Photoelectrochromic window with Pt catalyst

Thin Solid Films 502 (2006) 246 – 251 www.elsevier.com/locate/tsf

Photoelectrochromic window with Pt catalyst A. Georg a, A. Georg b,*, U. Opara Kraxovec c a

Freiburg Materials Research Centre, Freiburg, Germany Fraunhofer Institute for Solar Energy Systems, Freiburg, Germany Faculty of Electrical Engineering, University of Ljubljana, Ljubljana, Slovenia b

c

Available online 31 August 2005

Abstract Photoelectrochromic windows represent a special kind of switching windows. The energy for colouring is provided by sunlight, so that a voltage supply is not required. The transmittance can be decreased on illumination and can be increased again in the dark. In contrast to photochromic devices, the system is externally switchable under illumination. Our photoelectrochromic window consists of several components: a dye-covered nanoporous TiO2 layer, which is situated on a nanoporous electrochromic layer, such as WO3, two glass substrates coated with a transparent conductive oxide, of which one is coated with Pt, an iodide/tri-iodide redox couple and Li+ ions in a solid ion conductor. All the layers can be kept quite thin, so that they are transparent. The pores of the TiO2 and WO3 layers are filled with the electrolyte. This configuration is a particularly advantageous combination of the dye solar cell and an electrochromic element. The colouring time is independent of the area, the transmittance can be varied also in the illuminated state, and the system can also be switched by an auxiliary external voltage. Initial samples with solid electrolyte change their visible transmittance from 62% to 1.6%, their solar transmittance from 41% to 0.8%. The time for colouring and bleaching is about 15 min. D 2005 Elsevier B.V. All rights reserved. Keywords: Smart windows; Photoelectrochromic

1. Introduction Photoelectrochromic systems combine electrochromic layers [1,2] and dye solar cells [3,4]. Electrochromic layers change their transmittance reversibly when electrons and cations are injected. In photoelectrochromic systems, the dye solar cell provides the energy for the coloration of the electrochromic layer. Thus, the transmittance of the photoelectrochromic device can be decreased under illumination and can be increased again when illuminated or in the dark. An external voltage supply is not required. Applications of these devices include, for example, switchable sunroofs in cars or smart windows in buildings. We developed the photoelectrochromic configuration illustrated in Fig. 1, which is a particularly advantageous device. It consists of several components: a dye-covered

* Corresponding author. E-mail address: [email protected] (A. Georg). 0040-6090/$ - see front matter D 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2005.07.291

nanoporous TiO2 layer, a porous electrochromic layer, such as WO3, two glass substrates coated with a transparent conductive oxide (TCO), of which one is coated with Pt, an iodide/triiodide redox couple and Li+ ions in an organic solvent. Both the TiO2 and the Pt layers can be kept quite thin, so that they are transparent. The pores of the TiO2 and WO3 layers are filled with the electrolyte. During illumination (upper part of Fig. 1), a dye molecule absorbs a photon of the incident light. Then an electron is rapidly injected from the excited state of the dye into the conduction band of the TiO2 and diffuses to the WO3. Ionised dye molecules are reduced by I in the electrolyte according to the reaction: 3I Y I3 + 2e . Li+ ions intercalate into the WO3 and keep the charges balanced. Because of the injection of electrons, the WO3 changes its colour from transparent to blue. If electrons are allowed to flow via an external circuit from the WO3 via a TCO layer to the Pt electrode (lower part of Fig. 1, external switch closed), then the Pt catalyses the reverse reaction, i.e., the reduction of I3 to I . Li+ leaves

A. Georg et al. / Thin Solid Films 502 (2006) 246 – 251

247

2. Experimental WO3 TiO 2

TE

Pt I-

e-

substrate

TE

I3light

Li+

substrate ILi+

e-

I3-

edye

e-

Fig. 1. Construction and operating principle of the photoelectrochromic device. The upper part shows the coloration in open circuit (switch open) and the lower part shows the bleaching in short circuit (switch closed).

the WO3, and the WO3 is bleached fast. This process occurs also during illumination. If the external switch is open, electrons can leave the WO3 only by loss reactions. This process is very slow. With a liquid electrolyte, the device’s visible (solar) transmittance under 1000 W/m2 of illumination changes from 51% to 5% (35% to 1.5%) with switching times of about 3 min. Using a solid electrolyte, a visible transmittance change from 62% to 1.6% and a solar transmittance change from 41% to 0.8% are achieved with switching times of about 15 min. The colouring time is independent of the area. An alternative photoelectrochromic configuration was first published in [5]. The colouring and the bleaching are competing processes, because the bleaching is possible only via loss reactions. Therefore, either fast colouring and bleaching with a small transmittance change [5] or a large transmittance change with slow bleaching is achievable [6], or an external voltage is used for bleaching [7]. In our new device, the materials can be optimised for colouring and bleaching independently, so it simultaneously allows fast colouring and bleaching and high contrast [8]. In [8], we introduced this new device and discussed the differences to the alternative photoelectrochromic system and the advantages of our new system. Experiments with different layer configurations of photoelectrochromic devices were reported in [9]. From these experiments, we concluded that the loss reactions of electrons from the TiO2 can be neglected compared to the loss reactions of electrons from the WO3. We investigated both liquid electrolytes [8,9] and solid electrolytes [10,14]. Liquid electrolytes allow a faster switching, but need good sealing to be stable on the long term, whereas solid electrolytes, especially polymer electrolytes, show slower switching properties but are more suitable for most window applications.

Samples with solid electrolyte were prepared as described in detail in [14], with liquid electrolyte in [9]. Peroxopolytungstic (P-PTA) sols were made by dissolving tungsten powder in hydrogen peroxide followed by thermal condensation in ethanol. Ormosil was made by an acylation reaction between isocianatopropyltriethoxy silane and diaminopolypropylene glycol 4000 in tetrahydrofurane. 1 mol% of ormosil diluted in ethanol (1 g of ormosil/10 ml of ethanol) was added to the P-PTA sol. The solution was slowly stirred for 30 min. Films were formed by dip-coating the transparent conductive glass substrate (TCO). After drying in air, the films were heat treated for 30 min at 450 -C. To make TiO2 layers, Ti(IV) isopropoxide was added to 10 ml of ethanol solution containing 2.6 g of ethyl acetoacetate. Separately, 1 g of ormosil (see above) was dissolved in 10 ml of ethanol and a hydrolyzation/condensation reaction was catalysed with 0.1 ml 0.1 M HCl. After 30 min of stirring, the Ti solution was added and the solution was further stirred for 30 min. Thin layers were deposited using the dip-coating technique and annealed for 30 min at 450 -C. Tungsten oxide and TiO2 layers were successively dipcoated onto the TCO-coated glass substrates. As TCO, we used F:SnO2-coated glass substrates from Flabeg with 8 V/square. The thickness of WO3 and TiO2 layers was about 600 and 150 nm, respectively. The layers were left for 2 h in 0.001 M ethanol solution of dye [cisbis(isothiocyanato)bis(2,20-bipyridyl-4,40-dicarboxylato) ruthenium(II) dye (by Solaronix). Dye solution was heated up to 60 -C. The Pt layers were sputtered with a thickness of about 2 nm. As a liquid electrolyte, 0.5 M LiI and 0.005 M I2 dissolved in propylene carbonate (PC) was used. The distance of the electrodes was about 1 mm. As a solid electrolyte, an ormosilane network was synthesised. Equimolar amounts of 3-isocyanatopropyltriethoxysilane and O,OVbis(2-Aminopropylpolyethyleneglycol) (ME 4000 g/mol) were reacted to form a precursor composed of poly(propylene)oxide bis-endcapped with triethoxysilane groups. Hydrolyzation/condensation reactions were catalysed with acetic acid. The concentration of LiI in the electrolyte was 0.5 M and an appropriate amount of I2 was added to obtain 0.005 M concentration. In addition, a cationic surfactant based on dodecyl acid was added to the ormosil in a mass ratio (surfactant/ormosil) of 1:3. A polymer foil (Surlyn from Dupont) on each electrode with a thickness of 25 Am served as a spacer and a glue between both electrodes. Electrolyte was doctorblade printed onto the electrodes. The electrodes were then pressed together and heated up to 90 -C for few min. The size of the PEC samples was 5  5 cm2 or 10  10 cm2.

248

A. Georg et al. / Thin Solid Films 502 (2006) 246 – 251

Fig. 2. Samples of photoelectrochromic devices with solid electrolyte in the bleached state (left, short circuit) and coloured by illumination equivalent to one sun (right, open circuit).

For the characterisation of the kinetics of the colouring and bleaching during illumination and bleaching in the dark, a measuring device was developed. A halogen lamp illuminates the PEC sample, and the intensity of the transmitted light is detected by a silicon photodiode. The light intensity of the halogen lamp on the surface of the PEC cell corresponds to 1 sun (1000 W/m 2). The mismatch factor of dye solar cells was taken into account. The two electrodes of the PEC device are connected via a variable shunt resistance and a switch. The TiO2 layer is always directed toward the lamp, so that the colouring of the WO3 does not alter the light intensity of the TiO2. The shunt resistance was chosen to be 10 V, which is similar to the resistance of the TE layer. With this construction, the voltage in the open-circuit state (U oc) and the current in the short-circuit (I sc) state were measured. For the dark state, an optical filter was placed between the lamp and the sample. This filter absorbs all the light with wavelengths below 715 nm. Above this wavelength, the dye is not sensitive and no electrons are excited, as we demonstrated by spectral response measurements. For the illuminated state, the filter was removed and placed between the sample and the photodiode detector. It was necessary to install several collimators in order to suppress scattered light from the environment. In this way, the optical signal

is the same for both filter positions, and the transmittance signals for the dark and illuminated states are equivalent. Transmittance spectra were measured with Perkin-Elmer 330 Spectrometer after colouration or bleaching of the PEC devices.

3. Results and discussion 3.1. Optical properties Fig. 2 shows a sample with solid electrolyte in the bleached state (left, short circuit) and the coloured state (right, after illumination in a solar simulator). The corresponding transmittance spectra are shown in Fig. 3. The transmittance in the coloured state depends strongly on the thickness of the WO3 layer. It should be noted that the thickness of the Pt layer and the amount of the dye are small enough to allow a transmittance of 62% in the bleached state for the photopic response spectrum, and 41% for the solar spectrum. The main losses of transmittance are due to the TCO layers (especially in the

transmittance / %

70 60

bleached Tvis=62%, Tsol =41%

50 40 30 20

coloured Tvis=1.6%, Tsol =0.8%

10 0 400

600

800

1000

pecsol.opj

1200

1400

wavelength / nm Fig. 3. Transmittance spectra of the photoelectrochromic device (with solid electrolyte) in the coloured and bleached states.

Fig. 4. SEM image of nanoporous layers of WO3 (centre) and TiO2 (right) on TCO (SnO2:F, left).

A. Georg et al. / Thin Solid Films 502 (2006) 246 – 251

Ti

atomic ratio

1,0

W

249

Sn

0,8

150nm TiO2 0,4 0,6

600nm WO3

F:SnO2

0,2 0,0 0

200

400

600

800

1000

time of sputtering / s Fig. 5. Auger electron spectroscopy depth profile on the TiO2/WO3/F:SnO2 layers.

infrared range) and the redox electrolyte, which can be made thinner. 3.2. Structural properties The diameter of the particles in the WO3 layers is around 20– 30 nm, in the TiO2 10 nm, as displayed by SEM (scanning electron microscopy) measurements (Fig. 4). Auger electron spectroscopy revealed TiO2 within the WO3 layer (Fig. 5, [9]). This is due to the preparation method, that first, WO3 is deposited, and then the nanoporous WO3 layer is dip coated by the TiO2 sol, which allows the TiO2 sol to penetrate the pores of the WO3. HR-TEM (high-resolution transmittance electron spectrosopy) analysis connected with energy dispersive X-ray spectroscopy (EDXS) analysis of the WO3 layer (without TiO2) was performed [15]. HR-TEM shows the presence of roughly 5 nm thick layer of an amorphous phase connecting adjacent monoclinic WO3 (m-WO3) grains (Fig. 6). The EDXS spectra analysis of the crystalline and amorphous phase confirmed the presence of silica in

Fig. 7. Schematic drawing of the structure of the WO3 grains as a result of structural investigations with SEM, Auger electron spectroscopy, HRTEM and EDXS.

amorphous phase. This is also supported by IR spectroscopy [15]. Fig. 7 illustrates these results schematically. The WO3 particles consist of a crystalline monoclinic core (mWO3), which is surrounded by an amorphous phase (aWO3). Because of the preparation process, TiO2 and SiO2 are left inside the WO3 layer, mainly situated in the amorphous phase. The content of TiO2 increases inside the amorphous phase from the inner to the outer parts of the WO3 grain, as could be concluded by similar HRTEM and EDXS measurements on WO3 layers, which were coated by TiO2 [16]. 3.3. Kinetic properties The photoelectrochromic device, as it is described in this paper, allows various switching modes (Fig. 8). The device colours on illumination with open circuit and it

open circuit

short circuit

short circuit

open circuit Fig. 6. HR-TEM picture of the WO3 layer, showing amorphous regions (A) and crystalline regions (monoclinic, C). Circles indicate the electron beam diameter during EDXS analysis.

Fig. 8. Various switching modes under illumination (sun) or in the dark (cloud) with open circuit and short circuit. The top three modes take about 10 min, whereas the last one (dark, open circuit) takes about 10 – 100 h.

0,3

0,6

0,2

ODvis

0,4

0,1

U OC pecsol.opj

0,2 0

0,0 15

5 10 time / minutes

OD vis

0,8

0,2

I SC 0,6 0,1 0,4 0,2 0

5

pecsol.opj

10

0,0 15

ISC / mA/cm2

0,8

1,0

short circuit current density

0,4

0,3

optical density ODvis

1,0

open circuit voltage UOC / V

A. Georg et al. / Thin Solid Films 502 (2006) 246 – 251

optical density ODvis

250

time / minutes 2

Fig. 9. Left (a): colouring in open circuit illuminated by an intensity of 1 sun (1000 W/m ). Right (b): bleaching in short circuit in the dark (solid electrolyte).

WO3 to the I3 in the electrolyte. For solid ion conductors, an even longer time (100 h instead of 10 h) of selfbleaching can be achieved. Fig. 11 demonstrates the switching during constant illumination by switching between open and short circuit conditions, with open circuit voltage and short circuit current density, respectively. 3.4. Solar-cell properties As mentioned before, the photoelectrochromic device includes a dye solar cell. Fig. 12 shows the characteristic current –voltage curve of a device with the configuration, glass|TCO|TiO2(dye)|liquid electrolyte|Pt|TCO|glass. The layers were prepared in the same way as for the photoelectrochromic device. It should be not surprising that a device which has a high transmittance cannot produce a high power. The efficiency of the implemented solar cell corresponds to 0.054% with an open circuit voltage of 0.58 V, a short circuit density of 0.16 mA/cm2, and a power at maximum power point of 54 AW/cm2. This low power output is enough to operate the electrochromic WO3 layer. It is possible to increase the amount of dye in the device, e.g., by increasing the thickness of the TiO2 layer. This leads to a higher current and power output, but also

ODvis 0,6

UOC

0,2

0,4 0,1

0,2 pecsol.opj

0

5

10

OC

optical density OD

0,3

0,8

open circuit voltage UOC / V

optical density ODvis

1,0

SC

OC

SC

ISC OC

SC

1,0

1,0 0,8 0,6 0,4

0,5

0,2 pecsol.opj

0

10

20

30

40

time / minutes

50

0,0 60

open circuit voltage UOC / V

UOC

OD

short circuit current density ISC/ mA/cm2

bleaches in the dark with short circuit within about 10 min with a solid ion conductor. It is possible to adjust the electrolyte such that the device bleaches with short circuit under illumination or that it retains its colour. Slow bleaching occurs with open circuit conditions in the dark (about 10 h for liquid electrolyte, up to 100 h for solid electrolyte). Thus, the only impossible process seems to be a colouring in the dark, but this is usually not needed. However, if needed, the device still acts as an electrochromic device, i.e., it can be coloured and bleached by applying an external voltage, independent of the conditions of illumination. The dominating kinetic processes were investigated in detail, with a focus on liquid electrolytes as a model system. The results will be published soon. For solid ion conductors, the colouring and bleaching is shown in Fig. 9. For open circuit, one can measure the voltage of the device, which reaches about 0.5 V. For short circuit, one can measure the current density with respect to the area of the device, and integrate to get the charge. The charge is proportional to the optical density, the coefficient of proportionality being the coloration efficiency. The curves displayed in Figs. 10 and 11 were measured applying a liquid electrolyte. Fig. 10 shows the bleaching in open circuit in the dark due to loss reactions, which are mainly electron transfers from the

time / hours Fig. 10. Bleaching in open circuit in the dark (liquid electrolyte).

Fig. 11. Colouring in open circuit (OC) and bleaching in short circuit (SC) with constant illumination (1 sun, liquid electrolyte).

current density / mAcm-2

A. Georg et al. / Thin Solid Films 502 (2006) 246 – 251

0,20

251

Thicker layers of TiO2 will result in a window, which can be used as a solar cell as well as being switched in its transmittance.

0,15 0,10

Acknowledgements

0,05 0,00 0,0

0,1

0,2

0,3

0,4

0,5

0,6

voltage / V Fig. 12. I – U curve of the photovoltaic cell in the photoelectrochromic device (without WO3 layer, liquid electrolyte).

to a reduction of the transmittance. We expect that a thickness of the TiO2 layer of about 1 Am increases the power output significantly (roughly by a factor 6) without decreasing the transmittance too much, because now the main losses in transmittance are due to the TCO layer and the redox electrolyte. If one goes further, even higher current densities and power outputs can be achieved; in principle, up to the values achieved for dye solar cells, e.g., about U oc = 0.6 – 0.8 V, I sc = 10 –20 mA/cm2, efficiency 5 – 10% [11 – 13]. For some applications, like sunroofs in cars, it may be of interest to have an optically switchable window which changes its transmittance for example from 20% down to 2%, but which still allows a power output from the solar cell with an efficiency which may be then still be about 5%.

4. Conclusions A photoelectrochromic device has been presented, which combines an electrochromic layer of WO3 with a dye solar cell. The layers show a complex nanostructure. The high porosity allows the electrolyte to penetrate into the layers of WO3 and TiO2, even for polymer ion conductors. The device can be switched under illumination as well as in the dark. For a cell with solid electrolyte, the visible (solar) transmittance changes from 62% (41%) to 2% (1%) with switching times of about 15 min.

This work was supported financially by the University of Freiburg, Germany, and by the German Ministry of Education and Research BMBF.

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