Self-assembled Monolayer-modified ITO For Efficient Organic Light-emitting Diodes On Interfacial And Electroluminescent Properties

Organic light-emitting diodes?OLEDs?have attracted considerable attention as their applications in character display,solid state lighting,flexible display,and so on.Interfacial engineering between the ITO electrode and the overlying organic layers is an important process to obtain the high performance of the diode devices.This paper aimed at the study on the effect of different types of self-assembled molecules on the properties of ITO surface,and characterized the photoelectric properties by preparing OLED devices.Through the use of organosilane self-assembled molecules with different chain lengths,the effect of perfluorosilane self-assembled monolayers on the properties of ITO surface was studied systematically.The contact angle,AFM,XPS,UV-vis and PES tests were used to characterize the properties of ITO surface.The modification of the organosilane molecular have effect on the trend of surface water contact angle and work function was HF₁₇DES-ITO>HF₁₃DES-ITO>TFPMS-ITO.The contact angles of TFPMS-ITO,HF₁₃DES-ITO and HF₁₇DES-ITO increased to 79.67°,89.40°and108.9°,respectively,and the work functions increased to 5.01 eV,5.14 eV and 5.51 eV,respectively.The experimental results also show that the performance of device is significantly improved after the modification of the organosilane molecular.And the performance of device based on the TFPMS-ITO is optimal,the maximum current efficiency is 5.0 cd/A,and the maximum brightness is 28291.50 cd/m².Herein,self-assembling monolayers?SAMs?based on aromatic phosphonic acid?F₁BPA and F₅BPA?and aliphatic phosphonic acid(HF₁₇DPA and HF₂₁DPA)were formed on ITO surfaces.The work function of ITO was elevated from 4.82 eV to 5.00eV,5.20 eV,5.54 eV and 5.81 eV by introduction of F₅BPA and HF₂₁DPA,respectively,which could facilitate hole transport into the device,and therefore improve charge balance in OLEDs.In addition,the water contact angles of ITO also increased from 46.45°to 85.33°,87.09°?109.7°and 110.6°,respectively,which could contribute to better-matched interfacial surface energy,and prevent the interface incompatibility.The surface properties of ITO after modification were also studied by using X-ray photoelectron spectroscopy,atomic force microscope and UV-visible spectra measurements.Besides,compared with the OLEDs using bare ITO anodes,the aliphatic SAM-modified devices showed improved brightness?29245.49 cd/m²?,higher luminous efficiency?6.09 cd/A?,and smaller turn-on voltage?2.8 V?.The aromatic SAM-modified devices showed improved brightness?27251.86 cd/m²?,higher luminous efficiency?3.62 cd/A?,and smaller turn-on voltage?3.0 V?.With mixed monolayers prepared from DPA and HF₂₁DPA in particular?1:0,2:1,1:1,1:2,0:1?,the work function of ITO can be tuned over a range from 5.10 to 5.81 eV,depending on the monolayer composition.For a device with the structure of ITO/Mixed-SAM/NPB?25 nm?/Alq₃?60 nm?/LiF?1 nm?/Al?100 nm?,when the composition ratio of DPA/HF₂₁DPA is 1:2,the performance of the device is the best.When 25 nm TCTA was used as the hole-transporting layer rather than NPB,the single component HF₂₁DPA-ITO is used as the anode,and the device performs best.