Post-doctoral researcher Congcong Wu, who is working in the lab of Shashank Priya, the Robert E. Hord Jr. Professor of Mechanical Engineering, holds up a layer of the flexible solar panel the group is working on. The process to adhere a thin film of titanium oxide to the panel takes less than 10 seconds using screen-printing technology.
In the very near future, recycling light energy may be easier than recycling any other item in your house. Led by Shashank Priya, a team at Virginia Tech is producing flexible solar panels that can become part of window shades or wallpaper that will capture light from the sun as well as light from sources inside buildings. Solar modules less than half-a-millimeter thick are being created through a screen-printing process using low-temperature titanium oxide paste as part of a 5-layer structure that creates thin, flexible panels similar to tiles in one’s bathroom. These tiles can be combined together to cover large areas; an individual panel, roughly the size of a person’s palm, provides about 75 milliwatts of power, meaning a panel the size of a standard sheet of paper could easily recharge a typical smart phone.
Most silicon-based panels can absorb only sunlight, but the flexible panels are constructed to be able to absorb diffused light, such as that produced by LED, incandescent, and fluorescent fixtures, according to Priya, the Robert E. Hord Jr. Professor of Mechanical Engineering in the College of Engineering. “There are several elements that make the technology very appealing,” said Priya. “First, it can be manufactured easily at low temperature, so the equipment to fabricate the panels is relatively inexpensive and easy to operate. Second, the scalability of being able to create the panels in sheet rolls means you could wallpaper your home in these panels to run everything from your alarm system, to recharging your devices, to powering your LED lights.” The panels can also be made to any design, so they could become window shades and curtains as well, absorbing sunlight through windows.
Currently, the efficiency of the cells is nearly on par with the heavier, rigid silicon structures, but at panel-level there is some research required. Still, it is likely the new flexible panels will overtake their rigid cousins soon. “Amorphous silicon is a fairly mature technology running at about 13-15% efficiency,” he said. “Our panels right now operate around 10% at the panel size. At smaller, less-useful sizes, the efficiency increases, so we can see a potential for much greater energy collection efficiencies.”
Though formamidinium lead triiodide (FAPbI3) possesses a suitable band gap and good thermal stability, the phase transition from the pure black perovskite phase (α-phase) to the undesirable yellow nonperovskite polymorph (δ-phase) at room temperature, especially under humid air, hinders its practical application. Here, we investigate the intrinsic instability mechanism of the α-phase at ambient temperature and demonstrate the existence of an anisotropic strained lattice in the (111) plane that drives phase transformation into the δ-phase. Methylammonium bromide (MABr) alloying (or FAPbI3-MABr) was found to cause lattice contraction, thereby balancing the lattice strain. This led to dramatic improvement in the stability of α-FAPbI3. Solar cells fabricated using FAPbI3-MABr demonstrated significantly enhanced stability under the humid air.
The flexible panels, as they approach the conversion efficiency of rigid silicon and glass, can also be incorporated into products that the older technology cannot compete with – such as military uniforms and backpacks, items Priya’s lab is working on now with the U.S. Army’s Communications-Electronics Research, Development, and Engineering Center. By adding flexible panels to these items, soldiers will become their own recharging stations, resulting in reduction of the logistical footprint of a fighting force in the field, as well as the weight each individual soldier must carry on his or her back.
By creating panels that capture a wide variety of light wavelengths, Virginia Tech engineers are opening a door to an entirely new area of light and energy recycling that could make saving energy as easy as hanging a curtain. Another paper demonstrating the stability of the cells will be published by ACS Energy Letters later in October under the title, “Improved Phase Stability of Formamidinium Lead Triiodide Perovskite by Strain Relaxation.”
http://vtnews.vt.edu/articles/2016/10/me-flexiblesolarpanel.html http://pubs.acs.org/doi/abs/10.1021/acsenergylett.6b00457
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