Preferential CH₃NH₃⁺ Alignment and Octahedral Tilting Affect Charge Localization in Cubic Phase CH₃NH₃PbI₃

Abstract

Hybrid organic–inorganic perovskites (CH3NH3PbI3) have gained prominence in recent years due to their fascinating electronic properties and potential for commercial application in photovoltaics and optoelectronics. One of their intriguing features is in the structure itself and the role played by the organic cation CH3NH3 + (MA+). In this study, we implement first-principles-based methods to take a static look at this dynamic system, which may shed some light on the preferential orientation of MA+ and its impact. We find there is a lattice energy gain in cubic CH3NH3PbI3, when going from a pristine host supercell with [100] MA+ orientation to a distorted host consisting of preferentially aligned MA+ and tilted PbI6 octahedra. Reoriented MA+ and octahedral tilting are also accompanied by larger number of (N–H3)···I hydrogen bonds. This lattice reconfiguration may support charge localization as evidenced by larger 216-atom supercell calculations. The localization behavior is a consequence of lattice polarization (reoriented MA+ and tilted octahedra) which spreads across multiple unit cells and thus may not be strongly bound small polarons.

Significance of hydrogen bonding and other noncovalent interactions in determining octahedral tilting in the CH3NH3PbI3 hybrid organic-inorganic halide perovskite solar cell semiconductor

“…Both in- and anti-phase tilting of the PbI 6 4− octahedra have been crystallographically observed in o-CD 3 ND 3 PbI 3 and o-CH 3 NH 3 PbI 3 perovskites 31,32 (see below), and confirmed theoretically 24–26,32 for the orthorhombic polymorph of CH 3 NH 3 PbI 3 perovskite. The tilting is prominent along the a and b mutually perpendicular crystallographic axes described by a − b + a − in Glazer notation 33 .…”

“…Structural elements 21 , including the tilting of the PbI 6 4− octahedra of the inorganic core 22–24 , play an important role in dictating the function of the material, and is known to be driven by noncovalent interactions. The extent of tilting of the MY 6 4− octahedra in BMY 3 halide perovskites (B = monovalent organic or inorganic cation, M = divalent metal, and Y = Cl, Br, I and their mixed derivatives) is usually quantified in terms of the deviations of the M—Y—M bond angles from 180° along the three crystallographic directions 21–26 . The origin of such tilting of the octahedra in inorganic and organic-inorganic perovskites has also shown to be a consequence of a (rotational) lattice disorder 22,27,28 , which is associated with order-disorder dynamics 29,30 .…”

“…MAPbI 3 exists in three main temperature phases (orthorhombic, T < 165 K; tetragonal, 165 < T < 327 K; and cubic, T > 327 K) 4,18,19,25,26,31,32,36 . Several studies have endeavored to shed light on geometrical aspects of the three phases of the system that are driven by different modes of hydrogen bonding.…”

Excitations Partition into Two Distinct Populations in Bulk Perovskites

“…The other hypothesis is that polaron formation may localize carriers, giving rise to the second population. Polaron formation in organic–inorganic lead halide perovskites has been proposed by several groups . The dipole moments of nearest neighbor CH 3 NH 3 + may orient toward a single Pb 2+ , reducing the electrostatic potential energy of electrons around the Pb 2+ and favoring the formation of a small polaron.…”

Multi-scale model for the structure of hybrid perovskites: analysis of charge migration in disordered MAPbI3 structures

“…The exact relationship between this dynamical structure and the material's PV properties remains debated. Suggested mechanisms include for example charge-carrier pathways formed by ferroelectric domains of ordered MA + cations [18], the effects of the lattice distortion on charge localization [19] and potential fluctuation effects on the charge-carrier mobility due to dynamical MA + disorder [20]. However, detailed computational studies of these processes require electronically accurate models, which significantly restricts the size of the studied systems.…”