## 3 Results and Discussions
### Electrochemical and Photophysical Properties  
The HOMO energy level of TPA-1A was \(-\)5.78 eV, and showed little lower energy value with the NPB material (\(-\)5.5 eV). The HOMO energy level is lying between the anode (\(-\)4.80 eV) and the emission layer (\(-\)5.90 eV). The suitable HOMO energy level helps to transport holes effectively from the anode (Table 1). The HOMO energy difference between TPA-1A and the emission layer is 0.1 eV, while the energy difference of NPB is 0.4 eV. Lowering the energy difference can enhance the efficiencies through providing an effective hole-hopping path. The LUMO energy was \(-\)2.45 eV, which was calculated by adding the band gap to the HOMO energy. The LUMO energy level is higher than that of the electron transporting layer (\(-\)2.80 eV), which is beneficial for acting as an electron blocking layer. The frontier molecular orbital (FMO) energy levels were matched with the adjacent layers of the fabricated OLED device to ensure the effective charge transportation. Figure 3 describes the UV-VIS absorption and photoluminescence (PL) spectra of TPA-1A, which showed a maximum absorption peak at 319 nm. The above absorption is attributed to the \(\pi\)-\(\pi^{\circ}\) transition of the aromatic rings. The band gap value of TPA-1A was 3.33 eV, which was calculated from the on-set absorption at 372 nm. We did not notice any prominent absorption in the visible region. The PL emission at room temperature revealed a maximum at 425 nm. The triplet energy was 2.58 eV, which was measured from the low temperature PL emission (77 K) at 481 nm and which is lower than that of spiro-based molecules. Commonly, spirobiluorene without other molecular attachment has a higher triplet energy of 2.87 eV, when another molecule attaches via the para position leading to lowering the triplet energy around 2.52 eV compared to the ortho position (2.77 eV). In the case of the ortho position, pi electron delocalization is restricted, which has a higher triplet energy. Moreover, when a molecule attached through the para position makes a long conjugation length by delocalizing more electrons, it affects the triplet energy. In our molecule, arcidine attached with triphenyl amine through the para position which extended the conjugation length while decreasing the triplet energy [40]. [PERSON] et al. reported on a green phosphorescence OLED with the higher triplet energy material \begin{table}
\begin{tabular}{c c c c c c c c c} \hline \multirow{2}{*}{**HTM**} & \(\mathbf{T_{g}}\)1  & \(\mathbf{T_{d}}\)1  & **UV-Vis1 ** & **PL max2 ** & **HOMO ** & **LUMO ** & **Eg ** & **Et ** \\  & **(\({}^{\circ}\)C)** & **(\({}^{\circ}\)C)** & **(nm)** & **(nm)** & **(eV)** & **(eV)** & **(eV)** & **(eV)** \\ \hline TPA-1A & 176 & 474 & 319 & 425 & \(-\)5.78 & \(-\)2.45 & 3.33 & 2.58 \\ \hline \end{tabular}
\end{table} 
Table 1: Thermal, photophysical, and electrochemical properties of 4-(9,9-Diphenyllacridin-10(9H)y!)-\(N\)-(4-(9,9-diphenyllacridin-10(9H)-y! )phenyl)-\(N\)-phenylaniline (TPA-1A). Figure 2: Scanning electron microscope (SEM) images of the TPA-1A-based device cross-section. The current density-voltage-luminescence (J-V-L) characteristics are depicted in Figure 5. The TPA-1A-based device showed a higher turn-on voltage of 4.4 V when compared to the NPB-based device (3.4 V). The green PhOLED device showed similar power efficiencies of 30.85 and 31.62 \(\mathrm{Im}\)/W for NPB and TPA-1A, respectively, at turn-on voltage. The power efficiencies at 1200 \(\mathrm{cd}\)/\(\mathrm{m}^{2}\) for TPA-1A was 27.56 \(\mathrm{Im}\)/W, which is much higher than that of the reference NPB-based device (19.88 \(\mathrm{Im}\)/W). The above results explained that the power efficiency of the TPA-1A-based device not only depends on the turn-on voltage, but also the balance of electrons and holes in the device. The current efficiency of the TPA-1A-based green PhOLED at the turn-on voltage was 44.29 \(\mathrm{cd}\)/A, but the NPB-based reference device showed lower efficiency of 33.38 \(\mathrm{cd}\)/A.