Since their debut in photovoltaic cells in 2009, organometallic halide perovskites have emerged as one of the most rapidly advancing photovoltaic technologies in history. Perovskite solar cells today exhibit a power-conversion efficiency of 22%, on par with that of commercial silicon-wafer cells. Perovskites can be printed as flexible polycrystalline thin films by solution processing, one of the least expensive methods available (see Physics Today, May 2014, page 13). But the polycrystalline films contain a lot of atomic vacancies, interstitials, and other defects that trap photoinduced charge carriers and reduce the flow of current. To passivate those defects—that is, to tie up the dangling bonds that create trap states—a collaboration led by MIT and Cambridge University researcher Samuel Stranks has developed a treatment that could hardly be simpler: a half-hour exposure to white light in humid air.

When applied to thin films of the solution-processed methylammonium lead iodide, the...

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