Temperature Dependence of the Urbach Energy in Lead Iodide Perovskites

Abstract

To gain insight into the properties of photovoltaic and light-emitting materials, detailed information about their optical absorption spectra is essential. Here, we elucidate the temperature dependence of such spectra for methylammonium lead iodide (CH 3 NH 3 PbI 3 ), with specific attention to its sub-band gap absorption edge (often termed Urbach energy). On the basis of these data, we first find clear further evidence for the universality of the correlation between the Urbach energy and open-circuit voltage losses of solar cells. Second, we find that for CH 3 NH 3 PbI 3 the static, temperature-independent, contribution of the Urbach energy is 3.8 ± 0.7 meV, which is smaller than that of crystalline silicon (Si), gallium arsenide (GaAs), indium phosphide (InP), or gallium nitride (GaN), underlining the remarkable optoelectronic properties of perovskites.

Investigating the Tetragonal‐to‐Orthorhombic Phase Transition of Methylammonium Lead Iodide Single Crystals by Detailed Photoluminescence Analysis

“…Regarding the extracted temperature dependent Urbach energies of both phases, we find that the values of E u,0,tetra and θ tetra agree well with the results from Ledinsky et al, [ 40 ] who quantified the temperature dependent Urbach Energies of the tetragonal phase of MAPbI 3 thin films also on the basis of PL measurements. Also, our observation that θ tetra and θ ortho exhibit identical values indicates, that the electronically active phonon modes are the same for orthorhombic and tetragonal phase and that their energy does not change upon the phase transition.…”

“…The reflection probability at the perovskite‐helium interface is estimated to be 0.85 by using the Fresnel equations and averaging over all incident angles. For the absorption spectra of the tetragonal phase of MAPbI 3 , we combine the room temperature absorption coefficient reported by Crothers et al [ 29 ] with the temperature‐dependent absorbance data of the Urbach tail reported by Ledinsky et al [ 30 ] Since during the phase transition tetragonal and orthorhombic phase coexist, the total absorption spectrum of the crystal is a combination of a certain fraction of tetragonal phase absorption and orthorhombic phase absorption. For the latter, the absorption coefficient is approximated by absorption data from thin films and an extrapolated Urbach tail based on transmission measurements of a single crystal (see Section S3 in the Supporting Information for full details of the modelling approach).…”

“…[ 37 ] The parameter X = 〈 U 2 〉 x /〈 U 2 〉 0 , with 〈 U 2 〉 0 being the zero‐point uncertainty of the atomic positions, is related to the structural disorder in the system and would be zero for a perfect crystal. [ 37,39 ] Assuming a perfect crystal ( X = 0), the term θ/(2 σ 0 ) can be referred to as the static disorder E u,0 , and Equation () becomes [ 40 ] …”

Defect Passivation in Perovskite Solar Cells by Cyano‐Based π‐Conjugated Molecules for Improved Performance and Stability

“…Metal–halide perovskites have attracted significant attention as a promising photovoltaic technology due to their high optical absorption coefficient, [ 1 ] steep sub‐bandgap absorption edge, [ 2 ] long charge carrier diffusion length. [ 3 ] To date, the PCE of lab‐scale single‐junction PSCs has already surpassed 25%, underlining the potential of this technology to meet high device performance at a low cost.…”

Transferless Inverted Graphene/Silicon Heterostructures Prepared by Plasma-Enhanced Chemical Vapor Deposition of Amorphous Silicon on CVD Graphene

“…It is usually described by the so-called Urbach energy [40]. This parameter directly determines minimal losses in the open-circuit voltage in the finalized solar cell [41]. As shown in Figure 3, the a-Si:H films deposited at temperatures below 100 • C show large deep-defect densities, as can be seen from the high mid-gap absorption.…”