RGB color textile-based flexible and transparent OLEDs considering aesthetics

doi:10.21203/rs.3.rs-1760082/v1

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RGB color textile-based flexible and transparent OLEDs considering aesthetics

https://doi.org/10.21203/rs.3.rs-1760082/v1

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Recently, as state-of-the-art displays, wearable electronic devices and free-form displays have begun to attract attention, so that much research on textile-based organic light-emitting diode (textile OLED) displays as their ultimate platform has been actively conducted. However, although many studies have reported textile OLEDs with high current efficiency and maximum wearability, most research has left for the future improvements that require the use of transparent OLEDs to improve aesthetics and applications in various fields such as fashion displays and military products. Herein, RGB full color transparent organic light emitting diodes (trOLEDs) were designed using a dielectric-metal-dielectric transparent electrode to overcome the limitations of previous works and improve the aesthetic elements of textile displays. Opto-electronic performance and mechanical characteristics of textile RGB trOLEDs were analyzed; furthermore, to quantify the normalized acquisition rate of information that can be obtained from the textile substrate when transparent OLEDs on textile substrate are not operated, this study suggests a new method and novel equation for optical analysis.

Dielectric-metal-dielectric electrode

textile display

transparent display

information acquisition rate

wearable display.

As the Internet of Things (IoT) has become mainstream, display technology has rapidly been developing in parallel. With current technological innovation in the area of displays, devices that have a simple, rigid appearance, such as computers and mobile phones, are evolving to become thinner, lighter, and more flexible by using curved, bendable, foldable, and even rollable displays. In addition, not only for simple one-dimensional information transmission from devices to human, but to transmit information among people and devices in both directions for various purposes, display technology has achieved form-factor innovation into various kinds of smart devices, especially into wearable displays. Among such innovations, clothing-type displays are attracting attention as very effective intercommunication methods for communication among humans and devices, and the most human-friendly technology suitable for the movements of the human body^1–7. Moreover, clothing-type displays are state-of-the-art technology including multi-functionality and aesthetics at the same time, applicable to numerous application fields such as smart textiles, military goods, and automobile interior walls^8–12.

With these trends of technological development, clothing-type displays using organic electroluminescence (EL), fabricated by several techniques such as inkjet printing¹³, screen printing¹⁴, or thermal evaporation on PDMS planarization layer,¹⁵ have been reported. However, with the limitations of the light emitting mechanism of EL materials, there are disadvantages in that devices are driven at high voltage of 165 V to 400 V, which could be considered a severe problem for applications to wearable displays. Moreover, the maximum luminance achieved in those works was much less than 1000 cd m^-2, which is considered an insufficient value for outdoor displays.

In this regard, Choi et al. reported textile OLEDs that solved those limitations of clothing-type EL displays by using OLEDs for clothing-type displays¹⁶. Maximum values of luminance of 7968 cd m^-2 and current efficiency of 14.6 cd A^-1 show sufficient electrical and optical performance for outdoor clothing-type displays. Also, the report showed that the fabricated planarized textile substrate had almost same the mechanical flexibility as that of bare textile, which results in excellent bending and wrinkling performance for textile displays. As such, these novel approaches have provided electrical and versatility advantages for clothing-type displays; however, for better applicability to various fields such as fashion displays, in which aesthetic factors are important, the application of transparent OLEDs to textile displays was found to be essential.

Yin et al. reported textile-based transparent green OLEDs by using ultra-thin 7 nm Au film as anode and 9 nm Ag film as cathode to improve the aesthetic factors of textile OLEDs¹⁷. However, although this work successfully solved limitations of previous works, it was limited to single-color (green) displays, which are insufficient for information displays. In addition, the metal film was so thin that the surface morphology of electrode had slightly poor film quality, which could result in low product yield and unsuitability for large area displays. Also, transparency optimization was not fully realized, and optical analysis of textile OLEDs is lacking for full application to clothing-type displays. In transparent textile displays, not only the transparency of the trOLEDs, but also the clarity of the patterns on the textile substrate when trOLEDs are not operating, is a very important factor and should be considered for optimizing textile trOLEDs .

Unlike other transparent displays, in textile trOLEDs, trOLED films of n % transmittance do not guarantee that n % of the information on the textile can be identified; this is because trOLEDs directly and entirely cover textile substrates, so external light experiences multiple stages of light loss; when light passes through trOLED film, it is reflected by the substrate, and passes through the trOLED film again. Therefore, it is necessary not only to accurately determine the transmittance of the trOLEDs, but also to analyze how visible the information on the textile is; numerical quantification of information acquisition from non-operating textile trOLEDs thus should be done.

Here, to realize more practical application of clothing-type displays, red, green, blue (RGB) full color transparent OLEDs with high transmittance values are obtained by optimizing the thicknesses of the dielectric-metal-dielectric electrode based on the transfer matrix formula; these were fabricated on patterned textile substrates and showed outstanding electrical performance. Furthermore, by in-depth analysis and using novel equations to quantify the clarity of patterned textile when trOLEDs are not operating, universal optical analysis method for textile trOLED displays is suggested. In addition, using a multifaceted approach to the mechanical properties of textile trOLEDs, such as cantilever and bending tests, complete analysis of the flexibility of textile trOLEDs was conducted.

The most important part of the structure of transparent OLEDs is the electrodes, and representative materials widely investigated as transparent electrodes include ITO (Indium Tin Oxide), PEDOT: PSS, metal nanowires, graphene, carbon nanotubes, etc.^18–22. However, because each material has its own downsides, such as insufficiency of flexibility, low transmittance, or high sheet resistance, they can be considered unsuitable for application to flexible wearable displays, especially textile trOLEDs. On the other hand, multilayer electrodes consisting highly refractive index dielectric layers/thin metal films/high refractive index dielectric layers (DMD) have advantage in that they exhibit high transmittance, high flexibility, and low sheet resistance²³.

DMD electrodes have excellent flexibility because of their inside thin metal films. Furthermore, due to their unique structural and optical characteristics, such as suppressing surface plasmon coupling at the dielectric/metal interface^24,25, the transmittance of thin metal films with relatively low transmittance can be dramatically increased (Supplementary Fig. 1). Also, lower dielectric layers acting as metal seed layers to prevent porous metal deposition, allow the deposition of continuous thin metal films, resulting in low sheet resistance and improved transmittance²⁶. With their suitability for wearable displays, especially for textile displays, DMD electrodes were used as transparent electrodes for textile RGB trOLEDs.

RGB full color textile trOLEDs: Optimized transparency and opto-electronic performance

Figure 1a shows the proposed textile trOLEDs, which are not operating; the patterns on the textile substrate can be clearly identified. Figure 1b shows textile RGB trOLEDs operating at 5 V.

For the fabrication of the proposed textile trOLEDs, as shown in Fig. 1c,f,i, RGB trOLEDs were fabricated on a patterned polyester textile substrate.

A transparent DMD electrode made of ZnS/Ag/MoO3 was used for the anode, and an electrode made of ZnS/Cs2CO3/Ag/ZnS was used for the cathode for RGB trOLEDs.

In the trOLED structure, and especially in the DMD electrode structure, the layer that has the greatest influence on the transparency is the thin metal layer²⁷. Here, due to its good conductivity and lowest absorptivity in the visible wavelength band, Ag is used for the thin metal layer in DMD electrodes for the cathode and anode²⁸. A silver electrode of too high thickness can cause a decrease in transmittance (Supplementary Fig. 2); one of too low thickness can cause difficulty in fabrication and a decrease in sheet resistance²⁹. As such, an optimized minimum thickness of 8 nm Ag is fixed for the DMD anode and cathode.

To design an effective DMD electrode, it is essential to consider a dielectric material with very high refractive index values; among dielectric materials that satisfy this condition, ZnS has relatively high surface energy, so it can effectively prevent diffusion from adjacent Ag film, resulting in a smoother Ag thin film on the ZnS layer. ZnS acts as an excellent seed layer for thin Ag film. For the anode, 5 nm of MoO₃ is used as capping dielectric layer due to its capability as hole injection layer (HIL) for the anode. For the cathode, if the seed layer of ZnS is too thick, it could interfere with proper electron injection from the Ag cathode to the emitting layer; as such, 5 nm of ZnS is used as seed layer and 1 nm of Cs₂CO₃ is used for improved electron injection capability³⁰. For the capping dielectric layer of the cathode, ZnS is used to improve transmittance.

After the thickness of the other organic/inorganic layers is optimized considering the charge balance, an optical simulation was conducted using the measured refractive index value of each material and MATLAB software (MathWorks) based on the transfer matrix formula³¹; optical transmittance simulation was conducted in relation to the top and bottom ZnS thicknesses, as shown in Fig. 1d,g,j for the RGB trOLEDs. Based on the calculated results for the optimized top and bottom ZnS thicknesses (top ZnS: 25, 26, 21 nm, bottom ZnS: 16, 15, 20 nm for RGB trOLEDs respectively), RGB trOLEDs were fabricated on glass substrate; theoretical calculations and experimental results of transmittance are compared in Fig. 1e,h,k. With the similarities of the theoretical calculations and the experimental results, validity is assigned to both optical transmittance simulation and actually-measured refractive index values; with high transmittance results for the RGB trOLEDs (63.94, 77.26, 64.99% at 550 nm for RGB trOLEDs respectively), the applicability to textile trOLEDs can be confirmed.

The optimized RGB trOLEDs were fabricated on textile substrate and showed sufficient opto-electronic performance for outdoor displays; Fig. 2a,b,c show maximum luminance values of 5511, 4534.9, and 1338 cd/m²; Fig. 2d,e,f show current efficiency values of 6.07, 3.79, and 2.11 cd/A for RGB textile trOLEDs.

Furthermore, textile RGB trOLEDs show very low microcavity effect by 8 nm thin Ag film,^32,33 so that there is almost no change in peak wavelength, as shown in Fig. 3a,b,c; peak wavelength follows a nearly Lambertian emission, as shown in Fig. 3d,e,f for textile RGB trOLEDs. These results show that textile displays is hardly distorted by the viewer’s angle, which could be considered a strong advantages for wearable displays.

With the well-optimized transparency and outstanding opto-electronic properties, RGB full color textile trOLEDs can become the new mainstream of wearable displays, leading a new IoT era.

Information acquisition from non-operating textile trOLEDs

To numerically quantify the information on non-operating textile trOLEDs, the reflectance values observed in the visible light wavelength band by UV-vis spectrometer (UV-2550) were judged as numerically quantified values of information³⁴. For example, as shown in Fig. 4a for various bare colored textiles (white, red, green, and blue colored textiles), reflectance results show how much information bare textiles have at each wavelength from 400 nm to 700 nm. It can be seen that white colored textiles have information over the entire wavelength range; each different colored textile has information within a narrow wavelength band (400 nm ~ 470 nm for blue, 480 nm ~ 580 nm for green, and 590 nm ~ 700 nm for red colored textiles); there is little information in other wavelength bands. However, when trOLEDs are fabricated directly on textile substrates, the reflectance values of textile trOLEDs contain not only information on textile substrates but also light reflected by thin trOLED films, as shown in Fig. 4b. Therefore, it is necessary to remove the reflected light at thin trOLEDs films from the reflectance values of the entire surface of the textile trOLEDs, so as to allow calculation of how much information on the textile substrate can be acquired. Here, by subtracting the reflectance of the thin trOLEDs film from that of entire textile trOLEDs³⁵, it is possible to obtain the information acquisition rate from non-operating textile trOLEDs, which can be expressed as follows:

Information acquisition rate from non-operating textile trOLEDs = R_to- R_of,

where R_to is reflectance of entire textile trOLEDs and R_of is reflectance of trOLED film, as shown in Fig. 4c,d,e.

The information acquisition rate represents the spectrum of information on the textile substrate when trOLEDs are applied; to calculate how much information is acquired compared to the information the bare textile originally had (acquired rate of information considering information of bare textile as 100%), it is necessary to normalize this result according to the information spectrum of the bare textile, which can be expressed as:

Normalized information acquisition rate = (R_to- R_of) / R_bt,

where R_bt is the reflectance of the bare textile, as shown in Fig. 4a.

Figure 4f,g,h show calculated normalized information acquisition rate from RGB textile trOLEDs when textile colors are white, red, green, and blue, respectively. These results have two implications: the first is that not only the transmittance of the trOLED film, but also information acquired from the non-operating textile trOLEDs, can be expressed as numerically quantified values in all visible wavelength bands. It is thus suggested that which colored textiles are most advantageous in terms of information acquisition when trOLEDs are fabricated on textile substrates; optimization to select textile color for textile trOLEDs is thus possible. The second point is that the normalized information acquisition rate from red, green, and blue colored textile trOLEDs showed tendencies similar to that of white colored textile. This result shows that the normalized information acquisition spectrum does not change significantly depending on the color of the textiles even when trOLEDs are fabricated. In other words, this result means that only the transmittance performance of trOLED film affects the information acquisition spectrum; the textile color does not. In the wavelength range of about 520 ~ 610 nm, it can be seen that information acquisition is lower than that of white colored textile, which can be interpreted as following the tendency in which the amount of information the bare textile originally had decreased in that wavelength range, as shown in Fig. 4a; to extract information from textile trOLEDs, the bare textile used for substrate must have more than a certain amount of information. Through these two implications, it is expected that calculating the normalized information acquisition rate will become a standard in the universal optical analysis method for textile trOLEDs applied to various fields in which aesthetic factors are very important.

Mechanical characteristics of textile trOLEDs

The mechanical characteristics of textile trOLEDs were evaluated by two types of tests: cantilever test (ASTM:D1388)³⁶ to evaluate mechanical flexibility and bending test to determine bending reliability.

As shown in Fig. 5a, as the textile specimen is slid along the front and back sides in the direction of the incline, one side of specimen is bent downward by gravity and finally touches the slope. The length of the slid part until one side touches the slope is defined as the cantilever length. The shorter the cantilever length, the more flexible the textile specimen. This means less force is required to bend the textile specimen a certain radius, and there is less wear-induced stress when device is actually worn. The cantilever length of the bare polyester textile, the textile substrate, the textile substrate to which DMD electrodes are applied, the textile substrate to which the entire trOLED structures are applied, and other specimens of flexible materials such as PET films and A4 paper, are shown in Fig. 4b. Furthermore, the bending rigidity (flexural rigidity), which is a measure used in the evaluation of actual mechanical flexibility and considers weight differences of individual specimens, were calculated (based on supplementary Equations 1,2)^37,38 ; results are shown in Fig. 4c. These results indicate that textile trOLEDs showed superior flexibility compared to other flexible substrates and mechanical flexibility almost identical to that of the bare textile, confirming that trOLEDs hardly affect the flexibility of bare textiles, so that wear-induced stress can be minimized when device is actually worn.

Figure 4d shows textile trOLEDs when bent and wrinkled. To evaluate the bending reliability of textile trOLEDs, bending test was conducted in two ways; normalized luminance and sheet resistance of DMD electrodes were analyzed after the bending test with bending radius in a range from 10 mm to 1.5 mm and repetitive bending cycle of 1 to 1,000 times at bending radius of 3 mm. As shown in Fig. 4e, textile trOLEDs can work stably up to a bending radius of 3 mm. They are still able to emit light at a radius of 2 mm but had many parallel vertical cracks in the bending direction, as can be seen in the right side inset figure in Fig. 4e. With a bending radius of 3 mm, textile trOLEDs work stably after up to 1000 repetitive bending cycles, as shown in Fig. 4f. These results confirm that textile trOLEDs can sufficiently endure not only ergonomic flexion and movement, but also harsh deformation that could occur in everyday life.

Textile substrate fabrication

The planarized textile substrate was fabricated based on the ultra-thin planarization layer implanting method. More details on the fabrication method and fabricated textile substrate are provided in Supplementary Figs. 3,4.

TrOLED fabrication

Phosphorescent red and fluorescence green and blue trOLEDs were fabricated on a patterned polyester textile substrate. The following structure was utilized for red: ZnS (16 nm) / Ag (8 nm) / MoO₃ (5 nm) / N,N′-Bis(naphthalen-1-yl)-N,N′-bis(phenyl)benzidine (NPB) (50 nm) / Bis(10-hydroxybenzo[h]quinolinato)beryllium (Bebq2) doped with Tris[1-phenylisoquinolinato-C2,N]iridium(III) (Ir(piq)3) (Bebq₂:Ir(piq)₃) (40 nm) / ZnS (5 nm) / Cs₂CO₃ (1 nm) / Ag (8 nm) / ZnS (25 nm), green: ZnS (15 nm) / Ag (8 nm) / MoO₃ (5 nm) / NPB (50 nm) / Tris(8-hydroxyquinolinato)aluminum (Alq₃) (50 nm) / ZnS (5 nm) / Cs₂CO₃ (1 nm) / Ag (8 nm) / ZnS (26 nm) and blue trOLEDs: ZnS (20 nm) / Ag (8 nm) / MoO₃ (5 nm) / NPB (45 nm) / 2-Methyl-9,10-bis(naphthalen-2-yl)anthracene (MADN) doped with p-di(p-N,N-diphenyl-amino-styryl)benzene (DSA-Ph) (MADN: DSA-ph) (25 nm) / ZnS (5 nm) / Cs₂CO₃ (1 nm) / Ag (8 nm) / ZnS (25 nm).

Refractive index measurement for transmittance simulation

Refractive index (n, k) values used in the transmittance simulation were determined by ellipsometry measurement. Refractive index values of ZnS, Ag, and MoO3 are plotted in Supplementary Fig. 5.

DATA AVAILABILITY

Data supporting the findings of this study are available from the corresponding author upon reasonable request.

ACKNOWLEDGMENTS

This work was supported by a National Research Foundation of Korea (NRF) grant funded by the Korean government (MSIT) (No. NRF-2019R1A2C3010012). The authors express their sincere gratitude to the National NanoFab (NNFC) for the measurements.

AUTHOR CONTRIBUTIONS

J. L and K.C.C. conceived and designed the research. J. L. conducted the main experiments and analysis, and prepared figures and the manuscript. S.C. contributed to experiments on planarization of textile substrate. T.W.L. and Y.P contributed to optical simulation for optimizing transmittance of trOLEDs; Y.H.H. helped with optical analysis for normalized intensity profiles. S.Y.J. contributed to measuring the mechanical bending characteristics of the textile trOLEDs.

COMPETING INTERESTS

The authors declare no competing financial or non-financial interests.

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RGB color textile-based flexible and transparent OLEDs considering aesthetics

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Abstract

Introduction

Results And Discussion

RGB full color textile trOLEDs: Optimized transparency and opto-electronic performance

Information acquisition from non-operating textile trOLEDs

Mechanical characteristics of textile trOLEDs

Methods

Textile substrate fabrication

TrOLED fabrication

Refractive index measurement for transmittance simulation

Declarations

References

Additional Declarations

Supplementary Files

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