Abstract
Key factors to control the efficiency in iridium doped red and green phosphorescent light emitting diodes (PhOLEDs) are discussed in this review: exciton confinement, charge trapping, dopant concentration and dopant molecular structure. They are not independent from each other but we attempt to present each of them in a situation where its specific effects are predominant. A good efficiency in PhOLEDs requires the triplet energy of host molecules to be sufficiently high to confine the triplet excitons within the emitting layer (EML). Furthermore, triplet excitons must be retained within the EML and should not drift into the nonradiative levels of the electron or hole transport layer (resp., ETL or HTL); this is achieved by carefully choosing the EML’s adjacent layers. We prove how reducing charge trapping results in higher efficiency in PhOLEDs. We show that there is an ideal concentration for a maximum efficiency of PhOLEDs. Finally, we present the effects of molecular structure on the efficiency of PhOLEDs using red iridium complex dopant with different modifications on the ligand to tune its highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) energies.
Highlights
Achievements in organic light emitting diodes (OLEDs) lead to their integration as displays in several electronic devices and make them serious candidates to replace widely used LCD panels
We demonstrate that to achieve high efficiency phosphorescent light emitting diodes (PhOLEDs) a good confinement of the triplet exciton of phosphorescent dopant is needed with the use of an appropriate triplet energy gap host
We have presented four main elements that can have a direct control on the efficiency of iridium complex
Summary
Achievements in organic light emitting diodes (OLEDs) lead to their integration as displays in several electronic devices and make them serious candidates to replace widely used LCD panels. PhOLEDs harvest both singlet and triplet emissions, making it possible to reach high efficiency devices Their operation and efficient implementation are more complex than for the fluorescent OLEDs. To achieve high efficiency in PhOLEDs, the host material that forms the EML needs to satisfy a certain number of properties regarding the dopant as well as the adjacent layers. A high doping concentration minimizes the charge trapping but results in exciton quenching thereby lowering the efficiency of the light emitting device [31]. The objective of this paper based on our research activities is to present our investigations to control the efficiency in PhOLEDs through triplet exciton confinement, minimization of charge trapping, doping concentration, and dopant molecules design to limit self-quenching.
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