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Sputtered Si3N4 and SiO2 electron barrier layer between a redox electrolyte and the WO3 film in electrochromic devices
Solar Energy Materials & Solar Cells 159 (2017) 395–404
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Sputtered Si3N4 and SiO2 electron barrier layer between a redox electrolyte and the WO3 film in electrochromic devices Shankar Bogati, Andreas georg n, Wolfgang Graf Fraunhofer Institute for Solar Energy Systems (ISE), Heidenhofstraße 2, 79110 Freiburg, Germany
art ic l e i nf o
a b s t r a c t
Article history: Received 4 March 2016 Received in revised form 27 July 2016 Accepted 22 August 2016
The application of a redox electrolyte in hybrid type electrochromic devices allows a high charge capacity, thereby a high optical contrast can be achieved. However, the current between the redox electrolyte and the electrochromic layer (tungsten oxide, WO3) interface in equilibrium at colored state of device is critical for a window with large area. This loss current should be maintained below 10 μA/cm2 to avoid an inhomogeneous coloration of a large window caused by an ohmic voltage drop in transparent conductive oxide (TCO). The power consumption of such a loss current can be neglected. In this regard, we have investigated silicon nitride (Si3N4) and silicon oxide (SiO2) films as an intermediate electronic barrier layer on the top of the WO3 with thicknesses of 12, 35, 80 and 180 nm. These films were coated by direct current reactive magnetron sputtering technique. The electron barrier properties were studied for the iodide/triiodide ( I−/ I−3 ) and tetramethylthiourea/tetramethylformaminium disulfide dication (TMTU/ TMFDS2 þ ) redox couples for application in electrochromic devices. For both redox couples, Si3N4 showed an effective electronic barrier layer. The thickness of 80 nm of Si3N4 reduced the loss current from 240 down to 20 μA/cm2 for the I− /I−3 redox electrolyte with the visual (solar) transmission from 45% (33%) down to 0.7% (0.5%)at 1 V.Similarly, the loss current was reduced from 70 down to 7 μA/cm2with the visual (solar) transmission from 71% (54.5%) down to 3% (2%) for TMTU/TMFDS2 þ redox electrolyte. However no significant reduction of loss circuit current was achieved with SiO2 barrier layer for both redox couples. & 2016 Published by Elsevier B.V.
Keywords: Electrochromic Redox electrolyte Redox potential Coloration efficiency Optical density Charge transfer resistance Loss current
1. Introduction Electrochromic smart windows allow a significant reduction of annual energy consumption, controlling the heating loads, cooling loads, and lighting needs [1] by adjusting the intermediate levels of transmission through it. Typically, the oxide based electrochromic windows comprises two transparent electrodes, e.g., fluorine doped tin oxide (F:SnO2), on which an electrochromic layer (WO3)and a counter electroactive layer (metal oxide) are coated [2]. An ion conductor containing lithium salt is introduced in between two electrodes. Here a cathodic voltage for the coloration and an anodic voltage for the bleaching are applied at the WO3. The coloration occurs intercalating the charges (Li þ ions and electrons) into the WO3 and the bleaching is regained by extraction of the inserted charges, which are compensated by the counter electroactive layers. The energy consumption is mainly concerned during the coloration and the bleaching processes and can be neglected at the colored state as both electrodes are n
Corresponding author. E-mail address: [email protected] (A. georg).
http://dx.doi.org/10.1016/j.solmat.2016.08.023 0927-0248/& 2016 Published by Elsevier B.V.
separated by an electrolyte and a loss current can be avoided. The term “loss current” shall describe the current in between the two electrodes, which is needed to keep a certain coloration or, equivalently, potential. However, this work is focused on the hybrid type of electrochromic device [3]. It also has two transparent electrodes (F:SnO2 coated on glass), on which an electrochromic layer, e.g., WO3 in our case and a catalytic layer (Pt) are coated. An electrolyte comprising a lithium salt, a redox electrolyte like iodide/triiodide ( I− /I−3 ) is introduced in between them. x M þ (cation) þ x e þ WO3 (transparent) ⇌ MxWO3 (blue) first redox reaction (1) R – e ⇌ O þ e second redox reaction
(2)
Fig. 1 is a schematic of an electrochromic device with redox electrolyte without (a) [3] and with a barrier layer (b) on WO3. The charge insertion for the coloration with a cathodic voltage and extraction for the bleaching with an anodic voltage at WO3 are performed by a voltage across the electrodes as s.hown in upper and lower part of the Fig. 1a and b. Unlike the most common thin film battery type of EC devices, the charge compensations during the coloration and the bleaching are performed by a redox
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Fig. 1. Layers configuration of electrochromic device working principle of an electrochromic device with redox electrolyte as described by Georg et al. in 2009 [3] without barrier layer (a) with a barrier layer (b).
electrolyte. The Eqs. (1) and (2) explain the two reactions that take place in EC device during the coloration and the beaching. During the coloration the redox couple is oxidized at counter electrode (Pt) and provides the charge to the WO3 via external circuit. At the same time, Li þ ions intercalate in the WO3 for charge neutrality. In contrary, the bleaching is achieved by reversing the reaction. The redox electrolyte neutralizes the charge in the both process. During and after the coloration, the charge transfer occurs between the redox electrolyte and the WO3 layer, and, if WO3 is porous, TCO layer, which is below WO3 as shown in Fig. 1. However, this charge transfer is kinetically supressed. Nevertheless the charge transfer from WO3 or TCO to a redox electrolyte leading to a certain loss current in equilibrium state of coloration is critical, especially, for a large area application for homogenous coloration due to voltage drop in the TCO layer. The porosity of the WO3 layer can be varied by controlling the gas pressure and power density during the sputtering process [4,5]. Furthermore, the sol-gel derived WO3 and TiO2 have been intensively investigated for photoelectrochromic [6,7] and photochromic devices [8]. The sol-gel layers typically showed crystalline grains, surrounded by amorphous phase, together with a high porosity. The sputtered layers, if prepared to realize high porosity for fast switching, typically are amorphous in phase. It should be noted that the electrochochemical potential of the as-prepared WO3 should not be higher than that of the redox electrolyte for EC devices based on redox electrolyte to prevent the self-coloration of the device. The investigation of electronic barrier layer (selective ion transport layer) on sol-gel coated WO3 to suppress the loss or the leakage current with ferrocene based redox electrolyte has been described by M. Allemand et al [9]. They significantly reduced the loss current after coating of tantalum pentoxide (Ta2O5) or silicon dioxide (SiO2) onto WO3, which were prepared by sol-gel technique. Furthermore, the spin coated polystyrenesulphonate sodium salt (PSSNa) was also investigated for ferrocene and ethanol based electrolyte. In this regard, we have investigated two dielectric layers namely Si3N4 and SiO2 individually with various thicknesses of 12, 35, 80 and 180 nm as an electronic insulating layer for the I− /I−3 and the TMTU/TMFDS2 þ redox electrolytes to supress the loss current. The TMTU/TMFDS2 þ and I− /I−3 have been investigated as redox electrolyte for EC devices in previous works [3,10], respectively. The layer configuration of the coating is shown in the Fig. 1b. It is expected that there is no conductivity for electrons for these dielectric layers, but they have to allow permeation for Li þ ions by certain porosity. On the other hand, they should reduce the permeation of the I−3 or TMFDS2 þ ions, leading to a decrease in loss
current. A coverage of the surface of the WO3 layer by the dielectric layer is not enough, as the WO3 itself shows a certain porosity, which is needed to allow a fast Li þ diffusion. 1.1. Estimation of loss current There are two drawbacks generated by a high loss current: the energy consumption and ohmic voltage drop of the TCO on large area. For a loss current of 100 mA/cm2, the yearly energy consumption can be estimated to be 4 KWh/m2, when a voltage of 1.0 V is applied and the colored state is kept over half of the year. A typical range of energy saving due to application of electrochromic window is in the order of 20–40 KWh/m2 [11]. Therefore, the energy consumption due to a loss current of 100 mA/cm2 can be tolerated. However, the ohmic voltage drop on the applied TCO is more critical. For an area of 1 m2 and a sheet resistance of a TCO layer of 16 Ω/□, a current of 1 mA/cm2 corresponds to an ohmic drop of about 0.16 V, which may be acceptable, without creating too strong in-homogeneities of the optical contrast. Real windows typically have larger area, but due to highly conductive contacts on the edges, the resulting ohmic drop is reduced. Furthermore, one should note that it is expected that the loss current decreases by gelification or polymerization of the electrolyte, which has to be done for a real window product [12,13]. In this context we investigated the thin layer of Si3N4 or SiO2, which can be coated onto WO3 by DC-magnetron process and applied for large area application, as an electronic barrier layer. Therefore, we set the target for a maximum loss current with liquid electrolytes as 10 mA/cm2. Certainly, this is a very rough estimation, but may be used as a guideline for the order of magnitude.
2. Materials and methods 2.1. Materials Lithium iodide (LiI), Iodine (I2), tetramethylthiourea (TMTU), nitrosyl tetrafluoroborate (NOBF4) and gamma-butyrolactone (GBL) were purchased from Sigma Aldrich, Germany. Fluorine doped tin oxide (F:SnO2) coated glass having a sheet resistance of 16 Ω/□ was purchased as k-glass™ from Pilkington, UK used as the substrates for all experiments. Tungsten (W), silicon (Si) and platinum (Pt) targets were used to deposit WO3, Si3N4 and SiO2 and Pt, respectively. The electrochemical characterization work station (Zennium) was supplied by ZAHNER-elektrik GmbH & Co. KG, Germany.
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397 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12.
2.2. Preparation of redox electrolytes Two liquid electrolytes are prepared for individual redox couples. They are composed of 0.5 M reduced and 5 mM of oxidized forms and 0.5 M lithium salt (LiI and LiClO4 in case of I− /I−3 and TMTU/TMFDS2 þ , respectively) in GBL. Iodide and triiodide were prepared after dissolving iodine and lithium iodide as explained in reference [14], i.e. by formation of I−3 out of I and I2.
I− + I2 → I−3
(3)
The NOBF4 was used as an oxidizing agent to oxidize the TMTU to TMFDS2 þ as described in reference [15].
Short circuit Coloration voltage (-0.4 V) Short circuit Bleaching voltage (0.4 V) Short circuit Coloration voltage (-0.6 V) Short circuit Bleaching voltage (0.6 V) Short circuit Coloration voltage (-1 V) Short circuit Bleaching voltage (1 V)
Fig. 2. An electrochromic test cycle with different bias voltages.
y ( x)TMTU+( y)NOBF4 → TMFDS[(BF4)]2 + ( x − y)TMTU + y NO↑ (4) 2 2.3. Preparation of electrodes and assembling of cells The films of WO3, Si3N4, SiO2 and Pt are coated by DC-magnetron sputtering coating process. The thickness of the layers were measured by preparing an edge of the layer by a lift off process followed by atomic-force microscopy (AFM). The layers configuration is given in the Fig. 1. The thicknesses of the WO3 and the Pt were maintained to be 450 and 1 nm, respectively. First a layer of the WO3 was sputtered onto k-glass™ substrate subsequently the Si3N4 or the SiO2 were coated. The thickness of Si3N4 was coated as 12, 35, 80 and 180 nm. However the thickness of SiO2 coated as 12 and 35 nm. As a counter electrode Pt was coated onto k-glass™. The two electrodes were sandwiched together and a silicone frame of 2 mm was used as a spacer. An electrolyte as described in Section 2.2 was introduced. The external circuit was made with cupper wire placing it on the edge of k-glass™ for the switching of devices. The effective area of the device was 6 cm2. 2.4. Characterization methods The redox potential or the electrochemical potential of WO3 upon intercalation of Li þ ions was determined with three electrode arrangement. The sputtered WO3 on k-glass™ was used as a working electrode and a Ag/AgCl (144 mV versus standard hydrogen electrode, SHE) served as reference electrode and a Pt coated k-glass™ was used as counter electrode. The electrolyte composition of 0.5 M LiClO4 in GBL was for this experiment. The current was gradually increased certain value and reversed back to the zero. The resultant voltage drop was recorded with working and reference electrodes and transmission was recorded by a photo diode (OP301) and a red LED (wavelength 655 nm). As explained by Georg et al. [16], the optical density was used for the determination of x (number of intercalated Li þ per W atom in WO3) as shown in Eq. (8). An electrochromic (EC) test cycle with three colorations and bleaching voltages as well as six intermediate short circuit current was defined as in Fig. 2. At zero bias voltage the exchange of electron from redox electrolyte to EC layer may happen when the redox potential of redox electrolyte is lower than WO3 creating some self-coloration. Nevertheless, this difference in redox potentials can be compensated by the anodic voltage. The comparative evaluation of loss current will be based on the cathodic voltage at saturated colored condition. The resulting electrical current was recorded using the electrochemical workstation. In parallel, the transmission of the cell was recorded at a wavelength of 655 nm using photo diode (OPT301) and a red LED as shown in Fig. 3. From this set up the rate of change of the intercalated charge into the WO3 can be evaluated by defining an optical current [10] since there is no significant effect of coloration of the used redox electrolyte at this wavelength [10,17]. From this concept the difference between the
Fig. 3. Electrochromic test setup with LED and photodiode (OP301) and a test cell.
electrically measured current (electrical current) and the optically converted current (optical current) can be separated.
Iopt (A/cm2)=
∆Q opt ∆t
=
∆ OD CE
∆t
=
∆OD CE*∆t
I (loss current) ¼ I (electrically measured) – I sured)
(5) opt
(Optically mea(6)
Iopt: optical current; CE: coloration efficiency (54 cm2/C for DC magnetron sputtered WO3 at λ ¼ 655 nm [17]); ΔOD: change in optical density; Δt: change in time [s]. This loss current must be continuously applied in order to maintain the device at colored state. For the better understanding of insulating properties of Si3N4 and SiO2, loss current was compared at colored state in equilibrium of the cell. Additionally the speed of coloration i.e., the rate of change of optical density at the beginning of coloration is evaluated for all layer configurations. Finally, the spectra of the cell in UV, visible and near infrared regions are measured for optimized thickness of the dielectric layer with Bruker Fourier Transform Spectrometer (IFS 66).
3. Results and discussions 3.1. Morphology and FTIR spectroscopy analysis of deposited layers Before the deposition of selective ion transport layer onto WO3, the porosity and the growth of Si3N4 and SiO2 on k-glass™ have been analysed. The amorphous structure of magnetron sputtered WO3 , Si3N4 and SiO2 identified by X-Ray diffractions [18,19]. The
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Fig. 4. SEM pictures of sputtered WO3, Si3N4 and SiO2 on k-glass™ with top view (left side) and cross-section view (right side).
SEM pictures of the layers are shown in Fig. 4 with top view (leftside) and cross-section view (right side). The grain structure of the layers were influenced by the first layer (F:SnO2). Similarly the formation of the pores in between the columns is supported by the roughness of the F:SnO2 layer. On the other hand the growth of the SiO2 is different where no columnar structure could be detected. The infrared transmission spectra of sputtered WO3, SiO2 and Si3N4 deposited on silicon wafer with the thickness of 600 nm can
be seen in the Fig. 5. The presence of the different chemical bonds in the film can be detected by infrared spectroscopy. Georg el al. [4] have characterized four different types of sputtered WO3 films by IR-spectroscopy with the observation of different bonds involved in the films. Similar to their study the absorption peaks of the bonds such as W ¼O (970 cm 1), W-O-H (1420 cm 1), H2O (1620 cm 1), and O-H (2850 cm 1, 3010 cm 1, 3040 cm 1) can be found in the in the Fig. 5a and b. The infrared spectra pattern of
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Fig. 5. IR-transmittance spectra of the sputtered WO3, SiO2 and Si3N4 on silicon with the thicknesses of 600 nm.
the sputtered WO3 prepared for this study as in Fig. 5 is similar to type-D [4]. The absorption bands of Si3N4 for the same thickness can be seen in Fig. 5a and b. The band at 490 cm 1 is peculiar to Si3N4 [20] and absorption due to the stretching vibration modes of Si-N are typically in between 830 cm 1 and 870 cm 1 [21]. Furthermore O-H can be seen at 3300 cm 1. For the SiO2, the Si-O stretch peak and Si-O bend peak can be seen at 1070 cm 1 and 450 cm 1, respectively, as reported by Wright et al. [22]. Bi-Shiou et al. [23] suggested that
the position of the Si-O vibrational band are also around 840 cm 1. However no O-H ground absorption is seen for SiO2 around 3500 cm 1. 3.2. Influence of barrier layer on loss currents for iodide triiodide redox electrolyte As explained in the Section 2.3, the WO3 and the Pt coated on the k-glass™ glass were used as a working and counter electrodes,
Fig. 6. Loss currents during coloration (a and b) and the change in optical densities (c and d) of EC devices for 0.4, 0.6 and 1 V bias voltages with and without barrier layer for I−/I− 3 redox electrolyte.
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constant, temperature, elementary charge, respectively, and x represents the number of intercalated Li þ injected electrons per W atom in WO3. The constant a represents the binding energy of the Li þ ions and electrons in WO3, b represents the interaction between the incorporated charges, and the logarithmic term represents the increase in entropy due to the intercalation. For x 40.1, the logarithmic part can be neglected. This simplifies that the electrochemical potential of LixWO3 linearly depends on x, where x is directly proportional to the degree of intercalation or change in optical density of LixWO3. The electrochemical potential of LixWO3 versus optical density is measured in Fig. 7. The relationship between the optical density and x can be described as reported by Georg et al. [24].
x=
Fig. 7. Electrochemical potential (E) of a sputtered WO3 vs optical density.
respectively. And the defined test cycle was used for the EC tests. The resulting loss currents were measured for 0.4, 0.6 and 1 V bias voltages. Fig. 6a and b show the resulting loss current (the difference between electrically measured and optically evaluated currents) for three bias voltages. At zero bias voltage, the change in OD, in average, was found to be 0.12. This is due to the low redox potential of redox electrolyte compared to the redox potential LixWO3. Thus, to avoid this self-coloration the redox potential of WO3 has to be reduced for the combination with iodide triiodide redox electrolyte. The self-coloration can be eliminated with a anodic potential at WO3. The inset in Fig. 6d shows the slowing down of the coloration with increase in barrier layer thickness, likely due to the slow diffusion of Li þ ions through the barrier layers. However a coloration time of 5 min is still a sufficient value for large area window applications. For coloration at 1 V, there is a first decrease in loss current, and then an increase, again, as well as a first increase in optical density followed by a decrease. This is likely due to a phase change of WO3 [24]. As explained in [25], neglecting the phase changes in WO3, an electrochemical potential , E, of LixWO3 depends on the charge intercalation in WO3 as follows.
E( x) = a+ b x−ν Where a, b, and
Kb Temp e0
⎛ x ⎞ ⎟ ln⎜ ⎝1 − x⎠
(7)
ν are constants, Kb, Temp, e0 are the Boltzmann
M WO3 e0NAρLCE
OD ≈ 0. 21 * O. D.
(8)
MWO3: molar mass of WO3, e0: elementary charge, NA: Avogadro's number, ρ: density of our sputtered, WO3 (ρ ¼5 g/cm3), L: thickness of WO3 layer, (L ¼450 nm), CE ¼54 cm2/C [17]. For the x to be greater than 0.1, the optical density should be also greater than 0.46. That means, while the optical density increases, the difference between the electrochemical potential of LixWO3 and redox couple also increases. As result of this, a higher loss current is generated. The relationship between the applied voltage that creates the optical density larger than 0.46 and optical density, for specific thickness e.g., 450 nm in our case, can be formulated as follows Fig. 7. Δ O D (Change in optical density) ¼ 2 2 * E (applied bias voltage) þ 0.037 (9) Fig. 8a and b show the influence of the barrier layer on the loss current of electrochromic devices for different coloration potentials. A significant decrease in loss current can be seen with Si3N4 at 1 V bias voltage. However, it was not decreased for SiO2 for all bias voltages and for Si3N4 for 0.6 V and lower voltages. This corresponds to the limited diffusion of triiodide through the barrier layer for higher bias voltages. Therefore, for the change in optical density up to 1.5 at 655 nm of wavelength no barrier layer is needed. The speed of coloration of electrochromic devices with and without SiO2 barrier layer for different coloration voltages are logarithmically plotted in Fig. 9a. For all applied bias voltages the intercalation of Li þ ions was as fast as without any SiO2 layer. However, the speed of coloration was significantly decreased with Si3N4 for all coloration voltages, which indicates that the slow
Fig. 8. Loss current at 0.4, 0.6 and 1 V voltages in EC device with (a) SiO2 and (b) Si3N4 barrier layers.
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Fig. 9. Speed of coloration at 0.4, 0.6 and 1 V voltages with (a) SiO2, (b) Si3N4 barrier layer.
diffusion of Li þ ions through it, which can be also seen in Fig. 9b. But the influence on the total optical density is not affected. Hence the loss current is reduced for iodide triiodide redox couples for 1 V applied bias voltage by Si3N4 without reducing the optical density of device. The speed of the coloration is reduced by factor of 2 with Si3N4 at all three coloration voltages.
3.3. Influence of barrier layer on loss currents for TMTU/TMFDS2 þ redox electrolyte Fig. 10a and b show the loss current in the electrochromic devices with TMTU/TMFDS2 þ redox couple at 0.4, 0.6, 1 V coloration voltages and Fig. 10c and d represent the resulting
Fig. 10. Loss currents during coloration (a and b) and the change in optical densities (c and d) of EC devices for 0.4, 0.6 and 1 V bias voltages with and without barrier layer for TMTU/TMFDS2 þ redox electrolyte.
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Fig. 11. Loss current at 0.4, 0.6 and 1 V voltages in EC device with (a) SiO2 and (b) Si3N4 barrier layer.
optical densities response at corresponding bias voltages. Unlike in the cell with iodide/triiodide redox electrolyte, the self-coloration at zero bias voltage is found to 0.02 (around 1% change in transmission), which indicates that the electrochemical potential of LixWO3 has slightly higher redox potential compared to TMTU/ TMFDS2 þ redox couple. So, the redox potential of WO3 and of TMTU/TMFDS2 þ redox electrolyte matches each other nearly perfectly, which is not the case for I− /I−3 . A high charge transfer resistance of TMTU at Pt electrode [17] and high redox potential of TMTU/TMFDS2 þ may result in low change in optical density of the device compared to the device with iodide/triiodide redox systems. The slow bleaching at open circuit conditions for all cases may be due to the slow diffusion of TMFDS2 þ through the Si3N4 and WO3 film. As explained in the Section 3.2, the linear relationship between the change in O.D. (larger than 0.46) and applied bias voltage can be formulated as follow. Δ O D (Change in optical density) ¼ 2 6 * E (applied bias voltage) –0 72 (10) Fig. 11a and b show the loss current response with respect to the applied coloration voltages with and without barrier layer for TMTU/TMFDS2 þ redox couple. Similar to the iodide triiodide redox couple the influence in the loss current was not achieved with SiO2 barrier layer i.e., the diffusion of TMFDS2 þ is not influenced by it. That is why further experiments were not carried out with SiO2
layer. However, the loss current was significantly reduced from 70 to 4 μA/cm2 with 180 nm Si3N4 barrier layer. Unlike iodide/triiodide redox electrolyte it was also decreased below 10 μA/cm2 at 1 V even with 80 nm. One should note that a Si3N4 layer in principle acts as an antireflecting layer in between the highly refracting WO3 and the electrolyte with low refractive index. Then, 80 nm is very close to an optimized thickness of such an antireflective layer, but the refractive index of Si3N4 is still too high. A detailed optimization of this should apply a SiOxNy layer with optimized porosity, or, more precisely, permeability for TMFDS2 þ and Li þ . Fig. 12a and b show the speed of coloration of the electrochromic device with SiO2 and Si3N4 barrier layer at 0.4, 0.6 and 1 V coloration voltages. As shown in Fig. 12a, the speed of the coloration is not effected by the SiO2. However the speed of coloration with Si3N4 was decreased from 0.14 to 0.04. Similarly, the speed of coloration was reduced for all bias voltages. The bleaching at short circuit and the speed of coloration are low due to the slightly high charge transfer resistance of redox electrolyte at Pt layer and high redox potential of TMTU/TMFDS2 þ . 3.4. Influence of barrier layer thickness on loss currents for redox electrolytes The influence of the barrier layer thickness on the loss currents
Fig. 12. Speed of coloration at 0.4, 0.6 and 1 V voltages with SiO2 (a) Si3N4 (b) barrier layer.
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2þ Fig. 13. Loss current versus thickness of barrier layer in double logarithmic scale for I−/I− (b) at 1 V. 3 (a) TMTU/ TMFDS
Fig. 14. Transmission spectra of EC devices at bleached and colored states with 80 nm of Si3N4 barrier with TMTU/TMFDS2+ (a) and I−/I− 3 (b) redox system.
at 1 V are shown in Fig. 13a and b for iodide/triiodide and TMTU/ TMFSD2 þ , respectively. For both redox electrolytes, the influence of thicknesses of Si3N4 on the loss currents is plotted in double logarithmic scales. From the graph, the required thickness of barrier layer can be estimated for required loss current. The loss current can be further decreased by in increasing the thickness of Si3N4 for iodide triiodide redox electrolyte. Similarly loss current can be reduced down to 10 μA/cm2 with around 50 nm of Si3N4 at 1 V for TMTU/TMFDS2 þ . As explained in Section 3.2 the effect of the barrier layer is not significant in reduction of loss current for I− /I−3 redox electrolyte up to the change in densities of 1.5, which can be achieved by 0.6 V. However for the TMTU/TMFDS2 þ redox electrolyte, the Si3N4 of 80 nm thickness is sufficient for the reduction of loss current below the target value (10 μA/cm2). Therefore, for the optimized prototype optimized EC device the Si3N4 of 80 nm was coated on to WO3 coated k-glass™. Fig. 14a and b are the transmission spectra of the EC cells comprising I− /I−3 and TMTU/TMFDS2 þ redox systems, respectively. For both electrochromic devices, the low transmission in the infrared region is due to the absorption and reflectance of F:SnO2. Similarly, the low transmission in 480–520 nm is due to the absorption of triiodide [17] for the second electrochromic device Table 1. The optical density can be increased with applied voltage across the electrode. The self-coloration of device can be
Table 1 Visual and solar transmissions of EC cells with iodide/triiodide and TMTU/ TMFDS2 þ electrolytes with 80 nm Si3N4 barrier layer for different coloration voltages. TMTU/TMFDS2 þ
I− / I− 3 Applied voltage (V)
Tvis (%)
Tsol (%)
Loss current (μA/cm2)
Tvis (%)
Tsol (%)
Loss current (μA/cm2)
0.4 0 0.4 0.6 1.0
56 45 12 4 0.7
43 33 6 2 0.5
0.33 0 2 6 20
71.5 71 44 20 3
55 54.5 29 30 2
0.33 0 1 5 7
eliminated by applying the voltage of 0.4 V, which creates the negligible loss current (around 0.33 μA/cm2) as shown in Table 1 to keep it at bleached state. The lower redox potential of iodide/triiodide compared to the TMTU/TMFDS2 þ results in the higher change in transmission. The EC device comprising TMTU/ TMFDS2 þ system can be taken for the further optimization due its transparent nature in visible region and matching redox potential. 4. Conclusion The loss current, which creates inhomogeneous coloration in
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large EC devices with redox electrolyte, has been optimized by the addition of an electronic barrier layer. In this regard, two dielectric materials, namely SiO2 and Si3N4, were studied as an electronic barrier layer in between WO3 and redox electrolyte. The loss current was reduced with 80 nm of Si3N4 from 200 to 20 μA/cm2 for the I− /I−3 redox electrolyte system for the visual (solar) transmission change of 45% (33%) down to 0.7% (0.5%). Similarly, the loss current was significantly reduced from 70 to 7 μA/cm2, which is below the targeted value (10 μA/cm2) for TMTU/TMFDS2 þ redox systems with 80 nm of Si3N4 for the visual (solar) transmission change of 71% (54.5%) down to 3% (2%).
Acknowledgment We gratefully acknowledge the financial support from the EU FP7 project WINSMART under Grant no. 314407.
References [1] R. Baetens, B.P. Jelle, A. Gustavsen, Properties, requirements and possibilities of smart windows for dynamic daylight and solar energy control in buildings: a state-of-the-art review, Sol. Energy Mater. Sol. Cells 94 (2010) 87–105. [2] C.G. Granqvist, Handbook of Inorganic Electrochromic Materials, Elsevier Science B.V., Amsterdam, 1995. [3] A. Georg, A. Georg, Electrochromic device with a redox electrolyte, Sol. Energy Mater. Sol. Cells 93 (2009) 1329–1337. [4] A. Georg, W. Graf, R. Neumann, V. Wittwer, The role of water in gasochromic WO3 films, Thin Solid Films 384 (2001) 269–275. [5] A. Georg, W. Graf, V. Wittwer, The gasochromic colouration of sputtered WO3 films with a low water content, Electrochim. Acta 46 (2001) 2001–2005. [6] U.O. Krasovec, M. Topic, A. Georg, A. Georg, G. Drazic, Preparation and characterisation of nano-structured WO3-TiO2 layers for photoelectrochromic devices, J. Sol-Gel Sci. Technol. 36 (2005) 45–52. [7] A. Hauch, A. Georg, S. Baumgärtner, U. Opara Krašovec, B. Orel, New photoelectrochromic device, Electrochim. Acta 46 (2001) 2131–2136. [8] A. Georg, A. Georg, U. Opara Krašovec, V. Wittwer, Sixth International Meeting on Electrochromism, August 29th - September 2nd, 2004, in: j. Vondrák, V. Novák, J. Reiter (Eds.) sixth International Meeting on Electrochromism, 2004, pp. 146–149. [9] P.M. Allemand, R.F. Grimes, A.R. Ingle, J.P. Cronin, S.R. Kennedy, A. Agrawal, J.M.
Boulton, Electrochromic Devices, in, Google Patents, 2001. [10] S. Bogati, A. Georg, C. Jerg, W. Graf, Tetramethylthiourea (TMTU) as an alternative redox mediator for electrochromic devices, Sol. Energy Mater. Sol. Cells 157 (2016) 454–461. [11] P.F. Tavares, A.R. Gaspar, A.G. Martins, F. Frontini, Evaluation of electrochromic windows impact in the energy performance of buildings in Mediterranean climates, Energy Policy 67 (2014) 68–81. [12] A. Hauch, A. Georg, Diffusion in the electrolyte and charge-transfer reaction at the platinum electrode in dye-sensitized solar cells, Electrochim. Acta 46 (2001) 3457–3466. [13] U.O. Krašovec, A. Georg, A. Georg, V. Wittwer, J. Luther, M. Topič, Performance of a solid-state photoelectrochromic device, Sol. Energy Mater. Sol. Cells 84 (2004) 369–380. [14] V.A. Macagno, M.C. Giordano, A.J. Arvía, Kinetics and mechanisms of electrochemical reactions on platinum with solutions of iodine-sodium iodide in acetonitrile, Electrochim. Acta 14 (1969) 335–357. [15] D. Li, H. Li, Y. Luo, K. Li, Q. Meng, M. Armand, L. Chen, Non-corrosive, nonabsorbing organic redox couple for dye-sensitized solar cells, Adv. Funct. Mater. 20 (2010) 3358–3365. [16] A. Georg, D. Schweiger, W. Graf, V. Wittwer, The dependence of the chemical potential of WO3 films on hydrogen insertion, Sol. Energy Mater. Sol. Cells 70 (2002) 437–446. [17] S. Bogati, H. Orvalho, C. Jerg, A. Gerorg, W. Graf, TMTU as alternative redox couple for electrochromic and photoelectrochromic devices, in: A. Rougier, C. G. Granqvist, B. Dam, M. Pemble (Eds.), European Materials Research Society, Solar Energy Materials and Solar Cells, Lille-France, 2014. [18] A. Georg, W. Graf, V. Wittwer, Comparison of electrical conductivity and optical properties of substoichiometrically and electrochemically coloured WOx films of different crystallinity, Sol. Energy Mater. Sol. Cells 51 (1998) 353–370. [19] H.P. Löbl, M. Huppertz, Thermal stability of nonstoichiometric silicon nitride films made by reactive dc magnetron sputter deposition, Thin Solid Films 317 (1998) 153–156. [20] T. Serikawa, A. Okamoto, Properties of magnetron-sputtered silicon nitride films, J. Electrochem. Soc. 131 (1984) 2928–2933. [21] M.A. Signore, A. Sytchkova, D. Dimaio, A. Cappello, A. Rizzo, Deposition of silicon nitride thin films by RF magnetron sputtering: a material and growth process study, Opt. Mater. 34 (2012) 632–638. [22] S.W. Wright, J.C. Anderson, Trapping centres in sputtered SiO2 films, Thin Solid Films 62 (1979) 89–96. [23] W.-F. Wu, B.-S. Chiou, Properties of radio frequency magnetron sputtered silicon dioxide films, Appl. Surf. Sci. 99 (1996) 237–243. [24] A. Georg, A. Georg, U.O. Krašovec, V. Wittwer, Phase transitions of the WO3 layer in photoelectrochromic devices, J. New Mater. Electrochem. Syst. 8 (2005) 317–325. [25] R.S. Crandall, P.J. Wojtowicz, B.W. Faughnan, Theory and measurement of the change in chemical potential of hydrogen in amorphous HxWO3 as a function of the stoichiometric parameter x, Solid State Commun. 18 (1976) 1409–1411.
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Solar Energy Materials & Solar Cells journal homepage: www.elsevier.com/locate/solmat
Sputtered Si3N4 and SiO2 electron barrier layer between a redox electrolyte and the WO3 film in electrochromic devices Shankar Bogati, Andreas georg n, Wolfgang Graf Fraunhofer Institute for Solar Energy Systems (ISE), Heidenhofstraße 2, 79110 Freiburg, Germany
art ic l e i nf o
a b s t r a c t
Article history: Received 4 March 2016 Received in revised form 27 July 2016 Accepted 22 August 2016
The application of a redox electrolyte in hybrid type electrochromic devices allows a high charge capacity, thereby a high optical contrast can be achieved. However, the current between the redox electrolyte and the electrochromic layer (tungsten oxide, WO3) interface in equilibrium at colored state of device is critical for a window with large area. This loss current should be maintained below 10 μA/cm2 to avoid an inhomogeneous coloration of a large window caused by an ohmic voltage drop in transparent conductive oxide (TCO). The power consumption of such a loss current can be neglected. In this regard, we have investigated silicon nitride (Si3N4) and silicon oxide (SiO2) films as an intermediate electronic barrier layer on the top of the WO3 with thicknesses of 12, 35, 80 and 180 nm. These films were coated by direct current reactive magnetron sputtering technique. The electron barrier properties were studied for the iodide/triiodide ( I−/ I−3 ) and tetramethylthiourea/tetramethylformaminium disulfide dication (TMTU/ TMFDS2 þ ) redox couples for application in electrochromic devices. For both redox couples, Si3N4 showed an effective electronic barrier layer. The thickness of 80 nm of Si3N4 reduced the loss current from 240 down to 20 μA/cm2 for the I− /I−3 redox electrolyte with the visual (solar) transmission from 45% (33%) down to 0.7% (0.5%)at 1 V.Similarly, the loss current was reduced from 70 down to 7 μA/cm2with the visual (solar) transmission from 71% (54.5%) down to 3% (2%) for TMTU/TMFDS2 þ redox electrolyte. However no significant reduction of loss circuit current was achieved with SiO2 barrier layer for both redox couples. & 2016 Published by Elsevier B.V.
Keywords: Electrochromic Redox electrolyte Redox potential Coloration efficiency Optical density Charge transfer resistance Loss current
1. Introduction Electrochromic smart windows allow a significant reduction of annual energy consumption, controlling the heating loads, cooling loads, and lighting needs [1] by adjusting the intermediate levels of transmission through it. Typically, the oxide based electrochromic windows comprises two transparent electrodes, e.g., fluorine doped tin oxide (F:SnO2), on which an electrochromic layer (WO3)and a counter electroactive layer (metal oxide) are coated [2]. An ion conductor containing lithium salt is introduced in between two electrodes. Here a cathodic voltage for the coloration and an anodic voltage for the bleaching are applied at the WO3. The coloration occurs intercalating the charges (Li þ ions and electrons) into the WO3 and the bleaching is regained by extraction of the inserted charges, which are compensated by the counter electroactive layers. The energy consumption is mainly concerned during the coloration and the bleaching processes and can be neglected at the colored state as both electrodes are n
Corresponding author. E-mail address: [email protected] (A. georg).
http://dx.doi.org/10.1016/j.solmat.2016.08.023 0927-0248/& 2016 Published by Elsevier B.V.
separated by an electrolyte and a loss current can be avoided. The term “loss current” shall describe the current in between the two electrodes, which is needed to keep a certain coloration or, equivalently, potential. However, this work is focused on the hybrid type of electrochromic device [3]. It also has two transparent electrodes (F:SnO2 coated on glass), on which an electrochromic layer, e.g., WO3 in our case and a catalytic layer (Pt) are coated. An electrolyte comprising a lithium salt, a redox electrolyte like iodide/triiodide ( I− /I−3 ) is introduced in between them. x M þ (cation) þ x e þ WO3 (transparent) ⇌ MxWO3 (blue) first redox reaction (1) R – e ⇌ O þ e second redox reaction
(2)
Fig. 1 is a schematic of an electrochromic device with redox electrolyte without (a) [3] and with a barrier layer (b) on WO3. The charge insertion for the coloration with a cathodic voltage and extraction for the bleaching with an anodic voltage at WO3 are performed by a voltage across the electrodes as s.hown in upper and lower part of the Fig. 1a and b. Unlike the most common thin film battery type of EC devices, the charge compensations during the coloration and the bleaching are performed by a redox
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Fig. 1. Layers configuration of electrochromic device working principle of an electrochromic device with redox electrolyte as described by Georg et al. in 2009 [3] without barrier layer (a) with a barrier layer (b).
electrolyte. The Eqs. (1) and (2) explain the two reactions that take place in EC device during the coloration and the beaching. During the coloration the redox couple is oxidized at counter electrode (Pt) and provides the charge to the WO3 via external circuit. At the same time, Li þ ions intercalate in the WO3 for charge neutrality. In contrary, the bleaching is achieved by reversing the reaction. The redox electrolyte neutralizes the charge in the both process. During and after the coloration, the charge transfer occurs between the redox electrolyte and the WO3 layer, and, if WO3 is porous, TCO layer, which is below WO3 as shown in Fig. 1. However, this charge transfer is kinetically supressed. Nevertheless the charge transfer from WO3 or TCO to a redox electrolyte leading to a certain loss current in equilibrium state of coloration is critical, especially, for a large area application for homogenous coloration due to voltage drop in the TCO layer. The porosity of the WO3 layer can be varied by controlling the gas pressure and power density during the sputtering process [4,5]. Furthermore, the sol-gel derived WO3 and TiO2 have been intensively investigated for photoelectrochromic [6,7] and photochromic devices [8]. The sol-gel layers typically showed crystalline grains, surrounded by amorphous phase, together with a high porosity. The sputtered layers, if prepared to realize high porosity for fast switching, typically are amorphous in phase. It should be noted that the electrochochemical potential of the as-prepared WO3 should not be higher than that of the redox electrolyte for EC devices based on redox electrolyte to prevent the self-coloration of the device. The investigation of electronic barrier layer (selective ion transport layer) on sol-gel coated WO3 to suppress the loss or the leakage current with ferrocene based redox electrolyte has been described by M. Allemand et al [9]. They significantly reduced the loss current after coating of tantalum pentoxide (Ta2O5) or silicon dioxide (SiO2) onto WO3, which were prepared by sol-gel technique. Furthermore, the spin coated polystyrenesulphonate sodium salt (PSSNa) was also investigated for ferrocene and ethanol based electrolyte. In this regard, we have investigated two dielectric layers namely Si3N4 and SiO2 individually with various thicknesses of 12, 35, 80 and 180 nm as an electronic insulating layer for the I− /I−3 and the TMTU/TMFDS2 þ redox electrolytes to supress the loss current. The TMTU/TMFDS2 þ and I− /I−3 have been investigated as redox electrolyte for EC devices in previous works [3,10], respectively. The layer configuration of the coating is shown in the Fig. 1b. It is expected that there is no conductivity for electrons for these dielectric layers, but they have to allow permeation for Li þ ions by certain porosity. On the other hand, they should reduce the permeation of the I−3 or TMFDS2 þ ions, leading to a decrease in loss
current. A coverage of the surface of the WO3 layer by the dielectric layer is not enough, as the WO3 itself shows a certain porosity, which is needed to allow a fast Li þ diffusion. 1.1. Estimation of loss current There are two drawbacks generated by a high loss current: the energy consumption and ohmic voltage drop of the TCO on large area. For a loss current of 100 mA/cm2, the yearly energy consumption can be estimated to be 4 KWh/m2, when a voltage of 1.0 V is applied and the colored state is kept over half of the year. A typical range of energy saving due to application of electrochromic window is in the order of 20–40 KWh/m2 [11]. Therefore, the energy consumption due to a loss current of 100 mA/cm2 can be tolerated. However, the ohmic voltage drop on the applied TCO is more critical. For an area of 1 m2 and a sheet resistance of a TCO layer of 16 Ω/□, a current of 1 mA/cm2 corresponds to an ohmic drop of about 0.16 V, which may be acceptable, without creating too strong in-homogeneities of the optical contrast. Real windows typically have larger area, but due to highly conductive contacts on the edges, the resulting ohmic drop is reduced. Furthermore, one should note that it is expected that the loss current decreases by gelification or polymerization of the electrolyte, which has to be done for a real window product [12,13]. In this context we investigated the thin layer of Si3N4 or SiO2, which can be coated onto WO3 by DC-magnetron process and applied for large area application, as an electronic barrier layer. Therefore, we set the target for a maximum loss current with liquid electrolytes as 10 mA/cm2. Certainly, this is a very rough estimation, but may be used as a guideline for the order of magnitude.
2. Materials and methods 2.1. Materials Lithium iodide (LiI), Iodine (I2), tetramethylthiourea (TMTU), nitrosyl tetrafluoroborate (NOBF4) and gamma-butyrolactone (GBL) were purchased from Sigma Aldrich, Germany. Fluorine doped tin oxide (F:SnO2) coated glass having a sheet resistance of 16 Ω/□ was purchased as k-glass™ from Pilkington, UK used as the substrates for all experiments. Tungsten (W), silicon (Si) and platinum (Pt) targets were used to deposit WO3, Si3N4 and SiO2 and Pt, respectively. The electrochemical characterization work station (Zennium) was supplied by ZAHNER-elektrik GmbH & Co. KG, Germany.
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397 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12.
2.2. Preparation of redox electrolytes Two liquid electrolytes are prepared for individual redox couples. They are composed of 0.5 M reduced and 5 mM of oxidized forms and 0.5 M lithium salt (LiI and LiClO4 in case of I− /I−3 and TMTU/TMFDS2 þ , respectively) in GBL. Iodide and triiodide were prepared after dissolving iodine and lithium iodide as explained in reference [14], i.e. by formation of I−3 out of I and I2.
I− + I2 → I−3
(3)
The NOBF4 was used as an oxidizing agent to oxidize the TMTU to TMFDS2 þ as described in reference [15].
Short circuit Coloration voltage (-0.4 V) Short circuit Bleaching voltage (0.4 V) Short circuit Coloration voltage (-0.6 V) Short circuit Bleaching voltage (0.6 V) Short circuit Coloration voltage (-1 V) Short circuit Bleaching voltage (1 V)
Fig. 2. An electrochromic test cycle with different bias voltages.
y ( x)TMTU+( y)NOBF4 → TMFDS[(BF4)]2 + ( x − y)TMTU + y NO↑ (4) 2 2.3. Preparation of electrodes and assembling of cells The films of WO3, Si3N4, SiO2 and Pt are coated by DC-magnetron sputtering coating process. The thickness of the layers were measured by preparing an edge of the layer by a lift off process followed by atomic-force microscopy (AFM). The layers configuration is given in the Fig. 1. The thicknesses of the WO3 and the Pt were maintained to be 450 and 1 nm, respectively. First a layer of the WO3 was sputtered onto k-glass™ substrate subsequently the Si3N4 or the SiO2 were coated. The thickness of Si3N4 was coated as 12, 35, 80 and 180 nm. However the thickness of SiO2 coated as 12 and 35 nm. As a counter electrode Pt was coated onto k-glass™. The two electrodes were sandwiched together and a silicone frame of 2 mm was used as a spacer. An electrolyte as described in Section 2.2 was introduced. The external circuit was made with cupper wire placing it on the edge of k-glass™ for the switching of devices. The effective area of the device was 6 cm2. 2.4. Characterization methods The redox potential or the electrochemical potential of WO3 upon intercalation of Li þ ions was determined with three electrode arrangement. The sputtered WO3 on k-glass™ was used as a working electrode and a Ag/AgCl (144 mV versus standard hydrogen electrode, SHE) served as reference electrode and a Pt coated k-glass™ was used as counter electrode. The electrolyte composition of 0.5 M LiClO4 in GBL was for this experiment. The current was gradually increased certain value and reversed back to the zero. The resultant voltage drop was recorded with working and reference electrodes and transmission was recorded by a photo diode (OP301) and a red LED (wavelength 655 nm). As explained by Georg et al. [16], the optical density was used for the determination of x (number of intercalated Li þ per W atom in WO3) as shown in Eq. (8). An electrochromic (EC) test cycle with three colorations and bleaching voltages as well as six intermediate short circuit current was defined as in Fig. 2. At zero bias voltage the exchange of electron from redox electrolyte to EC layer may happen when the redox potential of redox electrolyte is lower than WO3 creating some self-coloration. Nevertheless, this difference in redox potentials can be compensated by the anodic voltage. The comparative evaluation of loss current will be based on the cathodic voltage at saturated colored condition. The resulting electrical current was recorded using the electrochemical workstation. In parallel, the transmission of the cell was recorded at a wavelength of 655 nm using photo diode (OPT301) and a red LED as shown in Fig. 3. From this set up the rate of change of the intercalated charge into the WO3 can be evaluated by defining an optical current [10] since there is no significant effect of coloration of the used redox electrolyte at this wavelength [10,17]. From this concept the difference between the
Fig. 3. Electrochromic test setup with LED and photodiode (OP301) and a test cell.
electrically measured current (electrical current) and the optically converted current (optical current) can be separated.
Iopt (A/cm2)=
∆Q opt ∆t
=
∆ OD CE
∆t
=
∆OD CE*∆t
I (loss current) ¼ I (electrically measured) – I sured)
(5) opt
(Optically mea(6)
Iopt: optical current; CE: coloration efficiency (54 cm2/C for DC magnetron sputtered WO3 at λ ¼ 655 nm [17]); ΔOD: change in optical density; Δt: change in time [s]. This loss current must be continuously applied in order to maintain the device at colored state. For the better understanding of insulating properties of Si3N4 and SiO2, loss current was compared at colored state in equilibrium of the cell. Additionally the speed of coloration i.e., the rate of change of optical density at the beginning of coloration is evaluated for all layer configurations. Finally, the spectra of the cell in UV, visible and near infrared regions are measured for optimized thickness of the dielectric layer with Bruker Fourier Transform Spectrometer (IFS 66).
3. Results and discussions 3.1. Morphology and FTIR spectroscopy analysis of deposited layers Before the deposition of selective ion transport layer onto WO3, the porosity and the growth of Si3N4 and SiO2 on k-glass™ have been analysed. The amorphous structure of magnetron sputtered WO3 , Si3N4 and SiO2 identified by X-Ray diffractions [18,19]. The
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Fig. 4. SEM pictures of sputtered WO3, Si3N4 and SiO2 on k-glass™ with top view (left side) and cross-section view (right side).
SEM pictures of the layers are shown in Fig. 4 with top view (leftside) and cross-section view (right side). The grain structure of the layers were influenced by the first layer (F:SnO2). Similarly the formation of the pores in between the columns is supported by the roughness of the F:SnO2 layer. On the other hand the growth of the SiO2 is different where no columnar structure could be detected. The infrared transmission spectra of sputtered WO3, SiO2 and Si3N4 deposited on silicon wafer with the thickness of 600 nm can
be seen in the Fig. 5. The presence of the different chemical bonds in the film can be detected by infrared spectroscopy. Georg el al. [4] have characterized four different types of sputtered WO3 films by IR-spectroscopy with the observation of different bonds involved in the films. Similar to their study the absorption peaks of the bonds such as W ¼O (970 cm 1), W-O-H (1420 cm 1), H2O (1620 cm 1), and O-H (2850 cm 1, 3010 cm 1, 3040 cm 1) can be found in the in the Fig. 5a and b. The infrared spectra pattern of
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Fig. 5. IR-transmittance spectra of the sputtered WO3, SiO2 and Si3N4 on silicon with the thicknesses of 600 nm.
the sputtered WO3 prepared for this study as in Fig. 5 is similar to type-D [4]. The absorption bands of Si3N4 for the same thickness can be seen in Fig. 5a and b. The band at 490 cm 1 is peculiar to Si3N4 [20] and absorption due to the stretching vibration modes of Si-N are typically in between 830 cm 1 and 870 cm 1 [21]. Furthermore O-H can be seen at 3300 cm 1. For the SiO2, the Si-O stretch peak and Si-O bend peak can be seen at 1070 cm 1 and 450 cm 1, respectively, as reported by Wright et al. [22]. Bi-Shiou et al. [23] suggested that
the position of the Si-O vibrational band are also around 840 cm 1. However no O-H ground absorption is seen for SiO2 around 3500 cm 1. 3.2. Influence of barrier layer on loss currents for iodide triiodide redox electrolyte As explained in the Section 2.3, the WO3 and the Pt coated on the k-glass™ glass were used as a working and counter electrodes,
Fig. 6. Loss currents during coloration (a and b) and the change in optical densities (c and d) of EC devices for 0.4, 0.6 and 1 V bias voltages with and without barrier layer for I−/I− 3 redox electrolyte.
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constant, temperature, elementary charge, respectively, and x represents the number of intercalated Li þ injected electrons per W atom in WO3. The constant a represents the binding energy of the Li þ ions and electrons in WO3, b represents the interaction between the incorporated charges, and the logarithmic term represents the increase in entropy due to the intercalation. For x 40.1, the logarithmic part can be neglected. This simplifies that the electrochemical potential of LixWO3 linearly depends on x, where x is directly proportional to the degree of intercalation or change in optical density of LixWO3. The electrochemical potential of LixWO3 versus optical density is measured in Fig. 7. The relationship between the optical density and x can be described as reported by Georg et al. [24].
x=
Fig. 7. Electrochemical potential (E) of a sputtered WO3 vs optical density.
respectively. And the defined test cycle was used for the EC tests. The resulting loss currents were measured for 0.4, 0.6 and 1 V bias voltages. Fig. 6a and b show the resulting loss current (the difference between electrically measured and optically evaluated currents) for three bias voltages. At zero bias voltage, the change in OD, in average, was found to be 0.12. This is due to the low redox potential of redox electrolyte compared to the redox potential LixWO3. Thus, to avoid this self-coloration the redox potential of WO3 has to be reduced for the combination with iodide triiodide redox electrolyte. The self-coloration can be eliminated with a anodic potential at WO3. The inset in Fig. 6d shows the slowing down of the coloration with increase in barrier layer thickness, likely due to the slow diffusion of Li þ ions through the barrier layers. However a coloration time of 5 min is still a sufficient value for large area window applications. For coloration at 1 V, there is a first decrease in loss current, and then an increase, again, as well as a first increase in optical density followed by a decrease. This is likely due to a phase change of WO3 [24]. As explained in [25], neglecting the phase changes in WO3, an electrochemical potential , E, of LixWO3 depends on the charge intercalation in WO3 as follows.
E( x) = a+ b x−ν Where a, b, and
Kb Temp e0
⎛ x ⎞ ⎟ ln⎜ ⎝1 − x⎠
(7)
ν are constants, Kb, Temp, e0 are the Boltzmann
M WO3 e0NAρLCE
OD ≈ 0. 21 * O. D.
(8)
MWO3: molar mass of WO3, e0: elementary charge, NA: Avogadro's number, ρ: density of our sputtered, WO3 (ρ ¼5 g/cm3), L: thickness of WO3 layer, (L ¼450 nm), CE ¼54 cm2/C [17]. For the x to be greater than 0.1, the optical density should be also greater than 0.46. That means, while the optical density increases, the difference between the electrochemical potential of LixWO3 and redox couple also increases. As result of this, a higher loss current is generated. The relationship between the applied voltage that creates the optical density larger than 0.46 and optical density, for specific thickness e.g., 450 nm in our case, can be formulated as follows Fig. 7. Δ O D (Change in optical density) ¼ 2 2 * E (applied bias voltage) þ 0.037 (9) Fig. 8a and b show the influence of the barrier layer on the loss current of electrochromic devices for different coloration potentials. A significant decrease in loss current can be seen with Si3N4 at 1 V bias voltage. However, it was not decreased for SiO2 for all bias voltages and for Si3N4 for 0.6 V and lower voltages. This corresponds to the limited diffusion of triiodide through the barrier layer for higher bias voltages. Therefore, for the change in optical density up to 1.5 at 655 nm of wavelength no barrier layer is needed. The speed of coloration of electrochromic devices with and without SiO2 barrier layer for different coloration voltages are logarithmically plotted in Fig. 9a. For all applied bias voltages the intercalation of Li þ ions was as fast as without any SiO2 layer. However, the speed of coloration was significantly decreased with Si3N4 for all coloration voltages, which indicates that the slow
Fig. 8. Loss current at 0.4, 0.6 and 1 V voltages in EC device with (a) SiO2 and (b) Si3N4 barrier layers.
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Fig. 9. Speed of coloration at 0.4, 0.6 and 1 V voltages with (a) SiO2, (b) Si3N4 barrier layer.
diffusion of Li þ ions through it, which can be also seen in Fig. 9b. But the influence on the total optical density is not affected. Hence the loss current is reduced for iodide triiodide redox couples for 1 V applied bias voltage by Si3N4 without reducing the optical density of device. The speed of the coloration is reduced by factor of 2 with Si3N4 at all three coloration voltages.
3.3. Influence of barrier layer on loss currents for TMTU/TMFDS2 þ redox electrolyte Fig. 10a and b show the loss current in the electrochromic devices with TMTU/TMFDS2 þ redox couple at 0.4, 0.6, 1 V coloration voltages and Fig. 10c and d represent the resulting
Fig. 10. Loss currents during coloration (a and b) and the change in optical densities (c and d) of EC devices for 0.4, 0.6 and 1 V bias voltages with and without barrier layer for TMTU/TMFDS2 þ redox electrolyte.
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Fig. 11. Loss current at 0.4, 0.6 and 1 V voltages in EC device with (a) SiO2 and (b) Si3N4 barrier layer.
optical densities response at corresponding bias voltages. Unlike in the cell with iodide/triiodide redox electrolyte, the self-coloration at zero bias voltage is found to 0.02 (around 1% change in transmission), which indicates that the electrochemical potential of LixWO3 has slightly higher redox potential compared to TMTU/ TMFDS2 þ redox couple. So, the redox potential of WO3 and of TMTU/TMFDS2 þ redox electrolyte matches each other nearly perfectly, which is not the case for I− /I−3 . A high charge transfer resistance of TMTU at Pt electrode [17] and high redox potential of TMTU/TMFDS2 þ may result in low change in optical density of the device compared to the device with iodide/triiodide redox systems. The slow bleaching at open circuit conditions for all cases may be due to the slow diffusion of TMFDS2 þ through the Si3N4 and WO3 film. As explained in the Section 3.2, the linear relationship between the change in O.D. (larger than 0.46) and applied bias voltage can be formulated as follow. Δ O D (Change in optical density) ¼ 2 6 * E (applied bias voltage) –0 72 (10) Fig. 11a and b show the loss current response with respect to the applied coloration voltages with and without barrier layer for TMTU/TMFDS2 þ redox couple. Similar to the iodide triiodide redox couple the influence in the loss current was not achieved with SiO2 barrier layer i.e., the diffusion of TMFDS2 þ is not influenced by it. That is why further experiments were not carried out with SiO2
layer. However, the loss current was significantly reduced from 70 to 4 μA/cm2 with 180 nm Si3N4 barrier layer. Unlike iodide/triiodide redox electrolyte it was also decreased below 10 μA/cm2 at 1 V even with 80 nm. One should note that a Si3N4 layer in principle acts as an antireflecting layer in between the highly refracting WO3 and the electrolyte with low refractive index. Then, 80 nm is very close to an optimized thickness of such an antireflective layer, but the refractive index of Si3N4 is still too high. A detailed optimization of this should apply a SiOxNy layer with optimized porosity, or, more precisely, permeability for TMFDS2 þ and Li þ . Fig. 12a and b show the speed of coloration of the electrochromic device with SiO2 and Si3N4 barrier layer at 0.4, 0.6 and 1 V coloration voltages. As shown in Fig. 12a, the speed of the coloration is not effected by the SiO2. However the speed of coloration with Si3N4 was decreased from 0.14 to 0.04. Similarly, the speed of coloration was reduced for all bias voltages. The bleaching at short circuit and the speed of coloration are low due to the slightly high charge transfer resistance of redox electrolyte at Pt layer and high redox potential of TMTU/TMFDS2 þ . 3.4. Influence of barrier layer thickness on loss currents for redox electrolytes The influence of the barrier layer thickness on the loss currents
Fig. 12. Speed of coloration at 0.4, 0.6 and 1 V voltages with SiO2 (a) Si3N4 (b) barrier layer.
S. Bogati et al. / Solar Energy Materials & Solar Cells 159 (2017) 395–404
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2þ Fig. 13. Loss current versus thickness of barrier layer in double logarithmic scale for I−/I− (b) at 1 V. 3 (a) TMTU/ TMFDS
Fig. 14. Transmission spectra of EC devices at bleached and colored states with 80 nm of Si3N4 barrier with TMTU/TMFDS2+ (a) and I−/I− 3 (b) redox system.
at 1 V are shown in Fig. 13a and b for iodide/triiodide and TMTU/ TMFSD2 þ , respectively. For both redox electrolytes, the influence of thicknesses of Si3N4 on the loss currents is plotted in double logarithmic scales. From the graph, the required thickness of barrier layer can be estimated for required loss current. The loss current can be further decreased by in increasing the thickness of Si3N4 for iodide triiodide redox electrolyte. Similarly loss current can be reduced down to 10 μA/cm2 with around 50 nm of Si3N4 at 1 V for TMTU/TMFDS2 þ . As explained in Section 3.2 the effect of the barrier layer is not significant in reduction of loss current for I− /I−3 redox electrolyte up to the change in densities of 1.5, which can be achieved by 0.6 V. However for the TMTU/TMFDS2 þ redox electrolyte, the Si3N4 of 80 nm thickness is sufficient for the reduction of loss current below the target value (10 μA/cm2). Therefore, for the optimized prototype optimized EC device the Si3N4 of 80 nm was coated on to WO3 coated k-glass™. Fig. 14a and b are the transmission spectra of the EC cells comprising I− /I−3 and TMTU/TMFDS2 þ redox systems, respectively. For both electrochromic devices, the low transmission in the infrared region is due to the absorption and reflectance of F:SnO2. Similarly, the low transmission in 480–520 nm is due to the absorption of triiodide [17] for the second electrochromic device Table 1. The optical density can be increased with applied voltage across the electrode. The self-coloration of device can be
Table 1 Visual and solar transmissions of EC cells with iodide/triiodide and TMTU/ TMFDS2 þ electrolytes with 80 nm Si3N4 barrier layer for different coloration voltages. TMTU/TMFDS2 þ
I− / I− 3 Applied voltage (V)
Tvis (%)
Tsol (%)
Loss current (μA/cm2)
Tvis (%)
Tsol (%)
Loss current (μA/cm2)
0.4 0 0.4 0.6 1.0
56 45 12 4 0.7
43 33 6 2 0.5
0.33 0 2 6 20
71.5 71 44 20 3
55 54.5 29 30 2
0.33 0 1 5 7
eliminated by applying the voltage of 0.4 V, which creates the negligible loss current (around 0.33 μA/cm2) as shown in Table 1 to keep it at bleached state. The lower redox potential of iodide/triiodide compared to the TMTU/TMFDS2 þ results in the higher change in transmission. The EC device comprising TMTU/ TMFDS2 þ system can be taken for the further optimization due its transparent nature in visible region and matching redox potential. 4. Conclusion The loss current, which creates inhomogeneous coloration in
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large EC devices with redox electrolyte, has been optimized by the addition of an electronic barrier layer. In this regard, two dielectric materials, namely SiO2 and Si3N4, were studied as an electronic barrier layer in between WO3 and redox electrolyte. The loss current was reduced with 80 nm of Si3N4 from 200 to 20 μA/cm2 for the I− /I−3 redox electrolyte system for the visual (solar) transmission change of 45% (33%) down to 0.7% (0.5%). Similarly, the loss current was significantly reduced from 70 to 7 μA/cm2, which is below the targeted value (10 μA/cm2) for TMTU/TMFDS2 þ redox systems with 80 nm of Si3N4 for the visual (solar) transmission change of 71% (54.5%) down to 3% (2%).
Acknowledgment We gratefully acknowledge the financial support from the EU FP7 project WINSMART under Grant no. 314407.
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