Tuning ITO Properties By Self-assembled Monolayers And The Application In OLEDs

Self-assembled monolayers(SAMs)is an important way to improve the performance of organic light-emitting diodes(OLEDs).In order to explore the relationship between the molecular structure of SAMs and the modification effect,this paper analyzes the characteristics of SAMs molecules from the four aspects,that is the head groups,electronegativity groups,conjugate degree and heteroatoms.First,SAMs were formed on ITO using BA,PPA and PMS which has a-COOH,-H₂PO₃ and-Si(OCH₃)₃,respectively.And the modification density of SAMs on ITO was obtained according to the contact angle and XPS.Specifically,the modification density of PPA is the highest,which is 3.5;that of BA is 1.7;and that of PMS is much lower,which is only 0.3.Considering the modification density and the difficulty of synthesis,carboxylic acid is more suitable for exploring the influence of molecular structure on the modification effect.On this basis,ITO was modified by SAMs molecules which has-CF₃,and devices whose structure were ITO/SAMs/NPB(25 nm)/Alq3(60 nm)/Li F(1 nm)/Al(100 nm)were prepared.The luminance,current density and turn-on voltage of the device indicated that the hole injection was significantly enhanced after the introduction of-CF₃.It is worth emphasizing that the turn-on voltage of3FBCA-OLED is only 2.5 V.Five carboxylic acid molecules with different degrees of conjugation were selected to form SAMs on ITO.The contact angle test showed that the surface energy of the modified ITO was reduced to some extent.The UPS test showed that the work function of ITO increased by 0.11 e V?0.30 e V.SAMs-ITO was used for the double-layer green fluorescent device.As the degree of conjugation of the SAMs molecules increases,the turn-on voltage of the device gradually decreased,and the current density at the same voltage increased.The device with CzCA-ITO has the best performance,whose turn-on voltage is only 2.6 V,and the maximum luminance is30418 cd·m^-2.In order to clarify the reason for the excellent modification effect of CzCA,FCA and FoCA with similar structures were selected to explore the effect of heteroatoms on the modificatio.According to Gaussian 09,it is found that the N made the HOMO electron clouds of SAMs molecules to the middle in the molecule,and the delocalized?bond made the HOMO electron clouds of CzCA into a special“through-band”.It is the reason for excellent modification effect of CzCA.