Buildings account for nearly 30% of global energy consumption, with windows and doors responsible for 30–50% of energy loss. Electrochromic (EC) smart windows, which can dynamically regulate solar light transmission under an external electric field, offer a promising solution to reduce cooling and heating loads. However, conventional EC systems still face key challenges: inorganic materials such as WO3suffer from sluggish ion diffusion and limited spectral tunability, while organic polymers often struggle with poor film-forming ability and insufficient interfacial stability. Achieving a unified device that combines high transparency in the bleached state, broad-spectrum (visible to near-infrared) darkening, fast switching, and long cycling stability remains a critical bottleneck.
To address the above challenges, Professor Meng Hong's research team at Peking University designed a dual-band electrochromic device architecture. They paired thermally cured WO3thin films with alkoxy-functionalized polycarbazole derivatives (CTPAO1 and CTPAO3) as complementary electrodes, constructing electrochromic devices ECD1 and ECD2, and systematically investigated their EC performance in the visible to near-infrared range.
Among them, ECD2 exhibited outstanding performance: it achieved an integrated optical contrast of 77.4% across the full spectrum (200–2350 nm) and a peak modulation of 87.5% at 730 nm. Its coloring time was 1.0 s and bleaching time only 0.7 s, demonstrating excellent switching kinetics. After 20,000 electrochemical cycles, ECD2 retained 95% of its initial contrast, while ECD1 retained only 85.7%, confirming that longer alkoxy chains significantly enhance stability.
Fig 1. Electrochromic performance. (a) Color change of ECD2 under different applied voltages; (b–c) Transmission spectra under different voltages; (d, g) Switching kinetics; (e–f) Stability over 20,000 cycles.
To evaluate practical energy-saving potential, the research team performed building energy simulations for nine typical climate cities using DeST software. The results showed that compared to conventional insulating glass units, ECD2 smart windows achieved energy savings in all cities, with annual total energy savings ranging from 13.1% to 50.8%. In Haikou City, the annual energy saving reached as high as 56.5 kW h/m2, with particularly significant cooling effects in summer. Box model experiments also demonstrated that ECD2 lowered indoor temperatures by 5.0 °C, validating its excellent thermal management capability.
Fig 2. Building energy simulations. (a) Building shading simulation; (b) Performance comparison; (c) Box model experiment; (d) Solar transmittance spectra; (e) Annual energy consumption comparison across nine cities; (f) Monthly energy consumption comparison for Haikou; (g) Cooling load saving ratio.
The related results were published in *Advanced Energy Materials* under the title "High-Performance Electrochromic Smart Window for Energy-Saving Applications Based on WO3and Polycarbazole".
Professor Meng Hong of the School of Advanced Materials, Peking University, is the corresponding author, and master's student Liu Xiong is the first author. This research was supported by the National Natural Science Foundation of China, the Shenzhen Science and Technology Program, the Guangdong Provincial Key Laboratory of Flexible Optoelectronic Materials and Devices, the Guangdong Provincial Engineering Technology Research Center for Multidimensional Optoelectronic Materials, and the Shenzhen Key Laboratory for Organic Optoelectromagnetic Functional Materials.
Link to the paper: https://doi.org/10.1002/aenm.70756
