Red Quantum Dots Light-Emitting Diodes Based On Organic Electron Transporting Layer

Quantum dot light-emitting diodes(QLED)have the outstanding advantages of tunable color and narrow spectrum,which have potential application in the next generation of ultra-high definition and wide gamut display.At present,highperformance QLED mainly use doped ZnO nanocrystals as inorganic electron transport layer.There are bottlenecks such as charge transport imbalance caused by high electron mobility and non-radiative Auger recombination caused by many defective states.Different from the inorganic electron transport layer,this thesis focuses on the red quantum dot light-emitting diode based on organic electron transport layer.Selects the triazines materials with the deeper lowest unoccupied molecular orbital(LUMO)as the organic electron transport layer.On this basis,thermal activated delayed fluorescence(TADF)polymer/small molecule interface sensitization and bulk sensitization were developed,and successfully breakthrough the efficiency of red quantum dot light-emitting diodes based on organic electron transport layer.And the results are as follows:1.Deal with the problem of low device efficiency caused by energy level mismatch between organic electron transport layer and quantum dots.We used pyridine as the peripheral unit and regulate the electron-withdrawing group of organic electron transport material,ultimately assembled high-performance red QLED based on organic electron transport layer.It is found that when the central core is changed from benzene to the pyrimidine and the triazine,the LUMO level gradually decreases from-2.28 eV to-2.79 eV and-3.07 eV,which promotes the electron injection and balanced of charge transport.At the same time,the organic electron transport layer can avoid the exciton quenching problem at interface caused by the inorganic electron transport layer defects,so that the fluorescence quantum yield(PLQY)of the QDs film is significantly increased from ZnO of 41%,to organic electron transport layer of 48-49%.Finally,QLED with the organic electron transport layer TmPPPyTz shows the highest performance,with the external quantum efficiency of 13.4%(18.8 cd A-1,23.9 lm W-1),with the emission peak at 624 nm,and the color coordinates of(0.68,0.32).2.Based on TmPPPyTz as the electron transport layer,TADF polymer was used as hole transport layer,and fabricate red QLED based on organic electron transport layer with TADF polymer interface sensitization.It was found that when TADF polymer Poly(ACBPCZ-TMP)instead of traditional polymer material as the hole transport layer,the triplet(T1)states can transfer to lowest singlet(S1)through the thermally assisted reverse intersystem crossing,and then the Si of TADF polymer layer were transferred to the QDs layer through Forster energy transfer.The interface sensitization of the R-QDs can significantly improve the exciton utilization rate,reducing the recombination resistance,and did not affect the hole injection and transport.At the same time,by changing the thickness of TADF polymer as hole transport layer to optimize the device performance,the maximum external quantum efficiency of 13.6%(21.6 cd A-1,15.2 lm W-1),with the emission peak at 621 nm,and the color coordinates are(0.67,0.32).3.Based on TmPPPyTz as the electron transport layer,TADF polymer or small molecules and quantum dots are blended to form a light-emitting layer.Red QLED based on organic electron transport layer fabricated with phase sensitization by TADF polymer/small particle.It was found that TADF material could dispersed QDs to suppressing the aggregation quenching phenomenon,which caused by the trapped states of the imperfect QDs.At the same time,the hole injection and transport are improved,final achieved a high performance red QLED without hole transport layer.Most importantly,TADF polymer can be used to realize bulk sensitization of quantum dots and significantly improve PLQY.When TADF Poly(ACBPCZ-TMP)was used as sensitizer,the PLQY of QDs film increased from 54.1%of pure film to 71.0%of sensitized film.The maximum external quantum efficiency of the corresponding red QLED is 14.9%(16.7 cd A-1,16.3 lm W-1),which is much better than that of the non-sensitized devices(3.3%).When the TADF small molecule DMAC-TRZ was used as the sensitizer,the external quantum efficiency was further increased to 16.3%(23.3 cd A-1,16.6 lm W-1),and the emission peak at 627 nm,and the color coordinates were(0.66,0.33).