| In phosphorescent organic light emitting diodes (PhOLEDs), the bipolar host materials are benefical for the balance of the hole/electron injection and transporting, which could improve the efficiencies of the devices and reduce the efficiency roll-off effectively. However, the strong interactions between the electron-donor and acceptor would reduce the triplet energy levels and the thermal stabilities, and further lead to their use in the practical applications. As we all know, the carbazole moiety possessed a high triplet energy level and a rigid structure. Using the carbazole as the electron-donor unit, and a series of bipolar host material were designed and synthesized by conjugating different electron-acceptor moiety to the cabazole. Meanwhile, the relationships between molecular structures and devices performance were studied. The bipolar host materials were synthesized through tuning the size of molecules (2nd Charptor), controlling the linkage positions (the 3rd to the 7th Chapter) and changing the articulations (the 8th and the 9th Chapter) between the carbazole units and the electron injection or transporting groups. Many new effictive methods were harvested for designing high performance bipolar host materials for PhOLEDs fabrication.1. By conjugating different generations of carbazole dendrimers to the 3,6-positions of the fluorene and linking benzene & 9-phenyl carbazole at the 9-positions of the fluorene, high triplet energy levels 3,6-fluorene derivatives were synthesized. All the host materials have the high thermal stabilities (Tg> 160℃), and the solution processable PhOLEDs using the host materials doped with the blue gust FIrpic as the emitting layers (EML) were fabricated. A maximum current efficiency (ηE,max) of 18.5 cd/A and a maximum power efficiency (ηpE,max) of 8.1 1m/W based on the CaGl as host were achieved, respectively. Morever, the devices displayed the stable spectra even at high driving voltages. The electron-injection properties of the host materials were limited due to the bulky 2nd gerneration carbazole dendrimer moiety leading to reduce the electron conductivity. All of this demonstrated that the bluky molecules influenced the conductivity and the thermal stability of the host materials, and further affected the efficiencies of the devices; 2. Three kinds of biplor host materials based on carbaozle were designed and synthesized:a. A series of bipolar carbazole host materials were designed and synthesized by conjugating different electron withdrawing groups to the carbazole. The triplet energies of the host materials accorrding to the following order:ortho>meta>para. PhOLEDs fabricated with the host materials doping with the guests such as FIrpic and Ir(ppy)3, a maximum external quantum efficiencies (ηEQE,max) of 18.7%were achieved for blue PhOLEDs using mBICP as host materials. The green PhOLEDs with phenanthroimidazole derivatives doping the Ir(ppy)3 as the EML shown aηEQE,maxas high as 21.0%. Morever, the efficiency roll-offs of all this PhOLEDs are low. Part of the device performances are among the best reported blue and green PhOLEDs based on FIrpic and Ir(ppy)3b. A series of bipolar host materials subsituted at the 3-position of the 9-phenylcabazole were designed and synthesized. The thermal stabilities of these host materials are higher than that of the 9H-carbazole host materials. A maximum external quantum efficiency (ηEQE,max) of 17.0%and 20.3%were harvested based on the mBIPhCP and mCPmPO, respectively. As same as the 9H-carbazole linked bipolar host materials, the efficiency roll-offs of the PhOLEDs are also low even at high brightness.c. A series of biploar 1,8-disubstituted and 1,3,6,8-tetrasubstituted carbazole phosphorescent host materials were synthesized through the traditional Suzuki reaction. All of these hosts show high thermal stabilities (Tg> 130℃). For the vacuum processable device based 1,8-disubstituted carbazole derivative (1,8-OXDCz), the device performance exhibits an ideal turn on voltage of about 2.5 V and a maximum power efficiency as high as 89.7 lm/W, corresponding theηEQE.max reached to 20.3%. And the solution-processable device based the 1,3,6,8-tetrasubstituted carbazle derivative (TTPhCz), a maximum current efficiency was reached to 41.0 cd/A, and the device displays excellent stabilities at different driving voltages.All of the above results suggest that controlling the linkage positions of the electron donor and acceptor of the bipolar hosts could improve the efficiencies of the PhOLEDs and reduce the efficiency roll-offs. 3. In order to improve the thermal stabilities and the bipolar transporting properties of the host materials, we changed the linkage modes between the carbazole and the electron-withdrawing group (diphenylphosphine oxide). The blue PhOLEDs fabricated with the TCTAPO as host exhibits a maximum power efficiency as high as 40.7 lm/W, corresponding the EQE reached to 17.7%. The PhOLEDs based on the bipolar star-shaped host material DmCzmPOPB, a maximum external quantum efficiency of 16.5%was also achieved for the bipolar transporting properties of the host materials. This results demonstrated that the thermal stabilities of the host materials and the efficiencies of the devices could be tunned by changing the linkage modes of the carbazole and the electron-withdrawing groups.
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