Abstract
In this thesis, we have developed and studied the deep-red phosphorescence organic light-emitting devices (PhOLEDs). Seven new deep-red phosphor and fifteen novel host materials were synthesized, and these materials were utilized in deep-red electroluminescent (EL) devices. These heteroleptic cyclometalated iridium(III) complexes show the emission maxima in the range 629-644 nm with high luminescence quantum yields of 28-89%. According to the DFT calculations and the measurement of the photophysical properties for these iridium phosphor, the ancillary ligand containing carboxylate group increases the MLCT character of the emission leading to high radiative quantum yields. In particular, the complex (tmpq)2Ir(tca) exhibits the emission maximum of 629 nm with highest emitting quantum yield of 89%. In the studies of the host materials, we have synthesized nine hosts using the 6H-indolo[2,3-b]quinoxaline group as a core structure incorporating the electron-donating or withdrawing functional groups. These nine hosts show the fluorescence emission maxima in the range 463-522 nm in room temperature and the phosphorescence emission maxima in the range 543-551 nm in 77K. In additional, these hosts exhibit the identical LUMO level of 2.9 eV, but different in the HOMO level between 5.3-6.0 eV according to the attaching functional groups. The devices using the host containing 6H-indolo[2,3-b]quinoxaline group and (piq)2Ir(acac) as the emitter shows the extraordinary maximum EQE in the range 18.3-22.9% with CIE coordinates of (0.68, 0.32). The exceptional performance could be attributed to the emission bands of hosts overlap very well with the MLCT absorption bands of (piq)2Ir(acac), providing an efficient route of energy transfer from the host to the dopant. The molecular weight of these host materials were further extended in the range 664~752 g/mol to increase the thermal stabilities, and the six new generation host materials containing 6H-indolo[2,3-b]quinoxaline or 11H-indeno[1,2-b]quinoxaline group were designed and synthesized. The new generation hosts exhibit high decomposition temperature (Td) of 428~502 ℃ and glass transition temperature (Tg) of 146~205 ℃. The best device using BIQS as a host and (tmq)2Ir(acac) as a emitter shows an maximum EQE of 25.9%, an maximum CE of 37.3%, an maximum PE of 32.9 lm W-1 with CIE coordinates of (0.67, 0.33). For real application, we have studied the operational lifetime (T50) of the devices. Preliminary results show that the operational lifetime of the device using BIQS as a host and Ir(piq)3 as a emitter is estimated to be 17553 h at an initial luminance of 500 cd m-2, and this device can also exhibit the excellent maximum EQE and CE of 17.1% and 22.0 cd A-1, respectively. Parts of the research were published in the journal “Advanced Materials” in 2011