Scientific advancements in antimony selenosulfide solar cells

Abstract

Antimony selenosulfide [Sb₂(S,Se)₃] is a scientifically interesting, and technologically intriguing photovoltaic (PV) material for the next generation of solar cells. Recently, power conversion efficiencies (PCEs) of 10.92% and 20.86% have been achieved in single-junction Sb₂(S,Se)₃ cells, under standard (AM1.5G) and indoor illumination (1000 lux), respectively. Prototype Si/Sb₂(S,Se)₃ and Sb₂Se₃/Sb₂(S,Se)₃ tandem solar cells have demonstrated PCEs exceeding 10%. However, various intractable factors, mainly the anisotropic carrier transport, anion-vacancy (V_S/Se) and cation anti-site (Sb_S/Se) defects, and non-optimized interfaces cumulate to notable photocurrent and photovoltage losses in Sb₂(S,Se)₃ solar cells. A comprehensive understanding of these performance-limiting factors can be instrumental in amplifying the PCE of Sb₂(S,Se)₃ solar cells, beyond state-of-the-art. In this context, this review provides a comprehensive discussion on the device engineering strategies and establishes a robust framework for the fabrication of high PCE (>15%) Sb₂(S,Se)₃ solar cells.

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Swapnil Barthwal

Swapnil Barthwal earned his MTech in Solid State Materials from the Department of Physics, Indian Institute of Technology (IIT) Delhi in 2019. He then joined Prof. Sandeep Pathak's research group in the Department of Energy Science and Engineering at IIT Delhi as a Junior Research Fellow (2019–2022), where he conducted research on the development of antimony chalcogenide solar cells. Subsequently, he continued his work in this domain within Dr K. Ramesh's research group at the Department of Physics, Indian Institute of Science (IISc), Bengaluru. His research is primarily focused on the development of novel nanomaterials for energy harvesting and storage.

Siddhant Singh

Siddhant Singh received his BSc in Physical Sciences from the University of Delhi in 2018 and his MSc in Physics from the National Institute of Technology Karnataka, Surathkal, in 2021. He is currently pursuing his doctoral research in the Photovoltaics Group at the Indian Institute of Science, Bengaluru, under the supervision of Dr K. Ramesh (Principal Research Scientist). His research focuses on the fabrication and characterization of optoelectronic devices based on solution-processed perovskite single crystals and thin films.

Kumar Haunsbhavi

Dr Kumar Haunsbhavi is currently a Postdoctoral Research Fellow in the Department of Physics at the University of the Free State, Bloemfontein, SA. Dr Haunsbhavi earned his PhD in Physics from the Department of Physics, Bangalore University, India (2022), where he conducted research on metal oxide (MO) thin films for photodetector and gas sensing applications, in collaboration with the Department of Physics at the Indian Institute of Science (IISc), Bangalore. Prior to this, Dr Haunsbhavi served as an Assistant Professor in the Department of Physics at SJB Institute of Technology and as a faculty member in the Department of Physics at JB Campus, Bangalore University. He has over ten years of experience in teaching and research and has authored more than ten peer-reviewed articles in reputable scientific journals. His current research focuses on the fabrication and development of multifunctional devices based on 2D quantum materials for photodetectors, solar cells, gas sensors, and other optoelectronic technologies.

V. Vadhana Sharon

Dr V. Vadhana Sharon is an Assistant Professor of Physics at Kristu Jayanti (Deemed-to-be) University, Bengaluru, India. She specializes in solar cells, nonlinear optics and nanomaterials. In 2024, she was honored with the institution's Silver Research Award for outstanding research achievements. Her recent publications highlight her significant contributions to advanced optical research. She recently convened the international conferences ICRTMS (2023–2025) on materials science.

David E. Motaung

Prof. David E. Motaung is a Professor at the University of the Free State, in the Department of Physics, South Africa. He was rated as a Top 1% Scientist in the World by Stanford University in 2024. He is presently embroiled in the teaching of undergraduate and supervision of postgraduate students. Prof. Motaung's current research interests include the design, synthesis and characterization of semiconductor metal oxide nanostructured materials and their application in gas sensing devices for air quality and muscle food quality monitoring and fabrication of organic–inorganic photovoltaic solar cells for energy generation. Prof. Motaung has published more than 150 peer-reviewed articles, including six comprehensive review articles, all leading to scientific impact contribution with an h-index of 44, 5500 citations and an i10 index of 110. Prof. Motaung has edited two books for Elsevier, and CRC Press. He has also contributed more than 20 book chapters. He has served as a Guest Editor of special issues in several journals.

Ramesh Karuppannan

Dr Ramesh Karuppannan is working as a Principal Research Scientist at the Indian Institute of Science, Bangalore, India. He completed his BSc (Physics) and MSc (Physics) at Bharathidasan University, Trichy, India. He obtained his Doctorate in 1999 from the Indian Institute of Science, Bengaluru, India. His main research areas are structural relaxation in chalcogenide glasses, IR-transmitting materials, phase-change memory materials, carbon nitrides, gas sensing, pressure-induced phase transitions, thermoelectric materials and photovoltaic materials based on chalcogenides and perovskites.

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1. Introduction

The global shift toward renewable energy sources is imperative to mitigate the impacts of climate change and decarbonize the existing energy infrastructure. Solar photovoltaics (PV) is at the forefront of this transformation, leveraging the vast availability of solar energy to produce electricity. As energy demand continues to escalate, PV technologies are integral to achieving carbon-neutral energy systems. Solar PV contributed 5.5% of the world's electricity in 2023, up from 1.1% in 2015.^1,2 Although conventional PV technologies (efficiency exceeding 20% at modular level), such as crystalline silicon (c-si), and thin-film solar cells (CdTe, CIGS), have dominated the global market, these technologies are incompetent to meet the demands for the next generation of PV devices, due to material and processing limitations.^1,3,4 Nonetheless, the scientific community is gravitating towards sustainable photon-harvesting materials and devices, that can offer a combination of high efficiency, flexibility, and low-cost fabrication.^5–8

In this context, antimony selenosulfide [Sb₂(S,Se)₃] has emerged as a promising photon-harvesting material for next-generation PV technology. This quasi one-dimensional (Q-1D) semiconductor offers the advantages of non-toxic and earth-abundant constituents, exceptional optoelectronic properties and high physicochemical stability. It exhibits a unique combination of strong optical absorption (α > 10⁵ cm⁻¹), tunable bandgap (1.1–1.7 eV, depending on the S/Se atomic ratio), low Urbach and exciton binding energy values, ultra-flexibility, and solution-processibility. These merits endorse it as a compelling alternative to the conventional PV materials.^9–14 Recent advancements have demonstrated significant improvements in power conversion efficiency (PCE) for Sb₂(S,Se)₃-based solar cells, achieving notable results in both single-junction and tandem configurations. Sb₂(S,Se)₃ solar cells are also gaining traction in niche applications such as indoor photovoltaics (IPV), building integrated photovoltaics (BIPV), and photo-integrated rechargeable batteries/supercapacitors, showcasing their versatility across diverse energy-harvesting environments.^15–23

Sb₂(S,Se)₃ films are readily obtained in the desired stoichiometry via various synthesis routes, such as vacuum-based processes (pulsed laser deposition,^24,25 sputtering,²⁶ thermal evaporation,²⁷ close space sublimation,²⁸ vapor transport deposition²⁹) and solution-processing techniques (hydrothermal,^6–16 spin coating,^30,31 chemical bath deposition³²). Despite the diversity in synthesis routes, all the high PCE (>10%) Sb₂(S,Se)₃ solar cells (summarized in Table 1) exclusively consist of hydrothermally synthesised, Sb-rich, n-type Sb₂(S,Se)₃ thin-films, sandwiched between CdS and Spiro-OMeTAD, serving as an electron and hole transport layer (ETL and HTL), respectively. The lowest open-circuit voltage (V_OC)-deficit and fill factor (FF) values achieved in Sb₂(S,Se)₃ solar cells are 0.48 V³³ and 73.14%,³⁴ respectively. These values disclose the availability of significant scope for further device optimization and efficiency enhancement.

Table 1 Device architecture, deposition techniques, novelty, and performance parameters of the state-of-the-art Sb₂(S,Se)₃ solar cells (PCE > 10%)

Device architecture	Deposition technique	Novelty	Absorber bandgap (eV)	Device parameters				V OC-deficit (V)	Ref.
				V OC (V)	J SC (mA cm^−2)	FF (%)	PCE (%)
FTO/SnO2/CdS/Sb2(S,Se)3/Spiro-OMeTAD/Au	Hydrothermal	Texturization of FTO substrate to enhance light scattering and maximize charge-carrier generation	1.30	0.569	27.38	70.04	10.92 (certified as 10.70)	0.48	33
FTO/CdS/Sb2(S,Se)3/Spiro-OMeTAD/Au		Incorporation of sodium selenosulfate for in situ defect passivation of deep-level cation antisite (SbSe) defect	1.44	0.633	24.91	68.60	10.81	0.54	10
FTO/CdS/Sb2(S,Se)3/Spiro-OMeTAD/Au		LiF doping in CdS to improve its morphology and conductivity, Li-diffusion into Sb2(S,Se)3 passivates VSe and SbS defects	1.53	0.673	24.45	65.39	10.76	0.59	35
FTO/CdS/Sb2(S,Se)3/Spiro-OMeTAD/Au		Ethanol assisted additive hydrothermal route, to synthesize Sb2(S,Se)3 thin-films with mitigated defect-density and reformed morphology	1.43	0.630	25.27	67.35	10.75	0.54	11
FTO/Zn(O,S)/CdS/Sb2(S,Se)3/Spiro-OMeTAD/Au		Alkali metal fluoride (NaF, KF, RbF, CsF) assisted solution post treatment of Sb2(S,Se)3 thin-films to enhance the crystallinity, conductivity, and energy-level alignment	1.51	0.673	23.7	66.8	10.70	0.57	12
FTO/CdS/Sb2(S,Se)3/Spiro-OMeTAD/Au		Na2HPO4 additive to promote [hk1]-oriented growth of Sb2(S,Se)3 films, along with mitigation of SbS defects	1.48	0.666	25.19	63.60	10.67	0.55	36
FTO/CdS/Sb2(S,Se)3/Spiro-OMeTAD/Au		Alkaline sodium borohydride induced reaction kinetics regulation to mitigate oxide impurities and passivate deep-level traps	1.48	0.662	25.07	64.0	10.62	0.55	37
FTO/SnO2/CdS/Sb2(S,Se)3/Spiro-OMeTAD/Au		Ultrathin SnO2 bridge layer to improve poor electrical and chemical contact at FTO/CdS interface, reduction in S-vacancy defect in CdS film	1.29	0.540	26.94	72.99	10.58	0.50	34
				0.552	26.31	73.14	10.48	0.49
FTO/SnO2/CdS/Sb2(S,Se)3/Spiro-OMeTAD/Au		Post deposition treatment of Sb2(S,Se)3 films using MgCl2, passivation of anion vacancy defects via (Cl-doping)	1.53	0.671	25.44	65.21	10.55	0.59	38
FTO/SnO2/CdS/Sb2(S,Se)3/Spiro-OMeTAD/Au		Temperature-gradient solution deposition to manipulate S/Se distribution and amend unfavorable band structure	1.46	0.675	22.92	68.13	10.55	0.52	39
FTO/CdS/Sb2(S,Se)3/Spiro-OMeTAD/Au		Synthesis of high-quality Sb2(S,Se)3 films by utilizing ethylenediaminetetraacetic acid (EDTA) as a strong coordination additive, to regulate the nucleation and growth process	1.57	0.664	23.8	66.3	10.5	0.63	9
FTO/CdS/Sb2(S,Se)3/Spiro-OMeTAD/Au		Post surface-treatment using ammonium sulfide [(NH4)2S], mitigation of SbS antisite defects	1.46	0.686	22.04	68.8	10.41	0.50	15
FTO/CdS/Sb2(S,Se)3/Spiro-OMeTAD/Au		Hydrazine hydrate (N2H4) solution assisted post-treatment to modify CdS ETL, removal of residual Cd oxychlorides, improving CdS/Sb2(S,Se)3 interfacial band alignment	1.52	0.678	23.63	66.07	10.30	0.57	16
FTO/CdS/Sb2(S,Se)3/Spiro-OMeTAD/Au		NH4F additive assisted regulation of the band-gap gradient in the Sb2(S,Se)3 layer and modification of the CdS/Sb2(S,Se)3 interface	1.44	0.630	24.76	65.95	10.28	0.55	17
FTO/CdS/Sb2(S,Se)3/Spiro-OMeTAD/Au		Exploring thermally evaporated, up-scalable, stable and inorganic MnS as the HTL, to potentially substitute Spiro-OMeTAD	1.49	0.646	24.29	64.5	10.14	0.58	18
FTO/CdS/Sb2(S,Se)3/Spiro-OMeTAD/Au		Regulation of heterogeneous nucleation via BaBr2 additive, and simultaneous mitigation of bulk and interface defects	1.51	0.668	23.90	63.0	10.12	0.57	40
FTO/CdS/Sb2(S,Se)3/Spiro-OMeTAD/Au		Novel deposition strategy to regulate the morphology, Se/S elemental ratio, and defects in Sb2(S,Se)3 films	1.49	0.630	23.7	67.7	10.10	0.59	19
FTO/CdS/Sb2(S,Se)3/Spiro-OMeTAD/Au		Ethylenediamine (EDA) solvent annealing of the CdS buffer layer to improve the CdS/Sb2(S,Se)3 interfacial bonding	1.54	0.651	23.71	65.23	10.10	0.62	41
FTO/CdS/Sb2(S,Se)3/Spiro-OMeTAD/Au		Surface treatment of the CdS underlayer by KI, modulating band-alignment at CdS/Sb2(S,Se)3 from cliff-like to spike-like	1.53	0.663	23.75	63.91	10.06	0.60	42
FTO/SnO2/CdS/Sb2(S,Se)3/Spiro-OMeTAD/Au		Employing a low-cost precursor (Na2SeSO3) to regulate the S/Se elemental ratio, tuning band gap, and suppressing defect density in Sb2(S,Se)3 films	1.35	0.551	26.01	70.1	10.05	0.54	20
FTO/SnO2/CdS/Sb2(S,Se)3/Spiro-OMeTAD/Au		CsI doping to modulate the energy level, carrier transport, and defect-density in Sb2(S,Se)3 films	1.55	0.671	22.93	65.3	10.05	0.60	21

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Existing review works on Sb₂(S,Se)₃ solar cells primarily emphasize the material and electrical properties of Sb₂(S,Se)₃, summarizing notable developments, strategies for suppressing open-circuit voltage (V_OC) deficits, and defect engineering.^43–50 This review is unique in providing a comprehensive and critical analysis of the latest scientific advancements in Sb₂(S,Se)₃ solar cells, with a particular focus on material engineering, device design, interface optimization and sustainability. It aims to bridge the knowledge gap and comprehensively address the key challenges that currently limit the device performance of Sb₂(S,Se)₃ solar cells, such as carrier transport anisotropy, defect management, and interface losses. Furthermore, it explores the potential strategies for surpassing PCE benchmarks for Sb₂(S,Se)₃ solar cells, making this review an essential reference for researchers aiming to advance the field of Sb₂(S,Se)₃ solar cells.

2. Fundamentals and recent developments in Sb₂(S,Se)₃ solar cells

Sb₂(S,Se)₃ solar cells are comparatively a newcomer in the field of PV, yet have shown unprecedented evolution in the PCE and exhibit significant commercialization potential. Despite considerable progress made in the development of Sb₂(S,Se)₃-based solar cells, there exists ambiguity in the understanding of the origin of high V_OC-deficit (>0.5 eV), and severe non-radiative recombination losses in Sb₂(S,Se)₃ solar cells. Owing to unconventional (low-symmetry) Q-1D crystal structure, the charge-transport dynamics in Sb₂(S,Se)₃ differ from that in other PV materials (such as Si, CdTe, CIGS, perovskites), which usually have a (high-symmetry) 3D crystal structure and (near) isotropic charge transport.^35–38 Highly anisotropic carrier transport, and low carrier mobilities, demand a strong built-in electric field to assist carrier extraction from Sb₂(S,Se)₃ thin-films. For this reason, the conventional architecture of Sb₂(S,Se)₃ solar cells consists of a superstrate (n–i–p) configuration, employing charge transport layers with high carrier densities and a fully depleted absorber layer, thus creating the necessary high built-in electric field for enhanced separation and collection of photogenerated carriers.

Advanced research trends in Sb₂(S,Se)₃ solar cells can be broadly grouped into three primary categories. First, the material optimization, which involves fine-tuning of stoichiometry, regulating bandgap gradients, controlling defect densities, and optimizing crystallographic orientation in Sb₂(S,Se)₃ thin films, which are critical to the performance of Sb₂(S,Se)₃ solar cells. Second, the focus on sustainability, where efforts are being made to develop CdS- and Spiro-OMeTAD-free Sb₂(S,Se)₃ solar cells, with the aim to address environmental and economic concerns, without any compromise with device performance. Third, significant progress is being made in the device engineering aspects, with innovative strategies like bulk/3D heterojunctions, bifacial solar cells, and tandem architectures. The following subsections detail these advancements in a comprehensive manner.

2.1. Material optimization-regulation of stoichiometry, bandgap gradient, defects, and crystallographic orientation in the Sb₂(S,Se)₃ absorber layer

S/Se elemental ratio creates a tradeoff between the short-circuit current density (J_SC) and open-circuit voltage (V_OC) in Sb₂(S,Se)₃ solar cells. An increment in the Se-content in the Sb₂(S,Se)₃ films tends to enhance the J_SC value due to band-gap narrowing, while the V_OC is significantly compromised. On the other hand, an increase in S-content leads to blue-shifting in the bandgap (and V_OC), while J_SC is compromised. The balance between J_SC and V_OC, driven by S/Se ratio, is a critical challenge in optimization of Sb₂(S,Se)₃ solar cell performance. Furthermore, S and Se have spatially non-uniform distribution in hydrothermally deposited Sb₂(S,Se)₃ thin-films. Formation of an unfavorable bandgap gradient (schematically shown in Fig. 1a and 2a) during film growth and the loss of S (and Se) during the annealing process, is a key challenge in Sb₂(S,Se)₃ thin-films and solar cells. Creation of a shallow bandgap gradient within the Sb₂(S,Se)₃ film is a straightforward strategy to form a favorable energy band structure, which in turn can promote efficient transfer of photogenerated charge carriers.

Fig. 1 Schematic of the energy-level alignment in the corresponding solar devices, fabricated via different routes: (a) HTD, (b) HTD + VTD (Se), and (c) HTD + VTD (Sb₂Se₃ + Se). (d) J–V characteristics, (e) EQE spectra and (f) dV/dJ versus (J + J_SC–GV) plots for the corresponding cells. Adapted with permission from ref. 51 Copyright 2023, Royal Society of Chemistry.

Fig. 2 (a) Schematic diagram of energy level alignment in Sb₂(S,Se)₃ devices with or without NaF–SPT. (b) SEM micrographs of the corresponding Sb₂(S,Se)₃ thin-films. (c) Statistical boxplots for PCE in Sb₂(S,Se)₃ solar cells. (d) J–V characteristics, (e) EQE spectra, and (f) SCLC spectra of Sb₂(S,Se)₃ solar cells treated with or without NaF–SPT process. (g) I–V characteristics of Sb₂(S,Se)₃ films. The schematics of the devices are shown in the insets. (h) Light intensity dependence of V_OC for control and NaF–SPT Sb₂(S,Se)₃ devices. Adapted with permission from ref. 12 Copyright 2022, Wiley-VCH.

Wang et al.⁵¹ demonstrated a hybrid growth method to regulate the bandgap gradient in Sb₂(S,Se)₃ thin-films. The novel strategy consisted of two steps: the first step involved hydrothermal deposition (HTD), followed by vapor transport deposition (VTD). The devices were fabricated in the superstrate architecture FTO/CdS/Sb₂(S,Se)₃/PTAA/Au. Leveraging the isomorphic structures of Sb₂S₃ and Sb₂Se₃, a favourable bandgap gradient in Sb₂(S,Se)₃ solar cells was attained by carefully engineering the S/Se elemental ratio. As a result of the benign bandgap gradient (Fig. 1b and c), devices fabricated via the hybrid methods (HTD+VTD) delivered better photovoltaic performance than the HTD devices (Fig. 1d–f). Li et al.⁵² and Liu et al.⁵³ achieved a V-shaped bandgap grading of Sb₂(S,Se)₃ films through dual-source vapor transport deposition and (Sb₂S₃ and Sb₂Se₃) co-sublimation techniques, respectively. The bandgap gradient strategy has been found to be strategic in the simultaneous improvement of short circuit current density and photovoltage, leading to a record PCE (of 9.02%) in CdS-free Sb₂(S,Se)₃ solar cells.⁵³

Zhao et al.¹² demonstrated a novel solution post-treatment technique (SPT) to regulate the S/Se atomic ratio, and to induce a shallow bandgap gradient within the hydrothermally deposited Sb₂(S,Se)₃ films. Alkaline metal fluoride (NaF, KF, RbF, and CsF) were used as additives, and a notable improvement was obtained in the morphology, crystallinity, and conductivity of the Sb₂(S,Se)₃ films. In particular, Sb₂(S,Se)₃ films with the NaF additive exhibited favorable energy level alignment for charge transportation (Fig. 2a), compact morphology with distinct grain boundaries (Fig. 2b), lower trap-density (Fig. 2f), and higher conductivity (Fig. 2g). As a result, the NaF–SPT synthesized champion device (FTO/Zn(O,S)/CdS/Sb₂(S,Se)₃/Spiro-OMeTAD/Au) exhibited a remarkable PCE of 10.70%. The low value of diode ideality factor (1.28) in the ameliorated device indicated the improved diode characteristics, and suppression of non-radiative recombination pathways on adopting the NaF–SPT technique for Sb₂(S,Se)₃ thin-film synthesis.

S/Se elemental ratio is not only decisive in dictating the charge carrier transport in Sb₂(S,Se)₃ films, but also creates a tradeoff in the photocurrent and photovoltage in Sb₂(S,Se)₃ solar cells.⁵⁴ The effect of Se-content on the absorption spectra, energy-level alignment, band-gap, and carrier-lifetimes can be visualized in Fig. 3a–d.⁵⁵ Although higher Se-content narrows the bandgap, it is acknowledged to mitigate the deep-level defects, lengthen the carrier lifetimes, and promote the desired [hk1]-orientation. This vertical alignment of grain boundaries is particularly beneficial for carrier transport and extraction in Sb₂(S,Se)₃ films, as it presents fewer barriers to the transport of charge carriers.

Fig. 3 (a) UV-Visible absorption spectra, (b) energy-level alignment (with respect to CdS and Spiro-OMeTAD), (c) Tauc plots, and (d) transient decay kinetics (scatter) and fitting curves (solid lines) monitored at 600 nm, for the Sb₂(S,Se)₃ films deposited on the glass/FTO/CdS substrates. Adapted with permission from ref. 55 Copyright 2024, Wiley-VCH.

Guo et al.⁵⁶ demonstrated an efficient strategy for regulating crystallographic orientation, optimizing energy band alignment, and mitigating of defects in Sb₂(S,Se)₃ thin film solar cells (Fig. 4a–f). A monoatomic Al₂O₃ layer was used to engineer the CdS/Sb₂(S,Se)₃ interface. It was revealed that in the absence of the Al₂O₃ interface layer, the Sb₂(S,Se)₃ films predominantly display the [hk0]-orientation. In contrast, following the introduction of a monoatomic Al₂O₃ layer, the Sb₂(S,Se)₃ films exhibit a highly preferred orientation, primarily dominated by the (2 1 1) and (2 2 1) crystallographic planes, along with compact, dense and pin-hole free grains. Benefitting from the favorable [hk1]-orientation, Sb₂(S,Se)₃ films deposited over Al₂O₃ exhibited higher tunneling current (210 pA), than the control sample (150 pA). Thicker Al₂O₃ films were found to be detrimental for the device, particularly deteriorating the photocurrent due to their insulating properties. Furthermore, the modified films exhibited intrinsic semiconducting properties, more conducive to suppression of recombination losses and improvement in carrier collection. Consequently, the Al₂O₃ engineered device exhibited a PCE of 9.39% (compared to 7.24% in the control counterpart), along with significant improvement in photocurrent (Fig. 4a and b). The device based on mono-atomic Al₂O₃ exhibited a lower diode ideality factor (1.65), lower dark-reverse current (3.61 × 10⁻⁶ mA cm⁻²), higher built-in voltage (0.94 V) and lower series resistance than the control sample (Fig. 4c–f). DLTS measurements revealed two types of hole traps (Sb_S1 and Sb_S3) in the control films, while the Al₂O₃ engineered sample exhibited only one hole trap (Sb_S3) along with mitigated defect-density. In a parallel study, the Al₂O₃ interface layer was also found to improve CdS/Sb₂(S,Se)₃ interface quality and promote [hk1]-orientation, leading to improve photovoltage and PCE (of 6.25%) in the FTO/CdS/Al₂O₃/Sb₂Se₃/carbon solar cell.⁵⁷

Fig. 4 (a) J–V characteristics, (b) EQE spectra, (c) dark J–V characteristics, (d) C–V plots, (e) N_C–V, and (f) EIS curves of the Sb₂(S,Se)₃ solar cells, with and without a monoatomic Al₂O₃ layer. Adapted with permission from ref. 56 Copyright 2024, Elsevier.

Recently, Ren et al.³⁶ proposed a chelate engineering strategy to deposit high-quality Sb₂(S,Se)₃ films with preferred [hk1]-orientation, and a low density of deep-level defects. Tetrahedral (PO₄)³⁻ introduced via dibasic sodium phosphate (Na₂HPO₄, DSP) additive adsorbed on the polar planes (101) of the underneath CdS layer, and promoted heterogeneous nucleation and vertical [h,k,1]-oriented growth of Sb₂(S,Se)₃ ribbons. (PO₄)³⁻ introduction also passivated the detrimental Sb_S1 antisite defects. The CdS (101) surface is known to be hostile for the [hk1]-oriented growth of Sb₂(S,Se)₃ ribbons, owing to poor bonding at the interface. Considering the adsorption of [(SbO)₃(PO₄)] on the CdS (101) planes, the bonding principle at CdS/Sb₂(S,Se)₃ is likely broken. Fig. 5a illustrates a schematic of the mechanism for growth of Sb₂(S,Se)₃ film. Without DSP addition, the Cd and S atoms on the CdS (101) facet form a connection with the S(e) and Sb atoms of the Sb₂(S,Se)₃ film, partly forming the [hk1]-oriented Sb₂(S,Se)₃ ribbons. With DSP introduction, the [(SbO)₃(PO₄)] is adsorbed on the CdS (101) surface, in which the Cd and S atoms on the CdS (101) surface preferentially bind with the O and Sb atoms of [(SbO)₃(PO₄)]. As the (PO₄)³⁻ forms a tetrahedron, the three SbO⁺ sets on the corners of the tetrahedron are distributed at ∼109.5° to each other. Subsequently, the S(e) and Sb atoms of the Sb₂(S,Se)₃ layer connect with the Sb and O atoms of the [(SbO)₃(PO₄)], generating the [hk1]-oriented Sb₂(S,Se)₃ because of the spatial orientation. Sb₂(S,Se)₃ films formed with DSP addition (W-DSP) exhibit higher crystallinity (lower FWHM), higher texture coefficients (Fig. 5b), and lower Urbach energy (Fig. 5f). As a result of improved crystallinity, [hk1]-oriented growth, and lower density of trap states, the W-DSP device exhibits improved PCE of 10.67% (Fig. 5c), outperforming the control counterpart (8.59%).

Fig. 5 (a) Schematic illustration of the growth mechanism of the Sb₂(S,Se)₃ film with DSP assistance during the hydrothermal deposition process. XRD patterns (b1) and amplified XRD patterns (b2) of Sb₂(S,Se)₃ films with and without DSP introduction. (b3) FWHM values extracted from the XRD patterns for the (120), (211), and (221) diffraction peaks. (b4) Texture coefficients for Sb₂(S,Se)₃ films with and without DSP introduction. (c) J–V profile, (d) EQE, (e) plots of d(EQE)/dE versus photon energy, and (f) Urbach energy calculation. Adapted with permission from ref. 36 Copyright 2025, Wiley-VCH.

It is imperative to develop novel reaction-kinetics regulation strategies to improve the electronic quality of the solution-processed Sb₂(S,Se)₃ films, and to address the persistent challenge of carrier recombination losses in Sb₂(S,Se)₃ solar cells. In this context, Fu et al.³⁷ introduced sodium borohydride (SB) into the precursor solution (schematic Fig. 6a). SB was found to be highly effective in accelerating the decomposition of selenourea (SU) and promoting Sb₂Se₃ phase formation through an alkaline effect. It simultaneously reduced SbO⁺ to Sb³⁺via a chemical reduction pathway. This dual action not only suppressed Sb₂O₃ impurity formation but also passivated deep-level defects such as sulfur-vacancies (V_S), and anti-sites Sb_S1, leading to improved crystallinity and a preferred [hk1]-orientation. Deep-level transient spectroscopy (DLTS) measurements revealed only one electron trap (V_S, density of 1.26 × 10¹³ cm⁻³) in the case of SB-added films; in contrast the control film exhibited hole traps (Sb_S1, density of 5.89 × 10¹³ cm⁻³) in addition to V_S (density of 9.46 × 10¹³ cm⁻³). Consequently, the regulated reaction kinetics significantly reduced recombination losses and enhanced carrier transport through better band alignment and defect control. As a result, the optimized devices achieved a power conversion efficiency of 10.62% (0.0684 cm²), comparable to the best reported for this material system (Fig. 6b and c).

Fig. 6 (a) Schematic representation of the reaction kinetics and the growth-orientation for the Sb₂(S,Se)₃ films without and with sodium borohydride (SB) addition. SU, STS and APT refer to selenourea (Se source), sodium thiosulfate (S source) and antimony potassium tartrate (Sb source), respectively. (b) The J–V characteristics, and (c) EQE profile of the control and optimized (SB additive based) devices. Adapted with permission from ref. 37 Copyright 2025, Wiley-VCH.

2.2. Role of the thin film deposition technique and device architecture in device performance

Various deposition techniques (schematically shown in Fig. 7) have been developed to synthesize device-quality Sb₂(S,Se)₃ films, including vapor-deposition, and solution-based techniques. Among these, solution processing has garnered significant interest due to its potential for low-cost, scalable manufacturing. Techniques like the one-step spin-coating method produce Sb₂(S,Se)₃ films with poor crystallinity and numerous pinholes, accelerating the carrier recombination in devices.^30,31 Therefore, interest has shifted to the hydrothermal and chemical bath deposition (CBD) approach for film deposition with high crystallinity and uniform morphologies. Although solution techniques are cost-effective and extensively used for Sb₂(S,Se)₃ film deposition, recent studies have started exploring the use of vapor-deposition processing to deposit Sb₂(S,Se)₃ films with desired orientation, stoichiometry, energy-level alignment and phase purity, crucial for the device performance.

Fig. 7 Schematic of the commonly adopted deposition strategies for Sb₂(S,Se)₃ thin film deposition. (a) Spin coating, adapted with permission from ref. 58 Copyright 2024, Wiley-VCH. (b) Chemical bath deposition (CBD), adapted with permission from ref. 59 Copyright 2023, Elsevier. (c) Hydrothermal deposition and (d) thermal evaporation, adapted with permission from ref. 56 Copyright 2024, Elsevier. (e) Solid state diffusion, adapted with permission from ref. 60 Copyright 2017, Wiley-VCH. (f) Rapid thermal evaporation (RTE) and (g) vapor transport deposition (VTD) adapted with permission from ref. 61 Copyright 2020, Wiley-VCH. (h) Closed space sublimation (CSS), adapted with permission from ref. 62 Copyright 2023, Wiley-VCH.

Solution processed Sb₂(S,Se)₃ thin films in general exhibit unfavourable [hk0]-orientation, compared to those fabricated using vapor deposition techniques. Moreover, the solution processed Sb₂(S,Se)₃ thin films contain secondary (oxide) phases in trace quantities, which are detrimental to device performance. In contrast, vapor deposition enables better control of phase purity, and stoichiometry. Key vapor deposition techniques developed for Sb₂(S,Se)₃ thin-film growth include Co-evaporation, rapid thermal evaporation, close surface sublimation, vapor transport deposition (VTD), magnetron sputtering, and pulsed laser deposition. It has been revealed that the increase in kinetic energy of vapor particles promotes the [hk1]-orientation in Sb₂(S,Se)₃ films. High deposition rates, and high adhesion coefficient for (hk1) planes leads to high growth rate along the [hk1]-directions. The vapor-pressure (and kinetic energy of vapor particles) can easily be regulated via controlling the (evaporation) source temperature, source-substrate distance, and vapor-density. Co-evaporation of Sb, S, and Se precursors, has demonstrated high-quality films with minimal defects.^26,31,63

As hydrothermal deposition remains the most-used method for Sb₂(S,Se)₃ thin film deposition, researchers have made meticulous effort to regulate the nucleation and growth of Sb₂(S,Se)₆ ribbons. Rapid hydrothermal deposition is kinetically more favourable to the [hk1]-oriented growth of Sb₂(S,Se)₆ ribbons. Tang et al.¹⁹ modified the hydrothermal recipe to regulate the growth kinetics and achieved >10% PCE in Sb₂(S,Se)₃ solar cells for the first time. Increasing the selenourea ratio in the precursor solution promoted the [hk1]-oriented growth of Sb₂(S,Se)₃ films. A recent study by Li et al.⁶⁴ demonstrates that by controlling the size and number of added presynthesized Sb₂S₃ particles in the hydrothermal precursor solution, the thickness, crystallinity and defects in the Sb₂S₃ can be effectively regulated. This strategy can be potentially investigated for hydrothermal deposition of electronic grade Sb₂(S,Se)₃ films.

Lu et al.⁶⁵ deposited Sb₂(S,Se)₃ thin films using VTD, employing two independent sources of Sb₂S₃ and Sb₂S₃. By optimizing the evaporation conditions, Sb₂(S,Se)₃ films with a bandgap of 1.33 eV were achieved, which matches closely with the optimal (detailed balance) bandgap (of 1.3 eV) for harvesting solar insolation (AM1.5G, 100 mW cm⁻²). The HTL free device (ITO/CdS/Sb₂(S,Se)₃/Au) delivered a PCE of 7.03%, along with a V_OC of 524 mV, J_SC of 25.2 mA cm⁻², and FF of 53.2%. Pan et al.⁶⁶ adopted the same HTL-free architecture to fabricate the champion device with a PCE of 7.1%. Sb₂(S,Se)₃ powders (with different S/Se elemental ratios) were synthesized via the ball milling method, and Sb₂(S,Se)₃ films with tuneable bandgaps were obtained by VTD.

2.3. Bulk and interface defect engineering

Passivation of bulk and interface defects has emerged as a critical factor in enhancing Sb₂(S,Se)₃ solar cell performance. The presence of deep-level defects (antisite defects and vacancies) has been theoretically predicted and experimentally confirmed. These defects induce Fermi-level pinning and reduce the minority carrier lifetimes. The high density of intrinsic defects can lead to non-radiative recombination and reduce device efficiency. These defects are the root cause to sluggish charge transport and underperformance of the device. Anion-vacancy defects (V_S/Se) and cation anti-site defects (Sb_S/Se) exhibit low formation energy and deep transition energy levels (Table 2). The competition among these defects leads to serious electron–hole compensation, and low charge-carrier densities. The most effective strategy to mitigate intrinsic point-defect density is post deposition surface treatments, such as sulfurization,⁶⁷ selenization²⁶ and potassium iodide (KI)⁶⁸ treatments. Passivation strategies, such as incorporating halide treatments (e.g., chlorine or bromine doping) and post-annealing processes, have shown success in reducing defect density and enhancing carrier lifetimes. However, instead of post deposition treatments, additive engineering into the precursor is found to be more convenient and instrumental in regulating the S/Se atomic ratio, and mitigation of the deep-level defects.

Table 2 The deposition technique, defect-characterization technique and defect-parameters for the Sb₂(S,Se)₃ thin-films. H and E depict hole and electron traps, respectively

Material	Deposition technique	Defect characterization technique	Trap	Type	E T (eV)	σ (cm^2)	N T (cm^−3)	Ref.
Sb2(S,Se)3	CBD	O-DLTS	H1	SbS1	E V + 0.616	6.03 × 10^−17	5.89 × 10^13	69
			E1	VS	E C − 0.764	1.4 × 10^−15	9.46 × 10^13
Sb2(S,Se)3	Hydrothermal		H1	VS1	E V + 0.416	4.62 × 10^−16	3.94 × 10^13	38
			H2	VS2	E V + 0.695	3.13 × 10^−15	8.08 × 10^13
MgCl2 treated Sb2(S,Se)3			H1	VS1	E V + 0.414	4.42 × 10^−17	3.50 × 10^13
			H2	VS2	E V + 0.694	3.06 × 10^−15	3.22 × 10^13
Li-doped Sb2(S,Se)3		O-DLTS	E1	VSe	E C − 0.452	6.60 × 10^−16	1.25 × 10^14	35
			H1	SbS	E V + 0.716	9.01 × 10^−15	1.97 × 10^14
Sb2(S,Se)3			E1	VSe	E C − 0.400	1.54 × 10^−15	4.19 × 10^14
			H1	SbS	E V + 0.726	8.57 × 10^−15	9.86 × 10^14
			H1	SbSe	E V + 0.779	3.32 × 10^−15	9.56 × 10^13	1
Sb2(S,Se)3			H1	SbS2	E V + 0.502	1.05 × 10^−17	2.24 × 10^14	9
			H3	SbS3	E V + 0.766	1.28 × 10^−15	1.21 × 10^15
		DLTS	E1	SbSe2	E C − 0.57	2.48 × 10^−12	1.45 × 10^15	68
			E2	SbSe1	E C − 0.71	4.39 × 10^−12	4.78 × 10^15
			H1	SbS	E V + 0.761	3.97 × 10^−15	6.45 × 10^15
KI treated Sb2(S,Se)3			E1	SbSe2	E C − 0.584	1.99 × 10^−13	6.49 × 10^14
			H1	SbS	E V + 0.784	1.6 × 10^−14	2.97 × 10^15
Sb2(S,Se)3		O-DLTS	H1	SbS1	E V + 0.500	1.99 × 10^−17	6.28 × 10^12	11
			H2	SbS2	E V + 0.671	5.91 × 10^−17	1.88 × 10^13
			E1	VS2	E C − 0.747	1.60 × 10^−16	2.47 × 10^13	67
			H1	SeSb	E V + 0.290	6.61 × 10^−17	6.55 × 10^12
			H2	SbS1	E V + 0.638	3.18 × 10^−15	4.15 × 10^12
			H3	SbS2	E V + 0.483	1.48 × 10^−16	4.64 × 10^14
			H4	SbSe2	E V + 0.682	3.67 × 10^−14	8.63 × 10^14
			H5	SbS3	E V + 0.790	4.87 × 10^−14	9.75 × 10^14
		DLTS	H1	SbS	E V + 0.560	5.98 × 10^−18	4.97 × 10^15	53
			H2		E V + 0.770	2.68 × 10^−16	6.40 × 10^15
			E1	SSb	E C − 0.563	1.40 × 10^−16	1.97 × 10^15
	Vertical transport deposition (VTD)	Temperature-dependent admittance spectroscopy	H1	VSe	E V + 0.203	2.43 × 10^−19	5.37 × 10^14	62
			H2	SbSe	E V + 0.364	3.85 × 10^−16	1.27 × 10^15

---

Zhao et al.¹⁰ proposed a novel in situ passivation (ISP) technique to passivate the Sb_Se defects and mitigate the non-radiative recombination in the Sb₂(S,Se)₃ solar cells. It was revealed that the additive (sodium selenosulfate, Na₂SeSO₃) induced in situ selinization in the Sb₂(S,Se)₃ films. This strategy produced Sb₂(S,Se)₃ films with improved crystallinity, morphology, orientation, and carrier lifetimes, along with significantly mitigated density of Sb_Se defects (Fig. 8a–h). Application of the ISP strategy improved the conductivity in the Sb₂(S,Se)₃ films from 2.93 × 10⁻⁵ to 6.61 × 10⁻⁵ S cm⁻¹ (Fig. 8f). O-DLTS, SCLC, steady state PL, and TAS studies echo the high quality (and lower defect density) of the passivated films. Moreover, this technique was found to be instrumental in fine-tuning the energy level alignment, and to promote efficient transport of photogenerated carriers. Benefiting from the efficient defect passivation and improved band-alignment, the champion device delivered a PCE of 10.81% (Fig. 8i), which is among the highest recorded PCEs for single-junction Sb₂(S,Se)₃ solar cells.

Fig. 8 The energy levels and defect levels of (a) the control-Sb₂(S,Se)₃ film and (b) the target-Sb₂(S,Se)₃ film. (c) The values of N_T (left) and (σ·N_T)⁻¹ (right) for Sb₂(S,Se)₃ devices. (d) The SCLC spectra (the device structural diagram is shown in the inset), (e) PL spectra and (f) I–V characteristics of Sb₂(S,Se)₃ films. TAS decay kinetics at 642 nm for (g) control-Sb₂(S,Se)₃ and (h) target-Sb₂(S,Se)₃ films. (i) The J–V characteristics of the Sb₂(S,Se)₃ devices. Adapted with permission from ref. 10 Copyright 2024, Wiley-VCH.

Recently, Su et al.³⁸ demonstrated a novel post-deposition, MgCl₂-assisted activation process to mitigate deep-level anion-vacancy defects, facilitating the formation of large grains and improving the band alignment at the Sb₂(S,Se)₃/HTL interface. As illustrated in Fig. 9a and b, two-hole traps H1 and H2 were identified in the pristine and MgCl₂ treated Sb₂(S,Se)₃ films, which were attributed to S-vacancies (V_S1 and V_S2, respectively). The MgCl₂ treatment of Sb₂(S,Se)₃ films were found to dramatically mitigate the H1 and H2 defects (Table 2), extend the (average) carrier lifetime (Fig. 9g and h), and improve photogenerated carrier transport, leading to an impressive PCE of 10.55%.

Fig. 9 (a) DLTS signals from the control and MgCl₂-treated Sb₂(S,Se)₃ devices. (b) Arrhenius plots derived from the DLTS signals. (c) The statistical histogram of calculated σ × N_T for different hole traps in the control and MgCl₂ treated Sb₂(S,Se)₃ devices. Schematic of (d) position of H1 (V_S1) and H2 (V_S2) defects in the Sb₂(S,Se)₃ lattice, (e) and (f) band edge positions and defect levels of the control and MgCl₂-treated Sb₂(S,Se)₃, respectively, including the Fermi level (E_F), and defect energy levels (H1, H2), relative to the vacuum level. (g) and (h) Transient kinetic decay (scatter) and corresponding biexponential curve fittings (solid line) monitored at 687 nm of the control and MgCl₂-treated Sb₂(S,Se)₃ films. Adapted with permission from ref. 38 Copyright 2025, Wiley-VCH.

Li et al.⁶⁸ investigated a novel KI-assisted post-deposition surface treatment for Sb₂(S,Se)₃ films. The strategy manipulated the crystallization process to form compact films with larger grain size, forming better band alignment and inhibiting the formation of Sb_Se defects in Sb₂(S,Se)₃ films. As a result, the heterojunction quality is significantly improved in the Sb₂(S,Se)₃ solar cells, leading to a PCE of 9.22%. Zhu et al.⁷⁰ investigated the effect of triethanolamine (TEA) additive in the hydrothermal deposition of Sb₂(S,Se)₃ films. TEA was found to be instrumental in regulating the S/Se elemental ratio, facilitating [021] and [061] crystallite orientation, and mitigating the detrimental V_Se defects, resulting in a PCE of 9.94%.

Recent advancements have highlighted the importance of interface engineering in improving the efficiency and stability of Sb₂(S,Se)₃ solar cells. One of the key challenges in Sb₂(S,Se)₃ devices is the high density of surface defects at the Sb₂(S,Se)₃/HTL interface, which can lead to charge recombination. To mitigate this, self-assembled monolayers (SAMs) and passivating interlayers have been introduced at the interfaces. Recently, Wu et al.⁷¹ for the first time developed two self-assembled monolayers (SAMs) of TPA-2Th and TPA-2Py, to mitigate surface defects and ameliorate band alignment at the Sb₂(S,Se)₃/HTL interface. The ameliorated device delivered a PCE of 8.21%. Other engineering strategies to improve the Sb₂(S,Se)₃/HTL interface include postdeposition treatment with thioacetamide,⁷² SbCl₃, CuCl₂,⁷³ organic chloride,⁷⁴ and metal fluorides.¹² In particular, organic chloride salt [diethylamine hydrochloride, DEA(Cl)]⁷⁴ modified Sb₂(S,Se)₃ films were found to have better morphology, phase-purity, mitigated surface defects and improved band-alignment, leading to a PCE of 9.17% in the Sb₂(S,Se)₃ solar cell. Alkali metal fluoride-assisted post-deposition treatment is also found to be instrumental in regulating the S/Se elemental ratio, improving the morphology, crystallinity, and conductivity of the Sb₂(S,Se)₃ thin film, leading to a PCE of 10.7%.¹² CdS layer post-treatment via CdCl₂,¹² KCl,⁷⁵ and LiF³⁵ was found to be instrumental in defect-passivation, and improving band alignment at the CdS/Sb₂(S,Se)₃ interface.

The CdS layer employed in the Sb₂(S,Se)₃ solar cell is usually deposited by CBD and air-annealed (at ∼400 °C), and exhibits high roughness (due to the high roughness of the underlying FTO), the presence of impurity phases (e.g., CdO, Cd(OH)₂) and moderate conductivity. Additionally, these CdS films have a high concentration of S-vacancies, poor crystallinity and low (free) electron-density, which is not conducive to efficient transport of photogenerated electrons from the Sb₂(S,Se)₃ layer to the FTO electrode. To counter these limitations, post-treatment of CdS with chloride salts, such as CdCl₂,⁷⁶ SbCl₃,⁷⁷ and ZnCl₂,⁷⁸ has been widely used. CdCl₂ treatment of CdS, however, leaves traces of Cd-oxychlorides, which can be eliminated by immersing it into hydrazine hydrate (N₂H₄) solution. This strategy has enabled regulation of the CdS/Sb₂(S,Se)₃ interface band alignment and recombination losses, leading to a PCE of 10.30%.¹⁶ Liu et al.⁷⁵ demonstrated KCl-assisted post-treatment of the CdS layer in Sb₂(S,Se)₃ solar cells. The surface engineering strategy improved the morphological and electrical properties of CdS films, leading to a PCE of 9.98%, along with a significant improvement in J_SC and FF in Sb₂(S,Se)₃ solar cells. KOH⁷⁹ incorporation was found to be effective in improving the transmittance and crystallinity of CdS films. Ishaq et al.⁴² employed a thin, spin-coated layer of KI at the CdS/Sb₂(S,Se)₃ interface and observed significant mitigation of interface and bulk defects. On thermal annealing, K majorly diffused into CdS, while I diffused preferentially into the Sb₂(S,Se)₃ layer. KI-treatment modulated the band-alignment at the CdS/Sb₂(S,Se)₃ interface from cliff-like to a more conducive spike-like; and mitigated the detrimental V_S/Se and Sb_S/Se defects, leading to a PCE of 10.06%.

Recently, Liu et al.³⁵ investigated LiF doping in CdS thin films, and observed (thermally induced) in situ diffusion of high mobility Li⁺ into the Sb₂(S,Se)₃ absorber. Small ionic radii of Li⁺ assisted its migration through the grain boundaries, entering the crystal structure, and settling at the gap between [Sb₄(S,Se)₆]_n ribbons, thus forming a Li⁺ gradient in the Sb₂(S,Se)₃ layer. Li⁺ effectively passivated the V_Se and Sb_S defects, via the formation of strongly coordinated Li–S and Li–Se bonds. This strategy not only improved the opto-electronic properties of the CdS and Sb₂(S,Se)₃ layers, but also created a favorable energy level alignment for the transportation of photogenerated charge carriers towards the respective electrodes. This strategy also modulated the band-offset at the CdS/Sb₂(S,Se)₃ interface from the normal cliff-like band-offset to a moderately spike-like structure, and suppressed the interfacial defect density (∼3.48 × 10¹² cm⁻²), inhibiting the interface and SCR recombination. The mitigation of defects and improvement in heterojunction quality culminated in improved PV performance (PCE of 10.76%) on adopting the lithiation strategy.

Al-doping was introduced to the CdS buffer layer, which facilitated tuning of the band alignment at the CdS/Sb₂Se₃ interface from the unfavorable cliff-like to (benign) spike-like.⁸⁰ Ethylenediamine (EDA) solvent treatment was developed to chemically engineer the underlying CdS film, and found to be instrumental in improving the bonding at the Sb₂(S,Se)₃/CdS interface.⁴¹ Ag⁸¹ and Cu⁸² doping in CdS layer strengthened its n-type semiconductivity, and mitigated the interface defect density at Sb₂Se₃/CdS. Recently, Peng et al.³⁴ employed an ultrathin layer at the FTO/CdS interface to mitigate S-vacancies in the CdS films, and a drastic improvement was observed in the crystallinity and electrical properties of the CdS thin films. As a result of interface engineering, the Sb₂(S,Se)₃/CdS interface defect density was reduced by more than 50%, and a PCE of 10.58% was achieved.

In contrast to post-treatment strategies, Zhang et al.²¹ demonstrated a CsI doping strategy to efficiently dope Sb₂(S,Se)₃ films, and simultaneously improve their morphology and mitigate the deep-level defects. Cs and I were found to co-ordinate with the Se/S and Sb-ions, respectively. The CsI-doped Sb₂(S,Se)₃ thin film solar cells exhibited a conducive band-alignment, suppressed carrier recombination, and improved photovoltaic performance (PCE of 10.05%). Recently, Ren et al.⁴⁰ demonstrated simultaneous passivation of bulk and interface defects, on employing BaBr₂ additive during hydrothermal deposition of Sb₂(S,Se)₃ films. Br⁻ was found to adsorb on the polar planes of CdS, selectively exposing the non-polar planes of CdS, which are conducive in facilitating [hk1]-oriented growth of Sb₂(S,Se)₃ grains. Ba²⁺ on the other hand, was found to co-ordinate with (S/Se)²⁻, and passivate the grain-boundaries. As a result of regulated nucleation kinetics, improved morphology and crystallinity of Sb₂(S,Se)₃ films, a PCE of 10.12% was achieved in the Sb₂(S,Se)₃ solar cell. Large cation (Bi³⁺,⁸³ Cs⁺,²¹ and Ce³⁺ (ref. 84)) doping is found to tune the high tensile strain in Sb₂(S,Se)₃ films to lower compressive strain, in addition to improve the [hk1]-orientation and mitigation of bulk, and grain boundary defects in Sb₂(S,Se)₃ films. (NH₄)₂S-induced hydrothermal sulfurization initially developed for reducing the trap density in Sb₂S₃ thin films,⁸⁵ was found to be equally effective for trap-passivation in Sb₂(S,Se)₃ films.¹⁵

A suitable choice of dopant can introduce p- or n-type semiconductivity in Sb₂S₃ and Sb₂Se₃ thin-films, enabling the fabrication of homojunction solar cells. Cu,⁸⁶ Sn,⁸⁷ Pb,⁸⁸ and Pt⁸⁹ serve as p-type dopants, while Bi,^90,91 I,^92,93 Cl,⁹⁴ Br,⁴⁰ and Te⁹⁵ serve as n-type dopants in Sb₂S₃ and Sb₂Se₃ thin-films. Recently, Chen's group demonstrated a homojunction Sb₂Se₃ solar cell with PCE of 10.15%. The p- and n-type semiconductivity was regulated on either side of the junction by regulating the Sb:Se stoichiometry.⁹⁶ Thus, the versatility in doping enables different conductivity types, making homojunctions based on Sb₂(S,Se)₃ promising pathways to highly efficient Sb₂(S,Se)₃ solar cells.

2.4. Light management via nanotexturization

In traditional planar heterojunction solar cells, there exists a tradeoff between the photon absorption, and charge carrier diffusion to the respective electrodes. To achieve sufficient photon harvesting while avoiding severe charge-carrier transport loss, one of the effective strategies is to construct 3D bulk heterojunction (BHJ) configurations. The BHJ configuration facilitates shortening of the carrier transport distance to alleviate the charge recombination, simultaneously providing a large heterojunction interface area for effective charge separation. To maximize light absorption in Sb₂(S,Se)₃ solar cells, researchers have explored the use of light management techniques such as anti-reflective coatings, textured substrates, and plasmonic nanoparticles. These approaches aim to increase the absorption of incident photons, thereby improving the photocurrent and overall device efficiency. Benefiting from previous studies on excellent photo-response exhibited by vertically [001]-oriented Sb₂S₃^97,98 and Sb₂Se₃⁹⁹ nanorod arrays, conceptually the light trapping can be significantly enhanced in nanorod array Sb₂(S,Se)₃ solar cells.

Nanorod textured Sb₂S₃ and Sb₂Se₃ thin film solar cells have been fabricated, with the aim of improving the light trapping and carrier-extraction.^99–102 Nanotexturization is a well-known engineering strategy to elongate the optical path for enhanced photon-trapping, and enlarged interface-area to facilitate charge extraction. Shi et al.⁸⁴ for the first time fabricated a nanorod-textured Sb₂(S,Se)₃ solar cell prepared using a hydrothermal method. As schematically shown in Fig. 10a, the Sb₂(S,Se)₃ layer was composed of vertically grown Sb₂(S,Se)₃ nanorods over a compact layer of Sb₂(S,Se)₃. CsF additive was employed to promote the crystallinity and [221]-orientation in the Sb₂(S,Se)₃ nanorod array. Compared to the control device, the nanorod-textured (CsF-40) solar cell demonstrated better carrier management (higher EQE) and diode-characteristics (lower η value of 1.51), leading to PCE improvement from 6.83% to 9.37% (Fig. 10b–g). The nanotexturization strategy was also found to reduce the band offsets at the CdS/Sb₂(S,Se)₃ and Sb₂(S,Se)₃/Spiro-OMeTAD interface, benefitting the photogenerated carrier transport (Fig. 10h and i).

Fig. 10 (a) Schematic illustration of an Sb₂(S,Se)₃ solar cell based on a nanorod-textured Sb₂(S,Se)₃ bilayer absorber. (b) J–V curves of the best devices with and without CsF additive. (c) Certified J–V and power density plots. (d) EQE spectra and integrated J_SC of the best devices with and without CsF additive. (e) Dependence of V_OC on different light intensities for the control and CsF-processed devices. (f) Histogram of PCE for 25 devices based on the control and CsF-processed Sb₂(S,Se)₃ films. (g) Nyquist plots of the control and CsF-40 Sb₂(S,Se)₃ solar cells obtained in the dark at 0.5 V bias voltage (inset: equivalent circuit used for Nyquist fitting). (h) and (i) Schematics showing the band alignments of the solar cells based on the control and CsF-40 Sb₂(S,Se)₃ absorbers. Adapted with permission from ref. 84 Copyright 2022, Elsevier B.V.

Recently, Dong et al.³³ demonstrated a novel charge-carrier management strategy to maximize photon absorption and charge-carrier collection. A textured FTO (T-FTO) substrate was employed as the front electrode, and a thin SnO₂ layer (deposited via ALD, spin-coating, and CBD) was used at the FTO/CdS interface to mitigate the shunt paths arising due to textured morphology. The insertion of the SnO₂ layer facilitated the growth of a conformal CdS layer and an optimal bandgap profile in the Sb₂(S,Se)₃ absorber layer. The T-FTO substrate exhibited higher roughness (Fig. 11a–d), and haze factor (Fig. 11e) than the FTO substrate with a micro-scale smooth surface (S-FTO). Higher haze factor indicates better photon harvesting ability and contributes to enhanced photon absorption by the absorber layer. In particular, the devices consisting of a T-FTO electrode, and ALD deposited SnO₂ layer were found to outperform those with S-FTO, and the SnO₂ layer deposited via spin coating (SC) and CBD (Fig. 11f). The small area (0.1225 cm²) device demonstrated a record PCE of 10.92% (certified as 10.70%, Fig. 11g), while the large area device (1 cm²) exhibited a PCE of 9.27% (Fig. 11h). Carrier loss analysis is illustrated in Fig. 11i. The carrier transport loss decreased from 13.39% in the reference cell (cell-ref) to 8.25% in the cell-target device, whereas non-radiative recombination loss decreased from 15.49% to 11.25%.

Fig. 11 Top-view SEM images of (a) S-FTO, and (b) T-FTO electrodes. AFM micrographs of (c) S-FTO, and (d) T-FTO substrates. (e) Haze factor of the S-FTO and T-FTO substrates. Inset: The average haze factor. (f) Statistical distribution of PCE in cells consisting of SnO₂ inter-layer deposited with different techniques. (g) J–V curve of the champion cell-target device. Inset: Cell configuration. (h) J–V curve of large-area (1.0 cm²) Sb₂(S,Se)₃ solar cell. Inset: Cell image. (i) The FF loss analysis of cell-ref and cell-target. Adapted with permission from ref. 33 Copyright 2025, Nature Springer.

2.5. CdS-free Sb₂(S,Se)₃ solar cells

The carcinogenicity, toxicity and parasitic absorption of the CdS layer is the key driving factor to develop an alternative ETL in Sb₂(S,Se)₃ solar cells. ZnS emerges as an ideal ETL in Sb₂(S,Se)₃ solar cells, owing to its non-toxicity, wide bandgap (∼3.8 eV) and high electron mobility (250 cm² V⁻¹ s⁻¹). ZnS shares a similar crystal structure and opto-electronic properties to CdS, both belonging to the II–VI semiconductor group with n-type semi-conductivity.^103,104 However, there is a high spike-like band offset at the ZnS/Sb₂(S,Se)₃ interface, which is not very conducive for transfer of photogenerated electrons from Sb₂(S,Se)₃ to ZnS. The offset/barrier can be suppressed significantly by partial oxidation of the ZnS layer. Li et al.¹⁰⁵ developed a molecular beam epitaxy (MBE) method to deposit a highly crystalline and conformal ZnS layer. The ZnS layer was further treated with O₂ plasma, and ZnCl₂ treatment (to enhance the crystallinity in films), followed by air-annealing (at 450 °C), to obtain ZnS:O films. The underlying ZnS:O films were found to be successful in promoting [hk1]-orientation in the Sb₂(S,Se)₃ layer, which is crucial of improvement in carrier-transport in Sb₂(S,Se)₃ films. The fabricated device FTO/ZnS:O/Sb₂(S,Se)₃/Spiro-OMeTAD/Au demonstrated a PCE of 5.15%.

Attempts to directly replace CdS with oxide-based ETLs have shown a drop in FF (and PCE), due to the chemical incompatibility and lattice-mismatch existing at the oxide/Sb₂(S,Se)₃ interface. Hu et al.⁶⁹ optimized the growth of Sb₂(S,Se)₃ films on TiO₂ film using additive engineering, to achieve a PCE of 8.52%. Dong et al.¹⁰⁶ demonstrated that in contrast to the crystalline/compact TiO₂ films, nanoparticle/amorphous TiO₂ films provide more anchoring sites, and are more conducive to the nucleation and growth of [hk1]-oriented Sb₂(S,Se)₃ films. Li et al.³⁴ optimized the co-sublimation method to achieve [hk1]-orientation, and a V-shaped double graded bandgap profile in Sb₂(S,Se)₃ films deposited over TiO₂ films. The conducive band-alignment and suppressed deep-level defects lead to an impressive PCE of 9.02%, which is the highest for CdS-free Sb₂(S,Se)₃ solar cells. Recently, Qin et al.¹⁰⁷ employed NaCl solution treatment to enlarge the bandgap and improve the quality of CBD grown TiO₂ films. The Na-ions diffused into the Sb₂(S,Se)₃ layer, passivated the Se-vacancies, and were found to play a key role in optimization of the TiO₂/Sb₂(S,Se)₃ interface. Wang et al.¹⁰⁸ investigated TiCl₄-passivated TiO₂ films, while Chen et al.¹⁰⁹ utilized atomic-layer deposited grown zinc tin oxide (ZTO) to engineer the band-alignment and defects at the ETL/Sb₂Se₃ interface; conceptually similar interfacial-engineering could be extended to Sb₂(S,Se)₃ solar cells.

Zhao et al.¹¹⁰ replaced the pristine CdS layer with a Zn(O,S)/CdS bilayer ETL. The novel device FTO/Zn(O,S)/CdS/Sb₂(S,Se)₃/Spiro-OMeTAD/Au demonstrated a PCE of 9.62%. This strategy enabled improvement in J_SC by 1.5 mA cm⁻², and 75% reduction in the usage of toxic CdS. In contrast, the device employing sole Zn(O,S) as the ETL exhibited high (dark) leakage current density, poor diode characteristics, and severe non-radiative recombination, resulting in a mediocre PCE of 4.20%. The SnO₂/CdS bilayer ETL has recently gained tremendous attention and has been frequently employed to achieve PCE values exceeding 10% in Sb₂(S,Se)₃ solar cells.

As illustrated in Table 3, attempts to substitute CdS layer has led to a drop in efficiency in Sb₂(S,Se)₃ solar cells. Another strategy is to replace the CdS layer (thickness ≈ 60 nm) with bilayer ETL such as Zn(O,S)/CdS and SnO₂/CdS.¹¹⁴ This strategy allows us to reduce the thickness of CdS to low levels (10–20 nm) without compromising the device performance. In particular, the SnO₂ nanoparticle dispersion is found to be successful in settling at the valleys at the topography of the FTO substrate, mitigating its roughness and the possibility of direct contact of FTO with the Sb₂(S,Se)₃ absorber.¹¹² NH₄F treatment has been applied to the dispersion of SnO₂ nanoparticles and increases the n-type conductivity of the SnO₂ film through fluorine doping (leading to PCE of 8.26% in Sb₂S₃ solar cells).¹¹⁵ Qi et al.¹¹⁶ demonstrated a PCE of 8.9% in the ZTO/CdS bilayer ETL-based Sb₂(S,Se)₃ solar cells, where an ultrathin and insulating polymer layer was employed at the HTL/Ag interface to mitigated the shunt paths.

Table 3 Photovoltaic performance of Sb₂(S,Se)₃ solar cells, consisting of either CdS-free, or ultrathin CdS ETL

Device architecture	Sb2(S,Se)3 deposition technique	ETL deposition technique	Device parameters				Ref.
			V OC (V)	J SC (mA cm^−2)	FF (%)	PCE (%)
FTO/TiO2/Sb2(S,Se)3/Spiro-OMeTAD/Au	CBD	Spin-coating	0.487	26.63	65.71	8.52	69
FTO/TiO2/Sb2(S,Se)3/Spiro-OMeTAD/Au	Hydrothermal	Spin-coating	0.600	21.43	54.79	7.08	106
FTO/Zn(O,S)/CdS/Sb2(S,Se)3/Spiro-OMeTAD/Au		CBD	0.656	22.74	64.54	9.62	110
FTO/Zn(O,S)/Sb2(S,Se)3/Spiro-OMeTAD/Au			0.418	20.15	49.85	4.20
FTO/TiO2/Sb2(S,Se)3/Spiro-OMeTAD/Au	Co-sublimation	Spin-coating	0.506	27.81	64.1	9.02	53
FTO/TiO2/BiI3 doped Sb2(S,Se)3/PEDOT : PSS/Au	Spin-coating	Spray pyrolysis + screen printing	0.520	21.5	63.0	7.05	83
FTO/TiO2/Sb2(S,Se)3/Spiro-OMeTAD/Au	Spin-coating	Spin-coating	0.478	25.8	58.1	7.15	111
FTO/TiO2/Sb2(S,Se)3/Spiro-OMeTAD/Au	CBD + spin coating	Spin-coating	0.560	19.48	52.34	5.71	60
FTO/SnO2/CdS/Sb2(S,Se)3/Spiro-OMeTAD/Au	Hydrothermal	Spin coating + CBD	0.72	21.69	55.35	8.67	112
FTO/SnO2/CdS/Sb2(S,Se)3/carbon/Ag	Hydrothermal + selenization	Chemical vapor deposition (CVD) + CBD	0.74	12.5	55.8	5.2	113
Mo/Sb2Se3/ZTO/ITO/Ag	Sputtering of Sb + selenization	ALD	0.440	32.84	60.77	8.63	109
ITO/TiO2/Sb2Se3/Au	VTD	Spin coating	0.394	25.5	52.9	5.33	108

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Recently, Dong et al.³³ comprehensively investigated the SnO₂/CdS bilayer ETL in Sb₂(S,Se)₃ solar cells, employing a SnO₂-layer deposited by ALD, CBD and spin-coating. The ALD grown SnO₂-layer was found to be the optimal buffer layer, enabling a record PCE of 10.92% (certified as 10.70%) in Sb₂(S,Se)₃ solar cells. This study elucidates that the underneath conformal SnO₂-coating promotes a conformal deposition of CdS. This strategy regulates the defects and creates an optimal bandgap profile in the Sb₂(S,Se)₃ absorber, highly conducive to the transport of photogenerated charge carriers.

The oxide/CdS bilayer ETL architecture has started gaining significant attention in Sb₂S₃ and Sb₂Se₃ thin-film solar cells as a strategy to overcome the limitations of a standalone CdS layer (∼70 nm thick). In this design, a thin oxide layer (typically SnO₂ or TiO₂) is first deposited on the FTO substrate—commonly by spin-coating a diluted colloidal nanoparticle solution- followed by chemical bath deposition (CBD) of the CdS layer. Liu et al.⁷⁶ employed a SnO₂/CdS bilayer ETL treated with Ce₂S₃, achieving an efficiency of 7.66%, while Wang et al.¹¹⁷ investigated a TiO₂/CdS nanoarray ETL treated with PbSe, which yielded 8.06% efficiency in Sb₂S₃ solar cells. The incorporation of ultrathin Ce₂S₃ and PbSe interfacial layers facilitated preferential crystallization of Sb₂S₃ along the [hk1] orientation and effectively passivated interface defects at the CdS/Sb₂S₃ junction. Su et al.¹¹⁸ revealed that the SnO₂/CdS underlayer promotes the growth of large grains, smoother surfaces, and reduced defect density in Sb₂S₃ films compared to those grown directly on CdS. SnO₂/CdS has shown promising performance in Sb₂Se₃ solar cells with PCE values approaching the 10% benchmark (9.50%, Che et al.;¹¹⁹ 9.37%, Sheng et al.;⁷⁷ and 9.28%, Zhao et al.¹²⁰).

2.6. Spiro-OMeTAD free Sb₂(S,Se)₃ solar cells

High cost ($600 per g), high moisture sensitivity, thermal instability, and lack of scalability in Spiro-OMeTAD deposition are key factors necessitating Spiro-OMeTAD-free Sb₂(S,Se)₃ solar cells. In addition to a complicated synthesis procedure, toxic and carcinogenic solvents like chlorobenzene and acetonitrile are used for the preparation of Spiro-OMeTAD precursor solution. The hydrophilicity of Spiro-OMeTAD (due to the hygroscopic nature of the Li–TFSI additive) is identified as the predominant source of PCE degradation in Sb₂(S,Se)₃ solar cells. Table 4 summarizes the device performance of Spiro-OMeTAD-free Sb₂(S,Se)₃ solar cells. Jiang et al.¹²¹ developed an inexpensive and highly stable dithieno[3,2-b:2′,3′-d]pyrrole-cored small molecule (DTPThMe–ThTPA) for selective hole transport in Sb₂(S,Se)₃ solar cells. The usage of the aforementioned HTL optimized the interfacial energy level alignment for hole transport, leading to a PCE of 9.7%, and increased stability. The HTL with electron-rich thiophene moieties coordinated strongly with the Sb-atoms at the interface Sb₂(S,Se)₃/DTPThMe–ThTPA, forming an efficient pathway for hole transport. Benefiting from the improved hole transport pathways and mitigated interface recombination, the champion device exhibited a PCE of 9.7%, along with highly improved stability. The improved interfacial charge transport was revealed by the Nyquist plots, where the DTPThMe–ThTPA-based device exhibited higher recombination resistance (R_rec, low-frequency component), and lower charge resistance (R_S, high-frequency component) than the Spiro-OMeTAD-based counterpart. Furthermore, the former device retained >90% of the initial PCE after storage in ambient air for 30 days, significantly higher than that in Spiro-OMeTAD-based counterparts (∼80%). The water contact angle at the DTPThMe–ThTPA surface was found to be 86°, significantly higher than that at Spiro-OMeTAD (67°), suggesting high hydrophobicity in the former (contributing to high resistance against the ingress of moisture).

Table 4 The photovoltaic performance of Sb₂(S,Se)₃ solar cells, employing alternate HTL to Spiro-OMeTAD

Device architecture	Sb2(S,Se)3 deposition technique	HTL deposition technique	Device parameters				Ref.
			V OC (V)	J SC (mA cm^−2)	FF (%)	PCE (%)
FTO/CdS/Sb2(S,Se)3/DTPThMe–ThTPA/Au	Hydrothermal	Spin-coating	0.638	23.18	65.50	9.69	121
FTO/CdS/Sb2(S,Se)3/MnS/Au		Thermal evaporation + air annealing	0.664	21.26	65.48	9.24	122
FTO/CdS/Sb2(S,Se)3/MnS (crystalline)/Au		Thermal evaporation + annealing in N2 environments	0.655	22.98	64.2	9.67	18
FTO/CdS/Sb2(S,Se)3/MnS (amorphous)/Au		Thermal evaporation	0.604	18.67	59.7	6.73
FTO/CdS/Sb2(S,Se)3/MnS/Au		Thermal evaporation + air annealing	0.664	21.26	65.48	9.24	122
FTO/CdS/Sb2(S,Se)3/MAPbBr3/Au		Spin coating	0.580	19.05	58.24	6.42	123
FTO/CdS/Sb2(S,Se)3/CsPbBr3/Au			0.620	21.50	58.55	7.82
FTO/CdS/Sb2(S,Se)3/PbS/C/Ag	Hydrothermal	Hydrothermal	0.650	18.8	65.06	8.0	124
FTO/CdS/Sb2(S,Se)3/Spiro-OMeTAD:TMT-TTF/Au		Spin-coating	0.650	20.63	67.66	9.02	125
FTO/CdS/Sb2(S,Se)3/CuPc doped P3HT/Au	Thermal evaporation	Spin-coating	0.491	27.7	60.9	8.25	126
FTO/CdS/Sb2(S,Se)3/NiO/Au	Hydrothermal	Spin-coating	0.644	19.12	60.14	7.40	127
ITO/CdS/Sb2(S,Se)3/Au	VTD	—	0.524	25.2	53.2	7.03	65
FTO/CdS/Sb2(S,Se)3/MoO3/Au	Hydrothermal	E-bean evaporation	0.684	16.53	63.75	7.20	128
FTO/CdS/Sb2(S,Se)3/MnS/Au		Thermal evaporation	0.623	23.7	64.5	9.50	129
FTO/CdS/Sb2(S,Se)3/MnS/ITO			0.541	23.4	58.3	7.41
FTO/CdS/Sb2(S,Se)3/MXene	Hydrothermal	Thermal spraying	0.64	20.9	62.7	8.29	130
FTO/CdS:O/Sb2(S,Se)3/PbS/C/Ag	Hydrothermal	Painting with carbon paste	0.518	25.98	67.21	9.05	131

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MnS emerges as a low-cost ($4 per g), environment-friendly, stable, and scalable HTL in Sb₂(S,Se)₃ solar cells. Qian et al.¹⁸ deposited (80 nm thick) an MnS layer over the Sb₂(S,Se)₃ films using thermal evaporation, followed by annealing to improve their crystallinity, mobility and conductivity. In addition, the annealing also improved the work-function of MnS, which contributed to improved built-in electric field and higher photovoltage. Although the MnS-based champion device (PCE of 9.67%) slightly underperformed compared to the device based on Spiro-OMeTAD (10.14%, Fig. 12a–c), it outperformed the latter in terms of reproducibility, longevity/stability (Fig. 12d) and cost-competitiveness. The unencapsulated MnS-based device exhibited PCE retention of >95% after 60 days of storage in ambient air (relative humidity of 15–40%), while the spiro- counterpart could retain only 58% of the initial PCE. Wang et al.¹²² proposed air annealing of thermally evaporated MnS, to improve its p-type conductivity and band-alignment with the Sb₂(S,Se)₃ layer. CuSbS₂ and CuSbSe₂ can be a potential choice for the HTL in Sb₂(S,Se)₃ solar cells, owing to their high p-type conductivity, matching the valence-band edges and low-lattice mismatch with Sb₂(S,Se)₃.¹³²

Fig. 12 (a) PCE distribution histograms of Sb₂(S,Se)₃ solar cells with different HTLs: as-deposited MnS HTL, post-annealed MnS HTL and Spiro-OMeTAD. (b) J–V curves; (c) EQE curves and the integrated current density of the Sb₂(S,Se)₃ devices with different HTLs. (d) Decay of normalized PCE over time of Sb₂(S,Se)₃ devices with different HTLs. Adapted with permission from ref. 18 Copyright 2022, Royal Society of Chemistry.

Among the inorganic HTL candidates, NiO stands out owing to its wide-bandgap (∼3.8 eV), deep valence-band edge (∼−5.3 V), thermal-stability, benign-synthesis, and low-cost.^133,134 Huang et al.¹²⁷ investigated NiO as the HTL in Sb₂(S,Se)₃ solar cells, and as a result of conducive band alignment, Sb₂(S,Se)₃/NiO achieved a PCE of 7.40%. The NiO-based device exhibited substantially higher stability than its Spiro-OMeTAD-based counterpart, by retaining 92% of the initial PCE after 700 h. Xing et al.¹²⁸ employed a non-toxic, high work-function and wide band-gap oxide MoO₃ as the HTL in Sb₂(S,Se)₃ solar cells. High purity MoO₃ thin films were obtained by e-beam evaporation and despite their n-type semiconductivity, the films were found to selectively extract holes and alleviate interface recombination. As a result, an improvement in built-in voltage and decrement in back contact resistance was obtained, leading to a PCE of 7.2%.

Li et al.¹³⁰ synthesized pure, uniform and highly conducting 2D MXene (Ti₃C₂T_x) nano-flakes and then coated it on the surface of Sb₂(S,Se)₃ thin film by thermal spraying. The negatively charged T_x group of MXene tightly attached and healed the sulfur-vacancy defects on the Sb₂(S,Se)₃ surface. This effect effectively reduced the recombination and enhanced the carrier transport at the back interface. As a result of ameliorated charge transport, the PCE of 8.29% was obtained in the MXene-based device, outperforming those with Au (4.0%) and carbon (2.8%) electrodes. A blend of Spiro-OMeTAD and lanthanide (Ln)-doped upconversion nanoparticles was found to serve as an extra light harvester and HTL, leading to a nominal PCE of 9.17%.¹³⁵

Carbon is known to serve a binary function (of HTL and back electrode),¹³⁶ and has been employed to fabricate noble metal-free Sb₂(S,Se)₃ solar cells. However, the devices yielded poor PCE (<3%), owing to high photovoltage loss at the Sb₂(S,Se)₃/C interface. Insertion of PbS¹²⁴ or P3HT¹³⁷ layers improved the PCE of the carbon-based device to 8.0% and 4.15%, respectively. A study by Ren et al.¹³⁸ suggests that the controlled oxidation of antimony chalcogenide films can promote amorphous phases (such as Sb₂O₃, Se, SeO_x) at the surface, and tune the otherwise Ohmic contact to a Schottky contact. Comprehensive studies on metal electrodes with Sb₂(S,Se)₃ are lacking and need to be performed for attaining the commercialization potential of Sb₂(S,Se)₃-based devices.

2.7. Bifacial, semitransparent and tandem solar cells

Zhang et al.¹³⁹ as a proof-of-the-concept explored Sb₂S₃ (band-gap of 1.74 eV) and Sb₂Se₃, with bandgaps of 1.74 and 1.22 eV, respectively, as the top and bottom subcell in a tandem solar cell. The tandem device exhibited a PCE of 7.93%, and made up the photovoltage losses in individual subcells, which is the biggest concern in antimony chalcogenide solar cells. This work provided impetus for further research on the investigation of Sb₂S₃, Sb₂Se₃, and Sb₂(S,Se)₃ for tandem solar cell applications. Sb₂(S,Se)₃ absorber materials are expected to have significant potential in space solar cells, building-integrated PV, and tandem solar cells. Qian et al.¹²⁹ developed a bifacial and semitransparent Sb₂(S,Se)₃ solar cell in the n–i–p architecture FTO/CdS/Sb₂(S,Se)₃/MnS/ITO. In this design, FTO served as the front electrode (cathode), while sputtered ITO acted as the back electrode (anode). The ultrathin device exhibited high transmission in the long wavelength region. Despite the formation of a detrimental Schottky junction at the back interface (MnS/ITO), the device delivered a PCE of 7.41% during front-side illumination and 6.36% during rear illumination, exhibiting excellent bifaciality of 0.86. Additionally, a 4-terminal Sb₂(S,Se)₃/Si tandem solar cell was fabricated, employing the Si-heterojunction solar cell as the bottom cell. J–V characteristics and EQE measurements reveal that the Si bottom cell efficiently harvests the long-wavelength light transmitted through the top Sb₂(S,Se)₃ cell, thus providing additional photoelectric conversion (Fig. 13a and b). Consequently, the tandem solar cell achieved a PCE of 11.66%, indicating a huge scope for achieving higher efficiency via optimization of this preliminary tandem cell design and fabrication process.

Fig. 13 (a) J–V curves of the Sb₂(S,Se)₃ semitransparent top cell and Si bottom cell. (b) EQE curves of the Sb₂(S,Se)₃ semitransparent top cell, Si bottom cell covered by the Sb₂(S,Se)₃ semitransparent top cell, and Si cell alone. Adapted with permission from ref. 129 Copyright 2023, Wiley-VCH.

Recently, Yang et al.¹⁴⁰ attempted to address the issue of severe sputtering damage during ITO deposition, and formation of a hole-extraction (Schottky) barrier at the MnS/ITO interface. An amorphous indium-zinc oxide (IZO) layer prepared by the soft sputtering method as the passivation layer to construct high quality ST-SSCs with minimal sputtering damage and excellent carrier transporting ability. This structure eliminates the hole extraction barrier at the MnS/ITO interface while protecting the underlying MnS from sputtering damage. In addition to alleviating the damage to the MnS layer, the IZO layer assists formation of the desired ohmic contact at the MnS/ITO interface. As a result, the semitransparent solar cell (FTO/CdS/Sb₂(S,Se)₃/MnS/IZO/ITO) delivered a record PCE of 8.26%. On coupling with the Sb₂Se₃ bottom cell, a PCE of 10.69% was achieved in the 4T tandem solar cell.

2.8. Sb₂(S,Se)₃-based indoor solar cells

The advent of the internet of things (IoT) and big data analytics has catalyzed an exponential increase in the demand for wireless devices. IoT technology has now expanded into a trillion-dollar sector, with projections indicating the deployment of approximately 75 billion devices globally by 2025. A significant proportion of these devices necessitate low power consumption, typically around 100 milliwatts, and are intended for installation in indoor environments. A promising approach to sustainably power these devices and achieve untethered, battery-free operation is the utilization of ambient light harvesting via indoor photovoltaic cells. The emission spectra of the commonly used indoor light sources, i.e., white LEDs and fluorescent lamps, have a narrow spectral distribution (400–700 nm) compared to AM1.5G, and the ideal bandgap for an absorber material to harvest indoor light is determined to be ∼1.8 eV. Perovskite-based indoor solar cells outperform all other competing technologies, achieving power conversion efficiencies surpassing 42%.¹⁴¹ Despite their superior performance, the commercialization potential of perovskites in ambient environments is limited due to concerns over lead toxicity and long-term material instability.

Chen et al.¹⁴² demonstrated Sb₂S₃ IPV minimodules (area of 5 cm², PCE of 12.82%) to power IoT wireless sensors and realized the long-term continuous recording of environmental parameters under WLED illumination in an office. The minimodule delivered competitive performance to the commercial a-Si : H IPV devices. Gao et al.⁵⁵ performed a systematic study on the effect of the optical bandgap of Sb₂(S,Se)₃ films on the comparative PV performance of the devices under AM1.5G and indoor light conditions. Sb₂(S,Se)₃ films with different bandgaps were synthesized by regulating the concentrations of S and Se source precursors in the hydrothermal deposition. The solar cells based on Sb₂(S,Se)₃ with 34.3% Se-content (and 1.50 eV bandgap) enabled an optimal PCE of 9.04% under standard AM1.5G illumination (Fig. 14b), while the device based on Sb₂S₃ with 1.74 eV bandgap achieved an optimal PCE of 20.34% under 1000 lux indoor illumination (Fig. 14e). The study concludes that though an appropriate Se/S atomic ratio is beneficial for improving the crystallinity of the Sb₂(S,Se)₃ film and passivating the trap states, the band gap remains a key factor in determining the suitability of this material for IPVs. Interestingly, PCE values exceeding 16% were obtained within a wide bandgap range from 1.5 to 1.7 eV, demonstrating the great prospects for Sb₂(S,Se)₃ as a light-harvesting material for indoor solar cells (Fig. 14d).

Fig. 14 (a) Cross-sectional SEM image of the Sb₂(S,Se)₃ solar cell. (b) J–V characteristics, and (c) EQE spectra of the Sb₂(S,Se)₃ solar cells under AM1.5G. (d) PCE distribution histograms of the Sb₂(S,Se)₃-based indoor solar cells. (e) J–V characteristics of the devices at the light intensity of 1000 lux under LED. (f) The relationship between output power and bandgap under AM1.5G and LED 1000 lux light sources. Adapted with permission from ref. 55 Copyright 2024, Wiley-VCH.

Cao et al.¹⁴³ investigated low Se-content (Sb/S/Se = 1 : 1.42 : 0.06) Sb₂(S,Se)₃ as an absorber material in indoor solar cells, fabricated in the architecture FTO/SnO₂/CdS/Sb₂(S,Se)₃/Spiro-OMeTAD:TMT-TTF/Au. Tetrakis(methylthio)tetrathiafulvalene (TMF-TTF) was introduced to prepare the Spiro-OMeTAD:TMT-TTF (36.6/1.0 mg) hybrid HTL. The composite HTL exhibited higher mobility than the pristine Spiro-OMeTAD. The Se-content in the Sb₂(S,Se)₃ films was found to be instrumental in regulating the favourable [hk1]-orientation, and in promoting charge transport properties. The champion device exhibited PCE values of 18.53, 17.62, 17.07, 17.30, 16.24, and 15.38%, under white LED illuminances of 1000, 500, and 200 lx with color temperatures of 3347 and 6103 K, respectively. Recently, Yang et al.¹⁴⁰ achieved a record PCE of 20.86% in a semitransparent Sb₂(S,Se)₃ solar cell, under 1000 lux indoor illumination.

3. Simulation studies in solar cells

Solar cell simulation is a pivotal tool in the advancement of photovoltaic technology, facilitating the modeling and analysis of solar cell performance under varied operational conditions. Software such as COMSOL Multiphysics, Silvaco Atlas, and PVsyst enables researchers to perform detailed numerical simulations that optimize design parameters, material properties, and operational efficiency. By employing these tools, scientists can accurately predict the behavior of monocrystalline, polycrystalline, thin-film and tandem solar cells, in homo-junction⁹² and hetero-junction^144–146 architectures. Additionally, simulation aids in the sensitivity analysis of temperature variations, irradiance levels, and other environmental factors, thereby improving the reliability of solar energy systems. The continued development and refinement of solar cell simulation software promise to significantly contribute to the evolution of high-performance solar technologies, fostering a transition towards sustainable energy solutions.³⁴

Cao et al.¹⁴⁷ performed a meticulous theoretical investigation to optimize triple-junction tandem solar cells, consisting of Sb₂S₃ (top cell)/Sb₂(S,Se)₃ (mid cell)/Sb₂Se₃ (bottom cell) stacking. A PCE of 32.98% was proposed via a band engineering strategy. Recently, Salem et al.⁶³ demonstrated a PCE of 22.00% (Fig. 14a–c) in a Se/Sb₂(S,Se)₃ dual-junction, monolithic, two-terminal (2T) tandem solar cell, using the Silvaco TCAD simulations. The proposed Sb₂(S,Se)₃ bottom was designed HTL-free, while ITO served as the interlayer. Dahmardeh et al.¹⁴⁸ demonstrated a PCE of 22.19% in Sb₂Se₃/Sb₂(S,Se)₃ tandem solar cells, employing the SCAPS simulation tool. The thickness of both absorber layers and ETL parameters were optimized to achieve current matching between the (top and bottom) subcells and facilitating efficient transport of photogenerated carriers (Fig. 15).

Fig. 15 Proposed monolithic HTL-free Se/Sb₂(S,Se)₃ tandem SC: (a) schematic of the HTL-free tandem SC design, emphasizing the removal of hole-transport layers from both sub-cells, (b) J–V characteristics of the HTL-free tandem configuration, comparing the top, bottom, and overall tandem cell performance, and (c) EQE curves of the HTL-free tandem. Adapted with permission from ref. 63 Copyright 2025, Elsevier.

Single junction Sb₂(S,Se)₃ solar cells have been extensively investigated using SCAPS simulation, and the key results are enlisted in Table 5. The simulation works prophesise attaining PCE values exceeding 20% in single junction Sb₂(S,Se)₃ solar cells, on adopting high levels of interface and bulk-defect passivation, and carefully tuning the band alignment at the ETL/Sb₂(S,Se)₃ and HTL/Sb₂(S,Se)₃ interfaces. Our previous computational work on Cd-free Sb₂(S,Se)₃ solar cells endorses ZnSe as a potential alternative to the toxic-CdS based ETL in Sb₂(S,Se)₃ solar cells.¹⁴⁴ On the other hand, CuSbS₂ has emerged as the most appropriate substitute to the Spiro-OMeTAD layer. The optimized devices based on ZnSe (ETL) and CuSbS₂ (HTL) exhibited a theoretical PCE of 20.02%, paving the way for boosting the PCE of Sb₂(S,Se)₃ solar cells beyond the current record of 10.92%.

Table 5 The optimized layer thickness and PV performance of single-junction Sb₂(S,Se)₃ solar cells, simulated via SCAPS simulation

Device architecture	Absorber thickness (µm)	V OC (V)	J SC (mA cm^−2)	FF (%)	PCE (%)	Ref.
FTO/CdS/Sb2(S,Se)3/Spiro-OMeTAD/Au	0.60	1.310	24.05	58.56	18.43	149
	0.30	0.654	24.04	63.87	10.04
ITO/Cds/Sb2(S,Se)3/MoS2/Mo	0.80	0.950	35.32	75.96	25.67	150
FTO/Cd0.6Zn0.4S/Sb2(S,Se)3/Spiro-OMeTAD/Au	0.50	0.881	26.67	74.22	17.43	151
FTO/ZnO/Sb2(S,Se)3/Cu2O/Au	0.55	0.951	27.70	68.00	18.00	152
FTO/ZnO/Sb2(S,Se)3/NiO/Au	0.50	0.940	27.60	67.00	17.30
FTO/Cd0.6Zn0.4S/Sb2(S,Se)3/Cu2O/Au	0.90	0.889	27.40	71.00	17.30
ITO/CdS/Sb2(S,Se)3/MnS/Au	0.30	0.722	25.29	69.54	12.70	153
Al:ZnO/Zn0.4S0.6/TiO2/Sb2(S,Se)3/MoSe2/Mo	2.00	0.638	32.34	75.75	15.65	154
FTO/ZnSe/Sb2(S,Se)3/CuSbS2/Au	0.50	0.930	28.64	74.54	20.01	144
FTO/CdS/Sb2(S,Se)3/Spiro-OMeTAD/Au	0.60	0.990	30.93	87.09	26.77	155
FTO/SnO2/CdS/Sb2(S,Se)3/CuI/Au	0.30	0.902	27.51	85.39	21.19	156
FTO/SnO2/CdS/Sb2(S,Se)3/CuI/C	0.30	0.897	23.20	85.55	17.82

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4. Summary and technological outlook

Considering the overall experimental and computational assessment, Sb₂(S,Se)₃ solar cells continue to demonstrate their potential as low-cost, high-efficiency alternatives to conventional photovoltaics. The key challenges impeding the commercialization potential of Sb₂(S,Se)₃ solar cells are – (i) moderate PCE values (plagued by high V_OC-deficits, and low FF), (ii) composition inhomogeneity due to difficulty in achieving the desired S/Se elemental ratio throughout the Sb₂(S,Se)₃ films, (iii) severe non-radiative recombination mediated via intrinsic (point) defects, especially anion vacancies (V_S/Se), and anti-sites (Sb_S/Se) present in high densities (>10¹⁵ cm⁻³), and (iv) limited research on large-area (>1 cm²) deposition and low-temperature fabrication protocols for Sb₂(S,Se)₃ solar cells. Although solution processing methods (mainly hydrothermal and CBD) remain to be widely adopted and optimized for the fabrication of lab-scale/small area Sb₂(S,Se)₃ solar cells with PCE exceeding 10%, these methods are not compatible with large area fabrication (modular levels). In this context, vapor deposition techniques, especially close spaced sublimation (CSS) and vapor transport deposition (VTD) seem more effective in the deposition of Sb₂(S,Se)₃ films with desired S/Se stoichiometry, [hk1]-orientation, conducive graded band-gap alignment, mitigated defects and large area homogeneity. There is a clear need for research works on flexible Sb₂(S,Se)₃ solar cells, which hold excellent commercialization potential. Considering the highly anisotropic charge transport in Sb₂(S,Se)₃ films, it is crucial to emphasize on the development of Sb₂(S,Se)₃ nano-array (bulk-heterojunction) solar cells, which exhibit better utilization of the incident spectrum via a photon trapping mechanism and better charge transport than planar-heterojunction Sb₂(S,Se)₃ solar cells.

MnS has emerged as the top choice to substitute Spiro-OMeTAD in Sb₂(S,Se)₃ solar cells, while CdS remains an indispensable choice for the ETL. As of now, adopting a bilayer ETL, consisting of a SnO₂ or TiO₂ layer decorated with an ultrathin (∼10 nm) CdS layer is a favored strategy to reduce the usage of CdS, without compromising the device performance. Various doping (particularly large cations and anions), post-deposition treatment, and additive engineering techniques (especially metal fluorides) have been found to be instrumental in alleviating the sluggish charge carrier dynamics in Sb₂(S,Se)₃ thin films, by regulating the defects and tuning the strain in the films. As the high temperature annealing induces the formation of Sb_S/Se anti-site defects, the development of low-temperature synthesis protocols can be crucial in mitigating the formation of scavenging deep-level defects. Prototype Sb₂(S,Se)₃ solar cells have shown PCE > 20% under indoor light conditions, suggesting great potential for these devices in indoor PV and building integrated PV. Future research should prioritize long-term stability, large-area manufacturing, and integration with tandem structures to unlock their full potential in the renewable energy landscape. The synergy of defect-engineering and band-alignment optimization is anticipated to achieve >20% PCE in Sb₂(S,Se)₃ solar cells, pushing the technology toward commercialization.

Author contributions

S. B. and S. S. conceptualized the work, wrote the original manuscript, and contributed equally to the work. K. H., R. G., V. V. S., S. M., and A. K. C. contributed to the review and editing of the manuscript. D. E. M., R. K. and K. R. were involved in the critical revision of the manuscript and supervised the work.

Conflicts of interest

All the authors have contributed to the work and declare no conflicts of interest.

Data availability

This review does not involve the generation of new datasets. All data supporting the findings of this papers are available in the publicly accessible sources cited within the manuscript.

Acknowledgements

This work received no specific grant from any funding agency in the public, commercial, or not-for-profit sectors.

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