Nanowire-based electrochromic devices

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1 ARTICLE IN PRESS Solar Energy Materials & Solar Cells 91 (27) Nanowire-based electrochromic devices S. Gubbala, J. Thangala, M.K. Sunkara Department of Chemical Engineering, University of Louisville, Louisville, KY 4292, USA Received 19 December 26; accepted 2 January 27 Available online 13 March 27 Abstract This report presents studies performed on high contrast electrochromic devices based on WO 3 nanowires. The devices were made in two formats. The first kind was made with vertically oriented nanowires grown on fluorinated tin oxide (FTO) coated glass substrates using a low pressure hot filament CVD process. The devices made with these substrates showed a highly reversible transmission modulation of over 7% with almost % transmission in the colored state, at a wavelength of 7 nm. In the second kind of devices, made using dispersed nanowires in a mat-like format, a transmission modulation of over 5% was observed in the same wavelength regime. The bleaching times of electrodes with high densities of nanowire arrays showed a large dependence of bleaching timescales based on the coloration timescales used. Beyond the observed enhancement in the optical transmission contrast characteristics, the coloration and bleaching timescales can be further improved by optimizing the nanowire array characteristics such as their densities and aspect ratios. r 27 Elsevier B.V. All rights reserved. Keywords: Electrochromics; Nanowires; CVD; WO 3 1. Introduction Nanoporous devices based on nanowires and columnar structures are gaining importance in various electrochemical device applications due to their high surface area and their potential to offer low resistance to charge and mass transport [1]. So, there is a need to synthesize nanowirebased films in a controlled and uniform manner and understand their behavior in a variety of electrochemical energy applications. So far, there have been only a few reports on the use of vertical arrays of nanowires for both dye sensitized solar cells and electrochromic devices [2 5]. But, it is not clear whether the nanowire-based porous films made using nanowire dispersions could be employed for high contrast electrochromic devices. Also, much more needs to be understood in order to optimize the use of vertical arrays for electrochromic devices. In addition, the ease of mat-like thin film preparation using nanowire dispersions is suitable for roll-to-roll manufacturing processes on an industrial scale and onto a wide variety Corresponding author. address: mahendra@louisville.edu (M.K. Sunkara). of substrates. So, we fabricated electrochromic devices both in the mat-like and nanowire array formats and investigated their electrochromic performance. Electrochromism is the phenomenon of inducing a reversible change in the optical properties of a material by the application of a small electric field [1]. Transmission modulation in these films is achieved by varying the oxidation state of the electrochromic material by an electric field assisted insertion and extraction of small alkali metal ions, like lithium (Li + ) [1]. Tungsten trioxide has been a choice for these applications for a long time owing to its high stability and good electrochromic behavior [6]. Development of electrochromic materials has gained increased importance in the last few years due to their potential to reduce air conditioning costs and thus the energy consumption in buildings by cutting off the infrared (IR) part of the solar spectrum, and also as information display devices [7]. Thin films of WO 3 deposited using various methods like sol gel technique, photochemical vapor deposition, vacuum evaporation, anodic oxidation, template assisted methods, etc. [8 12] have been successfully used in the past for making these devices. The performance of electrochromic devices is a function of the /$ - see front matter r 27 Elsevier B.V. All rights reserved. doi:1.116/j.solmat

2 814 ARTICLE IN PRESS S. Gubbala et al. / Solar Energy Materials & Solar Cells 91 (27) nature of the material (amorphous or polycrystalline), morphology, composition, as well as their water content [1]. Nanocrystalline forms of these materials offer higher porosity while still maintaining a high intercrystalline contact necessary for electrical conductivity. Such films have higher transmission and are expected to exhibit better electrochromic optical modulation compared to the conventional films due to their open structure [6]. There have been only two reports in literature on the fabrication of electrochromic devices based on nanowires [4,5]. The materials used in these studies were oxygen deficient W 18 O 49 and V 2 O 5 nanowires. In both the reports, the vertically oriented nanowires were directly deposited on the FTO substrates by thermal evaporation using powders as sources. Nanowires in the present study were synthesized using a scalable hot filament CVD process. This method of synthesis allows easy change in processing conditions to vary the density, diameters, and aspect ratio of the nanowires. 2. Experimental Fig. 1 shows a schematic of the hot filament CVD reactor setup used for the synthesis of tungsten oxide nanowires. The setup consists of a 2-inch diameter quartz tube housed in a tube-furnace. The ends of the quartz tube are connected to the necessary accessories for flow and pressure control. The filament used for the experiments is mounted on hollow ceramic rod as shown in Fig. 1. A.5 mm diameter tungsten wire was used as the source of tungsten in these experiments. The tungsten filament is heated up using an electrical feed-through to temperatures of about 195 K. The temperatures within our system were monitored using a dual wavelength pyrometer. Quartz boats were employed to curtail the deposition of tungsten oxide directly onto the tube walls. Typically, the substrates (FTO coated glass slides) were placed on the boat as shown in the schematic. The temperature of the substrate during the nanowire growth was 823 K at 77 mtorr and a flow of 11 sccm of air. The synthesis procedure and the nanowire growth mechanism is explained in more detail elsewhere [13]. In order to make dispersions of the synthesized nanowires, the as-synthesized nanowires were collected as dry powders by scraping the material from the quartz substrates. The as-obtained nanowire powder was dispersed into dimethyl formamide (DMF) with an ultrasonic horn for 2 min followed by sonication in a low energy density bath for about 15 min. The dispersions were allowed to settle for few minutes and the initial sediments were taken out and weighed. The initial sediments always contained nondispersed agglomerates and thicker nanowire bundles. The dispersions were allowed to settle over two days. The sediments were examined using SEM to observe the agglomeration patterns. The upper portions of the solution contained well-dispersed nanowires. This solution was used to make the mat-like electrodes. Electrodes in mat-like format were made by depositing 1 wt% dispersion of WO 3 nanowires in DMF solution on FTO substrates, over an area of approximately.25 cm 2. After depositing the nanowires, the electrodes were heated in ambient atmosphere at a temperature of 773 K, to oxidize the substoicheometric tungsten trioxide to WO 3. The electrodes after oxidation became highly transparent forming the WO 3 phase, which was confirmed by XRD. The counter electrode for this device was another FTO glass piece. The two electrodes were assembled face to face into a sandwich-like structure with 1 M LiClO 4 in propylene carbonate as the electrolyte between them. For this purpose, the electrodes were separated by a plastic spacer of 15 mm thickness. Electrochromic devices based on nanowire arrays were also made in a similar fashion. Schematic representations of these devices are shown in Figs. 2(a) and (b). 3. Results and discussion Fig. 3(a) shows the SEM micrograph of a substrate on which the nanowires were grown for 1 min, indicating a high density of vertically oriented nanowire arrays. The nanowires obtained had diameters ranging from 4 to 6 nm and lengths up to 2 mm. The number density of nanowires on these substrates was found to be nw/cm 2 (Fig. 3(b)). From the SEM image, it can be seen that there is a MFC's O 2 Ar Air Quartz To Vacuum Pump To Electrical Feed through Pressure Transducer Substrate Metal Source - Filament Fig. 1. Schematic representation of the CVD reactor used for the synthesis of WO 3 nanowires.

3 ARTICLE IN PRESS S. Gubbala et al. / Solar Energy Materials & Solar Cells 91 (27) Fig. 2. Schematic representation of the electrochromic devices made with (a) vertical arrays of WO 3 nanowires grown on FTO substrates, and (b) nanowires deposited from dispersions. Fig. 3. Scanning electron micrographs of (a) cross section of vertical arrays of nanowires grown on FTO substrates, (b) top view of the nanowires, and (c) nanowires deposited from dispersions. modification in the interface between the nanowires and the FTO substrate, compared to the bare FTO surface before the experiment. This modification could have helped in decreasing the contact resistance by making an alloy at the interface. However, the Li + intercalation/deintercalation characteristics forsuchalloysmaydifferfromwo 3. Also, the diffusion of sodium atoms into the FTO layer during the nanowire synthesis may have adversely affected the final characteristics of the electrode. The properties of the interface, although interesting with regard to the electrochromic behavior of the electrodes, have not been carried out at this point in time. The as-synthesized nanowires were blue in color. The X-ray diffraction (XRD) spectrum (Fig. 4(a)) indicated that the assynthesized bluish nanowire deposit was composed of an oxygen deficient monoclinic W 18 O 49 phase (JCPDS#5-392, a ¼ A, c ¼ A, andb ¼ A ). Oxidation of the as-synthesized nanowires in ambient atmosphere at a temperature of 5 1C for about 3 min changed the color of the nanowires to yellow, and the corresponding XRD spectrum (Fig. 4(b)) indicated that the phase of the nanowires changed from W 18 O 49 to WO 3 (JCPDS #2-1324; a ¼ A, b ¼ A, and c ¼ A ). At the same time, no structural damage or change is observed for nanowires after the oxidation. In comparison, the SEM of the electrodes with nanowires deposited on them from dispersions had a number density of 1 9 nw/cm 2 (Fig. 3(c)). The optical transmission measurements on these electrodes were performed using a Perkin-Elmer Lambda 95 UV vis spectrometer. An EG&G PAR 273A potentiostat was used to apply potentials to the WO 3 electrode with respect to the counter electrode (bare FTO glass). All the optical transmission measurements were performed with respect to a blank device which consisted of two FTO coated glass pieces sandwiched and sealed together in the same fashion as the devices and filled with the electrolyte. Thus, the transmission measurements included contributions from

4 816 ARTICLE IN PRESS S. Gubbala et al. / Solar Energy Materials & Solar Cells 91 (27) Intensity, arb units (1) (113) (211) (-415) (2) θ (1) %Transmission V V V Wavelength (nm) 6 5 Intensity, arb units (2) (2) (21) (22) % Transmission Time, Sec θ Fig. 4. XRD spectra of (a) as-synthesized nanowires and (b) after oxidizing the nanowires at 5 1C in air. The spectrum shows a change in the phase of the nanowires from oxygen deficient W 18 O 49 to WO 3 phase. Fig. 5. Optical photographs of the electrochromic device made with vertical arrays of nanowires in the bleached and colored states. the WO 3 films only. For coloration of the films, a voltage of 3.5 V was applied to the working (WO 3 ) electrode. During the bleaching process, a voltage of 2.5 V was applied to the electrode. Optical photographs of electrodes made with vertical arrays of nanowires in the colored and bleached states are shown in Fig. 5. All the spectra were collected in Fig. 6. (a) The optical transmission spectra of the devices made from vertically oriented nanowires with no applied bias, 3.5 V and 2.5 V. (b) The variation in transmission at 7 nm with a step variation in the potential. the wavelength range of 35 8 nm. Fig. 6(a) shows the variation of optical transmission through an electrochromic device made using a WO 3 vertical array of WO 3 nanowires on an FTO substrate. The plots show the transmission through the device at potentials of., 3.5, and 2.5 V. In this case, film electrode was kept at the coloration potential for 5 min. These films show a high change in the transmission before and after applying the coloration and bleaching potentials. From the transmission spectra of these devices, it can be seen that the films are highly electrochromic in the near IR wavelengths, with the transmission values falling to % from 76% on applying the potential. Upon applying bleaching potential, the optical transmission through the films go back to 71% transmission. It was found that although the coloration process seems to be fast in these films, the bleaching process takes a long time. In the above case, the coloration took place almost instantaneously, but the bleaching took almost 3 min to reach a value of 71%. The difference in the coloration and bleaching timescales was also observed when the electrodes were subjected to step potential changes at intervals of 5 min (Fig. 6(b)). As shown in Fig. 6(b), a transmission modulation of about 4% at 7 nm is observed. Although the slow

5 ARTICLE IN PRESS S. Gubbala et al. / Solar Energy Materials & Solar Cells 91 (27) bleaching is consistent with reports by Maruyama and Kanagawa [9] for CVD grown films at 3 1C, the reason attributed for it was the inclusion of microcrystalline particles, whereas the films prepared in this report have much smaller length scales. It was also observed that the transmission through the electrodes increased continuously with the application of a bleaching potential, reaching a saturation value after about 3 min. The initial steep rise in the bleaching behavior and a slowdown in the bleaching with time indicate that there are two different steps involved in the bleaching process, a fast component and a slow component. The channels in the nanowire array films get narrower in the interior parts as they go toward the substrate. As soon as the coloration potential is applied, all the Li + ions intercalate into the upper regions of the WO 3 nanowire arrays instantaneously. Most of the coloration in the first few seconds is observed mostly due to the intercalation of Li + ions in the upper parts of the nanowires closer to the bulk electrolyte solution. Further application of the potential makes Li + ions intercalate into the interior parts of the nanowires as more and more Li + ions diffuse into interior regions depleted with the Li + ions. The continued application of coloration potential and subsequent intercalation of Li + into the interior parts of the WO 3 nanowire arrays does not seem to effect the optical contrast significantly. During this period, a significant amount of Li + intercalation related current is observed. The rapid coloration of these films due to the top layer of the film is further confirmed by the XPS spectra of these films taken after 15 s of coloration. The XPS spectra of the films clearly show the well-resolved spin-orbit split doublet peaks which correspond to the W 4f 7/2 and W 4f 5/2. Fig. 7(a) shows the W 4f 7/2 and W 4f 5/2 peaks at 36.6 and 38.6 ev, respectively, which correspond to the W +6 oxidation state. Fig. 7(b) shows the peak positions at 36 and 38 ev, corresponding to the +5 oxidation state of W. The bleaching process seems to be most effected by the longer durations of the coloration. When bleaching potential is applied, the Li + deintercalates from the WO 3 and goes into the electrolyte solution. The Li + from the upper parts of the nanowires quickly deintercalates but the optical transmission does not change significantly because of intercalation of interior parts of the nanowire arrays. The subsequent deintercalation process from the interior parts of the WO 3 nanowire arrays occurs slowly for the following reasons: First, due to the Li + already present in the electrolyte solution, further deintercalation becomes difficult and due to the slow rate of mass transport of Li + from the interior parts of the film to the bulk, Li + ion concentration builds up, which slows down the deintercalation even further. Therefore, the color which remains for a long time during the deintercalation process is due to the Li + ions in the interior parts of the WO 3 film. The above dependence of the performance of these devices on the nanowire array density requires further optimization of these structures. Intensity, arb units Intensity, arb units W4f 7/2 W4f 7/2 W4f 5/2 W4f 5/ Binding Energy, ev Binding Energy, ev Fig. 7. XPS spectra of the nanowire arrays (a) before and (b) after Li + intercalation. Fig. 8. SEM image of nanowires grown on the FTO substrates for 2 min. The above measurements were also performed on WO 3 films grown for 2 min which had very high density of nanowires (Fig. 8). It was observed that the timescales associated with the transmission change through the

6 818 ARTICLE IN PRESS S. Gubbala et al. / Solar Energy Materials & Solar Cells 91 (27) electrodes was a strong function of the time for which they were subjected to a coloration potential. Fig. 9 shows the dependence of transmission modulation of these electrodes with time when a coloration potential of 3.5 V was applied for (a) 1 s, (b) 3 s, and (c) 5 min, followed by a bleaching potential for 5 min. These substrates had an even higher density of nanowires. The variation of bleaching time scales with different coloration times is again due to the high density of nanowires on the films which limits the mass transport of Li + out of the nanowires after applying long coloration potentials and indicates that the bleaching behavior is more dependent on the nanowire density, rather than the interface between the nanowires and the % Transmission a b FTO. The dependence of coloration and bleaching times on the porosity of tungsten trioxide films was studied recently in nanohole-array membranes by Nishio et al. [14] Their study showed that as the hole size in the films increased the coloration and bleaching times decreased significantly. This was attributed to the different rate of diffusion of Li + ions into the WO 3 membranes with different porosities. The dependence of bleaching times on the density of nanowires suggests that the density of these nanowires on the substrate has to be optimized in order to obtain bleaching times to less than 3 min irrespective of the coloration time scales and at the same time maintaining high contrast. Fig. 1 shows the cyclic voltammogram performed on one of these devices. The voltammograms shows cycles after 5 and 2 cycles. The electrochemical cycling performed on these electrodes at 1 mv/s scan rate in the potential range of 3.5 and 2 V does not indicate any degradation with lithium intercalation and deintercalation even after 2 cycles. The electrochromic performance of mat electrodes was also measured in a similar fashion. The optical photographs of these electrodes are shown in Fig. 11. Fig. 12(a) shows the transmission spectra of the electrode from 35 to 8 nm. The scans were taken with bias, 3.5 V, and 2.5 V. From the transmission spectra of these electrodes, it can be seen that these films also show an acceptable electrochromic behavior. The transmission at 7 nm for these films at bias is about 85%, which falls down to 32% upon Li + intercalation. The transmission goes back to 8% upon bleaching. The cycling behavior of these devices taken in 5 min intervals shows a transmission modulation of about 35% as shown in Fig. 12(b). These devices also show no electrochemical degradation with cycling similar to the data shown in Fig. 1 for devices made using nanowire arrays grown directly on FTO c Current (Amp) Scan #5 Scan # Time, seconds Potential (V) 2 4 Fig. 9. Optical transmission spectra of the devices made with nanowires grown for 2 min. Coloration potential was applied to these electrodes for (a) 1 s, (b) 3 s, and (c) 5 min, followed by 5 min of bleaching. The time scale for x-axis is the same for (a) (c). Fig. 1. Cyclic voltammetry performed on the electrode with vertically oriented nanowires at the 5th and 2th scans. The plot shows a very small variation in the currents indicating a good cycling stability of these electrodes.

7 ARTICLE IN PRESS S. Gubbala et al. / Solar Energy Materials & Solar Cells 91 (27) Mat Electrode Array Electrode Current, ma Fig. 11. Optical photographs of the electrochromic device made with matlike structures of nanowires in the bleached and colored states. a % Transmission b % Transmission 1 9 V V V Wavelength (nm) Time, sec Fig. 12. (a) The optical transmission spectra of the devices made from mat-like structures of nanowires with no applied bias, 3.5 V and 2.5 V. (b) The variation in transmission at 7 nm with a step variation in the potential. substrates. The high transmission through these films in the colored state is due to the low density of these wires on the FTO substrate, which allows the direct passage of light through the FTO, without interacting with the nanowires. The transmission through these electrodes in the colored Time, sec Fig. 13. Chronoamperometry on the array electrodes and mat-like electrodes. state can be decreased further by increasing the loading of nanowires on these substrates. As in the case of the electrodes with WO 3 vertical arrays, the bleaching times are again dependent on the time of coloration. Chronoamperometry was performed on both kinds of devices to study the currents during lithium intercalation and deintercalation. The plots (shown in Fig. 13) indicate a faster decay in the currents in the case of mat-like devices when compared to the array-based devices, both during coloration and bleaching. Although, this suggests that bleaching and coloration should be faster in mat-like configurations, it is observed that coloration is faster in the case of array electrodes, while bleaching is faster in the case of mat-like electrodes. This indicates that the change in the transmission of these electrodes is not a simple function of current. 4. Summary The results described here demonstrate that WO 3 nanowires can be used as high contrast electrochromic devices. The devices can be made from vertically oriented nanowires on FTO substrates or with nanowires deposited on FTO substrates from dispersions. The optical transmission modulation of the two devices was comparable, with the vertical arrays showing a relatively higher performance. The bleaching times of the devices made with nanowires in the mat-like format were found to be shorter than the ones made with vertical arrays. The transmission characteristics of devices made in mat-like format can be further improved by optimizing the nanowire density, diameters, and their aspect ratios. Acknowledgments The authors acknowledge financial support from the US Department of Energy for supporting the Institute for Advanced Materials (DE-FG2-5ER6471), Kentucky

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