Dynamic shortening of disorder potentials in anharmonic halide perovskites

Abstract

Halide perovskites are semiconductors that exhibit sharp optical absorption edges and small Urbach energies allowing for efficient collection of sunlight in thin-film photovoltaic devices. However, halide perovskites also exhibit large nuclear anharmonic effects and disorder, which is unusual for efficient optoelectronic materials and difficult to rationalize in view of the small Urbach energies that indicate a low amount of disorder. To address this important issue, the disorder potential induced for electronic states by the nuclear dynamics in various paradigmatic halide perovskites is studied with molecular dynamics and density functional theory. We find that the disorder potential is dynamically shortened due to the nuclear motions in the perovskite, such that it is short-range correlated, which is shown to lead to favorable distributions of band edge energies. This dynamic mechanism allows for sharp optical absorption edges and small Urbach energies, which are highly desired properties of any solar absorber material.

Hybrid Perovskite‐Organic Flexible Tandem Solar Cell Enabling Highly Efficient Electrocatalysis Overall Water Splitting

“…S1. However, the Urbach energy (E u ) of reference and target films were 78.3 meV and 69.6 meV, respectively, which implied the lower band edge disorder after PMABr treatment [16] (Fig. S8).…”

“…The restricted PCE was ascribed to the subsequent solvent erosion, and high-temperature processing of perovskite that injured the OPV layer. In 2016, Liu et al [9] fabricated perovskite-polymer tandem solar cell with a PCE 16.0%. But one issue of their device configuration is that the top and bottom sub cells possessed overlapped light absorption spectra, and the used photovoltaic materials shared similar bandgaps.…”

Polarons and Charge Localization in Metal‐Halide Semiconductors for Photovoltaic and Light‐Emitting Devices

“…There has been discussion around the suitability of the simple Fröhlich model to bulk 3D MHPs, given that it is based upon a harmonic approximation for lattice potentials, [ 24,33 ] with some arguing that lattice anharmonicity plays a more substantial role in soft 3D bulk MHPs than in most polar inorganic semiconductors. [ 52,126,127 ] A recent study by Mayers et al. [ 53 ] applied a molecular dynamics approach to account for large‐scale, low‐frequency anharmonic lattice motion; this approach replicated the temperature dependence of both the charge‐carrier mobility and bandgap in MAPbI 3 reasonably well.…”

The underappreciated lone pair in halide perovskites underpins their unusual properties

“…The relationship between chemistry, structural disorder, and electronic disorder in these compounds remains enigmatic, but this unusual antibonding VB may contribute to the defect tolerance, together with other factors such as compensatory ionic disorder 45 and lattice dynamical suppression of long-range correlations in the disorder potential. 46…”

“…Additionally, the anharmonicity associated with this bonding naturally has the effect of reducing lattice thermal conductivity, a point which has been covered extensively in the literature on thermoelectrics with lone-pair cations. Aside from the impacts on dielectric and thermomechanical properties, one expects that these unusual lattice dynamics would be linked to the electronic disorder in these structurally soft materials, 1,46 which in turn dictates many excited state properties.…”