Cellulose-based flexible organic light-emitting diodes with enhanced stability and external quantum efficiency† Qinghong Zheng, * abc Huixin Li, a Yiling Zheng, a Yinan Li, a Xi Liu, a Shuangxi Nie, b Xinhua Ouyang, * a Lihui Chen a and Yonghao Ni ad A cellulose-based flexible organic light-emitting diode (OLED) with enhanced stability and light extraction efficiency was prepared by using a PEDOT:PSS PH1000 transparent conductive electrode coated on regenerated cellulose film (RCF). The PH1000/RCF transparent conductive electrode exhibits extra-high bending stability as compared to the traditional ITO/RCF electrode, which can be attributed to the highly flexible nature of PH1000 and the strong interaction between PH1000 and cellulose. Finite-difference-time-domain simulations show that, due to the smaller refractive index mismatch between PH1000 and RCF, a small proportion of photons are confined in the optical waveguide. The external quantum efficiency of the PH1000/RCF OLED (3.65%) is enhanced by nearly 36.2%, as compared to the ITO/RCF OLED device (2.68%). The ITO-free cellulose-based flexible OLED with enhanced stability and external quantum efficiency would be attractive in a wide range of applications, such as flexible displays, wearable devices and so on. 1. Introduction Organic light-emitting diodes (OLEDs) have attracted wide attention in light sources and display screens in recent years because of their unique characteristics, such as light weight, energy saving, low cost, large viewing angle, high brightness, high contrast and high optical transparency. 1–5 With the concept of flexible products, OLEDs have gained more and more attention in flexible displays and wearable devices. 6–10 Up to now, flexible devices are mainly based on plastic polymers, 7,10,11 such as polyethylene terephthalate. However, they are not easily degraded after use, which may lead to environmental problems, such as white pollution. 12 Cellulose has many advantages, such as environmental compatibility, remarkable optical properties, non-toxicity, and thermal stability, making it a promising natural material. 13,14 The surface of cellulose is rich in hydroxyl groups, which makes it highly reactive and easy to prepare functional materials on it by various methods such as printing and sputtering. 15 The porosity, 16 electrical conductivity, 17 and hydrophilic/hydrophobic balance 18 of the cellulose materials can be tailored according to the device fabrication requirements. Moreover, the fabrication of a cellulose-based electronic device is compatible with roll-to-roll technology, making it ideal for mass production of electronic devices. Cellulose materials have been widely used in various types of electronic devices, such as solid batteries, 15 supercapacitors, 19 transistors, 20 fuel cells, 21 solar cells, 22 optical sensors, 23,24 and so on. Recently, efforts have been devoted to developing cellulose-based optoelectronic devices. 25–27 A common method is to deposit a layer of metal oxide, such as ITO, on the cellulose to form a transparent electrode for the optoelectronic devices. 15,26,28–30 However, ITO is an inor- ganic rigid thin film, which has poor compatibility with organic substrates and luminescent materials. Owing to brittleness 31–35 and without the support of the rigid substrate, the metal oxide film tends to crack during the bending process. Moreover, there is no strong interaction between the ITO film and the cellulose substrate. Therefore, the thermal expansion and contraction caused by the current may cause the ITO film to separate from the substrate, and the performance of the device is unstable. In addition, the refractive index (RI) mismatch between ITO (n E 2.0) 4,36,37 and cellulose (n E 1.55) 38,39 is very large, Cite this: DOI: 10.1039/d1tc00019e a College of Materials Engineering, Fujian Agriculture and Forestry University, Fuzhou 350002, People’s Republic of China. E-mail: fafuzqh@163.com, ouyangxh@fafu.edu.cn b Guangxi Key Laboratory of Clean Pulp & Papermaking and Pollution Control, School of Light Industry and Food Engineering, Guangxi University, Nanning 530004, People’s Republic of China c National Forestry & Grassland Administration Key Laboratory for Plant Fiber Functional Materials, Fuzhou 350002, People’s Republic of China d Department of Chemical Engineering, University of New Brunswick, Fredericton, New Brunswick E3B 5A3, Canada † Electronic supplementary information (ESI) available. See DOI: 10.1039/ d1tc00019e Received 3rd January 2021, Accepted 17th February 2021 DOI: 10.1039/d1tc00019e rsc.li/materials-c This journal is The Royal Society of Chemistry 2021 J. Mater. Chem. C Journal of Materials Chemistry C PAPER Published on 18 February 2021. Downloaded by Guangxi University on 4/6/2021 3:19:29 AM. View Article Online View Journal