# Quantum Dot-Based White Organic Light-Emitting Diodes Excited by a Blue OLED
## 3 Results and Discussion
### White OLED (WoLED) Devices  
White OLEDs have numerous advantages; they differ from classical or even inorganic light sources in terms of their properties. For instance, they emit diffused light that can illuminate an area based on their ultrathin thickness [30]. The focus of this study was on developing a white OLED device using a QD PR. The QD PR was employed as the color conversion layer (CCL), which was excited by the blue OLED (BOLED). To fabricate the QD-based white light, a green QD and red QD color conversion film were sequentially bonded onto the backside of the blue OLED substrate. To improve the white light chromaticity, two different light excitation architectures BOLED\(\rightarrow\)GQD\(\rightarrow\)RQD and BOLED\(\rightarrow\)RQD\(\rightarrow\)GQD were tested, as shown in Figure 14a,b, respectively. The first case was BOLED\(\rightarrow\)GQD\(\rightarrow\)RQD (blue light exciting GQD first) and the second case is BOLED\(\rightarrow\)RQD\(\rightarrow\)GQD (blue light exciting RQD first). Figure 15 illustrates the schematic device structure and the emitting photo of a white OLED. Figure 13: UV laser-excited PL spectrum of the following: (**a**) MADN host luminescent material; (**b**) v-DABNA dopant material; (**c**) EL spectrum of blue OLED Z-1; (**d**) CIE coordinates of blue OLED. However, luminance is related to lifetime, permanent burn-in, and power consumption. A high luminance requires a high luminous efficiency to avoid limiting lifetime and power consumption. Higher luminous efficiency means that a smaller current density is required for the same luminance. The WOLED has four subpixel structures. The white subpixel has high current efficiency, while red, green, and blue subpixels have low current efficiency. The filtering of primary colors results in almost 80 percent of light being lost while light made by the stack of blue and yellow OLEDs passes through without being filtered [31,32]. From the study, to improve the optical efficiency of a white OLED display with a high color gamut, a red or green patterned QD film was used as a color converting component and an LPF was used as a light-recycling component. The QD film absorbed the unnecessary light from the white OLED, which was then cut by the CF to downconvert to the essential light, while the LPF reused the unnecessary light selectively to enhance the amount of light absorbed by the QD. In addition, to reduce reflection loss caused by an air gap, a refractive index matching liquid was utilized between the components [23,33]. In this study, the RQD or GQD was spin coated on a separate glass substrate different from the BOLED. The white light shown in Figure 15 was achieved by placing a nanoimparted RQD or GQD PR on the back side of the BOLED glass substrate shown in Figure 14. Between the BOLED glass and the QD substrate, there was an air gap and reflection loss. The QD conversion efficiency excited by blue the OLED might be not high  
Figure 14: Light excitation architectures of (**a**) BOLED\(\rightarrow\)GQD\(\rightarrow\)RQD and (**b**) BOLED\(\rightarrow\)RQD\(\rightarrow\)GQD. Figure 15: The schematic device structure and the emitting photo of a white OLED. enough either. As a result, at an 8 V driving voltage, the BOLED light debilitated after exciting the (RQD) or (GQD). So in that case, the performance of white light (blue OLED mixed with RQD and GQD color conversion) was reduced when compared to a single-emitting layer blue OLED. Figure 16a,b illustrates the comparison of the luminance-voltage characteristic curve and electro luminance (EL) spectra at 8 V, respectively, for both white light excitation architectures. Based on both results, it was concluded that there was almost no difference in the luminance-voltage characteristics for both architectures.