Constructing Novel Electroluminescent Device With High-temperature And High-humidity Resistance Based On Flexible And Transparent Wood

Luminescent electronics,especially alternating current electroluminescent?ACEL?devices,are widely applied in display devices,electronic skins,and other fields owing to the simple structure,adjustable emission wavelength and low power consumption.As the basic support of the devices,the conductive substrate endows flexible ACEL devices with excellent mechanical properties.Traditional electronic devices are usually based on the rigid silicon or glass,which cannot meet the normal work under the conditions of tension,compression,bending,etc.Although the plastic substrate can not solve the problems of high brittleness and impact resistance,electronic devices are easy to deform in high-temperature and high-humidity environment due to the large thermal expansion coefficient?CTE?of plastic substrate.The above disadvantages will greatly shorten the service life of ACEL devices.Therefore,a flexible substrate material with low CTE has been widely concerned.In this study,commercial sliced birch wood was used as the raw material.By removing lignin from the sliced birch wood and impregnating the flexible epoxy resin under vacuum environment,the flexible and transparent wood with the advantages of bending,twisting,high transmittance and small coefficient of thermal expansion was successfully fabricated.By loading the reduced graphene oxide,spin-coating the conductive polymer?PEDOT:PSS?and spraying the silver nanowire,the flexible and conductive substrate was succcessfully prepared.By comparing the conductivity,transmittance and thermal stability,the preparation method of high transmittance and high conductivity was selected from the above three methods.Finally,the ACEL device was fabricated by pressing in the order of flexible conductive transparent wood/ZnS:Cu luminescent layer/flexible conductive transparent wood.Because the ZnS:Cu particles are directly dispersed in the flexible epoxy resin,the high viscosity of the epoxy resin enables a close bonding between the substrate and the luminescent layer.The ACEL device exhibits waterproofing,high-temperature and high-humidity resistance without additional sealing process.The results are as following.?1?The delignified wood was obtained by delignification in NaClO₂ solution for 2.5 h.Scanning electron microscopy?SEM?characterization showed that the vacuum-atmospheric cycles could make the epoxy resin fully filled with the hollow channels under the atmospheric pressure,and improve the optical transmittance of delignified wood.In addition,the natural interconnected porous structure of wood has a good anti-deformation capacity to restrict the volume expansion under thermal conditions.Finally,the flexible and transparent wood with a transmittance of 85%and a CTE of 4.08×10^-66 K^-1 was successfully fabricated.When the flexible and transparent wood was placed on the heating plate?100??for 30 min,the substrate still had the excellent thermal stability and dimensional stability.?2?After 4 min in-situ reduction with 55%hydroiodic acid,the flexible and transparent wood substrate with conductivity of 1.75 S/cm and transmittance of 65%was obtained.After spin-coating modified PEDOT:PSS polymer,the flexible and transparent wood substrate with conductivity of 50 S/cm and transmittance of 71%was obtained.Due to the black color of reduced graphene oxide and PEDOT:PSS polymer,the transmittance of flexible and transparent wood substrate was low.The light emitted by the ACEL device will be blocked and absorbed by the conductive layer,affecting the luminous efficiency of ACEL devices.?3?Silver nanowires were sprayed on the surface of transparent wood substrate by spraying process.When the area density of silver nanowire was controlled at 349 mg/m²,the transmittance was reached to 80.5%,and the sheet resistance was only 10?sq^-1.The physical adhesions of epoxy resin can tightly adhere the silver nanowires on the surface of the transparent wood substrate.The position of the AgNWs do not change during 500 cycles of bending and releasing operations,which ensures the stable electrical conductivity of the whole device.Compared with the former two methods,the flexible conductive transparent wood prepared by spraying silver nanowire has higher transmittance and better conductivity.Finally,the typical“sandwich”structure of the ACEL device was successfully fabricated.When the thickness of the luminescent layer was controlled at 31?m,the luminance approached a maximum value of 155.4 cd/m² at a voltage of 220 V?frequency at 1k Hz?,demonstrating its great potential for applications in signal expression and ambient lighting.CIE coordinates showed that the light emitted by the ACEL device was typical blue light emission;the luminance chroma was pure;and the quality was good.Due to the excellent interface characteristics of the transparent wood substrate and the middle luminescent layer,the ACEL device exhibited excellent stable under high-temperature and high-humidity environment without additional processes to seal.