## 3 Results and Discussion
### Negative Differential Resistance  
The effect of [Pt] on the charge transport in the Alq\({}_{3}\) layer was investigated from J-V characteristics in a monolayer device (Figure 4a). After performing sweeping voltage from 0 V to +14 V, the current abruptly increased in the range of 0 to 5 V then decreased in the range of 5 to 8 V. The peak of current is obtained for an applied voltage of 5 V. This shape of J-V is characteristic of negative differential resistance (NDR). It was only observed on [Pt] doped devices. By performing ten successive sweeping cycles (positive poling), the repeatable NDR behavior was observed with an increase in current peak. The intensity of this peak achieves a constant value after ten successive sweeping cycles (as shown in Figures S1-S3 of Supplementary Information). By performing negative poling (ten successive sweeping from 0 V to \(-\)14 V, as shown in Figures S2 and S3 of Supplementary Information), J-V characteristics recovered the initial shape, indicating that the electrical switching behavior is related to a reversible mechanism. At low voltage (below 8 V), the current could be switched between two electrical states: high-resistance state (OFF state) and low-resistance state (ON state). Thus, the current can be identified in three parts: (i) bistable area; (ii) Negative Differential Resistance (NDR) area and (iii) monostable area. An ON/OFF current ratio higher than \(10^{3}\) is achieved in the bistable area. The origin of NDR features of devices could be assigned to the formation of electrically active defects introduced by the [Pt] within the Alq\({}_{3}\) layer. Similar behavior was observed in devices based on organic layer doped by metal nano-clusters or fluorescent dyes [21; 22; 23; 24; 25]. It was proposed that charge accumulation within Alq\({}_{3}\) layer would play a key role in the bistable trend and NDR properties [21]. At low voltage (<5 V), the switching conductance of the active layer can be due to localized electronic states leading to electron like-traps. The filling of these trapping states leads to the formation of Alq\({}_{3}\)+[Pt]\({}^{-}\) dipole resulting in space charge polarization (SCP) of active layer. The SCP can modify charge injection properties at the electrode/active layer interface; promoting conductive pathways or space charge field inhibition of charge injections [26]. The J-V characteristics carried out after positive poling show a transition in conduction mechanism from injection limited (OFF) to bulk limited (ON) suggesting a variation in the electric-field distribution between OFF and ON states. Due to the distribution of the LUMO levels (estimated at \(-\)3.13 eV and \(-\)3.48 eV for Alq\({}_{3}\) and [Pt] respectively), the interface with the cathode is expected to be as ohmic contact for electrons (Figure 1b). Therefore, the asymmetrical J-V characteristics observed for OFF state could be explained by an energetic barrier at the interface (with the anode). That should result in injection-limited current in the device for negative applied voltages. After the positive poling, the charges are expected to be stored in the active layer due to the trap like behavior of [Pt] within Alq\({}_{3}\) matrix. Consequently, the trapped charges within active layer should be compensated by charges from the metal electrode. A portion of the compensated charges could lead to band bending (Figure 4b) and that would likely lower the energetic barrier for electrons at the interface with the anode [27; 28]. As a result, the current in the device increases to form the ON state in both polarity of the applied voltage  
Figure 3: Description of the host-guest confinement: the difference of HOMO and LUMO levels between the guest and host materials substantiates that HOMO and LUMO levels of [Pt] act as trapping levels (holes and electrons) within Alq\({}_{3}\) layer. (between \(-5\) V and \(+5\) V). It can be assumed that the injection properties of Anode/Alq\({}_{3}\):[Pt] would be modified by charges trapped within Alq\({}_{3}\):[Pt] layer.