Weak Organic Acids And Salts Modified PEDOT:PSS Electrodes And Its Application In Flexible OLEDs

In the past two years,with the upgrading of consumption and the rapid development of 5G network,terminal display technology is also developing towards ultra-high resolution,flexibility,light and beautiful appearance and low cost.Flexible transparent conductive electrode is very important to realize this kind of high-efficiency flexible optoelectronic devices.Many flexible transparent conductive electrodes have been widely studied to replace the traditional indium tin oxide(ITO)rigid electrode due to their excellent photoelectric properties,such as metal nanowires,metal nanopatterns,poly(3,4-ethylenedioxythiophene):poly(phenylethylenesulfonic acid)(PEDOT:PSS),carbon nanotubes,and graphene.PEDOT:PSS electrode is widely used in various flexible wearable devices and flexible optoelectronic devices because of its excellent solution processability,biocompatibility and tunable conductivity.However,there are still some problems when PEDOT:PSS electrode is applied in flexible organic light emitting diodes(OLEDs).For example,the pristine PEDOT:PSS solution has low conductivity(about 0.1～1 S/cm)and strong acidity(p H≈0.55).These problems will greatly affect the performance and stability of OLEDs.Therefore,improving the conductivity and alleviating the acidity of PEDOT:PSS simultaneously is key to realize high efficient and stable OLEDs.(1)PEDOT:PSS electrode with high conductivity and high stability was prepared by directly doping weak organic acids and salts.Because the traditional dopants are mostly acidic and high boiling solvents,the dopants are volatile and the post-treatment is complicated.Therefore,we chose 3-methylsalicylate(Na-Me Sal)solid powder as the dopant,which does not need to be removed in the post-treatment process.Besides,it shows weak alkalinity,which can improve the p H of the PEDOT:PSS solution.The conductivity of PEDOT:PSS modified by Na-Me Sal increased from 0.09 to 504 S/cm,and the p H increased from 0.55 to 3.18.The sheet resistance remained after 70 days storaged in the air atmosphere.The performance of flexible OLED devices(88.6 cd/A and 42.8 lm/W)based on Na-Me Sal modified PEDOT:PSS electrode is twice that of traditional DMSO modified devices(41.5 cd/A and 18.2 lm/W).Furthermore,the stability and bendability of Na-Me Sal modified devices have also been significantly improved.(2)Effect of benzoate cations on the conductivity of modified PEDOT:PSS electrodes and corresponding mechanism were investigated.Firstly,the effect of different benzoate cations on the conductivity of modified PEDOT:PSS electrodes were studied.The conductivity of pristine,H~+,Na~+and K~+modified PEDOT:PSS films were0.09,0.13,212.0 and 146.5 S/cm,respectively.It can be found that with the same anion,the higher the ionization of the metal cations,the smaller the radius and the smaller the shielding constant,the higher the improved PEDOT:PSS conductivity.The modification mechanism was analyzed and proposed by XPS,Raman and AFM.Flexible OLEDs based on the modified PEDOT:PSS electrodes were prepared.The luminous efficiency of Na~+and K~+modified devices are 91.2 cd/A(52.1 lm/W)and76.8 cd/A(37.1 lm/W),respectively,which are twice higher than that of traditional DMSO modified device(54.9 cd/A and 28.7 lm/W).The performance of Na~+modified devices with small radius is also significantly higher than that of K~+modified devices(3)The synergistic effect of silver benzoate on the conductivity of PEDOT:PSS electrodes.Since the conductivity of above mentioned modified PEDOT:PSS electrodes is still not high enough,the conductivity of PEDOT:PSS electrode should be further improved.In this section,silver benzoate is used to modify PEDOT:PSS electrode.Firstly,the effect of silver benzoate concentration on the conductivity of PEDOT:PSS films was systematically studied,which found that PEDOT:PSS doped with 10 mg/m L-silver benzoate led to the highest the conductivity of 551.5 S/cm.The structure of silver benzoate modified PEDOT:PSS was characterized by Raman and XPS.It was found that there was a doping effect of Ag in addition to the phase separation between PEDOT and PSS chains induced by the weak organic acids,due to the redox reaction between doped Ag~+and PEDOT:PSS.Finally,the Ag~+modified OLEDs obtained a luminous efficiency of 76.7 cd/A and 37.1 lm/W.In this thesis,PEDOT:PSS was modified by sodium 3-methylsalicylate,which not only improved the conductivity,but also adjusted the p H of PEDOT:PSS solution,leading to the performance and stability improvement of the flexible PEDOT:PSS electrodes and corresponding OLEDs.Furthermore,the mechanism of weak organic acids inducing phase separation between PEDOT and PSS was proposed based on the systematic study of various cations modified PEDOT:PSS.Additionally,a synergistic effect of weak organic salts with silver on the conductivity of PEDOT:PSS electrodes was found,resulting in a highly conductive PEDOT:PSS electrode.These studies may enlighten the related researches of flexible optoelectronic devices and wearable electronics based on PEDOT:PSS.