (400-450 nm), OLEDs.
This dissertation addresses both of these key OLED issues.
The outcoupling efficiency that is limited by the internal/ external waveguiding along with surface plasmon excitation loss was enhanced by using novel corrugated plastic substrates. The unique substrate structure enables enhanced outcoupling in the forward hemisphere by minimizing total internal reflection and surface plasmon excitation. The dissertation demonstrates OLEDs with an external quantum efficiency of 50%, meaning an outcoupling efficiency ≥ 50% for a green phosphorescent OLEDs which yields a > 2-fold enhancement over the conventional flat devices. Enhancements were observed for blue and white phosphorescent devices depending on the pitch of the corrugation and the features’ height.
Bright (14000 cd/m2) deep blue TADF exciplex-based OLEDs were fabricated using low cost abundant commercial organic small molecules, including triphenylphosphine oxide (PPh3O). The deep blue emission peaked at ~435 nm with the peak emission credited mainly to triplet exciplex emission at NPB/TPBi:PPh3O 5:1 interface. The triplet emission was confirmed by the strong quenching of the phosphorescence of the emitting layer in the presence of O2.