# Wet-Deposited TADF-Based OLED Active Layers: New Approaches towards Further Optimization
## 3 Results and Discussion
### Electroluminescence and Color Coordinates  
The electroluminescence (EL) spectrum for the OLEDs made with different solvents for the active layer is shown in Figure 7. It is possible to see the existence of two peaks for the devices made using the THF and chloroform as solvent and three peaks for chlorobenzene and toluene. The first peak that appears in the spectrum for the chlorobenzene and toluene, at about 380 nm, corresponds to the well-known PVK emission [53; 54], which causes a blue shift in the device emission with these solvents. The origin of this peak is related to the incomplete charge transferred from PVK to 2 PXZ-OXD. Since this spectrum was taken at 10 V, the electrical charge density is particularly high in the active layer and PVK starts to emit. The spectrum where THF was employed as solvent shows a central peak at 525 nm and a secondary peak at 492 nm. For the chlorobenzene, the principal peak appears at 499 nm and the secondary peak appears at 532 nm; this indicates that there exists a blueshift when this solvent was used, which is provoked by the PVK emission. For toluene, the spectrum shows a central peak at 498 nm and a secondary peak at 520 nm. Lastly, the spectrum obtained when chloroform was used as a solvent has two very close peaks; the first and principal peak appears at 507 nm and the secondary peak appears at 526 nm. The principal peak in these spectra corresponds to the TADF emission. The secondary peak is associated with a nonplanar vibration that happens in the oxazole group [55]. With the data obtained, it is possible to see that the use of different solvents changes the EL emission. This shift also indicates that the solvent will act in the recombination and charge-transport mechanisms. Figure 6: EQE as a function of the brightness of the devices. In Figure 8, it is possible to see the color coordinates for every OLED with different solvents that were used in the active layer. Joining the CIE color coordinates information with the EQE behavior (with brightness), it is naturally observed that the different solvents provoke changes in the color coordinates. If we move from THF to chlorobenzene and chloroform, it is possible to see that there exists a blueshift in the color coordinates. This can be an indication that the different solvents will make different changes in the molecular conformation. These modifications can be an indication that the use of different environments, in our case, solvents, can result in a modification in the molecular conformation, and these changes can be provoked by the different polarities in the environment. A first summary of these data is shown in Table 1.  
Figure 8: CIE chromaticity diagram with the color coordinates for every OLED made with the different solvents for the active layer. The correspondent color coordinates are (0.23; 0.47) for chloroform (black points), (0.28; 0.57) for THF (yellow point), (0.22; 0.50) for toluene (red points), and (0.22; 0.47) for chlorobenzene (grey point). Photo is of an OLED working at 10 V.  
Figure 7: Electroluminescence spectra obtained for an applied voltage of 10 V for every OLED made with the different solvents for the active layer. A detailed study of the charge-transport mechanisms on these devices was performed. For this purpose, p- and n-type-only devices were made. Considering the usual SCLC model previously described elsewhere [47, 56], we can extract valuable information regarding the electrical parameters involved in the electrical carriers' transport, namely, the Nt corresponds to the density of traps, Et to the energy of traps, uh to the hole mobility, and ue to the electron mobility. Table 2 summarizes these data. Looking at Table 2, it is possible to see that the OLED that used THF as solvent for the active layer materials shows the best equilibrium between the mobility of electrons and holes, with this situation being the closest to the ideal one. Ideally, both mobilities should be in the same order of magnitude.