Electrochromic device with self-diffusing function for light adaptable displays

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Electrochromic device with self-diffusing function for light adaptable displays ⁎

Seong M. Cho , Tae-Youb Kim, Chil Seong Ah, Juhee Song, Sang Hoon Cheon, Hojun Ryu, Joo Yeon Kim, Yong-Hae Kim, Chi-Sun Hwang Reality Device Research Division, ICT Materials & Components Research Laboratory, Electronics and Telecommunications Research Institute, 218 Gajeong-ro, Yuseong-gu, Daejeon 34129, South Korea

A R T I C L E I N F O

A B S T R A C T

Keywords: Electrochromism Display TiO2 nanostructure Light scattering

Light-adaptable (LA) displays combining the advantages of reflective and emissive displays, are considered as the future of display technology. Electrochromic devices are potentially strong candidates for reflective-mode LA displays because of their high transmittance in the transparent state and low power consumption. However, the narrow viewing angle of these displays in relation to a hybrid structure of the LA display, is a technical barrier that must be overcome for the successful application of electrochromic devices to light-adaptable displays. In the present study, an electrochromic device with a self-diffusing function was developed for application to lightadaptable displays. Si3N4 scattering particles were incorporated in the TiO2 nanostructure-based electrochromic device. Efficient forward diffuse scatterings were obtained without deteriorating the total transmittance of the device. The contrast ratio and color gamut of the electrochromic device were dramatically improved because of the addition of the scattering particles.

1. Introduction Nowadays, various kinds of displays are used in electronic devices. Based on the imaging method, these displays can be categorized into two groups: emissive and reflective displays. The emissive displays, such as organic light emitting devices (OLEDs) and liquid crystal displays (LCDs) with emissive backlights, produce images by selfemission of light. Although the image quality of this category of displays is excellent in indoor environments, their visibility becomes poor under very bright environments such as daylight, because of the surface reflection of the surrounding light. Reflective displays such as Epaper and electrochromic displays, on the other hand, produce images by reflecting surrounding light. These displays in general consume much less power than the emissive displays [1] with good visibility under daylight environments but very poor visibility in dark environments. A display that can combine the advantages of these two categories of displays is thus desirable for future display technologies [2–5]. This kind of display is called a light-adaptable display [2,3]. In its simplest form, a light-adaptable display should consist of a hybrid structure of an emissive device such as a transparent OLED and a reflective device such as an electrochromic device, connected in tandem. Electrochromic devices have been considered to be applicable to

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smart window, light shutter and display because of their unique property of being able to control the transmittance of light [6–9]. Electrochromic devices have a very high transmittance in their transparent state and hence, they are considered as strong candidates for the reflective part of a light-adaptable display. Typically, an electrochromic display consists of an electrochromic light shutter and a white reflector such as TiO2 [10,11]. However, the performance requirements of the transparent emissive part of the light-adaptable display, such as a transparent OLED, preclude the use of white reflectors in these systems. Mirror reflectors are required as backside reflectors in light-adaptable display applications, and hence, they are used as an alternative to white reflectors. Nevertheless, new problems arise with the use of mirror reflectors. For instance, because of the specular character of mirror reflection, the viewing angle of the display becomes very narrow. Therefore, to improve the viewing angle of the display, an additional diffusion layer is needed on the surface of the display. This diffusion layer induces the blurring of images with the blurring width being proportional to the distance between the image plane and the diffusion layer. If this distance decreases, the blurring will also decrease. Ideally, if the diffusion layer coincides with the image plane, the diffusion-induced blurring of images disappears. With these ideas in mind, an electrochromic device with diffusing function is proposed in the current study. With this kind of electro-

Corresponding author. E-mail address: [email protected] (S.M. Cho).

http://dx.doi.org/10.1016/j.solmat.2017.04.021 Received 2 December 2016; Received in revised form 7 April 2017; Accepted 10 April 2017 0927-0248/ © 2017 Elsevier B.V. All rights reserved.

Please cite this article as: Cho, S.M., Solar Energy Materials and Solar Cells (2017), http://dx.doi.org/10.1016/j.solmat.2017.04.021

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Fig. 1. Schematic diagram showing the structure of the TiO2 nanostructure-based electrochromic device.

electrode: nano-sized TiO2 particles, which provide a high specific surface area for the adhesion of the electrochromic molecules, and submicron-sized the scattering particles, which provide the diffusetransmitting function. In this latter structure, the scattering particles are fixed in the nanostructure, making the structure much more stable than that in the scattering electrolyte case. In view of this, the current work focuses on the latter type of structure [Fig. 2(b)]. According to the theory of light scattering, particle-induced light scattering phenomena are strongly influenced by the size of the scattering particles and can be classified into two categories: Rayleigh scattering and Mie scattering. Rayleigh scattering occurs when the size of the scattering particle is much smaller than the wavelength of the light (typically less than one-twentieth). The scattering has an isotropic character with very low efficiency. This implies that the backward and the forward scattering intensities are almost the same. In the present study, this is not a desirable character, because the backward-scattered light does not enter the device. As a result, the total transmittance of the device is decreased. If the particle size increases, the forward scattering increases, eventually leading to a transition from Rayleigh scattering (associated with smaller particles) to Mie scattering. This near-forward scattering is highly desirable in the present research. The contents of the scattering particles and the expected angular distribution of the scattered light can be roughly estimated by simulation. An open web MIE scattering calculator [20] was used to simulate Mie scattering, while the required optical constants of the TiO2 nanostructure were estimated using the effective medium approximation method [21]. The volume fraction of the TiO2 nanoparticles in the nanostructure was

chromic device, the blurring of image can be minimized and the device fabrication, much simpler. In the present study, this kind of electrochromic device was developed by modification of a TiO2 nanostructurebased electrochromic device, which has attracted much more attention because of its fast switching character [12–18]. 2. Simulation Fig. 1 shows the TiO2 nanostructure based electrochromic device used in the present work. The TiO2 nanostructure was formed with nano-sized TiO2 particles (less than 20 nm). Triphenylamine (TPA) molecules were used as the anodic electrochromic (EC) materials [18,19]. The TPA molecules were modified to have a phosphate anchor group and fixed on the surface of the nanostructure with the anchor group. A sputtered tungsten oxide film was used as the cathodic EC material. The main aim of the present research is to impart the diffusing function to the device. Two methods are proposed to achieve this aim. As shown in Fig. 2, the first approach is to make the electrolyte diffuse transmitting by adding some suitable scattering particles [Fig. 2(a)] while the second is to make the nanostructured electrode diffuse transmitting [Fig. 2(b)]. In the case of the former, no electrode modification is required. Hence, the fabrication process is very simple. However, problems regarding the stability of the scattering particles in the electrolyte arise. Specifically we observed some unstable electrochromic switching properties of the device with this structure in preliminary experiments. In the case of the latter, two kinds of particles are used to form the nanostructured

Fig. 2. Conceptual figures showing the methods to impart diffusing functions to the TiO2 nanostructure-based electrochromic device; (a) scattering electrolyte scheme and (b) scattering electrode scheme.

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90

2

120

150

1

0

30

180

1

20nm 100nm 200nm 300nm 400nm 500nm

60

0

-150

-30

-120

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-60 -90

Fig. 3. (a) Geometry of light scattering by a particle and (b) the simulated angular distribution of the scattered light.

estimated from the film thickness and the weight of the film measured with a chemical balance. The weight of the film was measured with 5 film samples coated on slide glasses at the same time, whose film area was 1.9 cm x 5 cm for each. The weight difference between the initial bare state and the state after calcination was considered as the weight of the films. The estimated volume fraction of the TiO2 nanoparticles was about 22% and the corresponding refractive index of the nanostructure, 1.65. The imaginary part of the refractive index was assumed to zero. The aforementioned values were applied in the simulation. Silicon nitride with a moderate refractive index of 2.0 was selected as the scattering particle. The refractive index plays a major role in determining the amount of particles required for sufficient scattering. On the other hand, the angular distribution of the scattered light is mainly governed by the size of the scattering particle. Fig. 3(a) and (b) show the simulation model geometry and the calculated angular distribution of the scattered light, respectively. In the case of 20 nmsized particles, the scattering is isotropic, as is typical of Rayleigh scattering. As the particle size increases, forward scattering becomes predominant, as is typical of Mie scattering. The same trend occurs for the simulated larger-sized particles up to the geometrical optical limit. In the geometrical optical limit, the forward scattering converges to transmission while the backward scattering converges to reflection. To obtain moderate forward scattering, a particle size range of 300–800 nm is considered suitable. If the particle is too large, the scattering becomes overly forward such that the diffusing effects are decreased. Large-sized particles are also very difficult to handle in colloidal processes. Fig. 4 shows the size distribution of the nitride particles used in the present research. The size distribution was

measured by a light scattering method (Mastersizer 300E, Malvern co. USA) in a colloidal state. The measured size distribution is observed to coincide well with direct observations from scanning electron microscopy (SEM) images. With this measured size distribution, the required amount of particles and the angular distribution of the scattered light can be estimated. Fig. 5 shows the simulation results using the measured particle size distribution of Fig. 4(a). Specifically, Fig. 5(a) shows the fraction of the scattered light as a function of the particle contents. From the figure, it is seen that a small amount of particles is capable of inducing significant scattering. For the simulation, the amount of the scattering particles in the experiments was set to the range of 0–3.0 vol%. Fig. 5(b) shows the expected angular distribution of the scattered light as obtained by a weighted summation of the effect of the scattered light for each particle size using the measured particle size distribution. The following equation was deployed:

I (θ ) =

∑ Ii (θ ) fi i

(1)

where I (θ ) is the total scattered light intensity at angle θ, Ii (θ ), the scattered light intensity at angle θ as produced by a scattering particle of size i and fi , the number fraction of particles of size i. The multiple scattering effects were neglected in the calculation. Fig. 5(b) clearly shows that the scattered light will have a smooth distribution up to 30°. The simulation results show that by incorporating the scattering particle into the nanostructured electrode, the diffuse transmittance of the electrochromic device is substantially improved.

Fig. 4. (a) Measured particle size distribution of the scattering particles and (b) SEM image of the scattering particles.

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Fig. 5. Simulation results: (a) fraction of the scattered light, (b) angular distribution of the scattered light.

3. Fabrication

4.2. Diffuse transmittance

Silicon nitride particles (SII08PB, Kojudo chem. co., Japan) were used as the scattering particles. Before commencing the actual experiments, the particle size of the raw powder was first reduced by ball milling with zirconia balls in ethanol medium for 48 h. The particle size distribution and the SEM image of Fig. 4 are based on the ball milled powder. The key issue in the fabrication was to obtain uniform dispersion of the scattering particles. The scattering particles were added during the TiO2 paste preparation phase using a colloid mixing process in ethanol medium. The TiO2 paste was supplied by ENB Korea. The prepared paste was bar-coated on a fluorine-doped tin oxide (FTO, 15 Ω/□) on glass substrate and calcined at 400 °C for 30 min to burn out the residual organic polymers. The final thickness of the nanostructure film was 6.5 µm. TPA molecules (modified to have a phosphate anchor group) were then fixed on the surface of the nanostructure with this anchor group in ethanol medium. A WO3 film was used as the cathodic electrochromic material. The WO3 film was deposited on an Indium tin oxide (ITO, 15 Ω/□) on glass substrate by DC reactive sputtering with a thickness of 300 nm. The DC reactive sputtering was performed with a W metal target with an Ar/O2 mixed plasma [22]. The process pressure was 20 mTorr. The prepared electrodes were jointed together with the melting film (Surlyn, Dupont co.). Thereafter, the electrolyte (0.1 M LiClO4 in propylene carbonate) was injected. The window-type cells and mirror-type cells were prepared separately. For the case of the mirror-type cells, an Ag film was deposited on the opposite side of the substrate with the WO3 film.

The most important device parameter in the present research is the diffuse transmittance, which can be measured with an integrating sphere. Fig. 7 shows the geometry of the measurement with the integrating sphere (UPK-100-F, Gigahertz-Optik co., Germany). The inner diameter of the integrating sphere was 10 cm while the diameter of the exit port was 1 cm. A mixed source of a deuterium lamp and a halogen lamp (DH2000-BAL, Ocean optics co. USA) was used as the light source. A spectrometer (QE66Pro, Ocean optics co. USA), connected to the detector port of the integrating sphere with an optical fiber, was used as the detector. The total transmittance and diffuse transmittance of the device can be measured with this integrating sphere by changing the cap of the exit port with a white cap or a light trap. When a white cap is applied to the exit port, the total transmittance can be measured while the diffuse transmittance can be measured when a light trap is applied. The total transmittance is the sum of the linear transmittance and the diffuse transmittance, as given in Eq. (2) [23]:

TTotal = TLinear + TDiffuse

(2)

where TTotal is the total transmittance, TLinear is the linear transmittance and TDiffuse is the diffuse transmittance. Viewing angle improvements can be obtained by improving the diffuse transmittance. From Eq. (2), it is seen that for constant total transmittance, the linear transmittance will be decreased by increasing the diffuse transmittance. Therefore, the basic direction of the present research was to improve the diffuse transmittance at the expense of the linear transmittance without deteriorating the total transmittance. Fig. 8 shows the measured results with the integrating sphere with the first part (Fig. 8(a)) showing the diffuse transmittance and the second part (Fig. 8(b)), the total transmittance. As can be seen from the figures, the diffuse transmittance was significantly improved by the addition of the scattering particles, with minimal changes in the total transmittance. These results imply that forward diffuse scattering was predominant in our device as significant backward scattering would have resulted in the decrease of the total transmittance.

4. Characterization and discussion 4.1. Dispersion of the scattering particles Assuring uniform dispersion of the scattering particles in the nanostructure is essential in the fabrication of the nanostructured electrode. If coagulation between the scattering particles were occurred, it eventually makes sedimentation of the scattering particles and the efficiency of the scattering particles would be decreased. Fig. 6 shows SEM micrographs of the fabricated nanostructured electrodes. As can be seen from the figure, no appreciable agglomeration was observed regarding the scattering particles. The dispersion of the scattering particles was also very good.

4.3. Reflectance and contrast ratio Fig. 9(a) shows the measurement geometry for the angular distribution of the scattered light and Fig. 9(b) shows the measured results for the mirror-type electrochromic device at a wavelength of 550 nm. The measurement was made with an LCD evaluation system (Otsuka optics co. Japan). The reflectance was calibrated with a BaSO4 white reference. This measurement geometry mimics the actual structure of 4

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Fig. 6. SEM micrographs of the fabricated TiO2 nanostructured electrodes with various contents of the scattering particles; (a) 0 vol% (b) 1.0 vol% (c) 2.0 vol% (d) 3.0 vol%.

figure, the reflectance spectrum of the bleached state was dramatically improved by the addition of the scattering particles. As a result, the contrast ratio was also improved. The contrast ratio can be evaluated with the luminous reflectance, defined as the weighted average of the reflectance with the CIE (International Commission on Illumination) 1931 eye sensitivity function [24];

RL =

∫ R (λ) V (λ) dλ ∫ V (λ) dλ

(3)

where RL is the luminous reflectance, R (λ ), the measured reflectance spectrum, and V (λ ) is the CIE 1931 eye sensitivity function. The luminous reflectance of the bleached state was improved from 5.7% to 45.3% by the addition of the 3.0 vol% of the scattering particles while the contrast ratio was improved from 5.0 to 11.5. 4.4. Color spectrum and color gamut Fig. 11 shows the measured color spectra of the electrochromic devices, with Fig. 11(a) specifically showing the geometry of the measurement and Fig. 11(b) and (c) showing the measured spectra for the devices without the scattering particles and with 3.0 vol% of scattering particles respectively. The measurement geometry was almost the same as that of Fig. 10, the only difference being the addition of a color filter on the surface of the devices. The measurements were made with the detector angle at 15° and 30°. As can be seen in the figures, the color spectra were also dramatically improved by the addition of the scattering particles. The color gamut of the devices can be evaluated with these color spectra. The color gamut means that portion of the color space that can be reproduced by the display and it usually represented as an area in the CIE1931 color space [25]. The color gamut of a display is the area of the triangle by the three primary color; red, blue and green.

Fig. 7. Schematic diagram showing the measurement geometry with the integrating sphere.

the display. As it can be seen from the figure, a dramatic increase in the reflectance in non-specular angle was observed by the addition of the scattering particles. These results imply that the viewing angle of the device can be dramatically improved by adding the scattering particles. Fig. 10(a) shows the results for the device without the scattering particles and Fig. 10(b) shows the results for the device with 3.0 vol% of scattering particles. The spectra were measured in bleached and colored states at a measurement angle of 15°. As can be seen from the 5

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Fig. 8. Measured results with the integrating sphere: (a) the diffuse transmittance and (b) the total transmittance.

Fig. 9. Angular distribution of the scattered light measured in the reflection geometry; (a) the measurement geometry and (b) measured reflectance at a wavelength of 550 nm.

Fig. 10. Reflectance spectra of the electrochromic devices measured at a detector angle of 15°: (a) spectra without the scattering particles and (b) spectra with 3.0 vol% scattering particles.

ground. As can be seen from the Fig. 11, the background noises increase steeply in a wavelength region below 450 nm. For the case of the device without the scattering particles this background noise seriously affects the color gamut of the device because the color reflectance is very low for the device. Especially, the color purity of the green and the red are seriously affected by the noise and it was the reason of the low color gamut of the device. To the contrary, for the case of the device with the scattering particles, the color reflectance was sufficiently larger than the

Fig. 12 shows the change of the color gamut, normalized with the National Television System Committee (NTSC) standards as a function of the contents of the scattering particles. It is clear that the color gamut of the device can be improved by the addition of the scattering particles. In the case where the detector angle was 15°, the color gamut was improved from 7.6% to 44.0% by the addition of 3.0 vol% of scattering particles. The low color gamut of the device without the scattering particles is related the low reflectance of the device compared to the noise back6

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Fig. 11. Color spectra of the electrochromic device: (a) the measurement geometry, (b) the measured color spectra without the scattering particles, (c) the measured color spectra with 3.0 vol% scattering particles.

The coloration reaction between dication – radial cation was utilized in the present research. The dication state is transparent and the radical cation state has a yellowish color. The radical cation TPA molecules absorb the residual blue color of the WO3 film and the combined device was yellowish gray in the colored state. Fig. 13 shows the electrochromic properties of the window-type devices without a backside Ag film, with Fig. 13(a) showing the spectra of the devices and Fig. 13(b) showing the switching characteristics at a wavelength of 550 nm. All the samples were operated with the same driving voltage of −1.4 V for coloration and +0.8 V for bleaching. At first glance, the transmittance of the bleached state appears to decrease as a result of the addition of the scattering particles. This is because only the linear transmittance was measured here. The changes in the total transmittances were very small as can be seen in Fig. 8(b). And, any noticeable difference in the coloration efficiency was not observed by the addition of the scattering particles (S1). These results clearly show that the scattering particles do not affect the electrochromic properties of the device. The scattering particles are considered to be inert in the electrochromic function of the devices.

Fig. 12. Color gamut evaluated with the measured spectra of Fig. 11.

noise and the color purities of the green and red colors were not seriously affected by the noise. The improvement of the color gamut by the addition of the scattering particle was due to the improvement of color purities of the green and red colors.

5. Conclusion An electrochromic device with a self-diffuse-transmitting function has been developed by modifying a TiO2 nanostructure-based electrochromic device. Silicon nitride particles were incorporated into the TiO2-nanostructured electrode to induce diffuse transmission. Improvements in the diffuse transmittance were observed without loss of total transmittance. Improvements in the viewing angle were also observed. The developed electrochromic device is applicable to hybridtype displays. This technology also potentially can be applied to design

4.5. Electrochromic properties One of the most important requirements for the present research is that the scattering particles should not affect the electrochromic properties. The TPA molecules have an anodic electrochromic character and undergo two step oxidation/reduction reactions as follow [18]:

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