Douglas Fabini1 2 Mitchell Koerner3 2 Geneva Laurita5 Ram Seshadri1 4 2

1, University of California, Santa Barbara, Santa Barbara, California, United States
2, University of California, Santa Barbara, Santa Barbara, California, United States
3, University of California, Santa Barbara, Santa Barbara, California, United States
5, Bates College, Lewiston, Maine, United States
4, University of California, Santa Barbara, Santa Barbara, California, United States

Inorganic and hybrid organic–inorganic main-group halides that adopt the perovskite structure combine excellent performance in photovoltaic applications, ease of preparation, and abundant constituent elements, but the origins of their remarkable properties are a matter of debate [1]. Here, we address two unusual aspects of the crystal structure and dynamics of these materials which suggest the primacy of the group IV–halogen sublattice in dictating performance, and apply these insights to discover new optoelectronic materials via high-throughput computational screening.

First, X-ray scattering studies reveal local off-centering of the group 14 cations within their coordination octahedra across the materials class reflecting a preference for lower symmetry coordination than that implied by crystallographic approaches [2,3]. Ab initio calculations, optical measurements, and analogies to existing theory implicate the ns2 lone pair electrons as the origin of this phenomenon, which we propose leads to enhanced defect screening, reduced thermal conductivity, and unusual temperature-dependence of the electronic bandgap [2]. We further demonstrate control of the strength of this phenomenon by chemical substitution on all sites of the perovskite [3].

Second, solid state nuclear magnetic resonance and dielectric spectroscopies reveal the full temperature-dependent dynamics of molecular reorientation in the high-performance formamidinium lead iodide [4]. Despite markedly different barriers for molecular rotation compared to those in the homologous methylammonium lead iodide, both systems exhibit similar dynamics at room temperature [4]. Together with the vastly different dipole moments for the two molecules, this result sheds light on emerging hypotheses of polaronic transport and transient Rashba–Dresselhaus effects.

Using design criteria based in part on these findings about the unusual electronic structure and lattice polarizability of the halide perovskites, we screen 54,000 compounds in the Materials Project database to identify candidate optoelectronic materials. Subsequent ab initio calculations and experimental preparation and screening are employed to test the validity of these criteria as predictors of high performance.

This work was supported by the U.S. Department of Energy, Office of Science, Basic Energy Sciences under award number DE-SC-0012541.

1. D. H. Fabini, J. G. Labram, A. J. Lehner, J. S. Bechtel, H. A. Evans, A. Van der Ven, F. Wudl, M. L. Chabinyc, R. Seshadri, Inorg. Chem. 56 (2017).
2. D. H. Fabini, G. Laurita, J. S. Bechtel, C. C. Stoumpos, H. A. Evans, A. G. Kontos, Y. S. Raptis, P. Falaras, A. Van der Ven, M. G. Kanatzidis, R. Seshadri, J. Am. Chem. Soc. 138 (2016).
3. G. Laurita, D. H. Fabini, C. C. Stoumpos, M. G. Kanatzidis, R. Seshadri, Chem. Sci. 8 (2017).
4. D. H. Fabini, T. A. Siaw, C. C. Stoumpos, G. Laurita, D. Olds, K. Page, J. G. Hu, M. G. Kanatzidis, S. Han, R. Seshadri, J. Am. Chem. Soc. 2017, DOI:10.1021/jacs.7b09536.