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Highly transparent Ni–Mg and Ni–V–Mg oxide films for electrochromic applications
Thin Solid Films 422 (2002) 1–3
Letter
Highly transparent Ni–Mg and Ni–V–Mg oxide films for electrochromic applications A. Azens, J. Isidorsson, R. Karmhag, C.G. Granqvist* ˚ ¨ Laboratory Uppsala University, P.O. Box 534, SE-751 21 Uppsala, Sweden Department of Materials Science, The Angstrom Received 2 September 2001; accepted 14 May 2002
Abstract Thin films of oxides based on Ni, Ni–V, Ni–Mg, and Ni–V–Mg were made by reactive d.c. magnetron sputtering. Electrochemical cycling showed pronounced anodic electrochromism. The Mg addition was capable of yielding a significantly enhanced optical transparency in the 400-l-500 nm wavelength range when the films were in their bleached state. 䊚 2002 Elsevier Science B.V. All rights reserved. Keywords: Electrochemical; Magnetron sputtering; Electrochromism
Electrochromic thin films can be used in several types of optical technology w1,2x, including in architectural ‘smart’ windows with a large energy savings potential provided that the control strategy is adequate w3,4x. Devices using a centrally positioned ion conductor joining a cathodically coloring W oxide film and an anodically coloring Ni oxide film have been studied extensively during the past few years w5–11x. This design is favourable with regard to electrochemical potentials and the possibility of having chemical compatibility to the electrolyte w11x; convenient device assembly can use gas treatment of the W oxide w12x and Ni oxide film w13x. A remaining problem with Ni oxide, however, has been the residual optical absorption in the 400-l-500 nm wavelength range, which has precluded a fully transparent state. The purpose of this Letter is to show that this absorption can be practically eliminated by an addition of Mg to the Ni or Ni–V oxide. Thin films of oxides of Ni, Ni–V, Ni–Mg, and Ni– V–Mg were made by reactive d.c. magnetron sputtering in a deposition system based on a Balzers UTT 400 unit. The vanadium addition renders the target non*Corresponding author. Tel.: 0046-18-471-3067; fax: 0046-18-500131. E-mail address: [email protected] (C.G. Granqvist).
magnetic, which facilitates magnetron sputtering. The targets were 5-cm-diameter metallic plates of Ni, NiV0.08, and Mg, all with 99.95% purity. Sputtering took place in an AryO2 yH2 mixture equal to 10y3y5 by volume. The gases were 99.998% pure. The total sputter pressure was 30 mTorr and the sputtering power was 200 W for the Ni and Ni–V targets and 100–150 W for the Mg target. Depositions took place onto substrates positioned 13 cm from the target. The substrates were glass plates precoated with a layer of ITO (i.e. In2O3:Sn), having a resistanceysquare of 15 V, for optical and electrochemical measurements; graphite substrates were used for compositional determinations using Rutherford backscattering spectrometry (RBS). Deposition rates were obtained from sputter time and ensuing film thickness recorded by surface profilometry; a typical value was 0.4 nmys. The films were approximately 200 nm thick. Elemental compositions were determined on selected films by RBS, specifically by analysing the backscattered yield following bombardment with 2.0 MeV alpha particles. Typical compositions were NiMg0.8O2.4 and NiV0.08Mg0.5O2.4. We expect the films to be hydrous, i.e. to be more or less hydrated, hydroxylated, oxyhydroxylated, etc. Preliminary X-ray diffraction data were unable to discriminate between the films being one-phase (NiMgO2) or two-phase (NiOØMgO). The
0040-6090/02/$ - see front matter 䊚 2002 Elsevier Science B.V. All rights reserved. PII: S 0 0 4 0 - 6 0 9 0 Ž 0 2 . 0 0 4 3 7 - 6
2
A. Azens et al. / Thin Solid Films 422 (2002) 1–3
Fig. 1. Spectral absorptance for 200-nm-thick films of the shown compositions.
Fig. 2. Spectral absorptance for 200-nm-thick films of the shown compositions.
films were immersed in a 1 M KOH electrolyte and underwent voltammetric cycling in a three-electrode arrangement with a Pt counter electrode and an Agy AgCl reference electrode. The films displayed readily observable electrochromism. MgO is electrochemically inactive, which leads us to favor a NiMgO2 structure, possibly mixed with NiO, for the films. Quantitative optical data were taken by spectrophotometry on films that had been withdrawn from the electrolyte and cleaned in deionized water, using a Perkin Elmer Lambda 9 instrument. Absorption was evaluated from 1–TyR, where T is the normal transmittance and R is the near-normal reflectance. Fig. 1 shows data for a film of NiMg0.8O2.4 in the 300-l800 nm range and, for comparison, also for a film of NiOq (whose oxygen content was not determined). With the films in their states of maximum transmittance, the absorptance is found to be significantly smaller for the Mg-containing film over the full spectral range, the difference being especially large at l- 500 nm. The strong absorptance at l- 350 nm is due to the semiconductor bandgap, which appears to be widened as a consequence of the Mg addition. Some weak absorptance maxima can be seen in the spectra; they are possibly associated with crystal field effects w14,15x. Fig. 2 illustrates analogous results for films of NiV0.08Mg0.5O2.4 and NiV0.08Oq; these samples display the same characteristic features as the ones reported on in Fig. 1. Color properties were quantified by use of chromaticity coordinates (x, y, z)—corresponding to red, green and blue primaries, respectively—using the CIE Colorimetric System w16x. The solid and dashed curves in Fig. 1 correspond to (0.337, 0.336, 0.327) and (0.339, 0.342, 0.319), respectively, with regard to a daylight illuminant. Color neutrality is represented by (0.333, 0.333, 0.333), and it is evident that the Mg changes the
visual appearance towards a more color neutral state primarily by increasing the contribution of the blue part of the spectrum (i.e. the z component). Charge capacities were evaluated by cycling at 10 mVys between a lower (bleaching) potential of y0.5 V vs. AgyAgCl and an upper (coloration) potential in the 0.45-Ucol-0.65 V vs. AgyAgCl range. Fig. 3 shows data for films of NiV0.08Mg0.5O2.4 and NiV0.08Oq, i.e. for the samples earlier reported on in Fig. 2. The films are electrochemically active. It is evident that similar charge capacities can be achieved provided that Ucol is set at a 0.05–0.1 V higher potential when Mg is present. Summarizing, we have found that a Mg addition to Ni oxide, with and without a small V content, gives a significant decrease of the optical absorptance, especially of blue light, and thereby an improved color neutrality,
Fig. 3. Charge capacity vs. coloration potential Ucol for 200-nm-thick films of the shown compositions. Electrochemical cycling in 1 M KOH took place between y0.5 V vs. AgyAgCl and Ucol at 10 mVys. Dots represent evaluated data and curves were drawn for convenience.
A. Azens et al. / Thin Solid Films 422 (2002) 1–3
at the same time as the shift of the electrochemical potential is so small that it is insignificant with regard to electrochromic device applications. We believe that these results are significant for architectural applications of energy efficient smart windows for which high transparency and color neutrality are important features w17x. Finally, we note that recent work on Ni–Mg hydrides have shown interesting optical switching with some principle similarities to the data reported in this paper w18,19x. Acknowledgments Two of us (A.A. and R.K.) want to thank the Swedish Foundation for Strategic Environmental Research and the National Energy Administration of Sweden for financial support. References w1x C.G. Granqvist, Handbook of Inorganic Electrochromic Materials, Elsevier, Amsterdam, 1995. w2x C.G. Granqvist, Solar Energy Mater. Solar Cells 365 (2000) 119. w3x C.G. Granqvist, Int. Glass Rev. 2 (2001) 67. w4x A. Azens, C.G. Granqvist, Proc. Soc. Photo-Opt. Instrum. Eng. 4458 (2001) 104.
3
w5x S.-H. Lee, S.-K. Joo, Solar Energy Mater. Solar Cells 39 (1995) 155. w6x J.G.H. Mathew, S.P. Sapers, M.J. Cumbo, N.A. O’Brien, R.B. Sargent, V.P. Raksha, R.B. Lahaderne, B.P. Hichwa, J. NonCryst. Solids 218 (1997) 342. w7x A. Azens, L. Kullman, G. Vaivars, H. Nordborg, C.G. Granqvist, Solid State Ionics 113–115 (1998) 449. w8x R. Lechner, L.K. Thomas, Solar Energy Mater. Solar Cells 54 (1998) 139. w9x J. Nagai, G.D. McMeeking, Y. Saitoh, Solar Energy Mater. Solar Cells 56 (1999) 309. w10x J. Karlsson, A. Roos, Solar Energy 68 (2000) 493. w11x A. Azens, G. Vaivars, M. Veszelei, L. Kullman, C.G. Granqvist, J. Appl. Phys. 89 (2001) 7885. w12x A.P. Giri, R. Messier, Mater. Res. Soc. Symp. Proc. 24 (1984) 221. w13x A. Azens, L. Kullman, C.G. Granqvist, Solar Energy Mater. Solar Cells, in press. w14x T.M.J. Nilsson, G.A. Niklasson, Proc. Soc. Photo-Opt. Instrum. Eng. 1272 (1990) 129. w15x R. Newman, R.M. Chrenko, Phys. Rev. 114 (1959) 1507. w16x D.L. MacAdam, Color Measurement: Theme and Variations, Springer Series of Optical Sciences, Vol. 27, Springer, Berlin, 1981. w17x M. Wigginton, Glass in Architecture, Phaidon, London, 1996. w18x T.J. Richardson, J.L. Slack, R.D. Armitage, R. Kostecki, B. Farangis, M.D. Rubin, Appl. Phys. Lett. 78 (2001) 3047. w19x J. Isidorsson, I.A.M.E. Giebels, M. Di Vece, R. Griessen, Proc. Soc. Photo-Opt. Instrum. Eng. 4458 (2001) 128.
Letter
Highly transparent Ni–Mg and Ni–V–Mg oxide films for electrochromic applications A. Azens, J. Isidorsson, R. Karmhag, C.G. Granqvist* ˚ ¨ Laboratory Uppsala University, P.O. Box 534, SE-751 21 Uppsala, Sweden Department of Materials Science, The Angstrom Received 2 September 2001; accepted 14 May 2002
Abstract Thin films of oxides based on Ni, Ni–V, Ni–Mg, and Ni–V–Mg were made by reactive d.c. magnetron sputtering. Electrochemical cycling showed pronounced anodic electrochromism. The Mg addition was capable of yielding a significantly enhanced optical transparency in the 400-l-500 nm wavelength range when the films were in their bleached state. 䊚 2002 Elsevier Science B.V. All rights reserved. Keywords: Electrochemical; Magnetron sputtering; Electrochromism
Electrochromic thin films can be used in several types of optical technology w1,2x, including in architectural ‘smart’ windows with a large energy savings potential provided that the control strategy is adequate w3,4x. Devices using a centrally positioned ion conductor joining a cathodically coloring W oxide film and an anodically coloring Ni oxide film have been studied extensively during the past few years w5–11x. This design is favourable with regard to electrochemical potentials and the possibility of having chemical compatibility to the electrolyte w11x; convenient device assembly can use gas treatment of the W oxide w12x and Ni oxide film w13x. A remaining problem with Ni oxide, however, has been the residual optical absorption in the 400-l-500 nm wavelength range, which has precluded a fully transparent state. The purpose of this Letter is to show that this absorption can be practically eliminated by an addition of Mg to the Ni or Ni–V oxide. Thin films of oxides of Ni, Ni–V, Ni–Mg, and Ni– V–Mg were made by reactive d.c. magnetron sputtering in a deposition system based on a Balzers UTT 400 unit. The vanadium addition renders the target non*Corresponding author. Tel.: 0046-18-471-3067; fax: 0046-18-500131. E-mail address: [email protected] (C.G. Granqvist).
magnetic, which facilitates magnetron sputtering. The targets were 5-cm-diameter metallic plates of Ni, NiV0.08, and Mg, all with 99.95% purity. Sputtering took place in an AryO2 yH2 mixture equal to 10y3y5 by volume. The gases were 99.998% pure. The total sputter pressure was 30 mTorr and the sputtering power was 200 W for the Ni and Ni–V targets and 100–150 W for the Mg target. Depositions took place onto substrates positioned 13 cm from the target. The substrates were glass plates precoated with a layer of ITO (i.e. In2O3:Sn), having a resistanceysquare of 15 V, for optical and electrochemical measurements; graphite substrates were used for compositional determinations using Rutherford backscattering spectrometry (RBS). Deposition rates were obtained from sputter time and ensuing film thickness recorded by surface profilometry; a typical value was 0.4 nmys. The films were approximately 200 nm thick. Elemental compositions were determined on selected films by RBS, specifically by analysing the backscattered yield following bombardment with 2.0 MeV alpha particles. Typical compositions were NiMg0.8O2.4 and NiV0.08Mg0.5O2.4. We expect the films to be hydrous, i.e. to be more or less hydrated, hydroxylated, oxyhydroxylated, etc. Preliminary X-ray diffraction data were unable to discriminate between the films being one-phase (NiMgO2) or two-phase (NiOØMgO). The
0040-6090/02/$ - see front matter 䊚 2002 Elsevier Science B.V. All rights reserved. PII: S 0 0 4 0 - 6 0 9 0 Ž 0 2 . 0 0 4 3 7 - 6
2
A. Azens et al. / Thin Solid Films 422 (2002) 1–3
Fig. 1. Spectral absorptance for 200-nm-thick films of the shown compositions.
Fig. 2. Spectral absorptance for 200-nm-thick films of the shown compositions.
films were immersed in a 1 M KOH electrolyte and underwent voltammetric cycling in a three-electrode arrangement with a Pt counter electrode and an Agy AgCl reference electrode. The films displayed readily observable electrochromism. MgO is electrochemically inactive, which leads us to favor a NiMgO2 structure, possibly mixed with NiO, for the films. Quantitative optical data were taken by spectrophotometry on films that had been withdrawn from the electrolyte and cleaned in deionized water, using a Perkin Elmer Lambda 9 instrument. Absorption was evaluated from 1–TyR, where T is the normal transmittance and R is the near-normal reflectance. Fig. 1 shows data for a film of NiMg0.8O2.4 in the 300-l800 nm range and, for comparison, also for a film of NiOq (whose oxygen content was not determined). With the films in their states of maximum transmittance, the absorptance is found to be significantly smaller for the Mg-containing film over the full spectral range, the difference being especially large at l- 500 nm. The strong absorptance at l- 350 nm is due to the semiconductor bandgap, which appears to be widened as a consequence of the Mg addition. Some weak absorptance maxima can be seen in the spectra; they are possibly associated with crystal field effects w14,15x. Fig. 2 illustrates analogous results for films of NiV0.08Mg0.5O2.4 and NiV0.08Oq; these samples display the same characteristic features as the ones reported on in Fig. 1. Color properties were quantified by use of chromaticity coordinates (x, y, z)—corresponding to red, green and blue primaries, respectively—using the CIE Colorimetric System w16x. The solid and dashed curves in Fig. 1 correspond to (0.337, 0.336, 0.327) and (0.339, 0.342, 0.319), respectively, with regard to a daylight illuminant. Color neutrality is represented by (0.333, 0.333, 0.333), and it is evident that the Mg changes the
visual appearance towards a more color neutral state primarily by increasing the contribution of the blue part of the spectrum (i.e. the z component). Charge capacities were evaluated by cycling at 10 mVys between a lower (bleaching) potential of y0.5 V vs. AgyAgCl and an upper (coloration) potential in the 0.45-Ucol-0.65 V vs. AgyAgCl range. Fig. 3 shows data for films of NiV0.08Mg0.5O2.4 and NiV0.08Oq, i.e. for the samples earlier reported on in Fig. 2. The films are electrochemically active. It is evident that similar charge capacities can be achieved provided that Ucol is set at a 0.05–0.1 V higher potential when Mg is present. Summarizing, we have found that a Mg addition to Ni oxide, with and without a small V content, gives a significant decrease of the optical absorptance, especially of blue light, and thereby an improved color neutrality,
Fig. 3. Charge capacity vs. coloration potential Ucol for 200-nm-thick films of the shown compositions. Electrochemical cycling in 1 M KOH took place between y0.5 V vs. AgyAgCl and Ucol at 10 mVys. Dots represent evaluated data and curves were drawn for convenience.
A. Azens et al. / Thin Solid Films 422 (2002) 1–3
at the same time as the shift of the electrochemical potential is so small that it is insignificant with regard to electrochromic device applications. We believe that these results are significant for architectural applications of energy efficient smart windows for which high transparency and color neutrality are important features w17x. Finally, we note that recent work on Ni–Mg hydrides have shown interesting optical switching with some principle similarities to the data reported in this paper w18,19x. Acknowledgments Two of us (A.A. and R.K.) want to thank the Swedish Foundation for Strategic Environmental Research and the National Energy Administration of Sweden for financial support. References w1x C.G. Granqvist, Handbook of Inorganic Electrochromic Materials, Elsevier, Amsterdam, 1995. w2x C.G. Granqvist, Solar Energy Mater. Solar Cells 365 (2000) 119. w3x C.G. Granqvist, Int. Glass Rev. 2 (2001) 67. w4x A. Azens, C.G. Granqvist, Proc. Soc. Photo-Opt. Instrum. Eng. 4458 (2001) 104.
3
w5x S.-H. Lee, S.-K. Joo, Solar Energy Mater. Solar Cells 39 (1995) 155. w6x J.G.H. Mathew, S.P. Sapers, M.J. Cumbo, N.A. O’Brien, R.B. Sargent, V.P. Raksha, R.B. Lahaderne, B.P. Hichwa, J. NonCryst. Solids 218 (1997) 342. w7x A. Azens, L. Kullman, G. Vaivars, H. Nordborg, C.G. Granqvist, Solid State Ionics 113–115 (1998) 449. w8x R. Lechner, L.K. Thomas, Solar Energy Mater. Solar Cells 54 (1998) 139. w9x J. Nagai, G.D. McMeeking, Y. Saitoh, Solar Energy Mater. Solar Cells 56 (1999) 309. w10x J. Karlsson, A. Roos, Solar Energy 68 (2000) 493. w11x A. Azens, G. Vaivars, M. Veszelei, L. Kullman, C.G. Granqvist, J. Appl. Phys. 89 (2001) 7885. w12x A.P. Giri, R. Messier, Mater. Res. Soc. Symp. Proc. 24 (1984) 221. w13x A. Azens, L. Kullman, C.G. Granqvist, Solar Energy Mater. Solar Cells, in press. w14x T.M.J. Nilsson, G.A. Niklasson, Proc. Soc. Photo-Opt. Instrum. Eng. 1272 (1990) 129. w15x R. Newman, R.M. Chrenko, Phys. Rev. 114 (1959) 1507. w16x D.L. MacAdam, Color Measurement: Theme and Variations, Springer Series of Optical Sciences, Vol. 27, Springer, Berlin, 1981. w17x M. Wigginton, Glass in Architecture, Phaidon, London, 1996. w18x T.J. Richardson, J.L. Slack, R.D. Armitage, R. Kostecki, B. Farangis, M.D. Rubin, Appl. Phys. Lett. 78 (2001) 3047. w19x J. Isidorsson, I.A.M.E. Giebels, M. Di Vece, R. Griessen, Proc. Soc. Photo-Opt. Instrum. Eng. 4458 (2001) 128.