Dielectric Response: Answer to Many Questions in the Methylammonium Lead Halide Solar Cell Absorbers

Abstract

Due to the unprecedented rapid increase of their power conversion efficiency, hybrid organic-inorganic perovskites CH 3 NH 3 PbX 3 (X = I, Br, Cl) can potentially revolutionize the world of solar cells. However, despite tremendous research activity, the origin of the exceptionally large diffusion length of their photogenerated charge carriers, that is, their low recombination rate, remains elusive. Using frequency and temperature-dependent dielectric measurements across the entire frequency spectrum, it is shown that the dielectric constant conserves very high values (>27) for frequencies below 1 THz in all three halides. This efficiently prevents photocarrier trapping and their recombination owing to the strong screening of charged entities. By combining ultrasonic and Raman spectroscopy with dielectric analysis, similarly large contributions to the dielectric constant are attributed to the dipolar disorder of the CH 3 NH 3 + cations as well as lattice dynamics in the gigahertz range yielding dielectric constants of ε stat = 62 for the iodide, 58 for the bromide, and about 45 for the chloride below 1 GHz at room temperature. Disorder continuously reduces for decreasing temperature. Dipole dynamics prevail in the intermediate tetragonal phase. The low-temperature orthorhombic state is antipolar. No indications of ferroelectricity are found.

Universal Dynamics of Molecular Reorientation in Hybrid Lead Iodide Perovskites

“…71 This glassy freezing of dipolar motion on cooling is in accordance with our prior report, 14 and in contrast with the abrupt loss of dipolar motion observed in dielectric measurements of MAPbI 3 at the β-γ transition. 14,17,18,20 Calorimetry about the motional freezing temperature reveals two closely spaced peaks in the heat capacity, C p , with a total transition entropy too small to be that of an abrupt order-disorder transition (details in the supporting information, Figure S2), suggesting a gradual loss of motion precedes the phase transition on cooling.…”

“…Since the first preparation of the hybrid lead halide perovskites, 12 the behavior of the molecular cations has been studied by calorimetry, 13,14 infrared spectroscopy, 13,15,16 dielectric spectroscopy, 14,[17][18][19][20] Raman spectroscopy, 21 quasi-elastic and inelastic neutron scattering, [22][23][24][25][26] nuclear magnetic resonance (NMR) spectroscopy, [27][28][29][30][31][32] isotopic substitution, 33 and ab initio methods, [34][35][36][37][38][39] and these findings have been correlated to the structure evolution from crystallographic techniques. 19,[40][41][42][43][44][45][46] Most of these efforts have focused on MAPbX 3…”

Creation and Annihilation of Nonradiative Recombination Centers in Polycrystalline Metal Halide Perovskites by Alternating Electric Field and Light

“…For an AC EF, ε goes down by one to two orders of magnitude with increasing of the frequency to 1 kHz . Literature discusses many reasons for such high value of ε and its frequency dependence . Local inhomogeneity in terms of capacity and conductivity as well as ion migration is considered to be an importnant factor leading to the complex dielectric response of MHPs .…”

“…Literature discusses many reasons for such high value of ε and its frequency dependence . Local inhomogeneity in terms of capacity and conductivity as well as ion migration is considered to be an importnant factor leading to the complex dielectric response of MHPs . Although there is no comprehensive picture yet, it is clear that a constant equilibrated EF inside the material should be much lower than an alternating EF at frequencies 0.1–1 kHz due to the difference in the apparent dielectric constant.…”

Dielectric Relaxation and Beyond Limiting Behavior of Alternating‐Current Conductivity in a Supermolecular Ferroelectric

“…These advantages make them suitable for av ariety of low-cost optoelectronic devices and materials,s uch as lightemitting diodes, [29][30][31] switching materials, [32][33][34][35][36] proton-conducting materials [37,38] and solar cells. [39][40][41] Meanwhile, much progress has been achieved for organic-inorganic hybridc ompounds in ferroelectricity.I nt he wide field of hybrid ferroelectrics, the hybrid halometallates of M II (M = Pb, Mn, Cd) represent at echnologically important class, since they preserve the structural characteristics of perovskites. Especially,t hey are comprised of av ariety of organic cations occupying the cavities enclosed by the MX 6 octahedra or MX 4 tetrahedron giving rise to largef reedom of motion.…”