Study of Electrochromic Devices Based on Complementary Conducting Polymer Films
By Benjamin C. Ihas, Jungseek Hwang, Irina Guirgui, David B. Tanner, John R. Reynolds
Abstract
Research into polymer-based electrochromic devices is demonstrating their potential for a variety of useful applications. Such devices, characterized by a variation in displayed reflectance controlled by an applied potential, have shown to possess high contrasts, sub-second switching times, high stability in atmospheric conditions, and repeatable behavior. Using both a grating spectrometer and Fourier transform interferometer, our research explored all of these properties, as well as performance dependence on wavelength, polymer film thickness, and choice of device window material. The results have yielded moderate to high contrast (peaking at a contrast of about 50% reflected intensity) throughout the spectrum between UV and FIR wavelengths.
Much recent attention has been paid to new electrochromic (EC) materials that exhibit long-term stability, high D %T (where T is transmittance), and rapid response times.1 Electrochromic devices are characterized by a device that exhibits a variation, controlled by an applied potential, in reflected light intensity. Practical application of EC devices is investigated, in part, by measuring the difference in percentage transmittance (D %T) between the maximum pale and colored states of the device.1 Inorganic EC materials, such as transition metal oxide films of nickel and tungsten, have been widely studied. Such oxide-based devices display high D %T, but are expensive to process, exhibit slow response times (on the order of one minute), and are not always stable over a large number of cycles.2
In comparison, conducting and redox electroactive polymers have recently attracted some attention due to several advantageous properties. Such polymers are relatively inexpensive, and, when used to create EC devices, exhibit sub-second response times, moderate to high contrasts, and low power requirements.3 These materials prove to be potentially useful in applications such as variable tint windows and mirrors, multicolored displays, and IR optical switches.4
Another advantage of such materials is that they hold much potential for automated production. This aspect holds great promise for their eventual industrial fabrication. Currently, however, the polymer-based ECs used in our experiments are hand made and are based on the two complementary materials poly[3,4-ethylenedioxythiophene] (PEDOT) and poly[3,6-bis(2-(3,4-ethylenedioxy)thirnyl)-N-methylcarbazole] (PEDOT-N-MeCz).4 This pairing satisfies the requirements that both polymers undergo a redox reaction, and are compatible with a single electrolyte.4 The monomers are first deposited, using a three electrode potentiostat, onto a 2.0" x 0.5" gold-coated Mylar strips, yielding a polymer film of 0.5" x 0.5" in area. Various polymer thickness’ are chose for investigation, and measured by counting the charge passed and comparing it to a predetermined quantity based on the charge passed to film thickness ration of 1 mC/cm2:2.67 nm.5
The PEDOT deposit, which faces outside of the final device, is responsible for the observed color shift. Before polymerization, its Mylar strip is slit with a razor blade across the deposition area. The slits, spaced about 1 mm apart and close to 0.5" in length, facilitate migration of the electrolyte between the rear and front polymers. More rapid device response times result because the PEDOT can now more easily communicate with the PEDOT-N-MeCz.
The EC device is constructed by stacking eight various layers.5 The PEDOT-N-MeCz deposited Mylar is placed, polymer side up, onto a thin polyethylene sheet that acts as the back shell of the device. Next, a four component gel electrolyte is gently spread onto the polymer so as not to damage the film.* A separator paper is then placed down to prevent shorting between the Mylar strips. More electrolyte is spread such that the paper is saturated. The PEDOT deposited Mylar is then pressed down, polymer face up, and electrolyte is again gently applied. Finally, the top window is added. This window is currently either composed of polyethylene or ZnSe. These window materials differ in infrared transmission properties and thus may be adjusted to suit the EC device’s application.
In order to measure the intensity of various wavelengths reflected by the device, two spectrometers are used. A microscope photometer is used to measure wavelengths ranging from the ultraviolet (UV) to near infrared (NIR) region of the electromagnetic spectrum. Both a xenon and a tungsten filament in halogen gas source are used to span the spectrum. A grating monochromator is then used to separate individual wavelengths and send them to the detector. For the NIR to FIR range, a Bruker Fourier transform interferometer is used. Our research continues to demonstrate the value of these EC devices. We have observed strong contrast throughout the observed spectra, experience repeatable results, and explored ways in which to improve the devices.
Contrast between the oxidized and reduced states of the devices is strong and easily observed. The redox reaction results from applying varoius potentails across the device. The following graph displas this trend.
Figure 1: Variable reflectance controlled by applied potentail
Figure 1 is a plot of the relative intensity of the light reflected off the device vs. wavelength. The intensity is defined as a ration of the difference between the object and parasitic intensity values over the difference between the standard and parasitic values. In this example, a single sample was observed between the UV and NIR spectrum while poltentials ranging from 0 V to –1.2 V were appled to the device. Notice that clear distinction exists between the states almost throughtout the UV, visible, and NIR spectrum. The noise towards either end is due to low source intensity at these wavelenghts.
Another observed trait of these devices is a reliable, repeatable returen after switching. A single sample was observed throught six deep cylce switches whereupon potentials of 1.0 V and –1.2 V were aternatly applied. The following fraph shows the sample’s stability as it continually returns to its respective maximum and minimum state.
Figure 2: Deep cycle switching of a single sample
Also, please notice the great contrast of the sample throughout the observed range. This contrast peaks at 1300 nm where the intensity of the reflected light differs by 90%.
Despite the promising results noted above, several improvements to the EC devices have been made during this project. One such improvement was with respect to the device fabrication technique. Originally, a soft cutting pad had been used to support he Mylar when it was cut with a razor knife. This method, however, seemed to be faulty. The soft pad allowed the Mylar to flex and stretch when being cut. This stretching made the top surface uneven and as a result, not all the light would return to the detector and thus, the experiments were not very repeatible. To alleviate this problem, samples were made by cutting the Mylar on a glass microscope slide. This attempt was designed to reduce the strecthing, and, was eventually observed to prove helpful. Results from two line scans, where, at a single wavelength, the spectrometer was moved across the samples perpendicularly to the slits, are shown below. Figure 3 depicts a line scan of a device whose Mylar had been cut on the cutting pad.
Figure 3: Line scan of stretched, uneven device
Each dip in figure 3 represenst a location of a slit. The higher plateaus, on a perfect device, should be at the same level, signifying that between the slits, the same amount of light is returning to the detector. The plateaus observed on this sample are very uneven and thus overall reflectance is reduced. Figure 4 on the other hand, depicts a line scan from a device made under the new process. Note the overall uniformity of both area between slits and height of plateaus. This device will have an overall greater reflectance because the Mylar is lying flat, and as a result more light is sent to the detector.
Figure 4: Line scan of a device whose Mylar had been cut on glass
Additionally, a ZnSe window is currently under use as an alternative to polyethylene for spectrum-specific applications. This window has been observed to greatly increase reflection in the IR spectra due to its better absorption properties in this wavelenght region. Other window materials may be investigate dint he future to further improve these devices.
In summary, polymer-based electrochromic devices hold great potential over other EC devices for a variety of reasons, and thus exhibit great potential. These devices are stable in the atmosphere, mechanically flexible, inexpensive, easy to produce, posess sub-second resonse times, rewuire low operating power, and posess long life. Such devices have many applications from anywhere between high contras, low power computer displays to safer, adjustable tint automotive mirrors. Currently, little limits the scope of these devices. Future investigation into application-specific windows as well as more paired polymers will only expand the potential for these versatile devices.
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