Fabrication of Color-Tunable Blue-Violet Organic Light Emitting Diodes for White Light Source

Abstract

In this study, blue-violet organic light emitting diodes (BV-OLEDs, ITO/MoO 3 (Y nm)/4,4 -bis(9-carba-zolyl)-2,2 -biphenyl (CBP) (X nm)/1,3,5-tris(N-phenylbenzimidazole-2-yl)benzene (TPBi) (100-X nm)/LiF (1 nm)/Al (100 nm)) were fabricated with the simple bilayer structure of CBP and TPBi, leading to a maximum luminance of 1280 cd/m 2 when the thicknesses of the MoO 3 , CBP, and TPBi were 4, 35 and 65 nm, respectively. The blue-shift of the electroluminescence was observed from various viewing angles, and is attributed to the micro-cavity effects. Further, it was observed that the Commission Internationale de l'Eclairage (CIE) chromaticity coordinates of the devices changed from (0.19, 0.102; at 4 V) to (0.333, 0.366; at 11 V) due to the electroplex effect between the CBP and TPBi at high voltage. Subsequently, simple white OLEDs (WOLEDs) were fabricated by combining the proposed BV-OLEDs with the down color conversion layer (CCL). The down CCL of the 5 wt% 5,6,11,12-tetraphenylnaphthacene (Rubrene)-doped 4,4 -bis(2,2-diphenylvinyl)-1,1 -biphenyl (DPVBi) had the highest maximum luminance of 1240 cd/m 2 , with measured CIE chromaticity coordinates of (0.333, 0.351) in the WOLEDs.The double-layer structure introduced by Tang et al., sparked intense interest in organic light-emitting diodes (OLEDs), and has therefore attracted much attention for application in solid-state lighting and flat-panel displays due to their low-voltage operation, wide viewing angles, high contrast, flexibility, and low cost. 1 In recent years, commercial products incorporating OLEDs include tablet computers, smart phones, and flat-panel displays, among others. However, OLEDs have certain shortcomings, such as low external quantum efficiency, insufficient lifetime, and low operational stability for display applications. 2-5 White OLEDs (WOLEDs) have several fabrication methods, as well as using high efficient phosphorescent materials and complicated device structures. Challenges in such structures, such as high cost, stability and productivity had to be overcome. Relatively, the blue-violet OLEDs (BV-OLEDs) had many merits such as conversing the emission wavelength to the visible light range, increased the fabricated methods in WOLEDs. Therefore, we used a simple BV-OLED combined with the down color conversion layer (CCL) for application in WOLEDs. Recently, many researchers have been turning to higher efficiency regions/wavelengths of visible-light, for example from green to blue devices. Be that as it may, blue-violet devices have scarcely been investigated, namely because the fabrication of such short-wavelength materials is challenging. Moreover, previous researchers have reported that blue-violet devices are not only inefficient, but also structurally complex. 6-15 Accordingly, BV-OLEDs require further attention to overcome these issues.In this paper, BV-OLEDs with simple structure were fabricated and their characteristics were investigated. In addition, the electroplex effect enables the proposed BV-OLEDs to be tunable by applying var...

Exciton-Induced Degradation of Carbazole-Based Host Materials and Its Role in the Electroluminescence Spectral Changes in Phosphorescent Organic Light Emitting Devices with Electrical Aging

“…On the other hand, the peak wavelength of the new band was at 612 and 562 nm for devices with the BmPyPb and Bphen ETLs, respectively. The fact that the wavelength of the new band spectra does not correspond to any emission from either CBP or ETL materials and varied with ETL materials suggests that the new bands may originate from some complex species involving both CBP and ETL molecules . We note that there is no clear correlation between the emission wavelength of such complex species and the lowest unoccupied molecular orbital (LUMO) level of these ETL materials.…”

“…The fact that the wavelength of the new band spectra does not correspond to any emission from either CBP or ETL materials and varied with ETL materials suggests that the new bands may originate from some complex species involving both CBP and ETL molecules. 25 We note that there is no clear correlation between the emission wavelength of such complex species and the lowest unoccupied molecular orbital (LUMO) level of these ETL materials. This suggests that factors other than the energy offset between the ETL LUMO and the CBP highest occupied molecular orbital play a role in forming this complex.…”