Organic light-emitting devices (OLEDs) have attracted a great deal of attention due to their potential use in lighting as well as future panel display applications. The basic structure of an OLED consists of a stack of two or more thin organic layers with a total thickness of about 1000 Ǻ sandwiched between a transparent anode and a metallic cathode.
The organic layers consist of a hole transporting layer (HTL), an emissive layer containing a dopant and a host material, and an electron transporting layer (ETL). When a potential is applied, the injected positive and negative charges recombine in the emissive layer to produce light (electroluminescence). Particular structures of the organic layers and the choice of anode and cathode should be considered to highly maximize the recombination process in the emissive layer, thus maximizing the light output from the OLED device.
The earliest OLEDs work was done at Eastman Kodak labs by Ching Wan Tang and Steven Van Slyke. They made a double layer device in which its structure comprised of ITO/diamine HTL/Alq3/Mg:Ag. A glass substrate was coated with a transparent indium tin oxide (ITO) acting as the anode. A diamine was used as the hole transporting material.
Alq3 (tris(8-hydroxyquinoline) aluminum (III)) served as both an electron transporting layer (ETL) and an emissive layer. Electrons were injected from a Mg:Ag alloy cathode with an additional layer of Ag to protect the Mg from oxidation. In this double layer device, electrons and holes combined at the diamine/Alq3 interface.
This double layer device utilized HTL and ETL to confine excitons and prevent leakage. The utilization of HTL and ETL could lead to efficient charge carrier injections. Nevertheless, this double layer device gave a fluorescent emission with poor external quantum efficiency (~1%). The problem could be that the excitons emitting from a dense, pure matrix typically go through significant self-quenching (Tang, C. W.; Van Slyke, S. A. Appl. Phys. Lett. 1987, 51, 913).In 1989, Tang, Van Slyke and Chen showed that it was possible to add small quantities of a highly fluorescent dye to a charge transporting material (Alq3) in the double layer device to easily tune the OLED emission color. Those OLED with doped emissive layer devices still had poor external quantum efficiency of about 2-3% (Tang, C. W.; Van Slyke, S. A.; Chen, C. H. Chen J. Appl. Phys. 1989, 65, 3610).
In an OLED, holes and electrons injected into the device recombine to form radiative excited states, or excitons. This electrically generated exciton can be either a singlet or triplet. Excitons in an OLED are believed to be created in a ratio of about 3:1, i.e., approximately 75% triplets and 25% singlets (Baldo, M. A.; O'Brien, D. F.; Thompson, M. E.; Forrest, S. R. Physical Review B, 1999, 60, 14422). In many cases, singlet excitons may readily transfer their energy to triplet excited states via "intersystem crossing," whereas triplet excitons may not readily transfer their energy to singlet excited states. As a result, 100% internal quantum efficiency is theoretically possible with phosphorescent OLEDs. A fluorescence device only utilizes singlet excitons while the energy of triplet excitons is generally lost to non-radiative decay processes that heat up the device, thus resulting in much lower internal quantum efficiencies. OLEDs utilizing phosphorescent materials that emit from triplet excited states are expected to result in higher internal quantum efficiency. That is why triplet emission in OLEDs matters.
In the initial stage of OLED development, fluorescent materials were typically used to produce fluorescence (emission from singlets), while most researchers avoided the use of phosphorescence (emission from triplets). The reason for this is a general assumption that phosphorescent materials would not emit efficiently at room temperature, and that the very long lifetime typically exhibited for phosphorescence would limit the utility of the devices. Phosphorescence generally occurs on the order of microseconds. Phosphorescence is referred to as a "forbidden" transition because the transition requires a change in spin states. Our group has demonstrated that phosphorescent emission could be achieved efficiently. In 1998, our group in collaborations with Prof. Stephen Forrest Group in Princeton University found out that one way to bypass the efficiency and lifetime problems associated with triplet emission was by utilizing heavy metal complexes, especially those containing iridium and platinum. Heavy atoms such as Ir and Pt can promote intersystem crossing by a mechanism known as spin-orbit coupling. Strong spin-orbit-coupling mixes singlet and triplet metal-to-ligand charge transfer (MLCT) states. Mixing of 1MLCT and 3MLCT states with the 3LC (triplet ligand-centered) state creates a hybrid 3(LC-MLCT). This mixing removes the spin-forbidden nature of the radiative relaxation of the triplet state thus leading to high phosphorescence efficiencies (Lamansky, S.; Djurovich, P.; Murphy, D.; Abdel-Razzaq, F.; Lee, H.-E.; Adachi, C.; Burrows, P. E.; Forrest, S. R.; Thompson, M. E. J. Am. Chem. Soc. 2001, 123, 4304-4312). Color tuning can be achieved either by changing LC or MLCT.
Our group has recently found that highly emissive Ir complexes can be formed with two cyclometallated ligands (abbreviated as C^N) and a single monoanionic, bidentate ancillary ligand (L^L). The emission colors from those Ir complexes are strongly dependent on the choice of cyclometallating ligand, ranging from green to red, with room temperature lifetimes on the order of microseconds. OLEDs have been made with (C^N)2Ir(L^L) phosphor dopants, giving efficient green, yellow or red emission (Lamansky et al., Inorg. Chem. 2001. ; Lamansky et. al., JACS 2001).