Layered BiOI single crystals capable of detecting low dose rates of X-rays

Abstract

Detecting low dose rates of X-rays is critical for making safer radiology instruments, but is limited by the absorber materials available. Here, we develop bismuth oxyiodide (BiOI) single crystals into effective X-ray detectors. BiOI features complex lattice dynamics, owing to the ionic character of the lattice and weak van der Waals interactions between layers. Through use of ultrafast spectroscopy, first-principles computations and detailed optical and structural characterisation, we show that photoexcited charge-carriers in BiOI couple to intralayer breathing phonon modes, forming large polarons, thus enabling longer drift lengths for the photoexcited carriers than would be expected if self-trapping occurred. This, combined with the low and stable dark currents and high linear X-ray attenuation coefficients, leads to strong detector performance. High sensitivities reaching 1.1 × 103 μC Gyair−1 cm−2 are achieved, and the lowest dose rate directly measured by the detectors was 22 nGyair s−1. The photophysical principles discussed herein offer new design avenues for novel materials with heavy elements and low-dimensional electronic structures for (opto)electronic applications.

Bandlike Transport and Charge-Carrier Dynamics in BiOI Films

“…We note that it has been hypothesized that self-trapping is particularly prominent in materials with reduced electronic dimensionality; ,, however, despite showing reduced electronic dimensionality and evidence for sizable electron–phonon coupling, charge carriers in BiOI clearly remain delocalized large polarons over their lifetimes, displaying bandlike transport. Suppression of such effects may result from the symmetry of the lattice phonons coupled to the excited state, which was recently found to be orthogonal to the quasi-2D charge-carrier density confined in the BiOI layers . As such, these findings thus enhance the prospects of other low-dimensional PIMs emerging with suitable electronic transport for photovoltaic applications.…”

“…Suppression of such effects may result from the symmetry of the lattice phonons coupled to the excited state, which was recently found to be orthogonal to the quasi-2D charge-carrier density confined in the BiOI layers. 44 As such, these findings thus enhance the prospects of other lowdimensional PIMs emerging with suitable electronic transport for photovoltaic applications. Furthermore, our findings suggest that significant defect densities may limit the photovoltaic performance of devices based on the BiOI thin films.…”

“…The first-principles simulation attributed the phonon modes at 1.65 and 2.88 THz to Bi–I vibrations, whereas the mode at 8.88 THz was suggested to originate from in-plane vibration of O atoms; however, no theoretical assignment of the peak we observe at 3.75 THz has been made to date. Complementary Raman investigations have been reported for single crystals of BiOI revealing additional Raman-active phonon modes at 50 cm –1 (1.50 THz) and 86 cm –1 (2.58 THz) …”

“…Crucially, BiOI also features high stability and a favorable electronic structure, i.e., antibonding nature of frontier orbitals, potentially leading to defect tolerance . BiOI crystallizes in a layered tetragonal matlockite structure with stacks of [I-O-Bi-O-I] sheets held together in the [001] direction by weak van der Waals interactions. , Owing to its layered structure, it is a highly anisotropic material with low effective charge-carrier masses along the [I-O-Bi-O-I] planes and high effective masses across the planes. ,,, Thus, crystal orientation (with planar layers growing perpendicular to the charge extraction layers) is crucial for efficient charge-carrier extraction in BiOI-based solar cells. However, despite a successful demonstration by Jagt et al of a highly oriented growth of BiOI with vertically aligned planes connecting electrodes, the device demonstrated a PCE of only 2%, making an investigation into the nature of the charge-carrier mobilities, localization, and recombination in thin BiOI films a highly topical subject of research.…”

“…Despite the highly promising photocatalytic properties of bismuth oxyhalides, − only a handful of studies have focused on photovoltaic applications. − Given its wide bandgap of around ∼1.8–1.9 eV, , bismuth oxyiodide (BiOI) is well suited to tandem photovoltaic applications, for example, as a top-cell absorber layer in combination with a Si-bottom cell . First-principles calculations have predicted BiOI to have an indirect bandgap (featuring the conduction band minimum in the Γ-R line and valence band maximum within the Z-R line of the Brillouin zone) ∼40 meV lower than the direct bandgap . While a direct bandgap is generally considered ideal for photovoltaic applications, the presence of an indirect bandgap lying close to the direct transition has been theoretically linked to an advantage in photovoltaic performance (facilitated by a balance between enhanced charge-carrier extraction and strong band-edge absorption) .…”