Article Cite This: ACS Appl. Electron. Mater. XXXX, XXX, XXX−XXX
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Polythiophene−PCBM-Based All-Organic Electrochromic Device: Fast and Flexible Anjali Chaudhary, Devesh K. Pathak, Manushree Tanwar, Priyanka Yogi, Pankaj R. Sagdeo, and Rajesh Kumar* Material Research Laboratory, Discipline of Physics & MEMS, Indian Institute of Technology Indore, Simrol, Indore 453552, India
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S Supporting Information *
ABSTRACT: A fast and flexible all-organic electrochromic device, fabricated using polythiophene and PCBM as active materials and plastic substrate, which shows very good power efficiency as well, has been reported here. The device shows quantifiable improvement in electrochromic performance using parameters like switching speed, coloration efficiency, color contrast, and cycle life. Spectroscopic investigations have been carried out using Raman and UV−vis to establish a bias induced redox switching based mechanism for the reported improvement in the performance. The device shows switching between magenta (OFF) and transparent states (ON) with a very small bias of ±1 V, an optical modulation of 50% and an absorbance switching contrast of 91%. An enhanced stability for a duration of longer than 2500 s and 250 cycles has been reported with an ultrafast response of few hundred milliseconds. A very high coloration efficiency of 321 cm2/C is achieved, making the proposed device one of the best reported P3HT-based electrochromic devices. KEYWORDS: electrochromic, organic electronics, flexible devices, polythiophene, PCBM
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INTRODUCTION Electronic materials with special functionalities are always in demand due to their capability of solving various scientific problems including directly either in energy storage and generation or in improving their performance.1,2 Electrochromic materials, displaying the phenomenon of reversible color switching with external electrical stimuli, are one such group of materials, which is important in view of the above statement. The phenomenon of such a bias-induced color switching refers to electrochromism.3−5 A recent development in this field, like other functional materials, also spiked due to rapid advances in the fields of nanoscience and nanotechnology.6,7 Various organic as well as inorganic materials have been extensively scrutinized for making electrochromic devices using different archetypes, and they have been exploited for other applications in nanodevices as well. Their applications lie in the fields of energy storage devices8 (supercapacitor, battery, etc.), solar cells,9 and other energysaving optoelectronic devices.10 In electrochromism, oxides like those of tungsten,11−14 titanium, cobalt, vanadium,15,16 and nickel17 display divergent colors depending how their optical properties (absorbance, reflectance, and transmittance) vary with applied electric bias. They usually lack these properties, displaying a true transparent state, which restricts them for application in a display-like18,19 system. Conducting polymers among many other organic materials display different shades of color20−22 when polyaniline is used. Polypyrrole,23,24 poly(3,4-ethylenedioxythiophene),25,26 and derivatives of polythiophenes and viologens27,28 also come © XXXX American Chemical Society
under the same classification of electrochromic materials. Unlike inorganic materials, which require a difficult fabrication processes, organic materials are solution-processable, which smoothens their path of device fabrication and makes them a suitable candidate for making a flexible device if appropriately designed. Poly(3-hexylthiophene) (P3HT) is one of the extensively discussed polymers having applications in solar cells,29 electrochromism,30 memory,31 and supercapacitors,8,32 among other things. Like most of the electrochromic materials, the color switching from P3HT arises because of a bias-driven redox process resulting in different absorption properties from different species. When used alone, the redox process relies only on the atmospheric moisture, thus taking a while for color switching, and does not sustain periodic switching of color for a longer duration. This problem can be addressed by incorporating an n-type material along with P3HT, like TTF or viologen,30,33,34 so as the combination of material enhances the overall electrochromic performance of the device. While reversing the color switching, the same redox process involving the electrode and redox activity at the counterion are involved, so not to help them in improving the switching speed at least in one cycle. This problem can be addressed by incorporating an appropriate material, which can act as a venue for electrons as well as a supportive redox species for temporary storage, and the same can be used while reversing the color switching cycle. Received: October 14, 2018 Accepted: December 31, 2018
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DOI: 10.1021/acsaelm.8b00012 ACS Appl. Electron. Mater. XXXX, XXX, XXX−XXX
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ACS Applied Electronic Materials
shown in the schematic (Figure 1). The device consists of layers of PCBM and P3HT on indium tin oxide (ITO)-coated flexible polyethylene terephthalate (PET) substrates with a layer of LiClO4 in a poly(ethylene oxide) (PEO) matrix. As apparent from the image (Figure 1), the device changes its color from magenta, when unbiased, to a transparent state by applying a voltage of 1 V, and the magenta color can be reinstalled with the same bias when applied with opposite polarity (i.e, −1 V). The most likely reason for this observed color change is conversion of P3HT to its polaron form on oxidation when biased with 1 V followed by a reduction of polaron to neural P3HT on bias reversal.30,37,38 Spectroscopic (absorption) variation corresponding to the above-mentioned bias-induced color switching can be seen using in situ UV−vis spectroscopy of the fabricated device (Figure 2a). Figure 2a shows absorbance spectra of the
This is very likely to reduce the time taken for this color change. Using a flexible substrate for fabricating the device will result in a flexible electrochromic device. In the present paper, an all-organic flexible electrochromic device has been demonstrated by using a combination of (ptype) P3HT and (n-type) PCBM35,36 ([6,6]-phenyl-C61butyric acid methyl ester) as active material on a PET substrate in the simple sandwich geometry. In the P3HT−PCBM combo, P3HT acts as the electrochromic material, whereas PCBM facilitates in improving the switching properties by playing a double role. For ease of characterization, the reported device parameters have been calculated from the same device fabricated on a glass substrate; however, a flexible device is also demonstrated qualitatively. A color switching between magenta and transparent states has been observed with the transparent state being opaque to IR, indicating its application as a heat shield. An improvement in the power rating, coloration efficiency, color contrast, and stability has been observed. Spectroscopic and electrochemical techniques have been utilized to investigate the switching mechanism and roles of individual constituent in the device.
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EXPERIMENTAL DETAILS
The following materials, commercially available chemicals from Sigma-Aldrich and Alfa Aesar, were purchased and used as received for device fabrication: poly(3 hexylthiophene-2,5-diyl) (P3HT, regioregular, Sigma-Aldrich), 1,2-dichlorobenzene (DCB, anhydrous, 99%, Sigma-Aldrich), acetonitrile (ACN, anhydrous, 99%, SigmaAldrich), [6,6]-phenyl-C61-butyric acid methyl ester (PCBM, >99.5%, Sigma-Aldrich), poly(ethylene oxide) (PEO, Alfa Aesar, MW = 100 000), lithium perchlorate (LiClO4, Sigma-Aldrich). For device fabrication, two solutions of 0.3 wt % P3HT and 0.5 wt % PCBM were prepared in 1,2-dichlorobenzene using a vortex. Complete details of the device fabrication process are given in the Supporting Information using a schematic in Figure S1. The fabricated device was further used for various electrochemical and spectroscopic measurements. The transmission spectra were recorded at room temperature using a UV−vis spectrophotometer (Cary 60 of Agilent). Raman spectroscopy has been carried out using a LABRAM HR spectrometer using a 633 nm excitation source. For electrochemical measurements, a Keithley 2450 workstation was used.
Figure 2. (a) In situ bias-induced absorption spectra from fabricated device along with real images and (b) schematic for showing the device as an IR radiation suppressor with arrows showing the direction of propagation of the light beam.
electrochromic device along with its actual images under different bias. First of all, when device is unbiased, in the “initial” state (red curve, unhooked to the power supply), maximum absorption is observed in the vicinity of 550 nm, indicating the absorption of green color, meaning that the presence of red and blue colors (appearing magenta) in the transmitted light is responsible for the magenta appearance to the device.37 At 1 V bias (black curve), the absorbance decreases and all the wavelengths very poorly get absorbed almost equally, which allows all the wavelengths on the visible region to transmit, thus making the device appear transparent to the visible spectrum. On bias reversal (−1 V), the magenta color gets reinstalled as is evident from the increase in the absorption back in the green region, mimicking the same spectrum as that of the unbiased device. The fabricated device shows more than 50% color contrast corresponding to 550 nm when switched between magenta and transparent states, which is better, as will be substantiated later quantitatively, in comparison to the previously reported device based on P3HT.37,39 The color contrast, defined by eq 1, has been calculated from the UV−vis spectra recorded in transmission mode as shown in Figure S2.
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RESULTS AND DISCUSSION An illustration of the flexible electrochromic device, prepared with an all-organic active layer, along with actual photographs of the device under ±1 V displaying color switching are shown in Figure 1. The convention for biasing the device is also
color contrast (%) =
Tn − Tf × 100 Tf
(1)
where T n and T f are the transmittance values (%) corresponding to ON and OFF states, respectively (Figure S2). It is also important to mention here that the device
Figure 1. Illustration of various components in flexible electrochromic device and actual photographs of device under bias of ±1 V along with the polarity convention used for biasing the device. B
DOI: 10.1021/acsaelm.8b00012 ACS Appl. Electron. Mater. XXXX, XXX, XXX−XXX
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ACS Applied Electronic Materials
Some quantification regarding the IR shielding has been carried out using an IR laser (Figure S4) and has been reported in the SI along with some experiments to rule out an alternate mechanism of color switching (Figure S5). The actual mechanism and performance of the device have been discussed below explaining the role of the polaron in switching color and the effect of PCBM on the switching speed along with the coloration efficiency of the device. As mentioned earlier, the electrochromic device, used here, consists of P3HT and PCBM for a predefined role as per design. To understand how their vibrational properties respond on application of bias, in situ Raman spectra of a fabricated device have been recorded. Figure 3(a) shows a Raman spectrum corresponding to the just fabricated device showing peaks at 1443 and 1380 cm−1, signatures of neutral P3HT40,41 (marked with *). When the device is turned ON (1 V) a peak shift is observed and appears at 1438 and 1376 cm−1(Figure 3b); additionally, two new peaks corresponding to the polaronic42 form of P3HT appear at 1186 and 1092 cm−1 (marked with #). The shift in peaks observed from the ON device disappears, and the neutral P3HT peaks get reinstalled when the device is turned OFF (−1 V), as shown in Figure 3c, along with disappearance of polaron Raman peaks. The shift in the main P3HT peaks as a result of applied bias does not resemble any other vibration modes of P3HT, indicating that the shifted peaks arise as a result of interaction between P3HT and PCBM, resulting in a quasi-state, which is reversible with bias reversal. The consequences of such an interaction on electrochromic switching will be discussed later. The in situ bias-dependent Raman spectroscopy and its correlation between absorption spectra and images, clearly indicates that bias-induced reversible switching between the neutral and polaron states of P3HT is responsible for the color
demonstrates a better color contrast in comparison to a control device consisting of only P3HT as shown in Figure S3a, which shows only a contrast of less than 35%. Another interesting feature here, which cannot be observed with naked eye and go unnoticed, is the higher absorbance by the device in the ON state in the IR region (650 nm−800 nm) as compared to the initial state. It means that when the device is in ON state, the transparent layer restricts the IR to pass through the device and thus will be very useful when used in smart windows for suppressing the heat and allowing only the visible light to enter. In other words, the ON device is opaque to the IR radiation and thus can be used in suppressing the transmission of heat, which is inherent to the radiation. This heat filter type observation has been depicted schematically in Figure 3b.
Figure 3. (a−c) In situ bias-induced Raman spectra of the electrochromic device showing peaks of neutral P3HT with * and polarons with # in (a) and (c), whereas unmarked peak in (b) shows a shifted P3HT mode.
Figure 4. (a) Variation in current flowing through device with voltage and time as varying parameters; (b) stability of absorption cycles of device up to 2500 s and 250 cycles along with a (c) zoomed-in view of one absorption cycle of the device showing the bias pulse (dashed line); (d) variation of optical density as a function of charge density used for calculation of coloration efficiency using eq 2. C
DOI: 10.1021/acsaelm.8b00012 ACS Appl. Electron. Mater. XXXX, XXX, XXX−XXX
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Figure 5. Schematic illustration of proposed switching mechanism involved in the color switching of the electrochromic device.
overall electrochromic performance by the device. The fast switching and improvement in efficiency can be understood as follows through the mechanism. It is understood that device performance parameters are the consequence of the operation mechanism; thus, the same needs to be understood first. Raman spectroscopic results (Figure 3) very clearly indicate that the transparent state of the device is achieved due to switching of P3HT species from its neutral (magenta) to polaron (transparent) state. It is worth mentioning here that along with P3HT (p-type material), PCBM (n-type material)43,44 is also incorporated in the device, helping P3HT to oxidize quickly by reducing itself as a counterion and thus enhancing its optical modulation and coloration efficiency. The color switching mechanism and electrochemical properties of P3HT and PCBM can be elaborated by looking at Figure 5, which pictorially depict the processes involved and the actual images of the device in a given state. Under the unbiased condition (initial state), the device appears magenta due to the presence of neutral PCBM and P3HT, which is overall magenta.39,45 While turning ON, the electrode containing the electrochromic species is connected to the positive terminal, while the other is connected to the negative terminal of the battery. The electrode connected to PCBM is connected to the negative terminal, and thus, it gets reduced, resulting in oxidation of P3HT to give a transparent polaron. During this process, the reduction of PCBM takes place, and it starts accumulating electrons received from P3HT after oxidation until it is oxidized intentionally. Charge storage is a well reported typical characteristic of such a nanostructure of carbon in the buckyball allotrope. This is shown in the middle panel of Figure 5. When the device is turned OFF (−1 V bias), the polarons take electrons, which are already available on PCBM (present in the vicinity), to get back to the neutral state to become magenta again. It is important here to understand that though the redox process taking place in P3HT is responsible for color switching, the turning ON and turning OFF process are different by means of the way they exchange electrons by leaving an electron and accepting an electron, respectively. The electrolyte layer containing LiClO4 and PEO plays a role in stabilizing the polarons formed during a bias of 1 V (middle panel of Figure 5). When biasing leads to polaron formation of P3HT, the ions in the electrolyte will get dissociated and be attracted toward the formed polaron in order to balance the charge in overall device. Such a stability helps in improving the performance of the device. While turning ON, the electron exchange has to take place from the power supply and needs the whole current cycle to be completed, whereas in the latter, electrons are already available in the close proximity of the polarons in the ON device. This difference in method of
switching with P3HT being the active color changing component and PCBM as the facilitating agent with other roles as mentioned below. This interaction can also be understood from the CV curve obtained from a solution containing P3HT and PCBM as shown in Figure S6. Performance of an electrochromic device must be tested for its real application through performance parameters including switching time, cycle life, coloring efficiency, and color contrast. To get an insight of the device performance, chronoamperometry as well as switching kinetics have been measured corresponding to the absorbance of the 550 nm wavelength by applying a square wave of ±1 V having 100 mHz frequency. The voltage pulse wave and corresponding current, flowing through the device, are given in Figure 4a, which shows that the maximum and minimum values of current flow are 0.9 mA and −1.43 mA, respectively. A longer time period square wave pulse has been used so that no information is missed; however, it will be clear that the switching times are much shorter than this time scale. The switching of absorbance as a result of applied bias shows very good stability for a period of 2500 s, during which 250 cycles of ON/OFF take place (Figure 4b). A persistency in absorption values is also observed, which is better in comparison to previously reported P3HT-based electrochromic devices as has been compared in Table S1. A closed view of a single absorption switching cycle of a device (Figure 4c) reveals that the device is switched on a subsecond scale. To be more precise, it takes 500 ms to switch from a transparent (ON) to magenta (OFF) colored state, and during this time, a 91% change in maximum absorption value is achieved. On the other hand, while turning the device ON (switching from magenta to transparent with a bias of 1 V, again it takes ∼1 s to go to the transparent state with absorption changes of 75%. It means that the device gets turned OFF faster that turning ON, unlike the reported device where it takes approximately the same time in switching ON or switching OFF. The reason behind this will be discussed later. The coloration efficiency (ηce)22 of the device is another important parameter, which is used to quantify the power efficiency of an electrochromic device and can be estimated using eq 2 below.
ηce = ΔOD/Q
(2)
where the variation in optical density ΔOD = Af − An, and Af and An are absorption values in the OFF and ON states, respectively (Figure 4c), and Q is the charge density.30 Figure 4d shows the variation of optical density as a function of charge density and the resulting coloration efficiency of 321 cm2/C using eq 2. This value means a very power-efficient device involving P3HT as can be appreciated from a comparison made in Table S1. All of the above results show a sign of good D
DOI: 10.1021/acsaelm.8b00012 ACS Appl. Electron. Mater. XXXX, XXX, XXX−XXX
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ACS Applied Electronic Materials electron exchange involved in the redox process leads to a faster reversal to the OFF state as compared to the initial state to turning ON and the same follows in the subsequent cycles. Thus, it takes longer for the device to turn ON as compared to turning OFF as mentioned above. Additionally, a bias-induced formation of a quasiP3HT−PCBM structure, as evident from Raman spectra (Figure 3), possibly induces electronic perturbations in the system, leading to increased IR absorption by the ON device (Figure 2), making it a possible candidate for heat shielding and providing a real application in smart windows in buildings and cars. A fast and flexible electrochromic device can be fabricated using all organic materials with very good power efficiency.
Pankaj R. Sagdeo: 0000-0002-2475-6676 Rajesh Kumar: 0000-0001-7977-986X Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS Facilities received from the Department of Science and Technology (DST), Govt. of India, under the FIST Scheme with grant number SR/FST/PSI-225/2016 are highly acknowledged. The authors are thankful to Dr. V. Sathe (UGC DAE CSR, Indore, India) for Raman measurements. One of the authors (D.K.P.) acknowledges the Council of Scientific and Industrial Research (CSIR) for financial assistance. Authors also acknowledge MHRD and DST, Govt. of India, for providing funding.
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CONCLUSIONS A flexible organic electrochromic device, fabricated using PCBM and P3HT as active materials and ITO deposited on PET as a flexible substrate, shows a fast electrochromic switching with switching times of 500 ms and a very good coloration efficiency of more than 320 cm2/C. Spectroscopic examinations reveal that the device switches from magenta, the initial state, to the transparent state, the ON state, when a bias of 1 V is applied to the P3HT side because of bias-induced redox switching of P3HT from its neutral state to the polaron state. The device can be restored back to the magenta state (or the OFF state) by reversing the bias polarity, which reduces the polarons back to the neutral state of P3HT. It is also predicted that the PCBM molecules act as storage for the electrons, which are released during P3HT to polaron formation when it was being turned ON. These electrons, available in the proximity of polarons (in the ON state), are used during polaron to P3HT conversion while turning OFF the device. The availability of these electrons in the proximity makes this process faster. The absorption spectra also reveal that the ON device can act as a heat filter, because IR absorption increases when the device is in the ON state possibly because of a change in the oxidation state of the PCBM−P3HT layer. The device shows an excellent stability for 250 ON/OFF cycles while switching between magenta and transparent states with a very low voltage of ±1 V. A good color contrast of more than 50% and absorption switching of more than 90% is also observed, thus showing an improved electrochromic device in the family of P3HT-based all-organic devices.
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsaelm.8b00012.
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REFERENCES
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Device fabrication method; comparison of performances of different devices; CV of combined active electrochromic material used for fabrication; some optical and electrical experiments; UV−vis spectra of control device (PDF)
AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. ORCID
Anjali Chaudhary: 0000-0002-8202-2697 Priyanka Yogi: 0000-0002-6639-014X E
DOI: 10.1021/acsaelm.8b00012 ACS Appl. Electron. Mater. XXXX, XXX, XXX−XXX
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DOI: 10.1021/acsaelm.8b00012 ACS Appl. Electron. Mater. XXXX, XXX, XXX−XXX
