Synthesis Of Indenocarbazole Host Materials And Application In Red Phosphorescent Organic Light-emitting Devices

Currently,there are three generations of organic electroluminescent materials for organic light emitting diodes,organic phosphorescence materials can simultaneously obtain singlet excitons and triplet excitons by using the heavy atom effect of iridium or platinum,and the exciton utilization rate is theoretically 100%.Therefore,organic phosphorescence materials are still considered as the most promising materials.However,due to the long exciton lifetime of phosphorescent materials,exciton concentration quenching is easy to occur at high brightness.Therefore,the host materials with good performance play an important role in improving the luminescence efficiency of organic phosphorescence materials.Presently,there are few host materials for red organic phosphorescence materials.This paper aims to develop the host materials for red organic phosphorescence materials which can replace foreign commercial applications.The carrier transport performance and energy level of the host materials are adjusted to achieve the balance of carrier transport,to improve the luminous efficiency of OLED and the stability of the device.The D-A host materials composed of electron donor group and electron acceptor group were designed and synthesized,and the synthesis and purification process of host materials were optimized.Indenocarbazole（FC）has strong electron donor ability,it has the following properties:1)Good hole mobility;2)multiple chemical modification sites,the structure is easy to modify;3)With a certain spatial configuration,the thermal stability of the material can be enhanced.Therefore,it can be used as an organic electroluminescent material to improve the electroluminescent properties of the materials.The main research contents are as follows:（1）In this part,FC was used as the core,quinazoline derivatives（PQZ）with strong electron-withdrawing ability was used as the A group,and carbazole,9-phenyl-9H-carbazole,diphenylamine and triphenylamine were used as D groups to design and synthesize four host materials,namely FC-Cz-PQZ,FC-BCz-PQZ,FC-DPA-PQZ and FC-TPA-PQZ.The triplet energy levels of these four materials range from 2.54 e V to 2.58 e V,which are higher than the red organic phosphorescent guest materials.With the enhancement of electron donating ability,the electron mobility of FC-DPA-PQZ and FC-TPA-PQZ reaches 10^-7（cm²/Vs）,which belong to bipolar host materials.The maximum EQE of FC-Cz-PQZ,FC-BCz-PQZ,FC-DPA-PQZ and FC-TPA-PQZ are 17.6%,17.2%,20.9%and 23.9%,respectively,FC-DPA-PQZ and FC-TPA-PQZ,The EQE of PQZ is higher than that of FC-Cz-PQZ and FC-BCz-PQZ,which are attributed to the carrier bipolar transport ability and expanding the recombination region of excitons in the light-emitting layer,reducing exciton concentration quenching and improving exciton utilization.The EQE of FC-TPA-PQZ is higher than that of FC-DPA-PQZ,because FC-TPA-PQZ has the suitable HOMO（-5.46 e V）and the LUMO（-2.46 e V）,The HOMO and LUMO of the hole transport layer is-5.50 e V and-2.48 e V,respectively,therefore,the OLED based on FC-TPA-PQZ has the highest EQE.（2）The best-performing host material（FC-TPA-PQZ）has bipolar transport properties,but the electron mobilitiy is still much smaller than the hole mobilitiy.For this reason,in this part of the work,the derivatives of pyrimidine（MD）and triazine（TRZ）with strong electron withdrawing ability were selected to replace PQZ as the A group,respectively,and the two were connected through the FC core.FC-TPA-MD and FC-TPA-TRZ,by changing different acceptors to further improve the carrier transport balance ability of host materials,achieving the goal for optimizing OLED performance.The two host materials have good thermal stability,suitable HOMO energy levels and high triplet energy level;The hole mobility of FC-TPA-MD and FC-TPA-TRZ is 10^-6（cm²/Vs）,the electron mobility is 10^-7（cm²/Vs）and10^-6（cm²/Vs）,respectively,indicating that the carrier transport balance of FC-TPA-TRZ is better than that of FC-TPA-MD,which is attributed the electron transport ability of the A group TRZ of FC-TPA-TRZ is higher than that of the acceptor MD of FC-TPA-MD.The EQE of OLED based on FC-TPA-MD host material（20.40%）is lower than that of OLED based on FC-TPA-MD（21.48%）,which is attributed to the not only bipolar carrier transport properties of FC-TPA-MD,but also suitable HOMO and LUMO energy levels.In addition,the hole and electron mobilities of FC-TPA-TRZ are both 10^-6（cm²/Vs）,which has a balanced carrier transport capability,however,the LUMO energy level of the hole transport layer and FC-TPA-TRZ is-2.48 e V and-2.58 e V,the LUMO energy level difference is small,the electron mobility of FC-TPA-TRZ is higher,and the hole transport layer fails to block electrons,so that excitons cannot be fully utilized.By optimizing the structure of the device,replacing the hole transport layer,the effect of blocking electrons can be achieved.（3）This part of work,TPA is the D group and TRZ is the A group.The two were connected by FC,the spatial structure were adjusted by different connection methods of pair,meta and ortho,in which study the effect of different spatial configurations on the properties of materials.Hence,three host materials of FC-TPA-p TRZ,FC-TPA-m TRZ and FC-TPA-o TRZ were designed and synthesized.They have excellent thermal stability,suitable HOMO and LUMO energy levels and high triplet energy levels.The hole mobilities of FC-TPA-p TRZ and FC-TPA-o TRZ are 10^-6（cm²/Vs）and 10^-5（cm²/Vs）,respectively,and the electron mobilities are 10^-6（cm²/Vs）and 10^-5（cm²/Vs）,respectively.Among them,FC-TPA-o TRZ has the best carrier transport balance,reaching a complete balance of 1:1.The maximum EQE of the three main materials reached 23.3%,23.9%and 23.7%respectively.The efficiency roll-off based on OLEDs with FC-TPA-p TRZ,FC-TPA-m TRZ and FC-TPA-o TRZ is 7.80%,4.60%and 1.48%at luminance of 1000 cd/m²,respectively.The lower efficiency roll-off of FC-TPA-o TRZ is attributed to having balanced carrier transport properties,enlarging the recombination region of excitons in the emissive layer,reducing the exciton concentration quenching of the device at high brightness.