Many an office worker, sitting in a building with large glass windows, in the middle of a sunny day, has wished their windows would tint when it’s sunny, but stay clear when its not. Two recent announcements bring that day one step closer, while also expanding opportunities for renewable energy generation. Independent groups led by Peidong Yang and Nathan Neale reported in Nature Materials and Nature Communications the development of window material that can change transparency upon external stimuli and work as a solar panel when opaque.
How smart are windows now?
Smart glass (or smart windows) are not a novel idea. Electrochromic windows that change light transmission properties upon the application of a low voltage have been commercialized. But, they cost nearly 50% more than normal windows and remain a novelty item. The new studies make it possible to recover the solar energy that is otherwise lost when the electrochromic glass is opaque. The researchers accomplished this through an ingenious use of an otherwise drawback of perovskite solar cells. Specifically, they leveraged the cells’ instability at temperatures close to 60oC, and performance reduction due to humidity.
Perovskite solar cells have achieved efficiency values of about 20% and are relatively easy to produce. Their core element features an absorber layer featuring a perovskite ABX3 crystal structure.
Yang et al: Stable, efficient energy conversion
Yang and co-authors report a thermochromic solar cell, where, upon heating, a fully organic CsPbI3-xBrx film turns into an effective absorber of a solar cell with low transparency (Fig. 1).
The inverse phase transition takes place when moisture is added and results in a highly transparent film. The group demonstrated that the optical properties of the two phases were stable over more than 100 switching cycles. With a composition of CsPbIBr2, the researchers achieved a solar cell efficiency above 4%. More than 85% of this efficiency was retained after 40 cycles. The transparent-to-opaque transitions were achieved by heating to 190oC and took around half an hour. The reverse process takes several hours.
Neale et al: Faster transitions, but less stable efficiency
Comparatively, Neale and colleagues achieved much faster transition times on the order of only a few minutes. They reported that sunlight is sufficient to form an MAPbI3 solar cell by dissociation due to photothermal heating (Fig. 3). The device was sealed in an atmosphere of CH3NH2 and argon and complex formed again when the device cools down. Although the device showed an initial efficiency above 10%, only 20% of this was retained after 20 cycles.
Transparency achieved with the inorganic CsPbI3-xBrx films of 82% in the visible spectrum is high when compared to thermochromic, gasochromic and liquid crystal systems, with transparencies of 75%, 77%, and 57% respectively. MAPbI3 has a comparable transparency of 68% as well. The transparency in the opaque state for CsPbI3-xBrx is 35%, which is quite high. But MAPbI3 showed a very low value of 3%. Both films had a reddish tint when opaque. This tint presents a potential barrier to commercial adoption.
No external control is currently possible for the MAPbI3 based system, but the windows turn opaque quasi-automatically once the temperature is above the threshold of 35oC. The CH3NH2 pressure can be potentially used to control the switching, where maintaining a high pressure results in transparency at a high temperature as well. But the question of adequate control during hot and cold days remains. While faster switching times is a crucial advantage of CsPbI3-xBrx, finding materials with lower phase transition temperatures without sacrificing stability will make them more practical.
Next steps: Improving cycling efficiency
For both technologies, cycling efficiency will need to improve considerably, in the range of 104-106 cycles. A wider range of operating temperatures and homogeneity over a large physical area are also needed.
Despite the drawbacks and challenges, the reported work presents a creative solution for the development of photovoltaic windows. Switching between transparent and solar cell modes solves the tradeoff between efficient switching and energy cost. Apart from the application in buildings, the technology may find acceptance in electric cars, where their large windows could be turned into solar cells during parking, recharging the batteries and simultaneously keeping the car’s interior cool.
Interested in exploring more breakthroughs in the renewable energy industry? Feel free to reach out to Kyle Gracey (kgracey@prescouter.com), PreScouter Project Architect and natural resources industry thought leader.
