Credit: Image courtesy of Seunghun Lee, University of Virginia
The atomic structure of organic-inorganic perovskite transitions from a tetragonal structure (top left) to a cubic structure (top right) upon heating. Black and purple spheres represent lead and iodine atoms, respectively. Also shown are the positively charged organic molecules (cations) and the rotational degrees of freedom (preferential or random). The cations in the cubic structure have more rotational freedom than in the tetragonal structure. Photoluminescence measurements (bottom left) and lifetimes (bottom right, t, nanoseconds) show the longer lasting the excited charge carriers in the cubic phase correlate well with higher rotational freedom.