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Tunable photovoltaic electrochromic device and module
Solar Energy Materials & Solar Cells 107 (2012) 390–395
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Solar Energy Materials & Solar Cells journal homepage: www.elsevier.com/locate/solmat
Tunable photovoltaic electrochromic device and module Lee-May Huang a,n, Chen-Pang Kung b, Chih-Wei Hu c, Cheng-Yu Peng a, Han-Chang Liu a a b c
Green Energy & Environment Research Laboratories, Industrial Technology Research Institute, Chutung, Hsinchu 31040, Taiwan Display Technology Center, Industrial Technology Research Institute, Chutung, Hsinchu 31040, Taiwan Department of Chemical Engineering, National Taiwan University, Taipei 10617, Taiwan
a r t i c l e i n f o
a b s t r a c t
Article history: Received 19 March 2012 Received in revised form 25 June 2012 Accepted 18 July 2012 Available online 17 August 2012
An innovative tunable photovoltaic electrochromic (PV–EC) device concept, in which planar distribution of silicon thin film solar cells (Si-TFSCs) structure with successively deposited electrochromic thin films (ECTFs) and electrolyte layer is demonstrated. The switching mechanism of a PV–EC device is realized by electrically separating the electrodes of the Si-TFSCs and ECTFs, and the connection of the two systems is achieved through an external switching apparatus that links each Si-TFSC and ECTF through one transparent conductive oxide (TCO) layer. According to the PV–EC device structure, Si-TFSCs arranged in stripes makes large area module fabrication feasible, while Si-TFSCs arranged in arrays allows pixel structure design possible. When illuminated by sunlight under short-circuit condition, the photopotential outputted from a single Si-TFSC drives ECTF layer to change color, simultaneously a portion of the photocurrent is stored in a capacitor. Once the ECTF is tinted, no power is needed to retain its colored state due to the bi-stable property of the electrochromic material. In order to revert the PV–EC device to its bleached state, the capacitor supplies reverse potential to cause discoloration of the ECTF. In addition, the monolithically integrated Si-TFSC module generates electricity to a connected load. In view of photoelectric conversion and optical modulation properties, the PV–EC device can both function as a Si-TFSC module and as a self powered smart glass, which has great advantages in green energy application. & 2012 Elsevier B.V. All rights reserved.
Keywords: Semi-transparent thin film solar cell Tunable solid type photovoltaic electrochromic device Solar powered smart glass Building integrated photovoltaics
1. Introduction An electrochromic (EC) device has reversible changes in the optical properties due to the electric field or current induced redox reaction of conductive materials and electrolyte [1–3]. The EC device provides tunable color changing that control glare and solar heat gain [4,5]. Electrochromic device can be divided into solid type and liquid type. A typical solid type EC device includes two transparent substrates that sandwiched five electroactive layers such as: a transparent conductive layer, an electrochromic layer, a solid electrolyte, an ion storage layer, and another transparent conductive layer deposited in-between the two substrates to constitute a battery-like structure [6]. The structure of a solution-type EC device is formed by two transparent conductive substrates, which are bonded by polymer adhesive in a direction facing an electrode layer, and an EC solution is disposed between the transparent conductive substrates [7–9]. The integration of photovoltaic (PV) and electrochromic (EC) device provides better efficiency in term of energy saving, for the photovoltaic electrochromic (PV–EC) device can achieve color
n
Corresponding author. Tel: þ886 3 5919389, fax: þ 886 3 5822157. E-mail address: [email protected] (L.-M. Huang).
0927-0248/$ - see front matter & 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.solmat.2012.07.021
change in EC layers from the sunlight converted electrical power. This self-powered smart glass can adjust the color of the EC device according to the intensity of sunlight irradiation to reduce indoor heat and to provide comfort. Until recently, there are two types of solar powered EC devices available, the first one is tandem structure silicon (Si) based PV–EC device [10–15] and the second one is dye-sensitized solar cell (DSSC) based photoelectrochromic (PEC) device [16–21], both of which belong to NREL technology. The silicon based PV–EC technology constitutes stacked layers of the photoabsorbers and EC active materials in one single device. However, this tandem-structure PV– EC device encounters problems such as low colored/bleached optical contrast, short circuit between the deposited layers and difficulty in producing large area device [15,20]. A photoelectrochromic technology separates the photoabsorbing layers of DSSC [17] and the EC layer to the anode and the cathode, respectively for constituting a device. This PEC device can be described as having EC material inserted into DSSC, and has become the mostly studied photovoltaic integrated electrochromic technology. However, to apply such a structure to practical applications, many problems need to be overcome, such as long-term stability of the photo-absorbing layers and the possibility of developing devices having larger sizes. An innovative solution type PV–EC device based on the integration of planar structure Si-TFSC and electrochromic solution has been
L.-M. Huang et al. / Solar Energy Materials & Solar Cells 107 (2012) 390–395
demonstrated in our previous paper [22]. The Si-TFSCs are arranged in stripes with laser scribing process, the cathode blocks are Si-TFSC cells, and the area outside the cathode blocks are anode which composes only of transparent conductive oxide (TCO). The overall transparency and the color contrast of this solution type PV–EC device are enhanced, as compared to tandem Si based PV–EC device [10–15]. However, this solution type PV–EC may cause erosion in SiTFSC, and unless redox couples are added to the EC solution [23], the self-bleaching duration is too long for practical applications. In this paper, a solid-type PV–EC device which can solve the problem encountered by solution type PV–EC device is introduced. The structure of the solid-type PV–EC device consists of planarly distributed semi-transparent Si-based TFSC (cells) with exposed transparent anode layers in-between the cathodes layers, configured in superstrate structure as shown is Fig. 1. Electrochromic thin films (ECTFs) are successively deposited on the anodes and cathodes of the Si-TFSC. An electrolyte layer is then formed on the surfaces of the Si-TFSC cells to contact the ECTFs simultaneously. The electronic current generated by the Si-TFSCs is transformed to ionic current in the electrolyte layer that migrates transversely along the electrodes of the Si-TFSCs in order to maintain charge neutrality in the EC system. The electrolyte must be a good ionic current conductor, but it must be an isolator for electronic current generated by the Si-TFSCs. In concordance with the structure of this solid type PV–EC device, Si-TFSCs arranged in stripes makes large area monolithic seriesconnected Si-TFSC module fabrication feasible, while Si-TFSCs arranged in arrays allows pixel structure design possible. Since the Si-TFSCs and EC system have common electrodes, the ECTFs change tint immediately upon sunlight exposure. The SiTFSCs generate photopotential and photocurrent, and at the same time the electric field of the Si-TFSCs stimulates redox reaction in the ECTFs. The cations and anions in the electrolyte are attracted to the anodes and cathodes of the Si-TFSCs, respectively, inducing color change in the ECTFs. Unlike our previously reported solution type PV–EC device that has self-bleaching property [22], for a solid type EC system, once an ECTF changes tint, a reverse potential is needed to bleach the ECTF. In this study, the idea of anodic coloring tunable PV–EC device is realized by electrically separating the electrodes of the Si-TFSCs and ECTFs, and the connection of the two systems is achieved through an external switching apparatus that links each Si-TFSC and ECTF through one transparent conductive oxide (TCO) layer [24]. The TCO layer is designed to be electrically isolated from the electrodes of the Si-TFSC. Here, two passivation methods have been investigated, the first method is to employ a laser scribing process that peels off a portion of an exposed area of the anode of the Si-TFSC to form a TCO layer as depicted in Fig. 2. The second one is to deposit a passivation layer such as SiOx or SiNx on a
Fig. 1. Working principle of a solid-type PV–EC device. PIN refers to Si-TFSC.
391
electrolyte EC (-) cathode EC (+)
PIN
TCO
anode glass
Fig. 2. A schematic cross-sectional view of an anodic coloring solid-type PV–EC device with a portion of the anode of the Si-TFSC peeled off to form a TCO layer. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
portion of the anodes of the Si-TFSC to isolate the TCO layer from the anode of the Si-TFSC as shown in Fig. 3. Once the passivation process is completed, an anodic coloring ECTF (EC(þ)) is disposed on the TCO layer. A tunable solid type PV–EC device is readily established by covering the anodic coloring ECTF and the cathode of the Si-TFSC with an electrolyte layer. A cathodic coloring ECTF (EC( )) can be optionally deposited on the cathode of the Si-TFSC to improve the performance of the PV–EC device. The on/off switching mechanism of the solar-powered anodic coloring PV–EC device is as explained in Fig. 4. In reference to the circuit diagram, PV represents a Si-TFSC and EC represents an electrochromic system. The output signal controls the connection between the anode of the Si-TFSC with the TCO layer which determine the color/bleach of the ECTF. A charge storing device (shown as a capacitor) is connected to the PV and the EC systems. The switching apparatus depicted in Fig. 4 can enter various control modes through switch control signals A, B, and C. Based on the aforementioned circuit design, there are two types of control modes for the anodic coloring PV–EC device. 1.1. The EC coloring and capacitor charging mode The charge-storing device is coupled to Si-TFSC with the same polarity. The TCO layer is connected to the anode of the Si-TFSC, acting as the anode of the ECTF, while sharing the same cathode as that of the Si-TFSC, as referred to Fig. 3A. When exposes to sunlight, the anodic coloring ECTF changes its tint, while concurrently stores the electrical power generated by the Si-TFSC. Once the ECTF tinted, no additional power is needed to retain the colored state due to the bi-stable property of the EC device and the electrical circuit is then disconnected. 1.2. The EC bleaching and capacitor discharging mode In order to revert the PV–EC device to its bleached state, the capacitor supplies reverse potential to cause discoloration of the ECTF, as illustrated in Fig. 3(b). In this mode, the anode of the SiTFSC is disconnected from TCO and the capacitor is switched to the opposite polarity of Si-TFSC. Here, the negative terminal of the capacitor is connected to the TCO layer, while its positive terminal in linked to the cathode of the Si-TFSC. In this case, the capacitor provides a reverse potential to bleach the PV–EC device. In addition to providing self-powered color changing, the series-integration of each Si-TFSC adds-up the photopotential of the Si-TFSC and can supply electrical power as a general Si-TFSC module does [24–26]. This dual functions PV–EC module provides an alternative solution to building integrated photovoltaic (BIPV) application, which demands not only power generation but also requires the balance of heat and light coming through the BIPV module [27,28].
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Fig. 3. A schematic cross-sectional view of an anodic coloring solid-type PV–EC device with a passivation layer deposited in between the TCO and the anode of the Si-TFSC. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
Control Signal C
a single PV–EC device. The structure of a PV–EC module is shown in Fig. 5.
Control Signal B Control Signal A
2. Experimental The concept of an anodic coloring solid-type PV–EC device was demonstrated by electrodepositing Prussian blue thin film (PBTF) on the TCO layer of a single Si-TFSC, and an electrolyte covered the PBTF layer and the cathode of Si-TFSC. 2.1. Semi-transparent Si-TFSC and module fabrication
Fig. 4. An on/off switching circuit diagram for a tunable PV–EC device. The on state (short circuit) of the control signal A and the control signal B corresponds to EC coloring and capacitor charging mode, while the off state (open circuit) of the control signal A and the on state of the control signal C correspond to EC bleaching and capacitor discharging mode. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
Although the electrical power generated by the PV–EC module takes into account the whole power output of the Si-TFSC module, the color/bleach of an ECTF is achieved by individually considering each pair of the anode and cathode of the Si-TFSC as an independent EC system [24]. The switching mechanism of each pair of the anode and the cathode of this monolithically series connected PV–EC module is similar to that of the previously explained PV–EC device. For the case of anodic coloring ECTF, a TCO layer that is successively deposited with ECTF, is isolated from the anode of the Si-TFSC. The ECTF and the cathode of Si-TFSC are then covered with an electrolyte layer for constituting
A Si-TFSC (cell) was fabricated by sputter and plasma enhanced chemical vapor deposition processes as described previously [29–33]. The structure of the Si-TFSC included a glass substrate; a ZnO:Al transparent conducting oxide layer which served as the anode; a P-type layer; a double junction a-Si:H/mcSi:H thin films which served as the photoelectric conversion layers; a N-type layer; and a ZnO:Al layer and a back reflector metal layer together to constitute the cathode. A pulse laser of 532 nm was then used to remove a portion of the silicon thin film to form the Si-TFSC in stripes, the ratio of anode and cathode blocks was 1:1, respectively. A process flow of fabricating a PV–EC module was similar to that of conventional TFSC module [26,34]. A continuous ZnO:Al layer was preliminary formed on a glass substrate. Then, a first laser scribing step called P1 was conducted to remove the ZnO:Al layer within the areas P1 so as to form a plurality of anode layers, followed by a plasma-enhanced chemical vapor deposition (PECVD) of silicon layer. A scribing step, called P2 was then carried out to completely cut through the silicon layer, forming a plurality of semiconductor thin film. Another ZnO:Al layer and (optionally a back reflector metal layer) were deposited successively, again followed by conducting a third laser scribing step P3 to remove the ZnO:Al layer and the metal layer, so as to form the
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2.2. Preparation of the Prussian blue electroplating solution The preparation of Prussian blue electroplating solution was as follows, 10 mM of K3Fe(CN)6 was added into 50 ml of DI-water, and 10 mM of FeCl3 and 10 mM of KCl were added into 50 ml of DI-water, so as to obtain two solutions. The two solutions were then mixed in a volume ratio of 1:1. 2.3. Electroplating of PBTF on the Si-TFSC A Prussian blue thin film (PBTF) was electrodeposited by traditional three electrode electrochemical method. The Si-TFSC was put into a standard electroplating process set-up. During the electrodeposition process, the PBTF was deposited on a TCO layer, which had been isolated from direct contact with the anode of the Si-TFSC, as shown in Fig. 2. The cathode area of Si-TFSC was masked to prevent contact with electroplating solution. An Autolab PGSTAT30 electrochemical analyzer with the TCO as working, platinum foil as counter and an Ag/AgCl as reference electrodes were utilized to carry out PBTF electrodeposition at a constant potential of 0.6V. The electrodeposition process was performed in dark room, in order to avoid additional photo-induced potential different of the Si-TFSC, which might disturb the uniformity of PBTF forming. During the process, the color of the TCO layer gradually changed from transparent to light blue, indicating that a PBTF had been plated on the TCO surface. The thickness of the PBTF was approximately 200 nm. A reverse potential of 0.2 V was then applied to the TCO layer to turn the PBTF into transparent colorless state. Finally, 0.1 M of LiClO4/PMMA/PC electrolyte with a thickness of about 80 mm was spread on a pair of Si-TFSC touching both the PBTF and the cathode of the Si-TFSC, so as to constitute a single solid type PV–EC device.
3. Results and discussion The photoelectric conversion and the electrochromic properties of a PV–EC device are presented in the following discussion. The IV characteristic of Si-TFSC and module shows insignificant changes in photoelectric properties after been subjected to Prussian blue electrodeposition process as shown in Table 1. For a-Si:H/mc-Si:H TFSC (cell), the measured Voc¼1.32 V, Isc¼27.65 mA, FF¼61.94%, Pmax¼ 22.65 mW and efficiency¼9.44%. As for the a-Si:H/mc-Si:H TFSC module, the measured Voc¼4.00 V, Isc¼26.57 mA, FF¼64.94%, Pmax¼69.09 mW and efficiency¼5.23%. Based on the circuit design described in Fig. 3, the PBTF can be tinted or bleached under sunlight irradiation. The optical contrast and spectral response of a PV–EC device is shown in Fig. 6 and Fig. 7. First, a control mode of the switching apparatus is set to charging and EC coloring mode with power supplied by the Si-TFSC as depicted in Fig. 3a. The PBTF experiences anodic coloration from transparent to blue, the film darkens to its lowest transmission state in 30 s. The bleaching state of PBTF is achieved by disconnecting the circuitry between TCO layer and the anode of the Si-TFSC, and setting the switching apparatus to discharging mode to supply reverse potential to the PBTF as
100
Bleached state 80
Transmittance (%)
cathode layers. When the processes were completed, adjacent SiTFSCs were interconnected with each other in series (as shown in Fig. 5). The transmittance of the Si-TFSC module was adjustable by controlling a P3 interval between the adjacent Si-TFSCs.
393
60
40
Colored state 20
0
2.4. Measurements
400
The Spectrophotometry data of the anodic coloring PV–EC device was obtained in-situ, using a spectrophotometer (model UV-1601PC, Shimadzu, Japan) and a solar simulator. The IV measurement in simulated sunlight followed IEC60904-9 standard.
500
600
700
800
900
Wavelength (nm) Fig. 6. The optical contrast of a Prussian blue thin film in a solid type PV–EC device under sunlight illumination. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
electrolyte EC(+) TCO passivation
cathode
P2
PIN
P1
anode P3
glass
Fig. 5. A schematic cross-sectional view of an anodic coloring solid-type PV–EC module with monolithically series connected Si-TFSC structure. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
Table 1 The characteristics of photoelectric conversion of a tandem type (double junction) Si-TFSC and module after been subjected to Prussian blue thin film electrodeposition process. Type of silicon thin film solar cell
Voc (V)
Isc (mA)
FF (%)
Pmax (mW)
Efficiency (%)
a-Si:H/mc-Si:H TFSC after PBTF electrodeposition a-Si:H/mc-Si:H TFSC module (3 cells in series) after PBTF electrodeposition
1.32 4.00
27.65 26.57
61.94 64.94
22.65 69.09
9.44 5.23n
n
The efficiency of TFSC module is calculated by considering the actual occupied area of the TFSC cells not including the area of the laser scribed anode.
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illustrated in Fig. 3b. The PBTF restores its transparency in 15 s. This result proves the feasibility of the concept of a tunable solid-state PV–EC device. The idea of dual functions PV–EC module with color changing and power generation capabilities is displayed in Fig. 8. The area of the Si-TFSC module (consisting three sets of Si-TFSC connected in series) is 3 cm 4 cm. The widths of the anode and the cathode of each set of the Si-TFSC (cell) are 0.5 cm, respectively. The PBTF layer on each Si-TFSC undergoes oxidation reaction and turns to light blue when exposed to sunlight. The PBTF returns to its transparent state under reduction reaction driven by a capacitor. This colored/bleached mechanism is already explained previously in Fig. 3(a) and (b). The electrochromic behavior of each individual ECTF layer is controlled independently by a switching apparatus, while the power generation capability of the PV–EC module works similarly as a conventional Si-TFSC module does. Here, the electrical production of the PV–EC module is verified by connecting the PV–EC module to a LED bulb. The outputted power of the PV–EC module and the LED bulb luminance intensity vary proportionally with respect to sunlight irradiation, as defined in Fig. 9. This tunable PV–EC module demonstrates for the first time that apart from generating power, Si-TFSC module is also capable to change its tint in response to the exposure to sunlight. The present study employs a-Si:H/mc-Si:H thin film solar cell to drive the ECTF color change. Due to the high photopotential of 100
Transmittance at 690 nm (%)
90
this double junction Si-TFSC (of about 1.3 V) which potential is more than enough to darken a PBTF. This ECTF tints immediately to its lowest transmission state under sunlight illumination. No gradual color change of the PBTF is observed under different solar irradiance, even for a minimum solar simulator irradiance power of 440 W/m2. However, in real application, the range of solar irradiance is much broader, and it is possible to induce gray-scale in the PV–EC module. This over photopotential problem can also be solved by the application of a single-junction hydrogenated amorphous silicon TFSC (a-Si:H), which Voc is closer to the redox potential of PBTF [35,36], and it is expected that the PBTF will have different gray-scale in response to sunlight exposure. In this paper, the concept of tunable PV–EC device and module is realized by electroplating an ECTF on a TCO layer. However, from the view of mass production, it is preferable to apply vacuum deposition such as sputtering process to fabricate the device. In this case, an ECTF (EC(þ)), an ion storage layer (EC( )) and a solid electrolyte can be successively vacuum deposited on the Si-TFSC in accord with the PV–EC device and module structure of Fig. 3 and Fig. 5, respectively. Though the current study applied Silicon TFSC as solar driven electrical source, however, other thin film solar cells including Copper Indium Gallium Selenide (CIGS) and Cadmium Telluride (CdTe) can also be integrated into the PV–EC device. With a different structure design of TFSCs, the PV–EC device and module might find applications in tunable self-powered smart glasses for building and transportation vehicles, tunable self-powered displays and optical sensors and so forth.
light on
80 70 60 50 40 30
light off
20 10 0 0
50
100
150
200
250
300
Time (secs) Fig. 7. The spectral response of a Prussian blue thin film in a solid type PV–EC device in the bleached and colored state at 690 nm wavelength. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
Fig. 9. The dependence of a PV–EC module power generation and LED bulb illumination intensity to solar irradiance. The PV power and LED light vary proportionally with respect to sunlight irradiation.
Fig. 8. Photograph of a dual function PV–EC module, the colored and bleached state of a single PBTF in a PV–EC device can be switched independently even under light exposure, while the PV–EC module generates power to light LED bulb. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
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4. Conclusions We have proposed an innovative tunable solid-type PV–EC device and module for Si-TFSC and smart glass applications. The concept of an anodic coloring solid-type PV–EC device is demonstrated by electrodepositing a PBTF on the TCO layer of a single SiTFSC, and an electrolyte covered the TCO layer and the cathode of Si-TFSC. The on/off switch control is accomplished by isolating the TCO from the anode of the Si-TFSC. When a control mode is set to charging and EC coloring mode, the PBTF can be tinted through light irradiation. The anode of the Si-TFSC is linked to the TCO layer, while coupling the anode and cathode of the Si-TFSC to a capacitor with similar polarity. Bleaching state is achieved by disconnecting the circuitry between TCO layer and the anode of the Si-TFSC, and setting the switching apparatus to discharging mode to supply reverse potential to the PBTF. The PBTF undergoes anodic coloration from transparent to blue under sunlight, the film darkens to its lowest transmission state in 30 s and restores its transparency in 15 s. An anodic coloring PV–EC module is built based on monolithic series-connected Si-TFSC, which fabrication method is compatible with the traditional Si-TFSC manufacturing process. The ECTFs are then simultaneously deposited on the electrodes (TCO and the cathode) of the Si-TFSC, both of which covered with an electrolyte. The electrochromic behavior of each ECTF layer is achieved by individually considering each pair of the anode and cathode of Si-TFSC (cell) as a single EC system and is independently controlled by a switching apparatus. The power generation of the PV– EC module is verified by connection to a LED bulb. The dependences of LED luminance intensity to various solar irradiances vary proportionally with the output power of the PV–EC module. This tunable PV–EC module proves for the first time that apart from generating power, Si-TFSC module is also capable to change its tint in response to the exposure to sunlight.
Acknowledgment Funding for this work was provided by (Department of Industrial Technology) Ministry of Economics Affairs, Taiwan.
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Solar Energy Materials & Solar Cells journal homepage: www.elsevier.com/locate/solmat
Tunable photovoltaic electrochromic device and module Lee-May Huang a,n, Chen-Pang Kung b, Chih-Wei Hu c, Cheng-Yu Peng a, Han-Chang Liu a a b c
Green Energy & Environment Research Laboratories, Industrial Technology Research Institute, Chutung, Hsinchu 31040, Taiwan Display Technology Center, Industrial Technology Research Institute, Chutung, Hsinchu 31040, Taiwan Department of Chemical Engineering, National Taiwan University, Taipei 10617, Taiwan
a r t i c l e i n f o
a b s t r a c t
Article history: Received 19 March 2012 Received in revised form 25 June 2012 Accepted 18 July 2012 Available online 17 August 2012
An innovative tunable photovoltaic electrochromic (PV–EC) device concept, in which planar distribution of silicon thin film solar cells (Si-TFSCs) structure with successively deposited electrochromic thin films (ECTFs) and electrolyte layer is demonstrated. The switching mechanism of a PV–EC device is realized by electrically separating the electrodes of the Si-TFSCs and ECTFs, and the connection of the two systems is achieved through an external switching apparatus that links each Si-TFSC and ECTF through one transparent conductive oxide (TCO) layer. According to the PV–EC device structure, Si-TFSCs arranged in stripes makes large area module fabrication feasible, while Si-TFSCs arranged in arrays allows pixel structure design possible. When illuminated by sunlight under short-circuit condition, the photopotential outputted from a single Si-TFSC drives ECTF layer to change color, simultaneously a portion of the photocurrent is stored in a capacitor. Once the ECTF is tinted, no power is needed to retain its colored state due to the bi-stable property of the electrochromic material. In order to revert the PV–EC device to its bleached state, the capacitor supplies reverse potential to cause discoloration of the ECTF. In addition, the monolithically integrated Si-TFSC module generates electricity to a connected load. In view of photoelectric conversion and optical modulation properties, the PV–EC device can both function as a Si-TFSC module and as a self powered smart glass, which has great advantages in green energy application. & 2012 Elsevier B.V. All rights reserved.
Keywords: Semi-transparent thin film solar cell Tunable solid type photovoltaic electrochromic device Solar powered smart glass Building integrated photovoltaics
1. Introduction An electrochromic (EC) device has reversible changes in the optical properties due to the electric field or current induced redox reaction of conductive materials and electrolyte [1–3]. The EC device provides tunable color changing that control glare and solar heat gain [4,5]. Electrochromic device can be divided into solid type and liquid type. A typical solid type EC device includes two transparent substrates that sandwiched five electroactive layers such as: a transparent conductive layer, an electrochromic layer, a solid electrolyte, an ion storage layer, and another transparent conductive layer deposited in-between the two substrates to constitute a battery-like structure [6]. The structure of a solution-type EC device is formed by two transparent conductive substrates, which are bonded by polymer adhesive in a direction facing an electrode layer, and an EC solution is disposed between the transparent conductive substrates [7–9]. The integration of photovoltaic (PV) and electrochromic (EC) device provides better efficiency in term of energy saving, for the photovoltaic electrochromic (PV–EC) device can achieve color
n
Corresponding author. Tel: þ886 3 5919389, fax: þ 886 3 5822157. E-mail address: [email protected] (L.-M. Huang).
0927-0248/$ - see front matter & 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.solmat.2012.07.021
change in EC layers from the sunlight converted electrical power. This self-powered smart glass can adjust the color of the EC device according to the intensity of sunlight irradiation to reduce indoor heat and to provide comfort. Until recently, there are two types of solar powered EC devices available, the first one is tandem structure silicon (Si) based PV–EC device [10–15] and the second one is dye-sensitized solar cell (DSSC) based photoelectrochromic (PEC) device [16–21], both of which belong to NREL technology. The silicon based PV–EC technology constitutes stacked layers of the photoabsorbers and EC active materials in one single device. However, this tandem-structure PV– EC device encounters problems such as low colored/bleached optical contrast, short circuit between the deposited layers and difficulty in producing large area device [15,20]. A photoelectrochromic technology separates the photoabsorbing layers of DSSC [17] and the EC layer to the anode and the cathode, respectively for constituting a device. This PEC device can be described as having EC material inserted into DSSC, and has become the mostly studied photovoltaic integrated electrochromic technology. However, to apply such a structure to practical applications, many problems need to be overcome, such as long-term stability of the photo-absorbing layers and the possibility of developing devices having larger sizes. An innovative solution type PV–EC device based on the integration of planar structure Si-TFSC and electrochromic solution has been
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demonstrated in our previous paper [22]. The Si-TFSCs are arranged in stripes with laser scribing process, the cathode blocks are Si-TFSC cells, and the area outside the cathode blocks are anode which composes only of transparent conductive oxide (TCO). The overall transparency and the color contrast of this solution type PV–EC device are enhanced, as compared to tandem Si based PV–EC device [10–15]. However, this solution type PV–EC may cause erosion in SiTFSC, and unless redox couples are added to the EC solution [23], the self-bleaching duration is too long for practical applications. In this paper, a solid-type PV–EC device which can solve the problem encountered by solution type PV–EC device is introduced. The structure of the solid-type PV–EC device consists of planarly distributed semi-transparent Si-based TFSC (cells) with exposed transparent anode layers in-between the cathodes layers, configured in superstrate structure as shown is Fig. 1. Electrochromic thin films (ECTFs) are successively deposited on the anodes and cathodes of the Si-TFSC. An electrolyte layer is then formed on the surfaces of the Si-TFSC cells to contact the ECTFs simultaneously. The electronic current generated by the Si-TFSCs is transformed to ionic current in the electrolyte layer that migrates transversely along the electrodes of the Si-TFSCs in order to maintain charge neutrality in the EC system. The electrolyte must be a good ionic current conductor, but it must be an isolator for electronic current generated by the Si-TFSCs. In concordance with the structure of this solid type PV–EC device, Si-TFSCs arranged in stripes makes large area monolithic seriesconnected Si-TFSC module fabrication feasible, while Si-TFSCs arranged in arrays allows pixel structure design possible. Since the Si-TFSCs and EC system have common electrodes, the ECTFs change tint immediately upon sunlight exposure. The SiTFSCs generate photopotential and photocurrent, and at the same time the electric field of the Si-TFSCs stimulates redox reaction in the ECTFs. The cations and anions in the electrolyte are attracted to the anodes and cathodes of the Si-TFSCs, respectively, inducing color change in the ECTFs. Unlike our previously reported solution type PV–EC device that has self-bleaching property [22], for a solid type EC system, once an ECTF changes tint, a reverse potential is needed to bleach the ECTF. In this study, the idea of anodic coloring tunable PV–EC device is realized by electrically separating the electrodes of the Si-TFSCs and ECTFs, and the connection of the two systems is achieved through an external switching apparatus that links each Si-TFSC and ECTF through one transparent conductive oxide (TCO) layer [24]. The TCO layer is designed to be electrically isolated from the electrodes of the Si-TFSC. Here, two passivation methods have been investigated, the first method is to employ a laser scribing process that peels off a portion of an exposed area of the anode of the Si-TFSC to form a TCO layer as depicted in Fig. 2. The second one is to deposit a passivation layer such as SiOx or SiNx on a
Fig. 1. Working principle of a solid-type PV–EC device. PIN refers to Si-TFSC.
391
electrolyte EC (-) cathode EC (+)
PIN
TCO
anode glass
Fig. 2. A schematic cross-sectional view of an anodic coloring solid-type PV–EC device with a portion of the anode of the Si-TFSC peeled off to form a TCO layer. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
portion of the anodes of the Si-TFSC to isolate the TCO layer from the anode of the Si-TFSC as shown in Fig. 3. Once the passivation process is completed, an anodic coloring ECTF (EC(þ)) is disposed on the TCO layer. A tunable solid type PV–EC device is readily established by covering the anodic coloring ECTF and the cathode of the Si-TFSC with an electrolyte layer. A cathodic coloring ECTF (EC( )) can be optionally deposited on the cathode of the Si-TFSC to improve the performance of the PV–EC device. The on/off switching mechanism of the solar-powered anodic coloring PV–EC device is as explained in Fig. 4. In reference to the circuit diagram, PV represents a Si-TFSC and EC represents an electrochromic system. The output signal controls the connection between the anode of the Si-TFSC with the TCO layer which determine the color/bleach of the ECTF. A charge storing device (shown as a capacitor) is connected to the PV and the EC systems. The switching apparatus depicted in Fig. 4 can enter various control modes through switch control signals A, B, and C. Based on the aforementioned circuit design, there are two types of control modes for the anodic coloring PV–EC device. 1.1. The EC coloring and capacitor charging mode The charge-storing device is coupled to Si-TFSC with the same polarity. The TCO layer is connected to the anode of the Si-TFSC, acting as the anode of the ECTF, while sharing the same cathode as that of the Si-TFSC, as referred to Fig. 3A. When exposes to sunlight, the anodic coloring ECTF changes its tint, while concurrently stores the electrical power generated by the Si-TFSC. Once the ECTF tinted, no additional power is needed to retain the colored state due to the bi-stable property of the EC device and the electrical circuit is then disconnected. 1.2. The EC bleaching and capacitor discharging mode In order to revert the PV–EC device to its bleached state, the capacitor supplies reverse potential to cause discoloration of the ECTF, as illustrated in Fig. 3(b). In this mode, the anode of the SiTFSC is disconnected from TCO and the capacitor is switched to the opposite polarity of Si-TFSC. Here, the negative terminal of the capacitor is connected to the TCO layer, while its positive terminal in linked to the cathode of the Si-TFSC. In this case, the capacitor provides a reverse potential to bleach the PV–EC device. In addition to providing self-powered color changing, the series-integration of each Si-TFSC adds-up the photopotential of the Si-TFSC and can supply electrical power as a general Si-TFSC module does [24–26]. This dual functions PV–EC module provides an alternative solution to building integrated photovoltaic (BIPV) application, which demands not only power generation but also requires the balance of heat and light coming through the BIPV module [27,28].
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Fig. 3. A schematic cross-sectional view of an anodic coloring solid-type PV–EC device with a passivation layer deposited in between the TCO and the anode of the Si-TFSC. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
Control Signal C
a single PV–EC device. The structure of a PV–EC module is shown in Fig. 5.
Control Signal B Control Signal A
2. Experimental The concept of an anodic coloring solid-type PV–EC device was demonstrated by electrodepositing Prussian blue thin film (PBTF) on the TCO layer of a single Si-TFSC, and an electrolyte covered the PBTF layer and the cathode of Si-TFSC. 2.1. Semi-transparent Si-TFSC and module fabrication
Fig. 4. An on/off switching circuit diagram for a tunable PV–EC device. The on state (short circuit) of the control signal A and the control signal B corresponds to EC coloring and capacitor charging mode, while the off state (open circuit) of the control signal A and the on state of the control signal C correspond to EC bleaching and capacitor discharging mode. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
Although the electrical power generated by the PV–EC module takes into account the whole power output of the Si-TFSC module, the color/bleach of an ECTF is achieved by individually considering each pair of the anode and cathode of the Si-TFSC as an independent EC system [24]. The switching mechanism of each pair of the anode and the cathode of this monolithically series connected PV–EC module is similar to that of the previously explained PV–EC device. For the case of anodic coloring ECTF, a TCO layer that is successively deposited with ECTF, is isolated from the anode of the Si-TFSC. The ECTF and the cathode of Si-TFSC are then covered with an electrolyte layer for constituting
A Si-TFSC (cell) was fabricated by sputter and plasma enhanced chemical vapor deposition processes as described previously [29–33]. The structure of the Si-TFSC included a glass substrate; a ZnO:Al transparent conducting oxide layer which served as the anode; a P-type layer; a double junction a-Si:H/mcSi:H thin films which served as the photoelectric conversion layers; a N-type layer; and a ZnO:Al layer and a back reflector metal layer together to constitute the cathode. A pulse laser of 532 nm was then used to remove a portion of the silicon thin film to form the Si-TFSC in stripes, the ratio of anode and cathode blocks was 1:1, respectively. A process flow of fabricating a PV–EC module was similar to that of conventional TFSC module [26,34]. A continuous ZnO:Al layer was preliminary formed on a glass substrate. Then, a first laser scribing step called P1 was conducted to remove the ZnO:Al layer within the areas P1 so as to form a plurality of anode layers, followed by a plasma-enhanced chemical vapor deposition (PECVD) of silicon layer. A scribing step, called P2 was then carried out to completely cut through the silicon layer, forming a plurality of semiconductor thin film. Another ZnO:Al layer and (optionally a back reflector metal layer) were deposited successively, again followed by conducting a third laser scribing step P3 to remove the ZnO:Al layer and the metal layer, so as to form the
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2.2. Preparation of the Prussian blue electroplating solution The preparation of Prussian blue electroplating solution was as follows, 10 mM of K3Fe(CN)6 was added into 50 ml of DI-water, and 10 mM of FeCl3 and 10 mM of KCl were added into 50 ml of DI-water, so as to obtain two solutions. The two solutions were then mixed in a volume ratio of 1:1. 2.3. Electroplating of PBTF on the Si-TFSC A Prussian blue thin film (PBTF) was electrodeposited by traditional three electrode electrochemical method. The Si-TFSC was put into a standard electroplating process set-up. During the electrodeposition process, the PBTF was deposited on a TCO layer, which had been isolated from direct contact with the anode of the Si-TFSC, as shown in Fig. 2. The cathode area of Si-TFSC was masked to prevent contact with electroplating solution. An Autolab PGSTAT30 electrochemical analyzer with the TCO as working, platinum foil as counter and an Ag/AgCl as reference electrodes were utilized to carry out PBTF electrodeposition at a constant potential of 0.6V. The electrodeposition process was performed in dark room, in order to avoid additional photo-induced potential different of the Si-TFSC, which might disturb the uniformity of PBTF forming. During the process, the color of the TCO layer gradually changed from transparent to light blue, indicating that a PBTF had been plated on the TCO surface. The thickness of the PBTF was approximately 200 nm. A reverse potential of 0.2 V was then applied to the TCO layer to turn the PBTF into transparent colorless state. Finally, 0.1 M of LiClO4/PMMA/PC electrolyte with a thickness of about 80 mm was spread on a pair of Si-TFSC touching both the PBTF and the cathode of the Si-TFSC, so as to constitute a single solid type PV–EC device.
3. Results and discussion The photoelectric conversion and the electrochromic properties of a PV–EC device are presented in the following discussion. The IV characteristic of Si-TFSC and module shows insignificant changes in photoelectric properties after been subjected to Prussian blue electrodeposition process as shown in Table 1. For a-Si:H/mc-Si:H TFSC (cell), the measured Voc¼1.32 V, Isc¼27.65 mA, FF¼61.94%, Pmax¼ 22.65 mW and efficiency¼9.44%. As for the a-Si:H/mc-Si:H TFSC module, the measured Voc¼4.00 V, Isc¼26.57 mA, FF¼64.94%, Pmax¼69.09 mW and efficiency¼5.23%. Based on the circuit design described in Fig. 3, the PBTF can be tinted or bleached under sunlight irradiation. The optical contrast and spectral response of a PV–EC device is shown in Fig. 6 and Fig. 7. First, a control mode of the switching apparatus is set to charging and EC coloring mode with power supplied by the Si-TFSC as depicted in Fig. 3a. The PBTF experiences anodic coloration from transparent to blue, the film darkens to its lowest transmission state in 30 s. The bleaching state of PBTF is achieved by disconnecting the circuitry between TCO layer and the anode of the Si-TFSC, and setting the switching apparatus to discharging mode to supply reverse potential to the PBTF as
100
Bleached state 80
Transmittance (%)
cathode layers. When the processes were completed, adjacent SiTFSCs were interconnected with each other in series (as shown in Fig. 5). The transmittance of the Si-TFSC module was adjustable by controlling a P3 interval between the adjacent Si-TFSCs.
393
60
40
Colored state 20
0
2.4. Measurements
400
The Spectrophotometry data of the anodic coloring PV–EC device was obtained in-situ, using a spectrophotometer (model UV-1601PC, Shimadzu, Japan) and a solar simulator. The IV measurement in simulated sunlight followed IEC60904-9 standard.
500
600
700
800
900
Wavelength (nm) Fig. 6. The optical contrast of a Prussian blue thin film in a solid type PV–EC device under sunlight illumination. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
electrolyte EC(+) TCO passivation
cathode
P2
PIN
P1
anode P3
glass
Fig. 5. A schematic cross-sectional view of an anodic coloring solid-type PV–EC module with monolithically series connected Si-TFSC structure. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
Table 1 The characteristics of photoelectric conversion of a tandem type (double junction) Si-TFSC and module after been subjected to Prussian blue thin film electrodeposition process. Type of silicon thin film solar cell
Voc (V)
Isc (mA)
FF (%)
Pmax (mW)
Efficiency (%)
a-Si:H/mc-Si:H TFSC after PBTF electrodeposition a-Si:H/mc-Si:H TFSC module (3 cells in series) after PBTF electrodeposition
1.32 4.00
27.65 26.57
61.94 64.94
22.65 69.09
9.44 5.23n
n
The efficiency of TFSC module is calculated by considering the actual occupied area of the TFSC cells not including the area of the laser scribed anode.
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illustrated in Fig. 3b. The PBTF restores its transparency in 15 s. This result proves the feasibility of the concept of a tunable solid-state PV–EC device. The idea of dual functions PV–EC module with color changing and power generation capabilities is displayed in Fig. 8. The area of the Si-TFSC module (consisting three sets of Si-TFSC connected in series) is 3 cm 4 cm. The widths of the anode and the cathode of each set of the Si-TFSC (cell) are 0.5 cm, respectively. The PBTF layer on each Si-TFSC undergoes oxidation reaction and turns to light blue when exposed to sunlight. The PBTF returns to its transparent state under reduction reaction driven by a capacitor. This colored/bleached mechanism is already explained previously in Fig. 3(a) and (b). The electrochromic behavior of each individual ECTF layer is controlled independently by a switching apparatus, while the power generation capability of the PV–EC module works similarly as a conventional Si-TFSC module does. Here, the electrical production of the PV–EC module is verified by connecting the PV–EC module to a LED bulb. The outputted power of the PV–EC module and the LED bulb luminance intensity vary proportionally with respect to sunlight irradiation, as defined in Fig. 9. This tunable PV–EC module demonstrates for the first time that apart from generating power, Si-TFSC module is also capable to change its tint in response to the exposure to sunlight. The present study employs a-Si:H/mc-Si:H thin film solar cell to drive the ECTF color change. Due to the high photopotential of 100
Transmittance at 690 nm (%)
90
this double junction Si-TFSC (of about 1.3 V) which potential is more than enough to darken a PBTF. This ECTF tints immediately to its lowest transmission state under sunlight illumination. No gradual color change of the PBTF is observed under different solar irradiance, even for a minimum solar simulator irradiance power of 440 W/m2. However, in real application, the range of solar irradiance is much broader, and it is possible to induce gray-scale in the PV–EC module. This over photopotential problem can also be solved by the application of a single-junction hydrogenated amorphous silicon TFSC (a-Si:H), which Voc is closer to the redox potential of PBTF [35,36], and it is expected that the PBTF will have different gray-scale in response to sunlight exposure. In this paper, the concept of tunable PV–EC device and module is realized by electroplating an ECTF on a TCO layer. However, from the view of mass production, it is preferable to apply vacuum deposition such as sputtering process to fabricate the device. In this case, an ECTF (EC(þ)), an ion storage layer (EC( )) and a solid electrolyte can be successively vacuum deposited on the Si-TFSC in accord with the PV–EC device and module structure of Fig. 3 and Fig. 5, respectively. Though the current study applied Silicon TFSC as solar driven electrical source, however, other thin film solar cells including Copper Indium Gallium Selenide (CIGS) and Cadmium Telluride (CdTe) can also be integrated into the PV–EC device. With a different structure design of TFSCs, the PV–EC device and module might find applications in tunable self-powered smart glasses for building and transportation vehicles, tunable self-powered displays and optical sensors and so forth.
light on
80 70 60 50 40 30
light off
20 10 0 0
50
100
150
200
250
300
Time (secs) Fig. 7. The spectral response of a Prussian blue thin film in a solid type PV–EC device in the bleached and colored state at 690 nm wavelength. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
Fig. 9. The dependence of a PV–EC module power generation and LED bulb illumination intensity to solar irradiance. The PV power and LED light vary proportionally with respect to sunlight irradiation.
Fig. 8. Photograph of a dual function PV–EC module, the colored and bleached state of a single PBTF in a PV–EC device can be switched independently even under light exposure, while the PV–EC module generates power to light LED bulb. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
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4. Conclusions We have proposed an innovative tunable solid-type PV–EC device and module for Si-TFSC and smart glass applications. The concept of an anodic coloring solid-type PV–EC device is demonstrated by electrodepositing a PBTF on the TCO layer of a single SiTFSC, and an electrolyte covered the TCO layer and the cathode of Si-TFSC. The on/off switch control is accomplished by isolating the TCO from the anode of the Si-TFSC. When a control mode is set to charging and EC coloring mode, the PBTF can be tinted through light irradiation. The anode of the Si-TFSC is linked to the TCO layer, while coupling the anode and cathode of the Si-TFSC to a capacitor with similar polarity. Bleaching state is achieved by disconnecting the circuitry between TCO layer and the anode of the Si-TFSC, and setting the switching apparatus to discharging mode to supply reverse potential to the PBTF. The PBTF undergoes anodic coloration from transparent to blue under sunlight, the film darkens to its lowest transmission state in 30 s and restores its transparency in 15 s. An anodic coloring PV–EC module is built based on monolithic series-connected Si-TFSC, which fabrication method is compatible with the traditional Si-TFSC manufacturing process. The ECTFs are then simultaneously deposited on the electrodes (TCO and the cathode) of the Si-TFSC, both of which covered with an electrolyte. The electrochromic behavior of each ECTF layer is achieved by individually considering each pair of the anode and cathode of Si-TFSC (cell) as a single EC system and is independently controlled by a switching apparatus. The power generation of the PV– EC module is verified by connection to a LED bulb. The dependences of LED luminance intensity to various solar irradiances vary proportionally with the output power of the PV–EC module. This tunable PV–EC module proves for the first time that apart from generating power, Si-TFSC module is also capable to change its tint in response to the exposure to sunlight.
Acknowledgment Funding for this work was provided by (Department of Industrial Technology) Ministry of Economics Affairs, Taiwan.
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