PHOTOVOLTAIC ELECTROCHROMIC MODULE ...

33rd European Photovoltaic Solar Energy Conference and Exhibition

PHOTOVOLTAIC ELECTROCHROMIC MODULE WITH UNIFORM COLOR CHANGE Lee-May Huang, Cheng-Yu Peng, Chih-Hung Chen, Han-Chang Liu, Chorng-Jye Huang Green Energy & Environment Research Laboratories, Industrial Technology Research Institute 195, Sec.4, Chung Hsing Rd., Chutung, Hsinchu, Taiwan 31057, R.O.C. Corresponding author: Lee-May Huang, Phone: +886 (3)5912252, E-mail: [email protected]

ABSTRACT: A novel photovoltaic electrochromic (PV-EC) module for building integrated PV (BIPV) applications has been developed. The PV-EC module includes planarly distributed silicon thin film solar cells (Si-TFSCs) with successively deposited electrochromic thin films and stripe-shaped electrolyte layers. In addition to providing selfpowered color changing, the series-integration of each Si-TFSC adds-up the photopotential of the Si-TFSCs and can supply electrical power as a general Si-TFSC module does. This dual functions PV-EC module provides an alternative solution to BIPV module, which demands not only power generation but also requires the balance of heat and light coming through the BIPV module. Keywords: Photovoltaic, Electrochromic, BIPV, planar structured PV-EC

1

INTRODUCTION

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.

Building integrated PV (BIPV) systems can provide crucial supplementary source of electrical energy from any surface available within the building’s envelope. Glazing integrated photovoltaics are among the most promising solutions due to heating and cooling savings in addition to electricity production [1]. Electrochromic (EC) device has been developed for retrofitting existing windows, to dynamically control transmitted light, glare, and solar heat, while allowing light in and preserving view out [2]. The integration of PV module and EC device provides better efficiency in term of energy saving, for the PV-EC device can achieve color change in the EC layers from the sunlight converted electrical power (as described in figure 1). 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.

Figure 2: Photograph showing the color changing of planar structure PV-EC device under exposure to sunlight, the active area of the PV-EC device is 5.5cm × 5cm, with each Si-TFSC stripe area=0.5cm × 5cm [7]. An innovative planar structure PV-EC device in which electrochromic materials are disposed between a semitransparent silicon thin film solar cells (Si-TFSCs) and a transparent non-conductive substrate, was recently developed by our group [7-9]. The structure of the PV-EC device consists of planarly distributed semi-transparent SiTFSCs with exposed transparent anode layers in-between the cathodes layers, configured in a superstrate structure, as shown in figure 2. The cathode of the Si-TFSCs are arranged in stripes, and the area outside the cathode blocks are anode which composed only of transparent conductive oxide (TCO). In solution type planar structure PV-EC device, organic electrochromic solution is disposed between a planarly distributed Si-TFSC and a transparent nonconductive substrate [7]. Figure 3 shows the working principle of the solution type PV-EC device. When sunlight enters the PV-EC device, the Si-TFSCs generate electron-hole pairs. The photopotential difference at the anode and cathode of the Si-TFSC induces electrochemical redox reaction of the electrochromic solution. Simultaneously, cathodic and anodic electrochromic color change occurs at the cathode and

Figure 1: The integration of see-through type BIPV module and EC smart glass. Until recently, there are two types of solar powered EC devices available, the first one is tandem structure silicon (Si) based PV-EC device [3,4] and the second one is dyesensitized solar cell (DSSC) based photoelectrochromic (PEC) device [5], both of which belong to National Renewable Energy Laboratory (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 [3]. A photoelectrochromic technology separates the photoabsorbing layers of DSSC [6] and the EC layer to the anode and the cathode respectively for constituting a device. This PEC device can be described as having EC

2577


anode of the Si-TFSC. The electronic current produced by the Si-TFSC is converted into an ionic current in the electrochromic solution. Thus, even though the anode and the cathode both contact the electrochromic solution when conducted, the problem of short circuit would not occur. Since the Si-TFSC cells are designed to be evenly distributed on a transparent substrate, the electric fields in the peripheral area and the center area are uniform, with a result that the color density in the peripheral and the center areas of the PV-EC device are the same, regardless of the size of the device. When the intensity of the sunlight declines, the electrons and holes generated by the Si-TFSC are reduced and the photopotential and photocurrent of the solution type PV-EC device decreases gradually. Due to the selferasable property of the solution type electrochromic material, the color of the electrochromic materials fade out gradually and resume to the original transparent state.

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/μcSi: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.

electronic current (thin film solar cell)

PV

Red/Ox reaction ionic current (electrochromic system)

EC

Figure 4: Planar structure PV-EC module with two types of electrolyte configurations: (a). a continuous electrolyte layer covering the whole PV-EC module, and (b) as striped electrolyte layers covering each pair of Si-TFSC forming an individual EC system.

Figure 3: Working principle of solution type planar structure PV-EC device [7]. 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 [3,4]. However, this solution type PV-EC may cause erosion in Si-TFSC, and unless redox couples are added to the EC solution, the selfbleaching duration is too long for practical applications. A solid-type planar structure PV-EC device which can solve the problem encountered by solution type PVEC device is introduced [8]. Here, the electrochromic thin films (ECTFs) are successively deposited on the anodes of the Si-TFSCs. An electrolyte layer is then formed on the surfaces of the Si-TFSC cells to contact the ECTFs (EC(+) in figure 4) 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, the series-integration of each Si-TFSC adds-up the photopotential of the Si-TFSC, making large area PV-EC module fabrication feasible. In this paper, the effects of two different electrolyte configurations: continuous electrolyte layer and stripeshaped electrolyte layers, on the color change performance of sunlight irradiated PV-EC module are investigated.

2

A process flow of fabricating a PV-EC module was similar to that of conventional TFSC module]. 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 cuts 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 cathode layers. When the processes were completed, adjacent Si-TFSCs were interconnected with each other in series (as shown in Figure 4). The transmittance of the Si-TFSC module was adjustable by controlling a P3 interval between the adjacent Si-TFSCs. 2.2 Preparation of the Prussian Blue electroplating solution The preparation of Prussian Blue electroplating solution was as follows, 10mM of K3Fe(CN)6 was added into 50ml of DI-water, and 10mM of FeCl3 and 10mM of KCl were added into 50ml of DI-water, so as to obtain two solutions. The two solutions were then mixed in a volume ratio of 1:1.

EXPERIMENTAL 2.3 Electroplating of PBTF on the Si-TFSC A Prussian Blue thin film (PBTF) was electrodeposited by traditional three electrode electrochemical method [8]. The Si-TFSC was put into a

2.1 Semi-transparent Si-TFSC and module fabrication A Si-TFSC (cell) was fabricated by sputter and plasma enhanced chemical vapor deposition processes. The

2578


standard electroplating process set-up. During the electrodeposition process, the PBTF (EC(+) in figure 4) was deposited on a TCO layer, which had been isolated from direct contact with the anode of the Si-TFSC. 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 electroplating 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 -0.2V was then applied to the TCO layer to turn the PBTF into transparent colorless state.

Although the electrical power generated by the PVEC 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 [8]. 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.

2.4 Spreading electrolyte on Si-TFSC 0.1M of LiClO4/PMMA/PC electrolyte was spread as: (a). a continuous layer covering the whole PV-EC module, and (b) as striped layers covering each pair of Si-TFSC, touching both the anode PBTF and the cathode of the SiTFSC, forming an individual EC system, as shown in figure 4. 2.5 Measurements 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.

3

Figure 5: Non-uniform color change of sunlight irradiated PV-EC module with continuous electrolyte configuration.

RESULTS AND DISCUSSION

When sunlight irradiates the PV-EC module, oxidation reaction occurs on the anode side of the EC system, and PBTFs start to change its tint from transparent colorless to light blue. However, figure 5 reveals that for the case of continuous electrolyte covering the Si-TFSCs (as shown in Figure 4(a)), as the duration of time exposure elapsed, the degree of color change of the four sets of the PBTFs are different. The non-uniform color change is expected to be induced by the potential difference of each set of the Si-TFSC connected in series, causing unequal redox reaction throughout the whole EC system. As a result, over-oxidation is likely to occur near the anode edge of the PV-EC module, and over-reduction appears near the cathode edge of the PV-EC module. However, for the case of stripe-shaped electrolyte layer, the PV-EC module changes its tint uniformly. Here, the cations and anions within each pair of the cathode/anode of the Si-TFSCs can be confined inside a single EC system, as shown in figure 4(b) and figure 5. Since the stripe-shaped electrolyte is mutually independent, the electrolyte of one EC system is not in contact with the adjacent electrolyte. Charge imbalance caused by covering a continuous electrolyte layer on the serially connected Si-TFSC module can be avoided. Under sunlight irradiation, the PBTFs change color evenly, no over-oxidation or over-reduction reactions is observed after 20 minutes of continuous sunlight irradiation, as indicated in figure 6.

Figure 6 Uniform color change of sunlight irradiated PVEC module with stripe-shaped electrolyte configuration 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 [8]. For aSi:H/μc-Si:H TFSC (cell), the measured Voc=1.32 V, Isc=27.65 mA, FF=61.94%, Pmax=22.65 mW and efficiency=9.44%. As for the a-Si:H/μc-Si:H TFSC module, the measured Voc=4.00 V, Isc=27.57 mA, FF=64.94%, Pmax=69.09mW and efficiency =5.23%.

2579


feasibility of the concept of a tunable solid-state PV-EC device with stripe-shaped electrolyte structure. Table 1: The characteristics of photoelectric conversion of a-Si:H/μc-Si:H-TFSC and module [8] Isc (mA)

FF (%)

Pmax (mW)

ɳ (%)

a-Si:H/μcSi:H TFSC

1.32

27.65

61.94

22.65

9.44

4.00

26.57

64.94

69.09

5.23*

* the efficiency of theSi-TFSC module is calculated by considering the actual occupied area of the TFSC cells not including the area of the laser scribed anode

100

Transmittance at 690 nm (%)

Voc (V)

a-Si:H/μcSi:H TFSC module

Figure 7: Photograph of a dual function PV-EC module, the colored and bleached state of a single PBTF in a PVEC device can be switched independently even under light exposure, while the PV-EC module generates power to light LED bulb [8].

90

Type of solar cell

The power generation of the PV-EC module is verified by connection to a LED bulb, as shown in Figure 9. The dependences of LED luminance intensity to various solar irradiances vary proportionally with the output power of the PV-EC module

light on

80 70 60 50 40 30

4

light off

20

The present study shows that planar structure PV-EC module having a plurality of stripe-shaped electrolyte with each single stripe individually covering each pair of SiTFSC can effectively mitigate the phenomenon of charge imbalance, so as to have a better uniformity in color change during light irradiation. 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 anode of the Si-TFSC. Each pair of the Si-TFSC are covered with stripe-shaped electrolyte. The electrochromic behavior of each ECTF layer is achieved by individually considering each set of the anode and cathode of Si-TFSC (cell) as a single EC system. This 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.

10 0 0

50

100

150

200

250

CONCLUSIONS

300

Time (secs)

Figure 8: The spectral response of a PBTF in a solid type PV-EC device in the bleached and colored state at 690 nm wavelength [8].

5

ACKNOWLEDGEMENT CONCLUSIONS

The financial support provided by Bureau of Energy (Grant No. 106-D0104) is gratefully acknowledged. 6

Figure 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 [8].

REFERENCES

[1] G. Gorgolis, D. Karamanis, Sol. Energy Mater. Sol. Cells 144 (2016) 559. [2] P.M.S. Monk, R.J. Mortimer, D.R. Rosseinsky, Electrochromism and Electrochromic Devices, Cambridge University Press, Cambridge, 2007. [3] K. Benson, H.M. Branz, Design goals and challenges for a photovoltaic powered electrochromic window covering, Sol. Energy Mater. Sol. Cells 39 (1995) 203211. [4] W. Gao, P. Liu, R.S. Crandall, S.H. Lee, D. K. Benson, Approaches for Large Area a-SiC:H PhotovoltaicPowered Electrochromic Window Coatings, J. NonCryst. Solids 266-269 (2000) 1140-1144.

Based on the circuit design described in previous study, the PBTF can be tinted or bleached under sunlight irradiation [8]. The optical contrast and spectral response of a PV-EC device are depicted in figure 8 and figure 9, respectively. The PBTF experiences anodic coloration from transparent to blue, the film darkens to its lowest transmission state in 30 seconds. The PBTF restores its transparency in 15 seconds. This result proves the

2580


[5] C. Bechinger, S. Ferrere, A. Zaban, J. Sprague, B.A. Gregg, Photoelectrochromic windows and displays, Nature 383 (1996) 608-610. [6] B.O’ Regan, M. Gratzel, A low-cost, high-efficiency solar-cell based on dye sensitized colloidal TiO2 films, Nature 353 (1991) 727-740. [7] L.M. Huang, C.W. Hu, H.C. Liu, C.Y. Hsu, C.H. Chen, K.C. Ho, Sol. Energy Mater. Sol. Cells, 99 (2012) 154. [8] L.M. Huang, C.K. Kung, C.W. Hu, C.Y. Peng, H.C. Liu, Sol. Energy Mater. Sol. Cells 107 (2012) 390. [9] L.M. Huang, C.W. Hu, C.Y. Peng, C.H. Su, K.C Ho, Sol. Energy Mater. Sol. Cells, 145 (2016) 69.

2581