Effect Of The Treatment And Modification Of Electrode On The Performance Of OLED

Organic light emitting diodes(OLEDs)are attracting more and more attention due to their advantages of self-illumination,wide viewing angle,slimness and low power consumption compared with traditional display technologies.In the recent three decades,OLED technology has been rapidly developed,its device structure,life expectancy and luminescence properties have reached the requirements of commercial production.OLED technology is widely regarded as the most promising next-generation display technology.Now,OLED technology is mainly applied in small size display and lighting technology.The conventional OLED device consists of a multilayer organic thin film functional layer sandwiched between an indium tin oxide(ITO)anode and a metal cathode having a low work function.ITO as an anode used in OLED devices,there are some problems.First,there is a large energy barrier between the ITO and the hole transport layer,which makes hole injection difficult and affects the efficiency of carrier recombination in the device.Second,ITO surface is a hydrophilic with poor contact properties with the organic layer,which makes the organic layer has poor thermal stability.This will affect the operating life time of the OLED device.Finally,in the OLED device structure,the refractive index difference between glass/air and ITO/ organic,resulting in total reflection and waveguide in the device.Total reflection and waveguide make the device internal optical loss up to 80%,seriously affecting the device optical coupling output efficiency.For further enhancing the performance of the OLED device.Enhancing the carrier balance or increase the light extraction of the device substrate are used to improving the light output efficiency of the OLED device.Therefore this article mainly studies the following two parts:(1)The larger hole injection barrier and the poor contact between ITO/HTL make the OLED device performance bad.In order to improve the performance of the OLED,the ITO electrode needs to be modified.Through literature review,it has been found that the use of molecular self-assembly(SAM)to treat ITO electrodes not only enhances the work function,but also improves the surface properties.In the experiment,p-chlorophenylacetic acid and p-fluoroacetic acid were used to modify ITO.The influence of SAM treatment on ITO electrode was analyzed by measuring the water contact angle and work function.The results show that after modification with p-chlorophenylacetic acid and p-fluorophenylacetic acid the work function of ITO is increased from 5.0e V to 5.24 e V and 5.16 e V respectively and the water contact angle increased.Different thicknesses of NPB films were deposited on the ITO electrodes and the surface morphology of NPB was measured by AFM.The SAM treatment was found improved the film-forming properties of the NPB film,making the NPB film more stable.The voltage-current density characteristic of the hole-transporting device shows that the SAM treatment improves the hole injection.Compared with the base device,after the p-chlorophenylacetic acid and p-fluorophenylacetic acid SAM treatment,the maximum brightness of OLED devices increased from 15880cd/m2 to 21771cd/m2 and 22428cd/m2,respectively,increased by 37.09% and 41.23%.After SAM processing,the half-decay lifetime of the OLED device increased from a few minutes to 46 hours,and the stability of the device has been significantly improved.There are two main reasons for the improvement of device performance.First,because the SAM processing increases the work function of the ITO electrode,the hole injection barrier is reduced,thereby lowering the turn-on voltage of the device and improving the light-emitting brightness and the luminous efficiency of the device.Second,the SAM treatment improves the contact between the organic layer and the ITO electrode,so that the structure of the device is more stable during operation,thereby improving the lifetime of the device.(2)In order to further enhance the light output of the OLED device,a PS: PMMA hybrid film with randomly distributed nano-columnar structures was used as the substrate of the device.The result shows that the randomly distributed nano-columnar structure as the substrate can improve the brightness,increase the current efficiency and reduce the working voltage,and obviously improve the performance of the OLED device.Compared with glass substrate OLED device,the current efficiency of the OLED based on nano-columnar structure increased from 3.2cd / A to 4.1cd / A,while the maximum brightness increased by 37.2%.In OLED device,there are total reflection,waveguide effects and carrier imbalance which will make the device's emiitng and light output are very poor.In order to improve the light output efficiency of OLED devices,this paper discusses how the processing and modification on the anode side improve the performances of OLED device.The treatment and modification include the anode and the substrate in direct contact with the anode.The results show that the treatment and modification at the anode can significantly improve the light output of the device.