A Review of Cu₃BiS₃ Thin Films: A Sustainable and Cost-Effective Photovoltaic Material

Abstract

The demand for sustainable and cost-effective materials for photovoltaic technology has led to an increasing interest in Cu₃BiS₃ thin films as potential absorber layers. This review provides a comprehensive overview of the main physical properties, synthesis methods, and theoretical studies of Cu₃BiS₃ thin films for photovoltaic applications. The high optical absorption coefficient and band gap energy around the optimal 1.4 eV make Cu₃BiS₃ orthorhombic Wittichenite-phase a promising viable alternative to conventional thin film absorber materials such as CIGS, CZTS, and CdTe. Several synthesis techniques, including sputtering, thermal evaporation, spin coating, chemical bath deposition, and spray deposition, are discussed, highlighting their impact on film quality and photovoltaic performance. Density Functional Theory studies offer insights into the electronic structure and optical properties of Cu₃BiS₃, aiding in the understanding of its potential for photovoltaic applications. Additionally, theoretical modeling of Cu3BiS3-based photovoltaic cells suggests promising efficiencies, although experimental challenges remain to be addressed. Overall, this review underscores the potential of CBS thin films as sustainable and cost-effective materials for future PV technology while also outlining the ongoing research efforts and remaining challenges in this field.

1. Introduction

Humanity’s insatiable demand for energy to power modern civilization is undeniable. To meet this demand, we have been primarily depending on the finite reserves of fossil fuels, which have significant environmental consequences that have led to the current climate emergency [1].

Renewable energies such as solar energy emerge as the clear solution due to their availability, cleanliness, and sustainability [2,3]. Keeping this in mind, photovoltaic (PV) devices, which transform solar energy into electricity, have attracted significant research interest and occupy a central position in current academic and technological efforts [4].

Thin film PV technologies show great promise due to their reduced raw material requirements, lower manufacturing costs, lighter weight, and ability to be used in locations and configurations unsuitable for traditional Si PV panels. In a PV film, the absorber layer is one of the most important components, usually composed of a p-type semiconductor. According to the Shockley–Queisser limit, for optimal efficiency, this semiconductor layer should ideally possess a band gap energy of approximately 1.4 eV [5].

Several p-type semiconductors have been investigated as absorbers for thin film PV, including copper indium gallium selenide (CIGS), copper zinc tin sulfide–selenide (CZTS), and cadmium telluride (CdTe) [5,6,7,8,9]. Among these, CIGS and CdTe stand out for their remarkable efficiency in thin film PV cells, achieving power conversion efficiencies (PCEs) of 23.6% and 22.6%, respectively [10,11,12,13]. However, the widespread adoption of these absorber layers is still deterred by the high cost and limited availability of In and Ga, as well as the Cd toxicity. CZTS has been explored as an alternative to address these issues, but it has not yet achieved the same efficiency levels, with a maximum reported efficiency of 14.9% [12,14,15].

Therefore, the pursuit of new, sustainable, and cost-effective semiconductor alternatives is crucial. Cu₃BiS₃ (CBS) Wittichenite-phase has emerged as a promising candidate for alternative thin film p-type absorber layers. Particularly, CBS comprises non-toxic and abundant elements with relatively low criticality. For instance, in 2022, according to the United States Geological Survey (USGS), the annual average domestic dealer price for Bi was approximately USD 8.70/kg, with a global production of 20,000 tons. In contrast, in the same year, the estimated average price of In was USD 250/kg, with a global refinery production of only 900 tons [16]. Moreover, Bi is also a byproduct of extraction mines for various other elements, including Cu, Zn, Mo, Sn, W, and Pb [5,17,18,19,20].

CBS presents excellent thermal stability above room temperature [5,21], along with high optical absorbance in the visible region $(\ge {10}^5 {\mathrm{c}\mathrm{m}}^{-1})$ and a direct optical band gap energy ($E_g$) occasionally reported around 1.4 eV, properties comparable to those of CIGS and CZTS [22]. Additionally, there is evidence of hole trapping, which facilitates an extended lifetime of electrons in the conduction band [23].

Despite these advantages, research on this material is still in its early stages, with numerous challenges remaining to be addressed, and competitive PV devices have yet to be developed [21,24,25,26,27]. Therefore, this review aims to offer a concise yet comprehensive overview of current synthesis methods and highlight key experimental and theoretical studies on CBS thin films as absorber layers in PV devices.

2. Structure and Properties of Cu₃BiS₃

Even though the Cu-Bi-S family can have several possible phases, namely, Cu₃BiS₃, CuBiS₂, CuBi₃S₃, and Cu₄Bi₄S₉ [18,28], Wittichenite-phase Cu₃BiS₃ (3:1:3) (and its off-stoichiometric chemical compositions [22]) is one of the most studied due to its high optical absorption coefficient $(\ge {10}^5 {\mathrm{c}\mathrm{m}}^{-1})$ and reported band gap energies close to 1.4 eV, making it particularly suitable for thin film PV devices [5,18,28,29,30].

Table 1. Main properties of Cu₃BiS₃ Wittichenite-phase.

Mineral	Wittichenite
Crystal System	Orthorhombic
Space Group	P212121
Lattice at 300 K [31]	a = 7.723 Å b = 10.395 Å c = 6.716 Å
Volume [31]	539.164 Å3
Density [31]	6.11 g.cm−3
Magnetic properties	Diamagnetic
Reported Eg	1.0–1.7 eV
Oxidation states	Cu+, Bi3+, S2−

summarizes the most relevant properties of CBS.

Cu₃BiS₃ Wittichenite-phase is a Cu-Bi-S ternary system that crystallizes into an orthorhombic lattice with a P2₁2₁2₁ space group (a = 7.723 Å, b = 10.395 Å, and c = 6.716 Å; α = β = γ = 90°) [31]. Figure 1 depicts the unit cell of Cu₃BiS₃, which contains four formula units and shows the different Cu¹⁺, Bi³⁺, and S²⁻ sites. The Cu ions occupy the center of a nearly planar trigonal polyhedron with S at each of its corners, with Cu-S bond lengths ranging from 2.255 to 2.348 Å [32,33]. The Bi and S ions form trigonal pyramids, with one Bi ion at the apex and three S ions at the base corners. The Bi-S bond lengths range from 2.569 to 2.608 Å [31,34].

The orthorhombic symmetry of CBS enables direct electronic transitions [35] and contributes to a high optical absorption coefficient [36]. Chalapathi et al. produced orthorhombic CBS thin films via evaporation and sulfurization, noting improved carrier mobility and a 1.32 eV band gap energy due to reduced grain size [29].

Hussain et al. achieved p-type electrical conductivity of 0.03 Ω⁻¹.cm⁻¹ in CBS thin films using a combined chemical and thermal process, highlighting its potential for PV cell production [37].

Fazal et al. demonstrated that chemically deposited CBS thin films exhibit optical band gap energies ranging from 1.0 to 1.25 eV, a Hall mobility of 34.9 cm² V⁻¹ s⁻¹, a charge carrier concentration of 6.07 × 10¹⁶ cm⁻³, and a resistivity of 7.22 Ω.cm at a thickness of 126 nm. These results suggest well-formed films with the potential for PV applications [38].

Future research can investigate how advancements in deposition methods affect the efficiency and stability of CBS thin films for photovoltaic applications [38,39,40]. Post-processing techniques like optimized heat treatments have been explored to enhance the chemical stability and conductivity of these films, rendering them more suitable for electronic devices and high-efficiency solar cells [18,34,41,42].

3. Density Functional Theory Studies

Density Functional Theory (DFT) first principles calculations serve as a powerful tool to gain deeper insights into how each constituent of the CBD Wittichenite phase influences its physical properties. By integrating experimental findings, DFT can expedite CBS research by elucidating underlying mechanisms and trends discerned from experimental data.

Whittles et al. carried out an exhaustive investigation into the alignment and width of the forbidden band, as well as the state of the valence band of CBS, employing DFT calculations considering the hybrid HSE06 functional with Spin-Orbit Coupling (SOC) in conjunction with X-ray absorption and X-ray photoemission spectroscopy (XPS) analyses [41]. Their findings reveal the presence of a type II band alignment between CBS and CdS. This suggests that using a more suitable n-type layer, which would create a type I heterostructure with a small conduction band offset, could lead to improved PCEs (Figure 2a). Furthermore, they predict a direct band gap energy of 1.18 eV at the Gamma point (Γ) and argue that it deviates from the often reported 1.4−1.5 eV due to the low density of states at the valence band maximum (VBM), as illustrated in Figure 2b. Nevertheless, this direct band gap value remains close to the optimal 1.4 eV, making it suitable for PV cell applications.

DFT studies also suggest that the VBM is primarily composed of Cu-d and Cu-sp states, with minimal influence from Bi-s states, whereas the conduction band minimum (CBM) is primarily composed of Bi-p and Bi-sp anti-ligand states [39,43,44].

Oubakalla et al. performed full-potential linearized augmented-plane-wave (FP-LAPW) DFT calculations considering the Generalized Gradient Approximation (GGA) with the Tran-Blaha modified Becke–Johnson (TB-mBJ) functional with a Hubbard potential (GGA + U + TB-mBJ) to investigate the optical and electronic properties of CBS thin films produced by co-electrodeposition in aqueous solution [43]. They predicted band gaps for CBS ranging from 1.42 to 1.52 eV, depending on the applied potential. These results aligned closely with experimental observations, suggesting the potential of this method to tune the CBS band gap.

Prudhvi Raju et al. explored the sonochemical synthesis of CBS quantum dots through a combined experimental and theoretical approach [44]. Employing FP-LAPW DFT calculations with GGA using the Perdew, Burke, and Ernzerhof (PBE) and TB-mBJ functionals, with and without SOC, they investigated the structural, electronic, and optical properties of the quantum dots. The results unveiled a transition in the band gap from direct to indirect due to the combination of Bi-p and Cu-d states in the conduction band spectra, leading to band degeneration with the inclusion of SOC, as illustrated by the electronic band structure in Figure 3a. Figure 3b shows that the VB Total Density of States (TDOS) calculated by DFT using the TB-mBJ functional is consistent with the experimental VB XPS spectrum, despite the underestimated band gap (≈1 eV) when compared to the experimental value (≈1.4 eV)

These findings highlight the substantial impact of SOC on the optical and electronic characteristics of CBS quantum dots, indicating their high potential for optoelectronic device applications.

Morales-Gallardo et al. employed DFT calculations with the GGA PBE functional to assess the electronic structure properties of CBS and determine photovoltaic parameters [45]. They focused on CBS band alignments with various n-type layers (CdS, ZnO, ZnS, CdS/ZnO, CdS/ZnS, and TiO2). The study predicts that the CdS/ZnS combination exhibits excellent energy alignment with a near-zero conduction band offset, making it the most promising composite material among these heterojunctions. The authors suggest that this theoretical approach could be valuable for designing heterojunctions to enhance solar cell performance.

Jia et al. utilized DFT calculations to explore the impact of two-dimensional coordination in CBS on out-of-plane phonon scattering, which resulted in remarkably low thermal conductivity [46]. Their analysis of the phonon spectrum indicated that this property renders CBS suitable for applications demanding thermal insulation, owing to its distinctive structure that facilitates phonon scattering. This suggests potential applications of CBS beyond photovoltaics.

4. CBS Film Synthesis Methods

Several studies propose synthesis methods for CBS films. Some rely on one-step processes, where the ternary phase forms in a single process. However, in most cases, subsequent annealing is required to improve CBS’s crystallinity.

Alternatively, two-step CBS synthesis methods are also reported. These involve depositing precursor layers (such as Cu and Bi metallic layers or Cu-S and Bi-S layers), followed by an annealing or sulfurization process to form the CBS phase [47].

4.1. One-Step Processing

4.1.1. Sputtering on Heated Substrate

Sputtering deposition can be used to produce compact CBS thin films with suitable optoelectrical properties in a single step by depositing CBS precursors onto a heated substrate. Haber et al. employed RF sputtering in a 5 mTorr Ar atmosphere to cosputter CuS and Bi targets at 80 and 10 W, respectively, onto fused silica substrates heated at 250 and 300 °C [17]. To obtain a 3:1:3 (Cu:Bi:S) stoichiometry, the sputtering of the CuS target remained active while the Bi sputtering was periodically switched on and off to achieve the correct elemental ratio. They observed that the film deposited at 300 °C was homogeneous throughout its depth, while the one deposited at 250 °C retained a layered structure.

Annealing processes performed within the 250–300 °C range for 2 h under a 50 Torr (1 Torr H₂S and 49 Torr Ar) atmosphere enhanced the crystallinity of the films produced by sputtering.

This single-step method, based on sputtering deposition on heated substrates, yielded CBS thin films with an average thickness of approximately 600 nm. These films exhibited an absorption coefficient ɑ ≈ 10⁵ cm⁻¹, sheet resistances as low as 1.6 × 10⁵ Ω, and band gaps close to the optimal 1.4 eV [17].

4.1.2. Thermal Evaporation on Heated Substrates

Hussain et al. used thermal evaporation to deposit CBS on soda–lime glass (SLG) substrates by co-evaporating anhydrous powder of copper (I) sulfide (Cu₂S) and bismuth sulfide (Bi₂S₃). In their study, besides room temperature depositions, the films were also deposited on substrates heated at temperatures ranging from 250 to 400 °C to promote the formation of the CBS Wittichenite ternary phase [37].

The authors observed that films deposited at room temperature were amorphous and, through Energy Dispersive X-ray Spectroscopy (EDS) analysis, they demonstrated substantial composition heterogeneity across the film surface, with variations in Cu and Bi composition as high as 33 and 12 percentual points, respectively. The CBS films deposited between 250 and 350 °C were still mostly amorphous. However, films deposited on substrates at 350 and 400 °C exhibited diffraction patterns corresponding to the desired CBS Wittichenite phase. For these films, the average crystallite size of the CBS films increased from 38.4 ± 0.1 nm to 44.2 ± 0.1 nm as the substrate deposition temperature rose.

The deposition temperature also impacted the morphology of the resulting CBS films, creating more compact and columnar structures, reducing PV losses due to recombination processes [48]. Additionally, Figure 4a illustrates how the deposition temperature significantly influenced the films’ band gap energies, decreasing from 2 eV for room-temperature deposition to a value near the optimal 1.4 eV for depositions conducted at 400 °C [49,50].

The authors also performed photocurrent–voltage measurements under AM 1.5G standard conditions of solar cells with the geometry shown in Figure 4b, using CBS films deposited at 375 and 400 °C. The I-V curves presented in Figure 4c demonstrate the enhanced PV performance of the cell utilizing the CBS film deposited at 400 °C compared to its 375 °C counterpart. The open circuit voltage ($V_{OC}$), short circuit current ($I_{SC}$), fill factor (FF), and PCE ($\eta$) improved from ${\left[V_{OC}\right]}_{375°C}=91 \mathrm{m}\mathrm{V}$, ${\left[I_{SC}\right]}_{375°C}=2.37 \mathrm{m}\mathrm{A}$, $FF=40\%,$ and $PCE=0.09\%$ to ${\left[V_{OC}\right]}_{400°C}=93 \mathrm{m}\mathrm{V}$, ${\left[I_{SC}\right]}_{400°C}=4.65 \mathrm{m}\mathrm{A}$, $FF=61\%,$ and $\eta =0.27\%$. Notwithstanding, these PCEs fall significantly short of the practical thresholds.

4.1.3. Spin Coating

Spin coating offers an interesting, cost-effective, one-step method for CBS synthesis. Jiajia Li et al. adopted a low-cost, solution-based process of dimethyl sulfoxide (DMSO) without the need for post-sulfurization, using CuCl, BiCl₃, thiourea, and DMSO for the precursor solutions, to successfully produce CBS films [51]. Mo-coated SLG samples were placed on a sample holder and spin-coated at 3000 rpm for 30 s. Subsequently, they were dried at temperatures ranging from 240 to 340 °C for 2 min in a N₂ atmosphere.

The formation of a pure CBS phase occurred during the precursor heating stage. While the CBS films exhibited good crystallinity and a close-to-ideal band gap energy $(E_g=1.47 \mathrm{e}\mathrm{V})$, they also displayed pores and cracks. These issues were mitigated by increasing the CBS grain size with adjustments to the drying temperatures.

The CBS films were utilized to fabricate PV cells incorporating a CdS n-type layer and a ZnO and ITO optical window, as illustrated in Figure 5a. Despite the reported high-quality CBS film, the resulting PV cells exhibited only a 0.17% PCE ($V_{OC}=190 \mathrm{m}\mathrm{V}, { J}_{SC}=2.37 \mathrm{m}\mathrm{A}/{\mathrm{c}\mathrm{m}}^2, FF=37.6\%$), which falls substantially behind the typical PV cell efficiency standards.

4.1.4. Chemical Bath Deposition

The chemical bath deposition (CBD) technique is widely utilized for depositing sulfides, selenides, oxides, and ternary compounds [52]. It offers several advantages over other deposition methods like thermal evaporation, magnetron sputtering, and spray pyrolysis. CBD does not require high vacuum conditions or high-power sources and it can be performed at low temperatures [53], making it cheaper than the aforementioned alternative processes. Moreover, CBD allows for the deposition of films over larger areas with relative ease of control over treatment parameters [54]. Its simplicity, replicability, and scalability make it appealing for commercial production processes [52].

Deshmukh et al. were the first to produce CBS films using CBD [54] successfully. They synthesized ternary Cu₃BiS₃ thin films on glass slides using C₄H₅CuO₄.H₂O, Bi(NO₃)₃.5H₂O, and CH₄N₂S (thiourea) precursors at 328 K for 50 min. These films exhibited high optical absorption within the visible range, with band gap energies ranging between 1.56 and 1.58 eV. X-ray diffraction (XRD) diffractograms revealed that the films were amorphous, although Scanning Electron Microscopy (SEM) micrographs displayed densely packed, fine, spherical nanoparticles measuring 70–80 nm in diameter across the sample surface, as depicted in Figure 6a,b. Still, EDS analysis indicated the presence of nearly stoichiometric CBS, with atomic percentage values for Cu, Bi, and S at 41.00%, 13.14%, and 45.82%, respectively.

The same researchers also utilized CBD to fabricate CBS films using CuCl₂.2H₂O, Bi(NO₃)₃.5H₂O, and CH₄N₂S (thiocarbamide) precursors [55]. By changing the concentrations of the solutions and subjecting them to subsequent annealing at 573 and 673 K, they produced CBS thin films with different morphologies, crystallinity, and band gap values ranging from 1.42 to 1.56 eV. In contrast to their initial findings, all these films exhibited crystalline phases, with those synthesized using lower reagent concentrations and higher annealing temperatures demonstrating increased crystallinity and band gap values closer to the ideal 1.34 eV, as per the Shockley–Queisser limit (Figure 6c,d).

Although CBD-produced films are promising and straightforward to produce, the absence of actual PV cells made from them hinders the assessment of their potential to yield competitive PV devices. Considering its simplicity, CBD could serve as a significant alternative to investigate the intrinsic defects observed in CBS films documented in the literature.

4.1.5. Spray Deposition

Chemical spray pyrolysis from methanol solutions using compressed air as the carrier gas offers another avenue for CBS film production, as demonstrated by Sheng Liu et al. They utilized a solution comprising CuCl₂, BiCl₃, and thiourea for this purpose [56]. Spray pyrolysis is known for its versatility and cost-effectiveness and can be implemented on a large scale [57,58].

Sheng Liu et al. observed that their CBS films, with an average thickness of 260 nm, displayed strong adhesion to the glass substrate. However, XRD analysis revealed the presence of secondary phases such as Cu_xS, Bi₂S₃, and CuBiS₂, and the morphology of the films exhibited poor uniformity [56].

Regarding their optical properties, CBS films deposited using spray pyrolysis presented band gaps ranging from 1.65 to 1.72 eV.

4.2. Two-Step Processing

4.2.1. Thermal Evaporation of Precursors and Post-Sulfurization

Chalapathi et al. used thermal evaporation to deposit metallic precursors in a Cu/Bi/Cu configuration, followed by sulfurization inside a graphite box placed in a quartz tube furnace [29]. The graphite box contained solid sulfur to facilitate the reaction kinetics required to produce the CBS Wittichenite phase. The quartz tube furnace was heated at a rate of 10 °C/min and maintained at 350, 400, and 450 °C for 60 min.

The characterization results demonstrated crystalline CBS films with minimal secondary phases, which became increasingly suppressed with higher temperatures (Figure 7a). Additionally, SEM micrographs showed that grain size increased with the sulfurization temperature. On the other hand, elevating the temperature during the second processing step led to a decrease in resistivity and band gap energy from approximately 1.4 to 1.3 eV (Figure 7b).

The temperature of synthesis also enhanced the mobility and charge concentration, as determined by Hall effect measurements. For instance, the sample sulfurized at 450 °C presented a resistivity of 29.80 ± 0.05 Ω cm, mobility of 6.0 ± 0.2 cm² V⁻¹ s⁻¹, and carrier concentration of 4.7 ± 0.5 × 10¹⁶ cm⁻³.

Yang et al. also employed a two-step synthesis method to prepare CBS thin films. Initially, they used co-evaporation to deposit Bi and CuS precursors in a Bi/CuS/Bi/CuS configuration [21]. The layer thicknesses were carefully controlled to achieve the ideal atomic ratio for forming a pure-phase CBS thin film [41,47,59]. Subsequently, S powders and a gas mixture of H and Ar were utilized in the annealing step, conducted at temperatures ranging from 350 to 475 °C, to facilitate the formation of CBS.

The authors reported that increasing the annealing temperature led to a rise in the number of grains assigned to the CBS film, with secondary phases present in all samples. In fact, for samples annealed at higher temperatures (400 to 450 °C), the formation of CuBiS₂ was observed.

Furthermore, the Cu/Bi ratio varied depending on the layer configuration from the initial step, affecting the film composition. A Cu/Bi ratio of 3 resulted in improved crystallinity of the Cu-Bi-S films.

Despite the films’ high absorption coefficients and p-type conduction, Hall effect measurements revealed low mobility of the charge carriers, reaching a maximum of 1.5 cm² V⁻¹ s⁻¹. Finally, the obtained CBS films were utilized to fabricate a PV cell with a Mo/CBS/CdS/ZnO/ZnO:Al/NiAl geometry, achieving an efficiency of 1.7%, the highest efficiency reported to date [21].

4.2.2. Sputtering Deposition of Precursors and Post-Sulfurization

Gerein et al. used DC sputtering to deposit multilayers of Cu and Bi precursors onto quartz substrates from metallic targets, ensuring a 3:1 Cu to Bi ratio by using equal sputtering times. Sulfurization of the precursors took place inside the deposition chamber with 50 Torr of H2S (99.9%) at temperatures ranging from 250 to 400 °C [60].

The authors investigated the impact of sulfurization temperature and duration, as well as subsequent annealing conditions in a H₂S environment, on the quality of the resulting films.

The utilization of backscattered electrons enabled the distinct visualization of two distinct phases, particularly noticeable in films subjected to higher sulfurization temperatures. This phenomenon arose from the evaporation of Bi from the films, which altered the overall reaction kinetics of the film formation process. The research revealed that conducting reactive annealing of binary precursor films at lower temperatures for prolonged durations favored the synthesis of CBS, with the most favorable outcome observed at 270 °C for 30 h, despite the challenges posed by Bi volatility.

4.2.3. Electrodeposition and Sulfurization

Electrodeposition has been effectively utilized to deposit metal precursors, subsequently converted into chalcogenides through a thermal process, yielding good results for CZTS devices [61]. One of the main features of this method is the quality and uniformity of the deposited films.

Colombara et al. employed electrodeposition to deposit Cu/Bi/Cu layers on Mo-coated SLG [33]. Subsequently, the multilayer films obtained through electrodeposition were subjected to annealing in a graphite box with excess sulfur (0.05 g) using an AS-Micro Rapid Thermal Processor (RTP) (AnnealSys). Annealing temperatures ranged from 270 to 550 °C over periods spanning from 5 to 960 min and heating rates ranged from 5 and 600 °C min⁻¹.

The authors observed that films sulfurized below 400 °C solely exhibited binary phases (CuS and Bi₂S₃). Conversely, temperatures exceeding 400 °C facilitated the required reaction kinetics to form the ternary CBS Wittichenite phase and increased crystallinity in films sulfurized at 450 and 500 °C. Compact and homogeneous CBS films were achieved by sulfurizing the precursors at 500 °C for 30 min with a heating rate of 5 °C.min⁻¹. However, higher heating rates led to film peeling off.

A summary of all synthesis methods here presented can be found in

Table 2. Comparison between Cu₃BiS₃ thin film synthesis methods.

	Synthesis Method	Methodology Summary	T (°C)	Eg (eV)	Observations	Refs.
One-step processing	Sputtering on Heated Substrate	Sputtering in Ar atmosphere, requires heated substrate	250–300	1.4	Homogeneous at 300 °C, layered structure at 250 °C	[17]
	Thermal Evaporation on Heated Substrate	Co-evaporation of precursors on heated substrate	250–400	1.4	CBS phase at 350–400 °C, compact columnar structure	[37,48,49,50]
	Spin Coating	Precursor solution spin-coated, followed by drying	240–340	1.47	Good crystallinity, pores and cracks mitigated with temperature	[51]
	Chemical Bath Deposition	Low-cost aqueous solution deposition	55–70 (328 K)	1.42–1.56	Spherical nanoparticles of 70–80 nm	[54,55]
	Spray Pyrolysis	Solution sprayed and decomposed on hot substrate	250–400	1.65–1.72	Presence of secondary phases, non-uniform	[56,57,58]
Two-step processing	Thermal Evaporation + Sulfurization	Metal evaporation followed by sulfurization	350–450	1.3–1.4	Crystalline, grain size increases with temperature	[29,41,47,59]
	Sputtering + Sulfurization	Metal sputtering followed by sulfurization	250–400	N.A.	Distinct phases, CBS favored at 270 °C	[60]
	Electrodeposition + Sulfurization	Electrochemical metal deposition followed by sulfurization	270–550	N.A.	Compact and homogeneous at 500 °C	[33]

.

5. CBS PV Cell Modeling

Theoretical modeling is a powerful tool to guide the study and refinement of PV devices based on novel absorber layers. Nevertheless, research on CBS as a p-type absorber layer for PV devices is limited to only two simulation studies. The first, by Mesa et al., used the updated version AMPS (Analysis of Microelectronic and Photonic Structures), wxAMPS, and SCAPS-1D (solar cell capacitance simulator) to perform the simulations [62], while the other, by Capistran-Martinez et al., relied solely on SCAPS-1D [27].

Mesa et al. claimed $V_{OC}=0.712 \mathrm{V}, J_{SC}=36.25 \mathrm{m}\mathrm{A}/{\mathrm{c}\mathrm{m}}^2, FF=79.54\%,$ and $\eta =19.86\%$ based on the simulated J-V curves shown in Figure 8a [62]. However, they did not provide details on the parameters adopted or the PV geometry, including the n-type layer and the optical window used.

On the other hand, Capistran-Martinez et al. conducted a comprehensive study comparing two different heterojunctions (CBS/CdS and CBS/Bi₂S₃) to assess the impact of n-type layers on the PV device. While CdS is commonly used as an n-type layer in p-n heterojunctions, issues with the CBS/CdS interface have been widely reported. [27,41]. To address this, the authors opted for Bi₂S₃ as the n-type layer in their SCAPS simulation. Bi₂S₃ was chosen because Cu⁺ ion diffusion enhances its n-type conductivity. Moreover, both CBS and Bi₂S₃ share the orthorhombic phase, which facilitates lattice matching at the p-n interface [27].

They used SCAPS to model glass/Mo/CBS/CdS/ZnO/ITO and glass/Mo/CBS/Bi₂S₃/ZnO/ITO PV cells (Figure 8b), incorporating CBS parameters derived from experimental analyses including XRD, UV–Vis spectroscopy, and Hall effect mobility measurements. The corresponding J-V curves are presented in Figure 8a and reveal FF values of 0.72 and 0.58 and PCEs of $\eta =6.8\%$ and $7.4\%$ for cells employing CdS and Bi₂S₃, respectively. The energy band diagram depicted in Figure 8c illustrates superior band alignment between CBS and Bi₂S₃, potentially elucidating the higher efficiency observed in the CBS/Bi₂S₃ heterojunction compared to CBS/CdS.

It is noteworthy that despite the promising outcomes from PV cell simulations featuring a CBS absorber layer, the highest experimental reported efficiency stands at only $\eta =1.7\%$, significantly lower than the simulated results by about one order of magnitude [51]. This indicates the existence of significant technical challenges in experimental implementation that must be addressed before attaining the simulated efficiencies.

6. Conclusions

This review highlights the promising potential of CBS as an alternative sustainable, cost-effective absorber layer in thin film PV cells. While numerous studies have explored its optoelectronic properties, there are still many unanswered questions regarding its optimal synthesis process, structural characteristics, and device performance.

Despite being relatively unexplored compared to other generations of PV materials, CBS is gaining significant attention from the scientific community due to its sustainability, environmental friendliness, and potential cost-effectiveness.

However, for CBS to fulfill its promise as the next big PV material, higher-quality and more focused studies are required. Addressing fundamental questions surrounding film synthesis, stoichiometry control, interface engineering, and device optimization will be crucial steps towards unlocking the full potential of CBS-based PV devices. Nonetheless, the fertile research landscape surrounding CBS suggests that it may indeed emerge as a leading PV material in the near future, offering significant advantages in terms of sustainability and affordability.