Organic Charge Transport Materials With Wide Bandgap And High Mobility

Electron and hole transport layers are important functional layers in organic light-emitting diodes（OLED）devices.The development of transport materials with high mobility is conducive to reducing the driving voltage of the device and even improving the quantum efficiency and lifetime of the device.This paper systematically studied two types of electron transport materials（ETMs）with high mobility and high T₁ energy and a new type of non-amine hole transport materials（HTMs）.The research content includes the following three aspects:1.TPPO-based ETMs with the bridge groups of ether,diphenyl silicon,bis（trifluoromethyl）methylene,phosphine oxide,and sulfone were systematically investigated.The ability of the bridging group to push and pull the electrons affects the electronic coupling between the TPPO groups,which governs the electron mobility and T₁ energy level of these materials.If the electron withdrawing ability of the bridging group is weaker than that of the TPPO group,the conjugation between TPPO will be interrupted,so the T₁ energy of the molecule is high but the electron transfer rate is low.On the contrary,if the electron withdrawing ability of the bridging group is stronger than that of the TPPO group,the TPPO groups can be conjugated with each other,so that the T₁ energy of the molecule is reduced.The bis（trifluoromethyl）methylene bridging group has a moderate ability to push and pull electrons,the SOMO of the bisphosphine molecule（A3）based on this group has the widest delocalization range in the reduced state,and a large proportion is distributed on the bridge C atom,which takes into account both the intermolecular and intramolecular electron transport.The T₁ energy of A3 is as high as 3.3 e V,and the electron mobility of the film state is nearly two orders of magnitude higher than that of the classic electron transport material DPEPO.The device using A3 as host and electron transport layer has a maximum EQE（22.3%）equal to that of DPEPO-based devices,while the current density under the same voltage is more than 6 times that of DPEPO-based devices.2.Pyridine substituted spirothioxanthene（T3Py SS）was proved to be an electron transport material with wide bandgap and high mobility,and its electron mobility was equivalent to that of traditional electron transport materials with long conjugated structure.The sulfur atoms and quaternary carbon atoms in thioxanthene partially break the conjugation of the fragment,making T3Py SS have a high T₁ energy level（>2.7 e V）.At the same time,the rigid and symmetrical structure of the molecule ensures that there is a smaller reorganization energy and a larger orbital overlap integral during the electron transport process.The investigation on the bridge atom and the number of functional groups in this type of ETM revealed that balanced inter-and intramolecular electron coupling was critical to achieving high-mobility,wide-bandgap ETM.Since T3Py SS had both high electron mobility and shallow LUMO level,the use of T3Py SS as the electron transport layer in blue-light devices facilitated electron injection into the light-emitting layer with wide bandgap and improved the carrier balance,therefore T3Py SS based device showed higher efficiency and better stability than devices based on classical ETMs.T3Py SS-based deep-blue fluorescent device（CIEy=0.07）achieved a very low driving voltage of 3.2V at a current density of 20 m A/cm² as well as a high EQE of 8.0%and a LT90 of 63h.In addition,the stability of green Ph OLEDs employing T3Py SS was also superior to that of devices based on traditional ETMs with high triplet energy.3.Four novel non-amine HTMs with high hole mobility and wide bandgap were obtained by using spirothioanthene and spiroxanthene groups as cores and connecting phenyl,fluorophenyl or naphthyl at positions 2 and 7.The HOMO of these molecules is delocalized on the entire molecule,the rigid and orthogonal molecular structure guarantees a small hole reorganization energy（0.1 e V）.The single crystal results show that sulfur atoms can introduce multiple intermolecular S-S and CH-S interactions,which can increase the intermolecular electronic coupling.The hole transfer integral of the molecular pair with S-S interaction reaches 39.7 me V,and there are many channels with hole transfer integral greater than 1 me V in a unit cell,which indicates that the rigid and orthogonal spherical molecular structure is conducive to the formation of multiple charge transfer channels,thus improving the mobility.The hole mobility of these four HTMs is higher than the classic HTMs m CBP,and the hole mobility of phenyl substituted spirothioxanthene（TPSS）reaches the level of 10^-3cm² V^-1 s^-1.TPSS with a wide bandgap（3.42 e V）and deep HOMO energy（5.6 e V）was used as hole transport layer and electron blocking layer of blue OLED.The performance of TPSS based blue fluorescent and thermally activated delayed fluorescent devices was not inferior to that of comparative devices based on traditional amine hole transport materials,which proves the design feasibility of non-amine hole transport materials.