With the enhancement of people's awareness of energy conservation and environmental protection, electrochromic devices (ECDs), used in buildings and vehicles have attracted increasing attention in recent years^[1,2]. Compared to other chromogenic materials, electrochromism is capable of reversibly switching between colored and bleached states corresponding to internal ion inserting and extracting^[3,4]. Taking advantage of that, ECDs play a major role in many fields, such as image display^[5], smart windows^[6] and stealth^[7], which are expected to realize industrialization ahead of other chromogenic materials. However, the application of ECDs are limited due to their low fabrication efficiency, unaffordable cost and complex packaging technique. Therefore, study on all- solid-state ECDs with both high performance and high sputtering efficiency has gradually become a trend^[8,9,10].

Some metrics are important factors affecting the performance of all-solid-state ECDs, such as optical contrast, cycle stability, especially sputtering efficiency and switching speed^[11]. Inorganic all-solid-state ECDs are typically composed of five superimposed layers^[12]: electrolyte layer is laminated between two EC layers, such as cathodic colored WO₃ and anodic colored NiO, individually connected with a transparent conductive layer. However, such typical five-layer ECDs based on monolayer electrolyte system are often limited by ordinary optical transmittance and inadequate sputtering efficiency, which is adverse to practical application and mass production. Generally, the magnetron sputtering efficiency of ceramic target with RF power supply is unsatisfactory^[13,14]. In contrast, there are few reports about solid electrolyte sputtered directly by metal or alloy targets, which can be applied to large-scale coating process^[15]. Furthermore, low optical transmittance of traditional all-solid-state ECDs is sometimes far from meeting the standards for practical applications^[16,17]. Optical design for anti-reflection simulation of multilayer electrolyte film can be applied to realize the theoretical maximum value of optical transmittance^[18]. Therefore, it is necessary to develop ECDs with a given multilayer structure which have high sputtering efficiency, high optical transmittance and satisfactory service performance^[19,20].

Taking the above factors into consideration, LiAlO_x/ Ta₂O₅/LiAlO_x (ATA) sandwich structured electrolyte with LiAlO_x supplying enough lithium ions and Ta₂O₅ serving as ion transport layer is requisite for optimizing the properties of the novel structured ECDs. The electron- blocking behavior, high ionic conductivity and chemical stability of Ta₂O₅ makes it an ideal ion transport layer in ECDs. Meanwhile, LiAlO_x holds both high optical transmittance and ionic conductivity. Due to the difference of refractive index between LiAlO_x and Ta₂O₅, the sandwich structure electrolyte with alternating high and low refractive index can be designed, which is expected to improve the transmittance using the anti-reflection principle^[21]. Thus, the structure makes full use of the excellent ionic conductivity and stability of Ta₂O₅, as well as enough lithium ions to meet the demand for fast color switching.

In this study, a seven-layer all-solid-state ECD (ITO/NiO/LiAlO_x/Ta₂O₅/LiAlO_x/WO₃/ITO) incorporated with ATA sandwich structured electrolyte of high sputtering efficiency, high optical transmittance and satisfactory coloration efficiency was designed. Due to effective lithium ions immigration channels provided by ATA sandwich structured electrolyte, the ECD hires remarkably improved switching speed and optimized optical transmittance. Moreover, the sputtering efficiency of ECDs was greatly improved by means of DC continuous magnetron sputtering using metal or alloy targets. The ATA based all-solid-state ECDs by optical design bring a further insight into the mass production and marketization of high-performance ECDs.

NiO film and LiAlO_x/Ta₂O₅/LiAlO_x sandwich structured electrolyte were deposited on 20 mm×20 mm indium tin oxide (ITO) glass substrates in sequence via reactive DC magnetron sputtering. LiAlO_x film was conducted in the flow of Ar/O₂ (40 sccm/10 sccm). The sputtering used Li-Al alloy target (99.9%, diameter of 10.16 cm) with 200 W DC power for 45 min at room temperature (RT). Then the middle layer of Ta₂O₅ was deposited at RT using Ta metal target (99.9%, diameter of 10.16 cm). The sputtering was performed under the condition of 200 W DC power and Ar/O₂ ratio of 45:5. For the next step, an amorphous WO_x film was deposited in quick succession. Finally, a layer of ITO film was deposited on top of it acting as a top electrode. The detailed deposition parameters of each layer was simulated for max optimization of optical performance in Table 1.

The section microstructure of ECDs was shown in the cross-section micrographs by field-emission scanning electron microscope (FESEM, FEI Magellan 400). To monitor the surface roughness of monolayer electrolyte film, atomic force microscope (AFM, SII Nano Technology Ltd, Nanonavi P) was examined. Later, the electrochromic property indexes including coloration efficiency, optical contrast, and cycle durability were studied by a CS350 electrochemical workstation (Corrtest, Wuhan). Lastly, UV/Vis/NIR Spectrometer (Hitachi, UV-4100) was used to characterize the in-situ transmittance spectrum of ECDs ranging from 350 to 2600 nm with the scanning speed of 1200 nm/min. In addition, optical modeling was executed via software of Essential Macleod to optimize the experimental spectra, based on the theory of optical interference by utilizing the optical constants of LiAlO_x and Ta₂O₅.

The surface microstructure of as-prepared electrolyte was characterized by AFM. The as-deposited LiAlO_x and Ta₂O₅ films exhibited smooth surface with root-mean- square roughness (RMS) of 0.45 and 1.14 nm, respectively (Fig. 1(a-b)). The ECD containing LiAlO_x/ Ta₂O₅/LiAlO_x sandwiched electrolyte was fabricated by optical optimization design. The film thickness, local enlarged view and corresponding element distribution of each layer in ECDs can be observed in Fig. 1(c, d, f). In Fig. 1(f), the element distribution diagram corresponds to intuitive expression for the cross-section EDS of ECDs, which represented the element distribution of each layer collated with the SEM photographs in Fig. 1(c). The three middle layers consist of LiAlO_x/Ta₂O₅/LiAlO_x sandwich structure with the thickness of nearly 100, 200 and 100 nm, respectively. Besides, surface SEM image of ATA electrolyte display the view of isolated island particles (Fig. 1(e), which indicates the loose porous structure of LiAlO_x layer. Thus, the ATA electrolyte constructed the effective conductive channel, which made the most of its excellent properties of ion conduction and stability.

To study O₂ flow ratio dependent optical transmittance of Ta₂O₅, different O₂ contents (O₂/(Ar+O₂)) (0, 5%, 10% and 15%) were set for Ta₂O₅ sputtering in Fig. 2(a). When the O₂ ratio corresponded to 10%, its optical transmittance was best optimized, which was applied to the sputtering of ATA electrolyte. Fig. 2(b) shows the in-situ transmittance spectra during 350-2600 nm with the applied voltage varying from -3 V to 3 V for ECDs. The initial state was highly bleached. With the increase of positive voltage, the optical transmittance decreased gradually due to its EC coloration effect. Until the bias reaches 3 V, the optical transmittance decreased even below 10%. Especially, ECDs presented excellent color retention as the external voltage was removed. Finally, ECDs returned to its original bleached state when the reverse voltage was applied.

In addition, optical simulation was performed for LiAlO_x/Ta₂O₅/LiAlO_x sandwich structured electrolyte. The optical constants (n and k) of LiAlO_x and Ta₂O₅ were used for calculation, which was made for different thickness combinations of LiAlO_x and Ta₂O₅, and an optimized structure was chosen upon the maximum integrated luminous transmittance. Then the experimental and calculated transmittance spectra of sandwich structured electrolyte were tested (Fig. 2(c-d)), which fit well with each other. By means of the optical calculation via software of Essential Macleod, the optical transmittance of ATA reach over 90% at the thickness of 100 nm/ 200 nm/100 nm, which is superior to that of LiTaO₃ at the same thickness. The structure of ATA electrolyte was optimized with the optical constants of LiAlO_x and Ta₂O₅ via software of Essential Macleod in Fig. 2(e), which shows the relationship between the luminous transmittance of ATA electrolyte and the film thickness of top and bottom LiAlO_x layer, that is, “LiAlO_x (bottom layer, thickness t₁)/Ta₂O₅ (200 nm)/ LiAlO_x (top layer, thickness t₂)”. When t₁=t₂=0, namely only Ta₂O₅ monolayer, the visible light transmittance corresponds to the lowest trough. When the top and bottom layer of LiAlO_x was introduced, the visible light transmittance increases in a wavy manner and reaches the maximum value over 90% under certain conditions (t₁=t₂=100 nm). Additionally, the designed structure was calculated according to the transparent state of Ta₂O₅ when lithium ions locate on the side of electrochromic layer under electric field. Therefore, LiTaO₃ (LTO) is equivalent to Ta₂O₅ (TO), which has little difference in optical constants without lithium ions.

One of the highlights of this work is to summarize the sputtering efficiency of electrochromic devices, which is rarely summarized in previous work. Generally speaking, DC power sputtering is relatively more efficient than RF power in the industrial production^[22]. However, there lacks of a quantitative comparison to guide the cost and efficiency in mass production. Taking the above into consideration, a continuous sputtering process of metal or alloy targets using DC power supply was proposed in this work. As displayed in Fig. 3, thin films of WO_x, NiO, LiAlO_x, LiTaO₃ and Ta₂O₅ were sputtered using DC and RF power supply, respectively. Here, we introduce a parameter, sputtering efficiency, which can be defined as the ratio of sputtering rate to power density. In Table 2, the sputtering efficiency of DC supply are obviously improved related to that of RF supply for each deposited layer. Especially, the sputtering efficiency of WO_x increased by nearly 14 times from RF to DC sputtering, mutually verified with Fig. 3.

As shown in Fig. 4(a), by means of optical design, the optical transmittance of the sandwich structured electrolyte (ATA) has been greatly improved on the premise that the ionic conductivity is not lower than that of LiTaO₃ at the same thickness. The designed ATA based ECDs have satisfactory coloring efficiency of 79.6 cm²/C while changing color reversibly (Fig. 4(b)). The inset of Fig. 4(b) reflects reversible color change between the bleached and colored states of the as-prepared ECD with the size of 20 mm×20 mm. Moreover, its response time for coloring and bleaching is as short as 1.9 and 1.6 s, respectively (Fig. 4(c)), which is mainly due to the convenient transport channels and rich ion reservation of ATA sandwich structured electrolyte. At the meantime, it keeps stable optical modulation after 200 cycles with in-situ testing units fixed wavelength at 630 nm (Fig. 4(d)).

In summary, we have developed the sandwich structured electrolyte with high transparency of luminous transmittance over 90% and remarkable sputtering efficiency by DC reactive magnetron sputtering using alloy and metal target. Additionally, ATA sandwich structured electrolyte as the ion conducting layer has been introduced into a seven-layer all-solid-state ECD consisting of ITO/NiO/LiAlO_x/Ta₂O₅/LiAlO_x/WO₃/ITO. Furthermore, ATA-based ECDs realized satisfactory coloration efficiency of 79.6 cm²/C, fast switching speed as short as 1.9 s for coloring and 1.6 s for bleaching, and excellent cycling stability over hundreds of cycles. Hence, ATA-based ECDs by optical design are expected to realize mass production and practical application in the near future.