# Wet-Deposited TADF-Based OLED Active Layers: New Approaches towards Further Optimization
## 3 Results and Discussion
### Impedance Spectroscopy Characterization
Since the capacity is a measure of charge accumulation, this may indicate that, in this OLED, the interface can accumulate more charge than in the other devices. Additionally, this device also shows the highest relaxation time. The OLED that used THF for the active layer was the one that showed the lower value for C\({}_{1}\). Therefore, if this device has a lower capacity, this can indicate the lower charge accumulated in the interface. The lowest relaxation time for the interface appeared in the OLED that used chlorobenzene for the active layer. This one also showed the second-lowest capacitance. Considering the other RC circuits, it is possible to see that the capacitance values, C\({}_{2}\), are much lower than the ones for the interface region. Normally, the recombination phenomena happen in this region. With this, it is expected to have a lower charge accumulation, since most of the charges are recombining. Therefore, if we have high values for the capacity, this can indicate that there is more charge accumulation and, therefore, less recombination. The values for C\({}_{2}\) are very close but there is a little difference for the OLEDs where chloroform and chlorobenzene were used as solvents for the active layer. The OLED that used chlorobenzene for the active layer is the one with lower C\({}_{2}\) values. The lower value can be an indication of the lower charge accumulation and, consequently, more charges for the recombination process. The OLED that used toluene as a solvent for the active layer also has lower C\({}_{2}\) values; this is in accordance with the results for the EQE presented in the previous sections, because in this device there is lower charge accumulation and there are more charges for the recombination process. On the other hand, the OLED that used chloroform as a solvent for the active layer is the one with higher C\({}_{2}\) values. The fact that this value is a little higher than the ones for the other devices can give information about the fact that the charge accumulation is higher and, as a consequence, the charges for the recombination process will be lower. The second relaxation time presented in the table corresponds to the relaxation that appears for higher frequencies in the capacitance and loss plots. These relaxation phenomena can be related to the physical properties of the active layer. Figure 11: Capacitance (C) and loss (G/\(\omega\)) data for the different kinds of OLEDs fabricated. Open circles: experimental data; lines: fitting using the double RC circuit. (**a**) THF as solvent, (**b**) chlorobenzene as solvent, (**c**) toluene as solvent, and (**d**) chloroform as solvent. The existence of traps in the devices implies that there exists a finite time for the charge interaction with electrically active levels (some certainly arising from defects). Therefore, the device with a lower relaxation time is the one that used toluene for the active layer, and the device with a higher relaxation time, for the high-frequency relaxation, is the one that used THF as solvent for the active layer. Considering these results, it is possible to assume that in the OLED that used toluene for the active layer, the charge interaction with the traps is possibly lower than in the other devices. In summary, with every consideration made, it is possible to conclude that the solvent will induce changes in the molecular conformation. If we look at the guest: host energy transfer process, the photophysical data indicate that the solvent will not induce changes, and, therefore, the electrical charge transport and accumulation need to be the fundamental explanation for the differences obtained. Finally, considering the impedance spectroscopy, it is possible to conclude that differences in the charge accumulation and in the interaction time between the charges and the trap levels are consequences of the different molecular conformation at the active layer because of the different solution properties employed in the deposition.