{Electrochromism in tungsten oxide thin films prepared by chemical bath deposition}
doi:10.5599/jese.357 27
J. Electrochem. Sci. Eng. 7(1) (2017) 27-37; doi: 10.5599/jese.357
Open Access : : ISSN 1847-9286
www.jESE-online.org
Original scientific paper
Electrochromism in tungsten oxide thin films prepared by
chemical bath deposition
Julijana Velevska1,, Nace Stojanov1, Margareta Pecovska-Gjorgjevich1,
Metodija Najdoski2
1Institute of Physics, Faculty of Natural Sciences and Mathematics, University Sts Cyril and
Methodius, Arhimedova 3, 1000 Skopje, Republic of Macedonia
2Institute of Chemistry, Faculty of Natural Sciences and Mathematics, University Sts Cyril and
Methodius, Arhimedova 5, 1000 Skopje, Republic of Macedonia
Corresponding author: julev@pmf.ukim.mk;Tel.: +389 2 3 249 860
Received: November14, 2016; Revised: February14, 2017; Accepted: February 15, 2017
Abstract
Tungsten oxide (WO3) thin films were prepared by a simple, economical, chemical bath
deposition method onto fluorine doped tin oxide (FTO) coated glass substrates. The
electrochemical properties of the films were characterized by cyclic voltammetry. The
obtained films exhibited electrochromism, changing color from initially colorless to deep
blue, and back to colorless. Visible transmittance spectra of (WO3) films were recorded in-
situ in their both, bleached and colored states. From those spectra, absorption coefficient
() and the optical energy gaps were evaluated. The dependence of the optical density on
the charge density was examined and the coloration efficiency () was calculated to be
22.11cm2C-1. The response times of the coloring and bleaching to an abrupt potential
change from -2.5 V to +2.5 V and reverse, were found to be 9.3 and 1.2 s respectively. The
maximum light intensity modulation ability of the films, when the AM1.5 spectrum is
taken as an input, was calculated to be about 50 %.
Keywords
Optical materials; cyclic voltammetry; response time; solar light modulation
Introduction
Electrochromism is a unique property of the material to change reversibly its optical properties
when it is electrochemically reduced or oxidized [1]. Electrochromic materials exhibit color change
between the clear transparent state and a darkened colored state, or between two colored states.
At the same time, there are materials that exhibit multiple colored states and are described as
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J. Electrochem. Sci. Eng. 7(1) (2017) 27-37 TUNGSTEN OXIDE THIN FILMS
28
polyelectrochromic. The classification of the electrochromic materials is related to the potential at
which the coloration process occurs. Materials with cathodic coloration exhibit coloration at
negative potential, i.e. they darken upon reduction (charge insertion). Anodic materials, on the
other hand, exhibit coloration at positive potential, i.e. they darken upon oxidation (charge
extraction). When the electrochromic material is integrated in device, it could modulate the
reflectance/transmittance of the incident illumination [2]. Electrochromic materials are currently
attracting much interest in academia and industry for both their spectroelectrochemical and
commercial applications [3]. Electrochromism is known to exist in many types of materials, both
organic and inorganic. Common inorganic electrochromic materials are transition metal oxides and
metal hexacyanometallates, while viologens, phthalocyanines, conducting polymers and
metallopolymers are common organic and polymer electrochromic materials.
Transition metal oxides have attractive technological importance for electrochromic applications
because they show considerable variation in stoichiometry, and can be quite easily deposited in a
form of thin film which is appropriate for device manufacturing. Among them, tungsten oxide (WO3)
is of intense interest and has been extensively investigated due to its appreciable electrochromic
properties in the visible and infrared region. It exhibits large optical modulation, good durability,
low power consumption, less stress for the viewer’s eyes, and relatively low price [4].
Eectrochromic WO3 thin films have been prepared by a large number of techniques, such as
thermal evaporation, electrodeposition, spray pyrolysis, chemical vapor deposition, electron beam
evaporation, magnetron sputtering, sol-gel methods [5 - 16] etc. Among these techniques, chemical
bath deposition methods have many advantages: they do not require sophisticated expensive
equipment, various substrates including metals, semiconductors or insulators can be used, the
starting chemicals are commonly available and cheap, and the preparation parameters are easily
controlled [17 - 21]. These methods have benefit of being easily realizable from the point of view of
industrialization, especially on large area devices, with the required electrochromic proper-
ties [22, 23]. The electrochromic properties of WO3 thin films, like the transmittance modulation
(T), coloration efficiency (), switching time (), and cyclic durability, strongly depends on their
structural, morphological and compositional characteristics [1, 2], which, in turn, depends directly
on the deposition method and deposition conditions.
The target of this research is to investigate electrochromic properties of WO3 thin films prepared
by a simple chemical bath deposition method [24] and their possible application for solar light
modulation.
Experimental
Tungsten oxide films were deposited onto fluorine doped tin oxide (FTO) coated glass substrates
commercially available with transparency of about 80 % for visible light, and sheet resistance of
about 10 – 20 Ω□-1. Before the deposition, the substrates were immersed in acetone and ethanol
to be degreased in an ultrasonic bath, and then rinsed in deionized water and dried in air. WO3 films
were deposited from a chemical bath with optimized composition and process conditions.
The bath solution was prepared by dissolving 1.65 g Na2WO4·2H2O in 90 ml deionized water. The
substrates were immersed in the beaker filled with deposition solution, and vertically supported
against its walls. Then, the whole system was heated slowly, up to 95C with continuous stirring.
The preparation of the bath solution and the deposition of the thin films have been explained in
details in Ref. [24]. The thickness of the films depends on the deposition time. In this work the
deposition time was 20 min, and the thickness of the films was 150 nm.
J.Velevskaet al. J. Electrochem. Sci. Eng. 7(1) (2017) 27-37
doi:10.5599/jese.357 29
The electrochemical properties of WO3 films were characterized by cyclic voltammetry
measurements performed using Micro AUTOLAB II equipment (Eco-Chemie, Utrecht, Netherlands)
in one compartment three electrodes electrochemical cell with WO3 film as working, platinum wire
as counter, and saturated calomel electrode (SCE) as a reference electrode. The cycling was carried
out in 1 moldm-3KCl aqueous solution as an electrolyte. The voltage scan rate was 10 mV s-1, and the
film working area was 1 cm2.
Electrochromic investigations were performed in situ in an electrochromic device (ECD) consisted
of: home-built glass cell (4 ×2.5×4 cm), WO3 film deposited on FTO substrate as working electrode
(WE), blank FTO substrate as counter electrode (CE), and 1 moldm-3KCl aqueous solution as an
electrolyte. The distance between the electrodes was about 1.5 cm, the volume of the electrolyte
was about 20 ml, and the active surface area of the electrodes was approximately 6 cm2.
The optical transmittance spectra were recorded by using Varian CARY 50 Scan UV-Visible
spectrophotometer in the wavelength range from 300 to 900 nm, in both, the completely colored
and bleached states of the film. An electrochromic cell with two clean FTO substrates filled with
electrolyte was measured as 100 % background. Coloration and bleaching of the film were
performed with -2.5 V and +2.5 V respectively. Spectra were recorded 3 min after the voltage was
applied. In order to obtain intermediate states of coloration, the film was also colored with
coloration potentials of -1.5 V and – 2 V.
The visible transmission spectra were used for evaluation of the optical band gaps Eg of the WO3.
For that purpose, the absorption coefficient () was evaluated from the transmittance data (T) and
the film thickness (t), using the equation [25]:
Tt
1
ln
1
(1)
The optical band gaps of the film were evaluated from the absorption coefficient by fitting the
data to the relation [26 - 28]:
ngEhAh (2)
where, A is a constant, h is the energy of the incident photon, Eg is the optical energy gap, and n is
a number which determines the type of electron transition causing the absorption. The value of n is
1/2 for direct allowed, 3/2 for direct forbidden, 2 for indirect allowed, and 3 for indirect forbidden
transitions.
The coloration efficiency () of the WO3 was calculated from the optical density change (OD) at
a wavelength of 700 nm, and the charge density (Q/S) during coloration after the films were fully
bleached:
Q
TTS
cblog (3)
where Tb and Tc are the transmittance of the fully bleached and colored states respectively, Q is
the injected charge, determined by the applied current and the time of its application, and S is the
active area of the electrochromic film.
The time needed for the electrochromic film to reach some fraction (usually above 70 %) of its
maximum colored or bleached state (response time, ) was examined as a change in the
transmittance at 700 nm due to abrupt voltage change between +2.5 V and -2.5 V.
For an electrochromic material to be practically successful, it must have the ability to switch
between its bleached and colored states frequently, whilst maintaining other important features
J. Electrochem. Sci. Eng. 7(1) (2017) 27-37 TUNGSTEN OXIDE THIN FILMS
30
consistently. Cycle life is defined as the number of cycles completed before the material fails, and
measures material stability. The cycling behavior of chemically deposited WO3 films during
electrochromic switching was directly observed spectroscopically by the in situ measurements of
the transmittance at 700 nm of the fully bleached and colored states of the film after some number
of cycles. The cycling was performed by alternatively applying potential of 2.5 V.
The ability to switch between two states (bleached and colored) in a relatively short response
time makes the tungsten oxide films a possible candidate for transmittance modulation device.
Taking the solar irradiance spectrum AM1.5 for a normal incident illumination on tungsten oxide
based electrochromic device (ECD = glass/FTO/WO3/electrolyte/FTO/glass) and the absorption
coefficient spectra of the WO3 film in its bleached and colored states, the output integral of the
spectral intensity and the integral of the spectral modulation could be calculated [29].
Results and discussion
The WO3 films investigated in this work exhibited good electrochromic behavior. They could be
repeatedly colored and bleached by alternative application of a negative and positive potential
respectively, versus a counter electrode. WO3 is cathodically coloring material which means that it
possesses a reduced colored state. It is transparent in oxidized state (positive potential), and has a
deep blue color in reduced state (negative potential). The X-ray diffraction (XRD) analysis showed
that the films were crystalline [24].
Figure 1. Cyclic voltammetric curves (six cycles) of chemically deposited WO3thin film.
Arrows indicate direction of the potential scan.
The electrochromic behavior of the films was examined by cycling voltammetry. The cyclic
voltammetric (CV) curves were obtained by sweeping the potential in the range of - 0.8 V to 0 V vs.
SCE at scanning rate of a 10 mV s-1. In Fig. 1 are presented six CV curves of the WO3 thin film. As can
be seen all CV curves (except the first one) have a same shape, which means that the films exhibited
good stability. The CV curves also showed an increase of the cathodic current density
to -0.86 mA cm-2 at -0.8 V, due to the reduction process occurring in the film and its switching to the
blue color, whereas the anodic peak is observed at around - 0.541 V due to the oxidation process
and bleaching of the film. The coloring process is followed by reduction of WVI ions and double
injection of potassium ions and electrons, and the bleaching process is followed by oxidation of WV
ions and double extraction of potassium ions and electrons. The following equation can express the
coloring/bleaching process:
J.Velevskaet al. J. Electrochem. Sci. Eng. 7(1) (2017) 27-37
doi:10.5599/jese.357 31
(blue)WOKxexKttransparenWO 3x
-
3
(4)
It was observed [30] that during the coloration process, the XRD peaks change in the position
and in the intensities indicating a structural transition associated with intercalation of ions. When
the ions were deintercalated by applying reverse potential, the crystalline structure reverts to the
initial lattice. However, this phenomenon remains to be studied in our future research.
The optical transmittance of the films was recorded to understand the type of electron transition
and it was used to evaluate optical energy gap. By analyzing the optical transmission spectra, it is
also possible to determine whether the optically induced transition is direct or indirect, and allowed
or forbidden.
The optical transmission spectra of the WO3 film in the wavelength range from 350 to 900 nm in
both, bleached and colored states, taken in situ, are presented in Fig. 2. One can see significant
transmittance difference (more than 60%) that occurs in the red region of the visible spectrum with
tendency to continue in the near infrared (NIR) region.
Figure 2. In-situ visible transmittance spectra of chemically deposited WO3 thin film
in the bleached and colored states.
Fig. 3 shows the optical transmittance of the ECD constructed by using chemically deposited WO3
film in its bleached and colored states for an applied bleaching potential of +2.5 V and coloring
potentials of -1.5 V, -2 V, and -2.5 V. The recorded transmission spectra for the device in the
bleached state for positive potentials lower than +2.5 V are indistinguishable from the spectrum of
the device bleached at +2.5 V, and hence are not shown. Also, no difference was observed when
negative potentials higher than -1.5 V were applied. The film started to change its color at -1.5 V,
and one can see that the spectra recorded at -1.5 V, -2 V and -2.5 V are distinguishable from each
other, and from the spectrum of the bleached state of the film. No significant difference was
observed between the spectra recorded at coloration potentials of -2.5 and -3 V, which means that
the film is wholly reduced at -2.5 V. Actual photographs of the device in the bleached (+2.5 V) and
colored (-1.5 V, -2 V, and -2.5 V) states are presented in Fig. 4. From these photographs one could
clearly notice the different shades of blue obtained by different coloring potentials.
J. Electrochem. Sci. Eng. 7(1) (2017) 27-37 TUNGSTEN OXIDE THIN FILMS
32
Figure 3. In-situ visible transmittance spectra of chemically deposited WO3 thin film bleached
at +2.5 V and colored -1.5 V, -2 V, and -2.5V.
A
B
C
D
Figure 4. Photographs of ECD A: bleached at +2.5 V, B: colored at -1.5 V,C: -2 V and D:-2.5 V.
The optical energy gaps of the films were evaluated utilizing the transmittance data and the
equations (1) and (2). The plots of (h)2 versus h for the chemically deposited WO3 thin film in
both, bleached and colored states, are shown in Fig. 5. The film showed a better fit for n = 0.5 which
shows the direct electron transition mechanism in both states (bleached and colored) of chemically
deposited WO3 films. The energy gaps were calculated from the linear parts in Fig. 3 as intercepts
with the photon energy axis. The evaluated band gaps for the film in its bleached and colored states
were 3.38 eV and 3.32 eV respectively. These values are in good agreement with the reported values
on tungsten oxide thin films [31].
Figure 5. The plots of (h)2 vs. hfor WO3 thin films prepared by chemical bath deposition.
J.Velevskaet al. J. Electrochem. Sci. Eng. 7(1) (2017) 27-37
doi:10.5599/jese.357 33
To investigate in more detail the optoelectrochemical properties of the chemically deposited
WO3 thin films, the optical density change (OD) was plotted against the charge density change
(Q/S), and displayed in Fig. 6. The coloration efficiency at 700 nm was extracted as the slope of
the line fitted to the linear region of the curve. The calculated value was found to be 22.11 cm2 C-1.
The value obtained in this work is higher when compared to the values obtained for
electrodeposited and sol-gel coated WO3 thin films [32-35], but lower compared to the values
obtained for the sputtered WO3 [36].
Figure 6. Optical density variationwith respect to the charge density measured at 700 nm
In order to investigate the transition response time between coloration and bleaching, the
transmittance was measured in-situ through the ECD. The applied potential was switched between
2.5 V (transparent state) and - 2.5 V (blue state). Fig. 7 shows the dynamic coloration/bleaching
characteristics of the ECD, recorded at the wavelength of 700 nm. The coloration and bleaching
times (c and b), defined as time required for achieving 70 % of the total transmission change
[37, 38] was found to be 9.3 s and 1.2 s respectively, which means that the coloring kinetics is slower
than the bleaching one. The faster bleaching time is due to the good conductivity of the tungsten
bronze (KxWO3) and the conductor (KxWO3) to semiconductor (WO3) transition. On the other hand,
the slower coloration time is due to the higher resistance during WO3 to KxWO3 transition [39].
Figure 7. Switching time characteristics (at 700 nm) between the colored and
bleached states for ECD, measured at ±2.5 V.
Charge density, C cm-2
J. Electrochem. Sci. Eng. 7(1) (2017) 27-37 TUNGSTEN OXIDE THIN FILMS
34
Fig. 8 shows the transmittance at 700 nm of the ECD in the bleached and colored states up to
10000 color-bleach cycles. As can be seen from Fig. 8, the transmittance shows insignificant
variations which means that the electrochemically deposited WO3 films are stable and could be
electrochemically switched for 104 cycles without serious deterioration. Unfortunately, after the 104
cycles the optical transmittance change rapidly decreased, so we could say that the device durability
is up to 104 cycles of bleaching and coloring.
Figure 8. In-situ transmittance (at 700 nm) of chemically deposited WO3 thin film in the
bleached and colored states vs. number of cycles.
Finally, the irradiance of the solar spectrum AM 1.5 [40] and the absorption coefficient spectra
(calculated from the transmittance spectra) of the chemically deposited WO3 films in their bleached
and colored states (Fig. 9), were taken as input parameters. The output spectral intensities
transmitted across the WO3 films were calculated and presented in Fig. 10. The results of the
numerical integration for the spectral intensity within the visible region (350 – 900 nm) are
presented in Table 1.
Table 1. Integral transmitted intensity from 350 to 900 nm(It) through
the WO3 films in their bleached and colored states.
State It/ W m-2
Bleached 506
Colored 253
The relative change of the integrated intensity (the visible transmitted intensity and the light
modulation) could be calculated by the equation:
bleached
coloredbleached
Modulation
t
tt
I
II
(5)
Using the results from the Table 1 and the equation (5), the integrated intensity modulation of
about 50 % was achieved, which is considerable value that gives the opportunity for implementation
of the chemically deposited WO3 films in electrochromic devices such as electrochromic windows.
J.Velevskaet al. J. Electrochem. Sci. Eng. 7(1) (2017) 27-37
doi:10.5599/jese.357 35
Figure 9. Absorption coefficient spectra of the chemically deposited WO3 film
in the bleached and colored states.
Figure 10. Spectral intensity of the transmitted AM 1.5 solar irradiance spectrum
through the WO3 film in bleached and colored states
Conclusions
Tungsten oxide thin films investigated in this work were deposited onto FTO coated glass
substrates by chemical bath deposition method. The method is simple, economical, and has benefit
of being easily realizable from the point of view of industrialization, especially on large area devices,
with the required electrochromic properties. The obtained films exhibited good electrochromic
properties. They were stable and exhibited excellent reversibility, with color changed from originally
colorless into deep blue when negative potential was applied, and back to colorless when the
potential was reversed. Transmittance difference of more than 60 % was achieved in the red region
of the visible spectrum. Also, by controlling the coloring potential, intermediate states of coloration
were achieved. Optical energy gaps were evaluated from the transmittance measurements for the
both, bleached and colored states of the films, assuming a direct semiconductor transition
mechanism. The coloration efficiency (at 700 nm) was found to be 22.11 cm2C-1, the value higher
compared with those obtained for electrodeposited and sol-gel coated WO3 thin films, but lower
compared to the values obtained for sputtered WO3 thin films. The switching times between
transparent and blue states of the WO3 thin film were found to be 9.3 s for coloring, and 1.2 for
bleaching. The maximum light intensity modulation ability of the films, as the AM 1.5 spectrum is
taken for an input, was calculated to be about 50 % which is considerable value which makes
chemically deposited tungsten oxide thin films suitable for application in electrochromic devices.
J. Electrochem. Sci. Eng. 7(1) (2017) 27-37 TUNGSTEN OXIDE THIN FILMS
36
References
[1] K. Bange, Solar Energy Materials & Solar Cells, 58 (1999) 1-131
[2] C.G. Granqvist, Handbook of Inorganic Electrochromic Materials, Elsevier, Amsterdam,
Holland, 1995, p.
[3] D.R. Rosseinsky, R.J. Mortimer, Acvanced Materials13(11) (2001) 783-793
[4] E. S. Lee, D. L. Di Bartolomeo,Solar Energy Materials & Solar Cells, 71(4)(2002)465-491
[5] C. G. Granqvist, Solar Energy Materials & Solar Cells, 60(2000)201-262
[6] M. C. Rao, O. M. Hussain, Research Journal of Chemical Sciences1 (7) (2011) 92-95
[7] P. M. S. Monk, Critical Reviews in Solid State and Materials Sciences24 (1999) 193-226
[8] A. J. More, R. S. Patil, D. S. Dalavi, S. S. Mali, M. G. Gang, J. H. Kim, Materials Letters134
(2014) 298-301
[9] C. Y. Kim, S. G. Cho, S. Park, D. K. Choi, Journal of Ceramic Processing Research10 (6) (2009)
851-854
[10] A. A. Joraid, Current Applied Physics 9 (2009) 73-79
[11] R. U. Kirss, L. Meda, Applied Organometric Chemistry 12 (1998) 155 – 160
[12] D. Gogova, K. Gesheva, A. Szekeres, M. Sendova-Vassileva, physica status solidi (a)176
(1999) 969-984
[13] C. M. Wang, C. Y. Wen, Y. C. Chen, K. S. Kao, D. L. Cheng, C. H. Peng, Integrated
Ferroelectrics: An International Journal158 (1) (2014)62-68
[14] T. S. Yang, Z. R. Lin, M. S. Wong, Applied Surface Science252 (2) (2005) 2029-2937
[15] F. Zhang, H. Q. Wang, S. Wang, J. Y. Wang, Z. C. Zhong, Y. Jin, Chinese Physics B23 (9) (2014)
098105(1-6)
[16] A. P. Baker, S. N. B. Hodgson, M. J. Edilisinghe, Surface and Coatings Technology153 (2)
(2002) 184-193
[17] M. Ristova, J. Velevska, M. Ristov, Solar Energy Materials & Solar Cells 71 (2002) 219-230
[18] T. Todorovski, M. Najdoski, J. Velevska, International Journal of Pure and Applied
Chemistry1(4) (2006) 549-552
[19] R. Neskovska, M. Ristova, J. Velevska, M. Ristov, Thin Solid Films515 (2007) 4717-4721
[20] S. Demiri, M. Najdoski, J. Velevska, Material Research Bulletin46 (2011) 2484-2488
[21] J. Velevska, M. Pecovska-Gjorgjevich, M. Najdoski, N. Stojanov, Silpacorn University Science
and Technology Journal5 (1) (2011) 34-42
[22] R. S. Mane, C. D. Lokhande, Materials Chemistry and Physics65 (2000) 1-31
[23] J. Cheng, D. B. Fan, H. Wang, B. W. Liu, Y. C. Zhang, H. Yan, Semiconductor Science and
Technology, 18 (2003) 676-679
[24] M. Najdoski, T. Todorovski, Materials Chemistry and Physics104 (2007) 483-487
[25] J. M. O-Rueda de Leon, D. R. Acosta, U. Pal, L. Castaneda, Electrochimica Acta56 (2011)
2599-2605
[26] R. Bhat, I. Bhaumic, S. Ganesamoorthy, A. K. Karnal, M. K. Swami, H. S. Patel, P. K. Gupta,
Physica Status Solidi A 209 (1) (2012) 176-180
[27] F. I. Ezema, A. B. C. Eqwealor, R. U. Osuji, Superficies Vacio21 (2008) 6-10
[28] A. Tumuluri, K. Lakshun Naidu, K. C. James Raju, International Journal of ChemTech
Research6 (6) (2014) 3353-3356
[29] M. Ristova, R. Neskovska, V. Mircevski, Solar Energy Materials & Solar Cells91 (2007) 1361-
1365
[30] Y. Djaoued, S. Balaji, R. Brüning, Journal of Nanomaterials, 2012 (2012) Article ID 674168, 9
pages
[31] K. Srinivasa Rao, B. RajiniKanth, G. Srujana Devi, P.K. Mukhopadhyay, Journal of Materials
Science-Materials in Electronics22 (2011) 1466-1472
J.Velevskaet al. J. Electrochem. Sci. Eng. 7(1) (2017) 27-37
doi:10.5599/jese.357 37
[32] J. R. De Andrarde, I. Cesarino, R. Zhang, J. Kanicki, A. Pawlicka, Molecular Crystals and
Liquid Crystals604 (2014) 71-83
[33] M. Deepa, M. Kar, S. A. Agnihotry, Thin Solid Films, 468 (2004) 32-42
[34] C. G. Kuo, C. Y. Chou, Y. C. Tung, J. H. Chen, Journal of Marine Science and Technology20 (4)
(2012) 365-368
[35] C. Y. Kim, S. Park, Asian Journal of Chemistry25 (10) (2013) 5874-5878
[36] S. H. Lee, H. M. Cheong, C. E. Tracy, A. Mascarenhas, A. W. Czanderna, S. K. Deb, Applied
Physics Letters75 (11) (1999) 1541-1543
[37] J. Velevska, M. Pecovska-Gjorgjevich, N. Stojanov, M. Najdoski, International Journal of
Sciences: Basic and Applied Research25 (3) (2016) 380-392
[38] Z. Jiao, X. W. Sun, J. Wang, L. Ke, H. V. Demir, Journal of Physics D: Applied Physics 43
(2010) 285501 (6pp)
[39] Z. Jiao, J. Wang, L. Ke, X. Liu, H. V. Demir, M. F. Yang, X. W. Sun, Electrochimica Acta 63
(2012) 153-160
[40] ASTM GI73-03 Standard Tables for Reference Solar Irradiances: Direct Normal and
Hemispherical on 370 Tilted Surface (2012)
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