Research On Inverted Top-emitting Organic Light-emitting Devices

At present, organic light-emitting devices(OLEDs) have achieve great success in industrialization. Inverted organic light-emitting devices(IOLEDs) play a crucial role to promote OLED into practice. However, it main characteristics, such as current efficiency, turn-on, are not as good as the conventional OLEDs, which is caused by its bad behavior on electron injection. Top-emitting OLEDs could achieve nearly 100% aperture ratio and are more compatible with the TFT below compared with the bottom-emitting devices. It is also the hot topic in recent years. But when it is applied to fabricate the white OLEDs, the microcavity effect existed could lead to the shift of peaks of the spectrum, increase or decline of efficiency of some part of spectrum and weak the stability of spectrum changing with the angle. White OLEDs have great potential application in large area display and lighting, nevertheless, the process of fabrication is relatively complex compared with the single color devices.The White top-emitting IOLEDs is demonstrated in this work. In the first part of this paper, the single color green top-emitting IOLED was manufactured, which is the preparation for white top-emitting IOLED. To modify the electron injection of IOLEDs, the Ag doped into Bphen interlayer was used as electron injection layer between the cathode and electron transportation layer. Then high efficient green guest Ir(ppy)3 was doped into the host CBP to fabricate the high efficient device. By comparing with the device without this electron injection layer, the current destiny of devices with this interlayer, at the same voltage, are much higher. And the highest current efficiency is 76.4 cd/A, which is 2.36 than the conventional device. The reasons could be attributed to two points. The first point could be this electron injection layer has modified the electron injection, which promotes the balance of carrier transportation. The second point is that the top-emitting structure and high reflective electrode contribute to the formation of strong microcavity effect, which could strengthen the specific peak of spectrum at special thickness of the whole device. The intent-doped and self-doped processes have been applied. The devices with the intentionally –doped process behaves slightly better than devices with the self-doped process. It proves that the self-doped process could be used in the practice without influencing the characters greatly.In the next part, the white top-emitting emitting device was fabricated. Firstly, white devices still faced the problem of electron injection, which lead to the high turn-on. To improve the behavior of electron injection, the gradient Ag doped into Bphen interlayer was applied and the doping ratio changed from to. When comparing with the device without the gradient electron injection layer, the current density at the same voltage is much higher, which could be treated as the evidence to prove that this gradient interlayer has improve the electron injection of IOLED further. Then though changing the structure of emitting layers, the location of excition formation layer have been found near the interface between the electron transportation layer and emitting layer. Continually, after the modification of the thickness and concentration of emitting layers, the white top-emitting devices have been made but its spectra changed dramatically with varied voltage, which changes from(0.489, 0.395) to(0.387, 0.377). The current efficiency is 21.6cd/A. When the red guest was changed into the yellow host, the voltage-stable spectra have been achieved. Additionally, the 50 nm NPB coppering layer has been utilized to achieve the angle-stable character. The CIE coordinate only changes(0.022,0.015), when angle changes from 0° to 60° .