## 3 Results and Discussion  
To examine the EL performance of **Fluo-2**, two standard device configurations were investigated, namely ITO/MoO\({}_{3}\) (5 nm)/**Fluo-2** (40 nm)/TPBi (30 nm)/LiF (1 nm)/Al (80 nm) (device configuration A) or ITO/MoO\({}_{3}\) (5 nm)/**Fluo-2** (40 nm)/BCP (10 nm)/Al\({}_{3}\) (20 nm)/LiF (1 nm)/Al (80 nm) (device configuration B) (see Figure 2) [19; 20; 21]. The luminance-voltage (\(L\)-\(V\)) curves obtained for the two device structures are presented in the Figure 3. Interestingly, major differences could be found. First, a turn-on voltage of 3.4 V was determined for devices fabricated with the configuration A whereas it decreases to only 2.8 V for the second configuration. Conversely, the highest luminance was found for the first configuration, the brightness peaking at 23,800 cd/m\({}^{2}\) at 10.4 V whereas a maximum luminance of only 3050 cd/m\({}^{2}\) was determined of type B-devices. Compared to previous results, a luminance about 17 times higher than that previously reported in the literature was determined for **Fluo-2**-based devices. While examining the EL spectra (see Figure 4), a pure emission of **Fluo-2** was obtained for type B-devices, whereas a contribution of TPBi could be detected for type A-devices with a shoulder at 377 nm. Therefore, it can be concluded that **Fluo-2** exhibits a hole-dominated transportation so that some holes  
Figure 2: Device configurations and formulas of the different materials used to fabricate organic light-emitting diodes (OLEDs). can drift from the emissive layer into the adjacent electron-transport layer and recombine with electrons, resulting in the TPBi emission. This result also explains the low device characteristics reported in the literature for **Fluo-2** (but also for **Fluo-1** which is of similar structure), the hole-dominated transportation of **Fluo-2** (but also of **Fluo-1**) favoring the electron-hole recombination at the cathode interface, leading to quenching and a significant reduction of the overall device efficiency [22]. Indeed, in these two studies, no hole blocking layer was used. As mentioned in the introduction section, naphthalene-based emitters are often characterized by an electron-dominated transportation whereas in the present case, holes could drift inside the electron-transport layer i.e., TPBi (type A-devices). This counter-intuitive result can be assigned to the device structure that was selected as well as to the materials chosen to fabricate OLEDs. Indeed, smaller energy gaps could be found between layers enabling to inject and transport holes. In this context, the energy barriers being lower, holes can be injected and transported more easily than electrons, resulting in the electron-hole recombination close to the TPBi Layer. Ability of **Fluo-2** to easily transport holes was confirmed by examining the variation of the CIE coordinates with the applied voltage. As shown in the Figure 4, a significant variation of the CIE coordinates could be detected for an applied voltage higher than 10 V, evidencing a shift of the electron-hole recombination zone close to the TPBi layer at high voltage. This issue was addressed by introducing an additional BCP interlayer (10 nm) between the emissive layer and the electron-transport layer (device configuration C). An improvement of the EL performance with this second device configuration is also expected due to the suppression of the electron-hole recombination within the TPBi layer, what was confirmed experimentally (see Table 1). As shown in the Figure 5, addition of the BCP layer could efficiently confined electrons within the emissive layer since almost no variation of the dominant wavelength with the applied voltage was detected. A pure emission of **Fluo-2** could be observed in the EL spectra recorded at various driving voltages, evidencing a confinement of holes within the emissive layer. As anticipated, enhanced electron-to-photon conversions were obtained for type C-devices.