## 3 Results and Discussion
For further analysis, the change in luminance at \(J_{\text{D}}\) as a function of operation time is shown in Figure 6. In region 1, for the case of 2.5 \(\times\)\(J_{\text{D}}\) device operation, which is depicted in Figure 6a, the overall luminance increases whereas the applied voltage decreases with respect to the operation time. After 96 h, the luminance is almost saturated. This trend in device degradation is more pronounced under higher electrical stress (Figure 6b). The device efficiency is investigated next to identify why the luminance of the sample varies under electrical stress. Figure 4: Current densities at \(V_{\text{D}}\) as a function of operation time for the constant current stresses at (a) \(2.5\times\)\(J_{\text{D}}\) and (b) \(15\times\)\(J_{\text{D}}\). The external quantum efficiencies (EQE, \(\eta_{\rm EQE}\)) of the degrading devices under electrical stress are shown in Figure 7. It is noted in Figure 7 that there is little change in shape except the minute changes at very low current densities (\(\sim\)10\({}^{-2}\) mA/cm\({}^{2}\)), which reflects the initial current variation before 24 h in Figure 3.0 For the case of the stress test at \(2.5\times J_{\rm D}\) (Figure 7a), the EQE is almost the same during device operation up to 168 h, which shows the same tendencies as the current and luminance variation. In order to investigate further, the EQE curves are shown in Figure 8 after normalizing with respect to respective peaks. The reason why the normalized EQEs show this trend can be explained by considering the factors constituting the EQE. It is known that the EQE can be divided into the internal quantum efficiency (IQE, \(\eta_{\rm IQE}\)) and the outcoupling efficiency (\(\eta_{\rm out}\)) [15]:  
Figure 3.1  
Figure 3.2 Variation of the normalized \(\eta_{\rm EQE}\) as a function of current density for the constant current stresses at (**a**) \(2.5\times J_{\rm D}\) and (**b**) \(15\times J_{\rm D}\). Figure 3.3 Luminance at \(J_{\rm D}\) as a function of operation time for the constant current stresses at (**a**) \(2.5\times J_{\rm D}\) and (**b**) \(15\times J_{\rm D}\). Figure 3.4 Device EQEs under constant current stresses at (**a**) \(2.5\times J_{\rm D}\) and (**b**) \(15\times J_{\rm D}\). \(\eta_{\text{IQE}}\) is defined as the ratio of the number of generated photons per unit time inside the EML to the number of injected electrons per unit time. \(\eta_{\text{out}}\) indicates a fraction of photons escaping from the EML to free space. In general, \(\eta_{\text{out}}\) can be predominantly affected by the device structure. Many researchers have attempted to increase \(\eta_{\text{out}}\) by modifying the layer structure with such things as the layer grating, the microlens array, and the inserted scattering particles [16]. Since the device structure was not changed during the operation in this case, the variation of the efficiency curve is not attributable to \(\eta_{\text{out}}\). The analysis above implies that the variation of the efficiency curve should be caused by the change in \(\eta_{\text{IQE}}\). The IQE of an OLED device can be expressed as the product of three factors [15]:  
Figure 3.5  
where \(\gamma\) is the charge-carrier balancing factor, \(\eta_{\text{S/T}}\) is the spin singlet/triplet factor, and \(q_{\text{eff}}\) is the effective radiative efficiency. First of all, \(\eta_{\text{S/T}}\) is determined by how the EML layer forms excitons. In our experiments, \(\eta_{\text{S/T}}\) can be regarded as constant as the fluorescent light-emission process for our device is not changed [17]. Furthermore, the shape of the efficiency curve is the result of the competition between the radiative and nonradiative recombinations, which is related to \(q_{\text{eff}}\). As we can see in Figure 8, the shape does not change during the operation after the initial 24 h, indicating that \(q_{\text{eff}}\) has not been hampered under the electrical stress. In other words, the effect of electrical stress is outside the EML, i.e., in cladding layers.