Research On Low-Temperature Cross-Linkable Hole Transport Materials For QLED

Owing to the various advantages of excellent electroluminescence performance,solution processibility and so on,quantum dot light-emitting diodes(QLEDs)are considered to be one of the most promising next-generation display technologies with large-area panel and low manufacturing cost.Cross-linking strategies of hole transport materials(HTMs)have been widely investigated and used in various optoelectronic devices,mainly thanks to their excellent solvent resistance and thermal stability.More importantly,the energy level and charge mobility of cross-linkable HTMs can be fine-tuned through precise molecular design,which offers great opportunities to explore and screen efficient HTMs.However,most of the reported thermally cross-linked HTMs have high cross-linking temperatures.In addition,the external quantum efficiency of QLED devices based on cross-linkable HTMs,is far lower than the highest value obtained based on traditional HTMs.Therefore,from the perspectives of cross-linkable molecule design and device fabrication process,we will study how to obtain high-performance low-temperature cross-linkable HTMs.In this dissertation,we synthesized two cross-linkable HTMs,namely DV-SFCZ and DV-FLCZ.The main difference between them is the substituents on their cores,for DV-SFCZ with fluorene and DV-FLCZ with dimethyl groups,respectively.To investigate the effect of the central steric hindrance of these two HTMs on the properties,the basic optoelectrical properties,film formation ability and corresponding device performances are fully characterized.The cross-linking temperature of DV-FLCZ is only around 140°C,which is very close to the normal annealing temperature of traditional small molecule materials(around 150°C).In addition,DV-FLCZ shows higher hole transport property,which can be attributed to the smaller central steric hindrance and more compact stacking of molecules inside.Since the deep HOMO level of QDs,the-5.7 e V HOMO level of DV-FLCZ is deep enough in favor of hole injection into QDs layer.Finally,red,green and blue QLEDs with DV-FLCZ as HTL achieved the maximal EQE of 20.5%,16.6%and 8.5%,respectively,which represented the highest EQE values of QLED performance using cross-linkable materials alone as HTL.This work reveals that decreasing central steric hindrance of HTMs can lower the cross-linking temperature and thus enhance hole-transporting capabilities.Besides,by lowering the energy barrier between PEDOT:PSS,quantum dots,and cross-linkable HTMs,optimized charge balance can be achieved and thus improved device performance can be also obtained.Owing to the residual initiator,QLED devices with photo-crosslinking material as the hole transport layer tend to exhibit poor stability.The cross-linking temperature of thermally cross-linkable materials is normally controlled by its molecular structure.Considering the complex influence of the molecular structure on the cross-linking temperature,the molecular design of cross-linkable materials is of great challenges.By combining thermally cross-linking with ultraviolet-light assistance,the cross-linking temperature of HTMs will be largely lowered and the cross-linking time will also be substantially reduced.By using a photothermal synergistic method to achieve cross-linked DV-FLCZ film,we systematically evaluated the effects of different powers of UV light on the cross-linking temperature,film quality and device performance.Under the ultraviolet condition of 365 nm wavelength and 100 m W/cm~2,DV-FLCZ has been fully cross-linked at 100?for 20 min.Compared with the film prepared by thermal cross-linking,the cross-linked film induced by photothermal method,exhibited similar morphology and corresponding device showed close performance.To verify the universality of photothermal synergistic method for vinyl group based cross-linkable HTMs,the reported CBP-V and DV-FLBC were applied to this method.Under the same UV condition and duration of thermal cross-linking,the cross-linking temperature of CBP-V and DV-FLBC decreased by 50 and 80?respectively.These results demonstrate that photothermal synergistic cross-linking strategy can effectively lower the cross-linking temperature,which would lay a good technology foundation for future industrialization of QLEDs.