## 4 Conclusions  
[Pt(I)(di-_tert_-butylsalophen)] was used as dopant in Alq\({}_{3}\) matrix and produced deep red OLEDs (Amax = 660 nm). Monolayer based OLEDs showed a NDR characteristic with a figure of merit higher than 10\({}^{3}\), promoting them as potential candidates in memory device applications. NDR mechanism is interpreted in terms of space charge polarization linked to charge trapping/untrapping mechanism. By adding electron-blocking layer of NPD, NDR can be amplitude-modulated up to its annihilation. These results suggest that NDR mechanism is closely related to trapped negative charges that would modify the electronic properties at the interface between the emissive layer and the anode. The stability of OLEDs was evaluated under electrical stress. The SED model can be used to fit the decay lifetime by assuming at least two degradation mechanisms likely related to a SCP effect and an electrochemical process. The rapid decay observed in the initial stage is more pronounced in [Pt] based devices, which would be due to space charge effects (trapping-detrapping/dipole reorientation) induced by [Pt] embedded within Alq\({}_{3}\) layer. Doping active layer with [Pt] decreased the rate of the long-term decay. Therefore, the results suggest that [Pt] stabilize Alq\({}_{3}\) layer under electrical stress. Moreover, relaxation phenomena specific to [Pt] based devices were observed and might be related to the physical process governing the NDR mechanisms. Thus, the specific characteristic can be interpreted in terms of internal electric field. This study can help to elucidate NDR mechanisms in organic molecular devices.  
**Supplementary Materials:** The following are available online at [[http://www.mdpi.com/2076-3417/8/5/762/s1](http://www.mdpi.com/2076-3417/8/5/762/s1)]([http://www.mdpi.com/2076-3417/8/5/762/s1](http://www.mdpi.com/2076-3417/8/5/762/s1)), Figure S1: Evolution of NDR peak after several positive sweeping cycles, Figure S2: Evolution of NDR peak after several negative sweeping cycles, Figure S3: Variation the current density versus the number of sweeping voltage cycles, Figure S4: Retention time of both ON and OFF states measured at 5 V.  
Figure 7: (**a**) Decrease-resetting zone identified from normalized current vs. time curves of doped devices (according with Figure 6); (**b**) The lifetime curves measured at 30 \({}^{\circ}\)C and 80 \({}^{\circ}\)C; (**c**) Schematic illustration of the physical mechanisms suggested for describing the decrease-resetting behavior of the current upon electrical stress. The applied field (E\({}_{\text{app}}\)) results in the migration of electrons within Alq\({}_{3}\) layer and the accumulation of electrons within Alq\({}_{3}\) layer and in the vicinity of NPD/Alq\({}_{3}\) interface. (**i**) Drop in current: Dipole formation; (**ii**) formation of conductive pathways and (**iii**) Increase in current: Thermal energy release.  
**Author Contributions**: [PERSON] and [PERSON] conceived and designed the experiments; [PERSON] and [PERSON] performed the experiments; [PERSON], [PERSON] and [PERSON] analyzed and interpreted the data; [PERSON] and [PERSON] contributed materials/analysis tools; [PERSON] and [PERSON] wrote the paper. [PERSON] contributed to the redaction of the paper.  
**Acknowledgments:** Region Midi-Pyrenees and University Paul Sabatier are acknowledged for [PERSON]'s fellowship. [PERSON] SCHLEGEL [PERSON] is acknowledged for his contribution in the setting up of the cleaning part in the OLEDs making process.  
**Conflicts of Interest**: The authors declare no conflict of interest. The founding sponsors had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript, and in the decision to publish the results.