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Polythiophene -viologen bilayer for electro-trichromic device
Solar Energy Materials and Solar Cells 188 (2018) 249–254
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Polythiophene -viologen bilayer for electro-trichromic device
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Anjali Chaudhary, Devesh K. Pathak, Suryakant Mishra, Priyanka Yogi, Pankaj R. Sagdeo, ⁎ Rajesh Kumar Material Research Laboratory, Discipline of Physics & MEMS, Indian Institute of Technology Indore, Simrol 453552, India
A R T I C LE I N FO
A B S T R A C T
Keywords: Electrochromic Viologen P3HT
An electrochromic device, capable of three-level color switching, has been envisaged by exploiting different color switching properties of Ethyl viologen (EV) and Poly (3-hexylthiophene) (P3HT) to make them switch in congruence with each other. The device, consisting of an EV/P3HT bilayer in poly-ethylene oxide matrix, appears magenta, blue or transparent under an appropriate bias. The unbiased device shows magenta colored appearance and turns transparent at a bias of 1 V when P3HT layer is used as anode. The device turns blue on increasing the bias to 1.4 V without changing the polarity. The mechanism of multilevel switching has been identified using UV–Vis and Raman spectroscopic investigations which reveal that the 1 V bias converts the P3HT into polarons and 1.4 V converts the EV dications (EV2+) into free radical. The electrochromic device demonstrates better color contrast (~ 50%) in both the optical domains. The fabricated device shows better switching speeds of ~ 1.5 s and good stability and coloration efficiency along with three-level color switching. However the reported device uses ITO coated glass as substrate, it can be used for all-organic flexible electrochromic device.
1. Introduction Concept of electrochromism, a phenomenon known for long, refers to a reversible color switching under electrical bias [1–7], has instigated the search for new functional materials for making smart devices. Electrochromic devices, fabricated using various active materials by adopting different design paradigms, are in use in displays, smart buildings, and aircrafts as well as in supercapacitors.[8–15] In recent years, research activity in electrochromism has seen tremendous interest due to its direct as well as indirect involvement in addressing the increasing energy requirements. Electrochromism addresses this issue by making solar cells more energy efficient and also through direct involvement in application as supercapacitors. However, various organic, inorganic and hybrid electrochromic materials are available, continuous search of new materials is going on mainly due to two reasons, firstly, for making a device more and more energy efficient and secondly to give the device a capability to play multifunctional roles and exhibit multiple colors. The latter gives an additional advantage as this leads to the application of electrochromic devices as tunable filters. Materials exhibiting electrochromic properties are the mainly ones which show different optical properties in their different redox [16] states and include [9,16,17] different metal oxides as well various organic counterparts [18–20]. All of these materials have certain
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advantages and disadvantages thus chosen accordingly. The organic materials that show electrochromism include polyaniline [21], poly (3, 4-ethylenedioxythiophene) & other derivatives of polythiophene [22], polypyrrole, viologens etc. They have attracted special attention due to their unique advantages over inorganic metal oxides. Their fabrication processes does not involve complicated steps and can be easily processed into uniform, transparent and amorphous films using solution casting, spin coating or electro polymerization. Electrochromic materials are also classified as “cathodic” or “anodic” based on how their color changes on reduction and oxidation respectively. Prussian white changes its color from colorless to blue on oxidation [23–26] and thus come under the umbrella of anodic electrochromic materials while PEDOT, WO3 [23] are categorized as cathodic coloring electrochromic materials. Knowing the category of a given material helps in designing an electrochromic device and choosing the other counter species for supporting the process. An electrochromic material changes color between the transparent state and colored state on application of a bias of appropriate value and polarity, with a possibility of the colored state being the opaque state [27]. Under this known fact, if two materials, for which the colored states are different, are combined appropriately, the device will have the capability of switching between the two colored state with an intermediate transparent state thus will help in making a multi-level
Corresponding author. E-mail address: [email protected] (R. Kumar).
https://doi.org/10.1016/j.solmat.2018.08.029 Received 7 August 2018; Received in revised form 30 August 2018; Accepted 31 August 2018 0927-0248/ © 2018 Elsevier B.V. All rights reserved.
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right panels) with an intermediate transparent state at 1 V (central panel) bias. It is clear from the schematic (Fig. 1) that viologen and P3HT have been used as cathodic and anodic species respectively in the electrochromic device. In the unbiased state, device appears magenta which is the color of neutral P3HT as the viologen in PEO matrix is transparent. When a positive bias (1 V) is applied to the ITO electrode directly connected to P3HT film, as per the polarity arrangement shown in Fig. 1, the device turns transparent. The most likely reason for this switching (magenta to transparent) is the oxidation of P3HT from its neutral state to polaronic state due to applied bias. The same will be substantiated using various direct and indirect methods (Raman and CV) and will be discussed later. On further increasing the voltage from 1 V to 1.4 V with the same polarity (Fig. 1), the device turns blue from transparent likely due to the color switching of viologen layer. At this voltage, the transparent state of P3HT layer, achieved in the previous stage, is sustained whereas the conversion of viologen from dicationic state (EV2+) to free radical state (EV+•) take place on the cathode which appears blue [30]. The dynamic doping of P3HT, mentioned above, accompanied by reduction of EV2+ enables the bi-organic device, consisting of P3HT and viologen, to switch amongst three color states. Here the dynamic doping dominates the magenta to transparent switching whereas the latter manifests in transparent to blue switching of the device. The overall color switching of the device is actually a result of color switching at two different electrodes, sandwiched together between two electrodes rather than switching of the entire active region altogether. This has been established by carrying out bias dependent measurements by placing the two separate electrochromic electrodes in an electrochemical cell in the presence of electrolyte consisting of 0.5 M lithium perchlorate (LiClO4) in acetone. This mimics the same system as in the solid state device shown in Fig. 2. Fig. 2a shows the schematic arrangement of separated electrodes whereas actual images of both the electrodes under different bias conditions are shown in Fig. 2b-d. It is very clear that under the unbiased condition (Fig. 2b) the anode (P3HT) appears magenta due to the neutral polymer and cathode (EV) appears transparent due to EV2+. This situation is analogous to the magenta appearing device in solid state (Fig. 1). On application of a bias of one volt, the anode switches to transparent whereas cathode remains transparent (Fig. 2c). This color toggling of anode is due to oxidation of P3HT resulting in dynamic doping and the same process will lead to a transparent solid state device. When the bias is increased to 1.4 V, by keeping the polarity intact, the cathode's color changes from transparent to blue (Fig. 2d) due to reduction of viologen from EV2+ to EV+• with no effect on anode. This condition, in the device form, will lead to a blue appearing device (Fig. 1). The whole color change process can be seen in video available as supporting information (SI) uploaded on the web. The overall redox process leading to bias dependent multiple color change can be represented as shown below in Scheme 1 where it can be seen that conversion of P3HT to its polaron state takes place [31] at 1 V
switchable electrochromic device. Furthermore, if both of these materials are organic this will have a further advantage in making an allorganic flexible device. The colored state of polythiophene and viologen [28], both are solution processible, are magenta and blue respectively thus may be explored for abovementioned multilevel switching purpose. These two being the most studied organic materials due to their wide range of applications [29] gives an additional incentive for this exploration. Furthermore, from electrochromic application point of view, polythiophene and viologen have low operating voltages in range of around one volt while showing a good optical modulation. In the present work, electrochromic properties of Poly(3Hexylthiophene) (P3HT) and ethyl viologen diperchlorate (EV) based device has been studied by making device where a P3HT/EV bilayer has been sandwiched between two ITO coated glass electrodes. An appropriately designed device exhibit multiple color switching between magenta (when unbiased) and blue state (when biased) showing an intermediate transparent state at relatively lower bias as compared to the blue state without the need of an electrolyte layer. Absence of electrolyte makes the multilevel switching device power efficient. P3HT and EV acts as an electron donor /electron acceptor pair and improves the overall performance of device quantified through coloration efficiency, switching speeds and stability, directly connected to the cyclability of device. 2. Experimental details Commercially available chemicals from Alfa Aesar and Sigma Aldrich have been used for device fabrication. Polyethylene oxide (PEO, Alfa Aesar, MW=100,000), Ethyl viologen diperchlorate (EV, 98%, Sigma Aldrich), Poly (3 Hexyl thiophene-2, 5 diyl) (P3HT, regioregular, Sigma Aldrich), 1, 2- Dichlorobenzene (DCB, anhydrous, 99%, Sigma Aldrich) and Acetonitrile (ACN, anhydrous, 99%, Sigma Aldrich) are used as received. P3HT in DCB and EV have been deposited in PEO matrix on ITO glass substrates and then assembled together to form a device. Various steps involved in device fabrication have been discussed in Supporting information (SI) in detail (Fig. S1). The transmission spectra have been recorded at room temperature on a UV–Vis spectrophotometer (Cary 60 of Agilent make). Raman spectroscopy measurements have been carried out using LABRAM HR spectrometer with 633 nm excitation source. For Chronoamperometry measurements, Keithley 2450 workstation was used. 3. Results and discussion The actual photographs of the fabricated device under a given bias condition are shown in Fig. 1 along with the device schematic showing the biasing polarity arrangements. As hypothesized above, the device changes color from magenta to blue on application of 1.4 V (left and
Fig. 1. Schematic illustration of electrochromic device displaying various colors with applied bias along with corresponding actual photographs of the device. 250
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Fig. 4. In situ Raman spectra of fabricated device at various applied potential along with schematic of Raman scattering experimental geometry.
correspond to red and far blue color region respectively with relatively lesser transmission in the green region apparent as a dip in this region in black curve (Fig. 3). The combination of the reddish and blueish transmission is giving the magenta look to the device as seen in the corresponding image (Fig. 3). Under 1 V bias, device transmits all the wavelengths of the visible region equally thus appears transparent [32,33] (red curve, Fig. 3). On increasing the voltage further to 1.4 V, the transmission profile remains similar to the unbiased situation but with a maximum transmission corresponding to 440 nm [34] (blue) with very low transmission in green and red wavelength regions. Thus the device appears blue. The bias dependent variation in absorption spectrum is shown in SI (Fig. S2). Most likely reason for such a bias dependent color switching has been hypothesized and will be validated below. In order to understand the hypothesized mechanism responsible for color appearance of device in-situ Raman spectroscopy has been carried out. Fig. 4 shows the schematic of Raman geometry and in-situ Raman spectra from the fabricated device under different bias values recorded using excitation wavelength of 633 nm. Raman spectrum of unbiased device (0 V) shows peaks at 1380 cm−1 and 1443 cm−1 which correspond to neutral P3HT whereas another peak at 1648 cm−1 indicates the presence of EV+2. It clearly means that the unbiased device contains neutral P3HT [35] and viologen dication (EV+2). The EV2+ being transparent, the magenta color of unbiased device is mainly due to the P3HT as the UV–Vis spectrum of the device (Fig. 3) is similar to the UV–Vis spectrum from neutral P3HT in solution as shown in the SI (Fig. S3). On application of 1 V bias, to the device with P3HT electrode connected to the anode, a peak at 1090 cm−1 is observed which is very often identified as originating due to the polaron formation due to oxidation of P3HT. The polarons formed by the dynamic doping, a wellknown phenomenon [35–37], can get stabilized by the diperchlorate ions available in the device from the viologen layer. The polaron is known to be transparent [38] to the visible spectrum and thus the whole device appears transparent [37,39]. It is worth mentioning here that though the viologen is connected to the cathode, no reduction is taking place (no viologen free radical peak in Raman spectrum). This makes both the layers, and thus the whole device, transparent when biased with 1 V. On further increasing the bias to 1.4 V, by keeping the polarity unchanged, Raman spectrum responds with peaks appearing at 1029 cm−1, 1359 cm−1, 1530 cm−1, 1658 cm−1 which are well reported modes of free radical viologen(EV+•) which is known for its blue color [19,28,34,40]. Thus the blue switching on application of 1.4 V can be understood on the basis of simultaneous formation of polaron and viologen free radical because P3HT polaron appears transparent whereas viologen is blue in its radical form (EV+•). Here, P3HT acts as a counter ion for
Fig. 2. (a) Schematic of arrangements used for electrochromic measurements in electrochemical cell and actual photographs of the electrodes at bias of (b) 0 V, (c) 1.0 V and (d) 1.4 V.
Scheme 1. Demonstration of redox reactions associated with both the stage of color switching.
at anode while reduction of EV+2 to EV+• occurs at 1.4 V at cathode when present in solid state form. The abovementioned process will be validated later on. Supplementary material related to this article can be found online at doi:10.1016/j.solmat.2018.08.029. The color switching, reported above, can be correlated with the observed spectral changes using UV–Vis spectroscopy for validation. Fig. 3 shows transmission spectra and photographs of the device corresponding to the applied bias. The in-situ UV–Vis transmission spectra (Fig. 3) show variation in transmittance as a function of applied voltage from 0 V, 1 V and then 1.4 V. The unbiased device shows higher transmissions corresponding to 650 nm and 410 nm wavelength which
Fig. 3. Transmission spectra of fabricated device at various applied potential with corresponding photographs of device. 251
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Fig. 5. (a) Schematic of set up used for switching cycle and absorption of device. (b) Change in current flowing through the device with time and (c) corresponding variation in absorbance at 530 nm. (d) Switching time; coloration and bleaching time shown by device.
device performance which is defined by following Eq. (1):
viologen and helps in it's reduction by facilitating the electron transport mechanism without relying on the environmental moisture as necessary for viologen alone devices [34]. Thus in-situ Raman spectroscopy validates the color switching mechanism, as hypothesized above, which is the redox state induced modulation in optical behavior of various species present in the predesigned device. This further emphasizes that redox reaction of P3HT/viologen at various applied potentials is the basic pillar that induces the multi-color switching. Switching time and stability are two very important parameters to measure the performance of an electrochromic device as these two are directly related to the speed and cyclability of the device. Since the device displays multiple color switching from magenta to transparent in stage-1 (refer Scheme 1) followed by switching to blue (stage-2), thus variation in absorption with time has been studied for two different wavelengths (400 nm and 530 nm) by applying appropriate switching biases. The dynamic absorption measurement along with simultaneous chronoamperometry was used to determine the switching speed and stability for switching in both the stages. For stage-1 switching (magenta → transparent), a square wave of 1 V with 10 s time interval was applied to see chronoamperometric and absorption response (Fig. 5). Fig. 5a shows the schematic of the setup used. It can be clearly seen in Fig. 5(b) that the chronoamperometric response is very stable for 480 s with little variation in maximum and minimum current values. Fig. 5(c) shows the cyclic response of the device for the abovementioned duration showing a reversible change in absorbance of the 530 nm wavelength as a result of the 1 V applied bias. The device is seen to switch 24 times in 480 s without much loss to the absorbance values for the said wavelength. To understand the actual time taken for switching, a single cycle has been analyzed by looking at a zoomed portion of the absorption variation (Fig. 5d). It is evident from Fig. 5d that the device takes ~1.5 s to reach 77% of its maximum absorption i.e. from transparent to magenta color whereas takes ~2 s (bleaching time) to go back to 67% of the maximum absorption value (i.e. switch back to transparent state from magenta) after reaching the color saturation. A switching time of ~2 s is faster than previously reported P3HT based electrochromic devices [41]. Coloration efficiency [17] (ηec) is an important parameter to access
ηec = ΔOD/Q,
(1)
where ΔOD = Ac-Ab with Ac, Ab are the absorbance in colored and bleached states respectively (values obtained from Fig. 5c), and Q is the charge per unit area of electrode. For stage 1, coloration efficiency is 222 cm2/C and related to quality factor [42] (γ), defined in Eq. (2), which comes out to be 126 cm2 C−1 s−1for stage 1.
γ = ηec / τ , τ = (τb + τc )/2,
(2)
where τb & τc are time taken in bleaching and coloration process respectively. A comparison of the performance of present device with other similar thiophene based electrochromic devices has been provided in the SI. Similar to the stage-1 switching, a 600 s dynamic switching analysis has been done for stage-2 (Fig. 6). The bias induced time dependent absorption change corresponding to 400 nm in response to a square wave of 1.4 V with 5 s time interval can be seen in Fig. 6a (black curve) along with the corresponding chronoamperometric response showing 60 cycles without much loss to the absorption (left Y-axis) and current (right Y-axis) meaning a stable color switching. The device shows enhanced stability with little variation in maximum and minimum current values over a period of 600 s. Closer look at a single switching step is required for better understanding of switching response (Fig. 6b). A turning “ON” time, the time taken to switch the absorption to > 80% of the maximum value, of 1.5 s is observed. The device takes ~ 3 s to get completely saturated to blue color after which neither the color changes further nor the 400 nm absorption. The device takes 1.2 s (less than the switching time) to bleach out also when the device turns to magenta. During this bleaching time, the absorption by the device reduces to 75% of the saturated state value, color from its blue color state changes to initial (magenta) color in 1.2 s. A faster bleaching time is observed as the tendency for polaron to neutral P3HT conversion is high because P3HT is very stable in its neutral state. At the same time viologen free radical assists polaron to go back to neutral state so that viologen free radical can also go back to its dicationic state, its stable state. Stage 2 has a coloration efficiency of 247 cm2/C and quality factor (γ) of 252
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than 50 cycles has been observed with little loss to the contrast. Coloration efficiency of more than 200 cm2/C has been obtained in both the switching stages. The matrix is also chosen to demonstrate an electrolyte less device making it an improved multiple colored organic electrochromic device with enhanced functioning and easier fabrication process involved. On the whole, the EV, being an electron acceptor, and P3HT, being an electron donor, together provide an excellent combination for a fast, stable and energy efficient option for multicolor electrochromic switching. Acknowledgements Authors are thankful to Dr. V. Sathe (UGC DAE CSR, Indore, India) for Raman measurements. Authors acknowledge useful discussion with Prof. V.D. Vankar (IIT Delhi). One of the authors (D.K.P.) acknowledges Council of Scientific and Industrial Research (CSIR) for financial assistance. Authors also acknowledge MHRD and DST, Govt. of India for providing funding. Conflict of interest Authors declare no conflict of interest. Appendix A. Supporting information Supplementary data associated with this article can be found in the online version at doi:10.1016/j.solmat.2018.08.029. References [1] M.L. Moser, G. Li, M. Chen, E. Bekyarova, M.E. Itkis, R.C. Haddon, Fast electrochromic device based on single-walled carbon nanotube thin films, Nano Lett. 16 (2016) 5386–5393, https://doi.org/10.1021/acs.nanolett.6b01564. [2] A. Agrawal, J.P. Cronin, R. Zhang, Review of solid state electrochromic coatings produced using sol-gel techniques, Sol. Energy Mater. Sol. Cells 31 (1993) 9–21, https://doi.org/10.1016/0927-0248(93)90003-L. [3] I. Mjejri, C.M. Doherty, M. Rubio-Martinez, G.L. Drisko, A. Rougier, Double-sided electrochromic device based on metal–organic frameworks, ACS Appl. Mater. Interfaces 9 (2017) 39930–39934, https://doi.org/10.1021/acsami.7b13647. [4] I. Mjejri, A. Rougier, M. Gaudon, Low-cost and facile synthesis of the vanadium oxides V2O3, VO2, and V2O5 and their magnetic, thermochromic and electrochromic properties, Inorg. Chem. 56 (2017) 1734–1741, https://doi.org/10.1021/ acs.inorgchem.6b02880. [5] M. Barawi, G. Veramonti, M. Epifani, R. Giannuzzi, T. Sibillano, C. Giannini, A. Rougier, M. Manca, A dual band electrochromic device switchable across four distinct optical modes, J. Mater. Chem. A 6 (2018) 10201–10205, https://doi.org/ 10.1039/C8TA02636J. [6] D. Dong, W. Wang, G. Dong, F. Zhang, H. Yu, Y. He, X. Diao, Improved performance of co-sputtered Ni–Ti oxide films for all-solid-state electrochromic devices, RSC Adv. 6 (2016) 111148–111160, https://doi.org/10.1039/C6RA21961F. [7] N. Penin, A. Rougier, L. Laffont, P. Poizot, J.-M. Tarascon, Improved cyclability by tungsten addition in electrochromic NiO thin films, Sol. Energy Mater. Sol. Cells 90 (2006) 422–433, https://doi.org/10.1016/j.solmat.2005.01.018. [8] X. Yin, J.R. Jennings, W. Tang, T.J. Huang, C. Tang, H. Gong, G.W. Zheng, Largescale color-changing thin film energy storage device with high optical contrast and energy storage capacity, ACS Appl. Energy Mater. 1 (2018) 1658–1663, https://doi. org/10.1021/acsaem.8b00120. [9] S. Mishra, P. Yogi, P.R. Sagdeo, R. Kumar, TiO2–Co3O4 core–shell nanorods: bifunctional role in better energy storage and electrochromism, ACS Appl. Energy Mater. 1 (2018) 790–798, https://doi.org/10.1021/acsaem.7b00254. [10] P. Yang, P. Sun, W. Mai, Electrochromic energy storage devices, Mater. Today 19 (2016) 394–402, https://doi.org/10.1016/j.mattod.2015.11.007. [11] K. Gurunathan, A.V. Murugan, R. Marimuthu, U.P. Mulik, D.P. Amalnerkar, Electrochemically synthesised conducting polymeric materials for applications towards technology in electronics, optoelectronics and energy storage devices, Mater. Chem. Phys. 61 (1999) 173–191, https://doi.org/10.1016/S0254-0584(99) 00081-4. [12] K. Wang, H. Wu, Y. Meng, Y. Zhang, Z. Wei, Integrated energy storage and electrochromic function in one flexible device: an energy storage smart window, Energy Environ. Sci. 5 (2012) 8384–8389, https://doi.org/10.1039/C2EE21643D. [13] X. Wang, X. Lu, B. Liu, D. Chen, Y. Tong, G. Shen, Flexible energy-storage devices: design consideration and recent progress, Adv. Mater. 26 (n.d.), pp. 4763–4782 〈http://dx.doi.org/10.1002/adma.201400910〉. [14] X. Xia, J. Tu, Y. Zhang, X. Wang, C. Gu, X. Zhao, H.J. Fan, High-quality metal oxide core/shell nanowire arrays on conductive substrates for electrochemical energy storage, ACS Nano 6 (2012) 5531–5538, https://doi.org/10.1021/nn301454q.
Fig. 6. (a) Change in absorption and current flowing through the device with applied square wave of 1.4 V with 5 s time interval, (b) Switching time i.e. time taken for absorption change, shown by device.
170 cm2/C as calculated using Eqs. (1) and (2). Furthermore, the switching speed of the P3HT/EV bilayer device is better than other electrochromic device that contains viologen or P3HT as one of its active materials 4. Conclusion An organic electrochromic device consisting of ethyl viologen and P3HT in bilayer arrangement shows three level bias dependent color switching between magenta, transparent and blue states with a very low voltage of 1.4 V. The device, initially in magenta color when unbiased, switches to transparent state at 1 V external bias (stage-1) followed by a switching to blue state (stage-2) on increasing the bias to 1.4 V. A direct color switching from magenta to blue is possible with an applied bias of 1.4 V giving a flexibility to retain any of these three states. Raman and UV–Vis spectroscopies establish that the stage-1 switching is due to formation of P3HT polaron as a result of dynamic doping and stage-2 switching is caused due to redox switching of viologen molecules from its dicationic form to free radical state. The coloration and bleaching times for the switching stages, as measured from bias induced absorption response corresponding to 400 nm and 530 nm wavelengths are found to be ~ 1.5 s. A very good cyclability of more 253
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Polythiophene -viologen bilayer for electro-trichromic device
T
Anjali Chaudhary, Devesh K. Pathak, Suryakant Mishra, Priyanka Yogi, Pankaj R. Sagdeo, ⁎ Rajesh Kumar Material Research Laboratory, Discipline of Physics & MEMS, Indian Institute of Technology Indore, Simrol 453552, India
A R T I C LE I N FO
A B S T R A C T
Keywords: Electrochromic Viologen P3HT
An electrochromic device, capable of three-level color switching, has been envisaged by exploiting different color switching properties of Ethyl viologen (EV) and Poly (3-hexylthiophene) (P3HT) to make them switch in congruence with each other. The device, consisting of an EV/P3HT bilayer in poly-ethylene oxide matrix, appears magenta, blue or transparent under an appropriate bias. The unbiased device shows magenta colored appearance and turns transparent at a bias of 1 V when P3HT layer is used as anode. The device turns blue on increasing the bias to 1.4 V without changing the polarity. The mechanism of multilevel switching has been identified using UV–Vis and Raman spectroscopic investigations which reveal that the 1 V bias converts the P3HT into polarons and 1.4 V converts the EV dications (EV2+) into free radical. The electrochromic device demonstrates better color contrast (~ 50%) in both the optical domains. The fabricated device shows better switching speeds of ~ 1.5 s and good stability and coloration efficiency along with three-level color switching. However the reported device uses ITO coated glass as substrate, it can be used for all-organic flexible electrochromic device.
1. Introduction Concept of electrochromism, a phenomenon known for long, refers to a reversible color switching under electrical bias [1–7], has instigated the search for new functional materials for making smart devices. Electrochromic devices, fabricated using various active materials by adopting different design paradigms, are in use in displays, smart buildings, and aircrafts as well as in supercapacitors.[8–15] In recent years, research activity in electrochromism has seen tremendous interest due to its direct as well as indirect involvement in addressing the increasing energy requirements. Electrochromism addresses this issue by making solar cells more energy efficient and also through direct involvement in application as supercapacitors. However, various organic, inorganic and hybrid electrochromic materials are available, continuous search of new materials is going on mainly due to two reasons, firstly, for making a device more and more energy efficient and secondly to give the device a capability to play multifunctional roles and exhibit multiple colors. The latter gives an additional advantage as this leads to the application of electrochromic devices as tunable filters. Materials exhibiting electrochromic properties are the mainly ones which show different optical properties in their different redox [16] states and include [9,16,17] different metal oxides as well various organic counterparts [18–20]. All of these materials have certain
⁎
advantages and disadvantages thus chosen accordingly. The organic materials that show electrochromism include polyaniline [21], poly (3, 4-ethylenedioxythiophene) & other derivatives of polythiophene [22], polypyrrole, viologens etc. They have attracted special attention due to their unique advantages over inorganic metal oxides. Their fabrication processes does not involve complicated steps and can be easily processed into uniform, transparent and amorphous films using solution casting, spin coating or electro polymerization. Electrochromic materials are also classified as “cathodic” or “anodic” based on how their color changes on reduction and oxidation respectively. Prussian white changes its color from colorless to blue on oxidation [23–26] and thus come under the umbrella of anodic electrochromic materials while PEDOT, WO3 [23] are categorized as cathodic coloring electrochromic materials. Knowing the category of a given material helps in designing an electrochromic device and choosing the other counter species for supporting the process. An electrochromic material changes color between the transparent state and colored state on application of a bias of appropriate value and polarity, with a possibility of the colored state being the opaque state [27]. Under this known fact, if two materials, for which the colored states are different, are combined appropriately, the device will have the capability of switching between the two colored state with an intermediate transparent state thus will help in making a multi-level
Corresponding author. E-mail address: [email protected] (R. Kumar).
https://doi.org/10.1016/j.solmat.2018.08.029 Received 7 August 2018; Received in revised form 30 August 2018; Accepted 31 August 2018 0927-0248/ © 2018 Elsevier B.V. All rights reserved.
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right panels) with an intermediate transparent state at 1 V (central panel) bias. It is clear from the schematic (Fig. 1) that viologen and P3HT have been used as cathodic and anodic species respectively in the electrochromic device. In the unbiased state, device appears magenta which is the color of neutral P3HT as the viologen in PEO matrix is transparent. When a positive bias (1 V) is applied to the ITO electrode directly connected to P3HT film, as per the polarity arrangement shown in Fig. 1, the device turns transparent. The most likely reason for this switching (magenta to transparent) is the oxidation of P3HT from its neutral state to polaronic state due to applied bias. The same will be substantiated using various direct and indirect methods (Raman and CV) and will be discussed later. On further increasing the voltage from 1 V to 1.4 V with the same polarity (Fig. 1), the device turns blue from transparent likely due to the color switching of viologen layer. At this voltage, the transparent state of P3HT layer, achieved in the previous stage, is sustained whereas the conversion of viologen from dicationic state (EV2+) to free radical state (EV+•) take place on the cathode which appears blue [30]. The dynamic doping of P3HT, mentioned above, accompanied by reduction of EV2+ enables the bi-organic device, consisting of P3HT and viologen, to switch amongst three color states. Here the dynamic doping dominates the magenta to transparent switching whereas the latter manifests in transparent to blue switching of the device. The overall color switching of the device is actually a result of color switching at two different electrodes, sandwiched together between two electrodes rather than switching of the entire active region altogether. This has been established by carrying out bias dependent measurements by placing the two separate electrochromic electrodes in an electrochemical cell in the presence of electrolyte consisting of 0.5 M lithium perchlorate (LiClO4) in acetone. This mimics the same system as in the solid state device shown in Fig. 2. Fig. 2a shows the schematic arrangement of separated electrodes whereas actual images of both the electrodes under different bias conditions are shown in Fig. 2b-d. It is very clear that under the unbiased condition (Fig. 2b) the anode (P3HT) appears magenta due to the neutral polymer and cathode (EV) appears transparent due to EV2+. This situation is analogous to the magenta appearing device in solid state (Fig. 1). On application of a bias of one volt, the anode switches to transparent whereas cathode remains transparent (Fig. 2c). This color toggling of anode is due to oxidation of P3HT resulting in dynamic doping and the same process will lead to a transparent solid state device. When the bias is increased to 1.4 V, by keeping the polarity intact, the cathode's color changes from transparent to blue (Fig. 2d) due to reduction of viologen from EV2+ to EV+• with no effect on anode. This condition, in the device form, will lead to a blue appearing device (Fig. 1). The whole color change process can be seen in video available as supporting information (SI) uploaded on the web. The overall redox process leading to bias dependent multiple color change can be represented as shown below in Scheme 1 where it can be seen that conversion of P3HT to its polaron state takes place [31] at 1 V
switchable electrochromic device. Furthermore, if both of these materials are organic this will have a further advantage in making an allorganic flexible device. The colored state of polythiophene and viologen [28], both are solution processible, are magenta and blue respectively thus may be explored for abovementioned multilevel switching purpose. These two being the most studied organic materials due to their wide range of applications [29] gives an additional incentive for this exploration. Furthermore, from electrochromic application point of view, polythiophene and viologen have low operating voltages in range of around one volt while showing a good optical modulation. In the present work, electrochromic properties of Poly(3Hexylthiophene) (P3HT) and ethyl viologen diperchlorate (EV) based device has been studied by making device where a P3HT/EV bilayer has been sandwiched between two ITO coated glass electrodes. An appropriately designed device exhibit multiple color switching between magenta (when unbiased) and blue state (when biased) showing an intermediate transparent state at relatively lower bias as compared to the blue state without the need of an electrolyte layer. Absence of electrolyte makes the multilevel switching device power efficient. P3HT and EV acts as an electron donor /electron acceptor pair and improves the overall performance of device quantified through coloration efficiency, switching speeds and stability, directly connected to the cyclability of device. 2. Experimental details Commercially available chemicals from Alfa Aesar and Sigma Aldrich have been used for device fabrication. Polyethylene oxide (PEO, Alfa Aesar, MW=100,000), Ethyl viologen diperchlorate (EV, 98%, Sigma Aldrich), Poly (3 Hexyl thiophene-2, 5 diyl) (P3HT, regioregular, Sigma Aldrich), 1, 2- Dichlorobenzene (DCB, anhydrous, 99%, Sigma Aldrich) and Acetonitrile (ACN, anhydrous, 99%, Sigma Aldrich) are used as received. P3HT in DCB and EV have been deposited in PEO matrix on ITO glass substrates and then assembled together to form a device. Various steps involved in device fabrication have been discussed in Supporting information (SI) in detail (Fig. S1). The transmission spectra have been recorded at room temperature on a UV–Vis spectrophotometer (Cary 60 of Agilent make). Raman spectroscopy measurements have been carried out using LABRAM HR spectrometer with 633 nm excitation source. For Chronoamperometry measurements, Keithley 2450 workstation was used. 3. Results and discussion The actual photographs of the fabricated device under a given bias condition are shown in Fig. 1 along with the device schematic showing the biasing polarity arrangements. As hypothesized above, the device changes color from magenta to blue on application of 1.4 V (left and
Fig. 1. Schematic illustration of electrochromic device displaying various colors with applied bias along with corresponding actual photographs of the device. 250
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Fig. 4. In situ Raman spectra of fabricated device at various applied potential along with schematic of Raman scattering experimental geometry.
correspond to red and far blue color region respectively with relatively lesser transmission in the green region apparent as a dip in this region in black curve (Fig. 3). The combination of the reddish and blueish transmission is giving the magenta look to the device as seen in the corresponding image (Fig. 3). Under 1 V bias, device transmits all the wavelengths of the visible region equally thus appears transparent [32,33] (red curve, Fig. 3). On increasing the voltage further to 1.4 V, the transmission profile remains similar to the unbiased situation but with a maximum transmission corresponding to 440 nm [34] (blue) with very low transmission in green and red wavelength regions. Thus the device appears blue. The bias dependent variation in absorption spectrum is shown in SI (Fig. S2). Most likely reason for such a bias dependent color switching has been hypothesized and will be validated below. In order to understand the hypothesized mechanism responsible for color appearance of device in-situ Raman spectroscopy has been carried out. Fig. 4 shows the schematic of Raman geometry and in-situ Raman spectra from the fabricated device under different bias values recorded using excitation wavelength of 633 nm. Raman spectrum of unbiased device (0 V) shows peaks at 1380 cm−1 and 1443 cm−1 which correspond to neutral P3HT whereas another peak at 1648 cm−1 indicates the presence of EV+2. It clearly means that the unbiased device contains neutral P3HT [35] and viologen dication (EV+2). The EV2+ being transparent, the magenta color of unbiased device is mainly due to the P3HT as the UV–Vis spectrum of the device (Fig. 3) is similar to the UV–Vis spectrum from neutral P3HT in solution as shown in the SI (Fig. S3). On application of 1 V bias, to the device with P3HT electrode connected to the anode, a peak at 1090 cm−1 is observed which is very often identified as originating due to the polaron formation due to oxidation of P3HT. The polarons formed by the dynamic doping, a wellknown phenomenon [35–37], can get stabilized by the diperchlorate ions available in the device from the viologen layer. The polaron is known to be transparent [38] to the visible spectrum and thus the whole device appears transparent [37,39]. It is worth mentioning here that though the viologen is connected to the cathode, no reduction is taking place (no viologen free radical peak in Raman spectrum). This makes both the layers, and thus the whole device, transparent when biased with 1 V. On further increasing the bias to 1.4 V, by keeping the polarity unchanged, Raman spectrum responds with peaks appearing at 1029 cm−1, 1359 cm−1, 1530 cm−1, 1658 cm−1 which are well reported modes of free radical viologen(EV+•) which is known for its blue color [19,28,34,40]. Thus the blue switching on application of 1.4 V can be understood on the basis of simultaneous formation of polaron and viologen free radical because P3HT polaron appears transparent whereas viologen is blue in its radical form (EV+•). Here, P3HT acts as a counter ion for
Fig. 2. (a) Schematic of arrangements used for electrochromic measurements in electrochemical cell and actual photographs of the electrodes at bias of (b) 0 V, (c) 1.0 V and (d) 1.4 V.
Scheme 1. Demonstration of redox reactions associated with both the stage of color switching.
at anode while reduction of EV+2 to EV+• occurs at 1.4 V at cathode when present in solid state form. The abovementioned process will be validated later on. Supplementary material related to this article can be found online at doi:10.1016/j.solmat.2018.08.029. The color switching, reported above, can be correlated with the observed spectral changes using UV–Vis spectroscopy for validation. Fig. 3 shows transmission spectra and photographs of the device corresponding to the applied bias. The in-situ UV–Vis transmission spectra (Fig. 3) show variation in transmittance as a function of applied voltage from 0 V, 1 V and then 1.4 V. The unbiased device shows higher transmissions corresponding to 650 nm and 410 nm wavelength which
Fig. 3. Transmission spectra of fabricated device at various applied potential with corresponding photographs of device. 251
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Fig. 5. (a) Schematic of set up used for switching cycle and absorption of device. (b) Change in current flowing through the device with time and (c) corresponding variation in absorbance at 530 nm. (d) Switching time; coloration and bleaching time shown by device.
device performance which is defined by following Eq. (1):
viologen and helps in it's reduction by facilitating the electron transport mechanism without relying on the environmental moisture as necessary for viologen alone devices [34]. Thus in-situ Raman spectroscopy validates the color switching mechanism, as hypothesized above, which is the redox state induced modulation in optical behavior of various species present in the predesigned device. This further emphasizes that redox reaction of P3HT/viologen at various applied potentials is the basic pillar that induces the multi-color switching. Switching time and stability are two very important parameters to measure the performance of an electrochromic device as these two are directly related to the speed and cyclability of the device. Since the device displays multiple color switching from magenta to transparent in stage-1 (refer Scheme 1) followed by switching to blue (stage-2), thus variation in absorption with time has been studied for two different wavelengths (400 nm and 530 nm) by applying appropriate switching biases. The dynamic absorption measurement along with simultaneous chronoamperometry was used to determine the switching speed and stability for switching in both the stages. For stage-1 switching (magenta → transparent), a square wave of 1 V with 10 s time interval was applied to see chronoamperometric and absorption response (Fig. 5). Fig. 5a shows the schematic of the setup used. It can be clearly seen in Fig. 5(b) that the chronoamperometric response is very stable for 480 s with little variation in maximum and minimum current values. Fig. 5(c) shows the cyclic response of the device for the abovementioned duration showing a reversible change in absorbance of the 530 nm wavelength as a result of the 1 V applied bias. The device is seen to switch 24 times in 480 s without much loss to the absorbance values for the said wavelength. To understand the actual time taken for switching, a single cycle has been analyzed by looking at a zoomed portion of the absorption variation (Fig. 5d). It is evident from Fig. 5d that the device takes ~1.5 s to reach 77% of its maximum absorption i.e. from transparent to magenta color whereas takes ~2 s (bleaching time) to go back to 67% of the maximum absorption value (i.e. switch back to transparent state from magenta) after reaching the color saturation. A switching time of ~2 s is faster than previously reported P3HT based electrochromic devices [41]. Coloration efficiency [17] (ηec) is an important parameter to access
ηec = ΔOD/Q,
(1)
where ΔOD = Ac-Ab with Ac, Ab are the absorbance in colored and bleached states respectively (values obtained from Fig. 5c), and Q is the charge per unit area of electrode. For stage 1, coloration efficiency is 222 cm2/C and related to quality factor [42] (γ), defined in Eq. (2), which comes out to be 126 cm2 C−1 s−1for stage 1.
γ = ηec / τ , τ = (τb + τc )/2,
(2)
where τb & τc are time taken in bleaching and coloration process respectively. A comparison of the performance of present device with other similar thiophene based electrochromic devices has been provided in the SI. Similar to the stage-1 switching, a 600 s dynamic switching analysis has been done for stage-2 (Fig. 6). The bias induced time dependent absorption change corresponding to 400 nm in response to a square wave of 1.4 V with 5 s time interval can be seen in Fig. 6a (black curve) along with the corresponding chronoamperometric response showing 60 cycles without much loss to the absorption (left Y-axis) and current (right Y-axis) meaning a stable color switching. The device shows enhanced stability with little variation in maximum and minimum current values over a period of 600 s. Closer look at a single switching step is required for better understanding of switching response (Fig. 6b). A turning “ON” time, the time taken to switch the absorption to > 80% of the maximum value, of 1.5 s is observed. The device takes ~ 3 s to get completely saturated to blue color after which neither the color changes further nor the 400 nm absorption. The device takes 1.2 s (less than the switching time) to bleach out also when the device turns to magenta. During this bleaching time, the absorption by the device reduces to 75% of the saturated state value, color from its blue color state changes to initial (magenta) color in 1.2 s. A faster bleaching time is observed as the tendency for polaron to neutral P3HT conversion is high because P3HT is very stable in its neutral state. At the same time viologen free radical assists polaron to go back to neutral state so that viologen free radical can also go back to its dicationic state, its stable state. Stage 2 has a coloration efficiency of 247 cm2/C and quality factor (γ) of 252
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than 50 cycles has been observed with little loss to the contrast. Coloration efficiency of more than 200 cm2/C has been obtained in both the switching stages. The matrix is also chosen to demonstrate an electrolyte less device making it an improved multiple colored organic electrochromic device with enhanced functioning and easier fabrication process involved. On the whole, the EV, being an electron acceptor, and P3HT, being an electron donor, together provide an excellent combination for a fast, stable and energy efficient option for multicolor electrochromic switching. Acknowledgements Authors are thankful to Dr. V. Sathe (UGC DAE CSR, Indore, India) for Raman measurements. Authors acknowledge useful discussion with Prof. V.D. Vankar (IIT Delhi). One of the authors (D.K.P.) acknowledges Council of Scientific and Industrial Research (CSIR) for financial assistance. Authors also acknowledge MHRD and DST, Govt. of India for providing funding. Conflict of interest Authors declare no conflict of interest. Appendix A. Supporting information Supplementary data associated with this article can be found in the online version at doi:10.1016/j.solmat.2018.08.029. References [1] M.L. Moser, G. Li, M. Chen, E. Bekyarova, M.E. Itkis, R.C. Haddon, Fast electrochromic device based on single-walled carbon nanotube thin films, Nano Lett. 16 (2016) 5386–5393, https://doi.org/10.1021/acs.nanolett.6b01564. [2] A. Agrawal, J.P. Cronin, R. Zhang, Review of solid state electrochromic coatings produced using sol-gel techniques, Sol. Energy Mater. Sol. Cells 31 (1993) 9–21, https://doi.org/10.1016/0927-0248(93)90003-L. [3] I. Mjejri, C.M. Doherty, M. Rubio-Martinez, G.L. Drisko, A. Rougier, Double-sided electrochromic device based on metal–organic frameworks, ACS Appl. Mater. Interfaces 9 (2017) 39930–39934, https://doi.org/10.1021/acsami.7b13647. [4] I. Mjejri, A. Rougier, M. Gaudon, Low-cost and facile synthesis of the vanadium oxides V2O3, VO2, and V2O5 and their magnetic, thermochromic and electrochromic properties, Inorg. Chem. 56 (2017) 1734–1741, https://doi.org/10.1021/ acs.inorgchem.6b02880. [5] M. Barawi, G. Veramonti, M. Epifani, R. Giannuzzi, T. Sibillano, C. Giannini, A. Rougier, M. Manca, A dual band electrochromic device switchable across four distinct optical modes, J. Mater. Chem. A 6 (2018) 10201–10205, https://doi.org/ 10.1039/C8TA02636J. [6] D. Dong, W. Wang, G. Dong, F. Zhang, H. Yu, Y. He, X. Diao, Improved performance of co-sputtered Ni–Ti oxide films for all-solid-state electrochromic devices, RSC Adv. 6 (2016) 111148–111160, https://doi.org/10.1039/C6RA21961F. [7] N. Penin, A. Rougier, L. Laffont, P. Poizot, J.-M. Tarascon, Improved cyclability by tungsten addition in electrochromic NiO thin films, Sol. Energy Mater. Sol. Cells 90 (2006) 422–433, https://doi.org/10.1016/j.solmat.2005.01.018. [8] X. Yin, J.R. Jennings, W. Tang, T.J. Huang, C. Tang, H. Gong, G.W. Zheng, Largescale color-changing thin film energy storage device with high optical contrast and energy storage capacity, ACS Appl. Energy Mater. 1 (2018) 1658–1663, https://doi. org/10.1021/acsaem.8b00120. [9] S. Mishra, P. Yogi, P.R. Sagdeo, R. Kumar, TiO2–Co3O4 core–shell nanorods: bifunctional role in better energy storage and electrochromism, ACS Appl. Energy Mater. 1 (2018) 790–798, https://doi.org/10.1021/acsaem.7b00254. [10] P. Yang, P. Sun, W. Mai, Electrochromic energy storage devices, Mater. Today 19 (2016) 394–402, https://doi.org/10.1016/j.mattod.2015.11.007. [11] K. Gurunathan, A.V. Murugan, R. Marimuthu, U.P. Mulik, D.P. Amalnerkar, Electrochemically synthesised conducting polymeric materials for applications towards technology in electronics, optoelectronics and energy storage devices, Mater. Chem. Phys. 61 (1999) 173–191, https://doi.org/10.1016/S0254-0584(99) 00081-4. [12] K. Wang, H. Wu, Y. Meng, Y. Zhang, Z. Wei, Integrated energy storage and electrochromic function in one flexible device: an energy storage smart window, Energy Environ. Sci. 5 (2012) 8384–8389, https://doi.org/10.1039/C2EE21643D. [13] X. Wang, X. Lu, B. Liu, D. Chen, Y. Tong, G. Shen, Flexible energy-storage devices: design consideration and recent progress, Adv. Mater. 26 (n.d.), pp. 4763–4782 〈http://dx.doi.org/10.1002/adma.201400910〉. [14] X. Xia, J. Tu, Y. Zhang, X. Wang, C. Gu, X. Zhao, H.J. Fan, High-quality metal oxide core/shell nanowire arrays on conductive substrates for electrochemical energy storage, ACS Nano 6 (2012) 5531–5538, https://doi.org/10.1021/nn301454q.
Fig. 6. (a) Change in absorption and current flowing through the device with applied square wave of 1.4 V with 5 s time interval, (b) Switching time i.e. time taken for absorption change, shown by device.
170 cm2/C as calculated using Eqs. (1) and (2). Furthermore, the switching speed of the P3HT/EV bilayer device is better than other electrochromic device that contains viologen or P3HT as one of its active materials 4. Conclusion An organic electrochromic device consisting of ethyl viologen and P3HT in bilayer arrangement shows three level bias dependent color switching between magenta, transparent and blue states with a very low voltage of 1.4 V. The device, initially in magenta color when unbiased, switches to transparent state at 1 V external bias (stage-1) followed by a switching to blue state (stage-2) on increasing the bias to 1.4 V. A direct color switching from magenta to blue is possible with an applied bias of 1.4 V giving a flexibility to retain any of these three states. Raman and UV–Vis spectroscopies establish that the stage-1 switching is due to formation of P3HT polaron as a result of dynamic doping and stage-2 switching is caused due to redox switching of viologen molecules from its dicationic form to free radical state. The coloration and bleaching times for the switching stages, as measured from bias induced absorption response corresponding to 400 nm and 530 nm wavelengths are found to be ~ 1.5 s. A very good cyclability of more 253
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