Table 1 Summary of EL performance of all WOLEDs in this study V on a(V) CEmax b(cd/A) PEmax c(lm/W) CEdat 1,000 cd/m2(cd/A) PEeat 1,000 cd/m2(lm/W) CIE at 10 V ( x , y ) Reference device 3.52 10.7 5.5 10.6
5.2 (0.38, 0.45) Device A 3.56 16.4 8.3 16.2 8.1 (0.32, 0.45) Device B 3.76 11.0 4.4 10.9 4.2 (0.32, 0.45) Device C 3.82 8.1 3.5 8.0 3.1 (0.24, 0.35) aTurn-on voltage; bmaximum current efficiency; cmaximum power efficiency; dcurrent efficiency at 1,000 cd/m2; epower efficiency at 1,000 cd/m2. Figure 3 The schematic energy PD0325901 level diagram of WOLEDs with the portion of EMLs. (a) device A. (b) device B. (c) device C. Black circle and white circle express electron and hole, respectively. The numbers indicate the Selleckchem BMS 354825 LUMO and HOMO energies relative to vacuum (in eV). Here, LUMO and HOMO are cited from [18–20]. Figure 4 The EL spectra of all WOLEDs under various voltages. (a) Reference device, (b) device A, (c) device B, and (d) device C. Another two MQW structure WOLEDs have low efficiencies compared to device A, even lower than that of the reference device. Devices A, B, and C offer a peak luminance of 17
700, 13,200, and 8,489 cd/m2, respectively. The difference between luminances indicates the different recombination efficiencies because luminance is generally decided by the recombination degree between electrons and holes [21]. Table 1 summarizes the EL performances of all devices. Such a large difference between their EL performances could be understood from different alignments between LUMO/HOMO energy levels of EML/PBL due to the use of different PBL materials. First, let us see the schematic energy level diagrams of WOLEDs with the portion of EMLs that are shown in Figure 3. Device A with TPBi as PBL belongs to the foregoing type-I MQW structure, and LUMO/HOMO energy levels (bandgap) of each EML located within LUMO/HOMO energy levels of TPBi and
two carriers are confined in the EML, while devices B and C belong to the type-II MQW structure with Bphen and BCP as PBL, respectively. The LUMO/HOMO Etofibrate energy levels of PBL and EML are staggered, and only a single carrier is confined in the EML. For device A, there is a 0.2-eV barrier at the interface of either [LUMO]EML/[LUMO]TPBi or [HOMO]EML/[HOMO]TPBi, and such an energy level alignment makes electrons and holes distribute uniformly in the EMLs that act as potential wells under electrical excitation. All the electrons and holes could be confined in EMLs due to the presence of a suitable energy level of TPBi, which would increase a recombination possibility between the two carriers and produce more excitons in EML [22]. For device B, the potential well of holes is the EML with a 0.4-eV barrier at the [HOMO]EML/[HOMO]Bphen interface; injected holes could easily be confined within the HOMO energy level of EML.
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