Interplay of Orientational Order and Electronic Structure in Methylammonium Lead Iodide: Implications for Solar Cell Operation

Abstract

We report on ab initio electronic structure and Car–Parrinello molecular dynamics simulations for several structural models of the prototype MAPbI3 perovskite for solar cells applications. We considered both configurations having a preferred orientation of the MA cations, giving rise to a net dipole alignment, and configurations with an isotropic distribution of the MA cations, respectively representative of polar (ferroelectric) and apolar (antiferroelectric) structures. Our calculations demonstrate the preferred stability of a set of polar structures over apolar ones, with an energy difference within 0.1 eV and a conversion barrier within 0.2 eV per unit cell (four MAPbI3), thus possibly accessible at room temperature. Ferroelectric-like orientations lead to a quasi I4cm structure for the inorganic component, characterized by lack of inversion symmetry, while the antiferroelectric-like orientations are associated to a quasi I4/mcm structure. Ab initio molecular dynamics simulations on the polar structures show no molecular rotations in the investigated time-scale, while several MA rotations are observed in the same time scale for the considered apolar structure, which is thus characterized by a higher disorder. The I4cm and I4/mcm types of structure have markedly different band structures, despite showing a relatively small band gap variation. Simulations carried out on finite surface slabs demonstrate that a net orientation of the MA cations gives rise to a strong bending in the valence and conduction bands, which could definitely assist charge separation and reduce carrier recombination, provided one is able to effectively stabilize polar compared to apolar structures. We believe our results could contribute an important step toward an in-depth comprehension of the basic properties of organohalide perovskites, assisting a further optimization of their photovoltaic response.

Giant Rashba Splitting inCH3NH3PbBr3Organic-Inorganic Perovskite

“…These values are amongst the highest ones reported [26]. Our findings point out the critical role of local inversion-symmetry breaking fields in OIPCs, and of the resulting spin splitting [18][19][20]. We hope our study will stimulate further investigations of spin structure in OIPCs to elucidate the role of spin splitting and possible spin polarization in existing and future applications.…”

“…We find α o = 7 ± 1 eVÅ and α c = 11 ± 4 eVÅ for the orthorhombic and the cubic phase. These range at the higher end of calculated values for OIPCs [19] which give α = 0.1...10 eVÅ. Since ARPES is a surface-sensitive technique, the reduced symmetry at the surface may enhance non-centrosymmetric fields and the measured Rashba splitting.…”

“…Enhancement of carrier lifetimes by one to two orders of magnitude were predicted [13,15] for CH 3 NH 3 PbI 3 and a static Rashba splitting in the conduction band of E 0 = 0.1...0.2 eV. The VB Rashba splitting is mediated by the SOC of the Pb 6p orbital [10] for both CH 3 NH 3 PbI 3 and CH 3 NH 3 PbBr 3 , but is sensitive to details of the crystal structure [19,53]. In most OIPCs, the Rashba splitting is stronger in the conduction band than in the VB [12,14,53].…”

“…Spin splitting could be strong enough to contribute to the long carrier lifetimes in OIPCs [13][14][15], and to allow for OIPCbased spintronic devices [16,17]. However, the spin splitting found in calculations [18][19][20] is extremely sensitive to the orientation of the organic cation and to distortions of the inorganic cage, with calculated Rashba parameters of energetically similar structures [19] ranging from < 0.1 eVÅ to almost 10 eVÅ. As direct measurements of the electronic structure are lacking [21], the actual extend of Rashba splitting in OIPCs remains unknown.…”

Dynamic Stereochemical Activity of the Sn2+ Lone Pair in Perovskite CsSnBr3

“…The short range spatial coherence of Sn 2+ fluctuations suggested by our ab initio calculations implies the creation of polar nanodomains that will tend to separate electrons and holes in real space, thus reducing bimolecular recombination. 5,92,93 Additionally, Sn 2+ displacements along certain directions split the otherwise degenerate conduction band minima in momentum due to the local symmetry-breaking. Such momentum-splitting, arising either from local distortions 94,95 or spin-orbit coupling 96,97 has been proposed to limit carrier recombination.…”