Preparation And Properties Of Flexible Stretchable Organic Light-Emitting Diodes

Organic light-emitting devices（OLEDs）have gained increasing attention due to their inherent advantages,including self-emission,low cost,wide viewing angles,low power consumption,comprehensive color gamut,and fast response time.They are regarded as the optimal candidates for future display and lighting technologies.Notably,flexible OLEDs exhibit excellent flexibility,lightweight construction,slim profile,and wearable features,offering vast prospects in areas such as foldable devices and wearable electronics.Nevertheless,opportunities and challenges go hand in hand.When compared to conventional rigid devices,flexible OLEDs still face limitations in terms of device brightness,current efficiency,and stability,which hinder their practical applicability.This paper aims to enhance the efficiency and stability of flexible/stretchable organic electroluminescent devices through the development of high-performance flexible/stretchable transparent electrodes,integration of non-periodic micro/nanostructures into elastic substrates,and control of the conformation of luminescent polymers.The research has yielded several innovative outcomes,and the main areas of investigation are outlined below:1.Conductive polymers have limited their application in the field of optoelectronic devices due to their low conductivity.Large-area,high-performance flexible transparent electrodes have been realized through Mayer-rod printing.Tunable control of PEDOT main chain orientation,local ordering,and phase separation is demonstrated via control of the Mayer-rod printing process parameters.The highest conductivity of the transparent electrode is approximately 2200 S/cm,achieving an optimal sheet resistance of around 40Ω/sq with a transmittance of approximately 86%.The electrode exhibits excellent mechanical flexibility,with minimal changes observed in both sheet resistance and morphology compared to the initial electrode after undergoing 5000 cycles of bending tests.Based on the flexible transparent electrode,a flexible green OLEDs was constructed.The device achieved a maximum current efficiency of 69.6 cd/A.Importantly,the flexible green OLEDs also demonstrated excellent mechanical flexibility and durability.Even after bending 2000 times with a bending radius of approximately 5 mm,the device retained approximately 82%of its original brightness.2.At present,the performance of flexible/stretchable OLEDs is still limited by the photoelectric performance,flexibility,and mechanical stability of flexible transparent electrodes.A printed embedded metal composite electrodes（PEMCE）strategy is presented,which provided a new method for realizing flexible/stretchable OLEDs with high performance and stability.The resulting ultra-flexible TCEs achieve a low resistance of 7Ω/sq with 85%transmittance and a low root-mean-square（RMS）roughness of 1.22 nm,outperforming the performance of commercial ITO/PET electrodes manufactured through sputtering.Additionally,the ultra-flexible transparent composite electrodes（TCEs）demonstrate outstanding mechanical flexibility and stability,enduring 50,000 mechanical flexures with a curvature of 3 mm.White OLEDs based on the ultra-flexible TCEs have been constructed,achieving a maximum luminance of 34787 cd/m² and a current efficiency of 77.4 cd/A,which are the best results among the reported flexible/stretchable white OLEDs.Moreover,this CE_maxis about 1.2 times higher than that of control rigid devices fabricated from ITO/glass substrates.The enhancement of the current efficiency is attributed to the light extraction effect of the silver（Ag）mesh and the excellent optoelectronic properties of the electrodes.The flexible/stretchable white OLEDs maintained～83%of the original brightness after bending for 2000 cycles at a radii of 0.5mm.The device demonstrates an impressive capability to withstand approximately 100%tensile strain.Remarkably,even under a challenging 90%tensile strain,there is no noteworthy decline in the optoelectronic properties of the device.The current efficiency and luminance remain consistently at85%or higher,highlighting the device’s robust performance under extreme stretching conditions.3.Due to the difference in refractive index between materials in OLEDs,the optical coupling output efficiency is only 20%,and its brightness and efficiency are much lower than those of rigid devices.Through the microstructure morphology design of the elastic substrate surface,the preparation of high efficiency and high brightness stretchable OLEDs is realized,and suppress the problem of the angle dependence of the device.Through the combination of experiment and theory,the reason why the non-periodic micro/nano-structures improves the device performance is clarified,which provides a new idea for the design of stretchable optoelectronic devices with high performance and high stability.Flexible white OLEDs with non-periodic micro/nano-structures achieve luminance of over 60,000 cd/m² and current efficiency of up to 127.4 cd/A.More importantly,the intrinsically stretchable OLEDs with non-periodic micro/nano-structures achieved an efficiency of 15.7 cd/A,demonstrating an improvement of approximately 50%compared to devices utilizing glass or planar substrates.Eventually,the intrinsically stretchable OLEDs maintained a good luminance state even when subjected to a 40%strain.4.Typically,an improvement in the stretchability of luminescent polymers is accompanied by a decrease in the charge transport ability,thus resulting in a significant decrease in device efficiency.We propose to enhance the stretchability and charge transport ability by doping intrinsically stretchable electroluminescent elastomers with self-confinement effect into luminescent polymers to form polymer interpenetrating networks.The mechanism of enhancing the stretchability and charge transport ability of luminescent polymers by intrinsically stretchable electroluminescent elastomer with self-confined effect was elucidated.The self-confinement effect between the rigid segments in the intrinsically stretchable electroluminescent elastomer ensures good charge transport,while the flexible segments enhance the stretchability of the films.Furthermore,using a stretchable hole transport layer doped with sliver bis（trifluoromethane sulfonimide）（Ag TFSI）and an integrated non-periodic micro/nano-structure substrate,we realized an intrinsically OLEDs with a turn-on voltage of3.2 V and a high CE of 18.7 cd/A.This provides a new idea for the development of high performance intrinsically stretchable OLEDs,and also has an important reference significance for the manufacturing of high efficiency stretchable photovoltaic,stretchable photodetector and other photoelectric devices.