Recent progress in the research on using CuSbS₂ and its derivative CuPbSbS₃ as absorbers in case of photovoltaic devices

Abstract

Thin-film solar cells show considerable application potential as alternative photovoltaic technologies. Cuprous antimony chalcogen materials and their derivatives, represented as CuSbS₂ and CuPbSbS₃, respectively, exhibit the advantages of low cost, massive elemental abundance, stability, and good photoelectric properties, including a suitable bandgap and large optical absorption coefficient. These advantages demonstrate that they can be used as light absorbers in photovoltaic applications. In this study, we review the major properties, fabrication methods, and recent progress of the performance of the devices containing CuSbS₂ and CuPbSbS₃. Furthermore, the limitations and future development prospects with respect to the CuSbS₂ and CuPbSbS₃ solar cells are discussed.

Key words： CuSbS₂; CuPbSbS₃; properties; fabrication; performance

1 Introduction

Recently, photovoltaic technology utilizing solar energy has been lauded as a clean and renewable energy provider. In particular, crystalline Si (c-Si) and other semiconductors based on thin-film solar cells have been widely explored in photovoltaic technology. Since the single-crystal CuInSe₂-based solar cell was first reported in 1975 with a power conversion efficiency (PCE) of 12% [1], Cu-based solar cells have rapidly developed. The series of breakthrough technologies, such as alloying with gallium (Ga), has also improved the device performance. Thus, the new semiconductor Cu(In,Ga)(S,Se)₂ (CIGS) has become one of the most promising light-absorber material for photovoltaic application. To date, the CIGS thin-film solar cell has achieved a PCE of more than 23.35% (area of 1 cm²) [2], which denotes that the thin-film photovoltaic technologies have an impressive future. However, the prices of the raw materials required to develop CIGS, such as indium (In) and gallium (Ga), are increasing because of elemental scarcity. This is hindering the further development of such solar cells. Meanwhile, as a quinary compound, CIGS has low thermodynamic stability, which increases the difficulty of accurately controlling the composition and defects in a CIGS-based system. Therefore, alternative cheap, stable, and Earth-abundant absorber materials with good photoelectric properties are needed for obtaining efficient thin-film solar cells.

The chalcostibite (CuSbS₂) semiconductor has attracted attention as a light absorber in solar cells owing to the obvious advantages of being cheap and abundant. As a naturally stable mineral with massive reserves, CuSbS₂ is abundant and cheap. The elemental abundances of Cu, Sb, and S are 60, 0.2, and 350 ppm, respectively (1 ppm= 1.0 mg/kg) [3], and can be purchased at 5624, 5456, and 102 US dollar/ton, respectively (data from London Metal Exchange (LME)). In terms of cost and reserve, CuSbS₂ is clearly superior to In, Ga, Se, Cd, and Te in the conventional photovoltaic absorbers, i.e., CIGS and CdTe (Fig. 1). The nontoxicity of CuSbS₂ is a further advantage in commercial applications. As a ternary compound, CuSbS₂ is similar to CuInSe₂ but exhibits better properties in case of photovoltaic applications. Theoretical simulations and experimental results have confirmed a suitable direct bandgap in CuSbS₂ (1.38–1.5 eV), indicating the high efficiency limit of single-junction solar cells [4–7]. Further, the spectroscopic limited maximum efficiency (SLME) was calculated; thus, the theoretical efficiency limit of CuSbS₂ was observed to be greater than that of CuInSe₂ by 23% when the absorber thickness was 0–500 nm [8]. In addition, due to the 5s² lone pair electron effect on Sb³⁺, the electronic transition probability from orbits d to p and s to p effectively increased, leading to a higher optical absorption coefficient when compared with that of CuInSe₂. Accordingly, the CuSbS₂ semiconductor material has received much focus as the light absorber.

With the increasing exploration of the CuSbS₂ absorber, its derivative, bournonite (CuPbSbS₃), has also drawn considerable attention. CuPbSbS₃ is derived from a CuSbS₂ system in which a PbS structure is incorporated, resulting in the three-dimensional (3D) crystal structure of CuPbSbS₃. This new semiconductor is abundantly available and stable. Pb has a natural abundance of 14 ppm and costs 1878 US dollar/ton in the market, achieving lower-cost photovoltaics than CIGS and CdTe (Fig. 1). CuPbSbS₃ possesses a suitable direct bandgap, a larger optical absorption coefficient than that of CuSbS₂, and other good photoelectric properties [9]. In addition, the melting points of CuPbSbS₃ and CuSbS₂ are 522°C and 551°C, respectively [10,11], considerably lower than that of CIGS (1070°C) [12]. Therefore, micron-sized grains of CuPbSbS₃ and CuSbS₂ are easily obtained at sintering temperatures of 300°C–400°C [13–15], indicating the possibility of low-temperature and large-scale manufacturing that would reduce the cost of photovoltaic applications.

Herein, we will review the major factors associated with CuSbS₂ and CuPbSbS₃, including their photoelectric properties and stabilities, and the recent progress with respect to photovoltaic devices. The future developmental potential and limitations of photovoltaic technologies are also discussed.

Fig.1 Comparison of the elemental abundance versus price

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2 Material properties

2.1 Crystal structure and electronic dimensionality

CuSbS₂ is an orthorhombic system having a two-dimensional (2D) layered structure comprising twisted Cu–S pentahedra and Sb–S tetrahedra. Each unit cell contains four Cu atoms, four Sb atoms, and eight S atoms, where the Sb atom is three-coordinated with the surrounding S atoms and the Cu atom is four-coordinated with the S atoms (Fig. 2). The lattice parameters are a = 6.018 Å, b = 3.796 Å, c = 14.495 Å, and α= β =γ = 90°. The 2D monolayered crystal structure prevents the overlap of the electron clouds between the layers, allowing efficient transportation of the carriers in an intra-layer manner but blocking their transportation along the out-of-plane direction. Consequently, the transportation of carriers is limited within CuSbS₂.

To construct a 3D crystal structure and achieve better transportation of the carriers, the strategy of incorporating PbS into CuSbS₂ is proposed [13]. After the incorporation of PbS, CuSbS₂ transforms to the new phase of CuPbSbS₃, where an orthorhombic system is maintained but the dimensionality of the crystal structure is altered from 2D to 3D (bulk; Fig. 2). The CuPbSbS₃ structure comprises twisted Cu–S pentahedra, Pb–S octahedra, and Sb–S tetrahedra. Each unit cell contains four Cu atoms, four Pb atoms, four Sb atoms, and twelve S atoms. The lattice parameters are a = 8.153 Å, b = 8.692 Å, c = 7.793 Å, and α = β = γ = 90° [10,16]. Within such a 3D structure, the carriers can move in all the directions, improving the transportation properties of CuPbSbS₃.

Fig.2 Crystal structures of CuSbS₂ and CuPbSbS₃

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It is better to account for the photovoltaic properties, such as bandgap, via electronic dimensionality than via structural dimensionality [17]. A high electronic dimensionality is preferred in case of high-performance photovoltaic absorber materials, whereas a 3D crystal structure is a requirement for ensuring high electronic dimensionality. As a 2D structural material, the electronic dimensionality of CuSbS₂ is restricted. However, the 3D CuPbSbS₃ is expected to demonstrate advanced electronic dimensionality.

The band structure obtained via density functional theory (DFT) (Fig. 3) was demonstrated to explain the electronic dimensionality of CuPbSbS₃. The conduction band minimum (CBM) and valence band maximum (VBM) connect three-dimensionally and disperse along all the directions, confirming 3D electronic dimensionality and leading to the transportation of mobile charge carriers along all the directions. The high electronic dimensionality of CuPbSbS₃ enables an isotropic current flow and improved performance. Therefore, CuPbSbS₃ is a promising semiconductor absorber in high-efficiency solarcells.

Fig.3 Band structures of (a) CuSbS₂ and (b) CuPbSbS₃ obtained via DFT [13]. Copyright 2020. Reproduced with permission from Elsevier

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2.2 Optical absorption and band energy

Previous theoretical studies have predicted that CuSbS₂ and CuPbSbS₃ have good optical absorption. Meanwhile, their absorption coefficients were obtained by fitting to the experimental transmission spectra. The absorption coefficient α is related to the transmission T as

(1)\alpha ={\displaystyle \frac{\ln (\mathrm{1}/T)}{d}},

where d is the thickness of the material. Figures 4(a) and 4(b) show the fitting results over the range of visible wavelengths, where the absorption coefficients of CuSbS₂ and CuPbSbS₃ exceed 7 × 10⁴ and 4 × 10⁵ cm⁻¹, respectively. These absorption coefficients are higher than those of most photovoltaic absorbers, demonstrating that the CuSbS₂- and CuPbSbS₃-based devices achieve high photoelectric conversion with nanometer-scale thickness.

The experimental bandgaps of CuSbS₂ and CuPbSbS₃ have been fitted using the absorption coefficient α. The fitting curves (Figs. 4(a) and 4(b) inset) demonstrate bandgap values of 1.4 and 1.31 eV, respectively, which are in agreement with the previous theorical values [11,13,18]. Hoang and Mahanti comprehensively considered multiple factors, including the bandgap, material absorption spectrum, and non-radiative transition, and reported that the maximum theoretical efficiency of CuSbS₂ solar cells can exceed 23% [8]. Meanwhile, the bandgap of CuPbSbS₃ (1.31 eV) is suitable with respect to solar cell absorbers. According to the relation between the bandgap and maximum efficiency of solar cells, this value could possibly foreshadow the optimal efficiency limit of 33% [19].

CuSbS₂ and CuPbSbS₃ were subjected to ultraviolet photoelectron spectroscopy (UPS) for determining the band energy (Figs. 4(c) and 4(d)). The Fermi energies E_F of CuSbS₂ and CuPbSbS₃ are 4.86 eV (ultraviolet photon energy: 21.2 eV, He–I excitation). The linear fittings of the UPS spectra of CuSbS₂ and CuPbSbS₃ in the long tails generate extrapolations of 0.39 and 0.38 eV, respectively, which correspond to the distance between E_F and VBM. The band energy could be calculated according to bandgaps of 1.4 and 1.31 eV. CuSbS₂ has the VBM at −5.25 eV and CBM at −3.85 eV, whereas CuPbSbS₃ has the VBM at −5.24 eV and CBM at −3.93 eV. In addition, CuSbS₂ and CuPbSbS₃ have been confirmed as p-type semiconductors. Owing to their band energy structures, CuSbS₂ and CuPbSbS₃ easily match with the major buffer layers such as CdS. Table 1 summarizes the common material parameters and properties of CuSbS₂ and CuPbSbS₃.

Fig.4 (a) and (b) Optical absorption coefficients of CuSbS₂ and CuPbSbS₃, and their linear fittings that are extrapolated to the bandgaps (insets). (c) and (d) UPS spectra of CuSbS₂ and CuPbSbS₃. Insets show the magnified low-energy spectra and their linear fittings [11,13]. Copyright 2014. Reproduced with permission from ACS Publications; copyright 2020. Reproduced with permission from Elsevier

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2.3 Stability

As natural minerals, CuSbS₂ and CuPbSbS₃ are stable on Earth, indicating their stable physical and chemical properties, including thermal stability, moisture stability, and oxygen stability.

The thermogravimetric analysis (TGA) curves were employed to explain the thermal stability. Figure 5 shows the phase change process with temperature. Below 220°C, the Cu–Sb–S precursor of CuSbS₂ was endothermically decomposed via a two-stage process: evaporation of free hydrazine and dissociation of the hydrazinium species at 100°C–130°C and removal of excess sulfur at 130°C–220°C. This decomposition was followed by stable grain/film formation at 220°C–390°C. Meanwhile, CuPbSbS₃ decomposed by the dissociation of the Cu/Pb/Sb complex and volatilization of the organic solvent before 205°C, followed by stable grain/film formation from 205°C to 480°C. These processes demonstrate that CuSbS₂ and CuPbSbS₃ formed stable grains and steady films at 390°C and 480°C, respectively, indicating their high thermal stability.

Fig.5 TGA curves of (a) CuSbS₂ powder and (b) CuPbSbS₃ solution [11,13]. Copyright 2014. Reproduced with permission from ACS Publications; copyright 2020. Reproduced with permission from Elsevier

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In case of photovoltaic absorbers, one major factor is chemical stability, which depends on their reaction with the environment (oxygen, moisture, UV light, etc.). The perovskite solar cells deliver high performance but suffer from severe instability [20], mainly because perovskite is sensitively degraded by moisture and oxygen. Exposure to UV light would promote the decomposition of perovskite. In contrast, CuSbS₂ and CuPbSbS₃ resist decomposition and environmental reactions under normal conditions because their elements and valences are stable. Although Cu is monovalent in these compounds, the steady Cu–Sb–(Pb)–S bonds resist oxidization. Moreover, all these compositions are barely soluble in water. Therefore, both these materials present high chemical stability.

Furthermore, the device stability has been confirmed based on the aforementioned physically and chemically stable properties. Banu et al. evaluated the stability of the CuSbS₂ solar cells [21]. The PCE did not decrease after 3.5 months in an air atmosphere and slightly increased after 7 months of air storage. The stability of the CuPbSbS₃ solar cells has also been evaluated [13]. The PCE decreased from 2.23% to 1.80% after one month in air, mainly owing to the poor air stability of the Spiro-OMeTAD hole transport layer.

Tab.1 Material and optoelectronic properties of CuSbS₂ and CuPbSbS₃

		CuSbS2	CuPbSbS3
mineral species		chalcostibite	bournonite
crystal system		orthorhombic	orthorhombic
space group		Pnma (No. 62)	Pmn21 (No. 31)
lattice parameter/Å	a	6.018	7.885
	b	3.796	8.287
	c	14.495	8.816
crystal structure		2D layer	3D network
electronic dimensionality		<3D	3D
absorption coefficient/cm−1 (visible wavelength)		>7 × 104	>4 × 105
bandgap/eV		1.40	1.31
maximum efficiency limit		23%	33%
CBM/eV		−3.85	−3.93
VBM/eV		−5.25	−5.24
Fermi energy/eV		−4.86	−4.86
conduction type		p	p
hole mobility/(cm2·v−1·s−1)		49	7
carrier concentration/cm−3		2.66 × 1018	6.08 × 1014
dielectric constant		~13	7.1–7.6
melting point/°C		551	522
density/(g·cm−3)		5.03	5.63

3 Device fabrication and performance

Until now, the mainstream and effective methods for preparing CuSbS₂ films and devices are the vacuum and solution methods. Vacuum methods were employed to produce the CuSbS₂ film some time back. In 2008, Rabhi et al. fabricated CuSbS₂ film via a single-source vacuum thermal evaporation method [22]. The CuSbS₂ powder evaporated and deposited on unheated glass in vacuum, and the film was then annealed at 200°C for 2 h. Without the high-temperature heat treatment, the film exhibited an amorphous structure during characterization analysis. The CuSbS₂ film became polycrystalline only after annealing at temperatures of greater than 200°C. Although the fabrication is complex, this vacuum evaporation method achieved large-scale and uniform fabrication. In 2011, Garza et al. proposed a new evaporation method [23], in which the Cu film was deposited onto the Sb₂S₃ film formed by chemical bath deposition (CBD) on glass. After the Cu film was deposited by thermal evaporation, the Sb₂S₃/Cu layer was annealed at 300°C–380°C. The XRD result showed that the Sb₂S₃/Cu layer would transform to the pure orthorhombic CuSbS₂ film during the high-temperature annealing process. Meanwhile, the Hall effect measurements were performed to confirm the p-type conductivity of CuSbS₂. Wan et al. subsequently proposed a two-stage co-evaporation process to obtain the film and construct an efficient device [24]. In this process, an Sb-rich precursor was deposited by co-evaporating Cu, Sb, and S at a low substrate temperature (230°C). Sb and S were then co-evaporated at a higher temperature (370°C) to obtain the film. This process improves the crystallinity and phase purity of the CuSbS₂ film. When constructed into a solar cell device having a structure of Mo/CuSbS₂/CdS/ZnO/ZnO:Al/Ag, the film achieved an encouraging PCE of 1.9% with a high open-circuit voltage (V_OC) of 526 mV. The co-sputtering processes for CuSbS₂ were recently explored by Saragih et al. [25]. They obtained a CuSbS₂ film layer on the TiN-coated Mo/glass substrate by the co-sputtering technique at 300°C, with a Cu and Sb₂S₃ cermet target at 50–60 W and a Cu metal target at 2 W, and annealing was subsequently performed at 350°C–450°C for 1 h. Based on the application of GaN and In_0.15Ga_0.85N as n-type bilayers, a solar cell device was constructed, achieving a PCE of 2.99%. Welch et al. successfully fabricated a CuSbS₂ film after magnetron co-sputtering of the Cu₂S and Sb₂S₃ targets [26].

The solution methods were utilized to produce films and devices. The first complete photovoltaic device based on a CuSbS₂ absorber was fabricated via CBD in 2005. This device achieved a V_OC of 345 mV and a J_SC of 0.2 mA·cm⁻², which mean the first observable photovoltaic performance of CuSbS₂ solar cell [27]. Since that time, solution preparation methods for CuSbS₂ films and devices have proliferated, delivering encouraging performances. Some of the existing solution methods include spray pyrolysis, electrodeposition, and spin coating. Using a precursor of CuCl₂·2H₂O, (CH₃COO)₃Sb, and H₂NCSNH₂ (as the sulfur source) in an aqueous solution, Manolache et al. deposited a CuSbS₂ film by the spray pyrolysis process at 240°C [28]. The resulting film was free of pinholes and rich in antimony. In a demonstration of electrochemical deposition, Septina et al. [29] first deposited Cu in an electrolyte containing CuSO₄ and citric acid and then deposited an Sb layer in an electrolyte containing K₂(Sb₂(C₄H₂O₆)₂), and tartaric acid. After deposition, the metal precursor was sulfurized under H₂S flow, forming CuSbS₂. This approach yielded a solar cell with a glass/Mo/CuSbS₂/CdS/ZnO:Al structure, a PCE of 3.13%, and a high V_OC (490 mV). Spin coating is the most effective method for fabricating efficient devices. In 2016, Banu et al. constructed a glass/Mo/CuSbS₂/CdS/i-ZnO/n-ZnO/Al structure with a PCE of 3.22% [21], which was the highest PCE reported in CuSbS₂ solar cells at that time (Fig. 6(a)). Further, some treatments, such as post-annealing treatments, have been explored to improve the performance of the CuSbS₂-based solar cells [30].

The device applications of the CuPbSbS₃ derivative were first proposed several decades ago. In 1973, Frumar et al. synthesized the first single-crystal CuPbSbS₃ and studied its physical properties [10]. Recently, Tablero explored the optical properties of CuPbSbS₃ and proposed its usage in photovoltaic applications [9]. Also, Liu et al. reported the first CuPbSbS₃ solar cell with a glass/ITO/CdS/CuPbSbS₃/Spiro-OMeTAD/Au structure. This cell was produced in solution via the spin coating process [13]. The new butyldithiocarbamate (BDCA, C₅H₁₁NS₂) solution method prepares a high-quality CuPbSbS₃ precursor, yielding a high-performance device after spin coating and annealing. This device achieves a PCE of 2.23% and a V_OC of 699 mV (Fig. 6(b)). Table 2 summarizes the developmental progress of efficient devices (with PCEs>1.9%) based on the CuSbS₂ and CuPbSbS₃ solar cells since 2014.

Fig.6 (a) CuSbS₂ device structure exhibiting the best performance until now. (b) CuPbSbS₃ device structure exhibiting the best performance until now [13,21]. Copyright 2016 and 2020. Reproduced with permission from Elsevier

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Tab.2 Fabrication methods and performances of the efficient CuSbS₂ and CuPbSbS₃ devices (PCEs>1.9%) developed since 2014

absorber	device structure	fabrication	VOC/mV	JSC/(mA∙cm−2)	FF/%	PCE/%	year	Ref.
CuSbS2	glass/Mo/CuSbS2/CdS/ZnO:Al	electrochemical deposition	490	14.73	44	3.13	2014	[29]
CuSbS2	glass/FTO/TiO2/mp-TiO2/CuSbS2/HTM/Au	metal/thiourea+ spin coating	304	21.50	46.8	3.10	2015	[31]
CuSbS2	glass/Mo/CuSbS2/CdS/ZnO/ZnO:Al/Ag	two-stage co-evaporation	526	9.57	37.4	1.90	2016	[24]
CuSbS2	glass/Mo/CuSbS2/CdS/i-ZnO/n-ZnO/Al	spin coating	470	15.64	43.56	3.22	2016	[21]
CuSbS2	glass/Mo/TiN/CuSbS2/GaN/In0.15Ga0.85N/ITO	co-sputtering	295	33.78	30	2.99	2017	[25]
CuPbSbS3	glass/ITO/CdS/CuPbSbS3/HTM/Au	BDCA solution + spin coating	699	8.19	39	2.23	2020	[13]

4 Future developments and limitations

CuSbS₂ and CuPbSbS₃ are promising absorber materials in photovoltaic applications. Their main materials and photoelectric properties have been proved previously, but the exploration of devices is in the initial stages.

Owing to their different material properties, CuSbS₂ and CuPbSbS₃ have unique advantages and disadvantages. The simplicity of components, high hole mobility, and large dielectric constant of CuSbS₂ make it a better absorber candidate than CuPbSbS₃; as such, the former has attracted more attention. However, the high doping concentration causes excessive conductivity and degenerates the semiconductor behavior; moreover, the low electronic dimensionality of the 2D monolayered crystal structure constrains the transportation of charge carriers. CuPbSbS₃ is promising because of its large optical absorption coefficient, optimal bandgap, suitable doping concentration, and other beneficial properties; however, its further development must overcome two major hurdles, i.e., the difficulty of controlling and fabricating complex quaternary components and the toxicity of Pb. These problems may be overcome via the usage of advanced fabrication processes in the future. For example, device encapsulation may be a solution to prevent toxicity. Moreover, the large Pb atoms increase the density of CuPbSbS₃ and improve the absorption of high-energy photons. Therefore, this material is expected to be used in future X-ray detection applications.

With regard to device fabrication, CuSbS₂ and CuPbSbS₃ devices with the highest performances were fabricated via the solution process followed by spin coating. The components of ternary and quaternary compounds are better controlled by the solution methods than by the vacuum methods. However, the solution methods introduce more defects than the vacuum methods and decrease the transportation ability of the formed film. These problems can seriously limit the device performance. Although solution methods can become the future direction of device fabrication, decreasing the defects and improving the transportation of charge carriers within the absorber film are demanded.

5 Conclusions

The cuprous antimony disulfide (CuSbS₂) and its derivative, cuprous lead antimony trisulfide (CuPbSbS₃), are cheap, abundant, and stable. Furthermore, they exhibit good photoelectric properties, indicating their potential for application as light absorbers in future photovoltaic technologies. Solar cell devices have been developed based on these materials, and excellent performances have been achieved, encouraging further research and applications. We believe that further explorations will lead to new breakthroughs with respect to device performance. The cuprous antimony chalcogen materials and their derivative, represented by CuSbS₂ and CuPbSbS₃, respectively, are expected to become research hotspots in case of photovoltaic technologies.

Acknowledgments

This work was financially supported by the National Natural Science Foundation of China (Grant Nos. 61725401, and 61904058), the National Key R&D Program of China (No. 2016YFA0204000), the China Postdoctoral Science Foundation (Nos. 2018M642825 and 2019M662623), the National Postdoctoral Program for Innovative Talent (No. BX20190127), and the HUST Key Innovation Team for Interdisciplinary Promotion (Nos. 2016JCTD111 and 2017KFXKJC003). The authors would also like to thank the Analytical and Testing Center of HUST and the facility support of the Center for Nanoscale Characterization and Devices (CNCD), WNLO, HUST.

Conflicts of Interest

The authors declare no conflict of interest.

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