Research On Performance Regulation And Anticounterfeiting Application Of Color-tunable Organic Light-emitting Devices

Counterfeit activities have become an increasingly serious global problem.The development of high-security multilevel luminescent anticounterfeit technology to resist counterfeit activities is of great significance to economic development,public health,and many other aspects.At present,the multilevel luminescent anticounterfeiting technology relies on excitation light with different wavelengths or a combination of excitation light,heat,or chemical reagents,which makes them complex and not easily accessible to the public.These restrictions hold back development of multilevel luminescent anticounterfeiting technology.In contrast,popularity of mobile power supplies and standardization of interfaces provide accessible electrical energy,thereby offering convenience for the implementation of multilevel luminescent anticounterfeit labels based on electronic excitation.In addition,electroluminescent devices exhibit high luminous intensity,which are not easily affected by external light sources.Their production process requires specific materials,preparation processes and equipment,which significantly raise the threshold for counterfeiting.Therefore,it is necessary to develop an electroluminescent anticounterfeiting label with the advantages of easy verification,high luminous intensity and difficult to replicate.Due to their disruptive properties such as self-luminescence,energy-efficient,and flexibility,organic light-emitting device(OLED)have become a cutting-edge technology for high-quality flat panel displays and solid-state lighting in many electroluminescent devices.Among them,color-tunable OLED can achieve multiple variations in brightness and color by adjusting voltage,which is consistent with the characteristics of multilevel luminescent anticounterfeiting.However,employing color-tunable OLED for anticounterfeiting applications is hinder by the complex material and structural design.In addition,it is necessary to achieve larger color variations with smaller voltage changes to facilitate recognition and authentication.In response to the above two questions,we manipulate excitons within the emitting layer to achieve variations in relative emission intensities of host and guest in this paper.A single emitting layer color-tunable OLEDs with host-guest doping system is developed and physical mechanisms underlying the color tunability is analyzed.On this basis,a hybrid luminescent anticounterfeiting label with high security is constructed through a color-tunable OLED selectively pumping photoluminescent layer.The research includes the following three parts:(1)To address the complex structure and high cost associated with color-tunable OLEDs,we propose a strategy employing a single light-emitting thin layer with fluorescence host-guest doping.This approach enables regulation of exciton distribution in the host-guest/transport layer by utilizing carrier trap effect of guest and insufficient energy transfer between host and guest.Firstly,we investigated the influence of overlap area between host emission spectrum and guest absorption spectrum on spectral variations.The results indicate that reducing overlap area between host emission spectrum and the guest absorption spectrum can promote intensity of host emission and enable observation of voltage-dependent electroluminescence spectra.Additionally,we demonstrate that reducing the thickness of emitting layer can achieve greater changes in the relative intensity of host emission,leading to more significant spectral variations.This is because the decrease in emitting layer increases the number of excitons per unit volume,making it easier for the guest material to reach saturation and thus enhancing the utilization of excitons by the host.When the emitting layer thickness is reduced to 2 nm,the device also exhibits orange-red emission from the exciplexes under high voltage.This is attributing to the additional TCTA/Tm Py PB interfaces due to island growth mode of vacuum deposition.By tuning the green emission of guest molecule C545T,the blue emission of host molecule CBP,and the orange-red emission of exciplex,an all-fluorescent color-tunable OLED present voltage-dependent color tunability.Its CIE 1931 coordinates shift from(0.26,0.61)at4 V to(0.31,0.54)at 8 V,achieving a color coordinate variation of(0.05,0.07)within the 4 V voltage range.The chromaticity coordinates are demonstrated to beyond 5-step Mc Adam ellipses at the neighboring voltages(an interval of 2 V)such that the voltage-modulated color variations are easily distinguishable by the human eye.The maximum brightness and maximum external quantum efficiency(EQE)of device is 9373 cd/m~2and 2.8%,respectively.The research on all-fluorescent color-tunable OLED lays the foundation for further optimizing device performance.(2)A thin emitting layer with thermally activated delayed fluorescence(TADF)and phosphorescence doping is proposed to address the issue of low efficiency in all fluorescence color-tunable OLED.The distribution of excitons on the host-guest are affect by reverse intersystem crossing and intersystem crossing processes,resulting in color-tunable OLEDs with high efficiency and a wide range of color variations.Firstly,the performance of phosphorescent material Ir(ppy)3 and TADF material Cz DBA as guest materials in single emitting layer color-tunable OLEDs is investigated.It is found that devices based on TADF materials exhibited larger spectral variations,while devices based on phosphorescent materials did not show significant spectral changes.This is attributed to the fact that reverse intersystem crossing process in TADF materials not only enhances exciton utilization but also leads to a significant number of excitons on the host.In addition,proceed to determine the relative contributions of nonradiative processes to efficiency losses of devices Cz-2 and Cz-20 by analyzing densities of triplets and singlets at steady state.Simulations of relative contributions of nonradiative processes in devices demonstrate nonradiative decay of triplets play a key role in reducing the maximum EQE of Cz-2.Therefore,we further employed TADF material DMAC-DPS as host and phosphorescent material Ir(MDQ)2(acac)as guest to enhance the utilization of triplet.Through transient photoluminescence measurement,we demonstrated that the number of excitons on guest influences energy transfer between the host and guest.Color-tunable OLED with high efficiency,low driving voltage,and wide color variation range is achieved.The device exhibits a maximum EQE of 8.4%,with CIE 1931 color coordinates shifting from(0.54,0.40)at 3 V to(0.31,0.30)at 7 V.Within the voltage range of 4 V,a color coordinate variation of(0.23,0.10)is obtained.Additionally,the chromaticity coordinates are demonstrated to beyond 5-step Mc Adam ellipses at the neighboring voltages(an interval of 1V),indicating that the voltage-modulated color variations are easily distinguishable by human eyes.Furthermore,multilevel luminescent anticounterfeiting label is implemented by patterning mask process,demonstrating its potential in field of anticounterfeiting.(3)To construct high security electroluminescent anticounterfeiting labels,strategy of a hybrid luminescent anticounterfeiting based on color-tunable OLED is proposed.Through electroluminescence of color-tunable OLED selectively pumping red photoluminescent layer,a hybrid luminescent anticounterfeiting technology based on color-tunable OLED is developed which can realize dynamic changes of emissive region,brightness,and color.Furthermore,it can combine with QR code or physically unclonable function to achieve a high security anticounterfeiting label.Firstly,we design and optimize the photoluminescent layer,enhancing the photoluminescence intensity of thin film by adjusting the host material and doping concentration.The developed photoluminescent layer can be excited by blue light and re-emit red light.Then,through lateral excitation devices and optical simulated,we demonstrate that the photoluminescent layer is capable of extracting substrate and waveguide modes in blue OLED.Devices based on internal integration of a photoluminescent layer were fabricated using an array of ITO electrodes.These devices achieved white emission and exhibited higher external quantum efficiency compared to their corresponding blue device.These results demonstrate that the internal photoluminescent layer is capable of extracting trapped light within device.Finally,by integrating the photoluminescent layer and the color-tunable OLED,we achieved a hybrid luminescent device that only requires electrical stimulation.This is accomplished by selectively pumping the red photoluminescent layer using color-tunable property of the OLED.This approach enables dynamic variations in the emissive region,brightness,and color under voltage control.Hybrid luminescent label have significant advantages,which avoiding multiple excitation light sources and pattern distortion compared with photoluminescent and electroluminescent labels.Furthermore,its versatility and compatibility were demonstrated through the preparation of various patterns of labels,allowing customization of special patterns according to user needs.The hybrid luminescent labels can also serve as an anticounterfeiting platform.High-security anticounterfeiting technology is realized by combined with QR codes or fingerprinting with physically unclonable function,further showcasing its potential in the field of anticounterfeiting.