# Wet-Deposited TADF-Based OLED Active Layers: New Approaches towards Further Optimization
## 3 Results and Discussion
### Impedance Spectroscopy Characterization
The OLED that  
Figure 10: Cole–Cole plot for every OLED made with different solvents for the active layer. used toluene as a solvent for the active layer showed the biggest drop in the capacity value. These drops can be (even indirectly) correlated with the charges' lower mobility because the charge will have more difficulty following the AC signal. In opposition, the devices that used THF and chloroform as solvents for the active layer had a low drop in the capacity value, which can correspond to a lower charge accumulation, making it difficult to follow the AC signal. The OLED where chlorobenzene was used as a solvent for the active layer also had a very low drop. The loss plot also shows a decrease with the frequency increase. But between 104 Hz and 105 Hz, there is an increase in the loss plot until reaching the relaxation peak. In the inset plot of Figure 9a,b, is possible to see that this peak appears in different frequencies. For toluene, the peak appears near 5.9 \(\times\) 10\({}^{5}\) Hz, for chlorobenzene near 6 \(\times\) 10\({}^{5}\) Hz, for chloroform near 9.9 \(\times\) 10\({}^{5}\) Hz, and for THF at frequencies higher than the ones measured. From the loss tangent plot, the behavior for every solvent is very similar and is in accordance with the theoretical models presented before. The peak for high frequencies corresponds to the point where the capacity starts to decrease. However, there is no visible peak of relaxation. In the Cole-Cole plot, it is again possible to see the double relaxation process. There are remarkable differences among the devices; however, the devices that used chlorobenzene and chloroform for the active layer show the same behavior. Some correlation between the data obtained and the physical models considered for the equivalent circuit can be carried out. Therefore, examining the model considered for the data analysis, it is possible to see that for low frequencies the equation will go to a constant value that is given by C\({}_{0}\). That value will have a dependence on the capacitances and resistances. For high frequencies, the equation will go for another constant value, C\({}_{S}\), that only depends on both capacitances. Both of these constants do not have a frequency dependence. With the association of the experimental data with this model, it is possible to obtain the values for the resistances and capacitances, which can provide more information about the way that charge will flow and will accumulate in the layer. A full simulation of the equivalent electrical circuit can be performed (numerical convergence methods) to extract the physical data of the circuit. Figure 11 shows such numerical fits. Looking at these curves, it is possible to see that in every OLED for high frequencies, the model is well adjusted to the experimental data. For the OLED that used chlorobenzene as a solvent for the active layer, at medium frequencies, there is a little discrepancy between the simulated curve and the experimental data. This discrepancy can be related to some physical phenomena that can happen in the device for these frequencies. Also, there exists a discrepancy between the simulated curve and the experimental data at low frequencies. This can be correlated with the equipment limitations at low frequencies. However, we cannot eliminate the existence of other physical phenomena that can happen in the devices. Table 3 presents the values obtained for the capacitances, resistance, and relaxation times using the equivalent circuit considered for the data analysis and simulations. With this equivalent circuit, it is possible to consider that, in our case, the first circuit (R\({}_{1}\) and C\({}_{1}\)) can be related with any interface (although other effects are not out of consideration) and the second circuit (R\({}_{2}\) and C\({}_{2}\)) can be related with the bulk. In this table, it is possible to see that the OLED that used toluene as solvent for the active layer is the one with the highest C\({}_{1}\) value.