1. Introduction

Transparent displays have been received a remarkable attention as a next-generation display due to their innovative design and extensive availability in various fields and applications, including vehicle, interior, military, and medical [1–8]. The transparent nature of display allows us to perceive image on the front as well as rear sides of the screen simultaneously. Such optical property provides powerful merits, but deteriorates the visibility of images thereby restricting the ability to have high-quality images from the transparent display. Favorably, this inevitable drawback can be overcome by implementing light shutter at rear side of the transparent display. For such a novel display system which termed as “smart window display”, the important attribute is to have high-performance smart window. A proposed concept of smart window display can manage the internal light by blocking visible light to letting pass through, provides comfort and privacy, and works as a TV screen. To realize smart window displays, a fully integrated window device system with transparent displays and light shutter with excellent transmittance control function are highly desirable.

Light shutter technology adjusts the amount of light transmitted according to the electrical signals. In order to apply light shutter to transparent display, driving characteristics such as high transmittance in transparent state, low transmittance in black state, appropriate switching time, stability against repeated driving, and reasonable driving voltage are highly required for light shutter device. Many techniques that can be utilized as light shutters are already known. Among them, liquid crystal based nematic liquid crystal (LC) [9,10], polymer dispersed liquid crystal (PDLC) [11], electrophoretic [12], electrowetting [13] and light absorbing dye based electrochromic devices using color particles are known as light shutters. However, these technologies could not perfectly fulfill the basic requirements of efficient smart window devices such as high transmittance in bleached state, excellent absorbance in colored state, low driving voltage, wide aperture ratio, fast response time and good driving stability. The nematic LC has a very fast response speed, but also exhibits significant initial transmission loss due to polarizer. PDLC has an excellent transmittance characteristic, but it requires very high driving voltage of tens to hundreds of volts. The high driving voltage is limited in the head mount display, which is a portable device with battery. In addition, electrophoretic and electrowetting devices use colored particles, however, the aperture ratio is considerably low in such devices, resulting in a significant loss in transmission and lack of sufficient response speed and driving stability. In the case of electrochromic device (ECD), the color changing (electrochromic) material in the cell changes from a colorless state to a colored state and absorbs light by itself, and thus the optical transmittance changing characteristic is remarkably excellent. These merits and demerits of various optical shutter technologies were well summarized in our previous reported paper [20]. However, the realization of a deep black color in a colored state and securing driving stability still remain as challenges for smart window display applications.

In this paper, we report an ideal concept to develop efficient smart window display with promising characteristics. Here, the proposed smart window display is fabricated by TOLED with light shutter. The full smart window display not only shows excellent optical properties but also demonstrated attractive ambient contrast ratio (ACR). In addition, light blocking screen revealed high transmittance in bleached state and negligible transmittance in colored state with suitable response time, excellent driving stability, and reasonable driving voltage.

2. Result and discussion

The performance of ECD completely depends on the electro-optical properties of electrochromic materials. In order to achieve desired optical properties of ECD, we used a fluoran derivative for the fabrication of light blocking screen. Usually, fluoran derivatives are widely used in thermal imaging system as recording materials [14–16]. Recently, very few studies have been reported on the fluoran derivatives for ECD applications [17,18]. The fluoran derivatives exhibit both bleached and colored state in accordance with the chemical status of lactone ring, which is determined from the reaction of fluoran derivatives with the acid species such as proton donor compound [16,19]. The color in fluoran derivative molecule is variable depending on the substituents attached to dibenzopyran moiety. For the light shutter application, electrochromic materials must show strong optical absorbance and high transparency in the overall visible wavelength region at colored and bleached states, respectively. Thus, a fluoran derivative, 6'-(diethylamino)-2'-[[3-(trifluoromethyl)phenyl]amino]spiro[isobenzofuran-1(3H),9'-[9H]xanthene]-3-one (DATFMF), is selected due to its high transparency in lactone ring-closed form and black color in lactone ring-opened form.

To realize an ECD electrically switchable between the bleached and colored states using DATFMF compound, the amount of acid species acquired from the proton donors must be controllable through the adjustment of electrical potential on the ECD electrodes. Generally, hydroquinone derivatives have a very stable redox reaction. Most importantly, it produces protons in the oxidized state. Hence, one of hydroquinone derivatives, 2,3-dimethylhydroquinone (DHMQ), for the use of proton generator was introduced. With strong electrochemical properties, DMHQ works as an electrochemically selective proton donor causing the lactone ring opening reaction at the interface of a working electrode as shown in inset of Fig. 1(a). An electrolyte gel in 60 um thick cell of ECD contains solvent (2-ethoxyethanol), supporting electrolyte (tetra-n-butylammonium tetrafluoroborate (TBTF)), polymer (polyvinyl butyral), electrochromic dye (DATFMF), proton generator (DMHQ). The fabrication method of ECD is explained in Experimental section and detail regarding electrochromic mechanism with electrochemical reactions between the constituent materials in the ECD system is reported elsewhere. The fabricated ECD shows excellent performance in controlling optical transmittance. Figure 1(b) shows the absorption spectrum of ECD with respect to the applied voltage. The absorption spectrum of ECD in bleached state (blue dotted line) shows the DATFMF absorption peak in the UV region, indicating the large optical band gap (3.6 eV) and very small amount of light absorption is measured in the visible wavelength region. Whereas, at higher applied voltage, the absorbance of ECD is enhanced due to increased concentration of lactone ring-opened DATFMF in the electrochromic gel. Indeed, two broad absorption peaks at 438 and 589 nm start to be generated at a voltage of 0.9 V, and as the voltage is gradually increased, the intensity of the absorption peaks increases. As shown in Fig. 1(c), the initially transparent ECD shows the back side image, but the image is gradually blocked by the ECD with increasing applied voltage due to increased absorptivity of ECD in the visible light region at higher applied voltage. The exceptional optical properties of two chemical states allow DATFMF to show the high optical contrast (ΔT) as shown in Fig. 1(a). Transmittance spectra of fabricated ECD in bleached and colored states indicate the best possible values compared to those of earlier reported ECDs. Our ECD displays very high transmittance as high as 85.56% at 562 nm and more than 70% for other visible wavelengths. Furthermore, the transmittance of ECD in colored state is almost 0% in the visible region except at 480 nm ~520 nm and over 630 nm of wavelength region, resulting in an excellent ΔT of 85.50 at 562 nm (maximum: 85.50 at 562 nm; complete visible region: over 70). This superior optical performance of our ECD demonstrates that it is one of the best light-shutter device reported so far.

 

Fig. 1 (a) Optical transmittance of ECD in bleached state (blue dotted line) and colored state (red solid line) and optical contrast (ΔT) of ECD for the different wavelength range. Inset figures present two chemical state of DATFMF (blue: neutral state, red: oxidized state) and working pictures of ECD on bleached state and colored state. (b) UV-visible absorption spectra of ECD with stepwise increase in applied voltage from 0.0 to 1.9 V. Blue dotted line and red solid line represent absorption spectrum of bleached state (neutral state) and colored state (fully oxidized state), respectively. (c) Photographs of ECD with respect to different applied voltages. (d) Relative transmittance change of ECD according to the applied voltage signal (1 cycle). (e) Transmittance change of ECD at 590 nm in bleached state (applied voltage of 1.7 V) and colored state (applied voltage of 0.0 V) depending on the driving cycle.

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To confirm the response time of ECD, the relative change in transmittance of ECD at 589 nm with respect to applied-voltage time was measured as shown in Fig. 1(d). Here, the response time is defined as the time taken for relative change in the transmittance by 80% from the point of applied voltage. The response times for colored and bleached states are found to be 1.9s and 4.1s, respectively. These values are reasonable for smart window application. Furthermore, to investigate the operational stability of ECD, transmittance change between bleached and colored states depending on on/off cycle was extensively measured. The sequence of square voltage pulse was applied to ECDs for 25,000 cycles. The sequence of applied square voltage pulse was 18s (1.7 V), 5s (−1.7 V) and 12s (0 V) as shown in Fig. 1(d). The transmittance of ECD was degraded only by −5.3% for bleached state and + 4.1% for colored state even after 25,000 cycles of repeated voltage pulse as shown in Fig. 1(e). This result demonstrates very high stability of studied ECD system and its feasibility towards commercialization.

For the fabrication of smart window display, transparent OLED is investigated with the focus on high transmittance cathode. The ultra-thin silver (12 nm) capped with 40-nm-thick NPB layer used as a transparent cathode to enhance the out-coupling efficiency and to achieve maximum transparency from OLED. The double electron transport layer (ETL) comprising exciton-confining ETL (TmPyPB) with high triplet energy (2.78 eV) [21] and n-doped ETL (Bphen:5% Li) is incorporated for enhancing the electron injection as well as efficiency of the transparent OLED. Figure 2(a) shows the optical transmittance of ITO glass, transparent cathode with organic layers and full device comprising ITO glass, organic layers, cathode, and encapsulation glass. Although the transparent cathode contains a thin metal layer, but still it shows very high average transmittance of 86.9% in the visible spectral region (435~700 nm), which is comparable to that of ITO (92.3%). The lower transmittance in the wavelength region below 435 nm is assigned to the light absorption from the organic layers underneath the thin Ag layer. Due to the high transmittance of transparent cathode, the luminance ratio of bottom (ITO side emission) to top emission (cathode side emission) at the same current density reaches 1:0.55 as shown in Fig. 2(b). As shown in Fig. 2(c) and (d), current density properties of bottom and top emission are same, while luminance and efficiency characteristics of bottom emission are better than those of top emission like common TOLED due to metal cathode. On the other hand, the optical transmittance of full device also exhibits very high transmittance of about 70% in the 450~650 nm wavelength range. The white light emission was realized by mixing blue fluorescent emission (bottom unit) and yellow phosphorescent emission (top unit) in the tandem OLED structure. Detailed chemical materials and fabricated structure are described in the Experimental section. The bottom emission of the TOLED shows a trivial yellowish white color with the CIE color coordinate of (0.34, 0.40) at 1,000 nit [see Fig. 2(e)]. Similarly, the top emission displays almost same emission characteristics with the bottom emission but it is slightly blue shifted (CIE color coordinates: 0.33, 0.39) because of the higher transmittance in the blue emission region (around 450nm light) of cathode and lower transmittance in the yellow emission region (around 560nm light) compared to ITO makes a slight difference in the top and bottom emission spectra as shown in Fig. 2(f).

 

Fig. 2 (a) Optical transmittance of ITO glass, transparent cathode comprising organic layers and full device comprising ITO glass, organic layers, transparent cathode and encapsulation glass. (b) Luminance-current density (L-J) characteristics of bottom and top emission. (c) Current density-voltage (J-V) and luminance-voltage (L-V) characteristics of bottom emission (ITO side emission) and top emission (cathode side emission) of TOLED. (d) Current efficiency-luminance (C.E) and power efficiency-luminance (P.E) characteristics of bottom and top emission. (e) CIE color coordinates of bottom and top emission. (f) Electroluminescence (EL) spectra of bottom and top emission.

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The full smart window display is realized by integrating TOLED with ECD using an adhesive layer. Herein, an adhesive layer not only provides a good adhesion property between the TOLED and ECD devices but also suppresses the optical loss generated from the interface of two devices. The smart window display exhibits a very high optical transmittance of 63% at 550 nm although the optical transmittance values of ECD and TOLED at 550 nm are 85% and 68%, respectively, as shown in Fig. 2 (a). A loss of 5% transmittance is attributed to the refractive index mismatching between the adhesive layer (RI = 1.4) and glass substrate (RI = 1.5). These results are validated by comparing the simulated and experimental optical transmittance spectra of ECD, TOLED, and smart window display [see Fig. 3]. Widely used SimOLED (Sim4tec, Germany) optical simulator was used to analyze the transmittance characteristics of smart window combined with ECD and TOLED. The simulated transmittance spectrum of ECD (Glass/ITO/Electrolyte/ITO/Glass) is correlated well with the empirical results. For the precise simulation, the refractive index of electrolyte was taken as 1.4 as 2-ethoxy ethanol (n_eth = 1.4) is present in large proportion in the electrolyte. In ECD, the light absorption is mainly caused by ferrocene. The simulated transmittance of TOLED is also in good agreement with the experimental transmittance results. The simulated transmittance of full smart window device was estimated for the different refractive index values (from 1.2 to 1.8) of adhesive layer. It is important to note that the thickness of adhesive layer was not seriously considered in the simulation because adhesive layer does not significantly absorb light in the visible wavelength region and amply thick adhesive layer (>10 um) merely induces incoherent light propagation in the layer. By considering simulation results, the high transmittance of full smart window display is obtained for the certain values of refractive index (RI n_adh = 1.4 ~1.5) of adhesive layer. Though, the highest transmittance of smart window display is attained at n_adh = 1.5 and the reduction of transmittance is under 0.15% in case of n_adh = 1.4. These results demonstrate that the simulated transmittance of full display device using adhesive layer with RI n_adh = 1.4 completely correlated with the experimentally measured transmittance.

 

Fig. 3 Simulated (red dotted line) and experimental (blue solid line) transmittance of (a) ECD, (b) TOLED, and (c) smart window display. (d) Simulation results of smart window display with respect to refractive index of adhesive layer.

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The schematic of fabricated smart window display structure [Fig. 4(a)] and its operating modes like transparent [Fig. 4(b)], opaque black [Fig. 4(c)], dual side emission [Fig. 4(d)] and single side emission [Fig. 4(e)] with respective photographs are shown in Fig. 4 and a video showing the operation of the smart window display is presented in Visualization 1. The transparent and opaque black modes of smart window display are shown in Fig. 4(b, c). In these modes, only ECD is in operating condition, whereas TOLED represents the OFF state, hence it can simply act as a smart window. In case of dual side emission mode, TOLED is in ON state and ECD indicates the OFF condition [Fig. 4(d)]. In this mode device concurrently shows both images on TOLED as well as on the rear side of the screen due to its excellent see-through property. However, image on the TOLED exhibited poor CR and visibility because it cannot represent black and also interrupted by the rear side light. On the other hand, the single side emission mode of smart window display denotes the ON state (TOLED, ECD = ON) and presents remarkably improved CR and visibility of image on the transparent organic display. Indeed, the ECD represents black state and prevents the interruption from back side/surrounding light [see (Fig. 4(e)]. As a result, more clear and high-quality image is achieved on the smart window display. Moreover, we also demonstrated the light-blocking effect of ECD by measuring ambient CR of smart window display at single and dual side emission mode in office lighting condition (~500 lux) with fluorescent lamp. Moreover, ambient light condition can affect CR of smart window display [22]. The ambient CR is calculated by using following reported equation [23].

 

Fig. 4 (a) Schematic of smart window display structure, and the operating modes of smart window display (See Visualization 1): (b) Transparent mode (TOLED: off state, ECD: on state), (c) Opaque black mode (TOLED: off state, ECD: on state), (d) dual side emission mode (TOLED: ON state, ECD: OFF state), (e) single side emission mode (TOLED: ON state, ECD: ON state).

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3. Experiment

Fabrication of ECD: ITO glasses were sequentially cleaned in acetone and isopropyl alcohol for 10 minutes using ultrasonic washer and then rinsed using deionized water followed by drying using nitrogen gas gun. 60 um thick spacer film was attached on the cleaned ITO glass substrate with active area of 3 × 3 cm². In the active area, the electrochromic gel was filled. Finally, the electrochromic gel contained ITO glass substrate was covered by another ITO glass and then both ITO glass substrates were tightly hold together by clamps.

Fabrication of TOLED: commercially available patterned ITO glass substrates were utilized as anode and ultra-thin silver covered with capping layer (NPB) was employed as a transparent cathode. For the white light emission, two-stack tandem architecture comprising fluorescent blue and phosphorescence yellow emitting units with lossless charge-generation layers (lithium doped Bphen/ HATCN/ TAPC) was adopted. The TOLED configuration is as follows: ITO (150 nm)/ HATNC (7 nm)/ TAPC (58 nm)/ MADN:blue fluorescent dopant(8%) (15 nm)/ TmPyPB (20 nm)/ Bphen:Li(5%) (25 nm)/HATCN (7 nm)/ TAPC (60 nm)/ Bepp2:yellow phosphorescent dopant (20 nm)/ TmPyPB (30 nm)/ Bphen:Li(5%) (10 nm)/ Ag (12 nm)/ NPB (40 nm). All the organic and inorganic materials such as 1,4,5,8,9,11-hexaazatriphenylene hexacarbonitrile (HATCN), 1,1-bis-(4-bis(4-methyl-phenyl)-amino-phenyl)-cyclohexane (TAPC), 2-methyl-9,10- (2-napthyl)anthracene (MADN), 1,3,5-tri(m-pyrid-3-yl-phenyl)benzene (TmPyPB). 4,7-diphenyl-1,10-phenanthroline (Bphen), bis[2-(2-hydroxyphenyl)pyridine]beryllium (Bepp2), N,N′-diphenyl-N,N′-bis(1-napthyl)-1,1′-biphenyl-4,4′-diamine (NPB), lithium (Li) were purchased from Luminescence Technology Corporation and Sigma-Aldrich. To fabricate TOLED, all the layers were thermally evaporated on the pre-cleaned ITO substrates in a high vacuum chamber at a base pressure of 10⁻⁷ Torr. Finally all devices were encapsulated using glass cover and UV-cured resin in nitrogen atmosphere.

Fabrication of Smart Window Display: As shown in Fig. 4(a), the smart window display device was fabricated by combining ECD and TOLED using an adhesion film. The adhesion film was obtained by mixing 2-ethoxyethanol and PVB.

Measurements: UV-vis transmittance and absorption spectra of all the fabricated devices were obtained by using spectrophotometer (Scinco, S-4100). Photographs of ECD and smart window display in different modes were recorded by a digital camera (FinePix J40, Fujifilm Corporation, Japan). All the photos are presented without optical parameter modification. The luminance versus current density (L-V) curves, electroluminescence (EL) spectra and Commission Internationale de l'Eclairage (CIE) 1931 color coordinate were attained using Konica Minolta CS-2000 spectroradiometer. All measurements were performed in ambient condition.

4. Conclusion

In summary, we demonstrated a next generation smart window display using simple and scalable process. The approach used in this study is valuable for fabricating a future large area fascinating smart window display. The fabricated smart window display exhibited excellent optical properties, especially high transmittance of more than 64%. Likewise, it also showed a good tunable ambient CR which allows to broadening the field of transparent display applications. The light blocking screen used in this smart window display presented a broad absorption spectrum at colored state and highly transparent in bleached state as compared to earlier reported ECDs. The ECD also shows an exceptional optical density, fast switching time and long term stability even after multiple switching cycles. The optical simulation results of advanced smart window display including ECD supported well with the experimental results. The proposed smart window display technology allows us to adjust the intensity of daylight entering the system quickly and conveniently. In addition, our ECD and smart window display structure is very simple and used inexpensive ECD materials as described in the Experimental section. Therefore, our ECD-based smart window technology would be cost competitive technology in the market. We strongly believe that the smart window display technology presented here is very encouraging to carry out further research in the direction of future attractive transparent display applications.