The Rise of Chalcohalide Solar Cells: Comprehensive Insights From Materials to Devices

Abstract

While lead-halide perovskites achieve high efficiencies, their toxicity and instability drive the search for safer materials. Chalcohalides, combining chalcogen and halogen anions in versatile structures, emerge as earth-abundant, nontoxic alternatives for efficient photovoltaic (PV) devices. A wide variety of chalcohalide materials, including pnictogen metals-, post-transition metals-, mixed-metals- and organic-inorganic metals-based chalcohalides, offer diverse structural, compositional, and optoelectronic characteristics. Some of these materials have already been experimentally synthesized and integrated into PV devices, achieving efficiencies of 4-6%, while others remain theoretically predicated. Despite these advancements, significant challenges must be addressed to fully realize the potential of chalcohalides as next-generation PV absorbers. This review provides a comprehensive insight of the fundamental properties of chalcohalide materials, emphasizing their unique structures, highly interesting optoelectronic and dielectric properties, to fuel further research and guide the development of high-efficiency chalcohalide solar cells. Various synthesis techniques are discussed, highlighting important and potentially overlooked strategies for fabricating complex quaternary and pentanary chalcohalide materials. Additionally, the working principles of different device structures and recent advances in fabricating efficient chalcohalide solar cells are covered. We hope that this review inspires further exciting research, innovative approaches, and breakthroughs in the field of chalcohalide materials.

Keywords: metal chalcohalides; perovskite‐inspired semiconductors; solar absorber; thin film photovoltaics.

Figure 1

The evolution of device efficiencies of various chalcohalide solar cells.

Figure 2

a) Structure of BiSI looking down the b‐axis at the ac‐plane. The black circle highlights the Bi–S 1‐D chain.^{[ 35a ]} b) Averaged cell representation (ACR) of the hexagonal P6₃ structure of Bi₁₃S₁₈I₂ viewed down the c‐axis. Inset: normal to the c‐axis to highlight bismuth (disordered/averaged dimer) sites, in hatched red.^{[ 36a ]} c) Schematic representation of the perovskite and antiperovskite crystal structure. Both are isostructural but with anions in place of cations and vice versa.^{[ 24c ]} d) Two views of the Cc monoclinic Pb₃S₂Cl₂ structure ([100] left, [111] right).^{[ 23i ]} e) Atomic structure of CuBiSCl₂.^{[ 23h ]} f) Crystal structures of Pb₂BiS₂I₃ (left) and Sn₂BiSI₅ (middle, right).^{[ 43 ]} g) Calculated crystal structures for Cmcm and Cmc2₁ polymorphs of Sn₂SbS₂I₃.^{[ 16c ]} h) Crystal structure of AgBiSCl₂, highlighting the coordination of both Ag and Bi atoms and the formation of alternate layers.^{[ 47a ]} i) Atomic structures of CH₃NH₃PbI₃ (left) and CH₃NH₃BiSeI₂ (right).^{[ 58 ]} j) Crystal structure of CH₃NH₃SbSI₂.^{[ 59 ]} Reproduced with permission.^{[ 35a ]} Copyright 2017, American Chemical Society. Reproduced with permission.^{[ 36a ]} Copyright 2017, American Chemical Society. Reproduced with permission.^{[ 24c ]} Copyright 2022, Wiley. Reproduced with permission.^{[ 23i ]} Copyright 2020, Nature. Reproduced with permission.^{[ 23h ]} Copyright 2022, Wiley. Reproduced with permission.^{[ 43 ]} Copyright 2016, American Chemical Society. Reproduced with permission.^{[ 16c ]} Copyright 2021, Royal Society of Chemistry. Reproduced with permission.^{[ 47a ]} Copyright 2023, American Chemical Society. Reproduced with permission.^{[ 58 ]} Copyright 2016, Royal Society of Chemistry. Reproduced with permission.^{[ 59 ]} Copyright 2018, American Chemical Society.

Figure 3

a) The dashed line represents a linear extrapolation of the trend toward BiSeI.^{[ 23a ]} b) Indirect bandgap of BiOBr_1−xI_x and BiSBr_1−xI_x.^{[ 15b ]} c) UV–vis–NIR absorption spectra of Bi₁₃S₁₈X₂ powder samples.^{[ 38 ]} Reproduced with permission.^{[ 23a ]} d) Absorption spectra of Ag₃SBr and Ag₃SI films by PDS.^{[ 23j ]} e) Bandgap as a function of x (Br amount).^{[ 23j ]} f) UV/vis absorption spectrum of polycrystalline powder of Pb₂BiS₂I₃, Sn₂BiS₂I₃, Pb₂SbS₂I₃, “Pb₂Sb_{1− x} Bi _x S₂I₃” (x ≈0.4), and Sn₂BiSI₅. The bump at ≈1.5 eV in the red spectrum (Sn₂BiS₂I₃) is due to the detector switch which we see frequently in different samples. g–i) projected electronic density of states of g) Pb₂BiS₂I₃ h) Sn₂BiS₂I₃ i) Sn₂BiSI₅.^{[ 43 ]} Copyright 2020, American Chemical Society. Reproduced with permission.^{[ 15b ]} Copyright 2016, Nature. Reproduced with permission.^{[ 38 ]} Copyright 2021, American Physical Society. d,e) Reproduced with permission.^{[ 23j ]} Copyright 2024, Royal Society of Chemistry. f–i) Reproduced with permission.^{[ 43 ]} Copyright 2016, American Chemical Society.

Figure 4

a) Indirect bandgap Tauc plots of solid‐state UV–vis Kubelka–Munk diffuse reflectance spectra for Sn₂SbS_2−xSe_xI₃.^{[ 44 ]} b) UV–vis spectra and Tauc plots of Ag₃BiI_6‐2xS_x.^{[ 31b ]} c) Density of states of CH₃NH₃PbI₃ and CH₃NH₃BiSI₂ (left) and calculated bandgaps of CH₃NH₃PbI₃ and CH₃NH₃BiSeI₂ (middle, right).^{[ 58 ]} d) Tauc plots of CH₃NH₃BiSI₂.^{[ 50 ]} Reproduced with permission.^{[ 31b ]} Copyright 2018, Wiley. Reproduced with permission.^{[ 44 ]} Copyright 2023, Royal Society of Chemistry. Reproduced with permission.^{[ 58 ]} Copyright 2016, Royal Society of Chemistry. Reproduced with permission.^{[ 50 ]} Copyright 2019, The Chemical Society of Japan.

Figure 5

a) formation energies per atom (upper panel), band gaps (middle panel), and effective masses of electron and hole (lower panel) for the 31 Bi and Sb‐based compounds. b) band gaps, hole and electron effective masses as a function of average electronegativity discrepancy between anions and cations for the 31 Bi and Sb‐based compounds; bottom left band gaps, bottom right effective masses of electron and hole as a function of bond valence sum of the cations. c) calculated dielectric constants (ε) of the considered bismuth/antimony oxyhalides and chalcohalides.^{[ 14a ]} a–c) Reproduced with permission.^{[ 14a ]} Copyright 2018, Nature.

Figure 6

Defect tolerance versus defect intolerance.^{[ 75c ]} Reproduced with permission.^{[ 75c ]} Copyright 2014, America Society Chemistry.

Figure 7

Mesoscopic structured solar cell versus n–i–p planar structured solar cell.

Figure 8

Schematic illustration of some solution processes for chalcohalide solar cell fabrication. a) One‐step and two‐step spin‐coating method, b) Chemical deposition method, and c) Spray‐pyrolysis method.

Figure 9

a) IPCE spectra of a film deposited at 275 °C.^{[ 23b ]} b) Short‐circuit IPCE values measured for films with various Se/(S + Se) levels measured in a two‐electrode cell containing I⁻/I₂ in acetonitrile.^{[ 23a ]} c) Photoelectrochemical I–V curves for films with various Se/(S + Se) levels measured in a two‐electrode cell containing I⁻/I₂ in acetonitrile under AM1.5G illumination.^{[ 23a ]} d) IPCE spectra of the BiSI electrode in an acetonitrile solution containing 0.1 M NaI at various applied potentials and absorption spectrum of BiSI.^{[ 15b ]} e) J–V characteristics of 16 devices under AM 1.5G simulated illumination and key performance metrics of the best device.^{[ 87 ]} f) Time‐resolved photoluminescence of BiSI films employing 633 nm excitation, revealing two time‐constants of 0.09 and 1.03 ns.^{[ 87 ]} g) I–V curves of 8 Bi₁₃S₁₈I₂‐based solar cells.^{[ 23c ]} h) current density–voltage (J–V) curves of BiSI.^{[ 85 ]} i) J–V curves of the five solar cell devices.^{[ 88 ]} Reproduced with permission.^{[ 23b ]} Copyright 2012, American Chemical Society. b,c) Reproduced with permission.^{[ 23a ]} Copyright 2012, American Chemical Society. Reproduced with permission.^{[ 15b ]} Copyright 2016, Nature. e,f) Reproduced with permission.^{[ 87 ]} Copyright 2019, American Chemical Society. Reproduced with permission.^{[ 23c ]} Copyright 2020, Royal Society of Chemistry. Reproduced with permission.^{[ 85 ]} Copyright 2020, American Chemical Society. Reproduced with permission.^{[ 88 ]} Copyright 2021, Wiley.

Figure 10

a) Incident photon to current conversion efficiency at a 0.25 V bias under monochromatic illumination.^{[ 69 ]} b) J–V curves under standard illumination conditions (100 mW cm⁻²) of air mass 1.5 global (AM 1.5G).^{[ 23d ]} c) external quantum efficiency (EQE) spectrum of the SbSI devices with various HTMs.^{[ 23d ]} d) photovoltaics efficiency of SbSI.^{[ 83a ]} e,f) IPCE spectrum and UV–vis absorption spectrum of the ASBSI devices fabricated via CBD of Sb₂S₃ for 2.5, 3.0, and 3.5 h, respectively.^{[ 30 ]} g) The cross‐sectional FESEM image of the device fabricated via CBD of Sb₂S₃ for 3.5 h.^{[ 30 ]} h) Schematic diagram to explain the effect of the ASBSI amount on the infiltration of HTM.^{[ 30 ]} Reproduced with permission.^{[ 69 ]} Copyright 2022, Wiley. b,c) Reproduced with permission.^{[ 23d ]} Copyright 2018, Wiley. Reproduced with permission.^{[ 83a ]} Copyright 2018, AIP publishing. e–h) Reproduced with permission.^{[ 30 ]} Copyright 2019, Wiley.

Figure 11

a) the schematic comparison of solution process and vapor process in formation mechanism, operating principle and cross‐sectional SEM image.^{[ 89 ]} b) two ways to enhance charge transfer.^{[ 89 ]} c) The architecture of photovoltaic device containing SbSI‐PAN nanocomposite (left) and its energy level diagram (right).^{[ 82 ]} d) J–V curves under standard illumination conditions (100 mW cm⁻²) of AM 1.5 G and e) IPCE spectra of SbSeI solar cells fabricated through 8, 10, and 12 spin‐coating cycles and thermal decomposition.^{[ 90 ]} a,b) Reproduced with permission.^{[ 89 ]} Copyright 2022, Wiley. Reproduced with permission.^{[ 82 ]} Copyright 2020, Elsevier. d,e) Reproduced with permission.^{[ 90 ]} Copyright 2021, Wiley.

Figure 12

a) Current–voltage curve under AM1.5G illumination for a solar cell Inset: Photo of a representative sample.^{[ 23i ]} b) Optical absorption spectra of the three samples.^{[ 23i ]} c) Absorption spectra of Ag₃SBr and Ag3SI films by PDS d) Bandgap as a function of x (Br amount).^{[ 23j ]} e) J–V curves under standard test condition (STC).^{[ 31a ]} f) Schematic diagrams of energy levels for the device to describe the role of trap states.^{[ 31a ]} g) (a) open‐circuit voltage, (b) short‐circuits current density, (c) fill factor, and (d) power conversion efficiency of the Au|PTAA|Ag₃BiI_6−2xS_x | m‐TiO₂|c‐TiO₂|FTO solar cells.^{[ 31b ]} a,b) Reproduced with permission.^{[ 23i ]} Copyright 2020, American Chemical Society. c,d) Reproduced with permission.^{[ 23j ]} Copyright 2024, Royal Society of Chemistry. e,f) Reproduced with permission.^{[ 31a ]} Copyright 2018, American Chemical Society. Reproduced with permission.^{[ 31b ]} Copy 2019, Wiley.

Figure 13

a) Current density–voltage curve of the champion Sn₂SbS₂I₃ solar cell.^{[ 31b ]} b) J–V curves in forward‐ and reverse‐scan modes of the champion MASbSI₂.^{[ 49 ]} c) The J–V curves without MASbSI₂.^{[ 49 ]} d) Forward and backward scanning of the reaction 30 min of MA₃Bi₂I_9−2xS_x.^{[ 48a ]} e) J–V curve of Bi₁₃S₁₈X₂ with optimal Bi₁₃S₁₈X₂ thickness.^{[ 38 ]} f) intensity–voltage curves of SbSeBr and SbSeI prototype solar cells.^{[ 40 ]} g) Optoelectronic parameters of MoSbSeI/CdS/i‐ZnO/ITO solar cells under different annealing conditions: PCE and V_oc (left), J_sc and FF (middle), EQE (right) (with applied 0 V and −0.5 V).^{[ 40 ]} h) J–V curve measured on a CuBiSCl₂ based solar cell. The inset shows the schematic structure of the device.^{[ 23h ]} Reproduced with permission.^{[ 31b ]} Copyright 2019, Cell. b,c) Reproduced with permission.^{[ 49 ]} Copyright 2018, American Chemical Society. Reproduced with permission.^{[ 48a ]} Copyright 2019, Wiley. Reproduced with permission.^{[ 38 ]} Copyright 2021, American Physical Society. f,g) Reproduced with permission.^{[ 40 ]} Copyright 2023, Royal Society of Chemistry. Reproduced with permission.^{[ 23h ]} Copyright 2019, Wiley.