Preparation Of Transparent Shape Memory PI Electrode And Its Application In OLED Devices

Compared with traditional electronic devices,flexible electronic devices(FEDs)have the dream characteristics of ultra-thin,transparent,lightweight,portable,flexible and bendable.With the development of flexible electronic technology,people are committed to the development of more diversified FEDs.The combination of shape memory polymers(SMPs)and flexible electronic technology enables FEDs to have the characteristics of variable stiffness and active deformation,which greatly enriches the functionality of FEDs,and also broadens the application fields of SMPs.In this paper,a shape memory polyimide with high transparency and high heat resistance(TSMPI)was developed.Two kinds of flexible transparent electrodes(FTEs),embedded bilayer metal mesh and CVD monolayer graphene,were fabricated using TSMPI as substrate.On this basis,a smart white OLED with active deformation,variable stiffness and shape reconstruction characteristics is obtained.Taking polyimide with high heat resistance as the research object,flexible dianhydride BPADA and fluorine-containing diamine TFDB are selected to synthesize TSMPI.The bridging ether bond contained in BPADA makes the molecular chain have a larger curved conformation,which is easier to twist and entangle each other,forming more physical cross-linking point,ensuring that TSMPI has excellent shape memory performance.The-CF3 group in TFDB has a strong electron withdrawing effect,which reduces the electron donating ability of diamines,increases the free volume of molecules,and can effectively inhibit the formation of charge transfer complexes,which plays a major role in improving the optical transmittance of TSMPI.The effects of molar ratio and thermal imidization temperature on properties were studied,the synthesis was optimized and the related mechanism was explained.PI270 with a molar ratio of 0.95 and a maximum thermal imidization temperature of 270°C has the best overall performance.The number average molecular weight of PI270 is 19.41 kg/mol,the imidization degree is 97%;the transmittance at 450 and 550 nm is 87%and 90%,respectively,which meets the requirements of optoelectronic devices on the light transmittance of substrate.The glass transition temperature of PI270 is 234°C,and the initial thermal decomposition temperature is 520°C,its heat resistance is higher than other reported transparent SMPs.PI270 has good mechanical properties,its tensile strength is 129 MPa,and its elongation at break is 7.4%.The shape fixation rate and shape recovery rate are 98%and 97%,respectively,PI270 has excellent shape memory performance.A flexible transparent metal grid electrode with embedded bilayer metal(Au/Ag)was fabricated by using TSMPI as flexible substrate(BMG4@TSMPI).The average sheet resistance of BMG4@TSMPI is 5.2Ω/sq,and the transmittance at 550 nm is85%.It has ultra-low surface roughness(Ra=2.7 nm),which meets the harsh requirements of OLED and other optoelectronic devices for surface roughness.Because of the strong adhesion between the substrate,it is difficult to destroy the integrity of the metal mesh by conventional physical damage.After 2000 times of continuous bending,its resistance did not change,indicating that it has excellent bending fatigue resistance.The smart white OLED prepared with BMG4@TSMPI as the anode has a starting voltage of 5.2 V,a maximum brightness of 6200 cd/m~2,and a maximum current efficiency of 4.3 cd/A.The shape memory effect of TSMPI endows flexible devices with active deformation and variable stiffness,which can be switched between"soft and hard",thereby realizing its reversible shape reconstruction.The flexible device can transform from a two-dimensional planar structure to a complex three-dimensional curved structure,and can maintain this stable temporary shape for a long time without external support.The reconstructed temporary shape can undergo shape recovery under external thermal stimulation and re-transform into the original two-dimensional planar shape.A flexible transparent electric heater(Alm@TSMPI)based on TSMPI substrate and embedded Al mesh was prepared.The response time is only 20 s,the highest steady-state temperature is as high as 235°C,and its electric heating performance is far superior to other reported flexible transparent electric heater.The conductive phase constructed with the embedded Al mesh can achieve the ultra-fast electrical response of Alm@TSMPI while keeping the light transmittance of the matrix unchanged.The electro-induced recovery time of Alm@TSMPI is only 13 s;its transition temperature is 230°C,which is the highest in the reported electro-response SMPs.Alm@TSMPI is an ultra-thin,transparent and flexible electric heating film,which can be closely adhered to any SMPs substrate.In theory,it can realize the electrical response of all SMPs whose transition temperature is lower than 230°C.Taking transparent shape memory styrene as a typical representative,it demonstrated the application of Alm@TSMPI as a flexible electric heater in other electro-responsive SMPs.The electrochemical polishing process was used to polish the surface of the copper foil.The polishing process parameters were optimized by analyzing the surface morphology of the polished copper foil.The working voltage of 4.5 V and the polishing time of 180 s are the best polishing conditions.Electrochemical polishing can greatly improve the surface flatness of copper foil,reducing its surface roughness Ra from 143 nm to 22 nm.With polished Cu foil as the growth substrate,the Cu-based single-layer graphene prepared by the CVD method has no obvious defect D peak,and the I2D/IG in Raman spectrum is as high as 2.3,and the quality is better than the graphene on the unpolished Cu foil.The graphene was transferred from copper foil to the surface of TSMPI substrate(p G@TSMPI)by a new transfer method:wet-stripping process.In this method,the target substrate is used to replace supporting layers such as PMMA,which avoids secondary pollution of graphene,and obtains graphene with an ultra-clean surface.The transferred p G@TSMPI replicated the micro morphology of the polished copper foil,with a surface roughness of 18 nm,a light transmittance at 550 nm of 86%,and an average sheet resistance value of 310±18Ω/sq.The flexible white OLED prepared with p G@TSMPI as transparent anode has a starting voltage of 5.8 V,a maximum brightness of 4790 cd/m~2,and a maximum current lumen efficiency of 4.9 cd/A.Relying on the shape memory effect of the TSMPI substrate,the graphene-based device also obtains active deformation,variable stiffness,and reversible shape reconstruction characteristics.