In this study, the WOLED which consists of the blue emission layer and the yellow emission layer was fabricated as the energy band diagrams of the multilayer of WOLED shown in Fig 4.5.
The Rubtene layer was inserted in the light-emitting layer of DPVBi to form the structure of DPVBi (10nm EML1)/ Rubrene (0.2nm)/DPVBi (30nm EML2) for the device A. The yellow emission of the device A was caused by the Rubrene layer. Figure 4.6 (a) shows the EL spectra of the device A at the applied voltage of 3~7V. The peak wavelengths of the DPVBi layer and the Rubrene layer were 436nm and 556nm, respectively. The intensity of blue emission was higher than that of the yellow emission, and the location of EL spectra for the blue and the yellow emission did not shift as the voltage increased.
In addition, the CIE coordinates of the device A at the applied voltage of 3~7V were shown in Fig 4.6 (b). It was found that the CIE coordinates for the device A changed from (0.269, 0.299) at
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3 V to (0.249, 0.259) at 7 V. By comparing for the CIE coordinates (0.330, 0.330) of standard white light, the error value of the CIE coordinates were about (- 0.081, - 0.071) at the 7V, i. e., the shift of the CIE coordinates is about (-7.4%, -13.4%) during the applied voltage of 3~7V.
That is to say, the CIE coordinates of the device A were unstable. As for the EL phenomenon in the applied voltage of 3~5V, some of the holes which were injected from the anode via Rubrene layer into the EML2 can be directly trapped by the Rubtene layer, which may be due to the effective hole trapping of rubrene molecules. However, the Rubtene layer has excellent charge carriers trapping property, but it is unable to trap lots of the injected holes. On the other hand, the electron mobility in BPhen: Cs2Co3 of 3.9×10-4 cm2/Vs was less than the hole mobility of 5.5×10-4 cm2/Vs in NPB, so lots of electrons from electrode inject just into EML2, i. e., the electrons recombine with holes in the EML2 close cathode. Therefore, the electrons were difficult to be directly trapped by the Rubtene layer at 3~5V. As a result, the great majority electrons and holes would meet in EML2. Similarly, for the voltage of 6~7V, more charge carriers were injected into Rubrene layer and EML2, so the amount of recombination for electrons and holes were also relatively increased in Rubrene layer and EML2, resulting in an enhancement in the intensity of peaks, as shown in Fig. 4.6 (a). However, the Rubtene layer in the device A cannot trap enough electrons and holes to generate exciton. Therefore, optimum complementary color of the blue and the yellow emission intensity was not achieved.
According to the results obtained above, the best recombination zone was in the EML2 of the
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device A. For the device B, the Rubrene layer inserted in and closed to the cathode to form the structure of DPVBi (34nm EML1)/Rubrene (0.2nm)/DPVBi (6nm EML2) was fabricated as the energy band diagrams of the multilayer of WOLED shown in Fig 4.7. However, by comparing 4.6 (a) and 4.8 (a), it is found that the yellow emission intensity of the device B was stronger than that of device A because of its position of the Rubrene layer being changed. Although the Rubrene layer was inserted into the best recombination zone and has excellent charge carrier trapping effect, the blue emission and the yellow emission did not reach the best complementary color.
As mentioned above, the optimal location is in EML2 of device A. Thus, the Rubrene layer was inserted into better recombination zone near the EML2/BPhen: Cs2Co3 interface of device B.
The difference between LUMO energy of Rubrene (-3.2 eV) and that of DPVBi (-2.8 eV) is 0.4 eV. Similarly, the difference between HOMO value of Rubrene (-5.4 eV) and that of DPVBi (-5.9 eV) is 0.5 eV [81, 82]. It is expected the electrons and holes can be trapped in the Rubrene layer as well as accumulated in the Rubrene/EML2 interface, leading to an enhancement in the intensity of yellow emission. Figure 4.8 shows the EL spectra and CIE coordinates of the device B. With the applied voltage increasing, most of the electrons and holes can be directly trapped and recombined to form exciton in the Rubrene layer. Moreover, the relative intensity of yellow is higher than that of the blue emission, i. e., the exciton number of Rubrene layer is more than that of DPVBi layer. In Fig. 4.8 (a), the intensity of both yellow emission and blue emission
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become large, but the ratio of the enhancement in yellow emission is larger than that of blue emission. Thus, the CIE coordinate gradually shifts toward an orientation of white color. For example, when the applied voltage is 3V and 4V, the location of CIE coordinates shows at (0.462, 0.481) and (0.425, 0.437), respectively. By comparing for the CIE coordinates of standard white light, the error value of the CIE coordinates were gradually reduced. It is confirmed that the better recombination zone of device B is in the Rubrene /EML2 interface.
With the applied voltage increasing to 5V, the recombination zone of electrons and holes gradually shifts toward the Rubrene /EML2 interface; the corresponding CIE coordinates is (0.391, 0.405). We thus conjecture that few excitons are generated in the EML2, resulting in an enhancement in the blue emission. On the other hand, more electrons and holes were trapped in the Rubrene, i. e., more excitons are generated in the Rubrene, leading to an enhanced yellow emission. At the voltage of 6~7V, CIE coordinates shift again from (0.368, 0.385) to (0.348, 0.365), and gradually shifts toward the CIE coordinates of standard white light. When a high voltage is applied, the concentration of electrons and holes increase which significantly influences the zone of the exciton generation. The movement of the electrons and holes are similar toward opposite electrode. Thus, the zone of the exciton generation in device B becomes broad as the EML1/ Rubrene and Rubrene /EML2 interfaces. This implies that the location of Rubrene not only improve color shift of CIE coordinates but also enhance carries recombination rate. By comparing for the CIE coordinates of standard white light, the error value of the CIE
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coordinates were about (0.018, 0.035) at the 7V, i. e., the shift of the CIE coordinates is about (-24.7%, -24.1%) during the applied voltage of 3~7V. With the results obtained above, the CIE coordinates of device B is near the CIE coordinates of standard white light by the ultra-thin Rubrene layer inserted into EML2, but the CIE coordinates of device B is still unstable during the applied voltage of 3~7V.