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npj Flexible Electronics volume 8, Article number: 55 (2024 ) Cite this article tigerstripe camouflage fabric
Materials that provide dynamically tunable infrared (IR) response are important for many applications, including active camouflage and thermal management. However, current IR-tunable systems often exhibit limitations in mechanical properties or practicality of their tuning modalities, or require complex and costly fabrication methods. An additional challenge relates to providing compatibility between different spectral channels, such as allowing an object to be reversibly concealed in the IR without making it appear in the visible range. Here, we demonstrate that conducting polymer-cellulose papers, fabricated through a simple and cheap approach, can overcome such challenges. The papers exhibit IR properties that can be electrochemically tuned with large modulation (absolute emissivity modulation of 0.4) while maintaining largely constant response in the visible range. Owing to high ionic and electrical conductivity, the tuning of the top surface can be performed electrochemically from the other side of the paper even at tens of micrometer thicknesses, removing the need for overlaying electrode and electrolyte in the optical beam path. These features enabled a series of electrically tunable IR devices, where we focus on demonstrating dynamic radiative coolers, thermal camouflage, anti-counterfeiting tags, and grayscale IR displays. The conducting polymer-cellulose papers are sustainable, cheap, flexible and mechanically robust, providing a versatile materials platform for active and adaptive IR optoelectronic devices.
Mid-infrared (mid-IR) light, with wavelengths between 3 and 30 μm, represents a spectral range of high relevance for both civilian and military applications1,2. Mid-IR light carries vital information spanning from fingerprints of molecules3 and features of localized charge carriers4,5, to thermal profiles of physical objects6, enabling multiple important applications from biosensors to thermal camouflage and radiative cooling. The latter utilizes spontaneous thermal emission to passively cool objects on Earth by radiatively transferring thermal energy to cold outer space7,8,9. Currently, the majority of IR devices are passive and therefore only provide pre-designed properties and functions10,11,12,13. This restricts their use from many applications, such as dynamic camouflage that can adapt to changing environments2,14,15, or tunable radiative coolers for smart thermal management16,17,18,19.
Besides realizing IR tunability through functional device integration (e.g., thermoelectric devices15), most approaches utilize dynamically tunable IR materials, such as phase change materials20,21,22,23 (e.g., Ge2Sb2Te5). Such materials provide modulation through thermal stimulation, including via light-induced heating24,25 or Joule heating20,21,26. For the 2D MXene Ti3C2Tx it was possible to tune the IR response in a large range via reversible water uptake through variations in the humidity27, however, with challenges related to tuning speed and large-scale homogeneity28. Another interesting approach is to use mechanically tunable materials, including systems inspired by cephalopods29,30 or mechanically stretchable conductors31. Also for these systems, challenges relate to low switching speeds, and also in difficulty in gradually varying the IR response with high accuracy. In that sense, micro-electromechanical systems (MEMS) form a related category that can provide fast switching speed and convenient control via electrical signals, but they rely on sophisticated and expensive microfabrication techniques32. An alternative approach is to use materials whose properties can be electrically tuned via reversible electrochemical doping, often also providing low-power consumption due to low operating voltages and bistability or hysteresis effects. Examples of materials include electrochromic polymers33,34,35,36,37,38,39,40 and graphene41,42,43. Importantly, electrochemical doping and dedoping requires the tunable material to be in contact with an electrolyte, which typically is opaque in the IR. To not supress or completely cancel the IR modulation of the device, the electrolyte therefore needs to be positioned below the tunable material (not in the beam path). However, most redox-tunable materials also require a supporting electrode to facilitate switching due to modest electrical conductivity. In turn, even thin electrodes are typically opaque in the IR range due to high density of free charge carriers, and would therefore also strongly limit the IR modulation if placed in the beam path44. Recent progress in the development of far-IR transparent conductors is highly interesting in this respect44, but so far the common approach has been to instead position structured electrodes (e.g., microporous electrode) that are permeable to ions under the tunable material. This ensures contact of the tunable material with both the electrode and the electrolyte, but often requires complex and costly fabrication procedures45,46,47. Another alternative is to develop IR-tunable materials that are sufficiently electrically conducting to be used as their own electrodes, such as multi-layer graphene41,42 or highly conducting polymer thin films based on PEDOT (poly[3,4-ethylenedioxythiophene])33,34,48. This removes the need for a supporting electrode in contact with the tunable material. This approach has been successful, but the modulation of IR properties of such thin film devices often also led to large changes in their visible response33,41,47. This is less desired for applications such as dynamic camouflage and anti-counterfeiting. For example, it would be problematic if a concealed object in the IR range becomes apparent in the visible upon tuning.
In this study, we demonstrate that a flexible and mechanically robust paper material made by blending cellulose with a conducting polymer can be used to overcome the above challenges. These papers possess high electrical conductivity (above 500 S/cm) and also provide vertical transport of ions. This allows for electrochemical tuning of the PEDOT redox states across the whole paper, for thicknesses as large as 150 μm and without any additional top electrode. In turn, modest electrical biases within ±1.5 V could tune the IR emissivity of paper-based devices in a range of about 0.4, which is higher than for previously reported systems based on thin PEDOT films33,34. Furthermore, the visible response of the devices remained largely constant during tuning, allowing for dynamic tuning of thermal radiation and apparent temperatures without changing or revealing information in the visible range. As depicted in Fig. 1, we demonstrate that these papers enable a range of applications, focusing on tunable radiative coolers, dynamic thermal camouflage, dynamic anti-counterfeiting tags, and grayscale UV-patterned IR displays. Our demonstrations include the integration of the papers with various functional films to achieve more complex and unique functions. The papers can be fabricated in a very simple way by blending and drying and can be easily scaled up to arbitrary sizes or thicknesses. In terms of large-scale use in commercial settings, we also emphasize that biobased cellulose is non-toxic, cheap, and more sustainable compared to commonly used synthetic fluorinated binders. We believe our conducting polymer-cellulose papers can find important use for various electrically tunable IR optical devices and applications.
Operation of the devices utilizes the mixed ionic-electronic transport properties of the conducting polymer paper, which enable efficient vertical transport of ions across the paper. Applying different electrical biases modulates the infrared properties of the papers, such as emissivity and surface reflection. In combination with additional functional layers, this opens for various applications ranging from radiative coolers and thermal camouflages to dynamic displays and anti-counterfeiting devices. For radiative coolers, varying the voltage could change the emissivity of the top paper and thus control the radiative cooling power and temperature of the overall device. The changes in emissivity also lead to changes in the apparent temperature captured by an infrared camera, enabling applications such as dynamic thermal camouflage and infrared displays. Anti-counterfeiting applications could further show visible patterns (e.g., letter “Y”) using an infrared-transparent dye and contain concealed information (e.g., letter “N”) that can be retrieved and observed in the infrared range by tuning the voltage.
We fabricated conducting polymer-cellulose (CPC) papers using poly(3,4-ethylenedioxythiophene):polystyrene sulfonate (PEDOT:PSS) and cellulose nanofibrils (CNF) as the main ingredients. The plasticizer glycerol and secondary dopant ethylene glycol (EG) were also added to improve the ionic and electrical conductivities of the paper as well as its mechanical properties49 (see chemical structures in Supplementary Fig. 1 and more details on the fabrication process in “Methods” section). We prepared papers with different thicknesses (in the range from tens to hundreds of micrometers) by varying the volume of the blend solution.
The intended IR applications require the papers to not only be electrically conducting but also ion permeable to allow for electrochemical tuning across the whole thickness of the paper, including the top interface that is not in direct contact with the electrolyte layer. To examine this, we used the basic paper device structure illustrated in Fig. 1, consisting of an electrolyte layer sandwiched between a top and a bottom CPC paper. The electrolyte was an ionic gel made of an ionic liquid [1-ethyl-3-methylimidazolium bis(trifluoromethylsulfoniyl), EMIM:TFSI] in the matrix of poly(vinylidene fluoride-co-hexafluoropropylene) (PVDF-HFP, see chemical structures in Supplementary Fig. 1). This solid-state gel layer can be used as a “cut-and-stick” electrolyte for various electrochemical devices due to its excellent ionic transport properties and good mechanical properties and stability in air50,51. Figure. 2a, b presents the specular reflection at different electrical biases for a device with a 47 µm thick top CPC paper. At 0 V bias, the device exhibited specular reflection of only ~5% between 400 and 1500 nm, while it increased to around 50% for longer wavelengths in the mid-IR. Applying a positive bias to the top paper did not change the overall spectral line shape but slightly increased the reflection in the near IR. These results agree with the PEDOT:PSS in the paper being in a high-conducting oxidized state after fabrication49, which leads to optically metallic properties and high reflectance at frequencies below its plasma frequency35. The results indicate that the paper is sufficiently conducting and smooth to provide high specular IR reflectance in a broad range from around 2500 nm to at least 17 µm. Applying a negative bias drastically decreased this broad IR reflectance. This is due to redox switching of the PEDOT to its neutral low-conducting state, for which the material is no longer optically metallic or reflective in the IR. Indeed, the dependence on bias potential of the IR surface reflection correlates with the non-monotonic variation in electrical conductivity (Fig. 2c) and the results also agree with the complex refractive index dispersion of PEDOT thin films in high- and low-conducting states (Supplementary Fig. 2). The de-doping process upon application of negative bias is governed by the following scheme:
where M+ is the cation provided by the electrolyte and e− is an electron. The high-conducting state of the polymer can be retrieved back by re-doping via oxidation using a positive electrical bias:
a Specular reflectance in the visible and near IR for a 47 µm paper device at different electrical bias. b Specular reflectance in the mid-IR for the 47 µm paper device at different electrical bias. c Bulk electrical conductivity of a 47 µm paper at different electrical bias. d In-situ electrochemical specular reflectance spectra with the application of electrical bias in the form of periodic square waves (±1.5 V and 180 s duration for each step). e Normalized reflectance at the wavelength of 3000 nm for a periodic square wave electrical bias (±1.5 V and 300 s duration for each step) with a few cycles. The bottom panel also indicates the method to extract oxidation and reduction times. f Response times for devices made from CPC papers with different thicknesses. g Normalized resistance-strain curves measured at a stretching speed of 0.5 mm/s for a CPC paper with thickness of 47 µm, length of 10 mm, and width of 8 mm. h Normalized resistance-bending cycle curves of a CPC paper (same dimensions as in (g)) with a bending radius of 3 mm and a bending cycle rate of 0.5 mm/s. i Cyclic voltammograms of a complete device, measured after the 1st, 10th, 44th, and 100th bending cycle. The CPC papers in the device had a thickness of 47 µm, length of 10 mm, and width of 30 mm. The bending radius was 10 mm and the bending cycle rate was 2 mm/s.
Importantly, the results confirm that switching occurs also at the outer interface of the device where reflection takes place (the optical penetration depth is far smaller than the paper thicknesses, as estimated for PEDOT:PSS in Supplementary Fig. 2d). Hence, we can conclude that the paper provides sufficient vertical ionic and electric mobility to enable switching across its whole thickness. Measurements for paper devices made using CPC papers with other thicknesses show successful switching up to at least 150 µm thicknesses (Supplementary Fig. 3). In the visible and near IR range (below 1500 nm), the tuning led only to slight changes with reflectance remaining below 5% for both states (Fig. 2a). This makes the material suitable for applications such as dynamic IR camouflage and anticounterfeiting with minimal changes in the visible52. Figure 2c and Supplementary Fig. 4 also confirm effective tuning of the electrical conductivity of the paper. The bulk conductivity of a 47 µm CPC paper was about 30 S/cm at negative bias of −1.5 V and increased to 560 S/cm at +1.0 V. It slightly decreased (to 523 S/cm) at a yet higher bias of +1.5 V, which might be related to mild degradation from over-oxidation53,54 or anti-ambipolar behavior of the conjugated polymer55.
To further understand the vertical ionic transport dynamics of the CPC paper, we carried out time-resolved in-situ spectroelectrochemical characterization where the electrical bias was applied in the form of periodic square waves (i.e., +1.5 V for 180 s and −1.5 V for another 180 s). Figure 2d presents the results for a device with 47 µm thick CPC top and bottom papers, showing simultaneous changes in the spectra upon every change in bias potential. The specular reflectance values agree well with the static measurements in Fig. 2a. We extracted the switching times for oxidation (from neutral state to oxidized state) and reduction (from oxidized state to neutral state) as the time to change the reflectance between 10% and 90% of the initial and final values (bottom panel in Fig. 2e). The oxidation time for a 47 µm CPC paper device was about 5.5 s, while the reduction time was 17 s. Figure 2f and Supplementary Fig. 5 present response times for devices with CPC papers of different thicknesses. Both oxidation time and reduction time increased with increasing paper thickness, following an approximate linear relationship (Fig. 2f). The switching times are remarkably short considering the large thicknesses of the papers, indicating high vertical ionic mobility. In fact, the switching times are not far from that of electrically tunable vertical devices based on thin films (with thickness of only a few hundred nanometers, such as multi-layer graphene41 and PEDOT:Tosylate films33), which exhibited switching times on the order of a few seconds (e.g., 4.6 and 3.3 s33,41). The results for our devices based on much thicker CPC papers (e.g., thickness of 47 µm) corresponds to about two orders of magnitude faster thickness-normalized switching. Recent studies on high-performance kirigami-type Au/polyaniline electrodes45 exhibited similar switching times (~16 s) as for our devices, but for lower thicknesses (<1 µm). It should also be stressed that the presented CPC paper devices possessed good optical memory function, in particular for the oxidized high-conducting state for which the normalized reflectance remained within 93% of the original value for more than 4 h after removing the applied bias (Supplementary Fig. 6). If turning off the device in the low-conducting state, the normalized reflectance increased within around 20 min to around the middle point between the lowest and highest values (50% normalized reflectance). This means that the stable reflectance was distinctly different if leaving the device unbiased for long times from the high or the low conductivity states. The corresponding optical memory effect does not only make our papers suitable for low-power operation, but also open for other applications such as neuromorphics56,57. We further note that these type of devices showed good durability upon repeated electrical modulation. The tuning range of the reflectance decreased <20% after more than 100 cycles between −1.5 and +1.5 V (Supplementary Fig. 7).
Most applications rely on robust mechanical and electromechanical properties, and many benefit also from stretchability or bendability. Using a custom-built four-point probe tensile strain setup, we could study the behavior upon stretching via the variation in electrical resistance across different CPC papers (see “Methods” section). As shown in Fig. 2g, the papers showed minor increases in their normalized resistance up to 22% strain. By comparison, previously reported CPC papers49 had an elongation at break of 13.4%, indicating that the addition of EG as a plasticizer enhanced the stretchability of the papers. To further test the mechanical properties of the papers, bending tests were performed at a bending radius of 3 mm for 100 cycles (see photographs in Supplementary Fig. 8). The normalized resistance fluctuated marginally during the first 83 cycles until partial mechanical failure at the 84th cycle (Fig. 2h). Encouraged by the mechanical robustness of the CPC papers, we tested the bending properties of a whole device as a function of its electrochemical performance (see Supplementary Fig. 9). Tape was applied around the edges of the device to improve adhesion between the layers (see details in “Methods” section). Cyclic voltammograms (CV) were measured in the flat state of the device after the 1st, 10th, 44th, and 100th bending cycle. The device showed good electrochemical stability with only a minor 4% variation in capacitance between the 1st (17.7 mF) and 100th (16.9 mF) bending cycle.
The large variation in specular reflection (Fig. 2b) implies that the devices may also provide variations in their thermal emissivity, which was confirmed through measurements of the total reflectance (specular + diffuse, \({R}_{{\rm{total}}}\) ) using an integrating sphere (Fig. 3a). Kirchhoff’s law of thermal radiation tells us that the thermal emissivity (\(\varepsilon \left(\lambda \right)\) ) is equal to the absorption at the same wavelength (\(A\left(\lambda \right)\) ). Since the CPC papers have negligible IR transmittance (Supplementary Fig. 10), the thermal emissivity of the devices is then obtained as \(\varepsilon \left(\lambda \right)=1-{R}_{{\rm{total}}}\left(\lambda \right)\) . Figure 3a exemplifies the results for a paper device with 47 µm thick CPC papers, showing successful broadband electrical tuning of the thermal emissivity by more than 0.38. For example, the emissivity varied between 0.29 and 0.67 at the wavelength of 10 µm. For both redox states, these emissivity values correspond to considerably higher reflectance than the specular reflection (Fig. 2b), highlighting the importance to account for diffuse back scattering and not only specular reflection when determining the absorption and emissivity. Figure 3b presents a summary of electrical emissivity modulation for devices with different paper thicknesses (at the wavelength of 10 µm), all exhibiting the same trend of the emissivity decreasing with increasing bias voltage. We found no clear dependence on thickness on the absolute emissivity or tuning range, but instead attribute the observed variations to natural differences between devices. Indeed, devices with the same paper thickness could present variations in the exact emissivity tuning range (Supplementary Fig. 11), but still showing the same tuning trend with the electrical bias. Papers with thickness of 47 µm were chosen as model system for the rest of the study.
a Diffuse reflectance, and emissivity measurement of a CPC paper device at different electrical bias (47 μm thick CPC paper). b Relation between the emissivity and electrical bias for paper devices with CPC papers of different thicknesses. c Outdoor radiative cooling tests for paper devices with CPC papers of different thicknesses (top: real temperature, bottom: temperature difference). d Schematic of the sky simulator and device structure (inset). Sky simulator radiative cooling tests for paper devices at different electrical bias for CPC papers with thicknesses of 23 µm (e) and 47 µm (f). In the top panels, the light blue lines are the raw data, while the dark blue lines are locally weighted smoothed data (LOESS, points of window 30). g Specular reflection of a 47 µm thick CPC paper after different PEI vapor treatment times. h Diffuse reflectance and emissivity of CPC papers after different PEI vapor treatment times. i Outdoor (bottom) and sky simulator (top) radiative cooling tests of PEI-treated 47 µm thick CPC papers.
The power of a radiative cooler is proportional to its thermal emissivity7,58, which has been utilized in different types of switchable or controllable radiative coolers45,48,59. We recently demonstrated temperature control of devices at ambient conditions via electrical redox tuning of a thin conducting polymer film, which provided emissivity modulation of around 0.2534. Motivated by the much larger modulation for our current system (around 0.4), we here studied the performance of CPC paper devices in terms of electrically tunable radiative cooling. Night-time outdoor cooling performance was first characterized for pristine CPC paper devices with three different thicknesses (23, 47, and 92 µm). All three devices showed clear sub-ambient cooling, with a temperature decrease of 1.0 to 1.5 °C compared to the surrounding temperature. Applying electrical biases did not result in clear changes in the absolute device temperatures. We stress in this respect that the absolute device temperature was monitored by thermocouples and not by a thermal camera, because a thermal camera also measures the change in emissivity itself41,59. Indeed, the electrically controlled variation in apparent temperature obtained by a thermal camera reached much larger values of above 12 °C for our devices (see sections below). Since outdoor measurements can be affected by factors such as wind or moving clouds, we also characterized our devices under more stable conditions in a sky simulator (see illustration in Fig. 3d). Our sky simulator was described in detail in our previous report60. In brief, it uses an IR-absorptive material cooled by liquid nitrogen to mimic the low emissive and absorptive IR behavior of the cold universe. Emitted infrared light from devices placed (upside down) at the top of the setup is reflected by sidewall mirrors and absorbed by the cold source (“the sky”). Although the setup cannot provide identical properties as the real sky34, it can provide a similar but stable environment for evaluating and comparing the radiative cooling performance of our devices. Figure 3e presents tunable cooling of a device with 23 µm thick CPC paper upon periodically switching the electrical bias between ±1.5 V. The results show clear and reversible tuning of the device temperature. At the oxidized state, the temperature was around 0.72 °C lower than the surroundings while the corresponding decrease in temperature from the surroundings was 1.04 °C for the reduced state. This gives a temperature tuning range of about 0.32 °C, which is higher than that reported for devices based on thin PEDOT films at ambient conditions34. This improvement is consistent with the superior emissivity modulation for the CPC paper devices. Indeed, the 47 µm CPC paper device (with yet higher emissivity modulation) showed an even larger tunable temperature modulation of 0.42 °C (Fig. 3f). Interestingly, the 47 µm paper device also showed transient upward temperature peaks for each switch in electrical bias, which is different from the device with thinner CPC paper and PEDOT thin film devices34. We attribute this behavior upon increasing CPC thickness to increased role of Joule heating in defining the temporal evolution of the device temperature. Joule heating is proportional to the square of the electrical current and the resistance, both of which increases with paper thickness. The bottom panel of Fig. 3f exemplifies this as a much higher current required to switch the 47 µm paper device compared with the 23 µm paper device, with maximal currents exceeding 50 mA/cm2. This is a natural consequence of the volumetric capacitance and more PEDOT material being switched for the thicker system.
The influence of oxidation state on radiative cooling performance was also investigated by gradual chemical reduction of CPC papers by exposure to vapor from a reducing agent (branched poly(ethylenimine), or (PEI))61. Previous reports verified that this chemical tuning method can achieve highly similar effects as electrochemical tuning and this chemical process is reversible35,36,37. We treated 47 µm thick CPC papers with PEI vapor for different time durations (20, 40, and 60 min) and characterized their specular and diffuse reflectance (Fig. 3g, h). The spectral variations upon chemical reduction are essentially the same as to those obtained by electrochemical tuning (Figs. 2a and 3a), providing an emissivity modulation at 10 µm wavelength from 0.33 (original without PEI vapor treatment) to 0.70 after 60 min PEI treatment. Sky simulator measurements show that increasing the PEI vapor treatment time led to better cooling power, with a difference in temperature decrease of around 1.2 °C between the pristine paper and that treated by PEI vapor for 60 min (top panel of Fig. 3i). Also for outdoor measurements, a 60 min PEI-reduced CPC paper showed a clear lower temperature compared to a non-reduced CPC paper, agreeing well with the measurement results from the sky simulator (bottom panel of Fig. 3i).
The large emissivity modulation of the CPC paper devices makes them suitable for dynamic camouflage applications, as here investigated by thermal imaging using an IR camera (Fig. 4a). Electrical redox tuning led to drastic changes in the apparent temperature, from an average of −9.6 °C at −1.5 V to −23.2 °C at +1.5 V (Fig. 4b, 47 µm CPC paper device). The temperature of the surrounding environment was around 0 °C. This large modulation of more than 13 °C results from the large emissivity variation, which could also be gradually controlled by the electrical bias (Fig. 4a, b). Switching the device for multiple cycles showed no observable degradation of the performance (Fig. 4c). As shown in Fig. 4d, increasing the CPC paper thickness lowered the average modulation range only slightly, from 13.7 °C (23 µm CPC paper device) to 11.4 °C (150 µm CPC paper device). The results are in line with the slight reduction in emissivity modulation for devices with thick CPC papers (Fig. 3b). Figure 4e presents the switching response times of the corresponding devices. The trend of increasing response times with increasing CPC paper thickness is the same as for emissivity modulation. The absolute numbers were slightly different in these experiments, likely due to variations between batches of devices. We further note that the switching response times are comparable to those reported for other IR electrochromic devices33,34,41,42.
a IR camera images of a device (size: 2.1 cm × 1.5 cm) at different electrical bias, based on CPC paper with 47 µm thickness. The temperature of the surrounding environment was around 0 °C. b Extracted average apparent temperature at different electrical bias of the paper device shown in (a). c Temporal variation in apparent temperature of the paper device for multiple cycles of square wave electrical bias (±1.5 V). d Modulation range of the apparent temperature for devices made from CPC papers of different thicknesses. e Response times for oxidation and reduction extracted from thermal camera images. f Schematic of camouflage measurement setup (left) and IR images of a paper device at negative (middle) and positive (right) electrical bias. Device size: 1.8 cm × 1.6 cm. The device was placed on a Peltier element (heated to 45 °C, measured by a thermal camera), which in turn was placed on metal plate (deep blue background color in the thermal images). A plastic foam with emissivity of about 1 was used to represent the temperature of the surrounding environment (located in the top part of the thermal image in yellow–green color). g Real-time apparent temperature variation of the paper device upon redox-switching for a concealed object with temperature of 45 °C (the Peltier element, same measurement as in (f)). h Same as in (g) but upon increasing the temperature of the concealed object in the range from 55 to 65 °C.
The above sections focused on a simple three-layer device architecture. From this section, we will discuss integrating the papers with additional functional films to realize more complex functions, including dynamic anti-counterfeiting tags and grayscale IR displays (see Section “Greyscale dynamic IR displays”). Anti-counterfeiting tags widely exist in our daily life, such as watermarks, holograms, and barcodes, all of which can provide information that cannot be seen by the naked eye and cannot be forged without understanding their working principles. Typical materials used for anti-counterfeiting tags include luminescent materials62, optical microstructured materials63, and hydro- or thermo-chromic materials64,65. Here, we will use our CPC paper devices with electrically tunable IR emissivity for such applications.
As illustrated in Fig. 5a, we used a device structure similar to those used above but with the gel electrolyte layer patterned with an intended hidden information to display on request. In the example, the intended information was “LOE”, which abbreviates “the Laboratory of Organic Electronics”. The information could not be discerned by the naked eye (or a normal camera) with or without electrical bias, thanks to the dark opaque top CPC paper in both redox states (Fig. 5b). When no bias or positive bias was applied to the top CPC paper, also the thermal camera showed a uniform image without revealing the hidden information. By contrast, a clear pattern of “LOE” emerged in the thermal image after applying a negative bias to the top CPC paper. The results are due to differences in redox state and thermal emissivity between non-patterned regions and patterned regions. In patterned regions (without electrolyte), the CPC paper could not be switched and therefore presented its low-emissivity oxidized state regardless of bias potential (and also a low apparent temperature, see Fig. 5b and Supplementary Video 1). As a result, the concept can provide hidden information that cannot be viewed in the visible and that only appears in thermal camera images upon an applied potential.
a Schematics of the device configuration, including a gel electrolyte layer with hollow patterns (details for fabrication can be found in Methods Section). b Sample images at ±1.5 bias obtained by a normal camera (left) and by a thermal camera (right). Device size: 3.0 cm × 1.2 cm. c A paper device with half of its area covered by an IR transparent blue paint. The panel shows a schematic (top left), visible image (top right), and thermal images at +1.5 V bias (bottom left) and −1.5 V bias (bottom right). The area covered by the dye has been marked by white dashed lines. Device size: 1.8 cm × 1.6 cm. d Temporal variations in the apparent temperature upon electrical switching of the device shown in (c), showing temperatures variations for both the region painted by the IR transparent dye and the bare paper region. e Anti-counterfeiting device configuration (top left) with both a paint pattern on the paper surface (top center) and a patterned tape spacer layer (top right). The panel presents a visible image of the device (middle left), and thermal images without (middle right) and with electrical biases (bottom). Device size: 2.4 cm × 1.1 cm.
An additional feature for anticounterfeiting can be to include fake information in the visible range. To realize this, we introduced IR transparent dyes with visible color. Prussian blue (PB, blue color) mixed with zinc oxide nanoparticles (ZN, white color) in different ratios was previously shown capable of producing blue colors with various brightness while maintaining high transparency in mid-IR66. We here deposited a paint made of PB mixed with ZN onto CPC papers by spin coating, and prevented deposition on half of the area using tape. The painted area (depicted by white dashed lines in the top right panel of Fig. 5c) showed a clear bright blue reflected color while the non-painted area remained dark in the visible. By contrast, the painted and non-painted areas presented very similar apparent temperatures, including upon redox-switching (bottom panel of Fig. 5c). The painted area showed only slightly higher average temperature of about 1 °C compared with the area without paint, confirming sufficient IR transparency of these dyes. We measured the response times of these devices and found that the apparent temperature changes were reversible and as rapid as without paints (Fig. 5d). This is promising for using these IR transparent dyes to create visible information that is independent of the information hidden in the dynamic IR image. To test this concept, we painted a cross sign with the IR transparent dye paint on the CPC paper surface and also introduced a combination of cross and tick signs in a patterned tape spacer layer between the top CPC paper and the electrolyte layer in the device (see configuration in the left panel of Fig. 5e). The patterned spacer layer limits the contact between the electrolyte and CPC paper to only certain regions, which therefore will be easily switched while others will not. (A different configuration with patterned electrolyte instead of patterned spacer also worked but with less good results, see Supplementary Fig. 12.) We first note that the visible information (painted dye tick) was clearly seen in the visible for both states, while only barely resolved in the thermal images. By contrast, the hidden information below the paper could be made appear or disappear in the thermal images by varying the electrical bias (Fig. 5e), while they remained invisible in the visible. The results demonstrate that the combination of IR transparent dyes and our paper devices are excellent candidates for anti-counterfeiting tags.
IR displays are useful for not only anticounterfeiting, but also various situations, such as smart road signs for autonomous vehicles or drivers at nighttime33 where vision in darkness is demanded. Above, we demonstrated only digital IR displays and with rather low resolution. As a final demonstration, we combined the CPC paper devices with the concept of UV patterning of conducting polymer thin films67,68,69 to realize dynamic grayscale IR images with high resolution. The UV patterning method is based on vapor phase polymerization of PEDOT thin films with an added UV exposure step of the polymerization precursors through a photomask67,68,69. The process flow is depicted in Fig. 6a and can be used to produce micro-sized grayscale patterns in thin PEDOT films. As demonstrated in our previous studies, UV patterning can lead to large differences in various film properties, including thickness, visible and IR absorption, and complex refractive index69. Here, we polymerized a UV-patterned thin PEDOT:Tosylate layer directly on the CPC paper (see details in the “Methods” section). Thermal camera imaging of regions exposed to different UV doses presented large differences in apparent temperatures (Fig. 6b). Tuning the UV transmittance of the photomask from 70% to 1% led to variation of the apparent temperature by almost 10 °C, from 14.5 to 5.6 °C (Fig. 6b). The results agree with previous reports showing decreased electrical conductivity of conducting polymer films prepared at increasingly higher UV exposure doses67,69, which makes them less reflective and more emissive in the IR (Fig. 6c). We further printed grayscale photomasks of the portrait of one of the co-authors (Fig. 6d), and created patterned PEDOT thin films on top of a CPC paper. The portrait could hardly be seen in the visible except for at certain angles (attributed to differences in reflection caused by variations in thickness). By contrast, the pattern was perfectly reproduced in the thermal image.
a Process flow of UV patterning method to create well-defined patterns on CPC paper devices. b Grayscale photomask design (left) and observed thermal image of a sample containing a UV-patterned PEDOT:Tosylate film made using that design. In the design, square pixels with different UV transmission were used (A: 1%, B: 3%, C: 22%, and D: 70%). The average apparent temperatures for pixels A–D were 5.6, 9.9, 14.1, and 14.5 °C, respectively. Device size: 2.4 cm × 2.4 cm. c Measured IR emissivity of the UV-patterned PEDOT:Tosylate films (without electrical bias) on the CPC paper with different UV doses (pixels A–D corresponding to the sample shown in b). d CPC paper with a UV-patterned PEDOT:Tosylate portrait (the original photo used was taken by Thor Balkhed, Linköping University. The copyright of the photo belongs to D.Z.). Device size: 3.0 cm × 2.5 cm. The bottom panels show visible and IR images and the top panels show the designed pattern and printed mask. Electrically tunable paper displays based on CPC paper with a portrait pattern at different electrical bias (e) and its real-time apparent temperature curve (f) for the hair and the forehead parts of the portrait (the original photo used was taken by Thor Balkhed, Linköping University. The copyright of the photo belongs to M.P.J.). Device size: 3.0 cm × 2.5 cm. The thickness of the CPC papers for these devices was 47 µm. The UV photomask was made using an office printer with resolution of about 100 µm.
To demonstrate electrical tunability, we applied ±1.5 V bias and monitored changes in the thermal image for a similar UV-patterned device (Fig. 6e and Supplementary Video 2). The thermal image disappeared at negative bias and re-appeared with the positive bias. The different parts of the images presented different modulation of the apparent temperatures upon redox tuning. For example, the forehead part made through high UV exposure dose only showed modulation range of 2.1 °C, while the hair part corresponding to low UV dose presented a higher modulation of 13.5 °C (marked with circles in the left panel of Fig. 6e, f). This can be explained by UV exposure leading to lower electrical conductivity and thereby higher IR emissivity also for the oxidized state of the material70. Indeed, both regions (high and low UV exposure doses) exhibited similar apparent temperature of about 10.0 °C in the reduced state, making it possible to (reversibly) erase the image by negative bias (Fig. 6f). Besides adding high resolution grayscale images, UV patterning therefore provides additional functionality which may be relevant for anti-counterfeiting systems.
In summary, this paper has demonstrated that CPC paper can be used as ion permeable and dynamically tunable electrodes for active IR optical devices. The CPC papers possess good mechanical properties and show outstanding out-of-plane ionic transport properties with short response times for electrical redox state tuning of CPC paper devices. Specular IR reflectance and thermal emissivity could be modulated by about 40% using low electrical bias of only ±1.5 V, representing one of the best IR-tunable materials. These properties enable multiple promising IR applications, where we demonstrated tunable radiative coolers, active thermal camouflage, dynamic anti-counterfeiting tags, and grayscale IR displays. We believe this special type of ion-permeable electrodes will become a powerful materials platform for IR optical devices, including not only the applications demonstrated herein but also concepts such as electrically tunable metasurfaces and neuromorphics.
Conducting polymer PEDOT:PSS (PH 1000, 1.3 wt%) was purchased from Heraeus Clevios. Cellulose nanofibrils (CNF, carboxymethylated, 0.52 wt% aqueous solution) were purchased from RISE Innventia AB. Other chemicals were purchased from Sigma-Aldrich. PEDOT:PSS, CNF, glycerol, and ethylene glycol were mixed at weight ratio of 130:130:1:7 or absolute weight ratio of 1.69:0.68:1:7. The blend solution was then homogenized using an ULTRA-TURRAX T-10 dispenser for a few minutes, followed by a degassing step of 1 hour using a vacuum desiccator or a vacuum oven. The solution was then poured into a container (e.g., petri dish), and dried either in a fume hood for 2 days or in a normal oven at 60 °C for 6 h. When the solvent was removed, the CPC paper could be easily peeled off from the container. We varied the volume of the mixed solution to obtain papers with different thicknesses.
The standard three-layer device configuration is depicted in Fig. 2a. The gel electrolyte [EMIM][TFSI] in PVDF-HFP was produced following previous reports50,51. In brief, PVDF-HFP, [EMIM][TFSI], and acetone were mixed at a weight ratio of 1:4:7 and stirred in a sealed vial overnight to obtain a homogeneous solution. The solution was drop-casted onto a clean glass substrate followed by a heating step at 60 °C on a hot plate for 2 h to remove residual solvent. The drop-casted layer could then be easily detached from the substrate and transferred onto a CPC paper to be used as the electrolyte layer. Another CPC paper was then placed on top of the electrolyte layer to form the three-layer device. To realize complex functions, the electrolyte layer and/or the top paper layer can be patterned. To create the type of devices shown in Fig. 5a, the patterns can be produced manually or through a flatbed cutter. We used the flatbed cutter to remove the “LOE” pattern in the electrolyte while the remaining parts formed a continuous gel film.
Additional layers can be deposited on top of the CPC paper device, where we in this study used IR transparent dyes and UV-patterned PEDOT:Tosylate layers. The IR transparent dyes were based on Prussian Blue (PB) and zinc oxide nanoparticles (ZN), as purchased from Sigma-Aldrich. The dyes were dissolved in ethanol and mixed to create the colored paint. The blue color used to produce the sample imaged in Fig. 5c were produced by a 20:1 mixture of PB and ZN. The dye was painted on the CPC papers by spin coating and using cleanroom scotch tape to prevent deposition onto the unwanted area (e.g., to create patterns). For the UV-patterned PEDOT:Tosylate layers, we followed the procedure reported in our previous studies69, following the simplified process flow shown in Fig. 6a (with UV exposure time of 20 min and polymerization time of 1 h). The UV mask was made using an office printer with resolution of about 100 µm. To operate the device, the top paper was connected to the positive electrode and the bottom paper to the negative electrode of the power source.
Bulk electrical conductivities of CPC papers were measured via the four-line probe method. Four-line gold/titanium electrodes with thickness of 100 nm/5 nm, width of 1 mm, and length of 10 mm were deposited onto glass substrates through thermal evaporation. Due to the hydrophilicity of the CPC papers, they could be easily attached to the glass substrates with Au electrodes, showing good adhesion. To measure the conductivities of the papers at different redox states, we directly attached a three-layer device onto the gold electrode glass substrate. The top and bottom papers were connected to the working electrode and counter/reference electrode of a potentiostat (Bio-logic SP-200). The gold lines were directly connected to four probe stations of a SCS parameter analyzer (Keithley 4200) in the configuration of four probe method. For each electrical bias, a stabilization time of 60 s were used to ensure the paper had reached a stable redox state. The electrical conductivity was then calculated via the formula of \(\sigma =1/({R}_{s}t)\) , where Rs is the sheet resistance measured by the four-line probe method and t is the paper thickness.
Static specular reflectance spectra at different electrical bias were collected using a Lambda 900 spectrometer (Perkin Elmer) for the visible and near IR and using a Spectrum 3 FT-IR (Perkin Elmer) for the mid-IR. For static measurements with electrical bias, a stabilization time of 60 s were used to ensure the paper had reached a stable redox state. For dynamic specular reflectance (shown in Fig. 2d), a fiber spectrometer (AvaLight-DH-S-BAL for light source and AvaSpec-NIR256-2.5 for spectrometer, Avantes) was used. To extract response times (Fig. 2e), the time drive mode of Lambda 900 was used with the resolution of 1 s.
Diffuse reflectance or emissivity measurements of CPC papers were carried out using a Bruker Vertex 70 FT-IR spectrometer with a downward-looking diffuse gold-coated integrating sphere (Labsphere A562). The angle of incidence for the integrating sphere was 9°. Since the paper is opaque in IR spectral range, we could calculate the emissivity of via \(\varepsilon =1-R\) , where \(R\) is the diffuse reflectance of the paper.
Thermal images were obtained through a ThermoVision A320G camera (FLIR Systems). The spectral range for this camera was 7.5–13.0 µm with a resolution of 320 × 240 pixels. In this study, all the images were captured outdoors during nighttime. For the apparent temperature measurement, we used a highly reflective metal and a highly absorptive polymer foam as the calibration targets.
We utilized our home-built setup for outdoor radiative cooling measurement, as reported in our previous studies71,72. In brief, the samples were placed in a chamber coated with Al tape with its opening covered by a piece of PE film. Three thermocouples (K type, Pentronic AB) were used to monitor the real-time temperature variation of the sample, reference objects, and air (surrounding). For indoor measurements, we used a sky simulator as described in our previous study34. A cross-section schematic of the setup is shown in Fig. 3d, where the inside wall of the setup chamber was covered by IR-reflective Al foils to guide the emitted thermal radiation into a thermoflask. This thermoflask was filled with liquid nitrogen and had an IR-absorbing layer at the bottom, to mimic the cold outer space that could absorb the emitted thermal radiation from the sample. The sky simulator resembles but does not perfectly simulate the real sky, as discussed in more detail in our previous study34,60. During measurements, the sample was embedded in a thermal insulation layer and two thermocouples were used to monitor the temperature variations on the backside of the sample and the front side (inside the chamber) of the sample, as shown in the inset of Fig. 3d.
PEDOT:PSS thin film samples were prepared by spin coating onto sapphire substrates and measured without or after 15 min PEI vapor treatment. The ellipsometry measurements were carried out at room temperature under normal ambient conditions, as reported in our previous studies5,36,37. RC2 and IR-VASE ellipsometers (J. A. Woollam Co.) were used for measurement in the visible-NIR range (400–1690 nm) and mid-IR range (2.0–30.0 μm). The data fitting was carried out through WVASE software (J. A. Woollam Co.) using an anisotropic Drude-Lorentz model5.
For tensile strain measurements, the CPC papers were clamped in a motorized linear stage (X-LSQ300A-E01, Zaber) with gold-coated four-point contact pads for resistance versus strain measurements. The resistance was acquired by a Keithley 2701 Ethernet Multimeter data acquisition system. The bending measurements were carried out in the same setup where the bending radius was approximated from the distance between two contact pads. In the test, the moving speed was 0.5 mm/s and the time for completing each cycle was 28 s. The fabrication of the paper devices for the bending test were adapted from Section “Fabricating the IR paper devices”. In brief, the ionic gel layer was sandwiched between two CPC papers and office scotch tape was used around the edges of the device to improve device stability during the bending tests. For these devices, the moving speed was 2 mm/s and cyclic voltammetry measurements were performed at a scan rate of 50 mV/s and a potential window between +1.0 and −1.0 V using a Bio-logic SP-200 potentiostat.
The data are available via Zenodo at https://doi.org/10.5281/zenodo.13629517.
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The authors gratefully acknowledge support from the Swedish Research Council (VR, 2020-00287, 2022-00211, 2022-06214, and 2019-04424), and the Knut and Alice Wallenberg Foundation, Linköping University and industry through the Wallenberg Wood Science Center. We also acknowledge the European Research Council (Consolidator grant, 101086683), the Swedish Foundation for International Cooperation in Research and Higher Education (STINT), and the Swedish Government Strategic Research Area in Materials Science on Functional Materials at Linköping University (Faculty Grant SFO-Mat-LiU No. 2009 00971). A.R. acknowledges support from the Marie Sklodowska-Curie Actions Seal of Excellent Fellowship program from the Sweden’s Innovation Agency (Vinnova grant 2021-01668). J.E. acknowledges support from the Digital Cellulose Center (Vinnova). M.P.J. and K.T. are Wallenberg Academy Fellows.
Open access funding provided by Linköping University.
Laboratory of Organic Electronics (LOE), Department of Science and Technology (ITN), Linköping University, Campus Norrköping, Norrköping, Sweden
Chaoyang Kuang, Shangzhi Chen, Mingna Liao, Aiman Rahmanudin, Debashree Banerjee, Klas Tybrandt, Dan Zhao & Magnus P. Jonsson
Wallenberg Wood Science Center, Linköping University, Campus Norrköping, Norrköping, Sweden
Mingna Liao, Aiman Rahmanudin, Debashree Banerjee, Klas Tybrandt & Magnus P. Jonsson
RISE Research Institutes of Sweden, Digital Systems, Printed-, Bio- and Organic Electronics, Norrköping, Sweden
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M.P.J., S.C., D.Z., and C.K. conceived and designed the research. C.K. fabricated the CPC paper with the assistance of J.E. A.R. carried out the mechanical characterization under the supervision of K.T. C.K. carried out the optical characterization and temperature measurements with the assistance of S.C, M.L., and D.B. C.K., S.C., and M.P.J. wrote and edited the manuscript. M.P.J. and D.Z. supervised the research. All the authors read and revised the manuscript.
Correspondence with Shangzhi Chen or Magnus P. Jonsson.
The authors declare no competing interests.
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Kuang, C., Chen, S., Liao, M. et al. Electrically tunable infrared optics enabled by flexible ion-permeable conducting polymer-cellulose paper. npj Flex Electron 8, 55 (2024). https://doi.org/10.1038/s41528-024-00339-7
DOI: https://doi.org/10.1038/s41528-024-00339-7
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