Hole transport materials play an important role in the light-emitting properties of OLEDs. They can effectively reduce the energy barrier during hole injection and improve the injection efficiency to improve the efficiency, brightness and lifetime of the device. In OLED devices, hole injection/transport materials (HIM/HTM) are indispensable. HIM is often in contact with ITO to facilitate the smooth injection of holes from the ITO electrode. Therefore, in order to reduce the potential barrier, HIM occupies the highest molecular orbital The (HOMO) energy level is often higher, while the HTM is located between the hole injection layer and the exciton blocking or light-emitting layer, so its HOMO energy level is often between the injection layer material and the exciton blocking layer material or the light-emitting layer material. between HOMO levels.
The molecular center of the linear structure is usually biphenyl, carbazole, fluorene, biphenylthiophene group and other structures. The commonly used linear structure hole transport materials are N,N'-diphenyl-N,N'-bis(3 -Methylphenyl)-1,1'-biphenyl-4,4'-diamine (TPD) and N,N'-diphenyl-N,N'-(1-naphthyl)-1,1'-Biphenyl-4,4'-diamines (NPB). By modifying the TPD molecule and replacing the side chain benzene ring with a biphenyl group, high thermal stability is imparted to the novel hole transport material.
Because the glass transition temperature of the linear structure molecules is low, crystallization is easy to occur under the influence of Joule heat generated during the operation of the device, which will destroy the uniformity of the film and reduce the efficiency and service life of the device. Generally, molecules with asymmetric and specific spatial structures can effectively reduce the intermolecular directional aggregation properties, improve the stability of hole transport materials, and reduce the possibility of crystallization. Depending on the core structure, star-structured HTMs can be divided into the following two categories:
Commonly used such HTMs are 4,4',4''-tris(diphenyl-amino)triphenylamine (TDATA) and 4,4',4''-tris[(3-methyl-phenyl) Phenylamino]triphenylamine (MTDATA), their glass transition temperatures are 89 °C and 75 °C, respectively.
The three star-shaped planar structure compounds B-DPP, T-DPP and BT-DPP can effectively reduce the hole injection barrier by growing molecules in the horizontal direction in the organic film in contact with the anode. It shows that this structure can be used as the material of hole transport layer and hole injection layer.
Trimeric indene is a rigid planar structure that is easy to synthesize. The periphery can be further modified by connecting different functional groups. Two dendrimers, Tr-TPA3 and Tr-TPA9, were synthesized based on the trimer core. Their high HOMO level and excellent film-forming properties make them suitable as a hole-transporting material for spin-coated devices.
The design of the molecular structure of small molecular weight compounds to achieve high Tg is very important. It is the original intention of the concept of spiro structure to improve the morphological stability of low molar mass materials while retaining their electronic properties. The spiro-structured material has a stable vertical structure due to the sp3-hybridized C atom in the center of the spiro ring.