A complementary electrochromic device based on Prussian blue and poly(ProDOT-Et2) with high contrast and high coloration efficiency

Solar Energy Materials & Solar Cells 95 (2011) 2238–2245

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Solar Energy Materials & Solar Cells journal homepage: www.elsevier.com/locate/solmat

Review

A complementary electrochromic device based on Prussian blue and poly(ProDOT-Et2) with high contrast and high coloration efficiency Kun-Chieh Chen a, Chih-Yu Hsu a, Chih-Wei Hu b, Kuo-Chuan Ho a,b,n a b

Department of Chemical Engineering, National Taiwan University, Taipei 10617, Taiwan Institute of Polymer Science and 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 13 August 2010 Received in revised form 7 February 2011 Accepted 19 March 2011 Available online 6 May 2011

A complementary electrochromic device (ECD) based on Prussian blue (PB) and poly(3,3-diethyl-3,4dihydro-2H-thieno-[3,4-b][1,4]dioxepine) (PProDOT-Et2) has been systematically investigated. PB is regarded as an anodic coloring material with high electrochemical stability, while PProDOT-Et2 is a cathodic coloring polymer with high contrast and high coloration efficiency (Z). The electro-optical properties of the two electrochromic (EC) materials are characterized separately in a 0.1 M LiClO4 in propylene carbonate (PC). A complementary ECD is assembled based on the two EC materials. The maximum transmittance of the ECD at 590 nm can be changed reversibly from 11.3% to 70.6% at the applied voltages of 1.2 and  1.3 V, and achieved a high coloration efficiency of 1214 cm2/C. Moreover, this ECD still remains at 98% of its maximum transmittance window (DTmax) even after 1,200 cycles, namely, the DT value decreases from 59% to 58%. & 2011 Elsevier B.V. All rights reserved.

Keywords: Coloration efficiency Cycling stability Electrochromic device Poly(ProDOT-Et2) Prussian blue

Contents 1. 2.

3.

4.

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2238 Experimental . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2239 2.1. Materials. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2239 2.2. Preparation of the electrochromic thin-film electrodes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2239 2.3. Assembly of the PProDOT-Et2/PB ECD . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2240 2.4. Electrochemical and spectroscopic measurements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2240 Results and discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2241 3.1. Electrochemical and spectral properties of PProDOT-Et2 and PB thin-film electrodes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2241 3.1.1. Electrochemical properties of PProDOT-Et2 and PB thin-film electrodes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2241 3.1.2. Spectral properties of PProDOT-Et2 and PB thin-film electrodes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2241 3.1.3. Coloration efficiencies of PProDOT-Et2 and PB thin-film electrodes. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2241 3.2. Performance of the PProDOT-Et2/PB ECD . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2242 3.2.1. Cyclic voltammograms . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2242 3.2.2. Absorbance spectra and coloration efficiency . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2242 3.2.3. The relationship between transmittance window and charge capacity ratio . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2243 3.3. Long-term cycling stability of the ECD . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2244 Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2244 Acknowledgment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2244 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2244

1. Introduction

n Corresponding author at: Department of Chemical Engineering, National Taiwan University, Taipei 10617, Taiwan. Tel.: þ886 2 2366 0739; fax: þ 886 2 2362 3040. E-mail address: [email protected] (K.-C. Ho).

0927-0248/$ - see front matter & 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.solmat.2011.03.029

Electrochromism is a phenomenon exhibited by certain electroactive materials with reversible and significant visible change in absorbance spectra. The color of an electrochromic (EC) material depends on its redox state. An electrochromic device (ECD) is composed of working electrode, counter electrode and electrolyte [1]. ECD can be categorized into three types, namely, solution

K.-C. Chen et al. / Solar Energy Materials & Solar Cells 95 (2011) 2238–2245

type, hybrid type and thin-film battery-like type. A complementary ECD is composed of one anodic and one cathodic coloring material [2]. The complementary ECDs offer great advantage, such as fast coloration, high coloration efficiency and low energy consumption [3]. Hence, the complementary ECD shows great potential for numerous future applications due to their high contrast in absorption. ECDs have been researched for anti-glare rear-view mirrors [4], large-area display devices [5], solar-attenuated windows [6] and many other applications based on their unique characteristics. The electrochromic materials can roughly be categorized into four major series. Firstly, transition metal oxides, such as WO3 [7], V2O5 [8], TiO2 [9], etc. Secondly, metal coordination complexes, such as Fe4[Fe(CN)6]3 (Prussian blue) [10,11] and CoFe(CN)6 [12]. Thirdly, organic monomeric compounds, such as viologen [13]. Lastly, organic conducting polymers, such as polyaniline [14], poly(3,4-propyldioxythiophene) (PProDOT) [15], poly(3,4-propyldioxy pyrrole) [15], poly(3-methyl thiophene) (PMeT) [16], and poly(3,4-ethylenedioxythiophene) (PEDOT) [17]. Organic conducting polymers exhibit rapid optical response, high color contrast, high coloration efficiency, and offer a variety of colors [16]; however, inorganic compounds, such as Prussian blue are known to possess better long-term stability [18]. With continuous development on the organic conducting polymers, some of them have already found applications and several new organic conducting polymers have been investigated in the past decade or so. PEDOT, one of the derivatives of polythiophene was first synthesized in Bayer AG research laboratories. PEDOT exhibits not only a low oxidation potential and high electrochemical stability, but also high conductivity and high optical density change (DOD) in the visible spectra [17]. PEDOT can be switched reversibly between sky blue in oxidized state and deep-blue in neutral state. With numerous advantages, PEDOT has been researched for many applications, such as antistatic coatings [19], capacitors [20], lightemitting diodes (LED) [21], biosensors [22], ECDs [17] and so on. In 2002, a derivative of PEDOT, PProDOT-Et2 has been successful synthesized by Reynolds group [23]. They reported a transmittance window (DT) of 75% at lmax, which is larger than that of PEDOT with a DT value of 54% at lmax. Besides, PProDOT-Et2 displayed more rapid switching [23]. It becomes apparent that PProDOT-Et2 is an EC material that is worth being studied.

2239

Even though organic conducting polymers have the required EC characteristic, inorganic Prussian blue (PB, iron (III) hexacyanoferrate (II)) has its own potential to be developed in ECDs [24]. PB abounds attractive electrochromic property and exhibits four redox states, including the colorless of Everitt’s salt (ES), sky blue of Prussian blue (PB), light green of Berlin green (BG) and yellow of Prussian yellow (PY). According to the previous research, PB shows fairly good stability in both ES and PB states, and it can be cycled reversibly for more than thousands times. Moreover, whether in aqueous or non-aqueous solvents, PB has been proven to have good electrochemical stability in these two extremely different conditions [10,11]. According to the above reason, we chose PProDOT-Et2 and Prussian blue to assemble a complementary ECD in this study. A partial list of complimentary ECDs reported in literatures pertaining to this work is summarized in Table 1.

2. Experimental 2.1. Materials All chemicals were ACS reagent grade. The solvents, acetonitrile (ACN) and propylene carbonate (PC), used in this work were purified before use. Potassium chloride (KCl), potassium ferricyanide (K3Fe(CN)6), iron (III) chloride hexahydrate (FeCl36H2O), lithium perchlorate (LiClO4), and 3,4-(20 ,20 -diethylpropylene)-dioxythiophene (ProDOT-Et2) were all purchased from Aldrich. Hydrochloric acid (HCl) was purchased from Fluka. All aqueous solutions were prepared using deionized water (DIW) with a resistivity of about 18.2 MO cm. All experiments were performed at room temperature in air. The electrode potential recorded was against a homemade Ag/Ag þ non-aqueous reference electrode. The Ag/Ag þ reference electrode contained a solution of 10 mM AgNO3 and 0.1 M tetrabuthylammonium perchlorate (TBAP) in ACN. The calibrated potential of the Ag/Ag þ reference electrode was 0.48 V vs. NHE. 2.2. Preparation of the electrochromic thin-film electrodes Optically transparent ITO glass substrates (Rsh ¼20 O/sq., Ritdisplay Corporation, Hsinchu, Taiwan) were used for the

Table 1 A partial list of complementary ECDs reported in literature. Cell configuration substrate/EC1/electrolyte/EC2/substrate

Operation voltage

Bleached/darkened states Tb/Td and DT (%)

Stability

Ref.

ITO/PBV/electrolytea/PB/ITO

 1.0  0.7 V  2.1  0.6 V

After 4000 cycles DT remains 56% After 5  104 cycles DT remains 42% After 1.5  104 cycles DT remains 12.6% After 2000 cycles DT remains 46.9% – After 2.3  104 cycles DT remains 34% After 1200 cycles DT remains 58%

[11]

ITO/PEDOT/electrolyteb/PB/ITO

73/8 65% @ 650 nm 15/63 48% @ 590 nm 64.4/22.3 42.1% @ 630 nm 67.8/9.6 58.2% @ 587 nm 0.6f @ 636 nm 58.6/14.2 44.4% @ 570 nm 70.6/11.3 59% @ 590 nm

PES/ITO/PANI-CSA/PEO-PC-LiClO4/PEDOT-PSS/ITO c

ITO/PBPMOM-ProDOT/electrolyte /BEDOT-NMCz/ITO d

e

 0.5  2.5 V  1.5  0.9 V

ITO/PANBS /electrolyte /PEDOT/ITO ITO/PANI/electrolyteg/PEDOT/ITO

 2.4  1.1 V  0.6  1.0 V

ITO/PProDOT-Et2/electrolyteh/PB/ITO

 1.3  1.2 V

a

[25] [26] [27] [28] [29] This work

Electrolyte: 0.04 M LiBF4 in succinonitrile. Electrolyte:1 M LiClO4/PCþ 10 wt% PMMA. c Electrolyte: 3.0 g PC, 7.0 g PEGMA, 1.0 g LITRIF, 17.5 mg DMPAP, and 5.0 mg glass beads (50 100 mm). d PANBS: poly(aniline-N-butylsulfonate)s. e Electrolyte: 0.06 g LiCF3SO3 in 0.3 g MPEGM, 0.6 g PEGDMe, 0.072 g Triallyl-1,3,5-triazine-2,4,6-(1 H,3 H,5 H)-trione, 0.06 g Darocure 1173, and 0.03 g Irgacure 784. f Absorption change. g Electrolyte:0.1 M LiClO4 þ1 mM HClO4/PC. h Electrolyte:0.1 M LiClO4/PC. b

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K.-C. Chen et al. / Solar Energy Materials & Solar Cells 95 (2011) 2238–2245

deposition of EC films. Before use, the ITO substrates (3.0  4.0 cm2) were ultrasonically cleaned in a 0.1 M detergent solution, deionized water (DIW), acetone, and IPA (isopropyl alcohol). Each step took about 15 min, and then the films were stored in air. The PB thin films were deposited galvanostatically. The composition of the plating solution was 10 mM FeCl3  6H2O(aq), 10 mM K3Fe(CN)6(aq), 0.1 M KCl(aq), and 0.1 M HCl(aq). When preparing the PB thin films, a constant cathodic current density of 20 mA/cm2 was applied for 170 s. The asdeposited PB electrodes were rinsed with DIW and dried with N2. The deposition solution for PProDOT-Et2 was composed of 10 mM ProDOT-Et2 monomer and 0.1 M LiClO4 in ACN. The PProDOT-Et2 thin films were electropolymerized onto the ITO glass substrates by applying a potential at 1.0 V (vs. Ag/Ag þ ) with a platinum counter electrode. The as-deposited PProDOT-Et2 electrodes were rinsed with CAN and dried with N2. 2.3. Assembly of the PProDOT-Et2/PB ECD The ECD was assembled with PProDOT-Et2 and PB electrodes by incorporating 0.1 M LiClO4 in propylene carbonate (PC). Both PProDOT-Et2 and PB electrodes were cycled five times by the CV method in 0.1 M LiClO4/PC solution to ensure that the electrooptical properties of the films reached steady state. After cycling,

the PProDOT-Et2 electrode was stepped to the doped state by applying 0.4 V for 180 s. Similarly, the PB electrode was stepped to colorless ES state by applying 0.9 V for 180 s. Following this potential pretreatment, the electrolyte containing 0.1 M LiClO4/PC solution was placed onto the PProDOT-Et2 electrode, and then the PB electrode was placed on the top. Subsequently, the two electrochromic layers, including the electrolyte, were carefully sandwiched together to form an electrochromic cell. The cell gap was controlled at 0.28 mm with an epoxy spacer tape. Finally, the cell was sealed with Torr Seals (Varian) cement around the four edges of the ITO glass. 2.4. Electrochemical and spectroscopic measurements The PProDOT-Et2 and PB thin-film electrodes were characterized electrochemically in a 0.1 M LiClO4/PC solution by the threeelectrode CV method. In-situ spectral measurements were also done. The transmittance spectra of the ECD were measured at different equilibrated applied voltages (PB vs. PProDOT-Et2). The applied voltage was 1.2 V lasting for 10 s and then changed to  1.3 V for 10 s at each cycle. The in-situ transmittance spectra at 590 nm were recorded for 300 cycles continuously.

2.0 0.7 V

0.3

0.1

PProDOT-Et2 (undoping) deep violet

0.0

tested in 0.1 M

1.5 Absorbance

Current density (mA/cm2)

0.2

PProDOT-Et2 tested in 0.1 M LiClO4 + PC

-0.2 scan rate = 100 mV/s -0.3 -0.5 0.0 -1.0

0.5 V 1.0

0.4 V

0.5

PProDOT-Et2:ClO4(doping) light violet

0.2 V 0.1 V -0.6 V

0.0 0.5

400

1.0

500

E (V) vs. Ag/Ag+

PB tested in

PB tested in

0.1 M LiClO4 + PC

0.1 M LiClO4 + PC

Everrit's salt (ES) colorless

0.0 PB light blue

-0.1

Absorbance

0.2 Current density (mA/cm2)

600 Wavelength (nm)

700

800

0.3

0.3

0.3 V 0V

0.2 -0.1 V

-0.2 V

0.1

-0.3 V -0.9 V

-0.2 scan rate = 100 mV/s -0.3 -1.0

LiClO4 + PC

0.6 V

0.3 V

-0.1

0.1

PProDOT-Et2

-0.5

0.0

0.0 0.5

E (V) vs. Ag/Ag+ Fig. 1. CVs for (a) PProDOT-Et2 scanned from 0.4 to  0.8 V and (b) PB scanned from 0.4 to  0.9 V in 0.1 M LiClO4/PC at a scan rate of 100 mV/s.

400

500

600 Wavelength (nm)

700

800

Fig. 2. Absorbance spectra of (a) PProDOT-Et2 thin-film electrode and (b) PB thinfilm electrode equilibrated in 0.1 M LiClO4/PC at different applied potential stepping from 0.7 to  0.6 V and 0.3 to  0.9 V with 0.1 V decrement, respectively.

K.-C. Chen et al. / Solar Energy Materials & Solar Cells 95 (2011) 2238–2245

2241

when the applied potential is more positive than 0.7 V, it is found that PProDOT-Et2 results in a capacitive charging and the absorbance does not change in the visible region. The wavelength at 590 nm corresponds to deep blue–violet region and is very sensitive to human eyes. Similarly, the spectra of PB equilibrated at different potentials are shown in Fig. 2(b), with the maximum absorption noticed at 690 nm. The PB spectra show no absorption characteristics when the applied potential is more negative than  0.6 V, which is consistent with the results of CVs. In contrast, when the applied potential is more positive than 0.3 V, the absorption reaches saturation. This is the reason to control the potential in between the first redox couple.

3. Results and discussion 3.1. Electrochemical and spectral properties of PProDOT-Et2 and PB thin-film electrodes 3.1.1. Electrochemical properties of PProDOT-Et2 and PB thin-film electrodes The typical CVs of PProDOT-Et2 and PB electrochromic (EC) electrodes are shown in Fig. 1(a) and (b), respectively. In Fig. 1(a), there are two coupled redox peaks for PProDOT-Et2. The two oxidative peaks locate at ca. 0.15 and  0.1 V, and the reductive peaks locate at ca. 0.18 and  0.42 V. PProDOT-Et2 is a cathodically coloring EC material; it shows deep blue–violet in the neutral state, and light blue in the doping state. On the other hand, in Fig. 1(b), there is a pair of redox peak for PB, which corresponds to the anodic peak at ca. 0.10 V and the cathodic peak at ca.  0.42 V. To avoid the formation of the second redox couple, the upper oxidative potential is controlled at 0.4 V.

3.1.3. Coloration efficiencies of PProDOT-Et2 and PB thin-film electrodes The coloration efficiency measurements provide a good way to quantify the electroactive properties of the ECD. The coloration efficiency is defined as the relationship between the charge capacity and the optical density change, DOD at a specific wavelength. Fig. 3(a) and (b) shows the relationships between DOD at 590 nm and the charge capacity, for both PProDOT-Et2 and PB electrodes, respectively. Meanwhile, we also measured the coloration efficiency of PB under its maximum absorption wavelength, i.e., 690 nm, as shown in Fig. 3(c). For the PProDOT-Et2 thin-film electrodes, the relationship was determined by switching voltages between a fixed bleached state (0.4 V) and different colored states (0.4 to  0.8 V, with an increment of 0.1 V); for the PB thin-film electrodes, the switching voltages were chosen between a fixed bleached state (0.3 V) and different colored states

3.1.2. Spectral properties of PProDOT-Et2 and PB thin-film electrodes Fig. 2(a) shows the absorbance spectra for the PProDOT-Et2 thin-film electrode equilibrated at different potentials in a 0.1 M LiClO4/PC solution. In the visible region, the intensity of the absorbance peak increases as the applied potential is increased. There are three characteristic peaks in the visible region, which locate at 545, 590, and 635 nm, with the maximum absorption noticed at 590 nm. The PProDOT-Et2 spectra show no absorption characteristics when the applied potential is more negative than 0.6 V, which is consistent with the results of CVs. In contrast,

0.4

PProDOT-Et2 tested in

0.3

ηPProDOT-Et2= 1,408 cm2/C

-0.8 V

0.1 M LiClO4 + PC -0.6 V

0.2

0.3 V

PB tested in 0.1 M LiClO4 + PC

0.3 ΔOD at 590 nm

ΔOD at 590 nm

0.5

ηPB = 54 cm2/C 0.2

-0.3 V

0.1

0.1 -0.3 V 0.0

0.4 V

0.00

-0.6 V

0.0 0V 0.05

0.10

-0.9 V 0.15

Charge capacity

ΔOD at 690 nm

0.6

0.20

0.25

0.30

0

1

(mC/cm2)

2

3

Charge capacity

4

5

6

7

(mC/cm2)

0.3 V

PB tested in 0.1 M LiClO4 + PC ηPB = 100 cm2/C

0.4

-0.1 V -0.3 V

0.2 -0.5 V 0.0

-0.9 V 0

1

2 3 4 5 Charge capacity (mC/cm2)

6

7

Fig. 3. Optical density change as a function of the inserted charge capacity of (a) PProDOT-Et2 thin-film electrode at 590 nm, (b) PB thin-film electrode at 590 nm and (c) PB thin-film electrode at 690 nm in 0.1 M LiClO4/PC at different applied potential stepping from 0.4 to  0.8 V and 0.3 to  0.9 V with 0.1 V decrement, respectively.

2242

K.-C. Chen et al. / Solar Energy Materials & Solar Cells 95 (2011) 2238–2245

(0.3 to  0.9 V, with a decrement of 0.1 V). Each voltage step lasted for 120 s, allowing the reaction to reach equilibrium. An integration of the I–t curve over the full 120 s, with each transient current collected at 0.1 s interval, shows the inserted charge capacity for each step. Thus, the relationship between DOD and the charge capacity can be obtained. Each data point shown in Fig. 3 represents the dynamic optical density change of the EC thin-film at a specific charge capacity injected. However, from the application point of view, we focus only on the overall coloration efficiency from the most bleached state to the most darkened state, which can be calculated from the slope of the two end points. Therefore, the coloration efficiencies for both thin-film electrodes can be calculated from the slope. For a PProDOT-Et2 thin-film with a passed charge capacity of 0.3 mC/cm2, the coloration efficiency at 590 nm was calculated to be 1408 cm2/ C, corresponding to the potential steps from 0.4 to  0.8 V. For a PB thin-film with a passed charge capacity of 6.0 mC/cm2, the coloration efficiencies at 590 and 690 nm were calculated to be 54 and 100 cm2/C, respectively, corresponding to the potential steps from  0.9 to 0.3 V. It can be seen from the CVs and spectral properties of both electrodes that the real coloration efficiency for each electrode depends on the applied potential. To keep the operating potential window within the safe range, 0.4 to  0.8 V was chosen for PProDOT-Et2; on the other hand, 0.9 to 0.3 V was chosen for PB. The coloration efficiencies for many conjugated conducting polymers are not constant. The nonlinear graph, as shown in Fig. 3(a), resulted from the conjugated polymers possessing different states while being doped. Accordingly, measurement of the coloration efficiencies for such systems becomes complicated. 3.2. Performance of the PProDOT-Et2/PB ECD 3.2.1. Cyclic voltammograms Fig. 4 shows the CVs performed for the PProDOT-Et2/PB ECD at a scan rate of 100 mV/s, which is slow enough to observe the gradual color change. So the CVs in the convoluted device correspond to the discrete redox processes. It shows no side reaction occurred during cycling, thus the operating potential window ( 1.3 to 1.3 V) is selected for the ECD considering both the maximum optical attenuation and the stability of the device. A three-stage reaction of the ECD was observed from the CV. The first reaction taking place between 0.1 and 0.3 V is contributed

from PProDOT-Et2 only. The second reaction taking place between  0.1 and 0.3 V is contributed from both PProDOT-Et2 and PB. The third reaction taking place between  0.5 and  0.7 V also is contributed from both PProDOT-Et2 and PB. Furthermore, the PProDOT-Et2/PB ECD showed a reversible CV from 1.3 to  1.3 V due to complementary reaction. The simplified working principle of the complementary PProDOT-Et2/PB ECD containing 0.1 M LiClO4 salt can be proposed. Eqs. (1) and (2) represent two EC reactions for PProDOT-Et2 and PB electrodes, respectively. PProDOT-Etn2 þ : nClO4 þ ne 2 PProDOT-Et2 þ nClO4 

ðDoping state, light violetÞ



ð1Þ

ðUndoping state, deep blue-violetÞ

Li2 FeII ½FeII ðCNÞ6 2LiFeIII ½FeII ðCNÞ6  þ Li þ þ e

ð2Þ

ðPB, blueÞ

ðES, colorlessÞ

where n is the stoichometric number of the counter ions. Thus, the overall electrochromic process in the ECD can be obtained by summing Eqs. (1) and (2) as PProDOT-Etn2 þ : nClO4 þ nES2PProDOT-Et2 þnPB þ nClO4 þ nLi þ 



ðBleach state, light blueÞ

ðColor state, deep blueÞ

ð3Þ 3.2.2. Absorbance spectra and coloration efficiency Fig. 5 shows the spectra of the ECD, which were equilibrated at each applied potential from 1.2 to 1.3 V (PProDOT-Et2 vs. PB). Starting from  1.3 V (bleach state), with an increment of potential at 0.1 V, the absorption increased due to coloration. Comparing with Fig. 2(a) and Fig. 5, it can be observed that the absorbance spectra of the ECD are similar to that of the PProDOT-Et2 thin-film. Therefore, the optical performance of the ECD is mostly contributed from PProDOT-Et2. As mentioned previously, the coloration efficiency of PProDOT-Et2 is 1408 cm2/C, which is much larger than that of the PB (54 cm2/C). Hence, the coloration efficiency of the ECD is predominant by PProDOT-Et2. According to Fig. 5, there are three characteristic peaks in the visible region, which are 545, 590 and 635 nm, with the maximum absorption occurring at 590 nm. The PProDOT-Et2/PB ECD spectra show no absorption characteristics when the applied potential is either more negative than  1.1 V or more positive than 1.0 V, which are consistent with the results obtained from the CVs. Furthermore, a deep blue–violet was observed at 590 nm.

1.2 PProDOT-Et2/PB ECD

0.1

PProDOT-Et2 (undoping) deep blue-violet

1.2 V

1.0

PProDOT-Et2/PB ECD ES colorless

0.0

0.8 Absorbance

Current density (mA/cm2)

0.2

0.6 0.4 0.2

PProDOT-Et2: ClO4(doping) light violet

-0.1 PB light blue

-1.3 V 0.0 400

-0.2 -1.5

-1.0

-0.5 0.0 0.5 EPProDOT-Et vs. PB (V)

1.0

1.5

2

Fig. 4. CV for the PProDOT-Et2/PB ECD scanned from 1.2 to  1.3 V in 0.1 M LiClO4/ PC at a scan rate of 100 mV/s.

500

600

700

800

Wavelength (nm) Fig. 5. Absorbance spectra of the PProDOT-Et2/PB ECD switched at different applied voltages stepping from 1.2 to  1.3 V with 0.1 V decrement. Voltage is the potential difference between the PProDOT-Et2 electrode and the PB electrode (PProDOT-Et2 vs. PB). PB electrode acts as the positive electrode.

K.-C. Chen et al. / Solar Energy Materials & Solar Cells 95 (2011) 2238–2245

The typical in-situ transmittance response was measured at 590 nm and the corresponding I–t transient response of the ECD is shown in Fig. 6(a) and (b), respectively. From Fig. 6(a), the applied voltage to color is 1.2 V, and to bleach is  1.3 V. The ECD achieves transmittances of 11.3% in the colored state and 70.6% in the bleached state, resulting in a DT of 59.3%. From Fig. 6(b), the darkening peak current density is larger than that of the blending one. This implies that the electron transfer in the undoping state is faster than the one in the doping state. The coloration efficiency of the ECD at 590 nm was calculated to be 1214 cm2/C from Fig. 6(a) and (b).

3.2.3. The relationship between transmittance window and charge capacity ratio Because our complementary electrochromic device has two thin-film electrodes, similar to the thin-film batteries, the charge capacity ratio of these two electrodes should be determined. In order to obtain the optimum charge capacity ratio, we prepared ECDs with different charge capacity ratios to observe the in-situ transmittance response at 590 nm and obtain the maximum transmittance window (DT) of the ECDs. Here, the charge capacity ratio R, is defined as the initial charge capacity of PB to that of

100 PProDOT-Et2/PB ECD

Transmittance at 590 nm (%)

90 80

-1.3 V

70

50 40 30 20 1.2 V

0 0

10

PProDOT-Et2 according to the following equation R¼

qPB qPProDOT-Et2

where qPB is the charge capacity of the PB (in C/cm2), and qPProDOTEt2 is the charge capacity of the PProDOT-Et2 (in C/cm2). During the experiment, the charge capacity of PProDOT-Et2 electrode was fixed at 0.07 mcm2/C and that of PB electrode was changed. The values of R were varied as 0.0, 0.19, 0.53, 0.74, 0.76, 1.31, 1.47, 1.68, and 1.88. Moreover, all samples were freshly darkened at þ1.2 V and bleached at  1.3 V. The voltage is the potential difference between the PProDOT-Et2 electrode and the PB electrode (PProDOT-Et2 vs. PB). These voltages were controlled within the electrochemically safe voltage limits, so as to achieve the maximum transmittance window. Fig. 7 shows the transmittance window at 590 nm as a function of the charge capacity ratio. The experimental data shown in Fig. 7 can be roughly classified into two regions, namely R%1.0, and R^1.0. For R%1.0, the DT increased with the increase in the charge capacity ratio; on the other hand, for R^1.0, the DT reached saturation, regardless of the change in the charge capacity ratio. In an earlier report, Chen et al. [30] established a general model for complementary ECDs. The theoretical model for the transmittance window can be summarized by the following expressions M DTð0 r R o1Þ ¼ TECD ½exp½ZE qE ð1RÞexp½qE ðZE þ ZP RÞ

ð5Þ

M DTð1 rRÞ ¼ TECD ½exp½ZP qE ðR1Þexp½qE ðZE þ ZP RÞ

ð6Þ

20 Time (s)

30

40

where represents the transmittance parameter depending on the thickness and the charge capacity of both the films. qE , ZE , and ZP represent the charge capacity of the PProDOT-Et2, the coloration efficiency of the PProDOT-Et2, and the coloration efficiency of PB, respectively. From Fig. 7, the model predicts the experimental results reasonably well. Note that we intentionally varied the charge capacity ratio (R) in Fig. 7 to obtain the transmittance window (DT). In fact, for all other experiments described elsewhere throughout this study (Figs. 4–6 and 8), we fixed the value of R at unity, so as to maintain the charge balance for both EC films. The experimental results can be explained by the following observations. Firstly, the coloration efficiency of the PProDOT-Et2, which is 1408 cm2/C, is nearly twenty times larger than that of PB

1.0

100

PProDOT-Et2/PB ECD

PProDot-Et2/PB ECD 80

0.5 -1.3 V

-1.3 V

∆T at 590 nm (%)

Current density (mA/cm2)

ð4Þ

M TECD

60

10

2243

0.0

-0.5

60 40 20

-1.0 1.2 V

1.2 V

model expt. data

0

-1.5 0

10

20

30 Time (s)

40

50

0.0

0.5 1.0 1.5 2.0 Charge capacity ratio ( R = qPB/qPProDOT-Et ) 2

Fig. 6. (a) In-situ transmittance response of the cell at 590 nm. Applied voltages were Vd ¼1.2 V (10 s) to color and Vb ¼  1.3 V (10 s) to bleach, and (b) the corresponding current density–time relationship of the cell in response to the applied voltages.

Fig. 7. Transmittance window at 590 nm as a function of the charge capacity ratio. Experimental data are shown by the diamond points while the model prediction is shown by the curve. Active area of the ECD is 4 cm2.

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K.-C. Chen et al. / Solar Energy Materials & Solar Cells 95 (2011) 2238–2245

coloring PProDOT-Et2 and an anodically coloring PB thin-film was first researched. According to this study, the PProDOT-Et2/PB complementary ECD was feasible, and can be switched between colorless and deep blue–violet reversibly. The transmittance window of the ECD at 590 nm can be changed from 70.6% (  1.3 V) to 11.3% (1.2 V), and the coloration efficiency at 590 nm was calculated to be 1214 cm2/C. Finally, this ECD still remained at 98% of its maximum transmittance window (DTmax) after 1200 repeated cycles, namely, the DT value had decreased from 59% to 58%. According to the reasonably good properties mentioned above, it is hoped that the PProDOT-Et2/PB ECD may offer possibility for practical applications.

Transmittance at 590 nm (%)

100 PProDot-Et2/PB ECD 80

60

40

Tb Td ∆T

20

0

Acknowledgment 0

300

600 900 Cycle number b

1200

1500

d

Fig. 8. Transmittance at bleached state (T ), at darkened state (T ), and the transmittance change (DT) at 590 nm as a function of cycle number shows the dynamic cycling stability of PProDOT-Et2/PB ECD during potential step between 1.2 and  1.3 V with 10 s step time for each.

(54 cm2/C). Although the coloration efficiency of PB was not measured at its maximum absorption wavelength, we still could make this conclusion by the following reasons. Firstly, the reacted charge capacity of PB film is already larger than that of PProDOTEt2, as can be seen from Fig. 3(a) and (b). This means that the thickness of the PB under the experimental condition is large enough. In fact, the coloration efficiency of an electrochromic material is a material’s optical property. It depends strongly on the wavelength, but depends less on the thickness. We have performed additional experiments for PB to collect its coloration efficiency at the maximum absorption wavelength, namely, 690 nm. This result is shown in Fig. 3(c). Although the coloration efficiency for PB at 690 nm is 100 cm2/C, which is higher than that at 590 nm, the coloration efficiency for PProDOT-Et2 at 590 nm is 1408 cm2/C, which is more than fourteen times larger than that of PB. In other words, the coloration is mostly contributed from PProDOT-Et2, and PB only plays the role of charge balancing layer in order to evoke a reversible optical modulation. Moreover, from Fig. 2(b), the coloration of PB is not obvious when PB had only a charge capacity of 6.0 mC/cm2. From the above observations, we can conclude that the PB plays the role of an ion-storage or charge balancing layer in this system. 3.3. Long-term cycling stability of the ECD Fig. 8 shows the in-situ transmittances in the bleached state (Tb), darkened state (Td), and transmittance window (DT) of the ECD at 590 nm as a function of the cycling number. The ECD was assembled at a charge capacity ratio of unity (R ¼1), and tested up to 1200 cycles. During the cycling tests, the coloring and bleaching applied voltages were set at 1.2 and  1.3 V, respectively, considering both the maximum optical attenuation and the stability of the device. To bleach, the time interval of each step was set at 10 s. It is apparent that there is no significant decay up to these cycles, and the DT still remained at 98% of its maximum transmittance window (DTmax) after 1200 repeated cycles, namely, the DT value decreased from 59% to 58%.

4. Conclusion From this work, a new organic–inorganic complementary ECD based on PProDOT-Et2 and PB was proposed and examined. The electrochromic characteristic of an ECD with a cathodically

The authors Industrial Park, substrates. This Science Council

wish to thank Ritdisplay Corporation, Hsinchu Taiwan, for providing the conductive ITO glass research is financially supported by the National of Taiwan.

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