Polyimide Substrates For Large-area Transparent Flexible Optoelectronic Devices

Along with the deepening of research on materials and devices structures,and the continuous development of preparation technologies,flexibility and transparency have become the goal for the development of organic optoelectronic devices,which was represented by OLED.In this case,the preparation of high performance transparent flexible substrate has been highlighted as the first barrier to surmount for organic optoelectronic devices to achieve flexibility and transparency.Transparent polymer films were considered to be the preferred choice for transparent flexible substrate owning to their tremendous advantages such as light weight,good bending performance,rich species,and compatibilities with the roll-to-roll process that widely used in mass productions.In addition to excellent optical performances,stable thermal properties and inertness to organic solvents were also required for transparent flexible substrates.Taking all these into account,polyimide films,known as 'the golden films',were considered to be the best choice for transparent flexible substrates.In order to achieve sufficient lifetime to meet the needs of commercialization,high demands of barrier properties were put forward by organic optoelectronic devices.Unfortunately,poor barrier properties were often witnessed on polyimide films due to their inherent porous surfaces and hygroscopicity.To solve this problem,flexible barrier layers with high performances must be deposited on the transparent polyimide substrates.Moreover,in order to avoid the influences on efficiencies and emitting spectra of devices,decent optical properties including high transmittance and good color neutrality,which could be achieved by reasonable barrier layer structure designed with the idea of broadband anti-reflective film,were among the requirements for barrier layers.Since ITO,which was widely used in the conventional optoelectronic devices and has become de facto standard,was brittle and not suitable for flexible devices,the development of novel transparent flexible electrode was the third barrier to surmount for organic optoelectronic devices to acheive flexibility and transparency.Metal meshes(Au,Ag,or Cu)were usually prepared by pattern processes,such as lithography,evaporation with shadow mask,and nanoimprinting,prior to the deposition of transparent conducting films to improve their conductivities.The part of the substrate covered by metal mesh was.opaque,a balances between the optical properties and conductivities of the composite electrodes could be found by tuning the coverage of metal meshes.However,like every coin has two sides,the visible meshes and inhomogeneity when the device was turned on would be one of the side effects,which was unacceptable in some certain practical applications.To overcome this problem,light scattering of nanoparticles doped in polyimide substates could be utilized to improve the haze of substrates.By choosing a proper doping condition,transparent flexible polyimide substrates with both high transmittance and high haze could be obtained.Focused on the preparation of large-area transparent flexible substrate,this thesis was organized with the following four aspects:1)Transparent polyimide films with different thicknesses made from polyimide resin and polyamic acid were prepared by spin coating.Further characterizations of surface properties and optical properties were conducted.It was found that films with desired thicknesses and smooth surfaces could be prepared by controlling the viscosities of solutions and spin speeds in spin coating.These two kinds of polyimide used here were negative birefringent materials.When they were prepared with thin thicknesses,there would be oscillations in transmittance spectra of the films,which were originated from interferences.Specific to these two kinds of polyimide used in this thesis,the range of available film thicknesses were given in thesis to avoid the influences of the oscillated transmittance spectra on the efficiencies and spectra of the light-emitting devices.2)In this thesis,the idea of broadband anti-reflective films from thin film optics was introduced into the design of barrier layers.Two four-layer alternating barrier layer structures consisted of SiNx and SiOx,which adapt to two kinds of polyimide,were designed and optimized with the TFcalc software under the compatible consideration of barrier properties,bending performances,optical properties and cost.Furthermore,these two barrier structures were experimentally deposited on two kinds of polyimide films,respectively.It was found that the transmittance of the composite films were remarkably improved in the whole desired band in addition to the improvements of barrier properties.The composite polyimide films exhibited smooth surfaces,high surface energies,and high pencil hardness,indicating a successful preparation of high performance barrier layers.3)Hexagon Ni/Au metal meshes with different sizes were prepared on the composite polyimide films with the combination of UV lithography,E-beam evaporation and lift-off process.The hexagon Ni/Au metal meshes showed excellent performances in electrical conductivity,light transmittance,and bending performance.What's more,the hexagon Ni/Au metal meshes would bring about a significant decrease of the sheet resistances of conducting films when they were buried in the conducting films,revealing their great potential in the applications of large-area transparent flexible conducting substrates and touch panels.4)Different inorganic nanoparticles were doped in the above two transparent polyimide films with the aim of increasing the haze of the substrates to hide the subsequent deposited metal mesh.The influences of relative refravtive indices between nanoparticles and polyimide films,nanoparticle sizes,and doping concentrations on the transmittance and haze of the polyimide films were analysed in detail.It was also found that the doped films with both high transmittance and high haze could be obtained under proper doping condition.This work paved a way to prepare polyimide substrates with desired transmittance and haze by tuning doping condition.