Phonon dispersion and solid-solutions of hybrid halide perovskites

Aron Walsh^a, Jarvist Frost^a, Federico Brivio^a

International Conference on Hybrid and Organic Photovoltaics
Proceedings of International Conference on Hybrid and Organic Photovoltaics 2015 (HOPV15)

Roma, Italy, 2015 May 11th - 13th

Organizer: Filippo De Angelis

Poster, Federico Brivio, 058
Publication date: 5th February 2015

A clear and complete understanding of the hybrid perovskite materials underpinning a new generation of solar cells is not yet known. Building on our initial work concerning dielectric and electronic properties [1-3], we use first-principles simulation techniques to provide insight into two aspects of the materials chemistry and physics: (i) phonon dispersion of CH₃NH₃PbI₃; (ii) solid-solutions on the CH₃NH_3,Pb and I lattice sites. Using the finite-displacement method within the Phonopy package, and the Density-Functional Perturbation Theory as implemented in VASP, we obtain information on the phonon density of states, structure stability, and simulated Raman and IR spectra, which can be compared directly with experimental measurements. The most interesting aspect is the emergence of a coupling between the motion of the organic component of the material (methylammonium), and the inorganic hosting cage. This coupling can lead to micro-domains of high and low electrostatic potential, which can contribute to the high photovoltaic efficiency (real space separation of electrons and hole carriers) and current-voltage hysteresis (a time and voltage dependence of the micro-structure) of this class of compound, as previously suggested [4]. The formation of alloys on the A, B and X sites of the perovskites structure have been used as a means to tailor the optoelectronic properties, as well as the structural stability of the hybrid perovskite compounds. We provide an in-depth statistical mechanical treatment of halide binary alloys. The large endothermic heat of solution for the binary I/Cl material supports the hypothesis that Cl is not incorporated into the CH₃NH₃PbI₃lattice. While halide substitution primarily alters the valence band energy, substitution of Pb can be used to tune the energy of the conduction band.

Brivio F.; Walker A.B. and Walsh A.; APL Materials, 2013, 1, 042111. Brivio F.; Butler K.T.; Walsh A. and van Schilfgaarde M.; Physical Review B, 2014, 89, 155204. Frost, J.M.; Butler K.T.; Brivio F.; Hendon C.H.; van Schilfgaarde M. and Walsh A.; Nano letters, 2014, 14, 2584-2590. Frost, J.M., Butler, K.T. and Walsh, A.; APL Materials, 2014, 2, 081506.

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