## 3 Results and Discussion
Therefore, it is thought that the change in cladding layers has caused the variation in \(\gamma\), relating to the observed transient behavior. As deduced from the analysis of the efficiency curves, one can ask what triggers this phenomenon. While each chemical should be stable enough to have a minimal impact on the device performance, it has been reported that the reaction between LiF/Al and Alq\({}_{3}\) is possible during deposition [18], as supported by the electron energy-loss spectroscopy. As a result of the reaction, the dissociated Li ions can migrate into the EML as the device is operated. Then the states of the migrated Li ions can be formed in the vicinity of the LiF/Al:Alq\({}_{3}\) interface. In this case, the migrated Li ions can play an important role in the observed transient behavior as shown in Figure 3.6 For the EML, it can act as defects, which degrades the device's lifetime. However, the reason why both the current density at \(V_{\text{D}}\) and the luminance at \(I_{\text{D}}\) increase (region 1) by the migration of Li ions should be deduced from the nature of organic semiconductors. Figure 3.7 Schematic diagram of Li ion migration in the device, leading to the enhancement of electron  
injection (LUMO: lowest unoccupied molecular orbital, HOMO: highest occupied molecular orbital, ETL: electron transport layer, HTL: hole transport layer). The bonding strength between organic molecules is dominated by the weak dipole interaction, also known as the van der Waals interaction [17]. Thus, the carrier transport based on the band theory may not be applicable because the band formation in organic solids is weak. In a disordered system, the main charge transfer mechanism is the hopping process via quantum-mechanical tunneling [19]. The injected charges may hop by emitting or absorbing phonons. In this phonon-assisted hopping, the hopping transition rate, \(W_{ij}\), from an initial state in a position \(\mathbf{r}_{i}\) with an energy \(E_{i}\) to a final state in a position \(\mathbf{r}_{j}\) with an energy \(E_{j}\) is given by the Miller-Abraham formulation [19]:  
Figure 3.8  
where \(v_{0}\) is the hopping frequency, \(\kappa\) is the inverse of the decay length, \(|\mathbf{r}_{ij}|\) is the distance between the states. The states induced by the migrated Li ions can lead to the interstate distance shortened, which would enhance the electron injection. Moreover, the work function of Li is ~2.3 eV, lower than the lowest unoccupied molecular orbital of Alq\({}_{3}\)[20; 21; 22]. Due to this nature of the work function, electron hopping assisted by Li ions can be facilitated. Consequently, the \(\gamma\) factor would increase owing to the enhanced electron transport as schematically described in Figure 3.9  
However, once Li ions migrate further into the EML, the effect of transport enhancement ceases to exist, thus reducing the current conduction as seen in region 2 in Figure 4.0 Although the migrated Li ions can assist the electron injection initially, excessive migration with further stress would eventually degrade the EML, thus detrimental to the device as illustrated in Figure 4.1  
As shown in Figure 4, the operation time when the current and luminance increase initially are shortened under higher stress current. It indicates that the rate of Li migration  
Figure 4.2 Schematic diagram of excessive Li ion migration in the device, quenching the exciton formation after 96 h of electrical stress. increases with the level of stress current. Although the effect of Li migration on the reliability of OLED may seem trivial, this effect is rather pronounced at the low-current operation. This investigation suggests that by optimizing the LiF layer or applying a more stable EIL, the lifetime of an OLED device could be extended further.