Failure Mechanism Of Organic Thin Films And Devices At High Temperature

Organic light-emitting diodes(OLEDs)have attracted much attention due to the fact that they are considered as potential candidates for full color display and solid-state lightings because of their unique properties such as lightweight,flexibility,high color contrast and fast response.OLEDs with layered structure of organic materials have recently found practical applications in displays for cellphones,computers and other devices.However,the low glass transition temperature(T_g)of organic functional materials suffers from low crystallization at high temperatures,leading to the unbalanced carrier transportation and degraded film morphology.Furthermore,the long operation at high temperatures could incur fast deterioration,consequently hindering their application in outdoor environment such as automobile taillights.Therefore,investigating the underlying degradation mechanism of both organic thin films and devices under high-temperature environment is still a pressing concern and provides a guideline to create long-lasting and reliable OLEDs in a practical manner.Firstly,to obtain the insight into the degradation mechanism of OLEDs at high-temperatures,this dissertation investigates and highlights the role of high temperatures on the optoelectric properties for carrier-transporting materials.The organic materials N,N'-bis(naphthalen-1-yl)-N,N'-bis(phenyl)-benzidine(NPB)and 1,3,5-tri[(3-pyridyl))-phen-3-yl]benzene(Tm Py PB),are known to be the low-T_g features,tend to be rugged after annealing at a certain temperature below T_g,leading to the increased film surface roughness.When the annealing temperature is beyond T_g,the films would be endured crystallization and followed by surface leveling.Subsequently,the intensity of photoluminescence(PL)of the resulting NPB and Tm Py PB films would be decreased at higher temperatures.The possible reason is due to the crystallization of low-T_g materials at high temperatures,excitons are allowed to migrate with longer distances in the crystallized film.The non-radiative efficiency could be further enhanced,resulting in an obvious decrease with PL spectrum intensity.Furthermore,unstable and variable electrical characteristics will be appeared under the condition of high temperature.However,the photoelectric properties of HAT-CN(dipyrazino[2,3-f:2',3'-h]quinoxaline-2,3,6,7,10,11-hexacarbonitrile)with high-T_g did not change significantly with high temperature annealing.Therefore,in order to improve the electronic stability of low-T_g materials,this thesis presents a method to create high-T_gorganic materials by using inorganic alkali metal compounds as dopants.For example,the ultrastable current intensity of the carrier-only cells prepared with NPB-doped Mo O₃ film can be observed at high temperatures.Theoretical calculations were conducted to clarify that the material with smaller molecular weight could absorb part of heating through vibration in the doped film.And the absorbed heating energy is inversely proportional to the ratio of the two different molecular weight.Thus,the organic materials with larger molecular weight will suffer less damage from heating,leading to the improved operational stability of the composite film.These results yield new insights into the mechanism of thermal stability for organic films and provide new possibilities for the realization of efficient and thermal-stable OLEDs.Secondly,we synthesized a deep blue material 4,5?-di-{[N,N,N?,N?-tetrakis-(4-tert-butyl-phenyl)]phenyl-1??,4???-diamine}-1,1?-binaphthalenyl,namely TPA-BN,which exhibits good thermal stability.More importantly,its energy level is well matched with the carrier transport layers,facilitating the exciton recombination process in the emitting layer.The thermodynamic properties of the resulting material were measured by thermogravimetric analysis and differential scanning calorimetry,showing the high melting temperature(T_m)and decomposition temperature(T_d),but insignificant T_g of the material.The surface roughness of TPA-BN film remained stable after being annealed at different high temperatures.And the UV-vis absorption and PL spectrum intensity of the film with different annealing temperatures didn't show significant shift,also demonstrating the superb film-forming property which could be prepared to homogeneous film with high morphological stability.To investigate the electroluminescence performances of TPA-BN,doped OLEDs with with different dopant concentrations were fabricated.The maximum current efficiency(CE)of the undoped device is 3.2 cd A^-1 and the external quantum efficiency of 4.5%.Finally,encouraged by the improved film quality,we sought to further investigate the impact of high temperature on the device performance and lifetime of OLEDs.The experimental results show that the high temperature significantly deteriorate the device performance and lifetime with enhanced turn-on voltage.To address the above phenomena,in this chapter,the optimized doping of the hole and electron transport layers in the device was evaluated and elucidated based on the previous chapters.The OLEDs with high temperature stability were prepared,and its device performance and lifetime at high temperature were systematically investigated.The maximum CE of the optimized device with the annealing temperature of 80?merely decreased by 12%and the device lifetime improves up to 208 h over the undoped reference device.The improved device stability is ascribed to the doped carrier transport layer,which effectively reduces the interfacial potential barrier,suppresses the interfacial charge accumulation caused by high temperature in addition to decreases the leakage current of the device after annealing at high temperature.The optimized organic film with doped inorganic materials prevents the crystallization of low-T_g materials,thus improving the device performance and lifetime.