# Solution-Processed Organic Light-Emitting Diodes Using a Lamination Method
## 3 Results
### Single-Substrate OLED Devices  
To further clarify the role of TPBi and PBD, TPBi was employed for its surface potential improvement through spontaneous orientation polarization (SOP), which facilitated better electron mobility by reducing the energy barrier at the interface. This characteristic, combined with its low HOMO energy level of 6.2 eV, significantly enhanced hole trapping within the emitting layer, thereby improving recombination efficiency and luminescence. On the other hand, PBD, with its biphenyl and oxadiazole structure, exhibited exceptional electron-transport and hole-blocking capabilities. The \(\pi\)-conjugated system allowed efficient delocalization of electrons, forming robust electron-transport pathways. Moreover, the low HOMO energy level of 6.0 eV effectively blocked holes, ensuring the optimized recombination in the emitting layer. These mechanisms collectively contributed to an increase in both current density and luminance, as demonstrated in Figures 5-7.  
After verifying the electron-transport ability of TPBi and PBD in single-carrier device experiments, single-substrate OLEDs were fabricated to evaluate the effect of the host-guest system on the luminescence performance. Figure 5 shows the current density and luminance graphs measured to find the optimal concentrations of TPBi and PBD, and the maximum values are summarized in Table 1.  
Figure 5: Current density and luminance according to the concentration of (**a,b**) TPBi and (**c,d**) PBD.  
was the highest at 6097 cd/m\({}^{2}\) at a concentration of 50 wt%. In the case of PBD, as the concentration increased, the current density increased to a maximum of 2489 mA/cm\({}^{2}\), and the luminance was the highest at 9965 cd/m\({}^{2}\) at a concentration of 50 wt%. It was confirmed that mixing TPBi and PBD with PDY-132 at a 1:1 ratio was the optimal condition; overall, the performance was higher than TPBi when mixed with PBD. Figure 6 shows the current density and luminance of a single-substrate OLED composed of TPBi and PBD as independent layers or applying a host-guest system based on the optimal concentration, and the maximum values are summarized in Table 2. The OLED composed of TPBi independently had a maximum current density of 1405 mA/cm\({}^{2}\) and a maximum luminance of 7103 cd/m\({}^{2}\). However, when mixed with PDY-132, both the current density and luminance decreased with the current density at 1088 mA/cm\({}^{2}\) and the luminance at 6505 cd/m\({}^{2}\). This means that the recombination efficiency decreases when mixed with TPBi. On the other hand, the OLED composed of PBD as an independent layer showed a maximum current density of 1607 mA/cm\({}^{2}\) and a maximum brightness of 8987 cd/m\({}^{2}\), and the OLED mixed with PDY-132 showed a maximum current density of 1725 mA/cm\({}^{2}\) and a maximum brightness of 9871 cd/m\({}^{2}\). When PBD was mixed with PDY-132, both the current density and brightness slightly increased, indicating that the recombination efficiency was improved. PDY-132, a PPV-based luminescent material with excellent hole transport properties, is interpreted to enhance recombination efficiency by incorporating PBD, which possesses a 1,3_A_-oxadiazole structure known for its high electron affinity and electron-transport capabilities [16].