Research On Ultrasonic Spray Coating Thin Films And Their Application In Organic Light-emitting Devices

Organic light-emitting device?OLED?has huge potential in display and lighting fields,but high manufacture cost is still hindering its further development.According to cost structure of OLED products,high manufacture cost is mostly caused by the commonly used vacuum thermal evaporation process for the fabrication of OLEDs,which needs complex equipment to keep high vacuum and has extremely low material utilization.However,ultrasonic spray coating technology with simple process,cheap equipment and high material ultilization could provide the possibility for the realization of low-cost OLEDs.Recently,the application of ultrasonic spray coating technology in OLEDs has not been reported.Thus,nanoscale thin film fabrication process based on ultrasonic spray coating technology is developed to explore its feasibility in fabricating OLEDs.On the basis of references and experiments,ultrasonic spray coating process is designed,including ultrasonic atomization,droplets coalescence?liquid layer formation?,solvent evaporation and solid film formation.To obtain an organic layer that meets the criterion of OLEDs,it is key to form a continuos liquid layer and keep its uniform distribution during the solvent evaporation.Poly?3,4-ethylenedioxythiophene?-poly?styrenesulfonate?dryre-dispersiblepellets]?PEDOT:PSS?is usually used to improve interfaceproperties of ITO transparent electrode in optoelectronic devices.The PEDOT:PSS is usually aqueous solution and aqueous solution cannot wet the ITO substrate very well so that it cannot form uniform liquid layer.To improve the wettability of PEDOT:PSS solution on the ITO substrate,the methanol with low surface tension is introduced to modify the properties of PEDOT:PSS solution.The modified PEDOT:PSS solution can quickly spread out to form uniform liquid layer,and the slild thin film is obtained after the rapid evaporation of solvent.The root-mean-square surface roughness?RMS?of ultrasonic spray coating PEDOT:PSS thin film is only 2.75 nm,which is comparable to spin coating PEODT:PSS thin film.Beisdes,due to the exisitence of methanol,there is obvious phase separation in the ultrasonic spray coating PEDOT:PSS films from methanol-modified solution,resulting in improvement of conductivity.When the ultrasonic spray coating PEDOT:PSS film is used as anode modified layer,to some extent,electroluminescent performances of OLEDs,such as current density,luminance,efficiency roll-off and so on,are improved.Because of low viscidity,solution of small organic molecules is not suitable for fabricating organic thin film by spin coating process.As a liquid phase deposition technology compatible to low-viscidity solution,ultrasonic spray coating process can solve this problem very well.To verify the feasibility of ultrasonic spray coating small molecular organic thin film,toluene solution of 4,4',4''-Tris?carbazol-9-yl?triphenylamine?TCTA?which can wet the PEDOT:PSS-coated substrate is used to fabricate ultrasonic spray coating small molecular organic TCTA films successfully.Ultrasonic spray coating TCTA films exhibit better microcosimic surface topography than vacuum thermal evaporating TCTA films,proving the superiority of liquid phase deposition process in the film formation mechanism of small molecular organic materials.Besides,the variation in impedance chracteristics of single-carrier devices caused by alternative small voltage with different frequency is proposed to evaluate carrier transport ability of organic thin films.Its feasibility is also analyzed by equivalent circuit model and formula derivation.OLEDs employing ultrasonic spray coating TCTA film as hole transport layer exhibit much lower leak current and higher luminous efficiency.Different with the former case,solution consisting of multiple organic materials can not wet the substrate surface very well and it also cannot ensure the whole coverage of liquid layer during the evaporation stage.At the time,redistribution of solute caused by complex flow behaviors exsisting in liquid layer,such as capillary flow and Marangoni effect,can not be neglected.In order to solve the problem,assistant solvent is introduced to build new stable tension equilibrium at the edge of liquid layer.During the solvent evaporation stage,the new tension equilibrium can keep unchanged to prevent the shrink of liquid layer.Through this method,uniform green luminescent thin films consisting of 9,9-Spirobifluoren-2-yl-diphenyl-phosphine oxide?SPPO1?doped with Tris[2-?p-tolyl?pyridine]iridium?III?[Ir?mppy?₃]are successfully fabricated.The ultrasonic spray coating luminescent thin film can be used as emitting layers to achieve efficient green OLEDs,which can be comparable with vaccum thermal evaporating OLEDs in efficiency.However,assistant solvent method still has limitations since it can not be used to fabricate large scale thin films.Thus,slope method is used to further modify ultrasonic spray coating process.In the slope method,gravity is used to remove excess solution of liquid layer so that only an ultrathin liquid layer is absorbed on the substrate.Due to the existence of Van der Waals force and electrostatic force,complex flow behavious in liquid layer could be restricted to ensure the formation of uniform multiple component luminescent thin films.Employing the ultrasonic spray coating process modified by solpe method,high quality uniform multiple components TCTA:Bis[2-?4,6-difluorophenyl?pyridinato-C2,N]?picolinato?iridium?III??Firpic?:Bis?2-methyldibenzo[f,h]quinoxaline??acetylacetonate?iridium?III?[Ir?MDQ?₂?acac?]and TCTA:Firpic:Ir?mppy?₃:Ir?MDQ?₂?acac?white luminescent thin films are successfully fabricated.Employing these thin films as emitting layer,efficient white OLEDs are fabricated and they are comparable to vacuum thermal evaporating devices in electroluminescent performances.According to above optimization,ultrasonic spray coating process deserves to be an efficient method to achieve low-cost and high performance OLEDs.Since inorganic materials have the properties of high carrier mobility and thermal stability,employing inorganic materials as transport layer would be beneficial to reduce turn-on voltage and improve power efficiency and stability of devices.Molybdenum oxide?MoO₃?that is usually used as electrode modified layer in optolectronic devices has huge potential to be efficient hole transport layer.However,due to the limitation of island growth mode,vacuum thermal evaporating MoO₃ thick films have bad film morphology and rough surface.Therefore,hybrid OLEDs employing vacuum themal evaporating MoO₃ film as hole transport layer cannot achieve efficient electroluminescent performances.Because of the superiority of liquid phase deposition process in the film formation mechanism,ultrasonic spray coating MoO₃ films havc much better film morphology,their cross-section surface undulation is restricted in the range of±3 nm,making it posiible to realize effect hybrid OLEDs.To avoid the quenching effect of MoO₃ films on excitons in emitting layer,excitons formation region is precisely controlled by the utilization of hole-transport-type host material.Finally,efficient hybrid OLEDs based on ultrasonic spray coating MoO₃ are successfully prepared.Compared with conventional devices,these hybrid OLEDs have an improvement in turn-on voltage,luminous efficiency and device stability,proving the potential of hyrbrid device structure in futher enhancing electroluminescent performances of OLEDs.Furthermore,in view of the huge potential of quantum dots light-emitting devices?QD-LEDs?in panel display field,top-emitting QD-LEDs are also fabricated by microcontact printing technology and their electroluminescent performances are studied.However,the commonly used PDMS stamp in microcontact printing technology is easily swelled by non-polar solvents.To prevent the damage of quantum dots solution,UV/ozone treatment is used to form a micron-sized SiO_x protecting layer on the PDMS transfer stamp.Based on the treated PDMS transfer stamp,uniform quantum dots thin films are transferred into devices to serve as emitting layers,and then top-emitting QD-LEDs are obtained.Compared with bottom-emitting devices,top-emitting QD-LEDs perform much better in luminance,luminous efficiency,efficiency roll-off and and color purity.Researches about top-emitting QD-LEDs would be beneficial to the development of quantum dots display products with high aperture ratio,wide color gamut and stable color.