Posted in | Solar Energy

Developing Solar Thin Films for 'Smart Windows'

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New research published in the journal Nature Communications has shown how mixed-halide perovskite thin films are ideal materials for the development of so-called ‘smart windows’ – windows that are capable of adjustable transparency, color change, and solar power generation.

The newly-published study explores structural phase transitions observed in mixed-halide perovskite thin films. Because of these transitions, solar thin films have two possible states, each with its own transparency and photovoltaic capabilities. Most perovskite thin films are made from organic materials, not inorganic halides.

This study, for the first time, demonstrates that inorganic halide perovskites semiconductors are suitable for this type of device because of their ‘soft’ and ‘dynamic’ lattice, which allow reversible phase transition without degrading the electronic properties.

Leitan Dou, Study Author and Assistant Professor of Chemical Engineering -Purdue University

Dou added that the properties observed in the study are fundamentally different from properties seen in similar materials, and discovery could open up new applications for semi-transparent solar cells.

“We can now tune their color,” Dou said.

The solar cells are capable of going through reversible, thermally-driven transitions that are moisture mediated at temperatures between 150 and 200 degrees Celsius. In the study, scientists discovered that the cells switch between a transparent, non-perovskite state, with 81.7 percent transparency and a low power yield, to a darkly-colored perovskite phase with a transparency of 35.4 percent and a higher return of solar power.

The study team said the solar cells they developed to reach their conclusion featured a high degree of thermal stability, as well as completely-reversible color and activity, both of which are essential for successfully incorporating the cells into a range of practical uses.

Smart windows, one of those potential uses, are a particularly auspicious green technology, with potential applications in buildings, vehicles, computer displays and other technologies. Existing semi-transparent cells are capable of providing transparency, shade and power production. However, these photocells cannot change colors. The new research showed smart windows could potentially offer all of the same features as semi-transparent photocells, and also permit changeable transparencies that lead to the capture and management of solar energy.

Dou, who played a significant role in developing the films used in the study, said the inorganic phase transition model his team developed is particularly robust.

The films can be switched over 100 times without obvious degradation, making the inorganic perovskites more suitable for commercial applications.

Leitan Dou, Study Author and Assistant Professor of Chemical Engineering -Purdue University

Despite the promising results of the study, the solar cell system developed by the team has one significant drawback – a range of transition temperatures that is too high for use in vehicles and buildings.

“The research team is now exploring new materials toward lower-phase transition temperatures and faster transition kinetics,” Dou noted.

The new study comes after the U.S. Department of Energy's National Renewable Energy Laboratory (NREL) announced the development of an extremely stable perovskite solar cell. The study team said a solar cell developed at NREL maintained 94 percent of its initial efficiency after 1,000 hours of constant usage. Researchers noted that a solar cell typically only functions when the sun is in the sky, and therefore most solar cells don’t see 1,000 hours of continuous sunlight.

Most perovskite solar cells decrease in efficiency by around 20 percent fairly quickly after they start being used and have a steady decline after that. The NREL researchers admitted that a lot more testing is necessary before the technology can be put into use. The typical solar cells in the field currently have a usable lifespan of around 20 years.

More information can be found in the news release.

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