## 3 Results and Discussion  
In Figure 2, the current density-voltage (\(J\)-\(V\)) curves of devices under electrical stress are shown. For the stress level of 2.5 \(\times\)\(I_{\text{D}}\), the device characteristics seem to be almost  
Figure 1: Schematic device structure of the OLED devices under study. unchanged during 168 h. On the other hand, the current variation under high current stress (\(15\times J_{\rm D}\)) increases at a fixed voltage, which shows the typical device degradation. More variation under higher stress level can also be found in the _L-J_ curve depicted in Figure 3. However, simple _L-J-V_ curves alone are not sufficient for analyzing the degradation phenomena in detail. In Figure 4, the current density at the operating voltage (\(V_{\rm D}\)) for each experiment is selected and plotted as a function of operation time. Commonly, the variation in current density can be divided roughly into two regions. In case of the stress test at \(2.5\times J_{\rm D}\) (Figure 4a), the overall trend shows that the current density at \(V_{\rm D}\) increases up to 96 h after an initial decrease in the current density at \(V_{\rm D}\) (region 1). After 96 h, the device degradation of decreasing current density with the operation time is observed (region 2). The similar tendency is observed for the stress test at \(15\times J_{\rm D}\) (Figure 4b). Due to the higher stress condition, the device begins to degrade earlier (after 48 h) than the case shown in Figure 4a. Although the intervals collecting the device degradation data are different for each experiment, the overall trends are clear. Furthermore, the reliability tests for 168 h seem to be sufficient for observing the degradation behaviors shown in Figure 4.  
To investigate the electrical properties further, the ideality factors, \(n_{\rm ideal}\), defined as  
\[n_{\rm ideal}=\frac{q}{k_{B}T}\bigg{(}\frac{\partial\ln J}{\partial V}\bigg{)} ^{-1} \tag{1}\]  
are plotted as a function of operation time in Figure 5. Here, \(q\) is the elementary charge, \(k_{\rm B}\) is the Boltzmann constant, and \(T\) is the absolute temperature. As seen in Figure 2, one can deduce that the overall shapes of _J-V_ curves during the device operation are quite similar since \(n_{\rm ideal}\) is related to the slope of \(\ln J\) vs. \(V\) curve. Figure 3: _L-J_ curves under constant current stresses at (**a**) \(2.5\times J_{\rm D}\) and (**b**) \(15\times J_{\rm D}\). Figure 2: _J-V_ curves under constant current stresses at (**a**) \(2.5\times J_{\rm D}\) and (**b**) \(15\times J_{\rm D}\). Furthermore, the value and tendency of \(n_{\text{ideal}}\) give clues about dominant carrier recombination and transport mechanisms [12; 13]. At low current density (\(<\)10\({}^{-3}\) mA/cm\({}^{2}\)), it is seen that \(n_{\text{ideal}}\) is \(\sim\)2, which indicates that the Shockley-Read-Hall (SRH) recombination inside the emissive layer (EML) is dominant in the device [13]. In general, it is known that the mobility for the hole transport is much higher than that for the electron transport in organic semiconductors [14]. Thus, the dominant recombination mechanism would be the SRH recombination by the electron capture in this region. As the current increases, \(n_{\text{ideal}}\) is seen to increase due to the additional voltage drop caused by the series resistance in the layers outside the EML. From \(n_{\text{ideal}}\) in the driving current region (indicated by a dotted line marked by \(J_{\text{D}}\)), the device resistance in Figure 5a seems almost constant under electrical stress, as indicated by overlapping curves. However, \(n_{\text{ideal}}\) at \(J_{\text{D}}\) for the higher current stress shown in Figure 5a is slightly increased due to the effect of rapid device degradation.