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Solid state Gd–Mg electrochromic devices with ZrO2Hx electrolyte
Electrochimica Acta 46 (2001) 2167– 2171 www.elsevier.nl/locate/electacta
Solid state Gd–Mg electrochromic devices with ZrO2Hx electrolyte P. van der Sluis *, V.M.M. Mercier Philips Research Laboratories, Prof. Holstlaan 4 (WA-11), 5656 AA Eindho6en, The Netherlands Received 22 August 2000; received in revised form 9 November 2000
Abstract A hydrogen based device, which consists mainly of a GdMgH5 electrochromic layer, a ZrO2Hx electrolyte and a WO3 electrochromic counter electrode, was made. When a positive voltage is applied to the GdMgH5 layer, the hydrogen moves towards the WO3 layer. The latter becomes blue while the GdMgH5 layer becomes metallic (GdMgH2), resulting in a non-transparent stack. When a negative voltage is applied to the GdMgH2 layer, the hydrogen moves back to the GdMgH2 layer. Both are eletrochromic layers and therefore the stack becomes transparent again. The devices switch reversibly up to 500 times with switching times in the order of minutes. This paper deals with the deposition and in-situ loading of the devices with hydrogen and contains some preliminary information on the switching behaviour and optical properties. © 2001 Elsevier Science Ltd. All rights reserved. Keywords: Switchable; Metal hydride; Electrochromic; Device; Lanthanide
1. Introduction In 1995, it was discovered that thin Pd films could be used as a protective coating and catalyst for the study of hydrogen in lanthanide (Ln) metals [1,2]. A drastic change in the optical properties of the lanthanide thin films was found, depending on the amount of absorbed hydrogen. When a Pd coated yttrium film is exposed to hydrogen, the shiny metallic film transforms into a slightly transparent dark blue phase with a composition near YH2. After further hydrogen uptake, the film approaches the composition YH3 and becomes transparent yellow [3]. Upon evacuation of the film the dark blue phase can be restored. Thanks to these optical effects, it is even possible to study lateral hydrogen diffusion and electromigration in these films simply by light microscopy [4]. * Corresponding author. Fax: +31-40-2744282. E-mail address: [email protected] (P. van der Sluis).
However, for applications, it is often desirable to switch from a completely non-transparent film to a transparent state which is colour neutral without any absorption in the visible wavelength range (380– 780 nm). This can be achieved with Pd coated LnMg alloys or LnMg multi-layers [5,6]. Moreover, the nontransparent state for these Mg containing films is highly reflective. Technologically, it is relevant that switching can also be carried out by electrochemical means in aqueous KOH solution [7,8]. This would offer a way to make laminated devices with a liquid- or gel-type electrolyte. However, for most applications, an all-solid-state device is more desirable. Such a device, with a GdMg hydride bottom electrode (i.e. near the substrate), a hydrated zirconium oxide electrolyte and a WO3 top electrode, has been demonstrated [9], but the device was hampered with a very high short circuit current. This results in very long switching times (16 h) and makes durability testing virtually impossible. Here, we report on a different device: WO3 is used as the bottom
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electrode, a hydrided zirconium oxide is used as electrolyte and a GdMg hydride layer as the top electrode. The GdMg layer is loaded with hydrogen during its deposition.
2. Experimental Deposition of the stacks was carried out in a custombuilt RF-magnetron sputtering system with a base pressure of about 8×10 − 7 mbar on float glass-substrates covered with commercial ITO (20 V/ ). The target diameter is 2 in. and the target-to-substrate distance is 3 cm. On each substrate (30 ×30 mm) four devices (A–D) were made simultaneously. All substrates hang in the deposition chamber with the surface down, to avoid trouble with dust to the maximum possible extent. Sputter gas pressures are controlled by individual mass-flow controllers for each gas. The WO3 layers are sputtered from a W target, the ZrO2Hx layers from a Zr target, the Pd layers from a Pd target and the GdMgH5 layers from a Gd target with Mg inserts. The number of inserts was adjusted to obtain a Gd/Mg ratio of about 1:1. Pre-sputtering was carried out at the deposition conditions until a constant bias voltage was obtained, except for the second ZrO2Hx layer (vide infra). Chemical compositions and elemental aerial densities were determined by Rutherford Backscattering Spectrometry (RBS). Physical layer thicknesses were determined by high resolution SEM or profilometry. Reflection and transmission data were recorded using a He–Ne laser (633 nm). Because of interferences the absolute values of transmission and reflection should be interpreted with some care since they were measured at only one wavelength. Electrical measurements were carried out with a four-point probe Keithly 2400 sourcemeter, controlled by a PC.
Fig. 1. Schematic representation of our device, both in crosssection and viewed from the substrate side. Area a is covered with ITO, area b with ITO/WO3, area c with ITO/WO3/ ZrO2Hx /ZrO2Hx /ZrO2Hy, area d with the complete stack, area e with ZrO2Hx /Pd/GdMg/Pd/ZrO2Hx /ZrO2Hy and area f with Pd/GdMg/Pd.
3. Stack deposition The different layers of the stack are shifted with respect to each other along the diagonal of the square (see Fig. 1) with a mask that can be moved under vacuum. This shifting is required for easy access to the bottom electrode for electrical contact and to minimize short circuits along the edges of the device. The first layer (ITO) is divided into four squares with photo-lithographic techniques, providing for four devices on the 30 × 30 mm glass substrate. After slightly shifting the mask, the WO3 hydrogen storage layer is deposited. Reactive sputtering is carried out at 52 mbar with 50 W RF power in a mixture of Ar/O2 (ratio 100:5) for 350 s. The WO3 layer thickness is about 600 nm. The high pressure was chosen in order to obtain a porous layer. The O2 pressure was chosen to obtain a W to O ratio in the film of at least 3. After shifting further the mask by about 1.5 mm, the electrolyte is deposited at 62 mbar with 50 W RF power in a mixture of Ar/O2/H2 (ratio 100:0.4:40) for 1100 s. The layer thickness is about 120 nm. The high pressure was again chosen in order to obtain a porous layer. The O2 pressure was the minimum pressure to obtain a Zr to O ratio of 2 in the film. The H2 pressure was, at those conditions, the maximum pressure that still yielded transparent films. RBS gives composition ZrO2 – 2.1H0.3 – 0.4. This means that a part of the hydrogen is not chemically bonded as water or hydroxyl or that part of the Zr4 + is Zr3 + . Transmission electron microscopy of a sample deposited on Si showed a porous columnar structure with very small crystallites. According to X-ray diffraction, the crystallites were mainly cubic zirconia, with a slightly enlarged unit cell. From the RBS aerial atomic density and the physical thickness from TEM, a porosity of about 30% is calculated. After another mask shift of 1.5 mm, a Pd/GdMgH5/ Pd stack is sputtered. All three layers are sputtered at 86 mbar with 30 W RF power in a mixture of Ar/H2 (ratio 20:100). A high hydrogen pressure is required to obtain a fully loaded GdMgH5 film. It should be noted that changing this pressure will also result in deviations in the Gd/Mg ratio in the film. The bottom Pd serves as a catalyst and protective layer against the electrolyte. The top Pd serves as a protective layer against the capping layer and as a catalyst to get extra hydrogen in during deposition of the capping layer. Sputtering times are 10, 600 and 5 s, respectively, and the layer thicknesses are about 1, 55 and 0.5 nm, respectively. The resistance of this Pd/GdMgHx /Pd stack is low enough to be used as a top electrical contact. After the mask shifting back over 1.5 mm the capping layer is deposited. It has to be shifted back, in order to be able to gain electrical access to the GdMgH5 layer. No pre-sputtering of the Zr target is carried out in this case to reduce the chance of hydro-
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pressure results in a denser ZrO2 layer, required for long term encapsulation of the hydrogen in the device. The total cap-layer thickness is about 60 nm.
4. Appearance of the stack
Fig. 2. Photograph of device A and device B from sample 1. The meaning of the letters a–f is explained in the text.
gen de-loading of the GdMgH5 layer back into the sputtering chamber. The first part of the capping layer is sputtered under the same conditions as the electrolyte. The high hydrogen-to-oxygen ratio (100:1) ensures that the GdMgH5 layer stays transparent. After sputtering for 180 s under these conditions, the pressure is reduced to 28 mbar by lowering the Ar flow and sputtering is continued for 400 s. The final mixture has an Ar/O2/H2 ratio of 20:0.4:40. This lower sputtering
Fig. 3. Cell voltage, current density, reflection and transmission of device B from sample 1.
Fig. 1 shows a schematic drawing of the stack, both in its cross-section and from the glass-substrate (bottom) side. Fig. 2 shows two of the four devices from a sample seen from the bottom (i.e. substrate) side: device A is still in the as-deposited state and device B has been switched to the non-transparent state. In the bottom view regions a – f can be discerned. Region a is the bare ITO for contacting the bottom of the device. This narrow transparent region cannot be discerned from Fig. 2. Region b shows the WO3 layer. As expected it is transparent, although a darker region is seen close to the border with region c. In this darkened region the WO3 layer has reacted with hydrogen from the sputtering atmosphere creeping under the mask during deposition of the electrolyte and capping layer. Region c is transparent because the electrolyte and capping are transparent and the WO3 does not contain a significant amount of hydrogen. The reaction of the WO3 in region c with hydrogen from the sputtering atmosphere cannot take place because after a short period of deposition of ZrO2Hx the WO3 layer is covered. Regions d and e are again transparent since the GdMgH5 alloy is supposed to contain all the hydrogen. The very high transmission of region e (about 60%) is possible because the non-transparent Pd layers are relatively thin: the bottom layer is only 1 nm and the top layer only 0.5 nm. This is much thinner than usually employed for switching metal hydride thin films in contact with gas or liquid electrolytes [10]. Comparison of the appearance of regions e and c shows that the Pd/GdMgH5/Pd stack is just as transparent as the WO3 layer. After storage of the device for over a year at room temperature this region is still transparent, showing that the quality of the capping layer is sufficient for laboratory purposes. Region f is not transparent because here the hydrogen can escape to the atmosphere via the top Pd layer shortly after deposition. The overlapping of the top and bottom electrode geometrically define the active region of the device as indicated in Fig. 1 (region d). After applying + 1 V over the device ( + on GdMgH5) the device becomes non-transparent in the active region, whereas the rest of the device remains unchanged. The small light spot just below the middle of the active region (in device B, Fig. 2) is the reflection from the laser to measure the optical data given in Fig. 3. At open circuit conditions, the
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contrast between the left and the right devices of Fig. 2 slowly fades due to an internal short circuit current. However, after one year at room temperature the contrast is still clearly visible to the naked eye. After applying −1 V to the device ( − on the GdMgH2 layer) the device becomes transparent, even more transparent than as-deposited. The final transmission of the active region reaches the transmission of region b. This means that during deposition some hydrogen from the Gd–Mg layer finds its way to the WO3 layer. It cannot come from the electrolyte layer, since region c is just as transparent as to the right of region b.
5. Optical measurements Fig. 3 shows the measured transmission and reflection from the bottom side of device B of Fig. 2. The measurement starts in the non-transparent state. The voltage is decreased to −1 V in 100 mV steps in order to avoid high current pulses through the device. In 100 s the transmission raises from about 5 to 40% and the reflection from 7 to about 20%. Most of the charge has at that moment passed through the device (equivalent to 20 mC cm − 2). In the following 500 s only 5 mC cm − 2 passes through the device, but this has a big influence on the transparency: it raises from 40 to 60%. From long equilibration at this potential, the maximum transmission was found to be about 65%. This is higher than reported in the previous solid state gadolinium– magnesium hydride device (about 20%, [9]) because our absorbing Pd layers are much thinner (a total of 1.5 nm instead of 15 nm). Note that the reflection is higher in the transparent state instead of being higher in the non-transparent state. The latter might be expected since GdMgH2 is a very good reflector [5]. This is
Fig. 4. Cyclic voltammogram of device A of sample 2. Sample 2 is deposited in the same conditions as sample 1 and is expected to be identical. The measurement starts after equilibration for 2 h at +1 V. Sweep rate was 66.6 mV s − 1.
because we measured the reflection from the substrate side where the dark coloured WO3 inhibits measurement of the reflection from the GdMgH2 layer. Switching is carried out with 9 1 V over the device. At these voltages the current just before switching has stabilised at still fairly high currents: − 30 and +65 mA cm − 2 for switching to transparent and non-transparent, respectively. To check whether the used voltages were appropriate for the device, we carried out cyclic voltammetry.
6. Electrical measurements Fig. 4 shows the cyclic voltammogram (CV) of another device made in the same conditions. It was switched a couple of times and kept at +1 V for 2 h before starting the slow voltage ramp at 67 mV s − 1. Cycle 2 is identical to subsequent cycles (four carried out) but is different from the first. Since this device was switched several times before this CV, it is unlikely that this difference would be due to initial switching effects. We believe that it is due to side-reactions happening at voltages beyond about 0.8 V. The shape of the CV also supports the same conclusion. At cell voltages higher than 0.8 V and lower than − 0.8 V the current increases instead of decreasing further. Therefore, we conclude that 90.8 V is a safe potential window for the device. However, the charge moving through the device at 91V is the same for both directions: 22.5 mC cm − 2. Apparently the processes beyond 9 0.8 V result in a reversible charge redistribution, but might still be damaging. The curve for switching to the transparent state shows three different peaks, whereas the curve for switching to the non-transparent state shows only one peak. Sm0.3Mg0.7Hx films show three peaks when cycled in liquid electrolyte [8]. We have not investigated this further, knowing that all potentials and currents are cell potentials and currents. In our all-solid-state device we cannot envisage a way to work with a reference electrode, which is required for a meaningful interpretation. Fig. 5 shows a durability test of the device. It was cycled with 9 0.8 V. To avoid high current peaks, the cell voltage is ramped during 100 s when going from one potential to the other. Subsequently the voltage was kept constant for half an hour. We noticed a daily rhythm which appeared to be coupled to the room temperature. With time the device becomes slower. After just more than 400 cycles we stopped the measurement, since in the half hour the amount of reversible charge decreased from 15 to 7.5 mC cm − 2. With longer switching times we could still reach 15 mC cm − 2. So, we conclude that the kinetics goes down without significant reduction of capacity. This could be related to a thin, growing oxide layer on the metal
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charge current is negligible. At the same time, the ionic resistance is low enough to obtain switching times of about 5 min in a complete stack. This is fast enough for at least some applications. However, the durability should be enhanced significantly before these materials can be used in applications.
Acknowledgements Part of the work was supported financially by the European TMR Research Network ‘Metal hydride films with switchable physical properties’ and by the Dutch BTS program ‘Switchable mirrors’. Fig. 5. Charge– time diagram of device A of sample 2.
References
hydride–electrolyte interface. Similar effects are seen when switching these metal hydride electrodes in liquid electrolytes [11]. Preliminary Scanning Auger Microscopy on a cycled device supports this conclusion. For technological applications 400 switches are still an insignificantly low number. However, research is under way to improve the durability of these materials [12]. The final currents just before a subsequent switch are +20 and −104 nA cm − 2, respectively. This means that after a while the charge–time curve slowly displaces downward. After about 200 cycles this current difference has accumulated to more than the reversible charge itself. Since this is not reflected in the optical behaviour (in that case, the device should not be switching anymore), we conclude that the observed final currents are mainly due to electrical current through the electrolyte or along the edges of the device. The self-discharge, corresponding to a current of that kind under open circuit conditions, is low enough so that after one year at room temperature, the contrast between transparent and non-transparent devices is still clearly visible to the naked eye.
[1] J.N. Huiberts, R. Griessen, J.H. Rector, R.J. Wijngaarden, J.P. Dekker, D.G. de Groot, N.J. Koeman, Nature 380 (1996) 231. [2] J.N. Huiberts, J.H. Rector, R.J. Wijngaarden, S. Jetten, D. de Groot, B. Dam, N.J. Koeman, R. Griessen, B. Hjo¨rvarsson, S. Olafsson, Y.S. Cho, J. Alloys Compd. 239 (1996) 158. [3] E.S. Kooij, A.T.M. van Gogh, R. Griessen, J. Electrochem. Soc. 146 (1999) 2990. [4] F.J.A. den Broeder, S.J. van der Molen, M. Kremers, J.N. Huiberts, D.N. Nagengast, A.T.M. van Gogh, W.H. Huisman, N.J. Koeman, B. Dam, J.H. Rector, S. Plota, M. Haaksma, R.M.N. Hanzen, R.M. Jungblut, P.A. Duine, R. Griessen, Nature 394 (1998) 656. [5] P. van der Sluis, M. Ouwerkerk, P.A. Duine, Appl. Phys. Lett. 70 (1997) 3356. [6] P. van der Sluis, Appl. Phys. Lett. 73 (1998) 1826. [7] P.H.L. Notten, M. Kremers, R. Griessen, J. Electrochem. Soc. 143 (1996) 3348. [8] M. Ouwerkerk, Solid State Ionics 113– 115 (1998) 431. [9] R. Armitage, M. Rubin, T. Richardson, N. O’Brien, Yong Chen, Appl. Phys. Lett. 75 (1999) 1863. [10] S.J. van der Molen, J.W.J. Kerssemakers, J.H. Rector, N.J. Koeman, B. Dam, R. Griessen, J. Appl. Phys. 86 (1999) 6107. [11] A.-M. Janner, P. van der Sluis, in: E.D. Wachsman, J.R. Akridge, M. Liu, N. Yamazoe (Eds.), Proceedings of the International Symposium: Solid State Ionic Devices, The Electrochemical Society, Pennington, 1999, p. 289. [12] A.T.M. van Gogh, S.J. van der Molen, J.W.J. Kerssemakers, N.J. Koeman, R. Griessen, Appl. Phys. Lett. 77 (2000) 815.
7. Conclusions We have made an all-solid-state switchable device with an electrochromic metal hydride electrode and a hydrided zirconium oxide electrolyte. The electrical resistance of this electrolyte is so high that the self-dis-
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Solid state Gd–Mg electrochromic devices with ZrO2Hx electrolyte P. van der Sluis *, V.M.M. Mercier Philips Research Laboratories, Prof. Holstlaan 4 (WA-11), 5656 AA Eindho6en, The Netherlands Received 22 August 2000; received in revised form 9 November 2000
Abstract A hydrogen based device, which consists mainly of a GdMgH5 electrochromic layer, a ZrO2Hx electrolyte and a WO3 electrochromic counter electrode, was made. When a positive voltage is applied to the GdMgH5 layer, the hydrogen moves towards the WO3 layer. The latter becomes blue while the GdMgH5 layer becomes metallic (GdMgH2), resulting in a non-transparent stack. When a negative voltage is applied to the GdMgH2 layer, the hydrogen moves back to the GdMgH2 layer. Both are eletrochromic layers and therefore the stack becomes transparent again. The devices switch reversibly up to 500 times with switching times in the order of minutes. This paper deals with the deposition and in-situ loading of the devices with hydrogen and contains some preliminary information on the switching behaviour and optical properties. © 2001 Elsevier Science Ltd. All rights reserved. Keywords: Switchable; Metal hydride; Electrochromic; Device; Lanthanide
1. Introduction In 1995, it was discovered that thin Pd films could be used as a protective coating and catalyst for the study of hydrogen in lanthanide (Ln) metals [1,2]. A drastic change in the optical properties of the lanthanide thin films was found, depending on the amount of absorbed hydrogen. When a Pd coated yttrium film is exposed to hydrogen, the shiny metallic film transforms into a slightly transparent dark blue phase with a composition near YH2. After further hydrogen uptake, the film approaches the composition YH3 and becomes transparent yellow [3]. Upon evacuation of the film the dark blue phase can be restored. Thanks to these optical effects, it is even possible to study lateral hydrogen diffusion and electromigration in these films simply by light microscopy [4]. * Corresponding author. Fax: +31-40-2744282. E-mail address: [email protected] (P. van der Sluis).
However, for applications, it is often desirable to switch from a completely non-transparent film to a transparent state which is colour neutral without any absorption in the visible wavelength range (380– 780 nm). This can be achieved with Pd coated LnMg alloys or LnMg multi-layers [5,6]. Moreover, the nontransparent state for these Mg containing films is highly reflective. Technologically, it is relevant that switching can also be carried out by electrochemical means in aqueous KOH solution [7,8]. This would offer a way to make laminated devices with a liquid- or gel-type electrolyte. However, for most applications, an all-solid-state device is more desirable. Such a device, with a GdMg hydride bottom electrode (i.e. near the substrate), a hydrated zirconium oxide electrolyte and a WO3 top electrode, has been demonstrated [9], but the device was hampered with a very high short circuit current. This results in very long switching times (16 h) and makes durability testing virtually impossible. Here, we report on a different device: WO3 is used as the bottom
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electrode, a hydrided zirconium oxide is used as electrolyte and a GdMg hydride layer as the top electrode. The GdMg layer is loaded with hydrogen during its deposition.
2. Experimental Deposition of the stacks was carried out in a custombuilt RF-magnetron sputtering system with a base pressure of about 8×10 − 7 mbar on float glass-substrates covered with commercial ITO (20 V/ ). The target diameter is 2 in. and the target-to-substrate distance is 3 cm. On each substrate (30 ×30 mm) four devices (A–D) were made simultaneously. All substrates hang in the deposition chamber with the surface down, to avoid trouble with dust to the maximum possible extent. Sputter gas pressures are controlled by individual mass-flow controllers for each gas. The WO3 layers are sputtered from a W target, the ZrO2Hx layers from a Zr target, the Pd layers from a Pd target and the GdMgH5 layers from a Gd target with Mg inserts. The number of inserts was adjusted to obtain a Gd/Mg ratio of about 1:1. Pre-sputtering was carried out at the deposition conditions until a constant bias voltage was obtained, except for the second ZrO2Hx layer (vide infra). Chemical compositions and elemental aerial densities were determined by Rutherford Backscattering Spectrometry (RBS). Physical layer thicknesses were determined by high resolution SEM or profilometry. Reflection and transmission data were recorded using a He–Ne laser (633 nm). Because of interferences the absolute values of transmission and reflection should be interpreted with some care since they were measured at only one wavelength. Electrical measurements were carried out with a four-point probe Keithly 2400 sourcemeter, controlled by a PC.
Fig. 1. Schematic representation of our device, both in crosssection and viewed from the substrate side. Area a is covered with ITO, area b with ITO/WO3, area c with ITO/WO3/ ZrO2Hx /ZrO2Hx /ZrO2Hy, area d with the complete stack, area e with ZrO2Hx /Pd/GdMg/Pd/ZrO2Hx /ZrO2Hy and area f with Pd/GdMg/Pd.
3. Stack deposition The different layers of the stack are shifted with respect to each other along the diagonal of the square (see Fig. 1) with a mask that can be moved under vacuum. This shifting is required for easy access to the bottom electrode for electrical contact and to minimize short circuits along the edges of the device. The first layer (ITO) is divided into four squares with photo-lithographic techniques, providing for four devices on the 30 × 30 mm glass substrate. After slightly shifting the mask, the WO3 hydrogen storage layer is deposited. Reactive sputtering is carried out at 52 mbar with 50 W RF power in a mixture of Ar/O2 (ratio 100:5) for 350 s. The WO3 layer thickness is about 600 nm. The high pressure was chosen in order to obtain a porous layer. The O2 pressure was chosen to obtain a W to O ratio in the film of at least 3. After shifting further the mask by about 1.5 mm, the electrolyte is deposited at 62 mbar with 50 W RF power in a mixture of Ar/O2/H2 (ratio 100:0.4:40) for 1100 s. The layer thickness is about 120 nm. The high pressure was again chosen in order to obtain a porous layer. The O2 pressure was the minimum pressure to obtain a Zr to O ratio of 2 in the film. The H2 pressure was, at those conditions, the maximum pressure that still yielded transparent films. RBS gives composition ZrO2 – 2.1H0.3 – 0.4. This means that a part of the hydrogen is not chemically bonded as water or hydroxyl or that part of the Zr4 + is Zr3 + . Transmission electron microscopy of a sample deposited on Si showed a porous columnar structure with very small crystallites. According to X-ray diffraction, the crystallites were mainly cubic zirconia, with a slightly enlarged unit cell. From the RBS aerial atomic density and the physical thickness from TEM, a porosity of about 30% is calculated. After another mask shift of 1.5 mm, a Pd/GdMgH5/ Pd stack is sputtered. All three layers are sputtered at 86 mbar with 30 W RF power in a mixture of Ar/H2 (ratio 20:100). A high hydrogen pressure is required to obtain a fully loaded GdMgH5 film. It should be noted that changing this pressure will also result in deviations in the Gd/Mg ratio in the film. The bottom Pd serves as a catalyst and protective layer against the electrolyte. The top Pd serves as a protective layer against the capping layer and as a catalyst to get extra hydrogen in during deposition of the capping layer. Sputtering times are 10, 600 and 5 s, respectively, and the layer thicknesses are about 1, 55 and 0.5 nm, respectively. The resistance of this Pd/GdMgHx /Pd stack is low enough to be used as a top electrical contact. After the mask shifting back over 1.5 mm the capping layer is deposited. It has to be shifted back, in order to be able to gain electrical access to the GdMgH5 layer. No pre-sputtering of the Zr target is carried out in this case to reduce the chance of hydro-
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pressure results in a denser ZrO2 layer, required for long term encapsulation of the hydrogen in the device. The total cap-layer thickness is about 60 nm.
4. Appearance of the stack
Fig. 2. Photograph of device A and device B from sample 1. The meaning of the letters a–f is explained in the text.
gen de-loading of the GdMgH5 layer back into the sputtering chamber. The first part of the capping layer is sputtered under the same conditions as the electrolyte. The high hydrogen-to-oxygen ratio (100:1) ensures that the GdMgH5 layer stays transparent. After sputtering for 180 s under these conditions, the pressure is reduced to 28 mbar by lowering the Ar flow and sputtering is continued for 400 s. The final mixture has an Ar/O2/H2 ratio of 20:0.4:40. This lower sputtering
Fig. 3. Cell voltage, current density, reflection and transmission of device B from sample 1.
Fig. 1 shows a schematic drawing of the stack, both in its cross-section and from the glass-substrate (bottom) side. Fig. 2 shows two of the four devices from a sample seen from the bottom (i.e. substrate) side: device A is still in the as-deposited state and device B has been switched to the non-transparent state. In the bottom view regions a – f can be discerned. Region a is the bare ITO for contacting the bottom of the device. This narrow transparent region cannot be discerned from Fig. 2. Region b shows the WO3 layer. As expected it is transparent, although a darker region is seen close to the border with region c. In this darkened region the WO3 layer has reacted with hydrogen from the sputtering atmosphere creeping under the mask during deposition of the electrolyte and capping layer. Region c is transparent because the electrolyte and capping are transparent and the WO3 does not contain a significant amount of hydrogen. The reaction of the WO3 in region c with hydrogen from the sputtering atmosphere cannot take place because after a short period of deposition of ZrO2Hx the WO3 layer is covered. Regions d and e are again transparent since the GdMgH5 alloy is supposed to contain all the hydrogen. The very high transmission of region e (about 60%) is possible because the non-transparent Pd layers are relatively thin: the bottom layer is only 1 nm and the top layer only 0.5 nm. This is much thinner than usually employed for switching metal hydride thin films in contact with gas or liquid electrolytes [10]. Comparison of the appearance of regions e and c shows that the Pd/GdMgH5/Pd stack is just as transparent as the WO3 layer. After storage of the device for over a year at room temperature this region is still transparent, showing that the quality of the capping layer is sufficient for laboratory purposes. Region f is not transparent because here the hydrogen can escape to the atmosphere via the top Pd layer shortly after deposition. The overlapping of the top and bottom electrode geometrically define the active region of the device as indicated in Fig. 1 (region d). After applying + 1 V over the device ( + on GdMgH5) the device becomes non-transparent in the active region, whereas the rest of the device remains unchanged. The small light spot just below the middle of the active region (in device B, Fig. 2) is the reflection from the laser to measure the optical data given in Fig. 3. At open circuit conditions, the
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contrast between the left and the right devices of Fig. 2 slowly fades due to an internal short circuit current. However, after one year at room temperature the contrast is still clearly visible to the naked eye. After applying −1 V to the device ( − on the GdMgH2 layer) the device becomes transparent, even more transparent than as-deposited. The final transmission of the active region reaches the transmission of region b. This means that during deposition some hydrogen from the Gd–Mg layer finds its way to the WO3 layer. It cannot come from the electrolyte layer, since region c is just as transparent as to the right of region b.
5. Optical measurements Fig. 3 shows the measured transmission and reflection from the bottom side of device B of Fig. 2. The measurement starts in the non-transparent state. The voltage is decreased to −1 V in 100 mV steps in order to avoid high current pulses through the device. In 100 s the transmission raises from about 5 to 40% and the reflection from 7 to about 20%. Most of the charge has at that moment passed through the device (equivalent to 20 mC cm − 2). In the following 500 s only 5 mC cm − 2 passes through the device, but this has a big influence on the transparency: it raises from 40 to 60%. From long equilibration at this potential, the maximum transmission was found to be about 65%. This is higher than reported in the previous solid state gadolinium– magnesium hydride device (about 20%, [9]) because our absorbing Pd layers are much thinner (a total of 1.5 nm instead of 15 nm). Note that the reflection is higher in the transparent state instead of being higher in the non-transparent state. The latter might be expected since GdMgH2 is a very good reflector [5]. This is
Fig. 4. Cyclic voltammogram of device A of sample 2. Sample 2 is deposited in the same conditions as sample 1 and is expected to be identical. The measurement starts after equilibration for 2 h at +1 V. Sweep rate was 66.6 mV s − 1.
because we measured the reflection from the substrate side where the dark coloured WO3 inhibits measurement of the reflection from the GdMgH2 layer. Switching is carried out with 9 1 V over the device. At these voltages the current just before switching has stabilised at still fairly high currents: − 30 and +65 mA cm − 2 for switching to transparent and non-transparent, respectively. To check whether the used voltages were appropriate for the device, we carried out cyclic voltammetry.
6. Electrical measurements Fig. 4 shows the cyclic voltammogram (CV) of another device made in the same conditions. It was switched a couple of times and kept at +1 V for 2 h before starting the slow voltage ramp at 67 mV s − 1. Cycle 2 is identical to subsequent cycles (four carried out) but is different from the first. Since this device was switched several times before this CV, it is unlikely that this difference would be due to initial switching effects. We believe that it is due to side-reactions happening at voltages beyond about 0.8 V. The shape of the CV also supports the same conclusion. At cell voltages higher than 0.8 V and lower than − 0.8 V the current increases instead of decreasing further. Therefore, we conclude that 90.8 V is a safe potential window for the device. However, the charge moving through the device at 91V is the same for both directions: 22.5 mC cm − 2. Apparently the processes beyond 9 0.8 V result in a reversible charge redistribution, but might still be damaging. The curve for switching to the transparent state shows three different peaks, whereas the curve for switching to the non-transparent state shows only one peak. Sm0.3Mg0.7Hx films show three peaks when cycled in liquid electrolyte [8]. We have not investigated this further, knowing that all potentials and currents are cell potentials and currents. In our all-solid-state device we cannot envisage a way to work with a reference electrode, which is required for a meaningful interpretation. Fig. 5 shows a durability test of the device. It was cycled with 9 0.8 V. To avoid high current peaks, the cell voltage is ramped during 100 s when going from one potential to the other. Subsequently the voltage was kept constant for half an hour. We noticed a daily rhythm which appeared to be coupled to the room temperature. With time the device becomes slower. After just more than 400 cycles we stopped the measurement, since in the half hour the amount of reversible charge decreased from 15 to 7.5 mC cm − 2. With longer switching times we could still reach 15 mC cm − 2. So, we conclude that the kinetics goes down without significant reduction of capacity. This could be related to a thin, growing oxide layer on the metal
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charge current is negligible. At the same time, the ionic resistance is low enough to obtain switching times of about 5 min in a complete stack. This is fast enough for at least some applications. However, the durability should be enhanced significantly before these materials can be used in applications.
Acknowledgements Part of the work was supported financially by the European TMR Research Network ‘Metal hydride films with switchable physical properties’ and by the Dutch BTS program ‘Switchable mirrors’. Fig. 5. Charge– time diagram of device A of sample 2.
References
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7. Conclusions We have made an all-solid-state switchable device with an electrochromic metal hydride electrode and a hydrided zirconium oxide electrolyte. The electrical resistance of this electrolyte is so high that the self-dis-
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