The Interface Study In Solution-processed Organic Light-emitting Diodes

Solution-processed organic light-emitting diodes(OLEDs)shows great advantages and application potential in fabricating the cheap,large area,and flexible displays and lighting products.However,the performance of solution-processed OLEDs is inferior to that of device based on vacuum evaporation technology.It is a huge challenge for wet process to construct the multilayer device structure like evaporated OLEDs.The common organic materials in OLEDs own similar solubility in organic solvents and so the underlayers are always be redissolved during the deposition of upper layers.Furthermore,PEDOT:PSS is the universal hole injection materials in solution-processed OLEDs,but it is known to have many drawbacks.To obtain the multilayer solution-processed OLEDs and aviod the negative effect of PEDOT:PSS on performance of devices,many researchers put forward strategies to carry out the interface engineering in solution-processed OLEDs.But these interface engineering often need to deal with the long time high temperature,high energy ray irradiation,complex chemical structure modification and sometimes solvent screening process,virtually increasing the complexity of the preparation process.In this paper,on the one hand,we chose the proper wide bandgap electroactive OLEDs materials as the monomer,and deposit in situ cross-linked interlayers on PEDOT:PSS by means of electrochemically oxidative coupling methode.The multilayer solution-processed OLEDs devices were fabricated based on the cross-linked interlayers,and the cross-linked interlayers significantly improved the optical and electrical performance of devices.On the other hand,to avoid using PEDOT:PSS,we prepared inverted polymer light-emitting diodes(PLEDs)based on a PBI-H-doped ZnO cathode interlayer,and obtained a stable inverted PLEDs device.First,a bipolar blue material functionalized by peripheral four carbazoles,TCPC,was selected as monomer,and we deposit in situ a cross-linked interlayer ECP-TCPC with smooth surface topography,controlled thickness and good solvent resistance on PEDOT:PSS.Then,a bilayer PLEDs device employing typical P-type P-PPV as emitter was fabricated based on ECP-TCPC interlayer.The insertion of ECP-TCPC effectively blocked the superfluous hole carriers injected from anode,reducing the hole leakage current in PLEDs.The performance of PLEDs device was significantly enhanced resulting from the more balanced hole and electron current,and when a ECP-TCPC interlayer of 12 nm was inserted between PEDOT:PSS and P-PPV,the LEmax was enhanced from 12.3 cd A-1 to 16.2 cd A-1,giving an increase of 32%?Second,we chose a four carbazoles functionalized phosphorescent host materials SimCP2 as the monomer,and deposit in situ a cross-linked interlayer ECP-SimCP2 on PEDOT:PSS.Then,a multilayer solution-processed green PhOLEDs was constructed,utilizing the bipolar hyperbranched electrophosphorescent polymer HBIr as the emitter and TmPyPb as the electron transport layer.The insertion of ECP-SimCP2 interlayer obviously avoided the quenching effect of PEDOT:PSS on the radiative decay process of long-life triplet excitons in emissive layer,enhancing markedly the PL intensity and PLQYs of HBIr.When a ECP-SimCP2 interlayer of 18 nm was inserted between PEDOT:PSS and HBIr,LEmax was enhanced from 24.4 cd A-1 to 32.5 cd A-1,and EQEmax was enhanced from 9.0% to 12%.Third,a novel six carbazoles functionalized phosphorescent host materials SimCP3-Ph was selected as the monomer,and we deposit in situ a cross-linked interlayer ECP-SimCP3-Ph with smooth surface topography,controlled thickness and good solvent resistance on PEDOT:PSS.Then,solution-processed multilayer blue and white PhOLEDs were fabricated,employing 26DCzPPy:10wt% Firpic and 26DCzPPy:10wt% FIrpic:0.2wt% PO-01 as the emissive layer,respectively.The J-V characteristics of PhOLEDs and hole only devices demonstrated that ECP-SimCP3-Ph effectively reduced the hole injection barrier between PEDOT:PSS and 26 DczPPy and the turn-on voltage of blue and white PhOLEDs,enhancing the power efficiency of devices.When a ECP-SimCP3-Ph interlayer of 15 nm was inserted between PEDOT:PSS and emissive layer,the turn-on voltage of blue PhOLEDs was reduced from 4.4 V to 3.9 V,LEmax was enhanced from 13.8 cd A-1 to 17.8 cd A-1 and PEmax was enhanced from 7.5 lm W-1 to 10.5 lm W-1;the turn-on voltage of white PhOLEDs was reduced from 4.6 V to 4.1 V,LEmax,PEmax,and EQEmax was enhanced from 19.2 cd A-1,9.4 lm W-1 and 8.8% to 28.1 cd A-1,14.8 lm W-1 and 12.9%,respectively.Fourth,we prepared inverted PLEDs devices based on red dyes PBI-H-doped ZnO,ZnO and ZnO/PFN cathode interlayers,respectively,using P-PPV as the emissive layer.The J-V characteristics of electron only device based on ZnO:PBI-H,ZnO and ZnO/PFN in dark and under illumination demonstrated that ZnO was N-doped by excited state of PBI-H,induced by electroluminescence of P-PPV,enabling ZnO:PBI-H a good electron injection cathode interlayer.The inverted PLEDs devices based on ZnO:PBI-H cathode interlayer achieved a high LEmax of 15.4 cd A-1 and a low turn-on voltage of 3.1 V.