## 3 Results and Discussion
### SC-O-SC Temperature-Dependent Transport Properties  
In Figure 4a, we present the temperature-dependent IV measurements of the SC-O-SC devices depicted in Figure 1a, with 5 nm Al and 30 nm-thick NbN. We use a thicker NbN electrode to strengthen its SC properties (e.g., TC increased to 11.4 K in this device) and proximity effect, since the main goal was to search for SC signatures in the transport properties of the junctions. The IV at 300 K shows non-linear current increase with voltage, similar to findings previously reported for MEH-PPV junctions, indicating space-charge-limited conductivity [25]. As the temperature is lowered, the resistivity of the organic layer increases, providing further evidence that the metal-semiconductor-metal device is not governed by pinholes. Looking at the IV behavior just below the NbN transition temperature, one observes a region with negative differential resistance (NDR) above 8 mA, the blue curve in Figure 4a, measured at 11 K. By comparing (Figure 4b) the voltage drop on the NbN electrode (purple) with the voltage across the SC-O-SC junction (blue), as a function of current, we observe that the NDR coincides with crossing the SC critical current of NbN. This behavior is expected, recalling that when NbN becomes normal there is excess Joule heating, due to the relatively high resistance of NbN [26], which increases at low temperatures. This means that upon reaching the critical current, dissipation in NbN contributes to the temperature increase in the junction, therefore lowering the resistance of the MEH-PPV. This also explains the difference in the MEH-PPV high-voltage characteristics when comparing the measurements at 15 K and higher temperatures with those at temperatures below 11 K. Evidently, when measuring with larger currents and above 15 K, Joule heating of the NbN heats the MEH-PPV and changes its IV measurement. For temperatures below 11 K, the NbN is superconducting and there is no excess heating of the junction. Thus, the junction temperature is lower, and the organic's resistance is higher. We note in passing that we have also measured light-emitting properties in a similar OLED device through the top electrode, where an additional inorganic blocking layer was added, see the Supplementary Material for details. This device showed light emission down to the lowest temperature measured of 5 K. This provides further support for the effectiveness of Al in preventing damage to the organic layer, since emission would be strongly quenched if pinholes were present. Figure 5 presents the main results of our study. In Figure 5a, we plot the low bias resistance of the SC-O-SC junction as a function of temperature (normalized to the maximal resistance). There is a considerable drop in resistance coinciding with the critical tempera  
Figure 4: (**a**) Current vs. voltage of the SC-O-SC junction for temperatures between 2 K and 300 K. IV curves show space-charge-limited conductivity for the MEH-PPV layer. (**b**) V vs. I measured at 11 K for NbN (purple) and MEH-PPV (blue). NbN displays a critical current of 8 mA. The onset of negative differential resistance in the device corresponds with the critical current. ture of NbN (11.4 K, right dotted line); then, there is a further resistance drop upon crossing the Nb critical temperature (8.1 K, left dotted line). The final resistance, however, is not zero, reaching 10% of the maximum resistance at our lowest attainable temperature of 2 K.  
Figure 5b provides a zoomed-in view, up to 12 mV, of the IV curves for temperatures between 2 K and 11 K. Generally, for these small currents, the organic semiconductor is in the ohmic regime, and far from the space-charge-limited behavior that we observed at higher voltages (Figure 5) that is common in polymers such as MEH-PPV [25]. For all temperatures above 11 K, the behavior is ohmic, but also for temperatures above 8 K the behavior is nearly ohmic.