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Integrated smart electrochromic windows for
energy saving and storage applications†
Zhong Xie,ab Xiujuan Jin,a Gui Chen,a Jing Xu,a Di Chen*a and Guozhen Shen*b
Received 16th October 2013,
Accepted 4th November 2013
DOI: 10.1039/c3cc47950a
www.rsc.org/chemcomm
A self-powered electrochromic smart window with tunable transmittance driven by dye-sensitized solar cells has been designed,
which also acts as a photocharged electrochromic supercapacitor
with high areal capacitance and reversible color changes.
As a promising energy-saving electronic device, an electrochromic
(EC) device can adjust its optical properties upon applying an
external voltage, which renders it an effective candidate for electronic displays, anti-glare mirrors, eye-glasses and especially energysaving smart windows.1–7 An electrochromic smart window (ECSW)
is an intelligent energy-saving window which can provide adjustable
light transmission by reversible colour changes. By controlling the
amount of transmitted light and heat radiation, the ECSW can use
sunlight and control the interior energy exchange more effectively
and thus greatly reduce energy consumption, lighting expenses and
cooling/heating loads, which has great potential for application in
intelligent buildings, car roofs and the aerospace field.8–14
With multifunctional energy performance, highly integrated
energy systems have recently sparked interest worldwide, among
which the self-powered integrated energy system is one of the most
significant systems. For example, Wang et al. designed a series of selfpowered systems driven by nanogenerators, including the selfpowered EC device, water splitting systems and sensors.15–18 Our
team also reported a self-powered photodetector by integrating a
lithium-ion battery as the power supply.19 Moreover, with the ability to
convert solar energy into electricity, silicon based photovoltaic cells
and dye-sensitized solar cells (DSSCs) were developed and integrated
into EC electronic devices to realize self-powered operation and
a
Wuhan National Laboratory for Optoelectronics (WNLO) and School of optical and
electronic information, Huazhong University of Science and Technology (HUST),
Wuhan 430074, China. E-mail: [email protected]; Fax: +86-27-87792225
b
State Key Laboratory for Superlattices and Microstructures, Institute of
Semiconductors, Chinese Academy of Sciences, Beijing 100083, China.
E-mail: [email protected]
† Electronic supplementary information (ESI) available: Experimental details, SEM
image and a scheme of the integrated device, the optical spectra and switching
time characteristics of the DSSC driven ECSW, i–t curves and voltage decay and
leakage current of the DSSC charging ECSC. See DOI: 10.1039/c3cc47950a
608 | Chem. Commun., 2014, 50, 608--610
maximize energy savings.20–22 As the EC device and supercapacitors
are both based on electrochemical reactions and both possess a
sandwich configuration, multifunctional EC supercapacitors which
integrate energy storage and EC function into one device were recently
designed.23,24 This new type of EC supercapacitor makes it possible to
design a multifunctional integrated system to realize energy conversion, storage and efficient usage simultaneously.
In this work, an integrated system combining a DSSC and a WO3
based EC smart system was designed to realize self-powered
operation, which provides controlled and reversible color changes
with high optical modulation and a fast coloration/bleaching
response. The as-designed system also acts as a multifunctional
photo-charged electrochromic supercapacitor (ECSC) driven by DSSCs
with a high areal capacitance (22 mF cm 2) and reversible color
changes. WO32H2O films, used as EC electrodes, were synthesized via
a solution process according to our previous report,25 and the photoabsorbing layers of the DSSC chosen commercial TiO2 (P25) as active
materials were fabricated by screen-printing (experimental details are
provided in the ESI†). The morphology of the WO32H2O thin film
fabricated on ITO glass was characterized by SEM (Fig. S1a, ESI†),
which shows densely packed nanoparticles with a size of B500 nm.
The self-powered smart window was assembled by integrating
the WO3 based electrochromic window and the DSSC as the power
supply (Fig. S1b, ESI†). Two types of self-powered smart windows
were designed, which are the smart window with an independent
electrochromic device and a DSSC device (Type I) and the smart
window with an integrated electrochromic device and a DSSC device
(Type II), as shown in Fig. 1. The circuits and the photographs of the
Type I device are shown in Fig. 1a. In this device, the coloration and
bleaching loops are controlled by a single-pole double-throw switch
(SPDT switch). When closing the SPDT switch to a coloration loop,
the WO3 EC window changes its color from colorless to blue.
Conversely, the EC window reverts to colorless with the bleaching
loop switching on conduction (Fig. 1a1–a6). The figures also revealed
that the as-fabricated Type I device possesses high transparency
whether in the coloration or bleaching state. When the sunlight is
strong, the smart window in the coloration state can selectively
absorb the incident light and reflect the thermal radiation, providing
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Fig. 1 Pictures of a DSSC driven EC smart window under illumination of
sunlight: circuits of (a) the independent device (Type I) and (b) the
assembled device (Type II).
a comfortable environment with controlled natural light intensity
(Fig. 1a3 and a4). The circuit of the Type II device and the photographs of the real device are shown in Fig. 1b. As the photoanodes of
the two DSSCs were, respectively, printed on the work electrode and
the counter electrode of the EC device, the circuit of the integrated
system can be simplified to connect two Pt electrodes of DSSCs to the
corresponding electrodes of the EC device. Using a double-pole
double-throw switch (DPDT switch) to control the coloration and
bleaching loops, the Type II device reversibly changes its color
between colorless and blue under the illumination of strong sunlight.
The electrochemical and optical properties of the Type II device
at different incident light intensities were further characterized and
discussed as follows. The photocurrent density–voltage ( J–V) curves
of the DSSC unit within the Type II device for different illumination
intensities are shown in Fig. 2a, which revealed that the photocurrent density is enhanced upon increasing the light intensity. The
corresponding UV-vis transmittance spectra of the Type II device
(Fig. S2, ESI†) revealed that the EC window unit within the device
displays high transmittance in the bleaching state and the stronger
the intensity of the light, the larger the generated photocurrent, and
the greater the contrast in transmittance. Fig. 2b shows the
Fig. 2 (a) The J–V curves of the DSSC unit at different light intensities.
(b) The simultaneous voltage, current density and in situ transmittance
spectra vs. time of the Type II device at a light intensity of 100 mW cm 2.
(c) The solar energy conversion efficiencies and coloration efficiencies of
the Type II device at different light intensities.
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simultaneous real-time responses of the voltage and the current
density of the device and the in situ transmittance of the device at a
light intensity of 100 mW cm 2. The switching time is 30 s for each
state and the in situ transmittance response is investigated at a
fixed wavelength of 670 nm. As revealed in Fig. 2b, the practical
voltage of the device is switched between 0.2 V and 0.2 V, and the
transient current density changes along with the switching voltage.
Obvious optical modulation (B25.9%) can be observed in the
in situ transmittance spectra during the switching. The switching
time characteristics of the device at illumination light intensities of
60, 80, 120, and 150 mW cm 2 were obtained and showed
enhanced optical modulation and a coloration/bleaching response
with the increased intensity of incident light (Fig. S3, ESI†).
However, when the light intensity is increased to 150 mW cm 2,
a slower response and decreased optical modulation emerged
during the switching process, which may be attributed to the
irreversible reaction in the EC window caused by strong light
illumination. Fig. 2c shows the curves of the photoelectric conversion efficiencies (Z) of the DSSC unit and the corresponding
coloration efficiencies (CE) of the EC window unit at different light
intensities. As shown in Fig. 2c, the solar energy conversion
efficiency and the coloration efficiency increase with increased
light intensity, which appear to have a nearly linear relationship.
The highest combination of the conversion and coloration efficiency is 1.27% and 61.6 cm2 C 1, respectively, which indicates that
the integrated Type II device can achieve a comparable electrochromic performance without external power.
Considering that the EC window and supercapacitors were both
based on the electrochemical reactions of the electrode materials, we
further assembled a multifunctional sandwich-like device for both
energy storage and electrochromism by stacking two WO3 films with
the H2SO4–PVA gel electrolyte as the binder. An integrated electrochromic energy storage (EES) system charged by the DSSC was thus
designed, as shown in Fig. 3a. When the switch is turned on,
electricity generated from the DSSC flows to the EES device, making
it charged and changing its color to blue (Fig. 3b). As the circuit is
open, the EES window was discharged and turned to its bleaching
state (Fig. 3c). Fig. 3d illustrates the transmittance spectra of the
window in the charged and discharged state, which indicates that
the blue charged window absorbs the light in the visible range to
decrease the transmittance of incident light.
The energy storage properties of the EES smart window were
characterized by electrochemical measurements. Fig. 4a depicts the
CV curves of the EES device measured at a series of scan rates, and the
potential window ranges from 0.5 V to 0.5 V. All the CV curves
exhibit approximately box-like shapes, which are the typical shapes of
a two-electrode supercapacitor based on a WO3 material. As shown in
Fig. 4b, the areal capacitance vs. scan rates of the EES device is calculated and a high areal capacitance of approximately 0.039 F cm 2 is
obtained at a low scan rate of 5 mV s 1. Even at a fast scan rate of
100 mV s 1, the areal capacitance of the EES device still reaches
a capacitance as high as 0.019 F cm 2. The illumination light intensity on the DSSC is changed to obtain different photocharging
current densities (Fig. S4, ESI†), the corresponding galvanostatic
charge–discharge curves of the EES device are shown in Fig. 4c.
The non-ideal triangular shape of the charge–discharge curves
Chem. Commun., 2014, 50, 608--610 | 609
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to realize the integration of energy conversion, storage and the energysaving window in a self-powered system.
In summary, an integrated self-powered electrochromic system is
designed based on a nanoparticle WO3 film as an EC electrode and the
DSSC as the power supply. In the integrated ECSW, the DSSC absorbs
sunlight and converts it into electricity to support the EC window
effectively regulating its transmittance by reversible color changes.
Moreover, when the EC device is extended to be a supercapacitor, a
multifunctional self-powered and energy-storing electrochromic smart
system was formed. The DSSC charged EC energy-storage device can
also reversibly change its color during the charge–discharge process,
which can be potentially applied in buildings, cars and displays.
This work was supported by the National Natural Science Foundation (21001046, 51002059, and 91123008), the 973 Program of China
(2011CB933300), and the Program for New Century Excellent Talents
of the University in China (grant no. NCET-11-0179). We thank the
Analytical and Testing Center of Huazhong University of Science &
Technology and the Center of Micro-Fabrication and Characterization
(CMFC) of WNLO for the measurements of the samples.
Fig. 3 (a) Schematic illustration of the DSSC driven EC device for energy
storage. (b) Image of the electrochromic energy-storage (EES) device charged
by DSSC (colored). (c) Image of discharged EES device (bleached). (d) The
transmittance spectra of the EES device in charged and discharge states.
Fig. 4 (a) CV curves at different scan rates. (b) Plot of areal capacitance vs.
scan rates. (c) Galvanostatic charge–discharge curves at different current
densities. (d) Plot of areal capacitance vs. current density.
exhibits a typical pseudo-capacitance characteristic, which is ascribed
to the redox process. Fig. 4d presents the calculated areal capacitances
based on galvanostatic charge–discharge curves, which show that
the EES device can deliver a high areal capacitance of 0.022 F cm 2
at a small current density of 0.106 mA cm 2 (photocharged at
100 mW cm 2). When the photocharged current density is enhanced
to 0.228 mA cm 2, the capacitance obtained is maintained at
0.015 F cm 2. The areal capacitances calculated from the photocharge–discharge curves are comparable to the capacitances obtained
from CV curves, which reveal that the EES device photocharged by the
DSSC is feasible and efficient, and also demonstrates the probability
610 | Chem. Commun., 2014, 50, 608--610
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