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Towards plastic smart windows: optimization of indium tin oxide electrodes for the synthesis of electrochromic devices on polycarbonate substrates Marco Laurenti, Stefano Bianco, Micaela Castellino, Nadia Garino, Alessandro Virga, Candido Fabrizio Pirri, and Pietro Mandracci ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b00988 • Publication Date (Web): 08 Mar 2016 Downloaded from http://pubs.acs.org on March 8, 2016

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ACS Applied Materials & Interfaces

Towards plastic smart windows: optimization of indium tin oxide electrodes for the synthesis of electrochromic devices on polycarbonate substrates Marco Laurenti,†,* Stefano Bianco,† Micaela Castellino,§ Nadia Garino,§ Alessandro Virga,† Candido F. Pirri,†,§ and Pietro Mandracci† †

Department of Applied Science and Technology, Politecnico di Torino, C.so Duca degli

Abruzzi 24, 10129 Torino, Italy §

Center for Space Human Robotics @ PoliTo, Istituto Italiano di Tecnologia, C.so Trento, 21,

10129 Torino, Italy KEYWORDS. ITO; polycarbonate; electrochromic windows; WO3; sputtering

ABSTRACT. Plastic smart windows are becoming one of the key elements in view of the fabrication of cheap, lightweight electrochromic (EC) devices to be integrated in the newgeneration of high energy-efficiency buildings and automotive applications. However, fabricating electrochromic devices on polymer substrates requires a reduction of process temperature, so in this work we focus on the development of a completely room-temperature deposition process aimed at the preparation of ITO-coated polycarbonate (PC) structures acting as transparent and conductive plastic supports. Without providing any substrate heating or

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surface activation pre-treatments of the polymer, different deposition conditions are used for growing Indium Tin Oxide (ITO) thin films by the radio-frequency magnetron sputtering technique. According to the characterization results the set of the optimal deposition parameters is selected to deposit ITO electrodes having high optical transmittance in the visible range (~90%) together with low sheet resistance (~8 ohm/sq.). The as-prepared ITO/PC structures are then successfully tested as conductive supports for the fabrication of plastic smart windows. To this purpose, tungsten trioxide thin films are deposited by the reactive sputtering technique on the ITO/PC structures, and the resulting single electrode EC devices characterized by chronoamperometric experiments and cyclic voltammetry. The fast switching response between colored and bleached states, together with the stability and reversibility of their electrochromic behavior after several cycling tests, are considered to be representative of the high-quality of the EC film but especially of the ITO electrode. Indeed, even if no adhesion promoters, additional surface activation pretreatments or substrate heating were used to promote the mechanical adhesion among the electrode and the PC surface, the observed EC response confirmed that the developed materials can be successfully employed for the fabrication of lightweight and cheap plastic EC devices.

1. INTRODUCTION Nowadays, one of the most compelling challenges for materials scientists and engineering is the development of novel smart materials and technologies for energy saving and recovery. Some examples include energy harvesting systems based on piezoelectric nanogenerators,1,2 photovoltaic devices,3 Li-ion batteries4 and electrochromic (EC) smart windows (SWs).5,6,7


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EC-SWs are of great interest for a wide range of applications, including vehicles and buildings of new generation. Such devices generally consist of multilayer structures, where a pair of electrochromic materials is coupled together through an ion conducting layer. 8 Tungsten trioxide (WO3) is a widely investigated electrochromic,9,10,11,12 thanks to its fast switching speed between coloring and bleaching states, as well as to the long and stable reliability of its electrochromic response.6,8 Most of the commercial EC devices are fabricated on glass supports. Nevertheless, the advent of portable and flexible electronics strongly encouraged the use of polymers as conformable, light-weight, and cheap substrate materials.13,14 Among them, polycarbonate (PC) is attracting much more attention because of its several interesting properties like unique high impact resistance and strength in a wide range of operating temperatures, transparency, low flammability, and good processing.15,16 Indium Tin Oxide (ITO, In2O3:Sn) is one of the most common transparent conductive oxides (TCO) used to fabricate transparent electrodes for photovoltaic devices, solar cells and ECSWs.6,17,18 The increasing demand for lightweight and flexible devices strongly stimulated the investigation of ITO depositions on a large variety of polymeric substrates,19,20,21 using different synthesis techniques such as sputtering,22 evaporation,23 and sol-gel.24 Among them, sputtering is one of the most investigated, mainly because of the possibility to get uniform and homogeneous materials on wide-area substrates, hence representing a promising synthetic route for the largescale production of ITO electrodes. Moreover, a strict control on the final electrical and optical properties of sputtered ITO can be achieved by simply acting on one or more deposition parameters, like using a reactive sputtering atmosphere, changing the gas pressure and flow, or tuning the energy of the sputtered atoms.25,26,27 The electro-optical properties of sputtered ITO thin films can be improved also by heating the substrates at temperatures higher than 200 °C .28,29


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Nevertheless, this represents a strong limitation when the substrate involved in the preparation of the TCO is a polymer. In such situation, fully room temperature deposition processes must be preferred. An alternative approach for increasing the electrical conductivity of ITO thin films is the deposition of multilayer ITO/metal/ITO structures.30,31,32 Despite involving low temperature depositions compatible with polymeric substrates, such synthetic method could strongly affect the optical transparency of the final electrode.33 Another critical point of ITO sputtered on polymers is the poor mechanical adhesion of the resulting electrodes, due to the low surface energy nature of polymer materials together with their pronounced surface roughness. Generally, this drawback is faced by pre-treating the polymer with plasma surface treatments,34 with the final aim of activating the polymer surface and promoting the adhesion as well. Another solution to such problems is represented by the use of either inorganic and organic buffer layers,35,36 which could increment the mechanical adhesion of ITO37 and prevent at the same time oxygen diffusion phenomena coming from the polymer towards the ITO layer,38 which could be detrimental for its electrical and optical properties. All the solutions discussed above, even if promising, strongly affect the competitiveness of the deposition process, increasing the overall costs and time production and thus reducing the possibility to easily transfer the ITO deposition process on wide-area substrates for large scale production. Moreover, the thermal stability and good gas barrier properties of ITO thin films,39,40 together with low oxygen and water vapor transmission rate,41,42 have been demonstrated. Hence, the presence of additional buffer layers could be avoided when devices operate in relatively low temperature regime (up to 80 °C). In this work we report about the optimization of the deposition process aimed at the preparation of transparent and conductive PC supports by using ITO thin films as TCO electrodes. The


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overall synthetic process was carried out by the radio-frequency (RF) magnetron sputtering technique without providing any substrate heating or performing surface-activating pre-treatment of the PC substrates. The influence of different deposition conditions (sputtering atmosphere, RF power, and gas pressure) on the morphology, crystal structure, electrical and optical properties of the resulting ITO thin films were considered and the best deposition conditions selected according to the characterization results. A further insight came from the investigation of the chemical composition both at the surface, along the whole film thickness, and at the ITO/PC interface. The good quality of the optimized ITO/PC structures was finally exploited to prepare conductive supports for the fabrication of smart windows. To this purpose, electrochromic WO3 thin films were deposited at room temperature on ITO/PC and ITO/glass supports by the reactive sputtering technique. Chronoamperometric and cyclic voltammetry (CV) measurements of the resulting single electrode structures were performed by studying their interaction with a 0.1 M lithium hydroxide solution. CV and current vs. time profiles highlighted the good electrochromic response of the WO3 thin film, which showed fast coloring/bleaching switching mechanisms. The fast switching and the stable/reversible EC response of the single electrode structures were thus considered to be strongly representative of the high-quality of the prepared ITO/PC structures, and further encouraged the use of ITO-coated plastic supports for the fabrication of stable and highly efficient plastic electrochromic devices.

2. EXPERIMENTAL SECTION The depositions of ITO and WO3 thin films were performed by the RF magnetron sputtering technique. Different substrates were considered, such as silicon (Si) wafers, glass slides, and PC


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supports. In the following a description of the different deposition conditions and of the characterization set-up is provided. 2.1. Substrates preparation Silicon and glass substrates were properly cleaned in ultrasonic baths, first with acetone and then with ethanol (10 min for each washing cycle), and dried under direct nitrogen flow. PC supports were first wiped with ethanol, then treated in an ultrasonic bath of ethanol for 10 min and finally dried with nitrogen flow. Before starting the deposition of WO3 thin films, part of the ITO/PC and ITO/glass structures was properly masked in order to get the contact area required for the electrochemical characterization of the electrochromic structures. 2.2. Sputter deposition of ITO and WO3 thin films ITO and WO3 thin films were deposited by the RF magnetron sputtering technique. Suitable vacuum conditions ranging between 2×10-5 Pa and 3×10-5 Pa were obtained through a rotary and a turbomolecular pump. A RF voltage, at a working frequency of 13.56 MHz, was employed to create the plasma. Before starting each deposition, the target was cleaned for 15 min with a sputtering process in a pure argon atmosphere, in order to prevent any incorporation of contaminants in the final thin films. All the depositions were performed with a fixed target-tosubstrate distance of around 8 cm, without providing any intentional heating to the substrates. ITO electrodes were deposited starting from a commercial 4” diameter mixed-oxide target (K. J. Lesker, In2O3/SnO2, 90/10 wt%), while a 4” diameter WO3 target (K. J. Lesker, 99.99% purity) was used for the deposition of the electrochromic thin films. All the deposition conditions used for growing ITO and WO3 thin films are reported in Table 1. In the following sections, each


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sample will be labeled according to the corresponding set of deposition parameters listed in Table 1, alongside with the particular kind of substrate. Table 1. Deposition conditions and average thickness for RF-magnetron sputtered ITO and WO3 thin films. Set of Sample sputtering name parameters

RF Power [W]

Ar flow [sccm]

O2 flow [sccm]

Gas pressure [Pa]

Deposition time [min]

Average thickness [nm]

#A

#A-Si #A-GL #A-PC

80

40

1

0.66

30

190

#B

#B-Si #B-GL #B-PC

80

39.6

0.4

0.66

30

215

#C

#C-Si #C-GL #C-PC

80

40

0

0.66

30

440

#D

#D-Si #D-GL #D-PC

80

0

1.33

30

430

#E

#E-Si #E-GL #E-PC

100

40

0

0.66

20

370

#F

#F-Si #F-GL

50

40

1

0.66

40

200

40

2.3. Single electrode EC devices preparation Single electrode electrochromic devices based on WO3/ITO/glass or WO3/ITO/PC (working electrode) as cathode, and ITO/glass or ITO/PC as anode (counter and reference electrode) were prepared and tested by chronoamperometric measurements. To evaluate the current vs. time profile of each EC device, a double potential step was provided to the structure (‫ܧ‬1 = −4.0 V, ‫ܧ‬2


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= +4.0 V versus ITO electrode; ‫ݐ‬1 = 30 s, ‫ݐ‬2 = 5 s). CV measurements were performed using a three electrode cell configuration, with WO3/ITO/PC as working electrode, platinum wire as counter and Ag/AgCl as reference electrodes. 2.4. Characterization set-up The morphology and average thickness of the developed materials was analyzed by means of a Carl-Zeiss Dual-Beam Field Emission Scanning Electron Microscope (FESEM) coupled with a Oxford Instruments X-Max 50 mm2 Silicon Drift Detector for Energy Dispersive X-ray (EDX) spectroscopy. The crystal structure and orientation was evaluated by X-Ray Diffraction (XRD) measurements, using a Panalytical X’Pert MRD PRO diffractometer in parallel beam configuration. Cu-Kα monochromatic radiation was used as the X-ray source with λ = 1.54059 Å. Hall-effect measurements were performed at room temperature by using a MMR K25003RSLP instrument. Optical properties of the investigated materials were determined by a Cary 5000 UV–vis–NIR spectrophotometer (Varian, Inc.). X-Ray Photoelectron spectroscopy (XPS) measurements were carried out with a PHI 5000 VersaProbe (Physical Electronics) system. The X-ray source was a monochromatic Al Kα radiation. XPS depth profile, by means of an Ar+ flux at 2 kV accelerating voltage, has been performed in an alternate mode with sputtering cycles of 1 min each. The relative atomic concentration (at.%) of each chemical element has been calculated from the High-Resolution (HR) spectra. The depth profile has been carried out until the interface between ITO and PC was reached. Chronoamperometric and CV measurements were performed using a CHI760D potentiostat instrument. Hence, the switching response of WO3/ITO/glass and WO3/ITO/PC structures was evaluated by investigating their interaction with a 0.1 M LiOH solution, under the application of a multi-potential step.


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3. RESULTS The following section aims to present all the results concerning the characterization of ITO and WO3 thin films. The best deposition conditions for growing high-quality ITO thin films are selected on the basis of their morphological and crystal structure characteristics, investigated by FESEM and XRD analyses, while Hall effect measurements and UV-Vis spectroscopy provided useful information regarding their electrical and optical properties. A further insight comes from XPS analyses, which allowed investigating the chemical composition and interface properties among ITO thin films and the underlying PC substrate. The morphology, chemical composition, crystal structure and the optical properties of WO3 thin films are evaluated as well by means of FESEM, EDX spectroscopy, XRD and UV-Vis spectroscopy, respectively. The optimized ITO electrodes grown on PC substrates are used as TCO for the fabrication of single electrode plastic electrochromic structures based on WO3 thin films, and the results compared to the ones fabricated on glass substrates. Therefore chronoamperometric measurements and cyclic voltammetry results are presented, showing the switching response of the prepared EC devices between colored and bleached states, and their stability after several cycling tests.

3.1. Characterization of ITO thin films

The influence of using different deposition parameters on the characteristics of the resulting ITO thin films deposited on Si substrates was first evaluated by means of FESEM analysis. The samples were investigated both considering the surface morphology and the cross-section nanostructure. Figure 1 shows the FESEM images of ITO samples #A-Si (Figure 1(a) and 1(c))


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and #D-Si (Figure 1(b) and 1(d)) grown in a mixed (Ar+O2) and inert (Ar) atmosphere, respectively. As it is clearly visible, the deposition atmosphere strongly influenced the morphological characteristics, since sample #A-Si showed the presence of small closely-packed rounded grains (see Figure 1(a)) while a flat and smooth surface without evident grains is found for sample #D-Si (see Figure 1(c)). These morphological differences are further clearly visible from the cross section analyses; the presence of columnar closely-packed nanocrystals is well evident in Figure 1(b) for the sample #A-Si grown in the reactive atmosphere while a compact and dense structure is observed in Figure 1(d) for the other case (sample #D-Si). FESEM results concerning the other samples are reported in Figure S1 of the Supporting Information (S.I.).

Figure 1. FESEM images showing the surface morphology and cross-sectional structure of samples #A-Si (a-b) and #D-Si (c-d), deposited in reactive and inert atmospheres, respectively.


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Apart from the deposition atmosphere, no relevant changes in the morphology of all the investigated ITO/Si samples are observed when considering the variation of the gas pressure from 0.66 Pa to 1.33 Pa among samples #C-Si and #D-Si. The increase of the RF power from 80 to 100 W turned the flat surface morphology of sample #C-Si into the nanostructured one of sample #E-Si, which showed the presence of rounded crystal grains (see Figure S1(c)-(e) of the S.I.).

Figure 2. Low (a) and high (b) magnification FESEM images showing the surface of ITO sample #D-PC, deposited on PC substrate with the optimized deposition parameters.

FESEM analyses were also carried out to investigate the morphology of ITO samples grown on organic PC substrates. Figure 2 shows the FESEM results of sample #D-PC while those of the other samples are shown in Figure S2 of the S.I. When PC was used as substrate, ITO films were generally characterized by the presence of cracks, meaning that a poor mechanical adhesion to the plastic substrate was present in these cases. However this was not observed for sample #DPC, which is shown in Figure 2(a) and 2(b) at lower and higher magnifications, respectively. No cracks or delaminating phenomena were detected in this case. Such aspects are quite remarkable, since no additional treatments were performed on the pristine polymeric surface to improve the


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adhesion of the TCO. Moreover, some differences of the surface morphology could be observed with respect to the corresponding sample grown on Si substrate. The compactness and uniformity previously evidenced in Figure 1(d) was confirmed but when the substrate changed from Si to PC the presence of small grains was detected. The chemical composition of ITO samples grown on PC substrates was evaluated by XPS analyses. The final aim was to estimate the chemical content of the different samples and also to investigate the interface properties between the ITO thin film and the underlying PC substrate. For this reason, XPS measurements were first carried out at the surface of the samples after cleaning from adventitious carbon. After that, depth-profile XPS analyses were performed to study the atomic distribution along the film thickness; the ITO coating was sputtered until the interface with the PC substrate was reached. At this point, the interface chemical states were evaluated. From the quantitative analysis of the ITO surface summarized in Table 2, a higher oxygen content was found for samples #A-PC and #B-PC, i.e., those grown in a mixed atmosphere. Table 2. Quantitative chemical composition of ITO thin films estimated by XPS measurements. Data refer to samples grown on PC substrates. Sample name

O1s at. %

In3d at. %

Sn3d at. %

#A-PC

44.4

53.1

2.5

#B-PC

43.2

54.2

2.6

#C-PC

42.5

55.4

2.1

#D-PC*

39.0

38.1

2.5

#E-PC*

41.9

52.2

1.9

*

: adventitious carbon not completely removed from the surface of the sample after 1 min cleaning process.


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The Sn atomic % was found almost equal for all the samples, independently of the deposition conditions. The distribution of O, In, and Sn species along the film thickness resulted to be quite uniform as visible from depth-profile results shown in Figure 3(a) for sample #D-PC, and in Figure S3 of the S.I. for the other samples. By the analysis of the High-Resolution (HR) C1s peak acquired at the ITO-polymer interface the presence of four different chemical bonds involving C and O atoms could be observed. Table 3 summarizes the area under the different C1s peak components for all the analyzed samples, while the HR C1s peak of sample #D-PC and of all the other samples are shown in Figure 3(b) and S4 of the S.I., respectively. C=O groups are polar chemical bonds involving oxygen and carbon atoms belonging to both the PC substrate and ITO as well, while C−O, π−π*, and C−C/CH components are representative of chemical bonds present in PC chemical structure. The highest % area of C=O polar groups was obtained for sample #D-PC.

Figure 3. (a) Depth-profile XPS analysis and (b) HR C1s peak related to sample #D-PC grown on PC substrate.


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Table 3. XPS results related to the acquisition of C1s peak contribution detected from depthprofile analyses performed on ITO thin films deposited on PC substrates. Sample name

C1s peak components, area [%] C=O

C−O

π−π*

C−C/CH

#A-PC

6.16

18.52

3.94

71.38

#B-PC

9.20

21.12

4.15

65.53

#C-PC

6.67

19.70

2.71

70.92

#D-PC

13.07

17.55

3.56

65.82

#E-PC

9.38

20.31

3.80

66.50

The crystal structure and orientation of ITO samples deposited on Si substrates were investigated by means of XRD analysis, and the corresponding patterns are reported in Figure 4(a). The diffraction spectra of reactively sputtered ITO samples #A-Si and #B-Si are typical of polycrystalline materials, showing multiple diffraction peaks located at different 2θ angles and ascribable to the In2O3 cubic phase. In these cases the presence of a preferential (222) orientation could be inferred from the presence of sharp and intense diffraction peaks positioned at about 30.52° and associated to the aforementioned crystal direction. On the contrary, when ITO thin films were grown in inert atmosphere, i.e., samples #C-Si, #D-Si, and #E-Si, no evidence of preferential crystal orientations was observed, since the corresponding diffraction patterns just showed a single weak and broadened peak, located at around 35.4° and ascribable to the (400) crystal planes. A similar reduction of the crystalline nature was also found for ITO electrodes grown on PC supports, whose XRD patterns are shown in Figure 4(b). Independently of the deposition


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parameters, all the investigated samples exhibited similar characteristics. The presence of weak (222) and (400) diffraction peaks, together with a broadened hump extending between them, witnessed the coexistence of either amorphous and In2O3 cubic bixbyite crystalline phase.43

Figure 4. XRD patterns of ITO thin films grown on (a) Si and (b) PC substrates. The different diffraction peaks were compared with those of cubic phase In2O3 (JCPDS card n. 89-4595) and the corresponding crystal planes properly identified.

The optical properties of ITO thin films were evaluated by means of UV-Vis spectroscopy. Figures 5(a) and 5(b) show the transmittance spectra acquired for all the samples deposited on glass and PC substrates, respectively. A general decrease of the transmittance (T) in the visible range was observed for the samples grown in a mixed Ar+O2 atmosphere. On the contrary, sputtering in a pure Ar atmosphere generally resulted in the improvement of the transmittance of the developed materials (samples #C-GL, #D-GL, #E-GL). To better notice all these aspects, the average T% values at 600 nm and 800 nm are summarized in Table 4 for either ITO/glass and ITO/PC samples. In particular samples #D-GL and #D-PC showed the highest transparency, with


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an average T in the order of 90%. Figure 5(b) also highlights that ITO/PC samples generally exhibited a lower T than pristine PC.

Figure 5. (a) UV-Vis spectra for ITO/glass samples. (b) UV-Vis spectra for pristine PC and ITO/PC samples.


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Table 4. Optical transmittance percentage detected at 600 nm and 800 nm for ITO electrodes grown on glass and PC substrates. Sample name

Optical transmittance (T) @ 600 nm [%]

Optical transmittance (T) @ 600 nm [%]

#A-GL #A-PC

74 78

89 75

#B-GL #B-PC

71 70

85 84

#C-GL #C-PC

88 83

83 84

#D-GL #D-PC

93 92

90 88

#E-GL #E-PC

79 70

81 80

The electrical properties of ITO/glass electrodes were investigated by Hall effect measurements to estimate their sheet resistance (s), resistivity (ρ), and carrier density (n). All the estimated values are summarized in Table 5. In all the samples electrons were found as majority carriers, as expected. Moreover, as clearly visible from the reported data, a remarkable decrease of s and ρ values was detected between the samples deposited in a reactive atmosphere and the other ones. In particular low ρ values comprised between 6.9 ohm/sq and 9.7 ohm/sq were found for samples grown in inert atmosphere.


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Table 5. Electrical characteristics for ITO electrodes grown on glass substrates. Sample name

Sheet resistance (s) [ohm/sq]

Resistivity (ρ) [ohm·cm]

Carrier density (n) [cm-3]

#A-GL

1.1×105

5.4

2.0×1017

#B-GL

6.4×104

3.2

4.5×1017

#C-GL

6.9

3.4×10-4

7.1×1020

#D-GL

8.1

4.0×10-4

4.7×1020

#E-GL

9.7

4.9×10-4

8.6×1020

3.2. Characterization of WO3 thin film

The morphology and average thickness of WO3 thin films grown by using the deposition conditions of set #F (see Table 1) were investigated by means of FESEM analysis, which pointed out the presence of a smooth, flat, and uniform surface. No columnar grains were observed from the cross-section analysis of the 200 nm-thick investigated sample (see Figure 6(a)). The chemical composition of the WO3 sample was also analyzed by performing EDX spectroscopy. The corresponding spectrum, together with the quantitative compositional analysis, are shown in Figure 6(b).


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Figure 6. (a) FESEM image showing the cross-sectional structure of WO3 thin film on Si substrate; (b) EDX spectroscopy of WO3 thin film, together with the quantitative compositional analysis.

Different chemical elements were detected. Oxygen and tungsten were the most abundant. However, small amounts of silicon and carbon were also observed and ascribed to the substrate contribution and to contaminants adsorbed on the sample surface, respectively. Finally XRD and UV-Vis measurements, reported in Figure 7(a) and 7(b) respectively, evidenced the amorphous nature of the WO3 thin film (absence of any diffraction peaks on the whole acquisition range) and its marked transparency in the visible region.


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Figure 7. (a) XRD pattern showing the amorphous nature of the analyzed WO3 thin film. (b) UV-Vis spectrum of WO3 thin film.

3.3. Characterization of the electrochromic behavior of WO3/ITO/PC and WO3/ITO/glass structures

Single electrode EC devices were prepared in order to test ITO/glass and ITO/PC structures as electrodes for the fabrication of plastic smart windows. To this purpose, ITO thin films were grown both on glass and PC supports with the set of deposition conditions labeled as #D (see Table 1). Then, a 200 nm-thick WO3 thin film was sputtered onto the ITO/glass and ITO/PC substrates, and the interaction of the prepared EC devices with a 0.1 M LiOH solution analyzed by chronoamperometric experiments. In this way, the switching response and stability of the device during cycling tests could be evaluated, as also the coloring/bleaching capacity of the considered systems. According to the literature,44 the electrochemical insertion of lithium cations into WO3 occurred in the selected potential range according to the following reaction: WO3 (colorless) + ‫ݔ‬Li+ + ‫ݔ‬e− ←→ Li‫ݔ‬WO3 (blue)


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The chronoamperometric profiles confirmed that the coloring/bleaching processes during the repeated potential cycling were reproducible and reversible, as visible from the current vs. time profile shown in Figure 8(a) and 8(b) for the two different structures.

Figure 8. Chronoamperometric profile of single electrode (a) WO3/ITO/glass and (b) WO3/ITO/PC electrochromic structures. Panels (c) and (d) provide a magnified view of the current peak during the single coloring and bleaching cycle, together with the corresponding EC structures in the colored state.

Electrochemical studies were also performed in order to understand the electrochromic properties of the investigated WO3 thin films. The corresponding cyclic voltammograms are shown in Figure 9. During the chatodic potential scan simultaneous intercalation of positive ions and electrons into WO3 film caused reduction of W6+ ions to lower valance W5+ ions, hence leading the film to the colored state. Conversely, during anodic potential scan deintercalation of


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counter ions and electrons occurred, inducing the bleaching of WO3 film that appeared completely transparent.45,46,47

Figure 9. Cyclic voltammograms for the WO3 thin film, at scan speed of 20 mV·s−1.

4. DISCUSSION The electrical conduction mechanism of ITO is based on the presence of oxygen vacancies and on the substitutional doping of In3+ ions with Sn4+ ones as well. Oxygen vacancies (OVs) provide a pair of conduction electrons, while each introduced Sb dopant contributes with a single free carrier. However, the introduction of both OVs and the dopant must be carefully controlled. Indeed, an excess of Sn metallic species and/or OVs results in the limitation of the optoelectronic properties of the TCO, which could thus be highly conductive but with a reduced transparency. Indeed, both OVs and Sn can strongly deteriorate the ITO crystal structure, introducing a high amount of defects which represent trapping sites for charge carriers moving along their conduction pathways, or induce color center traps and consequently decrease the optical


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transparency of the TCO itself. In order to get a correct trade off to maximize both the electrical conductivity and the optical transparency, suitable deposition conditions must be accordingly selected. In the particular case of the sputtering technique, several parameters can affect the final properties of the deposited material. In particular, the gas pressure and the RF power, as well as the sputtering atmosphere, are considered the key elements to be properly changed and selected for having high-quality TCO layers.25,26 Another important parameter is the deposition temperature; heating the substrates should help in growing more conductive polycrystalline ITO thin films.48,49 Nevertheless, the introduction of grain boundaries connected to the presence of single-crystal grains could strongly limit the overall ITO electrical and optical properties, since also these act as defect centers and their formation must be limited.49 Moreover, roomtemperature deposition processes are mandatory in view of using polymeric substrates. In the present work, one of the main factors affecting the final characteristics of ITO thin films is found to be the sputtering atmosphere. This resulted to strongly affect the morphology, crystal structure, chemical composition, and finally the resulting electrical and optical properties. FESEM analyses carried out on ITO/Si samples underlined that the presence of a reactive atmosphere promoted the columnar growth regime during the deposition (samples #A-Si and #BSi). In these cases the final surface nanostructure resulted in small closely-packed rounded grains (see Figure 1(a)). The reactive atmosphere influenced the cross-section nanostructure as well, which displayed closely-packed, vertically oriented nanocrystals, with the presence of a high grain boundary density (see Figure 1(b)). The ITO nanostructure strongly changed when the deposition process was carried out in inert atmosphere. Samples #C-Si, #D-Si, and #E-Si are representative of this situation. The observed surface morphology was indeed flat and smooth as shown in Figure 1(c). Furthermore, no evidence of vertical columnar grains was visible (see


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Figure 1(d)), thus grain boundaries formation is expected to be highly reduced. From the crosssection FESEM analyses, the average thickness of each ITO/Si sample was also estimated and reported in Table 1. Using a mixed Ar+O2 atmosphere significantly decreased the deposition rate with respect to the inert case, thereby affecting the final morphology. This aspect further helps in the comprehension of the observed morphological differences. Indeed, for lower deposition rates the formation of small crystal grains on the substrate surface is promoted from the earliest steps of the growth process. This finally results in the formation of the observed columnar thin films having nanometer-sized, closely-packed crystal grains. ITO thin films were also grown on PC supports. The resulting samples were analyzed by FESEM, which pointed out the influence of using a different substrate (PC instead of Si) on the adhesion properties of the resulting ITO thin films. ITO/Si samples did not show any problems of mechanical adhesion. On the contrary, the presence of delaminating phenomena and cracks was found for some samples grown in inert atmosphere (#C-PC and #E-PC). A better mechanical quality was instead visible for sample #DPC and for those grown in a reactive atmosphere. Nevertheless, it is worth mentioning that reactive atmosphere conditions resulted in reduced electro-optical properties, as previously observed. The differences in morphology of ITO samples grown on Si substrates with different deposition atmospheres are further supported by the analyses of the corresponding crystal structure. Generally the two main crystal orientations of ITO thin films are the (400) and (222),48 both related to the presence of a cubic In2O3 crystalline phase. The preferential growth along the former or the latter is often dependent on the presence or absence of reactive oxygen in the sputtering atmosphere. Better electrical properties and worse optical ones characterize (400)oriented ITO samples.21,28,50,51 On the contrary, when a preferred orientation along the (222)


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direction is present, the resulting ITO films show a remarkable optical transparency to the detriment of the electrical conductivity.27,28,51,52 Both these situations well describe the present case. From the XRD patterns shown in Figure 4(a) is evident that the addition of oxygen (samples #A-Si and #B-Si) promoted a polycrystalline structure with a preferential orientation along the (222) direction,53 as witnessed by the presence of the characteristic diffraction peak associated to the corresponding family of crystal planes. Such kind of nanostructure is further evidenced from the presence of vertical columnar grains pointed out from FESEM analyses. A weak crystal orientation along the (400) direction is then observed for ITO thin films deposited in inert atmosphere. However, the corresponding diffraction patterns clearly witness their reduced polycrystalline nature, and even suggest the presence of an amorphous phase, in agreement with the characteristics of the cross-sectional structure highlighted by FESEM results. In addition to FESEM results, the influence of using a PC substrate on the characteristics of the resulting ITO thin films is confirmed by XRD measurements as well. Figure 4(b) shows the diffraction patterns collected from ITO/PC samples. Independently of the deposition conditions, all the electrodes grown on PC supports showed the coexistence of either crystalline cubic In2O3 and amorphous phase, witnessing their reduced crystalline nature. Nevertheless, this aspect should not be considered a drawback at all, since better mechanical properties of amorphous ITO thin films than polycrystalline ones have been reported.54,55 Lastly, the observed dependence of either mechanical adhesion and crystalline nature of the resulting ITO/PC samples from the substrate material can be related to the inhibitory effect of amorphous PC towards ITO crystallization.56,57 The influence of the sputtering atmosphere is highlighted again from Hall measurements, which further confirmed that the presence of a mixed gas atmosphere decreased the electrical


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conductivity of ITO samples, being characterized by high resistivity and sheet resistance values in the order of 3−5 ohm·cm and 104 −105 ohm/sq., respectively. This should be merely due to the higher amount of oxygen incorporated in the ITO layers as confirmed by XPS data shown in Table 2, and also because of a high grain boundary density as well, which limited the motion of electrons along their conduction pathways.21,28,50,51 On the contrary, samples grown in inert atmosphere showed low sheet resistance values in the order of 7−10 ohm/sq. The resistivity values were lower too, ranging between 3.4×10-4 and 4.9×10-4 ohm·cm. The reduced presence of grain boundaries, together with the limited amount of oxygen are considered the main factors responsible for the improved electrical properties of this family of samples.27,48,58 A remarkable difference in the carrier concentration is also observed, since it changed from a minimum of 2×1017 cm-3 for samples grown in a reactive atmosphere to a maximum value of 8.6×1020 cm-3 for those grown in inert one. Hence, in this work, the key factor influencing the carrier concentration is believed to be mainly the amount of oxygen. Indeed, the oxygen deficiency of ITO samples grown in inert atmosphere induced a higher presence of oxygen vacancies, highly increasing the free carrier concentration with respect to the reactively grown oxygen-rich ones.21,28,50,51 Oxygen addition to the sputtering process should improve the optical transmittance of ITO thin films,50,51 however this aspect is not visible in the present work. Conversely, the optical transmittance of reactively sputtered ITO films is found to be reduced. Again, this aspect is considered to be merely due to the influence of the sputtering atmosphere on the morphological characteristics of the resulting thin films. Indeed, as previously observed, the reactive atmosphere promoted the columnar growth of polycrystalline films having closely-packed small rounded grains. Optical properties are influenced by crystal defects as well.59 Light-scattering


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phenomena occurring at the surface of the ITO layer are expected to be more pronounced in the case of rough surfaces characterized by small, closely-packed crystal grains,60 as in the case of the samples grown in a reactive atmosphere, thus limiting the overall optical transmittance of the material. On the contrary, for flat and smooth surfaces characteristics of ITO thin films grown in inert atmosphere, the aforementioned scattering events are reduced and consequently the optical transmittance results to be improved. In particular, an average transmittance equal or higher than 90% is found in the visible range for those samples grown by using the deposition parameters of set #D. Depth-profile XPS analyses were carried out to investigate the interface properties among the ITO samples and the underlying PC substrate. Generally direct depositions of ITO coatings on untreated PC substrates result in non-adherent layers, since PC shows a low surface free energy (SFE) and a porous, irregular microstructure.34 Different works demonstrate the possibility of improving the mechanical adhesion between ITO thin films and the PC surface after performing plasma pretreatments of the pristine polymeric substrate, which can be merely obtained by ion treatment processes.61,62,63 The final aim of these additional pretreatments is mainly to increase the SFE of the polymer itself. This is generally represented by the increased amount of C=O polar surface groups,62,63 which help in promoting the subsequent formation of a mechanically stable ITO thin film. From the analysis of the HR C1s peak acquired at the ITO/PC interface, it is noticed that the area under the C=O peak, i.e. the amount of polar C=O groups at the interface among the polymer and the ITO electrode, is maximum in the case of sample #D-PC. The absence of mechanical adhesion defects like cracks or delaminating phenomena highlighted from FESEM analyses can be thus ascribed to the predictable higher SFE energy of the PC substrate itself, which turned into the formation of most stable chemical bonds between the exposed PC


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surface and the deposited ITO thin film. However, no pretreatments were performed on pristine PC in our case. The reason for the increased presence of polar groups must be explained in a different way. Sputtering is a plasma-assisted deposition process. In the initial steps of the deposition process, the plasma present in the deposition chamber directly interacts with the uncoated PC surface, before that a continuous ITO network is formed on the surface itself. Thus, it is believed that the particular deposition parameters of set #D allowed to achieve suitable plasma density conditions able to increase the amount of polar C=O groups on the uncoated PC. This mainly depends on the energy of ions impinging on the uncoated PC substrate in the early moments of the deposition process. Therefore, we took advantage of the presence of plasma not only for depositing ITO thin films, but also to further activate the PC surface through the ionic bombardment of the sputtering process, which resulted in the appearance of a higher amount of polar C=O groups (see Table 3). In particular the set of deposition conditions used for sample #D_PC is retained to be the most promising for inducing a higher amount of C=O polar groups, and thus to increase the SFE of the PC surface, finally resulting in the improvement of the mechanical adhesion of the consequently deposited ITO electrode. Hence, the best compromise among the electrical and optical properties, together with mechanical adhesion, was found for those samples grown with the deposition parameters of set #D, since showing a high T in the order of 90%, an average s value of 8 ohm/sq., and a better mechanical adhesion to PC, as previously observed from FESEM results shown in Figure 2. Nevertheless it is worth noting that, although their better electrical conductivity, our optimized ITO/PC samples showed a slightly lower optical transparency if compared to pristine PC (~ 98%) and some commercial ITO-coated polymers.


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Finally, additional ITO/glass and ITO/PC samples, prepared according to the set of optimized deposition parameters, were tested as transparent conductive supports to be exploited for the future fabrication of plastic smart windows of new generation. To this purpose, single electrode EC devices were prepared by depositing a 200 nm-thick WO3 EC thin film onto both ITO/PC and ITO/glass substrates. The FESEM analysis of the EC film pointed out the absence of any columnar growth, as visible in Figure 6(a), suggesting the amorphous nature of the investigated sample. Moreover, EDX spectroscopy showed a sufficient incorporation of oxygen in the grown material (14.42 at.% W, 50.51 at.% O). This was pursued by growing the EC film in an oxygenrich sputtering atmosphere. The amorphous nature of WO3 was further confirmed by XRD measurements shown in Figure 7(a), and considered to be related to the room-temperature deposition process used for growing WO3. Indeed, no substrate heating was intentionally provided since the final goal is the development of functional materials which can be integrated in polymer-based EC devices. The deposition process of WO3 thin film in a reactive atmosphere also resulted in a good transparency of the grown EC layer, as visible from the UV-Vis spectrum shown in Figure 7(b). The prepared EC structures were tested by chronoamperometric experiments, evaluating the EC stability under several cycling tests and the switching response of the single electrode structures when interacting with a 0.1 M LiOH solution. The current vs. profiles shown in Figure 8(a) and 8(b) confirmed the stability and reversibility of the EC structures when repeatedly cycled under the application of a multistep potential, independently of the kind of substrate, i.e. glass or PC. Moreover, the reported measurements evidenced the fast coloring and bleaching of the overall structure, since it happened after just few seconds. The stability of the EC behavior was also highlighted, since a full recovery of the colored/bleached state was observed. The coloring of the WO3 film was quite pronounced and uniform on the


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whole area, independently of the substrate (see Figure 8(c) and 8(d)). Although this was not surprising for the glass-based case, this is instead quite remarkable for the ITO/PC structure, as it was not so obvious if considering all the problems that could arise from the irregularities of the PC surface and the consequent mechanical adhesion problems affecting the overlying ITO electrode. No mechanical degradation of both the glass-based and PC-based EC structures was observed after several cycling tests. Pictures representative of this condition are shown in Figure 8(c) and 8(d), where the EC samples are left in their colored state; the WO3 layer still appeared uniformly colored over the whole area. CV measurements shown in Figure 9 further highlighted the good electrochemical behavior of the single electrode EC devices based on PC substrates. The anodic peak was clearly visible in the range of -0.2 ÷ -0.3 V and the well-defined shapes of CV curves were maintained during cycling. This suggested electrochemical stability and good reversibility of ionic exchange with the electrolyte together with a fast Li+ diffusion that occurred at the WO3/electrolyte interface. All the described aspects are highly representative of the high-quality of ITO/glass and ITO/PC substrates developed according to our optimized deposition parameters, and further support their electrical, optical, and mechanical stability, capable of providing the required electrical and mechanical contact between the plastic substrate and the overlying electrochromic coating.

5. CONCLUSIONS This work focused on the development of a completely room-temperature deposition process to prepare ITO-coated polycarbonate (PC) structures suitable for the fabrication of lightweight plastic smart windows to be integrated in the new-generation of high energy-efficiency buildings


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and vehicles. Different deposition conditions were used for growing ITO thin films by the radiofrequency magnetron sputtering technique, without using any surface activation pre-treatment of the bare polymer supports nor heating the substrates during the depositions. According to the characterization results, the optimal deposition conditions were selected for growing high-quality ITO thin films with a 90% optical transmittance in the visible range together with a sheet resistance of 8 ohm/sq. The optimized ITO/PC structures were then successfully tested as conductive supports for the fabrication of smart windows based on plastic substrates. To this purpose, EC WO3 thin films were grown by the reactive sputtering technique on both ITO/glass and ITO/PC supports. The resulting single electrode EC devices were characterized by chronoamperometric experiments using a 0.1 M electrolytic solution of lithium hydroxide and their behavior were compared. CV and current vs. time profiles highlighted the good electrochromic response of the WO3 EC thin film, which showed fast coloring/bleaching switching mechanisms. The fast switching response, together with the stability and reversibility of the single electrode EC devices after several cycling tests, confirmed the high-quality of the ITO electrode, independently of the kind of support, i.e. glass or PC. Indeed, even if no adhesion promoters, additional surface activation pretreatments or substrate heating were used to promote the mechanical adhesion among the electrode and the PC surface, we showed from depth-profile XPS analyses that suitable sputtering deposition parameters can be selected to get an adequate plasma density able of increasing the surface free energy of the pristine PC surface in the early steps of the growth process. This resulted in the improvement of the mechanical adhesion among ITO and the polymer itself. The considered EC structures thus showed a remarkable mechanical and electrical stability after repeated cycling tests, confirming that the developed materials can be successfully employed for the fabrication of polycarbonate-based plastic EC devices and


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further encourage their exploitation for the improvement of the energy efficiency in the next generation of buildings and vehicles.

ASSOCIATED CONTENT Supporting Information. FESEM images of ITO thin films grown on Si and PC substrates. Depth-profile XPS analyses of ITO thin films grown on plastic PC substrates. High-Resolution C1s XPS peaks collected at the ITO/PC interface, after depth profiling. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author *Phone: +39 011 090 7393. E-Mail: [email protected] Author Contributions All authors have given approval to the final version of the manuscript. ACKNOWLEDGMENT This work has been developed as part of the WINFIRE project. The authors gratefully acknowledge Gallina s.r.l, Elettrorava s.p.a, and Pegaso s.r.l for their collaboration, and the Piedmont Region of Italy that partially funded this work. The authors gratefully acknowledge Dr. Marco Fontana for the useful support with FESEM measurements. ABBREVIATIONS


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EC, electrochromic; ITO, indium tin oxide; PC, polycarbonate; SWs, smart windows; ECs, electrochromics; WO3, tungsten trioxide; TCO, transparent conducting oxides; RF, radiofrequency; Si, silicon; GL, glass; FESEM, field-emission scanning electron microscopy; EDX, energy dispersive X-ray spectroscopy; XRD, X-ray diffraction; XPS, X-ray photoelectron spectroscopy; HR, high-resolution; UV-vis, ultraviolet-visible spectroscopy; OVs, oxygen vacancies; SFE, surface free energy.

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