Synthesis And Properties Of Bipolar Transport Host Materials For Efficient Phosphorescent OLEDs

Organic light-emitting diodes (OLEDs) have aroused broad interests in both academic and industrial field because of their great potential application in flat panel displays and lighting sources. And the electrophosphorescent devices which can achieve 100% of internal quantum efficiency theoretically have attracted increasing attention in OLEDs. However, electrophosphorescene based on the phosphorescent heavy metal complexes would suffer from triplet-triplet annihilation and concentration quenching because of the long life time of phosphorescent triplet emitters. Thus, to address this issue, the triplet emitters are normally doped into suitable host materials. In this thesis, we have designed and synthesized a series of novel bipolar transport host materials with ortho-, meta- and para- linkage between the hole-transport and electron-transport units, the structure-property relationships including their thermal, photophysical, electrochemical and electroluminescent properties have been systematically investigated.The main contents and results are described as follows:Recent progress of host materials in phosphorescent OLEDs is reviewed in Chapter 1. Firstly, we introduced the basic concept of OLED, including the development of OLEDs, the emission mechanism, device structures and different functional materials used in OLEDs. And then three approaches to achieve electrophosphorescence are illustrated. Finally, host materials classified by their different charge transporting properties are reviewed. The new approaches and achievements of developing highly efficient organic light-emitting host materials with novel structures are focused. The design strategies and main contents of this thesis are also outlined.In Chapter 2, a series of bipolar transport host materials, 2,5-bis(2-(9H-carbazol-9-yl)phenyl)-1,3,4-oxadiazole (o-CzOXD, 1), 2,5-bis(4-(9H-carbazol-9-yl)phenyl)-1,3,4-oxadiazole (p-CzOXD, 2), 2,5-bis(3-(9H-carbazol-9-yl)phenyl)-1,3,4-oxadiazole (m-CzOXD, 3) and 2-(2-(9H-carbazol-9-yl)phenyl)-5-(4-(9H-carbazol-9-yl)phenyl)-1,3,4-oxadiazole (op-CzOXD,4), are facilely synthesized through simple aromatic nucleophilic substitution reactions. The incorporation of oxadiazole moiety greatly improves their morphological stability, with Td and Tg in the range of 428-464℃and 97℃-133℃, respectively. The ortho- and meta-position of 2,5-diphenyl-1,3,4-oxadiazole linked hybrids (1 and 3) show less intramolecular charge transfer and higher triplet energy as compared to their para-position linked analogue (2). The four compounds exhibit similar LUMO levels (2.55-2.59 eV) to other oxadiazole derivatives; whereas the HOMO levels vary in a range from 5.55 eV to 5.69 eV, depending on the linkage modes. DFT calculation results indicate that 1,3, and 4 have almost complete separation of HOMO and LUMO at hole- and electron-transporting moieties, while 2 exhibits only partial separation of the HOMO and LUMO possibly due to the intramolecular charge transfer. Phosphorescent organic light-emitting devices fabricated by using 1-4 as the hosts and the green emitter Ir(ppy)3 or (ppy)2Ir(acac) as the guest exhibit good to excellent performance. Devices hosted by o-CzOXD (1) achieve the maximum current efficiencies (ηc) as high as 77.9 cd/A for Ir(ppy)3 and 64.2 cd/A for (ppy)2Ir(acac), respectively. The excellent device performance may be attributed to the well matched energy levels between the host and hole transport layer, the high triplet energy of the host and complete spatial separation of HOMO and LUMO energy levels.In Chapter 3, a new host tBu-o-CzOXD is facilely synthesized through simple aromatic nucleophilic substitution reaction between 3,6-di-tert-butyl-9H-carbazole and 2,5-bis(2-fluorophenyl)-1,3,4-oxadiazole. Its thermal, electrochemical, electronic absorption and photoluminescent properties are fully investigated. A high glass transition temperature (Tg) of 149℃is observed for tBu-o-CzOXD due to the introduction of bulky tert-butyl moiety, significantly higher than 97℃of o-CzOXD without tert-butyl substituent. Moreover, encapsulation of tert-butyl on the 3- and 6-positions of carbazole greatly enhances the electrochemical stability as compared to o-CzOXD. Green phosphorescent OLEDs hosted by tBu-o-CzOXD show a maximum luminance of 48293 cd/m2 at 17.1 V, a maximum current efficiency of 38.4 cd/A and a maximum power efficiency of 34.7 lm/W. Furthermore, the devices exhibit slow current efficiency roll-off. The device merits, together with the excellent morphological and electrochemical stability, make the new compound ideal host material for phosphorescent emitters.In Chapter 4, a bipolar transport compound, 2-5-bis(4-(9-2-ethylhexyl)-9H-carbazol-3-yl)- phenyl)-1,3,4-oxadiazole (CzOXD), incorporating both electron- and hole-transport functionalities, was synthesized, and fully characterized by 1H NMR, 13C NMR, elemental analysis and mass spectrometry. Its thermal, electrochemical, electronic absorption and photoluminescent properties were studied. The atomic force microscopy images indicate that smooth and homogeneous film can be obtained by spin-coating from the chloroform solution of CzOXD/Iridium complexes blends. Highly efficient small-molecule-based organic light emitting devices by wet process were fabricated. With the device structure of ITO/PEDOT:PSS/Ir complex:CzOXD/BCP/Alq3/LiF/Al, a maximum luminance of 15232 cd/m2 and current efficiency of 20 cd/A for yellow-emitting OLED, and 4896 cd/m2,4.6 cd/A for red-emitting OLED were achieved under ambient conditions.In Chapter 5, a series of triphenylamine/oxadiazole hybrids, p-TPA-p-OXD (1), p-TPA-m-OXD (2),p-TPA-o-OXD (3), o-TPA-p-OXD (4), o-TPA-m-OXD (5) and m-TPA-o-OXD (6), were designed, synthesized and characterized as bipolar transport host materials for phosphorescent organic light-emitting diodes (OLEDs). The ortho and meta-TPA linked hybrids (4,5 and 6) show less intramolecular charge transfer, blue-shifted emission, wider energy gap, and higher triplet energy as compared to their para-TPA linked analogues. These triphenylamine/oxadiazole derivatives show glass transition temperature arranging from 84 to 116℃, which are significantly higher than CBP host. Their HOMO levels are all at around 5.3 eV, which match well with the most widely used hole-transport NPB layer. Their LUMO levels varied according to different linkage mode. PhOLEDs fabricated by using these hybrids as the hosts and deep red emitter (piq)2Ir(acac) as the guest exhibit much higher EL performances with maximum external quantum efficiencies of 9.8-21.6% and lower turn-on voltages (2.7-3.1 V) compared with the reference device with common CBP as host (4.3%,5.3 V). The EQE of 21.6% is achieved by using o-TPA-m-OXD as host in deep-red electrophosphorescence. By manipulating the charge balance and confine the triplet excitons, a maximum EQE of 23.7% and power efficiency of 105 lm/W have been achieved in green PhOLED usting m-TPA-o-OXD (6):(ppy)2Ir(acac) as emitters. m-TPA-o-OXD is also applicable for other phosphorescent emitters, such as green-emissive Ir(ppy)3 and yellow-emissive. A yellow electrophosphorescent device based on m-TPA-o-OXD (6):(fbi)2Ir(acac) withηEQE, max of 20.6%,ηc, max of 62.1 cd/A, andηp, max of 61.7 lm/W, has been fabricated. To our knowledge, these are the highest efficiency ever reported for deep red (piq)2Ir(acac), green (ppy)2Ir(acac) and yellow PhOLEDs. This work demonstrates that tradeoffs among bipolar property, triplet energy, energy gap and energy level can be realized through judicious molecular design for a host in phosphorescent OLEDs.In Chapter 6, a series of 9,9’-spirobifluorene/oxadiazole hybrids with various linkages between two components, namely SBF-P-OXD (1), SBF-m-OXD (2) and SBF-o-OXD (3), are designed and synthesized through Suzuki cross-coupling reaction. The incorporation of rigid and bulky spirobifluorene moiety greatly improves their thermal and morphological stability, with Td (decomposition temperature) and Tg (glass transition temperature) in the ranges of 401-480℃and 136-210℃, respectively.2 and 3 with meta- and ortho-linkage display higher triplet energy, blue shifted absorption and emission than their para-linked analogue 1 due to the decreasingπ-conjugation between the two components. Their HOMO and LUMO energy levels depend on the linkage modes within the range of 5.57-5.64 eV and 2.33-2.49 eV, respectively. Multilayer deep red electrophosphorescent devices with 1-3 as hosts are fabricated, and their EL efficiencies follow the order of 3 (o)> 2 (m) >1 (p), which correlates with their triplet energy and the separation of HOMO and LUMO distributions at molecular orbitals. The maximum external quantum efficiencies of 11.7% for green and 9.8% for deep red phosphorescent OLEDs are achieved by using 2 and 3 as host materials, respectively.In Chapter 7, a series of new 1,2,4-triazole-cored triphenylamine derivatives with various linkages between triazole and triphenylamine (TPA) moieties were designed and synthesized through Suzuki cross-coupling reaction. The incorporation of rigid triazole moiety greatly improves their thermal and morphology stability, with Td and Tg in the range of 480-531℃and 106-155℃, respectively. The ortho-TPA-linked 7 and 8 show much less intramolecular charge transfer and blue shifted emission than their para-TPA linked analogues 1 and 3, respectively. The meta- and ortho-structured compounds display higher triplet energy and better electrophophorescent performances than their para-structured congeners. The significant improvement of electrophosphorescent performances can be achieved through subtle change of the host molecular structures, which could be attributed to the well matched energy levels between the host and hole transport layer, the high triplet energy of the host and complete spatial separation of HOMO and LUMO energy levels. Devices hosted by structure-optimized o-TPA-m-PTAZ achieve the best EL performance, with the maximum current efficiencies and maximum external quantum efficiencies as high as 12.4 cd/A and 16.4% for deep red electrophosphorescence, and 50.7 cd/A,14.2% for green electrophosphorescence.In Chapter 8, an overall conclusion for this thesis is made. Then the novelty and creativity of the whole PhD work are specially emphasized. Finally, an outlook for future work on phosphorescent host materials is proposed.