## 3 Results and Discussion
### Stability Investigations
#### 3.3.2 Doping Effect on the Device Stability  
The stability measurements showed that [Pt] within the emissive layer would stabilize OLEDs upon operation (decrease of the degradation rate at \(\tau\) = 1500 h) whereas the higher decrease rate of the initial decay (\(\tau\) = 8.5 h) assumes that the effect of space charge on charge transport would be more pronounced in doped devices than in non-doped devices. A decrease-resetting behavior of current was observed (Figures 6 and 7a) upon electrical stress. Three portions can be identified: (i) the current decreases from 88.5 A to 85 A with a rate of 0.130 h\({}^{-1}\) between 7 h and 15 h; (ii) the current reaches a minimum value of 84 A after 16 h; and (iii) the current increases with a rate of 0.138 h\({}^{-1}\) between 17 h and 25 h. This behavior would be related to the modulation of SCP within the active layer due to the polarization/ depolarization mechanisms of [Pt] [14; 47]. This relaxation mechanism could be explained by the internal electronic field model based on diffusion of ionic species and dipole reorientation/charge trapping [14]. The NDR behavior observed in Alq\({}_{3}\):[Pt] based devices is substantiated with polarization/depolarization assumption. A possible explanation is the formation of preferred current pathways in devices induced by the orientation of [Pt]\({}^{-}\)-Alq\({}_{3}\)\({}^{+}\) dipoles. Similar decrease-resetting behavior of current has already been observed in Pt based OLED by [PERSON] et al. [9]. It was explained by the formation of preferred current pathways in devices leading to hot spots under electrical driving measurement. It can be assumed that similar effect might occur in Alq\({}_{3}\):[Pt] based devices. However, the origin of this possible conductive pathway is not clearly established. The electron pathways within Alq\({}_{3}\) layer would lead to hot spots resulting in local increases of temperature as a consequence the higher electron conductivity. On Figure 7b, the lifetime curves measured at 30 \({}^{\circ}\)C and 80 \({}^{\circ}\)C highlight the influence of the temperature on the decrease-resetting characteristic, suggesting that it was related to thermally activated mechanisms. The following stages, depicted in Figure 7c, are proposed for explaining the lifetime curve of [Pt] based device:  
1. [label=()]
2. Dipole formation by charge transfer (CT)--The current decrease could be explained by charge accumulation: the electrons are trapped by [Pt] leading to the formation of complex negatively charged [Pt]\({}^{-}\) and adjacent molecule would be positively charged (Alq\({}_{3}\)\({}^{+}\)). The formation of charged species tends to create [Pt]\({}^{-}\)-Alq\({}_{3}\)\({}^{+}\) dipoles. The electron accumulation (within Alq\({}_{3}\) layer and at the vicinity of NPD/Alq\({}_{3}\) interface) would induce internal field (E\({}_{\text{int}}\)) opposite to applied field, which tends to reduce electron mobility within Alq\({}_{3}\) layer. 3. Formation of conductive pathways--The preferred orientation of [Pt]\({}^{-}\)-Alq\({}_{3}\)\({}^{+}\) dipoles would tend to create preferential electron pathways within Alq\({}_{3}\) layer favoring local electron charge transport. 4. Thermal energy release--The current resetting could be explained by electron-phonon and/or electron-electron scattering in conductive pathways [48; 49; 50]. The local heating would induce thermal assisted mechanisms: depolarization of [Pt]\({}^{-}\)-Alq\({}_{3}\)\({}^{+}\) dipoles (i.e., thermally activated relaxation of electron captured by [Pt]) and thermal assisted injection of electron from the Alq\({}_{3}\) layer to the NPD layer. The phenomena responsible for device aging could not be clearly attributed to a single process. The analysis of the lifetime decay of the devices can give a qualitative interpretation of physical mechanisms: formation of SCP and electrochemical reactions. Further investigations are required to elucidate clearly the origin of decrease-resetting behavior of the lifetime curve of [Pt] based devices.