Preparation Of Parylene C/Al₂O₃ Composite Encapsulation Films With High Adhesion By Atomic Layer Deposition

In recent years,with the continuous development of Internet of Things technology,optoelectronic devices,as key devices for Internet,optical communication,optical display and photovoltaic application systems,have received widespread attention due to their important role in people’s daily lives.Stimulated by the growing demand for high-performance and long-life optoelectronic devices,great breakthroughs have been made in their manufacturing technology and output.Among them,Organic light emitting diodes（OLEDs）are the first choice for the display industry due to their thin film,light weight,fast response time,high contrast ratio,wide viewing angle and low power consumption.Flexible OLEDs are considered to be the ultimate display for future wearable devices and foldable electronics due to their flexibility and mechanical durability.However,due to the poor chemical stability of organic materials,they are prone to degradation in the presence of water and oxygen leading to device failure.Thin-film encapsulation techniques have been developed to improve the long-term stability of devices by growing an encapsulation layer before and after the device is prepared to block out water and oxygen from the environment.parylene C,a hydrophobic polymer,is an ideal waterproof insulating material due to its good chemical inertness,high dielectric strength,low water vapour permeability and excellent biocompatibility and is used in a wide range of applications including semiconductor technology.However,due to weak electrostatic interactions,the interaction between the parylene C coating and other materials is limited to hydrogen bonding and van der Waals forces,which makes its adhesion poor and very prone to delamination and encapsulation failure.At the same time,parylene C has a limited barrier capacity to meet the barrier performance requirements of organic optoelectronic devices which are highly sensitive to water and oxygen.Coupling agent pretreatment is currently widely used to improve adhesion,but the complexity and potential impact on device performance that this approach entails,as well as the micro-toxicity of the coupling agent,significantly limits the potential use of parylene C for future applications in skin electronics,wearable optoelectronic devices and photodynamic therapy.There is therefore an urgent need to develop a method to improve adhesion performance without the need for coupling agent pretreatment.In this paper we have not used a coupling agent and have improved the adhesion of parylene C to the substrate by depositing a thin film of Al₂O₃ with residual tensile stress based on atomic layer deposition（ALD）.The incorporation of ALD-Al₂O₃ films not only inhibited the delamination of parylene C,but also improved the barrier properties.The adhesion of parylene C,parylene C/Al₂O₃ and coupling agent pretreated parylene C was first investigated by the pull-apart method.It is able to effectively increase the adhesion strength by a factor of 2.4 compared to pretreatment by a coupling agent.Based on FTIR spectroscopy,the chemical bond composition of parylene C and the permeation of ALD-Al₂O₃ in parylene C were analysed.During the preparation and application of thin films,residual stresses play an important role in their mechanical properties.The evaluation of residual stresses in films is therefore essential for assessing the adhesion properties of films.The tensile stresses between parylene C/Al₂O₃ composite films and the substrate were systematically investigated using the wafer curvature method and the average residual stresses in the films were calculated according to the Stoney formula.The water vapour transmission rate（WVTR）of parylene C was measured by electrical calcium testing at a bending radius of 3mm for10,000 bends,and the WVTR remained at 3.55×10^-4 g·m^-1·day^-1 after bending.The parylene C/Al₂O₃ composite films were applied to the encapsulation of the flexible OLEDs.The encapsulation was tested and found to have no negative impact on the device performance and the device maintained 56.4%of its initial brightness after 300hours of storage.In this study,we have prepared parylene C/Al₂O₃ composite films with good adhesion,high barrier properties,low surface roughness and high light transmission,which are expected to achieve a wider range of applications in the field of flexible electronics.