Air-stable bismuth sulfobromide (BiSBr) visible-light absorbers: optoelectronic properties and potential for energy harvesting †

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Introduction
2][3] The arrival of leadhalide perovskites (LHPs) onto the scene has not only disrupted the materials focus of the community, but also shied the emphasis of research from bandgap and absorption coefficient to more strongly encompass charge-carrier transport properties. 6,7This is because the key to the success of LHPs as photovoltaic materials is not only their strong absorption of light over the visible and near-infrared wavelength ranges, but also their long diffusion lengths on the micron scale, which allows efficient charge-carrier collection.Remarkably, these long transport lengths are achieved despite having higher defect densities (10 13 to 10 17 cm −3 in polycrystalline thin lms) than conventional semiconductors, such as silicon (10 8 cm −3 typical defect densities). 8,9This has prompted groups to explore whether such defect tolerance could be replicated in other classes of materials, especially those which can overcome the toxicity and stability limitations of LHPs.It is proposed that the defect tolerance of LHPs arises in part as a result of its unusual electronic structure at its band-edges, which comes about from the strong contributions of the valence 6s 2 electrons from the Pb 2+ cation. 10,11Thus, efforts at nding electronic analogs to LHPs have focused on materials containing heavy posttransition metal cations with stable valence s 2 electrons (e.g., Sn 2+ , Sb 3+ or Bi 3+ ), and these are termed 'ns 2 compounds'. 12lthough the majority of ns 2 compounds investigated are perovskites, or have a perovskite-derived structure (e.g., vacancy-ordered triple perovskites), we are in no way limited to perovskite compounds to achieve a qualitatively similar electronic structure at the band-edges as LHPs. 11A handful of nonperovskite materials have therefore been explored, such as BiOI. 13,14Many of the ns 2 compounds explored tend to have wider bandgaps than iodide-based LHPs. 15Whilst this limits their performance in solar cells, their bandgaps are ideal for indoor photovoltaics (IPVs), for which the optimal bandgap is 1.9 eV. 15 IPVs can improve the sustainability of the Internet of Things (IoT), which describes an ecosystem of devices connected together via the cloud, embedding intelligence into infrastructure. 16,179][20][21] Nonetheless, the majority of IoT devices are battery-powered, which presents two major challenges.Firstly, the practicality challenges associated with replacing or recharging primary and secondary batteries is expected to prevent >80% of the potential of the IoT from being realized. 22Secondly, replacing billions of batteries annually presents a substantial sustainability challenge due to the waste created and drain upon the earth's limited availability of elements required for fabricating batteries. 15,23These practicality and sustainability challenges can be addressed by using IPVs to harvest the energy widely available from indoor lighting, which has high energy density, is predictable and places no restrictions on the distance between the energy supply and harvester. 15,24Coupling together the IPV and battery on the IoT node can substantially extend the lifetime of the battery.Given the regular time intervals in which indoor lighting is available, the high energy density of batteries is not required in some applications, such that more sustainable energy storage devices (e.g., supercapacitors) can be used.However, the current industry-standard IPV material (hydrogenated amorphous silicon, a-Si:H) is inefficient (4.4-9.2%power conversion efficiency, PCE). 15Current alternatives with indoor PCEs >30% include dye-sensitized solar cells (DSSCs), organic photovoltaics (OPVs) and LHP PVs.However, these are limited by: (1) the use of toxic solvents in their synthesis (e.g., chloroform or N,Ndimethylformamide); (2) high synthetic complexity, increasing costs (OPV); and (3) the presence of toxic elements (Pb) and poor stability (LHPs). 15,25,26It is therefore critical to develop non-toxic alternative materials that not only can achieve high PCEs, but are also stable and can be processed using simple, cost-effective methods that do not make use of toxic precursors or expensive catalysts.Many inorganic Pb-free ns 2 compounds have simple chemistry along with high air stability, and very recently, some of them have been shown to exhibit high spectroscopic limited maximum efficiencies (SLMEs) under indoor lighting. 15,278][29][30] These materials are currently too early in their development for life cycle analyses, but the comparatively low temperatures used in their processing, along with the capability for many of these materials to be processed with low-toxicity solvents, or solvent-free, suggest that the CO 2 eq footprint and human health toxicity would be similar to or smaller than those of lead-halide perovskites and organic photovoltaics.The highest PCE of up to 10% has been reported in Cu 2 Ag(Bi,Sb)I 6 materials under indoor lighting. 31Whilst further optimization is necessary for these materials, it is essential at this early stage to search more broadly for promising ns 2 compounds that t in the eld of IPV.
3][34][35] However, the experimental indirect bandgap is 1.57eV, 34 falling below the optimal value (1.9 eV) for indoor light harvesting.This bandgap can be widened to 1.9-2.0eV by substituting I for Br. 36However, there have been very few reports of BiSBr.The current literature shows the use of BiSBr to harvest light from a ltered Xe lamp to produce a measurable photocurrent in photoelectrochemical systems. 36Computations also show that the upper valence band is comprised of bonding and antibonding orbitals between Bi 6s and the two anion p orbitals, while the lower conduction band is comprised of the overlap between Bi 6p and anion p orbitals. 37This electronic band structure is similar to that present in LHPs, 6 but the contribution from Bi 6s to the upper valence band in BiSBr is smaller than Pb 6s in LHPs, such that the valence band maximum (VBM) is atter in BiSBr.Nevertheless, the conduction band remains disperse, leading to a small average electron effective mass of 0.52m 0 (m 0 is the rest mass of electrons), which is much smaller than the hole effective mass (3.73m 0 ). 37Additionally, BiSBr also exhibits large spin-orbit coupling, due to the presence of the heavy, polarizable Bi 3+ cation.As a consequence of this, BiSBr is predicted to have a high ionic dielectric constant of ∼30 (compared to an ionic dielectric constant of 20 for MAPbI 3 ), 6,37 which could contribute to Coulombically screening out charged defect states from charge-carriers, thus reducing the non-radiative recombination rate.Furthermore, BiSBr has a quasi-one-dimensional crystal structure, with inorganic chains held together by weak van der Waals interactions (Fig. 1a).Such a structure may enable the surfaces or interfaces to avoid being terminated with dangling bonds, and thereby minimize surface recombination. 38Surprisingly, very little is known about the charge-carrier kinetics of BiSBr thus far, and this material has not been investigated for indoor light harvesting.Furthermore, despite the great efforts on BiSBr single crystals, nanocrystals, and powders, 36,39 BiSBr is still rarely fabricated as thin lms, which limits its ultimate application in photovoltaics.
In this work, we developed a solution processing route to realize phase-pure and stoichiometric BiSBr thin lms, as veried using X-ray diffraction (XRD), Raman spectroscopy and energy dispersive X-ray spectrometry (EDX) measurements.The environmental stability was evaluated by tracking the evolution of its XRD pattern and visual appearance over time when stored in ambient air without encapsulation, as well as over a 24 h period under 1-sun illumination, at 85 °C and 85% relative humidity.The optoelectronic properties were determined through absorption coefficient (using a combination of UVvisible spectrophotometry and photothermal deection spectroscopy) and photoluminescence (PL) measurements.We also calculated the spectroscopic limited maximum efficiency (SLME) of BiSBr under indoor lighting based on its absorption coefficient spectrum and nature of its bandgap.The chargecarrier kinetics were evaluated by measuring the PL lifetime.Finally, in order to evaluate the compatibility of BiSBr with other transport layers, its band positions were determined through photoemission spectroscopy and Kelvin probe measurements.

BiSBr thin lm synthesis
The solution preparation and spin coating processes were both conducted in a N 2 -lled glove box (O 2 < 0.1 ppm, H 2 O < 5 ppm).0.596 g bismuth(III) bromide (BiBr 3 , $98%, Sigma Aldrich), and 0.303 g thiourea (ACS reagent, $99%, Sigma Aldrich) were dissolved into 1 ml dimethyl sulfoxide (DMSO, anhydrous, $99.9%,Sigma Aldrich) to form a 45 wt% solution.All of the chemicals were used as received.The solution was stirred for 3 h at room temperature before being ltered through 0.45 mm PTFE membrane (Fisher Scientic).The substrates (glass or quartz) were cleaned by ultrasonication in soapy water, acetone and ethanol successively for 15 min each.Aer 20 min of UVozone cleaning (UV Ozone Cleaner UVC-1014), the substrates were taken into a N 2 glovebox for thin lm deposition.To deposit the lms, a 1 cm 2 substrate was placed onto a vacuumsealed chuck, and the 30 ml solution was spread onto the substrate before it was spun at 4000 rpm for 30 s.The substrates were then stored overnight in an evacuated antechamber before being annealed at 250 °C for 10 minutes.

Measurement and characterization
X-ray diffraction (XRD) measurements were performed on BiSBr thin lms deposited onto glass substrates.The measurements were taken in air at room temperature using a Bruker D8 Advance Eco instrument with Cu K a radiation.PESA (photoelectron spectroscopy in air) spectra were recorded in air using a KP Technology APS02 system with excitation wavelengths of ca.180-280 nm.Raman spectra were acquired from a commercial Raman microscope setup (Renishaw inVia).Before use, the system was auto calibrated from the soware using a c-Si Evolution of XRD patterns and visual appearance of (f) BiSBr films stored in ambient air under standard laboratory-bench conditions (50-60% relative humidity, as well as (g) BiSBr and (h) triple-cation perovskite (Cs 0.05 (MA 0.17 FA 0.83 ) 0.95 Pb(I 0.83 Br 0.17 ) 3 ) thin films under 1-sun illumination, at 85 °C and 85% relative humidity over 24 h.# means delta-phase peak for perovskite.
calibration standard to its reference Raman shi peak at 520 cm −1 .The samples were excited with a 532 nm wavelength continuous wave (cw) laser source (torus mpc3000, 750 mW) using a 20× objective lens at 5% laser intensity, acquisition time 5 s.A grating of 1800 lines mm −1 was used throughout the measurements.The PL spectrum was also acquired using the same system and conditions.The accelerated environmental stability test was performed using a BBZM-III solar simulator (Bobei Lighting Electrical Appliance Factory), calibrated to 1 sun illumination (AM1.5G) with a KG-5 silicon reference cell certied by Newport.Simultaneously, the thin lms were mounted on a hotplate at 85 °C, and a small cup of water was placed next to the thin lms to increase the relative humidity to 85%, which was measured by a humidity meter (S.Brannan & Sons Ltd).For photothermal deection spectroscopy (PDS) measurements, BiSBr lms were spin-coated on Spectrosil® 2000 quartz substrates and immersed in an inert liquid with a high thermo-optic coefficient, FC-72 Fluorinert (3M Company).The sample surface was illuminated perpendicularly with a monochromatic beam from a 250 W halogen lamp (Newport) integrated with a 250 mm focal length monochromator (CVI DK240) and modulated with a mechanical chopper at 10 Hz.Nonradiative recombination processes at the lm's surface would result in a temperature gradient and, consequently, a refractive index gradient in the surrounding liquid.A 670 nm CW diode laser beam was deected by the liquid's refractive index and detected by a quadrant photodiode, with the signal amplitude demodulated by a lock-in amplier (Stanford Research Systems SR830).XPS spectra was acquired using a Thermo Scientic XPS with a monochromated Al K a Xray (1486.7 eV) source.Photoluminescence (PL) imaging and PL lifetime measurements were performed using a home-built optical microscope.A 520 nm wavelength excitation was provided by inserting a 520 nm bandpass lter (Thorlabs) into a pulsed supercontinuum white light (Fianium Whitelase) source with a repetition rate of 5 MHz.The optical image was obtained under wide-eld excitation and then the laser beam was focused onto the samples using a 100× air objective lens with a numerical aperture of 1.25 for PL lifetime measurement.Then the collected beam was sent to either the EMCCD camera for optical imaging (Photometrics QuantEM 512SC) or Single Photon Avalanche Photodiode for TCSPC lifetime measurement through a 660 nm band pass lter.Secondary electron microscopy (SEM) images of as deposited lms were acquired with the Zeiss-Merlin FEG-SEM/EDS platform using an accelerating voltage of 10 keV and a probe current of 250 pA.Energy dispersive X-ray spectrometry (EDX) mapping was carried out in the same SEM chamber using an accelerating voltage of 13 keV and an Oxford Instruments X-max 150 detector.

Computations
Ab initio electronic structure calculations were carried out under the Kohn-Sham formulation of density functional theory (DFT) 40 using the regularized and restored SCAN (r 2 SCAN) metageneralized gradient approximation (mGGA) functional of Furness et al. 41 as implemented in the Vienna ab-initio Simulation Package (VASP) [42][43][44] using the projector augmented wave (PAW) method. 45Forces were found to converge to an average difference of <0.4 meV Å −1 on a 3 × 4 × 7 G-centered grid using a plane wave cut-off of 400 eV and a self-consistent eld (SCF) loop tolerance (EDIFF) of 10 −7 eV.Convergence runs were set up and analyzed using vaspup 2.0. 46The crystal structure was optimized from an experimental starting point (ICSD Coll.Code 31389) using the conjugate gradient algorithm of Press et al. 47 and a tolerance of 5 × 10 −4 meV Å −1 .The density of states (DOS) converged on a denser 7 × 6 × 15 k-point grid using the tetrahedron method with Blöchl corrections. 48Band structures were calculated using the zero-weighted method and a sampling density of 60 k-points/segment.Band structure plots and KPOINTS les were generated using sumo. 49

Results and discussion
Thin lm synthesis and air-stability of BiSBr Inspired by BiSI and chalcogenides, we proposed two possible solution processing routes for BiSBr thin lms: (1) using a thiolamine alkahest solvent, 50 and (2) the use of thiourea as the sulfur source dissolved in a polar aprotic solvent, mixed with the metal-halide salt. 51For the rst route, we dissolved Bi 2 S 3 and BiBr 3 in 1 : 10 v/v ethane-1,2-dithiol and ethylenediamine.However, it was found that the chalcohalide phase could not be achieved here.Instead, we obtained a mixed phase of the halide and chalcogenide precursors, which was partly due to the low solubility of Bi 2 S 3 .For the second route, we found BiBr 3 and thiourea precursors to be readily dissolved in dimethyl sulfoxide (DMSO), which is a polar aprotic solvent at room temperature.As will be explained in detail below, this process led to phase-pure chalcohalide thin lms.Unlike Pb, the precursors used for BiSBr are fully compliant with the EU Restriction of Hazardous Substances directive (RoHS), and are not known to be bioaccumulative.The DMSO solvent is substantially safer than the N,N-dimethylformamide (DMF) or chloroform solvents commonly used for LHP solar cells, OPVs, or DSSCs (note, however, that standard personal protective equipment should still be used when handling DMSO).The process we developed here also avoids the use of expensive catalysts.Given these factors, and the low synthetic complexity of BiSBr, there is potential for this material to be manufactured cost-effectively and sustainably in the future.
The as-deposited lms were crystallized over a range of annealing temperatures (from 200 to 270 °C) for 10 min and 30 min, and the effect on bulk stoichiometry, crystallinity and phase-purity were measured through EDX (Table S1, ESI †) and XRD (Fig. 1b).We found that lms annealed at low temperatures were Br-rich, and became closer to the ideal stoichiometry when the annealing temperature was increased to 250 °C (Table S1, ESI †).When annealed at this temperature for 10 min inside a N 2 -lled glovebox, the Bi : S : Br ratio reached was 1.00 : 1.01 : 1.12.Annealing for longer than 10 min or at higher temperatures led to the lms becoming non-stoichiometric again, which is due to the formation of phase impurities such as BiBr 3 , as veried from the XRD patterns of 270 °C-annealed (10 min) sample (Fig. S1, ESI †).
The diffraction pattern of the optimized BiSBr lm annealed at 250 °C for 10 min is shown in Fig. 1b, which closely matches with the reference pattern (ICSD database, Coll.Code: 31389).The texture coefficients were calculated (Table S2, ESI †).Although the values of the texture coefficient change with annealing conditions, the preferred orientation consistently remained (110).The preferred orientation may be altered by choosing different substrates for BiSBr deposition and controlling the deposition parameters in the future. 52We used Pawley tting 53 to examine the structure of these thin lm materials in more detail.The overall R wp was 4.4%, showing there to be a close t, with no major peaks unaccounted for by the BiSBr reference pattern (see Fig. S2, ESI †).This result indicates the lms obtained here to be phase pure within the detection limits of the measurement.The broad background spanning from 17 to 30°2q originates from the amorphous glass substrate these lms were deposited onto. 6he structure of BiSBr viewed from the c-axis and b-axis is depicted in Fig. 1a.This shows the quasi-one-dimensional nature of BiSBr (centrosymmetric orthorhombic space group, Pnma), with parallel double chains of [(BiSBr) N ] 2 held together by strong Bi-S and Bi-Br polar covalent bonds.In the constituents of these double chains, a Bi atom is connected in a zigzag conguration to three S and two Br atoms that are separated from the other parallel chain.Each of these double chains are linked together by weak van der Waals interactions.From the quasi-one-dimensional crystal structure of BiSBr, one may assume that the (110) preferred orientation we obtained in these thin lms is sub-optimal for photovoltaic devices with a conventional vertical architecture. 52However, investigations into the related 1D ns 2 semiconductors, Sb 2 S 3 and Sb 2 Se 3 , found that the electronic dimensionality between ribbons was higher than 1D, such that there was still efficient charge-carrier transport. 54Furthermore, a challenge with 1D semiconductors having (hk1) preferred orientation is that this can lead to a discontinuous nanorod morphology, resulting in shunting. 55,56Further work is needed to understand the electronic dimensionality of BiSBr, and approaches to tune its preferred orientation in thin lms.
To understand the effects of the annealing conditions on the structure of the BiSBr lms, we analyzed the line broadening of the diffraction patterns.The full width at half maximum (FWHM) of the main diffraction peaks (Fig. 1c) were obtained from prole tting on the XRD patterns.As the annealing temperature increased from 200 to 220 °C, there was an evident overall reduction in FWHM, which can arise from an increase in grain size or a decrease in strain in the lms.Subsequently, with further increases in the annealing temperature up to 270 °C, the FWHM became more stable across the different annealing temperatures.To understand these observations in more detail, we performed Williamson-Hall analysis on the FWHM to extract the crystallite size and microstrain for each sample (Fig. S3, ESI †); note that instrument broadening was accounted for, as detailed in the ESI.The grain size was obtained from the vertical intercept of the Williamson-Hall plot, while the microstrain was obtained from the slope, and the uncertainties were extracted from tting (Table S3, ESI †).This analysis, shown in Fig. 1d, reveals no signicant differences in crystallite size and microstrain above uncertainty among the various annealing temperatures, with the exception of an increase in grain size as the annealing temperature increased from 200 to 220 °C.Overall, these data show that the lms were well crystallized with low microstrain, with the lms annealed at 250 °C for 10 min giving the highest phase purity.We therefore used these lms for all subsequent characterization.
The phase-purity of this lm was further veried through Raman scattering measurements.The point group of BiSBr is D 2h 16 , which contains nine Raman active modes.These are: six A g modes (287, 250, 121, 92, 75, 41 cm −1 ) and three modes with B 1g , B 2g or B 3g symmetry (for convenience we refer to these are B g -type modes).In ref. 57, only B g -type modes at 234 and 46 cm −1 were detected.Interestingly, as shown in Fig. 1e, ve A g modes (at 288, 243, 122, 93, 73 cm −1 ), along with one B g -type mode (234 cm −1 ) were measured in our BiSBr lms.The low energy peaks at 41 (A g ) and 46 cm −1 (B g -type) could not be measured because of instrument limitations.There were no distinct peaks that were not accounted for, consistent with the lm being phase pure.We also note that the dominant Raman peak is the A g peak at 287 cm −1 , which is associated with a symmetric breathing mode, and this would dominate Fröhlich coupling in BiSBr.Finally, we evaluated the environmental stability of BiSBr.This was achieved through the standard approach of storing thin lms without encapsulation in ambient air.Over the course of 21 days, the temperature of the laboratory was 23 ± 2 °C, whilst the relative humidity varied between 50-60%.The visual appearance and diffraction pattern of the BiSBr lm remained unchanged (Fig. 1f) aer 14 days.As we extended the duration of the stability test to 21 days, we observed the appearance of a small Bi 2 S 3 phase impurity peak (Fig. 1f).For comparison, methylammonium lead iodide perovskite would degrade within only 5 days under similar conditions, 58 showing BiSBr to exhibit higher air stability.
Additionally, the environmental and photo-stability of unencapsulated BiSBr thin lms under 1-sun illumination, and storage at 85 °C and 85% relative humidity, was investigated with comparison to a state-of-the-art triple-cation perovskite thin lm reference (Fig. 1g and h).The BiSBr thin lm was synthesized by annealing at 250 °C for 10 min, and the perovskite thin lm was fabricated according to our previous publication. 59Aerwards, six thin lm samples (three for each material) were mounted on a hotplate at 85 °C, and a large transparent beaker cover was placed on top, with a small cup of water placed next to the thin lms to increase the humidity (detected by humidity meter) of the surrounding environment.Simultaneously, the thin lms were illuminated under 1-sun.We took out two of the samples (one BiSBr and one perovskite) aer 3, 8 and 24 h, respectively, and then took photographs and measured the XRD pattern.The visual appearance and diffraction pattern of the BiSBr lms remained unchanged aer 24 h of exposure to these conditions.In contrast, degradation products (delta phase, as well as more PbI 2 ) appeared as impurities in the lead-halide perovskite lm aer 8 h of testing, along with a color change from black to slightly reddish.At the end of the 24 h test, almost no perovskite peaks could be detected from XRD measurements, and the lm appeared yellow.Thus, BiSBr is signicantly more environmentally and photo-stable than triple-cation perovskite thin lms.

Optoelectronic properties and optical limit in efficiency
We determined the absorption coefficient of BiSBr through UVvisible spectrophotometry (UV-vis) and PDS.From UV-vis, we can determine the absolute absorption coefficient of BiSBr by measuring transmittance and reectance, along with measuring the thickness of the lms (Fig. S4, ESI †).However, this technique is not very accurate in the below-bandgap region because of substrate effects (namely substrate absorption and optical interference), which become dominant as the absorption coefficient decreases by orders of magnitude.We therefore used PDS to determine the sub-bandgap absorbance more accurately (Fig. S5, ESI †); see details of this technique in ref. 60.The Urbach energy estimated from these PDS measurements is 115 ± 4 meV (please see Fig. S6, ESI † for details).However, PDS cannot measure the absolute absorption coefficient, and saturates in the highly-absorptive region.We therefore used UV-vis-determined values of the absorption coefficient from 2.1 to 2.8 eV, and scaled the relative absorbance from PDS to match with the absorption coefficient from UV-vis at 2.1 eV to obtain the overall absorption coefficient prole shown in Fig. 2a.This prole demonstrates a gradual increase in absorption coefficient for photon energies >1.6 eV, followed by a sharp increase to >10 4 cm −1 at >2.1 eV.
From Elliott model tting, 61 the exciton binding energy of BiSBr was found to be 20 meV, which is below the thermal energy at room temperature.This result is also consistent with the absence of excitonic peaks in the absorption spectrum (Fig. 2a).We would therefore expect free charge-carriers to dominate the photogenerated species present in BiSBr.
Promisingly, we observed photoluminescence (PL) from BiSBr lms (Fig. 2a) at room temperature.This PL peak is centered at 1.9 eV, matching the optical bandgap of this material.The absence of a red shi in PL is consistent with the small exciton binding energy in this material.
To derive numerical values of the bandgap, we constructed Tauc plots using the PDS data rather than UV-vis data because PDS measurements provided a more accurate measure of the absorption onset, with reduced sub-bandgap absorption.As shown in Fig. 2b & S7, ESI, † the rst direct transition of the BiSBr thin lm is 2.03 ± 0.01 eV, and the rst indirect transition is 1.91 ± 0.06 eV.This conrms that the bandgap of BiSBr is indirect, which agrees with the computed band structure of this material (Fig. 2c).From this computed E vs. k diagram, BiSBr has a valence band maximum (VBM) between the G and Z points, and conduction band minimum (CBM) between the G and Y points.The computed band structure shown in Fig. 2c was determined using the Kohn-Sham formulation of density functional theory (DFT) and r 2 SCAN functional.Due to the wellknown bandgap problem of DFT, we cannot expect absolute bandgaps to correlate well with experiment at this level of theory, however the indirect nature of the band edge is correctly predicted.We calculated the difference between the rst direct transition and indirect bandgap to be 0.06 eV, which is within error of the experimentally determined value (0.12 ± 0.06 eV).Dispersion-corrected van der Waals functionals were also tested, however the inclusion of long-range interactions did not affect the nature of the band edges, as shown in Fig. S8 and Table S4, ESI.† The closeness in energy between the indirect bandgap and rst direct transition is consistent with the PL peak being coincident with the optical bandgap (Fig. 2a).
To evaluate the potential of BiSBr for indoor photovoltaics, we calculated the spectroscopic limited maximum efficiency (SLME) under a standard 1000 lux white light emitting diode (WLED) spectrum.The SLME model estimates the maximum efficiency that a light-harvesting material can potentially reach if all absorbed photons can be converted into free chargecarriers that can all be extracted. 62The level of non-radiative recombination is estimated based on the difference in energy between the bandgap and the rst direct transition, which we took to be 0.1 eV for this calculation.This method is an improvement over calculating the Shockley-Queisser limit for an indoor light spectrum because the experimentallydetermined optical absorption spectrum of the material is accounted for, and it is not assumed that all recombination processes are radiative.The detailed methodology for calculating SLME is given in our previous work, 27 and from this we determined the SLME (under WLED illumination) of BiSBr to be 43.6% (Fig. 2d), which exceeds that of a-Si:H (41.4%), and is close to that of methylammonium lead iodide (48.2%).Notably, the SLME of BiSBr signicantly exceeds the performance of the most efficient a-Si:H IPV (9.2%).Given that a-Si:H is a wellestablished material, there is signicant potential for BiSBr IPV to outperform the current industry-standard material.To put the SLME of BiSBr further into context, it also exceeds the highest PCE reported thus far for LHP IPV (41.2% under 1062 lux WLED light). 63The optical properties of BiSBr are therefore very well suited to applications in IPV, but it will be critical to determine whether the charge-carrier kinetics will enable BiSBr to approach its optical limits.

Charge-carrier kinetics
Achieving a high charge-collection efficiency in photovoltaic devices requires a sufficiently long minority carrier lifetime.This is because the minority carrier lifetime directly inuences the dri and diffusion lengths.In the screening of early-stage solar absorbers, 1 ns is typically considered to be the threshold lifetime for a material to be worth further development in photovoltaics. 6This was based on the correlation between the PCEs of established inorganic thin lm materials with their minority carrier lifetimes, which found that materials that have exceeded the 10% PCE (under 1-sun illumination) benchmark for new PVs to be promising for commercialization have had lifetimes exceeding 1 ns. 64o gauge the minority-carrier lifetime, we measured the decay in PL aer exciting the BiSBr sample with a pulsed excitation laser (520 nm wavelength).These measurements were obtained using a confocal microscopy-PL setup, which provided sufficiently high excitation uences for us to obtain resolvable PL signals.The advantage of using this setup is that we can also obtain the information from PL intensity mapping (Fig. 3a).This PL map matched the needle-like morphology of the sample (Fig. S9, ESI †).Interestingly, the PL signal was stronger at the edge of each needle rather than in the hollow regions between each microfeature.In addition, we can see from the PL maps and scanning electron microscopy (SEM) micrographs (Fig. S9,  S5 and S6, ESI † for details).The excitation wavelength was 520 nm, with a repetition rate of 5 MHz, incident on the film side.

Paper
Journal of Materials Chemistry A ESI †) that the lms grown by this solution processing route form a discontinuous morphology.This arises from its quasione-dimensional structure, and has been widely found in other 1D materials, such as Sb 2 S 3 and BiSI. 65,66Future efforts focusing on photovoltaics will therefore need to achieve a more compact morphology, and this can draw upon the success of antimony chalcogenide thin lms, potentially using similar fabrication routes. 38,67On the other hand, the discontinuous morphology is compatible with photoelectrochemical or photocatalytic applications, and the higher surface area to volume ratio enabled by this porous structure may be favorable for increasing the overall reaction rate.Herein, we selected the brightest region on the PL map, and measured the PL decay at two excitation uences.As shown in Fig. 3b, increasing the uence led to a faster decay.This is consistent with transitioning the recombination regime from radiative to Auger-dominated, which occurs at high uences.We tted the PL decays measured at 10.9 mW and 56.9 mW excitation laser power using a phenomenological triexponential model, from which we estimated the average time constants to be 1.86 and 1.48 ns, respectively.More information can be found in Table S5, ESI, † and it can be seen in Fig. S10, ESI † that a very close t was obtained to the measured data.We also tted the PL decays using a model that accounts for surface and bulk recombination, as well as the diffusion of charge-carriers from the excitation spot (see ref. 68 and 69).The ts to the data are shown as dashed lines in Fig. 3b, and the parameter ts are provided in Table S6, ESI.† The effective total lifetimes, accounting for bulk and surface recombination, were 4.7 and 2.9 ns for 10.9 mW and 56.9 mW excitation measurements, respectively.The charge-carrier kinetics of BiSBr therefore justies development in photovoltaics.

Band structure of BiSBr
To pave the way for future efforts to develop photovoltaic devices from BiSBr, we measured the band positions.For the valence band position, we used X-ray photoemission spectroscopy (XPS; performed under ultra-high vacuum) and photoelectron spectroscopy in air (PESA; performed in ambient air without any vacuum steps).From XPS, we could obtain the valence band to Fermi level offset (E VB − E F ) by tting a tangent to the primary edge of the valence band spectrum (Fig. 4a), which was 1.31 ± 0.07 eV.We determined the Fermi level position relative to vacuum level from macroscopic Kelvin Probe measurements of the samples in ambient air.The average value of the workfunction within 100 scans was 4.71 ± 0.09 eV (Fig. 4b).The ionization potential of BiSBr from XPS and Kelvin Probe measurements was therefore 6.0 ± 0.1 eV.We also determined the ionization potential directly from PESA measurements in air, and found it to be 5.93 eV (see Fig. 4c), which is within error of the measurements from XPS and Kelvin Probe.Furthermore, if we compare the E VB − E F to the bandgap of BiSBr (1.91 ± 0.06 eV), we can see that this material is weakly n-type.
To determine the electron affinity, we subtracted the bandgap from the ionization potential, obtaining a value of 4.1 ± 0.1 eV.These conduction and valence band positions are very well aligned with common electron and hole transport layers, as shown in Fig. 4d.The wide variety of options available for feasible charge transport layers (CTLs) means that early device fabrication efforts can focus more on CTL materials with suitable morphologies (e.g., mesoporous structured) and surface energies for obtaining compact thin lms of BiSBr.Furthermore, the band positions of BiSBr are well suited to carrying out H + and CO 2 reduction reactions, as well as the oxygen evolution reaction, 14 which opens up another opportunity for BiSBr in terms of photocatalytic and photoelectrochemical applications.We add that the anisotropic structure of BiSBr could lead to variations in the band position along different faces, and this should be studied in greater detail in the future.

Conclusion
To conclude, we successfully synthesized phase-pure BiSBr thin lms through a solution processing route using RoHScompliant precursors, and a low-toxicity solvent.We found that annealing these materials at 250 °C for 10 min inside a N 2 -lled glovebox led to highly stoichiometric lms (B : S : Br = 1.00 : 1.01 : 1.12).The phase-purity of these lms was veried through X-ray diffraction and Raman scattering measurements, and we demonstrated the material to be stable in ambient air without encapsulation for at least 14 days, and were also more environmentally-and photo-stable than triple cation perovskite thin lms.We determined the indirect bandgap to be 1.91 ± 0.06 eV, which matches the ideal bandgap for indoor light harvesting.Using the absorption coefficient spectrum we measured, we predicted that this material could reach up to 43.6% efficiency in IPVs.By estimating the PL lifetimes from both a phenomenological tri-exponential model, as well as a physically-relevant recombination-diffusion model, we found that the charge-carrier lifetime of BiSBr justies its development in photovoltaics.From photoemission spectroscopy and Kelvin probe measurements, we determined the band positions to be 4.1 ± 0.1 eV (conduction band minimum) and 6.0 ± 0.1 eV (valence band maximum), meaning that BiSBr devices can be fabricated using standard test photovoltaic device structures.Thus, from the analyses of the bulk properties, this work demonstrates BiSBr to be a promising material for IPV applications, with potential to surpass the performance of the current incumbent technology (a-Si:H).Whilst the morphology requires optimization before realizing photovoltaics from BiSBr, the high surface area to volume ratio of current lms may already be well suited to photocatalytic and photoelectrochemical applications, especially since the band positions are well suited to carrying out water splitting and CO 2 reduction reactions.

Fig. 1
Fig. 1 (a) Crystal structure of BiSBr viewed along the c-axis (top) and b-axis (bottom).(b) X-ray diffraction (XRD) pattern of BiSBr thin film annealed at 250 °C for 10 min.The Miller indices are labelled, and the measured diffraction pattern is compared to the reference pattern (ICSD Coll.Code: 31389).(c) Full width at half maximum (FWHM) of the main diffraction peaks from BiSBr annealed at different temperatures for 10 min.(d) Evolution of the average grain size and microstrain in BiSBr thin films for different annealing temperatures, as determined using Williamson-Hall analysis.(e) Raman spectra of BiSBr thin film annealed at 250 °C for 10 min.Evolution of XRD patterns and visual appearance of (f) BiSBr films stored in ambient air under standard laboratory-bench conditions (50-60% relative humidity, as well as (g) BiSBr and (h) triple-cation perovskite (Cs 0.05 (MA 0.17 FA 0.83 ) 0.95 Pb(I 0.83 Br 0.17 ) 3 ) thin films under 1-sun illumination, at 85 °C and 85% relative humidity over 24 h.# means delta-phase peak for perovskite.

Fig. 2
Fig. 2 (a) Absorption coefficient and photoluminescence (PL) spectra of BiSBr film on a quartz substrate, as well as Elliott model fitting of the absorption coefficient spectrum.The absorption coefficient spectrum was obtained from UV-vis (transmittance and reflectance) measurements (2.1-2.8 eV) and photothermal deflection spectroscopy (PDS) measurements (1.4-2.1 eV).(b) Tauc plot from PDS measurements to estimate the indirect bandgap.(c) Band structure of BiSBr calculated using r 2 SCAN functional.(d) Spectroscopic Limited Maximum Efficiency (SLME) of various materials in comparison to the radiative limit (RL) and maximum power conversion efficiency experimentally reported.LHP is lead-halide perovskite.

Fig. 3
Fig. 3 (a) Microscopy-PL image of BiSBr thin film sample.The brightfield optical micrograph and PL map of the sample were taken over exactly the same area.The optical micrograph is in black and white, while the PL map is shown in color, with the intensity of PL represented by the colorscale displayed.The PL map is superimposed over the optical micrograph.(b) Confocal microscopy-time-correlated single photon counting (TCSPC) measurements of BiSBr in air at 10.9 mW and 56.9 mW excitation laser power, along with the instrument response function (IRF) and fitted models (refer to TablesS5 and S6, ESI † for details).The excitation wavelength was 520 nm, with a repetition rate of 5 MHz, incident on the film side.

Fig. 4
Fig. 4 (a) Valence band spectrum from X-ray photoemission spectroscopy measurements of BiSBr thin films, showing our fit to the primary edge; (b) Kelvin probe measurements of the workfunction of BiSBr thin films deposited on FTO, as well as measurements of the FTO substrate.For each sample, 100 measurements were taken, and the average workfunction obtained, along with the uncertainty.(c) Photoemission spectroscopy in air (PESA) of BiSBr.(d) Schematic diagram of the band positions of BiSBr determined through photoemission spectroscopy, Kelvin probe and bandgap measurements compared with the band positions of common electron and hole transport layers. 4,5