Fabrication of Electrochromic Devices Using Double Layer Conducting Polymers for Infrared Transmittance Control

ABSTRACT: We report the performance improvement ofelectrochromic devices for modulating the transmittance contrast of long wavelength infrared light between 1.5 and 5.0 μm based on a double layer of conducting polymers. The device, fabricated with poly(3-hexylthiophene) (P3HT) and poly(3,4-ethylenedioxythiophene) (PEDOT) as the first and second layers, respectively, showed an transmittance contrast of 60% with a response rate under 5 s, which is greater than the transmittance contrast of cells based on only P3HT or PEDOT (approximately 40%). Electrochromism is defined as a reversible and visible change in the transmittance of a material by applying different voltages or electric currents, resulting in electrochemical oxidation or reduction [1]. Over the last several decades, a number of electrochromic devices based on various electrochromic materials such as tungsten oxide or nickel oxide have been discovered [2-6]. Electrochromic materials with the ability to modulate radiation in the visible and near infrared wavelengths have been used for optical displays, smart windows, and rear-view mirrors [7,8]. However, there have been few studies on infrared (IR) electrochromism, probably due to the relative difficulty in preparing IR transparent electrodes and the lack of interest in IR electrochromic devices [9]. Conducting polymer, known as p-conjugated polymers, have been extensively investigated due to their potential for application in various areas such as actuators [11], sensors [12], and light-emitting diodes [13]. In particular, conducting polymers have recently gained much attention for use in electrochromic devices because of their excellent coloration efficiency, rapid color-switching ability, and broad color availability [14,15]. A range of colors can be controlled and depend on the oxidized (doped) and reduced (undoped) states of the conducting polymers; thus, electroactive conducting polymers have the potential to be electrochromic materials. Here, we fabricated electrochromic devices using two kinds of conducting polymers, i.e., poly(3,4-ethylenedioxythiophene) (PEDOT) and poly(3-hexylthiophene) (P3HT). These materials were made into thin films to modulate the transmittance of IR light between 1.5 and 5.0 μm. It has been known that PEDOT changes in color from faint blue (in its oxidized state) to deep blue (in the reduced state), while P3HT changes from transparent (in its oxidized state) to red-pink (in the neutral state) in the visible range. However, IR electrochromism of these materials has been poorly reported. In this work, double layers of conducting polymers were applied to the device and compared with single layer device performance. The transmittance contrast of electrochromic devices was measured under alternating potential steps (±2.7 V) in the infrared region between 1.5 and 5.0 μm. The electrochromic device structure is made up of a double polished Si wafer/patterned Au grid/conducting polymer/liquid electrolyte with spacer/patterned Au grid/double polished Si wafer, as shown in Fig. 1a. The use of a double polished Si wafer as a substrate is essential due to its transparency to IR light. In order for electrons to travel in the device, an electrically conducting material should be coated on the wafer substrate. However, most electrically conducting materials, such as metal, carbon, indium tin oxide (ITO), and F-doped tin oxide (FTO), are not transparent to the IR spectral region. Thus, the metal patterned grid was fabricated using Au via a photolithographic method to minimize the IR light absorption of the substrate (Fig. 1b). The Au patterned Si wafer, with a line width of 20 μm and a line repeat period of 500 μm, showed an IR transmittance of 90%. Conducting polymer thin films (Fig. 1c) on the Au grid/Si wafer were prepared as an electrochromic material via solution polymerization. For example, PEDOT thin films were formed by spin coating the solution, which consisted of 3,4-ethylenedioxythiophene monomer, pyridine retardant, Fe(III)-tosylate oxidant, and 1-butanol as the solvent. After spin coating, polymerization was carried out at 80°C for 5 min. The electrolyte consisted of 1.0 M LiClO