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Ultra-high transparent sandwich structure with a silicon dioxide passivation layer prepared on a colorless polyimide substrate for a flexible capacitive touch screen panel
Solar Energy Materials & Solar Cells 207 (2020) 110350
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Ultra-high transparent sandwich structure with a silicon dioxide passivation layer prepared on a colorless polyimide substrate for a flexible capacitive touch screen panel Chia-Ching Wu Department of Applied Science, National Taitung University, Taitung, Taiwan, Republic of China
A R T I C L E I N F O
A B S T R A C T
Keywords: ITO/Ag/ITO X–Y multilayer electrode Passivation layer Transfer matrix Touch screen panel
Indium tin oxide (ITO)-based multilayer structures with ultrathin silver (Ag) deposition on colorless polyimide (CPI) were investigated as touch sensor electrode materials in this study. The electrical, optical, structural, and morphological properties of the ITO-based multilayer structures were compared. The figure of merit of the ITO/ Ag/ITO(160)/CPI multilayer structure (bottom ITO film deposited at 160 � C), in which the bottom ITO layer was deposited at 160 � C, was calculated using Haacke’s formula, and the value was found to be 63.6 � 10 3 Ω 1. Moreover, the sheet resistance and optical transmittance at 550 nm were 6.4 Ω/□ and 91.4%, respectively. Transmission electron microscopy measurements demonstrated that the continuous Ag layer had a uniform film of 10.9 nm. When an ITO/Ag/ITO(160) multilayer structure was used as a substitute for an X–Y ITO layer electrode for fabricating a flexible capacitive-type touch screen panel (FCTSP) and a low-refractive-index silicon dioxide (SiO2) layer was used as the passivation layer, the ultra-high optical transmittance of the SiO2/ITO/Ag/ ITO(160) multilayer structure was measured to be 94.6% (at 550 nm), and the average optical transmittance over the visual region (400–700 nm) was measured to be 92.2%. An FCTSP with multiple touch points was successfully fabricated using an X–Y ITO/Ag/ITO(160) multilayer electrode and SiO2 passivation layer. Finally, we also derive the related theoretical formulas to fix our experimental data. Moreover, based on the theory, we can predict the optimized thickness to support the desirable ultra-high transparency of multilayer structures. This auxiliary simulation also paves an alternative route to explore the possibilities of optimized multilayer flexible devices.
1. Introduction Traditional electronics are based on integrated circuits that are mostly manufactured using rigid and planar semiconductor wafers or glass substrates. These wafers or substrates cannot be used for irregular, soft, or moving objects [1]. This drawback of traditional electronics inspired the development of flexible electronic technology. In recent years, devices with novel concepts, such as wearable electronics and bendable displays, have advanced rapidly and have been used for various functions [2–6]. These devices require flexible or stretchable touch sensors to provide users with a convenient input system. The development of flexible, transparent electrode materials is essential for producing touch sensors for such flexible or stretchable devices. Trans parent electrodes have been used in various industries, and they are especially popular for application in optoelectronic devices. In addition to touch sensors, transparent electrodes have been used in flat panel
displays, light-emitting diodes, solar cells, and other devices [7–10]. In the early stage of development of a transparent electrode, a transparent conducting oxide (TCO) was deposited on a rigid substrate, such as glass, and applied to flat-type devices. To date, one of the most commonly used materials for TCO is indium tin oxide (ITO), which ex hibits high optical transparency (>90% at 550 nm), high electrical conductivity (<7 � 10 5 Ω-cm), and an appropriate work function for hole injection (4.4–4.5 eV) in the crystalline state [11–13]. Moreover, ITO exhbits high chemical and physical durability and high workability [14,15]. However, ITO is fragile and rigid when applied to flexible de vices. Thus, cracks can develop while bending the device, which in crease the electrical resistance. This increase causes the performance of the device to deteriorate, thus posing a challenge to researchers [16,17]. To address this problem, various transparent electrode materials and structures have been explored for use in flexible touch sensors. In recent years, many researchers have devoted an enormous amount
E-mail address: [email protected]. https://doi.org/10.1016/j.solmat.2019.110350 Received 30 September 2019; Received in revised form 3 December 2019; Accepted 8 December 2019 Available online 13 December 2019 0927-0248/© 2019 Published by Elsevier B.V.
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of effort to the development of transparent conductive films employing thin metallic films, conductive polymers, metal oxides, metal meshes, nanowire networks, carbon nanotubes (CNTs), and graphene [18–23]. However, these methods are not suitable for use in commercial products due to disadvantages such as high cost, difficult processing, difficult mass production, and low reliability. Therefore, the focus has shifted back to ITO materials. To eliminate the critical drawbacks of the ITO films, ITO sandwiching of thin metal films has been extensively inves tigated [24,25]. Compared with single-layer ITO films, the ITO/meta l/ITO multilayer structures can effectively suppress the reflection from the metal layer in the visible range. The ITO/metal/ITO multilayer structures have almost the same transmittance as that of an ITO film (<85%) and yield better electrical conductivity (<10 4 Ω-cm). The middle ductile metal layer can improve the flexibility of an ITO film, and thus, this domain has generated considerable research interest. Gold (Au), silver (Ag), copper (Cu), molybdenum (Mo), aluminum (Al), and other materials have been used in different studies as the intermediate metallic layers in TCO/metal/TCO multilayer structures. Therefore, ITO/metal/ITO [26,27], GZO/Cu/GZO [28], IZO/metal/IZO [29], ZnO/Cu/ZnO [30], AZO/Ag/AZO [31], NTO/Ag/NTO [32], and ZTO/Ag/ZTO [33] multilayer structures have been reported. However, Ag has been found to be the best candidate for this application due to its lowest absorption coefficient (5%) and lowest refractive index (n ¼ 0.05) in the visible region of 400–700 nm [34,35]. To date, many scholars have conducted studies to determine how to further increase the transmittance of the ITO film and ITO/metal/ITO multilayer film while reducing the resistivity through the buffer or passivation layers. Some researchers have considered the effect of introducing a buffer layer or a passivation layer such as SiO2, ZnO, Al2O3, or Nb2O5. The objective of this study is to evaluate the transmittance change in the ITO and ITO/metal/ITO multilayer films by using materials with high and low refractive indexes. In this study, we studied the structural, electrical, and optical properties of the SiO2/ITO/Ag/ITO multilayer structures that were sputtered on a colorless polyimide (CPI) substrate. In the structures, SiO2 was selected as the passivation layer material, because SiO2 has a low refractive index (n ¼ 1.46). The SiO2 passivation layer can integrate in touch screen panels (TSPs). The thickness effects of the Ag and ITO films in the ITO/Ag/ITO multilayer structures were investigated, and the possibility of using ITO/Ag/ITO multilayer electrodes was demonstrated as a replacement for conventional ITO electrodes. Under optimized conditions, we fabricated a flexible capacitive-type TSP by using an ITO/ Ag/ITO multilayer electrode and a SiO2 passivation layer. The multi touch panel is the mainstream interaction technology of mobile appli cations. Several approaches can realize this technology, such as surface acoustic waves, infrared rays, and capacitive sensing. Compared with other competitive methods of realizing TSPs, capacitive-type TSPs have low cost, high sensitivity, high response rate, and good durability. Therefore, capacitive-type TSPs have been used as substitutes for con ventional resistive-type TSPs and have been applied to portable devices, such as smartphones, tablet computers, and wearable electronics. Thus, this study provides a practical solution to the existing problems of characterizing flexible TCO materials and their TSP applications.
reaction gas, and the working pressure was maintained at 5 � 10 3 torr. To fabricate the ITO/Ag/ITO multilayer structures, the bottom ITO film was first deposited on the CPI substrates with a deposition power at 80 W and the substrates temperatures were set to room temperature or 160 � C during the deposition process. Second, the Ag thin metal films with various deposition times were deposited with a deposition power of 50 W. Finally, the top ITO film was deposited at room temperature at 80 W. To increase the optical transmittances of the ITO/Ag/ITO multilayer structures, a silicon dioxide (SiO2) thin film was deposited on the top ITO to form SiO2/ITO/Ag/ITO multilayer structures. The SiO2 thin film with various deposition times were deposited with a deposition power of 80 W at room temperature. All the samples were annealed in air for 1 h at 300 � C. The resulting multilayer structures were characterized by high res olution scanning electron microscopy (HR-SEM), high resolution trans mission electron microscopy (HR-TEM), and atomic force microscopy (AFM) to obtain their surface images, high-resolution cross-sectional images, and surface roughness, respectively. The crystallinity of the multilayer structures were inspected by X-ray diffraction (XRD) by using CuKα radiation (λ ¼ 1.5406 nm). By using a UV–Vis spectrometer, the optical transmittances of the multilayer structures were measured at wavelengths between 300 and 1100 nm. The resistivity (ρ), carrier concentration (n), and mobility (μ) of the multilayer structures were obtained from Hall-effect measurements. The mechanical parameters of the multilayer structures were investigated using a computer-controlled bending test machine. The depth profiles of the multilayer structure were measured using secondary ion mass spectroscopy with 1 keV Oþ2 primary ions. Now, it turn to theoretical part to investigate the elec tromagnetic field transmittance. We consider a monochromatic plane wave was irradiated upon multilayer structures. The simulation results of the optical transmittance of the multilayer structures were obtained transfer matrix. Fig. 1 depicts our studying system. The matrix, M0,1, denotes the wave from vacuum to the first layer with k0 being wave number in vacuum and k1 being wave number in the first material. Mi,j denotes the wave propagating from media i to media j. Each elements in these matrixs contain the complete information involving wave input and output. 3 2 k0 k0 1þ 1 6 k1 k1 7 16 7 M0;1 ¼ 6 (1) 7 24 k0 k0 5 1 1þ k1 k1
2. Experiments and theoretical simulations
where ε1 ¼ n2 k2 is the real part of the permittivity and ε2 ¼ 2nk is the imaginary part of the permittivity. The refractive index (n) and extinc tion coefficient (k) values were measured using the ellipsometric
2
ki 1þ kj 16 6 Mi;j ¼ 6 24 ki 1 kj
3 ki kj 7 7 7 ki 5 1þ kj 1
(2)
pffiffiffi pffiffiffi here, ki and kj are defined as ki ¼ εi k0 and kj ¼ εj k0 , where ε is the relative permittivity. The permittivity is calculated as follows:
ε ¼ ε1 þ ε2
The indium tin oxide/silver/indium tin oxide (ITO/Ag/ITO) multi layer structures were deposited on a colorless polyimide (abbreviated as CPI, HannsTouch Solution Incorporated) substrate with an area of 60 � 60 mm2 by using a radio frequency magnetron sputtering system. The 2inch ITO target used in this study was a 10 wt% SnO2-doped In2O3; a silver target was used for depositing the insertion layer between the ITO thin films. The distance between the CPI substrate and the target was approximately 5 cm. The sputtering deposition parameters of the ITO film and Ag thin metal film were obtained. The base pressure of the sputtering system is 3 � 10 6 torr. Then, we introduced argon as a
Fig. 1. The schematic diagram of the multilayer structure system. 2
(3)
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instrument in the range of 300–1100 nm. Now, we apply the transform matrix to final result as follows: " þ # " þ# " þ# Ein Ein Eout tot ¼ M5;0 Ml5 M4;5 Ml4 M3;4 Ml3 M2;3 Ml2 M1;2 Ml1 M0;1 �M Eout Ein Ein " tot #" þ # tot M 11 M 12 Ein ¼ tot tot M 21 M 22 Ein
11.4 nm, 14.2 nm, and 17.2 nm with deposition times of 20 s, 40 s, 60 s, 80 s, 100 s and 120 s, respectively. In this study, the increase in the Ag deposition times from 20 to 40 s decreased the optical transmittance of the ITO/Ag/ITO/CPI multilayer structures at 550 nm due to severe light scattering due to the separate islands randomly distributed between the ITO layers. Because the Ag islands act as a scattering source of incident light, the ITO/Ag/ITO/CPI multilayer structures present a bluish color. The TEM images of the ITO/Ag/ITO/CPI multilayer structures at Ag deposition times of 40 and 80 s are presented in Fig. 2 (b) and (c), respectively. Fig. 2(b) presents that separate Ag islands inset the ITO layers at the Ag deposition time of 40 s. However, as the Ag deposition time increased from 40 to 80 s, the Ag islands coalesced to a continuous film that remarkably increased the optical transmittance in the same wavelength region. In Fig. 2(c), the Ag continuous film inset between the ITO layers was obtained at the Ag deposition time of 80 s. When an optimum Ag layer is embedded between two oxide layers, the resulting oxide/metal/oxide multilayer structures can suppress reflections from the Ag thin metal film. The oxide/metal/oxide multilayer structures exhibit a high optical transmittance in the visible wavelength region [37–40]. Therefore, the optical transmittance of the ITO/Ag/ITO/CPI multilayer structures at 500 nm (88.7%) was obtained when the Ag deposition time was set to 80 s. As the Ag deposition time was further increased, the optical transmittance of the ITO/Ag/ITO/CPI multilayer structures decreases linearly due to the higher thickness of the Ag layer with the transparent region shrinking and shifting to lower wavelengths. This shift is mainly caused by the increase in the reflectance, as induced by the increase in the plasma resonance frequency (ωp) of the free electron gas within the inserted Ag layer [41]. Consequently, by inserting an Ag layer with different deposition times between the ITO layers, the effective plasma resonance frequency of the ITO/Ag/ITO/CPI multilayer structures can be tuned. To further explain such a phenomenon, a simulated optical trans mittance based on the transfer matrix theory is presented in Fig. 2(d). The simulation results indicate the changes in optical transmittance at different thicknesses of the Ag layer, which are identical to those ob tained from the measured optical transmittance except for 20 and 40 s Ag deposition times. As we assumed that the Ag layer is a continuous film in the simulation multilayer structure, the morphology of the Ag layer is a separate island film in the real structure. To further increase the optical transmittance of the ITO/Ag/ITO/CPI multilayer structures, the bottom and top ITO films with different thicknesses were investigated, as shown in Fig. 3. The optical trans mittance at 550 nm was 83.8%, 84.2%, 85.1%, 89.5%, 87.2%, and 88.4% at ITO deposition times of 90, 100, 110, 120, 130 and 140 s, respectively. The influence of the thickness of the ITO film was lower than the influence of thickness of the Ag film on the optical trans mittance in the ITO/Ag/ITO/CPI multilayer structure. The above results reveal that the high optical transmittance of the ITO/Ag/ITO/CPI multilayer structures are obtained at the Ag and ITO deposition times of 80 s and 120 s, respectively.
(4) here, to connect our developed formulas with experiment data, we only consider optical transmittance, as follows formula: � þ �2 � � �E � �detM tot �2 � � � T ¼ �� out (5) þ � ¼� tot � Ein M 22 To investigate the feasibility of using the ITO/Ag/ITO multilayer structures as a X–Y electrode for realizing a flexible capacitive-type touch screen panel (FCTSP), we fabricated a single-sided ITO (SITO) structure of the FCTSP. The ITO/Ag/ITO electrode was directly depos ited and then patterned using conventional photolithography and wetetching processes. The SiO2 passivation layer was used to enhance the optical transmittance of the ITO/Ag/ITO electrode. The flexible printed circuit board (FPCB) bonding with metal pattern progressed using the anisotropic conductive film, and the FPCB was connected to the inte grated circuit controller. 3. Results and discussion Fig. 2(a) presents the optical transmittance of the CPI substrate and the ITO/Ag/ITO/CPI multilayer structures as a function of the Ag deposition time. The thickness of the CPI substrate was 10 μm and the optical transmittance of the CPI substrate was 98.1% in the visible re gion and near-infrared region (NIR), as shown in Fig. S1. The optical transmittance of the ITO/Ag/ITO/CPI multilayer structures was criti cally influenced by the deposition time of the inserted Ag layer. The optical transmittance of the ITO/Ag/ITO/CPI multilayer structures present a sharp increase in the near-ultraviolet (NUV) range. The visible range decreases slightly with a subsequent transmittance increase in the NIR at the Ag deposition times of 20 and 40 s. The ITO/Ag/ITO/CPI multilayer structures present a high optical transmittance in the visible region with a subsequent slight decrease in the optical transmittance in the NIR as the Ag deposition time increases from 60 to 80 s. When the Ag deposition time is further increased from 100 to 120 s, the optical transmittance decreases again in the visible region and rapidly decreases in the NIR. The optical transmittance of the ITO/Ag/ITO/CPI multilayer structures at 550 nm were 74.5%, 69.2%, 77.9%, 89.5%, and 81.9% at Ag deposition times of 20, 40, 60, 80, 100, and 120 s, respectively. The optical and electrical properties of very thin metal films depend considerably on their structures [36]. The thicknesses of the Ag thin metal film measured by HE-SEM images were 2.9 nm, 5.7 nm, 8.5 nm,
Fig. 2. (a) Measurement, (d) simulation UV–Vis spectra, and (b)–(c) TEM images of the ITO/Ag/ITO/CPI multilayer structures as a function of Ag deposition time. 3
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Fig. 5. The resistivity (ρ), carrier concentration (n), and mobility (μ) of the ITO/Ag/ITO/CPI multilayer structures as a function of ITO deposition time.
Fig. 3. UV–Vis spectra of the ITO/Ag/ITO/CPI multilayer structures as a function of ITO deposition time.
multilayer structures increased by approximately two to three orders of magnitude upon insertion of a 10.9-nm Ag film compared with the carrier concentration of the structures when bare ITO film was used. As discussed by Kloppel et al., the metal interlayer can act as an electron source for the oxide layer in the TCO/metal/TCO structure [42]. The carrier concentration of the Ag metal film is approximately n(Ag) ¼ 1 � 1022 cm3. Therefore, the electrons in the Ag layer can be easily injected into the ITO layer due to the downward bending of the band at the interface of the Ag (ϕm ¼ 4.4 eV) and ITO layers (ϕs ¼ 4.5–5.1 eV) by using different work functions in the ITO/Ag/ITO/CPI multilayer structures [43,44]. The resistivity of the ITO/Ag/ITO/CPI multilayer structures decreased from 7.34 � 10 5 to 1.83 � 10 5 Ω-cm as the ITO deposition time increased from 90 to 140 s. The change in resistivity of the ITO/Ag/ ITO/CPI multilayer structures with increase in ITO deposition time can be explained using the following basic relation:
The XRD measurement results of the ITO/Ag/ITO/CPI multilayer structures as a function of the ITO deposition are presented in Fig. 4. When the Ag layer is absent, no diffraction peak can be observed. This implies that the as-deposited ITO film has an amorphous structure. Irrespective of the ITO films, all the ITO/Ag/ITO/CPI multilayer struc tures have peaks at 2θ values of 38.1� , 44.5� , and 64.7� corresponding to the (111), (200), and (220) phases of Ag (JCPDS card No. 04–0783), respectively. No second phase or unknown phase exists in the XRD re sults in the ITO deposition time from 90 to 120 s. Furthermore, for the ITO deposition times of 130 and 140 s, there is a broad diffraction peak around the 2θ value of 31.3� that seems to consist of several peaks at 2θ values of 30.7� and 35.63� that correspond to the (222) and (400) phases of In2O3 (JCPDS card No. 06–0416), respectively. Fig. 5 exhibits the resistivity ρ, carrier concentration n, and mobility μ of the ITO/Ag/ITO/CPI multilayer structures as a function of ITO deposition times. The carrier concentration and mobility moderately increase from 2.73 � 1020 to 7.49 � 1021 cm 3 and from 1.32 to 118.62 cm2/V as the ITO deposition time increases from 90 to 140 s, respec tively. As the ITO film is an amorphous structure, an increase in the ITO deposition time causes an increase in the thickness of the ITO film. This increase in thickness causes an increase in the carrier concentration and mobility. Moreover, the carrier concentration of the ITO/Ag/ITO/CPI
ρ¼
1 neμ
(6)
where ρ is the resistivity, n is the carrier concentration, e is the charge of the carrier, and μ is the mobility [45]. From this equation, we note that changes in the carrier concentration and mobility can affect the re sistivity. Moreover, the resistivity of the ITO/Ag/ITO/CPI multilayer structures at different ITO deposition times decreases by a wide range than the resistivity of a single-layer ITO thin film. In a previous study, as the Ag layer was less than 10 nm, the Ag was randomly distributed in unconnected “islands” and the Ag exhibited a fairly high resistivity. However, as the thickness of Ag layer is thicker than 10 nm, it completely covered the bottom layer and reduced the resistivity [46]. In this study, the thickness of the Ag layer is 10.9 nm. Fig. 6 presents the figure of merit (FOM), sheet resistances, and op tical transmission (550 nm) of the ITO/Ag/ITO/CPI multilayer struc tures. The sheet resistances of the ITO/Ag/ITO/CPI multilayer structures decreased as a function of the ITO deposition times. In this study, it assumed that the total resistance of the ITO/Ag/ITO/CPI multilayer structures can be represented simply as resistances coupled in a parallel structure of the bottom ITO, Ag, and top ITO layers. Rtotal ¼
2 1 þ RITO RAg
(7)
where Rtotal, RITO, and RAg are the sheet resistance of multilayer struc tures, ITO, and Ag, respectively. Equation (7) reveals that as the thick ness of the top and bottom ITO films increased, the sheet resistance of the top and bottom ITO films slightly decreased. Thus, the Rtotal values decreased. To obtain the best combination of high transmission and low
Fig. 4. XRD analysis of the ITO/Ag/ITO/CPI multilayer structures as a function of ITO deposition time. (a) 90 s, (b)100 s, (c) 110 s, (d) 120 s, (e) 130 s, (f) 140 s (■:ITO phase and ●:Ag phase). 4
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(160)/CPI multilayer structure presented in Fig. 6(a), the image presents that the bottom ITO film has a complete crystallization structure due to its deposition at a substrate temperature of 160 � C, and the amorphous structure was obtained in the top ITO film. This result reveals that the interfacial layer between top ITO, bottom ITO, and Ag layers exist without any chemical compound. To investigate the elemental distribution along the vertical path of the ITO/Ag/ITO(160) multilayer structure, the SIMS analysis and the results can be seen in Fig. 6(b). The main elements of the ITO/Ag/ITO (160) multilayer structure were In, Sn, O, and Ag. The concentration of Ag increases quickly in the middle region. Thus, the concentration of the middle region reaches that of the Ag layer. The thickness of the Ag layer was 10.9 nm at the deposition time of 80 s. Overall, the interface be tween the top ITO layer and the Ag layer and between the Ag layer and the bottom ITO layer is very clear, which can also be seen in Fig. 7(b). Based on the ITO/Ag/ITO(160)/CPI multilayer structure, the sheet resistance and optical transmittance at 550 nm were 6.4 Ω/□ and 91.4%, respectively, and the value of the FOM is 63.6 � 10 3 Ω 1. The resistance change (ΔR) in the ITO/Ag/ITO(160)/CPI multilayer struc ture was measured using the outer bending radius, and the bending radius was set to 3 mm based on the commercial standard. Changes in the resistance of the ITO/Ag/ITO(160)/CPI multilayer structure can be expressed as ΔR(%) ¼ (R1 R0)/R0, where R0 is the initial resistance and R1 is the measured resistance after bending. During 30,000 bending cycles, the ΔR value of the ITO/Ag/ITO(160)/CPI multilayer structure is 4.12%, as shown in Fig. S2. The ITO/Ag/ITO(160)/CPI multilayer structure exhibited less change in resistance after 30,000 bending cycles. This result can be attributed to the high strain failure of the metallic Ag thin interlayer between the ITO layers and the local delamination or crack formation in the ITO layers. The optical transmittance of the ITO/ Ag/ITO(160)/CPI multilayer structure was higher than that of the ITO (160)/Ag/ITO(160)/CPI and ITO/Ag/ITO/CPI multilayer structures (Fig. S3). This was caused by the crystallized bottom ITO film, which exhibited a higher optical transmittance and lower resistivity than the values of the amorphous film [49,50]. Fig. 8 presents the structure of the FCTSP that includes the CPI substrate, ITO bridge, insulator, X–Y ITO, and metal and passivation layers. In this study, the ITO/Ag/ITO multilayer was investigated to be used as a substitute for the X–Y ITO layer for enhancing the reliability of the optical transmittance and resistivity. The passivation layer covered both the ITO/Ag/ITO multilayer structure and a metal layer of the FCTSP, as illustrated in Fig. 7. It is noteworthy that the transmittance is more sensitively modulated by top ITO than bottom ITO in the ITO/Ag/ ITO multilayer structure. This is due to the top ITO layer being in contact with air, so the amplitude of the reflected wave at the interface is rela tively large, leading to a strong interference effect. Therefore, SiO2 is selected as the passivation layer because it is a low-refractive-index material. The optical transmittance of the SiO2/ITO/Ag/ITO(160)/CPI multi layer structures as a function of the SiO2 deposition times are shown in Fig. 9(a)–(b). The optical transmittance of the SiO2/ITO/Ag/ITO(160)/ CPI multilayer structures is influenced by the SiO2 deposition time. The
Fig. 6. The figure of merit (FOM), sheet resistances and transmission (550 nm) of the ITO/Ag/ITO multilayer structures as a function of ITO deposition time.
resistivity, the FOM of the films was calculated using the Haacke’s equation [47]: ϕTC ¼
T 10 av Rs
(8)
where Tav is the optical transmittance of the ITO/Ag/ITO/CPI multilayer structures at a wavelength of 550 nm and Rs is the sheet resistance. Tav can be estimated using eq. (9): R vðλÞTðλÞdðλÞ (9) Tav ¼ R vðλÞdðλÞ where T(λ) is the optical transmittance and v(λ) is the photonic luminous efficiency function defining the standard observer for photometry [48]. In Fig. 6, the maximum value of the FOM of 25.4 � 10 3 Ω 1 can be observed at an ITO deposition time of 120 s. The sheet resistance and optical transmittance at 550 nm were 13 Ω/□ and 89.5%, respectively. From the above results, the high optical transmittance and low resis tance of the ITO/Ag/ITO/CPI multilayer structure was obtained for the Ag and ITO deposition times of 80 and 120 s, respectively. To produce the ITO/Ag/ITO/CPI multilayer structures with higher optical transmittance, different deposition parameters were further investigated. We know that the crystallization ITO film exhibits higher transmission than the amorphous ITO film. Fig. 7 presents (a) the high resolution TEM (HR-TEM) image and (b) SIMS results of the ITO/Ag/ ITO/CPI multilayer structures at a deposition temperature of 160 � C for the bottom ITO film [ITO/Ag/ITO(160)/CPI]. For the ITO/Ag/ITO
Fig. 7. (a) TEM image and (b) SMIS analysis of the ITO/Ag/ITO/CPI multilayer structure with the deposition temperature at 160 � C for the bottom ITO thin film.
Fig. 8. Structure of the touch sensor with ITO/Ag/ITO multilayer structure and SiO2 passivation layer. 5
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Fig. 9. (a) Measurement and (b) Simulation UV–Vis spectra of the SiO2/ITO/Ag/ITO(160)/CPI multilayer structures as a function of SiO2 deposition time.
optical transmittance values at 550 nm were 93.5%, 94.6%, 91.8%, and 85.1% at SiO2 deposition times of 5, 10, 15, and 20 min, respectively. The average optical transmittance at 400–700 nm of the SiO2/ITO/Ag/ ITO(160)/CPI multilayer structures were 89.99%, 92.16%, 89.6%, and 83.2% at SiO2 deposition times of 5, 10, 15, and 20 min, respectively. The maximum optical transmittance and average optical transmittance were obtained at a SiO2 deposition time of 10 min. In the visible range, the average optical transmittance of the SiO2/ITO/Ag/ITO(160)/CPI multilayer structures is larger than that of the ITO/Ag/ITO/CPI (84.7%) and ITO/Ag/ITO(160)/CPI (85.8%) multilayer structures, as shown in Fig. 9(a). The SiO2 passivation layer enhances the optical transmittance at 550 nm and average optical transmittance in the range of 400–700 nm. This result was attributed to the low refractive index of SiO2 (n ¼ 1.46, air/SiO2/ITO/Ag/ITO/CPI/air). Moreover, the SiO2 passivation layer increases the optical transmittance in the NIR, as shown in Fig. 9 (b). The multilayer structure exhibited a reduced optical loss in the NIR, thus suggesting its potential use in plasmonic applications in addition to the use in touch panel application. The simulation results indicate the changes in the optical transmittance at different thicknesses of the SiO2 layer, as shown in Fig. 9(c). These changes are identical to those observed from the optical transmittance measurement. The cross-sectional TEM image of the SiO2/ITO/Ag/ITO(160)/CPI multilayer structure is shown in Fig. 10. The gray, black, and light-gray images clearly present the multilayer structure comprising the bottom ITO, Ag, top ITO, and SiO2 layers, as shown in Fig. 10. The thicknesses of the bottom ITO, Ag, top ITO, and SiO2 layers are 21.6, 10.9, 24.7, and 21.3 nm, respectively. This structure was applied to the proposed FCTSP. Fig. 11 illustrates the evolution of the surface morphology and roughness during the growth of the multilayer by investigating the
Fig. 10. TEM image of the SiO2/ITO/Ag/ITO(160)/CPI multilayer structure with the 10 min SiO2 deposition time.
build-up samples with SEM and AFM. As presented in Fig. 11 (a) and (d), the root-mean-square (RMS) value of the 21.6-nm-thick crystallized ITO film deposited on a CPI substrate at room temperature is 0.61 nm. The RMS value of the ITO film deposited at a deposition temperature of 160 � C is 0.76 nm, this value is higher than that deposited at room temper ature. The ITO/Ag/ITO(160)/CPI multilayer structures exhibits a higher RMS value of 1.39 nm, as shown in Fig. 11(c) and (g). This result was caused by the crystallized structure of the bottom ITO film and the Ag layer with the (111) orientation, which induces higher surface rough ness in the top ITO film. Finally, as presented in Fig. 11(d) and (h), the ITO/Ag/ITO(160)/CPI multilayer structures with an SiO2 layer of 23.3 nm of presents an RMS value of 0.96 nm. The SiO2 film is an amorphous structure and is deposited on the ITO/Ag/ITO(160)/CPI multilayer 6
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Fig. 11. SEM images and AFM analysis of the (a)(e) ITO/CPI, (b)(f) ITO(160)/CPI, (c)(g) ITO/Ag/ITO(160)/CPI and (d)(h) SiO2/ITO/Ag/ITO(160)/CPI structures. Scan area is 1 μm � 1 μm.
structure as the passivation layer. Thus, the surface roughness of the SiO2/ITO/Ag/ITO(160)/CPI multilayer structure decreases. The decrease in the roughness reduces the scattering effect and enhances the optical transmittance of the SiO2/ITO/Ag/ITO(160)/CPI multilayer structure. The multiple touch point operation of an ultra-high transmittance FCTSP was tested; the FCTSP was fabricated using an ITO/Ag/ITO(160) X–Y electrode and an SiO2 passivation layer, as shown in Fig. S4. The operation of the ultra-high transmittance FCTSP with the ITO/Ag/ITO (160) electrode indicates that the ITO/Ag/ITO(160) multilayer structure with a low sheet resistance and high reliability is a promising trans parent electrode that can be used as a substitute for the conventional ITO electrode. The ultra-high transmittance FCTSP was operated by exact sensing of X–Y coordinates and linearity characteristics. Before by connecting the ultra-high transmittance FCTSP with software, the touch panel was connected to the touch circuit controller, and the touch panel was operated. As presented in Fig. 12, an ultra-high transmittance FCTSP with a liquid crystal display (LCD) was fabricated; it was slightly similar to a Samsung Galaxy S7 edge. The inset of Fig. 12 presents the well pattern of the touch sensor with the ITO/Ag/ITO(160) electrode that was patterned using photolithography and wet-etching processes. The positive photoresist was used in photolithography process. The wet etchant is a hydrochloric acid and nitric acid mixture. To evaluate the practical usage of the FCTSP, we lightly touched the FCTSP, and the three touch points were displayed on a notebook monitor, as shown in Fig. 12. This test verified that this FCTSP can detect multiple touch points. In the near future, the ultra-high transmittance FCTSP will be combined with an organic light-emitting diode (OLED) to form flexible displays for use in flexible smartphones.
Fig. 12. FCTSP fabricated with the ITO/Ag/ITO(160) multilayer structure with SiO2 passivation layer demonstrates the multiple touch point function.
transmittance of the SiO2/ITO/Ag/ITO(160) multilater structure to 94.6% at 550 nm. Moreover, by using the transfer matrix theory, the optical transmittance trend of the multilayer structure was simulated. The simulation results are in agreement with the measurement results. These results suggest that the SiO2/ITO/Ag/ITO(160) multilayer struc ture can be used as an alternative to the ITO electrode. Moreover, the structure enhances the optical transmittance and resistivity and thus can be applied to wearable electronics and bendable displays.
4. Conclusion The ultra-high optical transmittance and low resistivity electrode of the ITO/Ag/ITO(160) multilayer structure was studied as a substitute for the conventional X–Y ITO electrode for fabricating an FCTSP. The tunable electrical and optical properties of the ITO/Ag/ITO(160) multilayer structures were critically influenced by the thickness and morphology of the inserted Ag layer because the Ag layer dominates the electrical properties and is the main parameter that affects the antire flection effect. During the 30,000 bending cycles, the change in the resistance (ΔR) of the ITO/Ag/ITO(160)/CPI multilayer structure was 4.12% because of the high strain failure of the metallic Ag thin interlayer between the ITO layers. In this study, SiO2, a low refractive index ma terial, was used as the passivation layer. This layer increases the optical
Author contributions section C.C. Wu participated in the Hall, HR-SEM, HR-TEM, UV-Via analysis of ITO/Ag/ITO multilayer structure. Notes The authors declare no competing financial interest.
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Acknowledgments
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The authors acknowledge the financial support of the Ministry of Science and Technology (MOST 107-2221-E-143-008 and 107-2622-E143-001-CC3) and 105CE02. The authors also acknowledge Hann sTouch Solution Incorporated for supplying the polyimide substrate and assisting with the fabrication of the flexible capacitive-type touch screen panel, and thanks Dr. Jeng Yi Lee for numerical simulation and discussions. Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi. org/10.1016/j.solmat.2019.110350. References [1] J.A. Rogers, T. Someya, Y. Huang, Materials and mechanics for stretchable electronics, Science 327 (5973) (2010) 1603. [2] J. Kim, M. Lee, H.-J. Shim, R. Ghaffari, H.-R. Cho, D. Son, Y.-H. Jung, M. Soh, C. Choi, S. Jung, K. Chu, D. Jeon, S.-T. Lee, J.-H. Kim, S.-H. Choi, T. Hyeon, D.H. Kim, Stretchable silicon nanoribbon electronics for skin prosthesis, Nat. Commun. 5 (2014) 5747. [3] M.-S. Lee, K. Lee, S.-Y. Kim, H. Lee, J. Park, K.-H. Choi, H.-K. Kim, D.-G. Kim, D.Y. Lee, S. Nam, J.-U. Park, Highperformance, transparent, and stretchable electrodes using Graphene metal nanowire hybrid structures, Nano Lett. 13 (2013) 2814–2821. [4] S. Lee, A. Reuveny, J. Reeder, S. Lee, H. Jin, Q. Liu, T. Yokota, T. Sekitani, T. Isoyama, Y. Abe, Z. Suo, T. Someya, A transparent bending-insensitive pressure sensor, Nat. Nanotechnol. 11 (2016) 472–478. [5] D.H. Ho, Q. Sun, S.Y. Kim, J.T. Han, D.H. Kim, J. Cho, Stretchable and multimodal all graphene electronic skin, Adv. Mater. 28 (2016) 2601. [6] M. Kaltenbrunner, T. Sekitani, J. Reeder, T. Yokota, K. Kuribara, T. Tokuhara, M. Drack, R. Schw€ odiauer, I. Graz, S. Bauer- Gogonea, S. Bauer, T. Someya, An ultra-lightweight design for imperceptible plastic electronics, Nature 499 (2013) 458–463. [7] M.G. Helander, et al., Chlorinated indium tin oxide electrodes with high work function for organic device compatibility, Science 332 (2011) 944–947, https:// doi.org/10.1126/science.1202992. [8] J. Wu, et al., Organic light-emitting diodes on solution-processed graphene transparent electrodes, ACS Nano 4 (2009) 43–48, https://doi.org/10.1021/ nn900728d. [9] K. Ellmer, Past achievements and future challenges in the development of optically transparent electrodes, Nat. Photonics 6 (2012) 809–817, https://doi.org/ 10.1038/nphoton.2012.282. [10] S.L. Hellstrom, H.W. Lee, Z. Bao, Polymer-assisted direct deposition of uniform carbon nanotube bundle networks for high performance transparent electrodes, ACS Nano 3 (2009) 1423–1430, https://doi.org/10.1021/nn9002456. [11] H.K. Lin, W.C. Hsu, Electrode patterning of ITO thin films by high repetition rate fiber laser, Appl. Surf. Sci. 308 (2014) 58–62. [12] K.H. Choi, J. Kim, Y.J. Noh, S.I. Na, H.K. Kim, Ag nanowire-embedded ITO films as a near-infrared transparent and flexible anode for flexible organic solar cells, Sol. Energy Mater. Sol. Cells 110 (2013) 147–153. [13] P. Nath, R.F. Bunshah, B.M. Basol, O.M. Staffsud, Electrical and optical properties of In2O3: Sn films prepared by activated reactive evaporation, Thin Solid Films 72 (1980) 463–468. [14] E.N. Dattoli, W. Lu, ITO nanowires and nanoparticles for transparent films, MRS Bull. 36 (2011) 782–788. [15] R.G. Gordon, Criteria for choosing transparent conductors, MRS Bull. 25 (2000) 52–57. [16] O. Tuna, Y. Selamet, G. Aygun, L. Ozyuzer, High quality ITO thin films grown by DC and RF sputtering without oxygen, J. Phys. D Appl. Phys. 43 (2010), 055402. [17] Y. Leterrier, L. M� edico, F. Demarco, J.-A.E. Månson, U. Betz, M.F. Escol� a, M. Kharrazi Olsson, F. Atamny, Mechanical integrity of transparent conductive oxide films for flexible polymer-based displays, Thin Solid Films 460 (2004) 156–166. [18] H.-G. Im, J. Jin, J.-H. Ko, J. Lee, J.-Y. Lee, B.-S. Bae, Flexible transparent conducting composite films using a monolithically embedded AgNW electrode with robust performance stability, Nanoscale 6 (2014) 711–715. [19] H.-G. Im, S.-H. Jung, J. Jin, D. Lee, J. Lee, D. Lee, J.-Y. Lee, I.-D. Kim, B.-S. Bae, Flexible transparent conducting hybrid film using a surface-embedded copper nanowire network: a highly oxidation-resistant copper nanowire electrode for flexible optoelectronics, ACS Nano 8 (2014) 10973–10979.
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Solar Energy Materials and Solar Cells journal homepage: http://www.elsevier.com/locate/solmat
Ultra-high transparent sandwich structure with a silicon dioxide passivation layer prepared on a colorless polyimide substrate for a flexible capacitive touch screen panel Chia-Ching Wu Department of Applied Science, National Taitung University, Taitung, Taiwan, Republic of China
A R T I C L E I N F O
A B S T R A C T
Keywords: ITO/Ag/ITO X–Y multilayer electrode Passivation layer Transfer matrix Touch screen panel
Indium tin oxide (ITO)-based multilayer structures with ultrathin silver (Ag) deposition on colorless polyimide (CPI) were investigated as touch sensor electrode materials in this study. The electrical, optical, structural, and morphological properties of the ITO-based multilayer structures were compared. The figure of merit of the ITO/ Ag/ITO(160)/CPI multilayer structure (bottom ITO film deposited at 160 � C), in which the bottom ITO layer was deposited at 160 � C, was calculated using Haacke’s formula, and the value was found to be 63.6 � 10 3 Ω 1. Moreover, the sheet resistance and optical transmittance at 550 nm were 6.4 Ω/□ and 91.4%, respectively. Transmission electron microscopy measurements demonstrated that the continuous Ag layer had a uniform film of 10.9 nm. When an ITO/Ag/ITO(160) multilayer structure was used as a substitute for an X–Y ITO layer electrode for fabricating a flexible capacitive-type touch screen panel (FCTSP) and a low-refractive-index silicon dioxide (SiO2) layer was used as the passivation layer, the ultra-high optical transmittance of the SiO2/ITO/Ag/ ITO(160) multilayer structure was measured to be 94.6% (at 550 nm), and the average optical transmittance over the visual region (400–700 nm) was measured to be 92.2%. An FCTSP with multiple touch points was successfully fabricated using an X–Y ITO/Ag/ITO(160) multilayer electrode and SiO2 passivation layer. Finally, we also derive the related theoretical formulas to fix our experimental data. Moreover, based on the theory, we can predict the optimized thickness to support the desirable ultra-high transparency of multilayer structures. This auxiliary simulation also paves an alternative route to explore the possibilities of optimized multilayer flexible devices.
1. Introduction Traditional electronics are based on integrated circuits that are mostly manufactured using rigid and planar semiconductor wafers or glass substrates. These wafers or substrates cannot be used for irregular, soft, or moving objects [1]. This drawback of traditional electronics inspired the development of flexible electronic technology. In recent years, devices with novel concepts, such as wearable electronics and bendable displays, have advanced rapidly and have been used for various functions [2–6]. These devices require flexible or stretchable touch sensors to provide users with a convenient input system. The development of flexible, transparent electrode materials is essential for producing touch sensors for such flexible or stretchable devices. Trans parent electrodes have been used in various industries, and they are especially popular for application in optoelectronic devices. In addition to touch sensors, transparent electrodes have been used in flat panel
displays, light-emitting diodes, solar cells, and other devices [7–10]. In the early stage of development of a transparent electrode, a transparent conducting oxide (TCO) was deposited on a rigid substrate, such as glass, and applied to flat-type devices. To date, one of the most commonly used materials for TCO is indium tin oxide (ITO), which ex hibits high optical transparency (>90% at 550 nm), high electrical conductivity (<7 � 10 5 Ω-cm), and an appropriate work function for hole injection (4.4–4.5 eV) in the crystalline state [11–13]. Moreover, ITO exhbits high chemical and physical durability and high workability [14,15]. However, ITO is fragile and rigid when applied to flexible de vices. Thus, cracks can develop while bending the device, which in crease the electrical resistance. This increase causes the performance of the device to deteriorate, thus posing a challenge to researchers [16,17]. To address this problem, various transparent electrode materials and structures have been explored for use in flexible touch sensors. In recent years, many researchers have devoted an enormous amount
E-mail address: [email protected]. https://doi.org/10.1016/j.solmat.2019.110350 Received 30 September 2019; Received in revised form 3 December 2019; Accepted 8 December 2019 Available online 13 December 2019 0927-0248/© 2019 Published by Elsevier B.V.
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of effort to the development of transparent conductive films employing thin metallic films, conductive polymers, metal oxides, metal meshes, nanowire networks, carbon nanotubes (CNTs), and graphene [18–23]. However, these methods are not suitable for use in commercial products due to disadvantages such as high cost, difficult processing, difficult mass production, and low reliability. Therefore, the focus has shifted back to ITO materials. To eliminate the critical drawbacks of the ITO films, ITO sandwiching of thin metal films has been extensively inves tigated [24,25]. Compared with single-layer ITO films, the ITO/meta l/ITO multilayer structures can effectively suppress the reflection from the metal layer in the visible range. The ITO/metal/ITO multilayer structures have almost the same transmittance as that of an ITO film (<85%) and yield better electrical conductivity (<10 4 Ω-cm). The middle ductile metal layer can improve the flexibility of an ITO film, and thus, this domain has generated considerable research interest. Gold (Au), silver (Ag), copper (Cu), molybdenum (Mo), aluminum (Al), and other materials have been used in different studies as the intermediate metallic layers in TCO/metal/TCO multilayer structures. Therefore, ITO/metal/ITO [26,27], GZO/Cu/GZO [28], IZO/metal/IZO [29], ZnO/Cu/ZnO [30], AZO/Ag/AZO [31], NTO/Ag/NTO [32], and ZTO/Ag/ZTO [33] multilayer structures have been reported. However, Ag has been found to be the best candidate for this application due to its lowest absorption coefficient (5%) and lowest refractive index (n ¼ 0.05) in the visible region of 400–700 nm [34,35]. To date, many scholars have conducted studies to determine how to further increase the transmittance of the ITO film and ITO/metal/ITO multilayer film while reducing the resistivity through the buffer or passivation layers. Some researchers have considered the effect of introducing a buffer layer or a passivation layer such as SiO2, ZnO, Al2O3, or Nb2O5. The objective of this study is to evaluate the transmittance change in the ITO and ITO/metal/ITO multilayer films by using materials with high and low refractive indexes. In this study, we studied the structural, electrical, and optical properties of the SiO2/ITO/Ag/ITO multilayer structures that were sputtered on a colorless polyimide (CPI) substrate. In the structures, SiO2 was selected as the passivation layer material, because SiO2 has a low refractive index (n ¼ 1.46). The SiO2 passivation layer can integrate in touch screen panels (TSPs). The thickness effects of the Ag and ITO films in the ITO/Ag/ITO multilayer structures were investigated, and the possibility of using ITO/Ag/ITO multilayer electrodes was demonstrated as a replacement for conventional ITO electrodes. Under optimized conditions, we fabricated a flexible capacitive-type TSP by using an ITO/ Ag/ITO multilayer electrode and a SiO2 passivation layer. The multi touch panel is the mainstream interaction technology of mobile appli cations. Several approaches can realize this technology, such as surface acoustic waves, infrared rays, and capacitive sensing. Compared with other competitive methods of realizing TSPs, capacitive-type TSPs have low cost, high sensitivity, high response rate, and good durability. Therefore, capacitive-type TSPs have been used as substitutes for con ventional resistive-type TSPs and have been applied to portable devices, such as smartphones, tablet computers, and wearable electronics. Thus, this study provides a practical solution to the existing problems of characterizing flexible TCO materials and their TSP applications.
reaction gas, and the working pressure was maintained at 5 � 10 3 torr. To fabricate the ITO/Ag/ITO multilayer structures, the bottom ITO film was first deposited on the CPI substrates with a deposition power at 80 W and the substrates temperatures were set to room temperature or 160 � C during the deposition process. Second, the Ag thin metal films with various deposition times were deposited with a deposition power of 50 W. Finally, the top ITO film was deposited at room temperature at 80 W. To increase the optical transmittances of the ITO/Ag/ITO multilayer structures, a silicon dioxide (SiO2) thin film was deposited on the top ITO to form SiO2/ITO/Ag/ITO multilayer structures. The SiO2 thin film with various deposition times were deposited with a deposition power of 80 W at room temperature. All the samples were annealed in air for 1 h at 300 � C. The resulting multilayer structures were characterized by high res olution scanning electron microscopy (HR-SEM), high resolution trans mission electron microscopy (HR-TEM), and atomic force microscopy (AFM) to obtain their surface images, high-resolution cross-sectional images, and surface roughness, respectively. The crystallinity of the multilayer structures were inspected by X-ray diffraction (XRD) by using CuKα radiation (λ ¼ 1.5406 nm). By using a UV–Vis spectrometer, the optical transmittances of the multilayer structures were measured at wavelengths between 300 and 1100 nm. The resistivity (ρ), carrier concentration (n), and mobility (μ) of the multilayer structures were obtained from Hall-effect measurements. The mechanical parameters of the multilayer structures were investigated using a computer-controlled bending test machine. The depth profiles of the multilayer structure were measured using secondary ion mass spectroscopy with 1 keV Oþ2 primary ions. Now, it turn to theoretical part to investigate the elec tromagnetic field transmittance. We consider a monochromatic plane wave was irradiated upon multilayer structures. The simulation results of the optical transmittance of the multilayer structures were obtained transfer matrix. Fig. 1 depicts our studying system. The matrix, M0,1, denotes the wave from vacuum to the first layer with k0 being wave number in vacuum and k1 being wave number in the first material. Mi,j denotes the wave propagating from media i to media j. Each elements in these matrixs contain the complete information involving wave input and output. 3 2 k0 k0 1þ 1 6 k1 k1 7 16 7 M0;1 ¼ 6 (1) 7 24 k0 k0 5 1 1þ k1 k1
2. Experiments and theoretical simulations
where ε1 ¼ n2 k2 is the real part of the permittivity and ε2 ¼ 2nk is the imaginary part of the permittivity. The refractive index (n) and extinc tion coefficient (k) values were measured using the ellipsometric
2
ki 1þ kj 16 6 Mi;j ¼ 6 24 ki 1 kj
3 ki kj 7 7 7 ki 5 1þ kj 1
(2)
pffiffiffi pffiffiffi here, ki and kj are defined as ki ¼ εi k0 and kj ¼ εj k0 , where ε is the relative permittivity. The permittivity is calculated as follows:
ε ¼ ε1 þ ε2
The indium tin oxide/silver/indium tin oxide (ITO/Ag/ITO) multi layer structures were deposited on a colorless polyimide (abbreviated as CPI, HannsTouch Solution Incorporated) substrate with an area of 60 � 60 mm2 by using a radio frequency magnetron sputtering system. The 2inch ITO target used in this study was a 10 wt% SnO2-doped In2O3; a silver target was used for depositing the insertion layer between the ITO thin films. The distance between the CPI substrate and the target was approximately 5 cm. The sputtering deposition parameters of the ITO film and Ag thin metal film were obtained. The base pressure of the sputtering system is 3 � 10 6 torr. Then, we introduced argon as a
Fig. 1. The schematic diagram of the multilayer structure system. 2
(3)
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instrument in the range of 300–1100 nm. Now, we apply the transform matrix to final result as follows: " þ # " þ# " þ# Ein Ein Eout tot ¼ M5;0 Ml5 M4;5 Ml4 M3;4 Ml3 M2;3 Ml2 M1;2 Ml1 M0;1 �M Eout Ein Ein " tot #" þ # tot M 11 M 12 Ein ¼ tot tot M 21 M 22 Ein
11.4 nm, 14.2 nm, and 17.2 nm with deposition times of 20 s, 40 s, 60 s, 80 s, 100 s and 120 s, respectively. In this study, the increase in the Ag deposition times from 20 to 40 s decreased the optical transmittance of the ITO/Ag/ITO/CPI multilayer structures at 550 nm due to severe light scattering due to the separate islands randomly distributed between the ITO layers. Because the Ag islands act as a scattering source of incident light, the ITO/Ag/ITO/CPI multilayer structures present a bluish color. The TEM images of the ITO/Ag/ITO/CPI multilayer structures at Ag deposition times of 40 and 80 s are presented in Fig. 2 (b) and (c), respectively. Fig. 2(b) presents that separate Ag islands inset the ITO layers at the Ag deposition time of 40 s. However, as the Ag deposition time increased from 40 to 80 s, the Ag islands coalesced to a continuous film that remarkably increased the optical transmittance in the same wavelength region. In Fig. 2(c), the Ag continuous film inset between the ITO layers was obtained at the Ag deposition time of 80 s. When an optimum Ag layer is embedded between two oxide layers, the resulting oxide/metal/oxide multilayer structures can suppress reflections from the Ag thin metal film. The oxide/metal/oxide multilayer structures exhibit a high optical transmittance in the visible wavelength region [37–40]. Therefore, the optical transmittance of the ITO/Ag/ITO/CPI multilayer structures at 500 nm (88.7%) was obtained when the Ag deposition time was set to 80 s. As the Ag deposition time was further increased, the optical transmittance of the ITO/Ag/ITO/CPI multilayer structures decreases linearly due to the higher thickness of the Ag layer with the transparent region shrinking and shifting to lower wavelengths. This shift is mainly caused by the increase in the reflectance, as induced by the increase in the plasma resonance frequency (ωp) of the free electron gas within the inserted Ag layer [41]. Consequently, by inserting an Ag layer with different deposition times between the ITO layers, the effective plasma resonance frequency of the ITO/Ag/ITO/CPI multilayer structures can be tuned. To further explain such a phenomenon, a simulated optical trans mittance based on the transfer matrix theory is presented in Fig. 2(d). The simulation results indicate the changes in optical transmittance at different thicknesses of the Ag layer, which are identical to those ob tained from the measured optical transmittance except for 20 and 40 s Ag deposition times. As we assumed that the Ag layer is a continuous film in the simulation multilayer structure, the morphology of the Ag layer is a separate island film in the real structure. To further increase the optical transmittance of the ITO/Ag/ITO/CPI multilayer structures, the bottom and top ITO films with different thicknesses were investigated, as shown in Fig. 3. The optical trans mittance at 550 nm was 83.8%, 84.2%, 85.1%, 89.5%, 87.2%, and 88.4% at ITO deposition times of 90, 100, 110, 120, 130 and 140 s, respectively. The influence of the thickness of the ITO film was lower than the influence of thickness of the Ag film on the optical trans mittance in the ITO/Ag/ITO/CPI multilayer structure. The above results reveal that the high optical transmittance of the ITO/Ag/ITO/CPI multilayer structures are obtained at the Ag and ITO deposition times of 80 s and 120 s, respectively.
(4) here, to connect our developed formulas with experiment data, we only consider optical transmittance, as follows formula: � þ �2 � � �E � �detM tot �2 � � � T ¼ �� out (5) þ � ¼� tot � Ein M 22 To investigate the feasibility of using the ITO/Ag/ITO multilayer structures as a X–Y electrode for realizing a flexible capacitive-type touch screen panel (FCTSP), we fabricated a single-sided ITO (SITO) structure of the FCTSP. The ITO/Ag/ITO electrode was directly depos ited and then patterned using conventional photolithography and wetetching processes. The SiO2 passivation layer was used to enhance the optical transmittance of the ITO/Ag/ITO electrode. The flexible printed circuit board (FPCB) bonding with metal pattern progressed using the anisotropic conductive film, and the FPCB was connected to the inte grated circuit controller. 3. Results and discussion Fig. 2(a) presents the optical transmittance of the CPI substrate and the ITO/Ag/ITO/CPI multilayer structures as a function of the Ag deposition time. The thickness of the CPI substrate was 10 μm and the optical transmittance of the CPI substrate was 98.1% in the visible re gion and near-infrared region (NIR), as shown in Fig. S1. The optical transmittance of the ITO/Ag/ITO/CPI multilayer structures was criti cally influenced by the deposition time of the inserted Ag layer. The optical transmittance of the ITO/Ag/ITO/CPI multilayer structures present a sharp increase in the near-ultraviolet (NUV) range. The visible range decreases slightly with a subsequent transmittance increase in the NIR at the Ag deposition times of 20 and 40 s. The ITO/Ag/ITO/CPI multilayer structures present a high optical transmittance in the visible region with a subsequent slight decrease in the optical transmittance in the NIR as the Ag deposition time increases from 60 to 80 s. When the Ag deposition time is further increased from 100 to 120 s, the optical transmittance decreases again in the visible region and rapidly decreases in the NIR. The optical transmittance of the ITO/Ag/ITO/CPI multilayer structures at 550 nm were 74.5%, 69.2%, 77.9%, 89.5%, and 81.9% at Ag deposition times of 20, 40, 60, 80, 100, and 120 s, respectively. The optical and electrical properties of very thin metal films depend considerably on their structures [36]. The thicknesses of the Ag thin metal film measured by HE-SEM images were 2.9 nm, 5.7 nm, 8.5 nm,
Fig. 2. (a) Measurement, (d) simulation UV–Vis spectra, and (b)–(c) TEM images of the ITO/Ag/ITO/CPI multilayer structures as a function of Ag deposition time. 3
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Fig. 5. The resistivity (ρ), carrier concentration (n), and mobility (μ) of the ITO/Ag/ITO/CPI multilayer structures as a function of ITO deposition time.
Fig. 3. UV–Vis spectra of the ITO/Ag/ITO/CPI multilayer structures as a function of ITO deposition time.
multilayer structures increased by approximately two to three orders of magnitude upon insertion of a 10.9-nm Ag film compared with the carrier concentration of the structures when bare ITO film was used. As discussed by Kloppel et al., the metal interlayer can act as an electron source for the oxide layer in the TCO/metal/TCO structure [42]. The carrier concentration of the Ag metal film is approximately n(Ag) ¼ 1 � 1022 cm3. Therefore, the electrons in the Ag layer can be easily injected into the ITO layer due to the downward bending of the band at the interface of the Ag (ϕm ¼ 4.4 eV) and ITO layers (ϕs ¼ 4.5–5.1 eV) by using different work functions in the ITO/Ag/ITO/CPI multilayer structures [43,44]. The resistivity of the ITO/Ag/ITO/CPI multilayer structures decreased from 7.34 � 10 5 to 1.83 � 10 5 Ω-cm as the ITO deposition time increased from 90 to 140 s. The change in resistivity of the ITO/Ag/ ITO/CPI multilayer structures with increase in ITO deposition time can be explained using the following basic relation:
The XRD measurement results of the ITO/Ag/ITO/CPI multilayer structures as a function of the ITO deposition are presented in Fig. 4. When the Ag layer is absent, no diffraction peak can be observed. This implies that the as-deposited ITO film has an amorphous structure. Irrespective of the ITO films, all the ITO/Ag/ITO/CPI multilayer struc tures have peaks at 2θ values of 38.1� , 44.5� , and 64.7� corresponding to the (111), (200), and (220) phases of Ag (JCPDS card No. 04–0783), respectively. No second phase or unknown phase exists in the XRD re sults in the ITO deposition time from 90 to 120 s. Furthermore, for the ITO deposition times of 130 and 140 s, there is a broad diffraction peak around the 2θ value of 31.3� that seems to consist of several peaks at 2θ values of 30.7� and 35.63� that correspond to the (222) and (400) phases of In2O3 (JCPDS card No. 06–0416), respectively. Fig. 5 exhibits the resistivity ρ, carrier concentration n, and mobility μ of the ITO/Ag/ITO/CPI multilayer structures as a function of ITO deposition times. The carrier concentration and mobility moderately increase from 2.73 � 1020 to 7.49 � 1021 cm 3 and from 1.32 to 118.62 cm2/V as the ITO deposition time increases from 90 to 140 s, respec tively. As the ITO film is an amorphous structure, an increase in the ITO deposition time causes an increase in the thickness of the ITO film. This increase in thickness causes an increase in the carrier concentration and mobility. Moreover, the carrier concentration of the ITO/Ag/ITO/CPI
ρ¼
1 neμ
(6)
where ρ is the resistivity, n is the carrier concentration, e is the charge of the carrier, and μ is the mobility [45]. From this equation, we note that changes in the carrier concentration and mobility can affect the re sistivity. Moreover, the resistivity of the ITO/Ag/ITO/CPI multilayer structures at different ITO deposition times decreases by a wide range than the resistivity of a single-layer ITO thin film. In a previous study, as the Ag layer was less than 10 nm, the Ag was randomly distributed in unconnected “islands” and the Ag exhibited a fairly high resistivity. However, as the thickness of Ag layer is thicker than 10 nm, it completely covered the bottom layer and reduced the resistivity [46]. In this study, the thickness of the Ag layer is 10.9 nm. Fig. 6 presents the figure of merit (FOM), sheet resistances, and op tical transmission (550 nm) of the ITO/Ag/ITO/CPI multilayer struc tures. The sheet resistances of the ITO/Ag/ITO/CPI multilayer structures decreased as a function of the ITO deposition times. In this study, it assumed that the total resistance of the ITO/Ag/ITO/CPI multilayer structures can be represented simply as resistances coupled in a parallel structure of the bottom ITO, Ag, and top ITO layers. Rtotal ¼
2 1 þ RITO RAg
(7)
where Rtotal, RITO, and RAg are the sheet resistance of multilayer struc tures, ITO, and Ag, respectively. Equation (7) reveals that as the thick ness of the top and bottom ITO films increased, the sheet resistance of the top and bottom ITO films slightly decreased. Thus, the Rtotal values decreased. To obtain the best combination of high transmission and low
Fig. 4. XRD analysis of the ITO/Ag/ITO/CPI multilayer structures as a function of ITO deposition time. (a) 90 s, (b)100 s, (c) 110 s, (d) 120 s, (e) 130 s, (f) 140 s (■:ITO phase and ●:Ag phase). 4
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(160)/CPI multilayer structure presented in Fig. 6(a), the image presents that the bottom ITO film has a complete crystallization structure due to its deposition at a substrate temperature of 160 � C, and the amorphous structure was obtained in the top ITO film. This result reveals that the interfacial layer between top ITO, bottom ITO, and Ag layers exist without any chemical compound. To investigate the elemental distribution along the vertical path of the ITO/Ag/ITO(160) multilayer structure, the SIMS analysis and the results can be seen in Fig. 6(b). The main elements of the ITO/Ag/ITO (160) multilayer structure were In, Sn, O, and Ag. The concentration of Ag increases quickly in the middle region. Thus, the concentration of the middle region reaches that of the Ag layer. The thickness of the Ag layer was 10.9 nm at the deposition time of 80 s. Overall, the interface be tween the top ITO layer and the Ag layer and between the Ag layer and the bottom ITO layer is very clear, which can also be seen in Fig. 7(b). Based on the ITO/Ag/ITO(160)/CPI multilayer structure, the sheet resistance and optical transmittance at 550 nm were 6.4 Ω/□ and 91.4%, respectively, and the value of the FOM is 63.6 � 10 3 Ω 1. The resistance change (ΔR) in the ITO/Ag/ITO(160)/CPI multilayer struc ture was measured using the outer bending radius, and the bending radius was set to 3 mm based on the commercial standard. Changes in the resistance of the ITO/Ag/ITO(160)/CPI multilayer structure can be expressed as ΔR(%) ¼ (R1 R0)/R0, where R0 is the initial resistance and R1 is the measured resistance after bending. During 30,000 bending cycles, the ΔR value of the ITO/Ag/ITO(160)/CPI multilayer structure is 4.12%, as shown in Fig. S2. The ITO/Ag/ITO(160)/CPI multilayer structure exhibited less change in resistance after 30,000 bending cycles. This result can be attributed to the high strain failure of the metallic Ag thin interlayer between the ITO layers and the local delamination or crack formation in the ITO layers. The optical transmittance of the ITO/ Ag/ITO(160)/CPI multilayer structure was higher than that of the ITO (160)/Ag/ITO(160)/CPI and ITO/Ag/ITO/CPI multilayer structures (Fig. S3). This was caused by the crystallized bottom ITO film, which exhibited a higher optical transmittance and lower resistivity than the values of the amorphous film [49,50]. Fig. 8 presents the structure of the FCTSP that includes the CPI substrate, ITO bridge, insulator, X–Y ITO, and metal and passivation layers. In this study, the ITO/Ag/ITO multilayer was investigated to be used as a substitute for the X–Y ITO layer for enhancing the reliability of the optical transmittance and resistivity. The passivation layer covered both the ITO/Ag/ITO multilayer structure and a metal layer of the FCTSP, as illustrated in Fig. 7. It is noteworthy that the transmittance is more sensitively modulated by top ITO than bottom ITO in the ITO/Ag/ ITO multilayer structure. This is due to the top ITO layer being in contact with air, so the amplitude of the reflected wave at the interface is rela tively large, leading to a strong interference effect. Therefore, SiO2 is selected as the passivation layer because it is a low-refractive-index material. The optical transmittance of the SiO2/ITO/Ag/ITO(160)/CPI multi layer structures as a function of the SiO2 deposition times are shown in Fig. 9(a)–(b). The optical transmittance of the SiO2/ITO/Ag/ITO(160)/ CPI multilayer structures is influenced by the SiO2 deposition time. The
Fig. 6. The figure of merit (FOM), sheet resistances and transmission (550 nm) of the ITO/Ag/ITO multilayer structures as a function of ITO deposition time.
resistivity, the FOM of the films was calculated using the Haacke’s equation [47]: ϕTC ¼
T 10 av Rs
(8)
where Tav is the optical transmittance of the ITO/Ag/ITO/CPI multilayer structures at a wavelength of 550 nm and Rs is the sheet resistance. Tav can be estimated using eq. (9): R vðλÞTðλÞdðλÞ (9) Tav ¼ R vðλÞdðλÞ where T(λ) is the optical transmittance and v(λ) is the photonic luminous efficiency function defining the standard observer for photometry [48]. In Fig. 6, the maximum value of the FOM of 25.4 � 10 3 Ω 1 can be observed at an ITO deposition time of 120 s. The sheet resistance and optical transmittance at 550 nm were 13 Ω/□ and 89.5%, respectively. From the above results, the high optical transmittance and low resis tance of the ITO/Ag/ITO/CPI multilayer structure was obtained for the Ag and ITO deposition times of 80 and 120 s, respectively. To produce the ITO/Ag/ITO/CPI multilayer structures with higher optical transmittance, different deposition parameters were further investigated. We know that the crystallization ITO film exhibits higher transmission than the amorphous ITO film. Fig. 7 presents (a) the high resolution TEM (HR-TEM) image and (b) SIMS results of the ITO/Ag/ ITO/CPI multilayer structures at a deposition temperature of 160 � C for the bottom ITO film [ITO/Ag/ITO(160)/CPI]. For the ITO/Ag/ITO
Fig. 7. (a) TEM image and (b) SMIS analysis of the ITO/Ag/ITO/CPI multilayer structure with the deposition temperature at 160 � C for the bottom ITO thin film.
Fig. 8. Structure of the touch sensor with ITO/Ag/ITO multilayer structure and SiO2 passivation layer. 5
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Fig. 9. (a) Measurement and (b) Simulation UV–Vis spectra of the SiO2/ITO/Ag/ITO(160)/CPI multilayer structures as a function of SiO2 deposition time.
optical transmittance values at 550 nm were 93.5%, 94.6%, 91.8%, and 85.1% at SiO2 deposition times of 5, 10, 15, and 20 min, respectively. The average optical transmittance at 400–700 nm of the SiO2/ITO/Ag/ ITO(160)/CPI multilayer structures were 89.99%, 92.16%, 89.6%, and 83.2% at SiO2 deposition times of 5, 10, 15, and 20 min, respectively. The maximum optical transmittance and average optical transmittance were obtained at a SiO2 deposition time of 10 min. In the visible range, the average optical transmittance of the SiO2/ITO/Ag/ITO(160)/CPI multilayer structures is larger than that of the ITO/Ag/ITO/CPI (84.7%) and ITO/Ag/ITO(160)/CPI (85.8%) multilayer structures, as shown in Fig. 9(a). The SiO2 passivation layer enhances the optical transmittance at 550 nm and average optical transmittance in the range of 400–700 nm. This result was attributed to the low refractive index of SiO2 (n ¼ 1.46, air/SiO2/ITO/Ag/ITO/CPI/air). Moreover, the SiO2 passivation layer increases the optical transmittance in the NIR, as shown in Fig. 9 (b). The multilayer structure exhibited a reduced optical loss in the NIR, thus suggesting its potential use in plasmonic applications in addition to the use in touch panel application. The simulation results indicate the changes in the optical transmittance at different thicknesses of the SiO2 layer, as shown in Fig. 9(c). These changes are identical to those observed from the optical transmittance measurement. The cross-sectional TEM image of the SiO2/ITO/Ag/ITO(160)/CPI multilayer structure is shown in Fig. 10. The gray, black, and light-gray images clearly present the multilayer structure comprising the bottom ITO, Ag, top ITO, and SiO2 layers, as shown in Fig. 10. The thicknesses of the bottom ITO, Ag, top ITO, and SiO2 layers are 21.6, 10.9, 24.7, and 21.3 nm, respectively. This structure was applied to the proposed FCTSP. Fig. 11 illustrates the evolution of the surface morphology and roughness during the growth of the multilayer by investigating the
Fig. 10. TEM image of the SiO2/ITO/Ag/ITO(160)/CPI multilayer structure with the 10 min SiO2 deposition time.
build-up samples with SEM and AFM. As presented in Fig. 11 (a) and (d), the root-mean-square (RMS) value of the 21.6-nm-thick crystallized ITO film deposited on a CPI substrate at room temperature is 0.61 nm. The RMS value of the ITO film deposited at a deposition temperature of 160 � C is 0.76 nm, this value is higher than that deposited at room temper ature. The ITO/Ag/ITO(160)/CPI multilayer structures exhibits a higher RMS value of 1.39 nm, as shown in Fig. 11(c) and (g). This result was caused by the crystallized structure of the bottom ITO film and the Ag layer with the (111) orientation, which induces higher surface rough ness in the top ITO film. Finally, as presented in Fig. 11(d) and (h), the ITO/Ag/ITO(160)/CPI multilayer structures with an SiO2 layer of 23.3 nm of presents an RMS value of 0.96 nm. The SiO2 film is an amorphous structure and is deposited on the ITO/Ag/ITO(160)/CPI multilayer 6
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Fig. 11. SEM images and AFM analysis of the (a)(e) ITO/CPI, (b)(f) ITO(160)/CPI, (c)(g) ITO/Ag/ITO(160)/CPI and (d)(h) SiO2/ITO/Ag/ITO(160)/CPI structures. Scan area is 1 μm � 1 μm.
structure as the passivation layer. Thus, the surface roughness of the SiO2/ITO/Ag/ITO(160)/CPI multilayer structure decreases. The decrease in the roughness reduces the scattering effect and enhances the optical transmittance of the SiO2/ITO/Ag/ITO(160)/CPI multilayer structure. The multiple touch point operation of an ultra-high transmittance FCTSP was tested; the FCTSP was fabricated using an ITO/Ag/ITO(160) X–Y electrode and an SiO2 passivation layer, as shown in Fig. S4. The operation of the ultra-high transmittance FCTSP with the ITO/Ag/ITO (160) electrode indicates that the ITO/Ag/ITO(160) multilayer structure with a low sheet resistance and high reliability is a promising trans parent electrode that can be used as a substitute for the conventional ITO electrode. The ultra-high transmittance FCTSP was operated by exact sensing of X–Y coordinates and linearity characteristics. Before by connecting the ultra-high transmittance FCTSP with software, the touch panel was connected to the touch circuit controller, and the touch panel was operated. As presented in Fig. 12, an ultra-high transmittance FCTSP with a liquid crystal display (LCD) was fabricated; it was slightly similar to a Samsung Galaxy S7 edge. The inset of Fig. 12 presents the well pattern of the touch sensor with the ITO/Ag/ITO(160) electrode that was patterned using photolithography and wet-etching processes. The positive photoresist was used in photolithography process. The wet etchant is a hydrochloric acid and nitric acid mixture. To evaluate the practical usage of the FCTSP, we lightly touched the FCTSP, and the three touch points were displayed on a notebook monitor, as shown in Fig. 12. This test verified that this FCTSP can detect multiple touch points. In the near future, the ultra-high transmittance FCTSP will be combined with an organic light-emitting diode (OLED) to form flexible displays for use in flexible smartphones.
Fig. 12. FCTSP fabricated with the ITO/Ag/ITO(160) multilayer structure with SiO2 passivation layer demonstrates the multiple touch point function.
transmittance of the SiO2/ITO/Ag/ITO(160) multilater structure to 94.6% at 550 nm. Moreover, by using the transfer matrix theory, the optical transmittance trend of the multilayer structure was simulated. The simulation results are in agreement with the measurement results. These results suggest that the SiO2/ITO/Ag/ITO(160) multilayer struc ture can be used as an alternative to the ITO electrode. Moreover, the structure enhances the optical transmittance and resistivity and thus can be applied to wearable electronics and bendable displays.
4. Conclusion The ultra-high optical transmittance and low resistivity electrode of the ITO/Ag/ITO(160) multilayer structure was studied as a substitute for the conventional X–Y ITO electrode for fabricating an FCTSP. The tunable electrical and optical properties of the ITO/Ag/ITO(160) multilayer structures were critically influenced by the thickness and morphology of the inserted Ag layer because the Ag layer dominates the electrical properties and is the main parameter that affects the antire flection effect. During the 30,000 bending cycles, the change in the resistance (ΔR) of the ITO/Ag/ITO(160)/CPI multilayer structure was 4.12% because of the high strain failure of the metallic Ag thin interlayer between the ITO layers. In this study, SiO2, a low refractive index ma terial, was used as the passivation layer. This layer increases the optical
Author contributions section C.C. Wu participated in the Hall, HR-SEM, HR-TEM, UV-Via analysis of ITO/Ag/ITO multilayer structure. Notes The authors declare no competing financial interest.
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Acknowledgments
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