“From our experiments, it can expand up to 30 percent in volume,” said Xiaokang Wang, a PhD student under assistant professor Kejie Zhao.  “That’s a huge amount.  These expansion-shrinkage cycles put a huge stress and strain on the materials of the windows.  It causes layers of the window to delaminate, and then the smart window doesn’t work.”  Wang’s paper on “mechanical breathing” has been published in the journal Nature Communications.

Smart windows come in many different configurations, but the most popular are called electrochromic devices, because they change color when a voltage is applied.  On the Boeing 787 Dreamliner aircraft, for example, passengers can gradually adjust the transparency of their windows using electrochromics.  Similar windows are now available for large-scale installation in buildings, programmed to gradually darken on sunny days to reduce energy costs. 

A typical electrochromic smart window has five layers, with two electrodes on either side, an ion storage layer, an electrochromic layer, and an electrolyte in the middle.  Jianguo Mei, assistant professor of organic chemistry at Purdue, had been experimenting with different chemical recipes for the thin-film electrochromic material.  But the laminated structure of a smart window makes it difficult to study individual components during operation; the entire 5-layer substrate is just 500 nanometers thick, less than one percent the thickness of a human hair.

He enlisted the help of professor Kejie Zhao to assess the mechanical properties of the film at nanoscale.  Zhao and Wang were shocked by the initial tests.  “The material expanded up to 30 percent in volume, but also became half as elastic and half as hard,” said Wang.  This “mechanical breathing” caused the material to wrinkle and push up against the other layers of the substrate.  The outer layers delaminated, preventing electrons from flowing, causing the electrochromic device to stop functioning.  “In our experiments with untreated samples, we saw failure after just 100 cycles.”

They began to tweak and strengthen the substrate material of the electrode, roughening the surface with silica nanoparticles.  This increased mechanical reliability, surviving up to 8,000 cycles.  But it’s not nearly enough.  “These devices need to be able to withstand more than 200,000 cycles,” said Wang.  “That’s a huge gap!”

Wang and Zhao, along with Dr. Mei’s group in the chemistry department, are continuing their studies in the hopes that these electrochromic devices can be made more reliable.  “From our experiments, we are building new theoretical models,” said Wang.  “Together with material scientists and chemists, we want to find new materials, new material processing methods, and new mechanical designs to achieve much higher lifetimes of these devices,” said Wang.

Smart windows have the potential to save huge amounts of energy, becoming a normal and reliable part of our homes, offices, and vehicles.  But until reliability is improved, Purdue researchers will continue their work.  “I can see a future where entire walls will be electrochromic smart windows,” said Wang.  “People can enjoy the beautiful outdoors, while decreasing energy consumption at the same time.  It’s a win-win!”

Writer: Jared Pike, jaredpike@purdue.edu, 765-496-0374

Source: Kejie Zhao, kjzhao@purdue.edu, 765-496-0224

Jianguo Mei, jgmei@purdue.edu, 765-494-7156

Mechanical breathing in organic electrochromics
Xiaokang Wang, Ke Chen, Luize Scalco de Vasconcelos, Jiazhi He, Yung C. Shin, Jianguo Mei & Kejie Zhao

The repetitive size change of the electrode over cycles, termed as mechanical breathing, is a crucial issue limiting the quality and lifetime of organic electrochromic devices. The mechanical deformation originates from the electron transport and ion intercalation in the redox active material. The dynamics of the state of charge induces drastic changes of the microstructure and properties of the host, and ultimately leads to structural disintegration at the interfaces. We quantify the breathing strain and the evolution of the mechanical properties of poly(3,4-propylenedioxythiophene) thin films in-situ using customized environ- mental nanoindentation. Upon oxidation, the film expands nearly 30% in volume, and the elastic modulus and hardness decrease by a factor of two. We perform theoretical modeling to understand thin film delamination from an indium tin oxide (ITO) current collector under cyclic loading. We show that toughening the interface with roughened or silica-nanoparticle coated ITO surface significantly improves the cyclic performance.