Increasing the energy efficiency of buildings is a key method of facilitating the transition to an energy economy based on renewable sources.¹ The implementation of electrochromic devices in the form of smart windows that dynamically modulate solar light and heat flux could significantly decrease the energy consumption of buildings.² Electrochromic materials, which change their transmission properties upon the application of a voltage, traditionally modulate visible light. The two most commonly studied classes of electrochromics are transition metal oxides (TMOs), exemplified by WO₃, and organic electroactive polymers, which include polyanilines and polythiophenes.³

Polymer-based electrochromic materials targeting visible light offer several potential advantages over TMOs. For instance, comprehensive work by Reynolds et al. and others over the last two decades has resulted in the synthesis of polymer materials with good absorption properties over a wide range of colors.⁴ Such color tuneability is typically more difficult to achieve with inorganic materials. In general, polymer electrochromics also possess faster switching speeds and higher coloration efficiencies than TMOs.

Recently, the Milliron group has focused on developing electrochromic materials that modulate near-infrared (NIR) light.⁵ The aim is to design these materials so they can be implemented in smart windows that dynamically control solar heat flux into buildings, thereby decreasing heating and cooling costs. Milliron et al. have explored the use of a wide range of metal oxide nanomaterials, which include tin-doped indium oxide (ITO), aluminum-doped zinc oxide, and WO_3-x nanoparticles, as materials that are electrochromic towards NIR light.⁶ Coupling these materials with traditional TMOs used for visible light modulation has allowed for the construction of dual electrochromic devices that can independently modulate visible and NIR light.⁷ Smart windows utilizing these composite materials could then simultaneously control building lighting and heating.

In this work, we construct proof-of-concept electrochromic devices that combine polythiophenes to modulate visible light with ITO nanoparticles that modulate NIR light. The devices are easily fabricated through the electropolymerization of a polythiophene on transparent electrodes modified with ITO nanoparticles. These polymer-nanoparticle hybrid devices can be operated in three distinct voltage regimes, each of which uniquely modulate visible and NIR light. We compare the performance of these composite devices utilizing two polythiophene isomers- poly(3-methylthiophene) (PMeT) and poly(3,3’-dimethyl-2,2’-bithiophenyl) (PMe₂T₂). Devices containing PMe₂T₂ exhibit superior performance due to the higher band gap and oxidation potential of this polymer. These hybrid devices exhibit contrast ratios on par with the polymer and nanoparticle individual components, good switching times, and modest durability. Taken together, these results represent a new avenue of research in composite electrochromic materials aimed at managing light and heat flux for smart window applications.

References:

1. Energy Efficiency Market Report 2014, International Energy Agency, 2014.

2. Granqvist, C. G. Thin Solid Films 2014, 564, 1.

3. Mortimer, R. J. Ann. Rev. Mater. Res. 2011, 41, 241.

4. Groenendaal, L.; Jonas, F.; Freitag, D.; Pielartzik, H.; Reynolds, J. R. Adv. Mater. 2000, 12, 481.

5. Garcia, G.; Buonsanti, R.; Llordes, A.; Runnerstrom, E. L.; Bergerud, A.; Milliron, D. J. Adv. Optical Mater. 2013, 1, 215.

6. Kim, J.; Ong, G. K.; Wang, Y.; LeBlanc, G.; Williams, T. E.; Mattox, T. M.; Helms, B. A.; Milliron, D. J. Nano Lett. 2015, 15, 5574.

7. Llordes, A.; Garcia, G.; Gazquez, J.; Milliron, D. J. Nature 2013, 500, 323.