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Sun Research Papers

New smart windows darken in the sun—and generate electricity at the same time

By Robert F. Service

Solar power has undergone a revolution in recent years, thanks to an upstart family of crystalline materials called perovskites. Now, perovskites are transforming windows, keeping them clear on cold days, but turning them dark in the hot summer sun. Two research groups report that they’ve created perovskite-tinted windows that not only transition based on the temperature, but also harvest power like solar cells. The new technology could one day help cool buildings by shading out sunlight and generating power to boot.

“Smart windows” that switch between transparent and opaque have been around for decades. For example, the Boeing 787 Dreamliner airplane employs so-called electrochromic windows, which require an outside power source to darken. But such windows have failed to make a broad impact in the building market, because of their higher cost, inconsistencies in their ability to block outside light, and, for some, their demand for external electrical power.

But perovskites offer a possible route to smart windows and solar windows at the same time. Perovskites are materials made of a mix of elements with a particular crystalline structure, and solar cells made from them are nearly as efficient at converting sunlight to electricity as state-of-the-art silicon solar panels: The best ones convert more than 22% of the energy in sunlight to electricity, compared with 25% for silicon. By changing the perovskites’ elemental components, researchers can also control their transparency. What’s more, the starting materials for perovskites are far cheaper than existing solar cells.

The first advance in combining all these features came in November 2017, when researchers reported they had created a lead-based perovskite solar window that switched from transparent to opaque when the temperature hit 60°C. At cooler temperatures, the perovskite in the center of the solar cell forms a complex with an organic compound called methylamine. When warmed, the methylamine vaporizes and breaks away from the perovskite, causing the latter to darken and absorb sunlight. When the heat dissipates, either when the sun goes down or during the winter, the methylamine vapor moves back into the perovskite, turning it transparent once again. The warmed perovskite transforms up to 11.3% of its energy into electricity.

Still, the windows have their drawbacks. The biggest is that the efficiency of the solar cells drops after only a few times switching back and forth, probably because the methylamine fails to fully return the perovskite to its starting crystalline arrangement. So for nearly a year, the window’s inventors—led by chemist Nathan Neale and mechanical engineer Lance Wheeler of the National Renewable Energy Laboratory in Golden, Colorado—have been working to come up with a different perovskite recipe that could switch without the methylamine chemical reaction.

Peidong Yang, a chemist at the University of California, Berkeley, beat them to the punch. Today, Yang reports in Nature Materials that his team has created a cesium-based perovskite solar window that turns opaque and produces electricity when heated, but without methylamine. That allows the windows to switch back and forth repeatedly without a drop in performance. “It’s an attractive idea that you would have the solar cell capability and the smart window at the same time,” says Michael McGehee, a materials scientist at Stanford University in Palo Alto, California, who studies both perovskite solar cells and smart windows.

The newest solar windows still have their downsides. For starters, they don’t switch from transparent to opaque unless heated to more than 100°C. Plus, their efficiency is only about 7%, well below conventional solar cells. McGehee notes that in addition to their stability and efficiency issues, all the perovskite windows have a reddish tint to them when opaque. “That’s crucial,” says McGehee, as most builders shy away from using colored windows.

However, Yang says he and his colleagues have already come up with a variation that switches between 50°C–60°C; they’re just waiting until they can improve its solar conversion efficiency. The hunt continues for new perovskites that satisfy all the demands for windows of the future.

doi:10.1126/science.aat0827

Robert F. Service

Bob is a news reporter for Science in Portland, Oregon, covering chemistry, materials science, and energy stories.

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How to write a good research paper on Raisin in the Sun

As the subject of a research paper, Lorraine Hansberry's play A Raisin in the Sun has a lot of potential. Themes in the play include aspiration, conflict and identity, all of which can be explored in an interesting way. If you're thinking about writing a paper on it here are a few things to consider.

  • The play takes its name from a line in the poem A Dream Deferred by Langstone Hughes, which is about the barriers to progress suffered by black Americans in the 1940s and 50s. All the members of the Younger family have aspirations and all face obstacles. Walter wants to be wealthy but lacks education; Mama wants to move into a house in a better area but faces racism; Beneatha wants to study medicine but can't afford to as a result of Walter's gullibility. The play also raises questions about whether assimilation helps blacks or holds them back; Walter is influenced by Beneatha's educated and wealthy boyfriend George, but Beneatha increasingly listens to the Nigerian student Joseph Asagai.
  • The play explores conflict on multiple levels. At the beginning the Youngers are arguing over what to do with a $10,000 life insurance payment for Mama's late husband. Each has their own plan; Mama, backed by Walter's wife Ruth, thinks the family will have more opportunities if they move from their small apartment into a house. Walter wants to invest in a liquor store with his friend Willie, an idea which the devout Mama disapproves of. Beneatha wants to use it to pay for medical school. Later conflict is introduced between Beneatha and George, whom she increasingly sees as denying his African origins; rejecting George, she is encouraged by Joseph to look to Africa for her identity, and he asks her to marry him and move there. Racial conflict also arises when Mama buys a house in a white neighbourhood and one of the residents offers the family money to live somewhere else.
  • Racial identity is a theme developed mainly through Beneatha and her male friends, and their influence on Walter. Walter admires George, who he sees as fully assimilated into American society, but George mocks him for his poverty and lack of education. Joseph, on the other hand, tells Beneatha that she herself is assimilating - she straightens her hair, for example - and encourages her to stay true to her own culture. At the end Walter, who initially considered accepting the offer to leave the white neighbourhood, rediscovers his pride in being black and refuses.

A Raisin in the Sun is interesting as it combines at the dynamics between family members, ethnic groups and social classes. Many works of fiction explore one or even two of these, but this play is one of the few that looks at them all together. For that reason it's easy to write a good, interesting paper on it.