## 3 Results and Discussion
### Stability Investigations
#### 3.3.1 Analysis of Characteristics Measured on Non-Doped Devices  
The stretched exponential fit traced the normalized decay curves accurately. The parameters are summarized in Table 1. On non-doped device, values for \(\beta\) = 0.59 and 1.1 with a decay time of \(\tau\) = 8.5 h and 500 h were applied to the fast and long-time decays, leading to a very good agreement of the SED fit with the measured data (Figure 6). The values of the parameter \(\beta\) are in agreement with those obtained in Alq\({}_{3}\) based diodes [32; 40]. We can assume that the charge transport would become dispersive during the operating time and is likely due to the creation of defects in organic devices [8]. The fast decay (occurring at the initial stage) could be attributed to the creation of the internal electrical field (due to SCP) likely caused by the presence of traps, diffusion of mobile ions, or reverse orientation of the dipoles [14; 32].  
It was suggested that the long-time exponential decay of the luminance, results from a degradation model governed by electrochemical reactions [14; 32]. Consequently, it can be assumed that, during the long-time decay, the degradation mechanism would be related to the formation of unstable cationic Alq\({}_{3}\) species [32; 41].  
During, the operation of diodes, Joule effect resulting from charge flow can lead to thermal degradation of OLEDs [42]. In addition to the effect of oxygen and the role of the temperature in the degradation process of organic devices, the charge flow during the device operation plays a role in the aging mechanisms. By applying a voltage, an internal polarization field is induced, which tends to oppose to the external field, which causes a decrease of the drive current. Several models are proposed to explain the mechanisms that cause polarization field. The first one is the trapping model [43] in which traps can be present and/or created in the organic layers. The second model is based on the permanent dipoles: the external field induces the reorientation of the permanent dipoles and can lead to the increase and/or decrease of internal field in the device [14; 33]. The third model is the redistribution of mobile ions in active layers under the external bias: the diffusion of mobile ions leads to electrochemical processes at the electrode/organic interface [44]. Other investigations infer that the charge transport in the active layer of devices causes degradation of the organic film [45; 46]. [PERSON] et al. suggested that the gradual degradation of organic film is related to the passage of electrons through polymer film [45]. They assumed that the microscopic origin would be some form of redox instability in the materials. However, investigations on Alq\({}_{3}\) based diodes suggest that the degradation of these devices is primarily caused by the injection of holes and the subsequent formation of the unstable cationic Alq\({}_{3}\) species in vicinity of the hole transport layer [14; 44]. It should be noted that the lifetime measurements were performed under electrical stresses and inert atmosphere. Oxidation of the organic materials can thus be excluded. SED suggests that the charge transport would become dispersive during the operating time. The OLED degradation could be explained in term of dipole formation and/or trap-controlled mechanism leading to the annihilation of the emission centers.  
\begin{table}
\begin{tabular}{c c c c c} \hline \hline  & \multicolumn{2}{c}{**Fast Decay**} & \multicolumn{2}{c}{**Long-Time Decay**} \\ \hline  & \(\tau\) _(h)_ & \(B\) & \(\tau\) _(h)_ & \(\beta\) \\ \hline Non-doped device & 8.5 & 0.59 & 500 & 1.1 \\ Doped device & 1.5 & 0.53 & 1500 & 1.2 \\ \hline \hline \end{tabular}
\end{table}
Table 1: Parameters obtained from stretched exponential fits related to fast decay and long-time decay.