## 3 Results and Discussion
### Analysis of the Degradation of Blue TADF OLEDs by Different HTL Structures
\begin{table}
\begin{tabular}{c c c} \hline \hline  & \(\Delta\mathbf{V}\)1  & \(\Delta\mathbf{C}\)2  \\ \hline Device 1 & 0.8 V & 3.7\% \\ Device 2 & 1.1 V & 1.3\% \\ Device 3 & 1.3 V & 7.4\% \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Peak voltage and peak capacitance shift values after degradation of devices 1, 2, and 3.  
electrically biased, triplet-triplet annihilation (TTA) and triplet-polaron quenching (TPQ) occur owing to the trapped carriers at the HTL/EML interface. These TTA and TPQ phenomena produce hot excitons and polarons, which are the main reasons for roll-off at high luminance and low lifetime in OLEDs [21; 22]. As shown in Figure 3b, the voltage at which the capacitance is the highest shifts from 7.7 to 8.8 V when device 2 degrades. As the thickness of the total HTL layer is smaller than that of device 1, a stronger electric field force is applied to the organic material. This causes a Joule heating effect and decreases the carrier mobility of the organic material, causing a larger number of trapped carriers at the TCTA/EML interface. In addition, the hole injection barrier being larger than device 1 also causes an increase in the number of trapped carriers. Consequently, the voltage of the maximum capacitance shifts more at device 2 than at device 1 by approximately 0.3 V, as shown in Table 3.  
However, the peak capacitance of device 2 shifts from 23.3 to 23.0 nF, which is approximately equal before and after degradation. As shown in Table 4, although the triplet energy (E\({}_{\text{T}}\)) of mCP is the highest, resulting in superior device performance, the glass transition temperature (T\({}_{\text{g}}\)) and bond dissociation energy (BDE) of TCTA are significantly higher than those of mCP. T\({}_{\text{g}}\) and BDE are typical indications of the operational stability of organic materials in OLEDs, because the degradation of organic materials results from bond breaking in chemical reactions. High T\({}_{\text{g}}\) materials ensure high operational stability by inhibiting destructive phase change and crystallization, which generate trapped carriers [23]. Thus, even if there are more trapped carriers, the TCTA/EML interface exhibits higher stability and is insensitive to trapped carriers compared to the mCP/EML interface. Therefore, the difference between the peak capacitance of device 2 before and after degradation was the smallest. The shift in the peak capacitance, rather than the voltage shift, demonstrates the chemical stability of the interface. As shown in Figure 3c, the peak capacitance and voltage shift of device 3 were the largest among the devices. The peak capacitance drops from 39.0 to 36.1 nF, and its voltage shifts from 8.5 to 9.8 V. The value of T\({}_{\text{g}}\) for NPB is higher than that of mCP; however, there is significant degradation at the NPB/EML interface because device 3 has the largest hole injection barrier (0.5 eV higher than that of the mCP/EML interface). Note that the peak capacitance value is the highest here (approximately 40 nF), which indicates that a large number of carriers accumulate at the NPB/EML interface. These accumulated carriers cause significant degradation; furthermore, the thinnest HTL structure promotes the strongest electric force, which also causes significant degradation. Moreover, as shown in Table 2, the turn-on voltage of device 1 is the lowest and that of device 3 is the highest. The voltages at which the capacitance starts to increase in fresh devices 1, 2, and 3 are 2.8, 4.5, and 6.0 V, respectively. The lower the turn-on voltage, the more injection of the carrier and formation of triplets there will be at a lower voltage. Therefore, the C-V plot shows that the voltage at which the capacitance starts to increase is associated with the turn-on voltage.