Novel flexible devices can switch between five different states

Electricity used in indoor space cooling represents a large and growing proportion of a building’s energy use. According to the International Energy Agency, in 2021 it accounted for nearly 16% of the buildings sector’s total electricity consumption (about 2000 TWh). Part of this is due to increased temperatures in many cities around the world. And part of it is the result of ‘solar gain’ – the increase in temperature inside a building that results from solar radiation passing through its windows. So, it’s no surprise that architects and developers are increasingly interested in ‘dynamic’ windows made with electrochromic glass, which can alter their transparency through the application of an electrical voltage, minimising solar gain.

Several types of dynamic windows are already on the market, with yet more advancing through the research phase. In a paper published in Nano Energy [DOI: 10.1016/j.nanoen.2023.108396], a group of researchers from Jinan University (China) report on their contribution to this effort – a flexible dynamic window (FFDW) that can switch between transparent, grey, and black tints, as well as two mirror states.

They used a technique called reversible metal electrodeposition to fabricate their device. They chose to focus on a combination of bismuth (Bi) and copper (Cu) because these metals can be reduced simultaneously, reducing the eventual device’s energy consumption. Starting with BiCl3, CuCl2, LiBr, HCl and PVA, they produced a colourless gel to act as the electrolyte. An ITO-coated piece of PET was used as the working electrode. The counter electrode was made from PET covered in copper tape. When sandwiched together and sealed, this produced a dynamic window measuring 5 cm x 5 cm.

To achieve five independent optical states, the researchers had to finely control the deposition voltage and time. At +0.8V, the FFDW was completely transparent and colourless. Applying a voltage of –0.8 V for 10 s turned the window a uniform grey colour. This resulted from Bi3+, Cu2+ and Cu+ ions in the electrolyte depositing onto the ITO/PET electrode. Extending the time to 30s (still under –0.8 V) turned the window black. The authors credit this to the non-uniform deposition of metal nanoparticles, resulting in scattering and a localized surface plasmon resonance (LSPR) effect. Applying a more negative potential (− 1.6 V) for 30 s resulted in a “silvery mirror state”. Using a voltage-step strategy (− 1.6 V for 0.5 s, followed by − 0.4 V for 30 s) produced what they call a “cupreous mirror state” – a mirror that appears slightly red.

Measuring optical spectra of all five states showed that the transparent window had a transmittance of > 60% and almost no absorption at visible wavelengths; for the grey window, transmittance was < 40%. In its black state, the window was truly opaque, with an ultra-low transmittance (~3%) and a reflectance of ~20%. The transmittance of both mirror states was less than 10%, which suggests that they can block visible light effectively. The silvery mirror was shown to reflect more than 70% of the incident light; comparable to the performance of a commercial mirror. The cupreous state shows partial absorption of light at wavelengths longer than 500 nm, which gave it its characteristic red tint.

The duration to switch between these states varied. For example, going from transparent to silvery mirror state took 12.51 s; the reverse took 18.78 s. Creating the cupreous mirror state was quicker, at 8.83 s. Going from black-to-transparent took 11.12 s, while switching from transparent to opaque black took just 2.34 s. In addition, the team showed that they could apply regionally-controlled voltages onto a single device, allowing different regions to simultaneously display different optical states.

The authors say that this is the first time that an “…all-in-one FFDW based on Bi/Cu bimetal reversible deposition” has been demonstrated, and that it “…is promising for the applications of smart windows, display devices and information devices.”


Chunhua Su, Zhijuan Zhao, Daoyi He, Huawei Song, Chuanxi Zhao, Wenjie Mai. “Five-state flexible dynamic windows,” Nano Energy 111 (2023) 108396. DOI: 10.1016/j.nanoen.2023.108396