Study On Degradation Mechanism Of Long-Lifetime Organic Electroluminescent Devices

After decades of development,organic light emitting diodes(OLEDs)becomes more and more mature,and various related electronic products are emerging one after another.However,the poor stability of blue and white light devices is still a major obstacle to their further commercial development.Taking blue pure-organic lightemitting molecules with cyanobenzene group as the research object,this paper has studied the key factors which would affect the stability of the device,including the thermal aggregation of blue-light film,emission of electromer,delayed fluorescence sensitization mechanism and tandem structure.A variety of strategies have been put forward to improve the stability of the blue and white OLEDs.All topics are closely connected,and always advance in the direction of extending the operation lifetime of OLEDs,the main achievements are as follows:(1)In the first research work,two cyanobenzene-based blue thermally activated delayed fluorescent(TADF)molecules(2CzPN and 3Ph2CzCzBN)were as the research objects.Their neat films were annealed to investigate the effect of Joule heat on their operation stability.For 2CzPN molecules with small molecular weight,large aggregate particles would appear on the surface of the film after heat treatment at 85℃ for 5 minutes.And XRD analysis showed that some crystals also appeared in the original amorphous film after heat treatment.Different from 2CzPN,3Ph2CzCzBN with large steric groups showed better film thermal stability.Under the same heat treatment,there was no obvious change on the surface of the neat film.The difference of thermal stability between the two films is mainly attributed to the difference of molecular structure and spatial configuration.The device results also show that the 3Ph2CzCzBN-based device possess a longer operating lifetime,which is 6.8 times higher than that of 2CzPN.(2)Based on the original 3Ph2CzCzBN blue-light device,we study the effect of electromer emission on the stability of the device.The reason for the generation of electromer is due to the charge leaks from the emitting layer to the exciton blocking layer(EBL,SF3-TRZ)during the long-term operation of the device.The positively charged SF3-TRZ+ and negatively charged SF3-TRZ-could conduct bimolecular effect for producing the emission of electromer.The generation of this electromer also indicated the occurrence of charge leakage and the degradation of exciton blocking material in the long-term operation of the device.To reduce the influence of electromer on device performance,an embedded EBL structure with distributed exciton barrier ability was adopted.The 10 nm SF3-TRZ was divided into 2,3 and 5 parts,i.e.,the thickness of single-layer SF3-TRZ was 5 nm,3.3 nm and 2 nm,and these thin EBLs were embedded into the light-emitting layer.The distributed exciton blocking structure can effectively reduce the generation of electromer and limit the aggregation of triplet excitons in the light-emitting layer.At the initial brightness of 1000 cd m-2,the lifetime of the blue TADF device was increased from 710 hours to 1435 hours.(3)In the third research topic,we began to study how to fabricate white OLEDs(WOLEDs)with long lifetime,high efficiency and high color stability.By using blue cyanobenzene-based TADF molecule(2tCz2CzBn)as sensitizer,orange TADF molecule(1PXZ-BP)or traditional fluorescent molecule(TBRb)as dopant,we prepared two types of TADF WOLEDs with different mechanisms,full-TADF device and TADF sensitized fluorescence(TSF)device,respectively.The tert-butyl group around the blue-light molecule could increase the distance between molecules and weaken the π-π stacking,thus the concentration quenching of sensitizer and short-range Dexter energy transfer process could be effectively suppressed.Both devices show nearly 100%exciton utilization efficiency.The difference was that the TSF device showed a lower efficiency roll-off and longer device lifetime.At the same time,TSF WOLEDs with different color temperatures could show excellent spectral stability because of the modulated energy transfer channel,matched radiation rates and PLQYs between blue TADF sensitizer and orange fluorescent emitter.(4)We further used the more stable blue TADF molecule(3Ph2CzCzBN)as the sensitizer and assisted host to prepare a stable TSF WOLED.Time-resolved spectra,emission spectra under different excitation light sources and transient PL delay curves were used to reveal the energy transfer process in TSF sensitization system.Finally,we obtained a white-light TSF device with high efficiency,long lifetime,low cost and good spectral stability.The maximum external quantum efficiency of the device was 20.9%,the CIE coordinates was(0.31,0.41),and its half-lifetime was more than 2800 hours at an initial brightness of 1000 cd m-2.(5)In addition,we discussed the effects of tandem structure and optical extraction technology on the stability of the blue-light and white-light device in Chapter 6.An organic/organic-type charge generation layer was chose as the source of charge generation,because it not only had high optical transmittance and good charge generation ability,but also possessed excellent stability.Only through simple stacking,the device lifetimes and the current efficiencies of blue-light and white-light devices can be doubled.Moreover,after adding the light extraction structure,the lifetime of blue-light and whitelight devices has reached～4600 and～12000 hours,respectively.These results are in the forefront of the reported stability of blue and white OLEDs,and can provide some reference values for scientific research and industrial production.