ABSTRACT
After discussing highly efficient organic light-emitting diode (OLED) architectures in detail, it is appropriate to describe device operational stability and material degradation processes after prolonged device operation. In organic semiconductors, although there are no dangling bonds that introduce defects as in inorganic semiconductors, the molecule stability and the interface between two different layers of organic material are critical due to differences in surface energy, dipole moment, exciton formation/migration rate, and carrier transport property. These aspects are crucial to device performance over time and central to industrial applications of OLED technology. 10.1 Efficiency Roll-OffOrganic light-emitting diode (OLED) devices show a characteristic efficiency roll-off (efficiency droop) under high current densities (high luminance levels), as shown in Fig. 2.1b. This is detrimental for high luminance applications such as general illumination sources that require ~3000-5000 cd/m2 luminance. In standard phosphorescent devices, the external quantum efficiency typically drops to ~50%–75% of its peak value at a luminescence of ~500
cd/m2, a brightness suitable for a flat-panel display. There are a number of physical processes responsible for such efficiency roll-off. The major processes include triplet-triplet annihilation (TTA), triplet-polaron annihilation (TPA), and drift in the charge carrier balance (CCB). 10.1.1 Triplet-Triplet AnnihilationTTA is one major quenching process in phosphorescent OLED devices.16 This process is traditionally based on the Dexter energy transfer mechanism described in Process [2.7]. When the triplet energy of the host is considerably higher than that of the guest without significant back energy transfer from the guest to the host, the predominant TTA process may occur between two guest molecules. However, if phosphorescent guest molecules are used, a strong spin-orbit coupling of the guest molecules may further enhance the TTA process via a long-range Förster mechanism.17 In the case when relatively lower-triplet-energy host materials are used, the triplet excitons’ (triplets) TTA among the host molecules could be significant. This would reduce the amount of host triplets that may be energy-transferred to the guests to provide more photons. In addition, since exciton density rises with increasing electrical current injection, it can be expected that under high current injection, the triplets on the guest are saturated and the amount of host triplets becomes sufficiently high so that TTA could even take place between host and guest molecules. One method to reduce TTA is to employ high-triplet-energy host materials and select guest phosphors with a short triplet lifetime so that most guest triplets would be emitted radiatively in a short period of time before significant accumulation in the system. 10.1.2 Triplet-Polaron AnnihilationTPA is another major quenching process in OLEDs. This process can be expressed as T1 + e-Æ S0 + e-* [10.8] T1 + h+ Æ S0 + h+*, [10.9]
where charged species (or polarons) are represented by electrons (e-) and holes (h+), respectively. These processes describe the annihilation of triplet excitons with free or trapped charge carriers by a Förster-type mechanism119 to promote the charged species into excited states (e-* and h+*). As mentioned above, the density of triplet excitons in the device becomes extremely high under high current densities particularly near the exciton formation regions of a phosphorescent device. For the charged species, the density and location of accumulated charges depend on organic-organic or organic-metal heterojunction interface where the energy barrier offsets are high.120 The proportion of the accumulated electrons and holes, in turn, depends on the electron/hole mobilities of the host and transport layers. One way to minimize TPA is then to construct devices where the exciton formation region is spatially apart from the carrier accumulating interfaces or simply eliminate the number of organic transport layers in the device. 10.1.3 Charge Carrier Balance
The CCB is a critical parameter influencing the device efficiency, as shown in Eq. 2.2. Any drift in the CCB during device operation would result in a waste of charge carriers and thus contribute to the efficiency droop. The CCB is considered a dynamic process depending on densities of each carrier type under different injection levels.17 This is due to multiple factors such as the difference in mobility and in injection barrier heights encountered by electrons and holes. The best approach to balance charge carriers is to optimize the electron and hole transport layer thickness as well as employ ambipolar host materials that offer comparable electron and hole mobilities. 10.2 Material DegradationWhile efficiency droop concerns with the efficiency value at practical brightness levels, material degradation determines the overall device lifetime. Major sources of degradation include cathode oxidation, anode degradation, electromigration, molecular aggregation, and molecular fragmentation.
