Study On The Blue Phosphorescent Organic Light Emitting Diode With Double Emitting Layers And The Electroluminescent Mechanisms

The blue phosphorescent organic light emitting devices with high performance are expected to be the key technology for mature and stable organic light emitting devices. However, compared to the red and green devices, the level of the blue devices is lower. As the triplet energy level of blue exciton is higher, larger host bandgap and higher host triplet energy level are required. Such, a large barrier easily forms between the light emitting layer and the carrier transport layer and hinders the improvement of the devices. Here, we utilize the double emitting layers structure and choose UGH2 with the triplet energy level of 3.18 e V as host and FIrpic as blue phosphorescent material to improve the performance of blue phosphorescent organic light emitting devices. At the same time, the electroluminescence mechanisms of the devices was studied. The work is as follows:1. We fabricate the organic light emitting device based on double emitting layers structure with m CP and UGH2 as host. Firstly, the optimal concentration is got that FIrpic is doped into UGH2 as 20 % by research. And the main electroluminescence mechanisms of this device is proved to be the direct charge-recombination mechanism of FIrpic. Then the device with structure of ITO / m-MTDATA: Mo Ox( 15 wt%, 10 nm) / m-MTDATA(30 nm) / TAPC(10 nm) / m CP: FIrpic(10 wt%, 10 nm) / UGH2: FIrpic(20 wt%, 10 nm) / Bphen(40 nm) / Li F(1 nm) / Al was fabricated. In this device, the double emitting layers structure improves the accumulation of hole before injecting into the UGH2, in turn, reduces the quenching of exciton. It is more important that it helps the device take advantage of the exciton repeatedly, eventually improves the device performance. Next, the exciton concentration at the different sites is also explored and the result is at the interface between two light emitting layers, the exciton concentration is the highest. The device with double emitting layers structure gets the maximum current efficiency, power efficiency and external quantum efficiency is 21.13 cd/A, 14.97 lm/W and 10.56 %. The maximum current efficiency increase by 46.03 % and 20.96 % respectively, compared to the device based on single emitting layer structure with m CP and UGH2 as host.2. We fabricate the organic light emitting devices based on double emitting layers structure with TAPC and UGH2 as host. The structure of the device is as follows, ITO / m-MTDATA: Mo Ox( 15 wt%, 10 nm) / m-MTDATA(30 nm) / TAPC(10 nm) / TAPC: FIrpic(10 wt%, 10 nm) / UGH2: FIrpic(20 wt%, 10 nm) / Bphen(40 nm) / Li F(1 nm) / Al. In this device, the holes transport layer and the host of a light-emitting layer are made of the same material. Such, the structure is simpler than the device based on double emitting layers structure with m CP and UGH2 as host. Then the concentration of doping FIrpic into TAPC is increased from 10 % to 20 %, but the maximum current efficiency of the two devices is similar to the device based on single emitting layer structure with UGH2 as host. This phenomenon attributes to the too high holes mobility of TAPC. In addition, in the device with the concentration of 20 % doping FIrpic into TAPC, the dominant electroluminescence mechanisms turns out to be the direct charge-recombination mechanism by the single electronics device.