## 3 Results and Discussion
### Stability Investigations  
The degradation behavior under constant current or voltage driving can be influenced by several independent degradation mechanisms [14; 32]. The lifetime curve could be interpreted from the analytical models of decays, which are related to several different sets of physical mechanisms. During  
Figure 5: J-V Characteristics of OLEDs with different thicknesses of NPD layer (x = 0 nm, x = 10 nm and x = 55 nm) by keeping the total thickness of OLED constant. (Inset) L-V characteristics measured for the different structures. The device configuration is ITO/PEDOT:PSS/NDP (x nm)/Alq\({}_{3}\):5% [Pt] (150 nm-x nm)/Ca with x the thickness of \(N\),\(N^{\prime}\)-Di(1-naphthyl)-\(N\),\(N^{\prime}\)-diphenyl-(1,1\({}^{\prime}\)-biphenyl)-4,4\({}^{\prime}\)-diamine (NPD) layer. OLED operation, the current and the luminance tend to decrease exponentially and this suggests that two main degradation mechanisms may occur:  
* A fast initial decrease (with a rate of the order of few hours) which is one of the characteristics of space charge effects as trapping-untrapping/dipole reorientation [14; 32; 33]
* A slow decrease (with a rate of the order of few hundred hours), which can be related to electro-chemical mechanisms (likely molecule dissociation) and/or chemical species diffusion within the OLED layers [14; 34]. The lifetime measurements were performed in order to evaluate the effect of [Pt] on devices stability under a constant driving voltage. The study of degradation was performed on doped and non-doped bilayers OLEDs (Figure 6). We observed: (i) a rapid decay in the initial stage of operation that may be related to SCP mechanism; (ii) Long-time decay related to electro-chemical mechanisms (likely molecule dissociation) and/or chemical species diffusion. We noticed that J-V characteristic measured on doped devices exhibited decrease-resetting zones. The luminance and the operating current behavior of a device were recorded in the dark at constant voltage of 8 V and at ambient temperature. The driving voltage was fixed above the turn-on voltage (estimated to be in the range 4 to 5 V) and below the voltage at which the decrease in quantum efficiency occurs (estimated at 10 V). In high driving condition, the active layer was probably completely filled with excited triplet states, favoring triplet-triplet annihilation leading to roll-off of phosphorescent device. The stretched exponential decay (SED), given by Equation (1), can be used to fit the lifetime curve obtained under different driving conditions [14; 30; 33]. \[J/J_{0}=exp\left(-\left(\frac{t}{\tau}\right)^{\beta}\right) \tag{1}\]  
in which \(\tau\) is the decay time, and \(\beta\) the dispersion factor. As the parameter \(\beta\) reflects the different initial luminance drop behavior for different device structures, i.e., materials and layer design, \(\beta\) is nearly constant for different current densities measured on the same structure. Some investigations performed on disordered semiconducting materials suggest that the analyze of the decay rate could provide information on density of trap states and trap release rates [35; 36; 37; 38]. In this model based on hopping mechanism, it was suggested that \(\beta\) might depend on the spatial distribution of localized states within the band gap of the semiconductor. The exponential  
Figure 6: Normalized current vs. time curves of non-doped (in green) and doped (in red) devices with the fitted lifetime curves using the stretched exponential decay (SED) model (respectively in blue and black). The Red square highlighting the decrease-resetting zone is zoomed in Figure 7.  
decrease might be explained by the reduction of the number of emissive centers over time, caused by the flow of charge carriers [32; 39]. SED analysis is a convenient tool for describing relaxation processes in disorder molecular systems allowing a qualitative analysis of OLED aging.