Simple treatment improves perovskite films

A team of researchers has uncovered a pathway to efficient perovskite solar cells with improved thermal stability starting from what students call “crappy films,” according to team leader Nitin P. Padture of Brown University (J. Am. Chem. Soc. 2016, DOI: 10.1021/jacs.6b02787). Researchers can easily deposit “crappy films” of methylammonium lead triiodide, or MAPbI₃, but these films possess undesirable morphologies that aren’t optimized for harvesting light. Furthermore, MAPbI₃ films can deteriorate at temperatures reached inside solar cells, Padture says. Last year, he and his collaborators developed a method to improve a MAPbI₃ film’s microstructure by exposing the material to methylamine gas, causing the film to liquefy. With the gas turned off, the film crystallized into a more efficient light-absorbing material (Angew. Chem. Int. Ed. 2015, DOI: 10.1002/anie.201504379). The team has now found that exposing improved films to formamidine gas at 150 °C drives a cation-displacement reaction that transforms MAPbI₃ to formamidinium lead triiodide, FAPbI₃, which has superior thermal stability. This reaction also preserves the morphology of the improved films. Photovoltaic devices made with these FAPbI₃ films achieved efficiencies similar to conventional silicon solar cells, the team reports.