## 3 Results and Discussion  
### Analysis of the Degradation of Blue TADF OLEDs by Different HTL Structures  
In this section, the operational stability of blue TADF OLEDs with different HTL structures is discussed. Devices 1, 2, and 3 were fabricated, and the neighboring HTLs to the EML were mCP, TCTA, and NPB, respectively. Figure 2 depicts the EL characteristics of undegraded devices 1, 2, and 3, and Table 2 summarizes the performance of each device. Considering the highest occupied molecular orbital energy level of each device's HTL and DPEPO materials (see Figure 1a), the hole injection barriers of devices 1, 2, and 3 are 0.2, 0.4, and 0.7 eV, respectively. The barrier causes carrier accumulation, which induces a space-charge region and increases the electric potential in the forward bias [20]. Consequently, device 1, with the smallest barrier, exhibits the lowest turn-on voltage. For the same reason, device 2 demonstrates better performance than device 3. \begin{table}
\begin{tabular}{c c c c c c c c} \hline \hline \multirow{2}{*}{**Devices**} & **HTL** & **HTL** & **HTL** & **EML** & **ExBL** & **ETL** & **EIL** & **Cathode** \\ \cline{2-8}  & **30 nm** & **20 nm** & **10 nm** & **30 nm** & **X nm** & **30 nm** & **2 nm** & **80 nm** \\ \hline \multirow{2}{*}{1} & \multirow{2}{*}{NPB} & TCTA & mCP & \multicolumn{2}{c}{DPEPO} & & \\  & & & & (10) & & & \\ \cline{2-8}  & & TCTA & — & \multicolumn{2}{c}{DPEPO} & & \\ \cline{2-8}  & & & & DMEPO & \multicolumn{2}{c}{(10)} & & \\ \cline{2-8}  & & — & \multicolumn{2}{c}{DPEPO:} & & \\ \cline{2-8}  & & — & \multicolumn{2}{c}{DMEPO} & & \\ \cline{2-8}  & & — & \multicolumn{2}{c}{DMEPO (10)} & & \\ \cline{2-8}  & & — & \multicolumn{2}{c}{DPS (10)} & & \\ \cline{2-8}  & & & & wt\% & \multicolumn{2}{c}{(10)} & \\ \cline{2-8}  & & TCTA & mCP & \multicolumn{2}{c}{—} & \\ \cline{2-8}  & & TCTA & mCP & \multicolumn{2}{c}{DPEPO} & \\ \cline{2-8}  & & TCTA & mCP & \multicolumn{2}{c}{(40)} & \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Structures of the fabricated blue TADF devices. \begin{table}
\begin{tabular}{c c c c} \hline \hline  & **Turn-On** & **Maximum External Quantum** & **Maximum Power** \\  & **Voltage (V)** & **Efficiency (\%)** & **Efficiency (lm/W)** \\ \hline Device 1 & 3.5 & 17.7 & 17.7 \\ Device 2 & 3.9 & 7.7 & 6.1 \\ Device 3 & 4.0 & 3.4 & 2.6 \\ \hline \hline \end{tabular}
\end{table} 
Table 2: EL characteristics of devices 1, 2, and 3. Figure 3 and Table 3 present the C-V characteristics of devices 1, 2, and 3 before and after electrical aging at a current density of 10 mA/cm\({}^{2}\) for 10 min. The graphs of the C-V characteristics of the devices are of the same shape; as the voltage increases, the capacitance decreases shortly after a slight increase. The voltage at which the capacitance starts to rise is relevant to carrier injection. As carriers and excitons are trapped in the EML and its interface, those induce an increase in capacitance. Subsequently, the capacitance decreases because the exciton formation occurs faster than the carrier injection, and the number of trapped carriers is reduced. As shown in Figure 3a, the voltage at which the capacitance starts to increase is the smallest among the studied devices because the hole injection barrier of device 1 is the lowest, as previously indicated. As device 1 degrades, the voltage at which the capacitance is at its highest shifts from 4.6 to 5.4 V. When organic materials are degraded, the carrier mobility deteriorates, and a higher voltage is required for the carriers to recombine. The peak value of capacitance decreases from 21.7 to 20.9 nF. This drop in the maximum capacitance indicates that the device was aged via an electrical bias. When OLEDs are  
Figure 3: C-V characteristics of blue TADF OLEDs before and after degradation: (**a**) device 1, (**b**) device 2, and (**c**) device 3. Figure 2: (**a**) J-V-L characteristics and (**b**) J-EQE characteristics of devices 1, 2, and 3.