Investigating IR switching of dioxythiophene-based conjugated polymers
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Goins, Callie Lin
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Abstract
Electrochromic materials that can dynamically switch between a highly transmissive and a highly absorptive state in response to an applied voltage in the visible, near-infrared, and mid-infrared regions of the electromagnetic spectrum are of interest due to their potential application in military camouflage, transportation, and traffic visualization. Specifically, developing materials with large optical contrasts in the 3-5 µm and 8-12 µm wavelength ranges would allow for on-demand switching at wavelengths where IR detectors operate. Conjugated polymers are examples of organic materials whose absorption properties in the visible wavelength range can be modulated electrochemically; however, few have been studied to exhibit large optical contrast in the mid-IR region. Here, we explore the mid IR-switching capabilities of a series of dioxythiophene-based conjugated polymers divided into three classes: donor-acceptor, alkyl side chain, and oligoether side chain. While these polymers have been widely explored for their visible color switching properties, little existing work has been done to investigate their mid-IR switching properties. These systems were selected based on their strong and stable electrochromic performance in the visible range and reversible switching for hundreds to thousands of switches, both of which are important properties as mid-IR switching materials. First, we compared contrast in thin vs. thick electrochromic devices to optimize device thickness. Then, we studied the IR spectra of the oxidized and neutral states of polymer thin-films (200-500 nm) spray-coated onto gallium arsenide. These thin-films demonstrated strong contrast in both the 3-5 µm range and the 8-12 µm range. Here, the donor acceptor polymers had the smallest contrast, followed by those with alkyl side chains, with the polymers with oligoether side chains having the largest contrast. Next, prototype reflective devices were constructed, demonstrating significant contrast up to around 60% in the 3-5 µm range at low voltages (+/- 1 V). Donor-acceptor polymers were compared against each other, and polymers of different structural groups were also compared. We propose that these materials are promising candidates for applications requiring on-demand and user-controlled adaptive mid-IR switching.
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Undergraduate Research Option Thesis