# Wet-Deposited TADF-Based OLED Active Layers: New Approaches towards Further Optimization
## 4 Conclusions  
This work aimed to develop and characterize OLEDs based on a 2 PXZ-OXD green TADF, in simplified structures (two organic layers), and their optimization. Solution-deposited OLEDs are particularly prone to producing organic layers with a high density of intrinsic defects that negatively impact the device's performance. Here, we tried to optimize the OLED active layer deposited using different solvents and tentatively establish an electrical model that can explain the different results. The focus was on achieving the best trade-off between device performance and structure optimization with the best possible molecular conformation. The work involved the wet manufacturing of OLEDs (spin-coating) and the electro-optical characterization, giving the figures of merit based on the applicable physical models for the injection/transport and recombination of electrical charge. From the analysis of the figures of merit, we can conclude that the best device was the one that used toluene as a solvent for the active layer, with figures of merit near the best reported in the literature but with a more complex device structure. This maximum brightness was 3240 cd.m\({}^{-2}\), n\({}_{\text{C,max}}\) of 24.3 cd.A\({}^{-1}\), n\({}_{\text{p,max}}\) of 16.9 lm.W\({}^{-1}\), and a maximum EQE of 14%. On the other hand, the worst device was the one that used chloroform as solvent. By using DC electrical characterization, it was possible to correlate the data with intrinsic defect formation and electrical balance of the carriers inside the active layer. By using impedance spectroscopy, we conclude that the system can be modulated by two RC parallel circuits in series. The capacitance vs. frequency plot shows differences for the four types of OLEDs; one factor that can explain these differences is the charge mobility in the device. With the numerical data fitting, it was possible to obtain the parameters to describe the equivalent circuit. A relationship between relaxation times and pure figures of merit can be obtained, helping to provide a better understanding of the molecular conformation differences in the OLED active layer, and opening new opportunities for further developments. \begin{table}
\begin{tabular}{c c c c c c c} \hline \hline
**Solvent** & \(\begin{array}{c}R_{1}\\ (\text{K}\Omega)\end{array}\) & \(\begin{array}{c}C_{1}\\ (\text{n}\text{F})\end{array}\) & \(\begin{array}{c}\tau_{1}\\ (\text{ms})\end{array}\) & \(\begin{array}{c}R_{2}\\ (\text{O})\end{array}\) & \(\begin{array}{c}C_{2}\\ (\text{n}\text{F})\end{array}\) & \(\begin{array}{c}\tau_{2}\\ (\text{ns})\end{array}\) \\ \hline Toluene & 90 & 7.5 & 0.675 & 40 & 0.3 & 12 \\ \hline THF & 320 & 1.2 & 0.384 & 200 & 0.3 & 60 \\ \hline Chlorobenzene & 750 & 1.7 & 0.128 & 100 & 0.2 & 20 \\ \hline Chloroform & 300 & 2.0 & 0.600 & 95 & 0.4 & 30 \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Values obtained with the fittings for the equivalent circuit resistances and capacitances, and the respective times of relaxation. **Author Contributions:** Conceptualization, L.P.; methodology, F.T., J.C.G. and L.P.; software, L.P.; validation, F.T., J.C.G. and L.P.; formal analysis, F.T., J.C.G. and L.P., investigation, F.T., J.C.G. and L.P.; resources, L.P.; data curation, writing--original draft preparation, F.T., J.C.G. and L.P.; writing--review and editing, J.C.G. and L.P.; visualization, F.T., J.C.G. and L.P.; supervision, J.C.G. and L.P.; project administration, L.P.; funding acquisition, L.P. All authors have read and agreed to the published version of the manuscript. **Funding:** F.M.T., J.C.G. and L.P. acknowledge the Institute for Nanostructures, Nanomodelling and Nanofabrication (i3N), Portuguese Foundation for Science and Technology (FCT) (UIDB/50025/2020) and UIDP/50025/2020). J.C.G. also thanks FCT/i3N/UA for the individual CEEC grant (2021.02056.CEECIND). **Institutional Review Board Statement:** Not applicable. **Infrared Consent Statement:** Not applicable. **Data Availability Statement:** Data are available upon reasonable request.