## 3 Results and Discussion
### Analysis of the Degradation of Blue TADF OLEDs by Different ExBL Structures  
In this section, the optimization of the blue TADF OLED device with respect to the ExBL thickness is discussed. Due to the high triplet energy of the blue TADF dopant (DMAC-DPS), ExBL is essential for the device [28]. Devices 1, 4, and 5 were fabricated with ExBL thicknesses of 10, 0, and 40 nm, respectively. The J-V-L and J-EQE characteristics of  
\begin{table}
\begin{tabular}{c c c c} \hline \hline  & **Average BDE 1 (eV)** & **T\({}_{\text{g}}\)** (\({}^{\circ}\)**C**) & **E\({}_{\text{T}}\) (eV)** \\ \hline mCP & 4.23 & 55 & 2.90 \\ TCTA & 5.24 & 151 & 2.76 \\ NPB & 5.23 & 95 & 2.29 \\ \hline \hline \end{tabular} \({}^{\dagger}\) Average BDE between C-N in cationic state.  
\end{table}
Table 4: Average BDE between C-N in cationic state.  
Many previous studies have demonstrated that DPEPO is an unstable material [29; 30]. To investigate the effect of the poor stability of DPEPO on OLED device stability, device 5 was fabricated with an ExBL thickness of 40 nm. As shown in Figure 5c, the voltage at which the peak capacitance appears shifts from 8.1 to 12.0 V. The peak capacitance decreases from 15.6 to 14.7 nF, shifting more than that of device 1, indicating that device 5 is more degraded. This implies that as the DPEPO thickness increases, the device becomes more unstable. In addition to the unstable characteristics of DPEPO, low carrier mobility is another major cause of the high degradation of the device. DPEPO has a low electron mobility of 7.03 \(\times\) 10\({}^{-8}\) cm\({}^{2}\)/V-s, compared to other organic materials. The hole mobility for the selected HTL materials was within the range of 10\({}^{-5}\)-10\({}^{-4}\) cm\({}^{2}\)/V-s [15; 20; 24]. Due to the large gap between the electron and hole mobilities, the electrons cannot reach the center of the EML, and carrier recombination occurs at the DPEPO/EML interface. Additionally, a slight increase in capacitance within the range of 3 to 6 V can be seen in Figure 5c. We consider this to be caused by the hole being trapped in the HTL/EML interface owing to the imbalanced injection of the hole and electron [31; 32].  
Device 4 was fabricated without ExBL to determine whether the poor stability and mobility of DPEPO made the device unstable. As shown in Figure 5b, the voltage at which the peak capacitance appears, increases from 8.3 to 10.4 V. The peak capacitance drops from 21.6 to 21.0 nF. The peak capacitance change of device 4 was the smallest. Owing to the removal of the unstable DPEPO and the resultant decrease in the gap between the electron and hole mobility, the charge balance improved significantly, and the number of trapped carriers decreased. However, the lack of ExBL results in a greater change in the voltage of the peak capacitance than in device 1, because further exciton loss occurs owing to the high triplet energy of the dopant. Poor exciton confinement and exciton loss are the EML-induced inferior EL characteristics, as shown in Figure 4. Furthermore, the increase in capacitance at a low voltage, which is a similar pattern to that of device 5, is also considered to be caused by trapped excitons owing to poor exciton confinement. Therefore, a 10 nm thick DPEPO layer is necessary to improve the performance of the device.