Javier Padilla

Use of polymer/ionic liquid plasticizers as gel electrolytes in electrochromic devices

The dual polymer configuration is commonly used when constructing electrochromic devices (ECDs) due to the expected electrochemical stability and enhanced optical properties. In this configuration, two different polymers are used which are optically complementary. Herein we report the construction and characterization of dual-type ECDs using poly(3, 4-ethylenedioxythiophene) (PEDOT) and poly[3, 6-bis(2-(3, 4-ethylenedioxy)thienyl)-N-methylcarbazole] (PBEDOT-NMCz) as the two complementary electrochromic polymers for the device. A variety of gel electrolyte solutions were prepared and evaluated for these devices. The use of ionic liquids within these gels imparted interesting properties, including long lifetimes, and thermal stability of devices. Switching speeds for the various devices, as well as optical contrasts, were also obtained for the gel electrolytes containing different amounts of ionic liquid as plasticizer.

Electrochromic variable transmission optical combiner

Complementary coloring conducting polymer based electrochromic devices have been designed, fabricated and tested for possible application as a variable attenuation combiner element for a see-through head mounted display or a variable trasnsmissive sand wind dust goggle lens. Electrochromic cells fabricated on both glass and polycarbonate substrates have been demonstrated to meet closely the desired goals of low power consumption, wide transmission range, fast switching speeds and long lifetime. Photopic transmissions of 34% in colored state and 67% in bleached state were achieved in a reproducible manner. The measured switching times are 0.6 sec (colored to bleached state) and 1.9 sec (bleached to colored state). The life cycle testing showed stability up to 92,000 switches. The measured power consumption of the fabricated devices was < 1 mW/cm . The electrochromic technology design effort has identified processes for obtaining the optimum layer thickness and selecting polymers and gel electrolytes necessary to obtain the widest transmission range, fastest switching speed and longest lifetime. Early environmental testing has been performed by subjecting prototype electrochromic cells to temperatures varying from -30°C to + 40°C with the results reported herein. Follow on work includes further optimization of electronic drive schemes as well as field testing of electrochromic lens equipped sand, wind dust goggles.

Photopatterned conjugated polymer electrochromic nanofibers on paper

Electrochromic nanofibers of conducting polymer (terthiophene) have been deposited over a conventional paper sheet by means of the electrospinning technique, and subsequently photopatterned by means of UV radiation. The synthesis of a processable precursor copolymer with a norbornylene matrix and pendant units of terthiophene makes the electrospinning process available, and allows for chemical or electrochemical crosslinking of the precursor copolymer to obtain a conducting polymer. The inclusion of photocrosslinkable units (methacrylate) in the precursor copolymer also allows for photopatterning of the material. This was applied to obtain patterns on the paper which can be chemically oxidized or reduced resulting in electrochromic characters. SEM images of the conducting polymer nanofibers together with the cellulose fibers show how these materials can be attached to textile fibers, adding new functionalities that are reminiscent of the chameleonic abilities of some living creatures.

Use of Ionic Liquids in Electrochromic Devices

Abstract The different roles that ionic liquids can play in several aspects of electrochromic technology have been reviewed, from the advantageous synthesis of electrochromic polymers to their key role as substantial components of liquid, semi-solid, or solid electrolytes. The performance of resulting devices has been analyzed, showing correct optical behavior and sometimes impressive lifetimes. From this review, it can be concluded that ionic liquids are versatile compounds that can improve the current performance of electrochromic and other electrochemical devices.

Electrochromic conducting polymers: optical contrast characterization of chameleonic materials

The optical characterization in the visible wavelength range was obtained for an electrochromic material, poly-3, 4-ethylenedioxy-thiophene (PEDOT), as a function of its redox charge density (charge consumed for the color change between its maximum and minimum absorbance states). The experimental procedure was kept very simple and all the information can be obtained from only one film, including the identification of the maximum achievable contrast for the material. Different films of the electrochromic material were tested in order to check the validity of the predicted values, showing excellent agreement.

Exploring unbalanced electrode configurations for electrochromic devices

Electrochromic cells consisting of a black-to-transmissive propylenedioxythiophene/benzothiadiazole copolymer active layer and a minimally colored dioxypyrrole-based charge storage layer were constructed, and the effect of the redox capacity ratio between the two layers was evaluated. This ratio was tuned between large excesses of either the active layer or the charge storage layer (an unbalanced configuration), to cells where both films had the same redox capacity (a balanced configuration), and the effects on cell voltage, optical contrast, switching time and switching stability were assessed. The inclusion of an auxiliary reference electrode allowed us to monitor the redox behavior of both the active layer and the charge storage layer during cell operation. With this setup, we show that the redox behavior of each layer is highly dependent on the redox capacity ratio between the two films. We also demonstrate that, in principle, any electrochromic material that has a colorless state can serve as an optically inactive charge storage material if it is present in sufficient excess in an electrochromic cell. Additionally, by monitoring the redox behavior of each individual layer during cell operation, we show that performance loss in electrochromic cells during long-term stability testing is not necessarily due to an irreversible electrochemical or structural degradation of the active material or the charge storage layer. Rather we observe a drift in the oxidation state of both layers, suggesting that the losses are reversible, and could be mitigated through proper electrochemical control of the cells. The main benefit of an unbalanced configuration is the substantial lowering of the operating voltage without compromising switching time or the contrast as long as transparency of the charge storage material is maintained.