Sunlight causes large changes to the underlying network of atoms that make up perovskites, a promising material for solar cells. Before being hit with light, six iodine atoms rest around a lead atom. Within 10 trillionths of a second after being hit with light, the iodine atoms whirl around each lead atom. These first atomic steps distort the structure and result in long-lived changes, similar in size to those observed in melting crystals. Further, the atoms' motions alter the way electricity moves and may help explain the efficiency of perovskites in solar cells.
In recent years, perovskites have become superstars in the solar cell industry. They are cheap and easy to produce. Despite their popularity, scientists don't know why perovskites are so efficient. This work shows how atoms in perovskites respond to light and could explain the high efficiency of these next-generation solar cell materials.
Although perovskite solar cell efficiencies have climbed above the 20 percent mark, the fundamental mechanism responsible for these efficiencies is not understood. To gain insights into the mechanisms, researchers created stop-motion movies of the atoms involved just after the light hits the hybrid perovskites, made from lead, iodine, and methylammonium. The iodine atoms are arranged in octohedra, eight-sided structures that look like two pyramids joined at their bases. The lead atoms sit inside the octohedra; the methylammonium molecules sit between octohedra. This architecture is common to many of the perovskites investigated for solar cell applications. At SLAC, researchers hit a perovskite film with two bursts from ultrafast lasers. The technique, called ultrafast electron diffraction, lets them reconstruct the atomic structure. By repeating the experiment with different time delays between the first and second pulse of electrons, the team created a stop-motion movement of the iodine atoms whirling around the lead atoms.That is, a rotationally disordered halide octahedral structure formed in the picoseconds after the light struck. This work shows the important role of light-induced structural deformations within the lead-iodine lattice. These structural changes could alter the way that charges (electrons and their associated holes) move in hybrid perovskites and provide new information about solar cell efficiencies.
SLAC National Accelerator Laboratory and Stanford University
This work was supported by the Department of Energy (DOE), Office of Science, Basic Energy Sciences (BES), Materials Sciences and Engineering Division. The ultrafast electron diffraction work was performed at SLAC mega electron volt ultrafast electron diffraction facility, which is supported in part by the DOE Office of Science BES Scientific User Facilities Division Accelerator & Detector Research and Development program, the Linac Coherent Light Source Facility, and SLAC. X.Z. acknowledges the DOE, Office of Science, BES for supporting the growth of the hybrid perovskite samples used in this study. L.Z.T. and A.M.R. acknowledge support from the Office of Naval Research. D.A.E. and L.K. were supported by the Austrian Science Fund and by a research grant from Dana and Yossie Hollander, in the framework of the Sustainability and Energy Research Initiative. T.H. acknowledges support from the Precourt Institute for Energy. H.I.K. thanks the Alfred P. Sloan Foundation for support. M.D.S. is supported by a National Science Foundation (NSF) Graduate Research Fellowship. Part of this work was performed at the Stanford Nano Shared Facilities, supported by the NSF.
X. Wu, L. Tan, X. Shen, T. Hu, K. Miyata, M. Tuan Trinh, R. Li, R. Coffee, S. Liu, D.A. Egger, I. Makasyuk, Q. Zheng, A. Fry, J.S. Robinson, M.D. Smith, B. Guzelturk, H.I. Karunadasa, X. Wang, X.Y. Zhu, L. Kronik, A.M. Rappe, and A.M. Lindenberg, "Light-induced picosecond rotational disordering of the inorganic sublattice in hybrid perovskites." Science Advances 3, e1602388 (2017). [DOI: 10.1126/sciadv.1602388]
SLAC press release: Atomic Movies May Help Explain Why Perovskite Solar Cells Are More Efficient