# Wet-Deposited TADF-Based OLED Active Layers: New Approaches towards Further Optimization
## 3 Results and Discussion
### Electrical Behavior: Current Density-Applied Voltage-Brightness and Efficiencies  
The J-V-L data for all kinds of samples are shown in Figure 4. From the J-V-L data, it is possible to see that all devices showed a very low turn-on voltage (V\({}_{\text{on}}\)), between 4 and 5 V, a remarkable performance for a simple two-organic-layer structure. The devices that used THF as a solvent for the active layer materials showed one of the best results (on average). The maximum brightness was about 4030 cd m\({}^{-2}\), the maximum EQE was 10.5%, the \(\eta\)c\({}_{\text{max}}\) was 20.0 cd A\({}^{-1}\), and the \(\eta\)p\({}_{\text{max}}\) was 8.7 lm W\({}^{-1}\). The devices (again, on average) with the best results were the ones that used toluene as a solvent for the active layer materials. This showed a maximum brightness of 3240 cd m\({}^{-2}\), a \(\eta\)c\({}_{\text{max}}\) of 24.3 cd A\({}^{-1}\), \(\eta\)p\({}_{\text{max}}\) of 16.9 lm W\({}^{-1}\), and a maximum EQE of 14%; this efficiency is very good and is very close to the best in the literature [52], corresponding to a fully evaporated device with a complex structure of four/five organic layers. Looking at these results, it is possible to see that the device that used toluene as a solvent for the active layer showed the lowest V\({}_{\text{on}}\) and the best equilibrium between the current efficiency and the power efficiency. This equilibrium results in an EQE value good enough for this type of device structure.  
Figure 3: (**a**) PLE spectrum for each kind of device obtained with the excitation wavelength corresponding to the central maximum of photoluminescence spectrum. (**b**) Photoluminescence spectrum (inset: the small emission of the host material).  
In opposition, the device that used chloroform as a solvent for the active layer showed a current efficiency of only 3.7 cd A\({}^{-1}\), a power efficiency of 7.9 lm W\({}^{-1}\), a maximum brightness of 2330 cd m\({}^{-2}\), and a V\({}_{\rm on}\) of 5 V. This device showed the lowest maximum EQE about 5.5%. The overall efficiency data are shown in Figure 5.  
Finally, in Figure 6, comparative data for the EQE are shown (also as a dependence on brightness). It should be noted that, instead of the usual behavior of the simple solution-deposited OLED device structure, we achieved a particularly interesting low roll-off) when we analyzed the EQE at maximum brightness and at 1000 cd cm\({}^{-1}\). The lower roll-off appeared in the device that used chloroform as a solvent for the active layer, about 1%. The device that used toluene as a solvent for the active layer materials was the one that  
Figure 4: Current density and brightness as a function of the applied potential for every kind of OLED made with the different solvents for the active layer.  
Figure 5: (**a**) Plot of power efficiency and (**b**) plot of current efficiency as a function of the brightness for every OLED made with the different solvents for the active layer. The fast decrease observed for brightness higher than \(\approx\)2000 cd m\({}^{-2}\) is due to the device’s irreversible degradation.  
showed the worst roll-off, about 32%. In the bright region analyzed, the OLED where we used THF as solvent for the active layer materials showed a roll-off of 25% and the device where we used chlorobenzene as solvent for the active layer materials showed a roll-off of 6%. With these results, we can observe that until 1000 cd m\({}^{-2}\) these devices are very stable, and we are not employing and blocking layers. These values also show that the device can support the increase in the applied voltage, reaching high brightness values with stability. Moreover, compared with some performances reported in the literature, based on such TADF emitters but with a much more complex structure (and with organic layers thermally evaporated, decreasing the structural defects formed), we are running at the edge as much as possible. A deeper discussion will be provided further ahead.