Research On Paper-based Organic Light-emitting Devices And The Applications In Anti-counterfeiting

In recent years,the shoddy products have caused huge economic losses to the society,so safe and reliable anti-counterfeiting technology is urgently desired by people.Various kinds of anti-counterfeiting technology are emerging,including luminescence anti-counterfeiting technology.We can divide luminescence anti-counterfeiting into photo responsive luminescence anti-counterfeiting,chemical responsive luminescence anti-counterfeiting,mechanical responsive luminescence anti-counterfeiting and heat responsive luminescence anti-counterfeiting according to different excitation sources,among which photo responsive luminescence anti-counterfeiting is the most widely used.At present,electro luminescence technology is rarely applied in luminescence anti-counterfeiting technology.At present,as a relatively mature light-emitting technology,flexible organic light-emitting devices(OLEDs)is very suitable for application in anti-counterfeiting technology.Over the past few decades,flexible OLEDs have been widely used in many fields,including display and lighting.Compared with conventional rigid OLEDs,flexible OLEDs have more flexible application methods and wider application scenarios,and are more suitable for application in anti-counterfeiting technology.In the research on flexible OLEDs,the related study on flexible substrates is an important part.At present,the flexible substrates commonly used in OLEDs include plastic substrates,polymer films or ultra-thin glass,while paper substrates commonly used in daily life are rarely used in flexible OLEDs.Paper substrate has many advantages,such as low cost,abundant raw materials,bendable,foldable and environment-friendly recycling.However,the paper substrate has a rough surface and poor vapor and oxygen barrier properties,which hinders its application in flexible OLEDs.Up to now,there are only a handful of studies on paper-based OLEDs.In this work,we have studied flexible OLEDs based on paper substrates,and applied paper-based OLEDs to anti-counterfeiting technology to prepare a variety of safe and reliable anti-counterfeiting labels.The specific contents mainly include the following three parts:(1)The paper substrate was surface modified by dip-coating method and a high-efficiency paper-based OLED was fabricated based on the modified paper substrate.Compared with other solution methods,the dip-coating method is easy to operate and prepare thicker films.Furthermore it is basically not limited by the area and shape of the substrate.The surface of the paper substrate was treated by dip-coating method,and the surface morphology and flexibility of the paper substrate before and after treatment were characterized.The surface of the paper substrate before treatment was rough and had many holes and sharp peaks,and the surface morphology of the paper substrate after treatment was significantly improved.The surface R_RMS of the stone paper substrate after dip-coating is 19.7 nm.Then we performed bending tests and adhesion tests on the paper-based electrodes.The results show that the paper-based electrode remains stable in resistance after multiple bending and has good adhesion.We design the device structure of paper-based OLEDs and compare various paper substrates.We believe that the top-emitting structure is more suitable for paper-based OLEDs,and stone paper works best among all paper substrates.We compared stone paper-based top-emitting OLEDs with various luminescent colors and glass-based OLEDs with the same structure,and characterized their electrical properties.It can be seen from the results that although paper-based OLEDs are inferior to glass-substrate OLEDs in terms of brightness and efficiency,their values can reach the same order of magnitude as glass-substrate OLEDs.The paper-based OLED in this work has reached the standard of practical application.Taking the paper-based top-emitting green light OLED as an example,its maximum brightness and maximum current are 71346 cd/m² and 64.2 cd/A,respectively,which is the current the best data in paper-based green OLEDs.Then,the paper-based top-emitting green OLED was characterized by its stability,and its operation lifetime and storage lifetime were tested.In fact,the operation lifetime of LT50 can reach 4000 hours when converted to an initial brightness of 100 cd/m²,which has a certain gap compared with 16000 hours of glass-based OLED.In addition,with an initial brightness of 4000 cd/m²,the paper-based OLED can maintain more than 90%of the initial brightness after 10 days of storage.We believe that the operation lifetime and storage lifetime of paper-based OLEDs will be further improved when combined with a more reliable flexible thin-film encapsulation technology.(2)The device structure and spatial distribution were designed,and a paper-based OLED-based optical/electrical multiple anti-counterfeiting label was fabricated.Firstly,we demonstrated that the dip-coating method does not cause damage to the information printed on the surface of the paper substrate,so the anti-counterfeiting label based on paper-based OLED can be combined with the paper outer packaging or instruction manual of the commodity.Then,we design the structure of the anti-counterfeiting label.We prepared three light-emitting units with different structures on the anti-counterfeiting label,which are light-excited light-emitting units,electrical-excited light-emitting units,and optical/electrical dual-excitation units.The light-emitting states exhibited by these three units in the photo excition state,the electrical excitation state and the photo/electric excitation state are different and the differences are obvious.Through the clearly visible light-emitting state difference under different states,the anti-counterfeiting label can achieve effective anti-counterfeiting.Flexible electroluminescence is difficult to imitate,with simple operation and high luminous intensity.Therefore,the anti-counterfeiting label in this work has very good reliability,practicability and identifiability.The anti-counterfeiting label in this work maintains the flammability of the paper substrate.Anti-counterfeiting labels are disposable electronic devices,and their massive applications are bound to bring a lot of electronic waste.Therefore,the environmentally friendly recycling of anti-counterfeiting labels is crucial.Paper-based security labels are flammable and can be recycled by combustion power generation.In addition,due to the good degradability of the paper substrate,the anti-counterfeiting labels can be disposed of in natural landfills after simple processing.Furthermore,the anti-counterfeiting label has good water resistance,and thus can be transported and stored in a relatively humid environment,and will not fail after being exposed to water for a short time.Good water resistance makes the anti-counterfeiting label not increase the cost of product transportation and storage.Lastly,paper-based security labels have very good flexibility.Compared with PET-OLED,the performance of paper-based OLED remains stable after many times of bending,and it can be repeatedly folded for many times.The good flexibility makes the paper-based anti-counterfeiting label have a wider application range and better reliability.(3)The angle-color tunable anti-counterfeiting label and the voltage-color tunable anti-counterfeiting label based on paper-based OLED were prepared by utilizing the angle-dependent discoloration characteristics of top-emitting OLEDs and the voltage-dependent exciton recombination region movement characteristics of double-emitting layer OLEDs.We design the electroluminescent part of the anti-counterfeiting label to increase the reliability of the anti-counterfeiting label.The OLED we use in the paper-based anti-counterfeiting label is a top-emission structure,which has the characteristic of changing the luminous color with the change of angle.This characteristic is disadvantageous for lighting and display applications.But in the anti-counterfeiting label,we take advantage of this characteristic and enlarge it through the design of the device structure to prepare the angle-color tunable anti-counterfeiting label based on the paper-based OLED.In order to increase the magnitude of the color change of OLEDs with increasing angle,we adopt a double-emitting layer structure and shorten the microcavity length of the device.The angle-color adjustable anti-counterfeiting label can have two luminous areas with different luminous colors,namely the background area and the logo area.As the angle increases,the luminescence spectra of both regions appear red shifted.This is in contrast to the blue shift of conventional top-emitting OLEDs.This special color change feature further improves the reliability and technical threshold of anti-counterfeiting labels.In addition,the exciton recombination region in the OLED gradually moves with the change of voltage.In double-emitting layer OLEDs,the color of their emission changes with voltage.We amplify this change and fabricate a paper-based OLED-based voltage-color tunable security label.By replacing the host of the light-emitting layer with a bipolar host,and inserting an energy transfer blocking layer and an electron blocking layer in the emitting layer,we prepared a voltage-color tunable security label.As the voltage was increased,the lable's emission color changed from red to white,and finally become blue at high voltage.The maximum value(?x,?y)of the color coordinate change is(0.309,0.154),and the change is clearly visible to the naked eye.By designing the electroluminescent part of the anti-counterfeiting label,we prepared two kinds of color-tunable anti-counterfeiting labels.Through structural design and spatial distribution design,these two anti-counterfeiting labels can be combined with optical/electrical multiple anti-counterfeiting labels to prepare anti-counterfeiting labels with more multiple anti-counterfeiting methods,which can improve the reliability of anti-counterfeiting label without losing practicability and identifiability.