# Synthesis and Electrical Properties of a New Bipolar Material Using Spacer Moiety
## 3 Results and Discussion
### Electroluminescence Properties  
Non-doped devices and 10-, 20-, and 30-wt% doped OLED devices were fabricated using the newly synthesized DTPCZ as a dopant for the emitting layer. The non-doped device had the following configuration: indium tin oxide (ITO)/dipyrazino[2,3-f:2\({}^{\prime}\),3-h] quinoxaline-2,3,6,7,10,11-hexacarbonitrile (HATCN) (5 nm)/N,N\({}^{\prime}\)-bis(naphthalene-1-yl)-N,N\({}^{\prime}\)-bis (phenyl)benzidine (NPB) (40 nm)/4,4\({}^{\prime}\)A-Tris(carbazol-9-yl)triphenylamine (TCTA) (5 nm)/DTTCZ (20 nm)/1,3,5-tri(1-phenyl-1H-benzo[d]imidazole-2-yl) phenyl (TPBi) (40 nm)/LiF (1 nm)/Al (200 nm). The doped device was fabricated with the configuration of ITO/HAT-CN (5 nm)/NPB (40 nm)/TCTA (5 nm)/4,4\({}^{\prime}\)-Bis(N-carbazolyl)-1,1\({}^{\prime}\)-biphenyl (CBP): 10-, 20-, or 30-wt% DTPCZ (20 nm)/TPBi (40 nm)/LiF (1 nm)/Al (200 nm). HATCN was used as the hole-injection layer, and NPB was used as the hole transporting layer. TCTA served as the exciton blocking layer, TPBi served as the electron-transporting \begin{table}
\begin{tabular}{c c c c c c} \hline \hline  & & & **PLmax (nm)** & & \\ \cline{2-6}  & **Hexane** & **Toluene** & **Chloroform** & **THF** & **DMSO** \\  & **(1.88) a** & **(2.38) a** & **(4.81) a** & **(7.58) a** & **(46.7) a** \\ \hline DTPCZ & 397 & 421 & 451 & 457 & 522 \\ Poly DTPCZ & NA b & 466 & 526 & 524 & 559 \\ \hline \hline \end{tabular}  
* Dielectric constant of each solvent. b not available.  
\end{table} 
Table 2: PLmax wavelength of DTPCZ and poly DTPCZ in hexane, toluene, chloroform, THF, and DMSO solvent (concentration: 0.5 wt%). layer and hole-blocking layer, and LiF and Al served as the electron-injecting material and the cathode, respectively. The fabricated non-doped device had an operation voltage of 4.0 V at 10 mA/cm\({}^{2}\) and showed low efficiency; specifically, it had a current efficiency (CE) of 1.64 cd/A, a power efficiency (PE) of 1.11 lm/W, and an external quantum efficiency (EQE) of 1.63% (Figure 6). The ELmax peak was observed at 423 nm, similar to the PL peak of the DTTCZ film. However, the CIE coordinates were measured as (0.18, 0.17); emission over the 500-600-nm range was observed in the EL spectrum, which was not visible in the film PL. These results indicate a problem whereby the recombination zone within the device occurs at the interface between DTTCZ and the neighboring TCTA. Upon examining the energy band diagram of the device, it is observed that there is almost no energy barrier for electrons to move from TPBi to EML via the LUMO. However, a significant energy barrier of 0.23 eV exists for holes to move from TCTA to EML via the HOMO (Figure 6f). Therefore, by varying the thickness and type of the hole carrier transport layer, we aim to shift the position of the recombination zone in the center of the emitting layer. We will observe the resulting performance changes and report them separately in the future. In order to match the carrier balance, a doped device was fabricated. Figure 7a-d shows the determined current density (J)-voltage (V)-luminance (L) curve, the CE, the EL spectra according to doping concentration, and the band diagram of the doped devices, respectively. The EL performances of the devices are summarized in Table 3. The operating voltage of the three fabricated devices was consistently low at 3.9 V. Among them, when DPTCZ was doped at a concentration of 20 wt%, it exhibited the highest EL performance with a CE of 1.17 cd/A, an EQE of 2.32%, and a PE of 0.68 lm/W (Figure S4). When doped at 10 wt%, the low dopant concentration resulted in insufficient energy transfer from the host to the dopant, leading to lower efficiency. According to the increase in doping concentration from 20 to 30 wt%, all EL efficiencies showed a decrease. This result occurred because the luminescence efficiency was reduced by the aggregation quenching effect at high doping concentrations [29,30].