## 3 Results and Discussions
ITO and Al were used for the anode and cathode, respectively, HATCN was used for the hole-injecting layer (HIL), Liq was used for the electron-injecting layer (EIL), TAPC was used as the hole-transporting layer (HTL), and TmPyPB was employed as the electron transport layer (ETL). Figure 3: Frontier molecular orbital distributions of FLU-TPA/PYR and FLU-TPA/TRZ. The current density-voltage, current density-power, and current efficiencies are depicted in Figure 5 and summarized in Table 2. The turn-on voltage of the FLU-TPA/PYR and FLU-TPA/TRZ-based devices was 5 V. The current efficiencies of the FLU-TPA/PYR and FLU-TPA/TRZ-based devices were 21.70 and 18.72 cd/A, respectively. Consequently, the power efficiencies was observed as 13.64 and 11.76 lm/W for FLU-TPA/PYR and FLU-TPA/TRZ, respectively. The external quantum efficiency (7.75%) of the FLU-TPA/PYR-based device was higher than that of the triazine-based FLU-TPA/TRZ device (6.44%). Though both devices exhibited a similar turn-on voltage, the device efficiencies of the pyrimidine-based FLU-TPA/PYR were considerably higher than those of FLU-TPA/TRZ. Both the devices showed lower efficiency roll-off, which is one of the important factors for longer operation devices. The electro luminescent (EL) spectra of both devices are depicted in Figure 6. The maximum emission of both yellow devices was similar at 562 nm, which substantiated the yellow emission from the yellow dopant. Figure 4: Device structure of yellow phosphorescent organic light-emitting diodes (OLEDs). Figure 5: Current density–voltage, luminescence–voltage, and external quantum efficiencies of yellow phosphorescent OLEDs. Moreover, we fabricated non-doped fluorescence devices to understand the performances of our molecules as emitters (Figure 7). The non-doped device structure was as follows: indium-tin oxide (ITO) (150 nm)/1,4,5,8,9,11-hexaazatriphenylenehexacarbonitrile (HATCN) (7 nm)/4_A_'-cyclohexylidenebis[N,_N_-bis(4-methylphenyl)benzeneamine](TAPC) (43 nm)/FLU-TPA/PYR or FLU-TPA/TRZ (20 nm)/1,3,5-tri(m-pyridin-3-ylphenyl)benzene (TmPyPB) (35 nm)/8-quinolinolato lithium (Liq) (1.5 nm)/aluminum (Al) (100 nm). The non-doped device performances of the triazine-based FLU-TPA/TRZ was much better than that of the pyridine-based FLU-TPA/PYR. The current, power and external quantum efficiencies of the FLU-TPA/TRZ-based device were 10.30 cd/A, 6.47 lm/W and 3.57%, respectively (Figure 8 and Table 2). The maximum EL emission was observed at 549 nm, which was related to the CIE color coordinates (x, y) of 0.41, 0.55 with the emission of green color (Figure 9). \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Device Characteristics** & \begin{tabular}{c} **FLU-TPA/PYR** \\ **(Yellow Device)** \\ \end{tabular} & \begin{tabular}{c} **FLU-TPA/TRZ** \\ **(Yellow Device)** \\ \end{tabular} & \
\begin{tabular}{c} **FLU-TPA/PYR** \\ **(Non-Doped)** \\ \end{tabular} &
\begin{tabular}{c} **FLU-TPA/TRZ** \\ **(Non-Doped)** \\ \end{tabular} \\ \hline Turn-On Voltage (V) & 5.0 & 5.0 & 6.5 & 5.0 \\ \hline Current Efficiency (cd/A) & 21.70 & 18.72 & 1.27 & 10.30 \\ \hline Power Efficiency (lm/W) & 13.64 & 11.76 & 0.61 & 6.47 \\ \hline EQE (\%) & 7.75 & 6.44 & 0.53 & 3.57 \\ CIE (x,y) & 0.49,0.50 & 0.49,0.50 & 0.41,0.51 & 0.41,0.55 \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Device characteristics of FLU-TPA/PYR and FLU-TPA/TRZ-based phosphorescent and non-doped fluorescent devices. Figure 6: Electroluminescent spectra of yellow phosphorescent OLEDs. Figure 7: Device structure of non-doped fluorescent OLEDs.