A comprehensive review on synthesis and characterizations of Cu₃BiS₃ thin films for solar photovoltaics

Abstract

Since the last few decades, light-absorbing materials based on CuInGaSe₂ (CIGS), CuInS₂ (CIS), and CdTe have dominated the research in thin-film solar cells. To fabricate large-scale solar cells from these materials, problems may arise due to limited availability of the constituents, viz. Se, In, Cd, and Te, and the toxicity of some of these elements. Hence, recent research efforts are attentive toward abundantly available non-toxic, larger value of absorption coefficient and non-conventional elements. The Cu₃BiS₃ having wittichenite orthorhombic structure is one the most promising absorber layer candidates for low-cost thin-film solar cells. It has suitable direct band gap (1.10–1.86 eV), large absorption coefficient (10⁵ cm⁻¹) with composition of earth abundant, and relatively non-toxic and cost-effective constituents. Till now, a majority work was done on the preparation of Cu₃BiS₃ thin films by various techniques. Therefore, a comprehensive review of recent literature of Cu₃BiS₃ is urgently required. This paper will review the various techniques that have been used to deposit Cu₃BiS₃ semiconductor with the hope of new paths for the beginner.

Introduction

The thin-film-based solar cell (TFSC) generally use polycrystalline copper indium diselenide (CIS), cadmium telluride (CdTe), or copper indium gallium selenide (CIGS) for commercialization with reported conversion efficiency ranging from 14.6 to 22.3% [1,2,3,4]. However, there are limitations to use these materials in the production of photovoltaic (PV) devices as an absorber layer due to toxicity of Cd and Se as well as limited availability of Te, Cd and In [5]. Bismuth is non-toxic and easily available as compared to indium. The US Geological survey assessed in 2014 reported that the world mine production of Bi is 7600 metric tons with world reserves of 320,000 metric tons as compared to In refinery production of 770 metric tons with no estimate of reserves. The cost of Bi in 2013 was $17.4 per kg, while for In it was $620 per kg [6]. Therefore, current research attentions are directed towards an absorbent material that is non-toxic, convenient, and cost-effective. In this context, Cu₃BiS₃ ternary semiconductor compound has emerged as one of the promising candidates for solar absorber material. All the constituents of Cu₃BiS₃ are low cost, non-toxic, earth abundant, and environmentally friendly.

Basics of Cu₃BiS₃ compound and solar cells

The compound Cu₃BiS₃ has been found in nature as the mineral wittichenite, from the Wittichen mine [7]. In the mineral form, the structure (Fig. 1) of the Cu₃BiS₃ compound has been determined as orthorhombic (P2₁2₁2₁: space group no.19) with a = 7.723 Å, b = 10.395 Å, and c = 6.716 Å [8]. The density of the synthesized Cu₃BiS₃ is 6.01 × 10³ kg/m³ and calculated mass density is 6.11 × 10³ kg/m³, the same as that of an ideal cell dimension with 4 (Cu₃BiS₃) per cell [9]. Cu₃BiS₃ is a ternary compound of I₃–V–VI₃ and is a key member of the copper-based multicomponent chalcogenides (CBMC) family. Cu–Bi–S system alloys are made up of 13 compounds due to variation in the stoichiometric ratio of three elements, which are stable at room temperature [10]. More importantly, Cu₃BiS₃ has a p-type conductivity, stronger absorption coefficient (>10⁵ cm) [11] in the visible wavelength region and a direct band gap of 1.10–1.86 eV [12, 13]. The copper–bismuth–sulfide forms in several different phases. The phase diagram of the Cu–Bi–S system at 573 K is shown in Fig. 2 [14]. The investigation on the phase diagram of the Cu–Bi–S system has shown that a single-phase Cu₃BiS₃ material can be formed only in a small region.

Fig. 1

The crystal structure of Cu₃BiS₃ compound (Taken from [8])

Fig. 2

Phase diagram of Cu–Bi–S system at 573 K [14]

For the synthesis of Cu₃BiS₃ thin films as a light absorber, many physical and chemical techniques have been employed, such as sputtering, thermal evaporation, spray pyrolysis technique, electrodeposition, solvothermal, cyclic microwave radiation, hydrothermal, facile biomolecule-assisted solvothermal, hot injection solution, and chemical bath deposition (CBD) technique respectively. The purpose of employing these methods is to develop a low-cost and highly efficient absorber layer for Cu₃BiS₃ thin-film solar cell. In the recent years, many research groups have studied the synthesis and characterization of Cu₃BiS₃ and few of them showed applications in thin-film solar cells. Therefore, all the different techniques employed for the synthesis of Cu₃BiS₃ thin films are summarized in this review.

The device configuration of SLG/Mo/Cu₃BiS₃/CdS/ZnS/Al:ZnO thin-film-based solar cell to study the PV performance is as shown in Fig. 3. The device consists of Mo-coated soda lime glass (SLG), an electrical contact, a layer of Cu₃BiS₃ as a light absorber layer which is in contact with n-type CdS or ZnO to form a p–n junction and thin-layer Al:ZnO on the top of the buffer layer playing the role of a window layer and electrical contact.

Fig. 3

Schematic structure of Cu₃BiS₃ thin-film-based solar cell

Synthesis techniques of Cu₃BiS₃

The techniques for the synthesis of Cu₃BiS₃ thin films can be classified into two categories: vacuum- and non-vacuum-based techniques. According to the method used for the synthesis of Cu₃BiS₃, each technique has a few sub classifications.

Vacuum-based deposition techniques

Vacuum-based deposition methods normally involve deposition of the constituent atoms of the Cu₃BiS₃ compound on a substrate either by evaporation/co-evaporation or by sputtering of the target sources under optimized pressure and temperature. These techniques have the benefit of fabrication of high-quality thin-film devices, good reproducibility, and easy and direct control over the chemical composition of the sample. These vacuum-based deposition methods can be sub-classified into sputtering, evaporation/co-evaporation, etc.

Vacuum-based sputtering deposition method

To deposit high-quality thin films, many researchers have used sputtering deposition technique. Different sputtering technologies viz. ion beam, argon beam, DC, RF, and reactive magnetron sputtering have been employed for the deposition of Cu₃BiS₃ thin films [11, 15,16,17]. Cu₃BiS₃ thin films have been deposited using two different methodologies: a single step without sulfurization and a two-step deposition of either metallic precursor Cu–Bi/Cu–S–Bi followed by a sulfurization.

In 2006, Gerein and Haber [15] for the first time deposited Cu₃BiS₃ thin films by sputtering. The Cu₃BiS₃ thin film was synthesised in an H₂S atmosphere where Cu–Bi metal precursor films and Cu–S–Bi metal sulfide precursor films were sputter deposited on fused silica substrate. It was reported that pure orthorhombic phase was obtained at the film thickness of 250–1000 nm. It is also observed that the precursor composition determines the reaction pathway which becomes the dominant factor in controlling the morphology of Cu₃BiS₃ thin films. The deposited Cu₃BiS₃ film had electrical resistivity ranges from 3 to 200 Ω cm. Later on, the same research group [11] reported the synthesis of Cu₃BiS₃ thin films on fused silica substrates in one-step process by reactive sputtering of Cu–S and Bi on hot substrates. The produced thin films of Cu₃BiS₃ are of crystalline phase, smooth, dense, and continuous with direct band gap of 1.4 eV, an absorption coefficient of 1 × 10⁵ cm⁻¹, p-type conductivity and electrical resistivity of 84 Ω cm. It is also reported that the crystallite size increased and electrical resistivity decreased to 9.6 Ω cm when Cu₃BiS₃ films are annealed in H₂S atmosphere.

Yakushev et al. [16] fabricated the Cu₃BiS₃ thin films by using magnetron sputtering in two-steps. At first, 0.3 μm thick precursor layers of Cu and Bi were sputtered on Mo-coated SLG from 5 N purity elemental targets. Thick films of 4 N purity sulfur of thickness 1.5 μm were thermally evaporated on these precursor layers followed by heating at 250 °C for 30 min in Ar atmosphere. The synthesized Cu₃BiS₃ thin film had orthorhombic structure, four Raman modes with dominant peak at 292 cm⁻¹, and two bright, broad emission bands at 0.84 and 0.99 eV in XRD, Raman and photoluminescence spectra, respectively. In 2014, the same research group [17] reported the structural, elemental composition, optical, and electronic properties of p-type Cu₃BiS₃ thin films by adopting the similar deposition procedure [16]. The deposited Cu₃BiS₃ thin films exhibit single orthorhombic phase with the stoichiometry Cu_3.0Bi_0.92S_3.02, photoreflectivity at 10 K expressed two band gaps at 1.24 and 1.53 eV and two broad bands at 0.99 and 0.84 eV in low-temperature PL spectra [17].

Vacuum-based evaporation deposition method

Evaporation techniques are the normal choice of every researcher and being used for the deposition of wittichenite Cu₃BiS₃ absorber due to the previous success of evaporated absorber materials like CdTe [18], CIGS [19], and CZTS [20]. Numerous evaporation methods such as co-evaporation, fast evaporation, thermal evaporation, and electron beam (EB) [21,22,23,24,25,26,27,28,29,30,31,32] have been employed for the deposition of Cu₃BiS₃ thin films. Cu₃BiS₃ thin films were deposited using two different attitudes:

• 1. a single-step: simultaneous deposition of all precursors followed by sulfurization

• 2. a two-step: sequential deposition of metallic Cu–Bi–Cu–S or CuS–Bi₂S₃, Bi _x S _x –Cu followed by a annealing/sulfurization.

First time, in 2009 Mesa and Gordillo [21] reported preparation of Cu₃BiS₃ thin films on a SLG substrate by co-evaporation in a two-step process. In the first step, Bi _x S _x layer was developed by simultaneous evaporation of Bi and S, keeping the substrate temperature at 300 °C. In the second step, Cu₃BiS₃ was formed by evaporating Cu in the sulfur environment on the Bi _x S _x layer at 300 °C. The XRD revealed that the film grown only in the orthorhombic Cu₃BiS₃ phase. The deposited Cu₃BiS₃ thin film had a p-type conductivity, a high absorption coefficient (>10⁴ cm⁻¹), and optical energy gap 1.41 eV, indicating Cu₃BiS₃ had best property to perform as an absorber layer in PV solar cell. Furthermore, Mesa et al. [22] deposited Cu₃BiS₃ thin films by a two-step evaporation process on glass substrates. In the first stage, a Bi thin layer was deposited with a flux of about 1 Å/s, and in second stage Cu is evaporated keeping the flux of about 0.8 Å/s. In both stages, S environment was produced by the evaporation of elemental sulfur at temperature of 383 K and substrate was kept at 573 K. It was reported that the energy required a carrier to jump from one localized state to another increased with the temperature and Cu content in the Cu₃BiS₃. The same group later [23] reported the preparation of Cu₃BiS₃ on SLG substrate in a two-step process by co-evaporation. In the first stage, a Bi₂S₃ layer was grown by simultaneous evaporation of Bi and S by keeping the substrate temperature 300 °C. In the second stage, Cu was evaporated in a sulfur environment over the Bi₂S₃ layer at temperature ~300 °C to form Cu₃BiS₃.

The proposed reaction mechanism for the formation of Cu₃BiS₃ is as follows:

$$ \frac{2}{3}{\text{S}}_{2} + 2{\text{Bi }}\mathop \to \limits^{{300\;^{ \circ } {\text{C}}}} {\text{Bi}}_{2} {\text{S}}_{3} $$

(1)

$$ 2{\text{Cu}} + \frac{1}{2}{\text{S}}_{2} \to {\text{Cu}}_{2} {\text{S}} $$

(2)

$$ {\text{Bi}}_{2} {\text{S}}_{3} + 3{\text{Cu}}_{2} {\text{S }}\mathop \to \limits^{{300\;^{ \circ } {\text{C}}}} 2{\text{Cu}}_{3} {\text{BiS}}_{3} $$

(3)

The result revealed that the Cu₃BiS₃ film had p-type conductivity, a high absorption coefficient (>10⁴ cm⁻¹) and energy band gap of 1.39 eV. It was also reported that grain size and electrical conductivity of Cu₃BiS₃ were influenced by the Cu mass ratio. Then, Mesa et al. [24] synthesized thin films of Cu₃BiS₃ by co-evaporation as described earlier [22, 23]. Hall Effect, Seebeck effect, and surface photovoltage (SPV) measurement showed that Cu₃BiS₃ had a p-type semiconductor with Hall mobility, free carrier concentration, and thermoelectric power of 4 cm²/V s, 2 × 10¹⁶ cm⁻³ and 0.73 mV/K, respectively. The work function of Cu₃BiS₃ was reported 4.37 ± 0.04 eV before and 4.57 ± 0.01 eV after deposition of In₂S₃.

Later on, Mesa et al. [25] investigated the formation of ZnS, In₂S₃, and CdS buffer layers on Cu₃BiS₃ for application as an absorber layer in solar cell. The Cu₃BiS₃ and ZnS (thickness 200 nm) thin films were deposited by the co-evaporation techniques reported previously [23, 26]. The buffer layer of In₂S₃ was deposited by co-evaporation of In and S on the substrate heated to ~300 °C having thickness ~150 nm. The CdS thin films of thickness ~80 nm were deposited on Cu₃BiS₃ from a solution containing thiourea and cadmium chloride as sources of S²⁻ and Cd²⁺, respectively. Kelvin probe force microscopy (KPFM) showed the granular structure of the buffer layers with small grains of 20–100 nm and considerably smaller work function distribution for In₂S₃ compared to that of CdS and ZnS. For In₂S₃ and CdS buffer layers, KPFM indicated negatively charged Cu₃BiS₃ grain boundaries. In 2012 Mesa et al. [27] presented the results from a study held on Al/Cu₃BiS₃/Buffer/ZnO, using a high-resolution transmission electron microscopy (HRTEM) with In₂S₃ and ZnS as a buffer layer. The Cu₃BiS₃ was prepared by two-stage co-evaporation process with film thickness of ~1 μm as reported by Mesa et al. [21, 22]. The ZnS buffer layer was deposited by chemical bath deposition (CBD) method using thiourea and zinc acetate as a precursor solution for S²⁻ and Zn²⁺ source, respectively, with ammonia and sodium citrate as complexing agents. In₂S₃ was deposited by co-evaporating In and S on a substrate heated at a temperature 300 °C. The ZnO thin films were deposited at room temperature on glass/Al/Cu₃BiS₃/Buffer system by chemical reaction between ionized zinc and oxygen: Zn⁺–1e + O⁻ + 1e⁻ → ZnO. The Cu₃BiS₃ deposited on Al has a nanocrystalline structure with grain size around 10 nm. It was reported that the buffer layer of In₂S₃ grows in a polycrystalline nature, whereas ZnS in an amorphous nature. The ZnO layer grew into a wurtzite-type hexagonal structure, used as an optical window layer.

Furthermore, Dussan et al. [28] prepared Cu₃BiS₃ thin films on SLG substrates by evaporating Cu and Bi species in a sulfur environment through a two-stage process by varying the synthesis temperature between 473–573 K. The thermally stimulated current (TSC) spectrum showed three trapping levels around 1.04 eV. In Cu₃BiS₃ semiconductor, these three trapping levels may be associated with the presence of structural defects and unintentional impurities. Thereafter, Murali et al. [29] in 2014 reported the deposition of Cu₃BiS₃ thin films by co-evaporation of the Cu, Bi elemental precursors on a Mo-glass substrate with in situ sulfurization using a quartz effusion cell. The XRD pattern of Cu₃BiS₃/Mo/SLG stack showed Cu₃BiS₃ film was polycrystalline with preferred (131) orientation. The obtained Cu₃BiS₃ film had a high absorption coefficient (>10⁴ cm⁻¹), an energy band gap of 1.45 eV, and can be used as an absorber layer for a near-infrared photodetector. Then, Mesa et al. [30] presented the electrical properties of co-evaporated Cu₃BiS₃ by varying precursors mass ratio m_Cu/(m_Cu + m_Bi) in between 0.43 to 0.49. Hall effect measurement of Cu₃BiS₃ showed that the concentrations of n charge carriers are in the order of 10¹⁶ cm⁻³ irrespective of the Cu/Bi mass ratio. Also, the mobility of Cu₃BiS₃ (μ is of the order of 4 cm² V⁻¹ s⁻¹) varies accordingly to the transport mechanism and depended on temperature. From SPV measurement, a high density of surface defects was observed, which can be passivated by superimposing a buffer layer over the Cu₃BiS₃.

Recently, Mesa et al. [31] in 2015 presented the growth of In₂S₃ onto Cu₃BiS₃ and glass substrates by using CBD and co-evaporation. For this, Cu₃BiS₃ thin films were deposited as reported earlier [23] and In₂S₃ films were deposited on Cu₃BiS₃ by co-evaporation of In and S precursors. For CBD growth at 60 °C of In₂S_3, thin film on Cu₃BiS₃, InCl₃ and thioacetamide was used as a source for indium and sulfur. The film thickness of In₂S₃ was observed in between 80–170 nm. The XRD showed In₂S₃ films had a higher crystallinity when grew by co-evaporation than by CBD. Also, thin films of In₂S₃ with thickness less than 170 nm deposited by CBD had amorphous nature. However, when increasing the thickness, the films exhibit two diffraction peaks along (103) and (107) planes of the β-In₂S₃ tetragonal structure. Films of In₂S₃ grown by CBD had better coverage performance being suitable, to use as a buffer layer with lower thickness compared with those prepared by co-evaporation. Estrella et al. [32] reported the formation of Cu₃BiS₃ by heating a chemically deposited CuS thin film on which Bi was thermally evaporated. The possible film formation reaction was 3CuS + Bi → Cu₃BiS₃. For the formation of CuS, 5 ml of CuCl₂·5H₂O (1 M), 9 ml of Na₂S₂O₃ (1 M), and 10 ml of 0.5 M dimethylthiourea were used, and the deposition was carried out at 60 °C for 5 h. Then, Bi thin film of 100 nm was deposited by thermal evaporation on CuS thin film. The Cu₃BiS₃ thin film had an optical absorption greater than 10⁵ cm⁻¹, p-type conductivity of 0.03 Ω⁻¹ cm⁻¹, mobility lifetime 10⁻⁶ cm² V⁻¹, and photoconductive. Figure 4 shows a graphical representation of the reported band gap energy in eV in different synthesis methods of Cu₃BiS₃.

Fig. 4

Band gap energy (eV) of Cu₃BiS₃ reported by various synthesis techniques

Non-vacuum deposition techniques

In the last few decades, the majority of the Cu₃BiS₃ thin-film formation had been reported by various vacuum techniques as discussed above. Nowadays, it proved that an inexpensive deposition, thin-film-based solar cells are offered by the majority of non-vacuum methods. The vacuum-based technique consumes a very high energy, suffers from low material utilization, a small area deposition, expensive, and requires sophisticated instruments, whereas non-vacuum deposition techniques are of low cost and convenient for large area deposition, consume low energy, and do not require sophisticated instruments. Therefore, non-vacuum techniques have been developed to synthesis a ternary Cu₃BiS₃ thin film. This includes spray pyrolysis, solvothermal route, hydrothermal, spin coating, electrochemical deposition, and CBD. The same techniques had been widely used for the synthesis of CdTe [33], CIGS [34], and CZTS [35] semiconductor thin films. The non-vacuum deposition techniques used for the synthesis of Cu₃BiS₃ thin films are listed below:

Spray pyrolysis deposition technique

Spray pyrolysis deposition (SPD) technique does not require vacuum at any stage during the deposition process of thin films of any kind. This method is suitable for mass production with good reproducibility of the films. Considering all these advantages of SPD, attempts have been made in the synthesis of ternary Cu₃BiS₃ thin films. In 2015, for the first time, Liu et al. [36] reported the synthesis of Cu₃BiS₃ thin films by chemical spray pyrolysis technique. A precursor solution which contained 10 mM copper (II) chloride (CuCl₂), 3.8 mM bismuth (III) chloride (BiCl₃), and 50 mM thiourea in methanol solution was used. The precursor solutions were sprayed onto a heated glass substrate at 250, 300, 350, and 400 °C with solution flow rate 5 ml/min. The XRD pattern showed that as-deposited Cu₃BiS₃ thin films were wittichenite typed. The direct band gap of Cu₃BiS₃ films was reported in the range of 1.65–1.72 eV.

Solvothermal synthesis technique

In solvothermal synthesis technique, the reactions proceed in the thermal (100–300 °C) region so that the reaction velocity is easily controlled. This is favorable for the formation of crystal and to control the crystallite sizes. Therefore, the solvothermal method is suitable for the synthesis of novel nanomaterials with good crystallinity. First time, in 2012 Yan et al. [37] reported a solvothermal route to synthesize good-quality Cu₃BiS₃ nanoparticles at 180 °C for 20 h in a Teflon-lined autoclave containing Cu(NO₃)₂·3H₂O (2.42 g), Bi(NO₃)₃·5H₂O (0.97 g), and thiourea (1.52 g) dissolved in 25 ml of ethylene glycol (EG). The XRD pattern indicated the phase purity of Cu₃BiS₃ with crystalline grains of ~40 nm. The orthorhombic phase of wittichenite Cu₃BiS₃ with multi-armed microrod morphology was obtained when hypocrellins were used as a template. Then, Zeng et al. [38] successfully synthesized flower-like Cu₃BiS₃ hierarchical nanostructure. In this synthesis, copper chloride dihydrate (0.4276 g), and bismuth chloride (0.1570 g) (stoichiometric ration 3:1) were dissolved into ethanol (35 ml) and glycerol (50 ml) under vital stirring for 20 min, and then thiourea (0.38 g) was added into the solution directly. The whole solution was sealed into a Teflon-lined stainless-steel autoclave and maintained at 180 °C for 12 h. The XRD pattern of Cu₃BiS₃ showed the formation of wittichenite orthorhombic phase. The stoichiometric ratio and band gap of Cu₃BiS₃ thin films was reported 42:12:44 and 1.2 eV, respectively. This synthesized flower-like Cu₃BiS₃ nanostructure may be applied in solar cells.

Further, Murali et al. [39] reported the structural and optical properties of Cu₃BiS₃ nanopowder synthesized by solvothermal. The synthesis of Cu₃BiS₃ nanopowder involved 4 mM copper (II) acetylacetonate, 0.01 M bismuth chloride and 0.1 M thioacetamide into an autoclave at 170 °C for 8 h. It was shown that the obtained Cu₃BiS₃ product had a rod-like structure having the diameter of 60–200 nm and 1–2 μm in length with optical band gap 1.4 eV. Also, the IR photoresponse in-plane geometry was higher compared to sandwich geometry. Later on, Murali and Krupanidhi [13] studied the facile technique to synthesize high-quality Cu₃BiS₃ for photodetector applications. The manufacturing of Cu₃BiS₃ nanopowder by solvothermal at 170 °C for 8 h involved 3 mM CuCl, 0.01 M BiCl₃, and 0.1 M CS(NH₂)₂ with ethanol as a solvent. The band gap energy of Cu₃BiS₃ particles was varied from 1.86 to 1.42 eV. The Cu₃BiS₃ containing the secondary phases showed relatively higher band gaps. The IR photodetection of Cu₃BiS₃ was also exposed in terms of photocurrent and photoresponse. Furthermore, in 2013, the solvothermal synthesis of Cu₃BiS₃ with precursor complexion was reported by Viezbicke et al. [40]. For the preparation of Cu₃BiS₃, firstly 3 mM Cu(NO₃)₂·3H₂O and 3 mM l-cystein was dissolved in 50 mL EG, and then 1 mL Bi(NO₃)₃·5H₂O with 1 mM l-cystein was dissolved in separate 50 mL EG with stirring. These two solutions were mixed under stirring into a two-necked flask and heated by mantle at 187 ± 3 °C for 4 h. The XRD pattern confirmed a pure wittichenite [Fig. 5] crystal structure. The synthesized Cu₃BiS₃ had a direct band gap of 1.5 eV. SEM photographs exposed varying morphology dominated by the nanorods and included particles which had aspect ratio 1:1.

Fig. 5

XRD pattern for pure Cu₃BiS₃ Wittichenite crystal (taken from [40])

Recently, Murali and Krupanidhi [41] reported the preparation and application of Cu₃BiS₃ nanorods in infrared photodetection. The preparation of Cu₃BiS₃ nanorods by solvothermal was described as earlier [13]. They reported that photocurrent was enhanced to threefold from 3.47 × 10⁻⁷ to 2.37 × 10⁻³ A at 1 V for 10 mg nanorods embedded in polymer devices. Responsivity of hybrid device was also enhanced from 0.0158 to 102 A/W. The optical band gap of Cu₃BiS₃ had a value of 1.4 eV. The Cu₃BiS₃ could be promising material in the nano switchable near IR photodetectors. Thereafter, Yin and Jia presented the synthesis and characterization of Cu₃BiS₃ nanosheets on a TiO₂/FTO by solvothermal route. For the preparation of Cu₃BiS₃/TiO₂ composite thin film, CuCl (0.3 mM), Bi(NO₃)₃·5H₂O (0.1 mM), and C₂H₆OS (0.6 mM) were added to a mixed solution of glycerol and ethanol (volume ratio 1:1), and synthesis was carried out at 180 °C for 1, 3, and 6 h in an electric oven. Cu₃BiS₃ nanosheets of 30 nm thickness were successfully synthesized on TiO₂ nanorods. The reported energy conversion efficiency of the Cu₃BiS₃/TiO₂ thin-film solar cell was 1.281% [42]. This is the first report on the efficiency of Cu₃BiS₃ thin-film-based solar cell. The flower-like Cu₃BiS₃ was deposited on the TiO₂ nanotubes via a solvothermal method by Zhong et al. [43] at 180 °C for 12 h in an electric oven. The preparative precursor contained CuSO₄·H₂O, BiCl₃ and thiourea at suitable concentrations. Flower-like Cu₃BiS₃ sensitized TiO₂ NTs was successively fabricated. Zhong et al. reported that flower-like structure of Cu₃BiS₃ was composed by nanosheets of thickness 30 nm.

Moreover, Santhanapriya et al. [44] in February 2016 reported the synthesis of Cu₃BiS₃ and Cu₃SbS₃ for different Cu concentrations using EG and triethanolamine (TEA) as a solvent at 180 °C for 20 h. To synthesis Cu₃BiS₃ nanoparticles, Cu(NO₃)₂, Bi(NO₃)₂, and thiourea were used with EG. The XRD analysis confirmed the pure single-phase orthorhombic structure of Cu₃BiS₃ with an average crystallite size of 38 nm. The FESEM images indicated a flower-like structure and photoluminescence analysis showed strong emission at 455 nm for Cu₃BiS₃.

More recently, Gao et al. [45] reported the solvothermal synthesis of Cu₃BiS₃ for high performance lithium–sulfur batteries. For the synthesis of Cu₃BiS₃, CuCl₂·2H₂O (0.4276 g), BiCl₃ (0.1570 g) and thiourea (0.38 g) were dissolved in ethanol (35 mL) and glycerol (50 mL), and the reaction was carried out 180 °C for 12 h. The Cu₃BiS₃ had 3D flower-like ball morphology composed by misoriented and 2D thin nanosheets with outer diameter of 1.5–3.0 μm. The XRD of Cu₃BiS₃ 3D flower-like ball showed that all the diffraction peaks could be indexed to the orthorhombic phase. It was also shown that the Cu₃BiS₃/S flower exhibited a high initial capacity of 1343 mA h g⁻¹ at the current rate of 0.2C. A high specific capacity of 487 mA h g⁻¹ with a coulombic efficiency of 90% could be obtained at 0.2 C after 100 cycles. Figure 6 shows a graphical representation of the maximum reported particle size (nm) in different synthesis methods of Cu₃BiS₃.

Fig. 6

Maximum particle size (nm) of Cu₃BiS₃ reported by several synthesis techniques

Hydrothermal synthesis technique

During the last decade, the chemical solution routes were evolving as an effective, less energy, convenient, and material-consuming synthesis methods for materials synthesis. Hydrothermal method is one of the greatest emergent chemical solution methods owing to its high degree of compositional control of the stoichiometry. Hu et al. [46] reported the synthesis of Cu₃BiS₃ at 100–150 °C for 10 h in an autoclave having precursor solutions CuCl, BiCl₃, and thiourea. As-prepared Cu₃BiS₃ consists of whisker-like particles with an average size of 50 × 10 nm². The X-ray diffraction profile showed the formation of orthorhombic Cu₃BiS₃ phase with average particle size of about 35 nm. The proposed chemical reaction which describes the formation of Cu₃BiS₃ was given as follows:

$$ {\text{CuCl}} + {\text{xTu}} \to \left[ {{\text{Cu}}\left( {\text{Tu}} \right)_{x} } \right]^{ + } + {\text{ Cl}}^{ - } $$

(4)

$$ {\text{BiCl}}_{3} + {\text{yTu}} \to \left[ {{\text{Bi}}\left( {\text{Tu}} \right)_{y} } \right]^{3 + } + 3{\text{ Cl}}^{ - } $$

(5)

$$ 3\left[ {{\text{Cu}}\left( {\text{Tu}} \right)_{x} } \right]^{ + } + \left[ {{\text{Bi}}\left( {\text{Tu}} \right)_{y} } \right]^{3 + } \to {\text{ Cu}}_{3} {\text{BiS}}_{3} + \left( {3{\text{x}} + {\text{y}}} \right){\text{Tu}} $$

(6)

Later on, Chen et al. [47] produced the Cu₃BiS₃ nanorods by a simple ethanol-thermal route. For this, appropriate amount of CuCl₂·2H₂O (0.005 M), BiCl₃ (0.001 M), and thiourea (0.01 M) were added into a Teflon-lined SS autoclave containing ethanol or EG or glycerine up to 80% of total volume, retained at 160 °C for 10 h. Cu₃BiS₃ nanorods with different aspect ratios had been produced using different solvents. It was reported that the ethanol, ethylene glycol, and glycerine solvents were the key factors for the production of Cu₃BiS₃ nanorods. The formation of pure phase of Cu₃BiS₃ with the average crystalline size of nanorods 23 nm was reported from XRD study. TEM analysis showed 35 nm diameter and 2–15 µm length of produced nanorods.

Electrodeposition technique

Electrodeposition is an alternative promising method used for the low-cost synthesis of different semiconductor thin films for a small amount in research as well as large amounts in industry approach. This technique has been widely used for the fabrication of thin-film absorber layer viz. CIGS [48], CZTS [49], and CdTe [50] solar cells. Cu₃BiS₃ films made by an electrodeposition method were firstly reported by Peter et al. [51]. The Cu₃BiS₃ layer was deposited (1–2 μm thick) on the Mo-coated glass from a solution containing Bi(NO₃) (9 mM), CuSO₄·H₂O (30 mM), NaOH (2 M), and 0.2 M Sorbitol at −0.75 V versus HgIHgO followed by annealing in the S vapor at 450–500 °C for 30 min. The XRD confirmed the formation of wittichenite phase of Cu₃BiS₃, and the cathodic photocurrent response showed p-type conductivity. Further, Colombara et al. [52] reported a novel low-cost method for formation of Cu₃BiS₃ thin films as an absorber layer in solar cell. Firstly, Cu:Bi 3:1 thin films from an aqueous solution onto Mo-coated glass [53] substrate was prepared. These metal precursor films were converted into Cu₃BiS₃ by adopting sulfurization process. The higher annealing temperature would promote the diffusion of the binary sulfide and enhance the crystallinity of the films.

Recently, the same research group [54] had prepared Cu₃BiS₃ thin films by conversion of stacked and co-electroplated Bi–Cu metal precursors in the presence of elemental sulfur vapor. The precursor solution contained 0.03 M CuSO₄, 0.01 M Bi(NO₃)₃, 2 M NaOH, and 0.1 M d-sorbitol, and the electrolytic cell used was a three-electrode configuration. Colombara et al. [54] reported the homogeneous and compact Cu₃BiS₃ film formation by sulfurization of the co-deposited (Cu₃Bi) precursors at 500 °C for 0.5 h. The acceptor density and band gap energy of Cu₃BiS₃ was reported 3.10¹⁷ cm⁻³ and 1.3–1.4 eV, respectively.

Chemical bath deposition technique

A number of research groups reported the successful synthesis of Cu₃BiS₃ thin films for solar cell application by various techniques. However, still there was no single report on the synthesis of Cu₃BiS₃ thin-film-based solar cells using chemical bath deposition technique. Thereby, bearing in mind todays need of low-cost, high-efficiency solar cells, Deshmukh et al. [55] introduced first time chemical bath deposition technique in the preparation of Cu₃BiS₃ thin films for solar cell applications. The chemical bath deposition technique does not require any sophisticated instrumentation like vacuum systems and other expensive equipment’s. The initial chemicals are commonly available and are cheaper materials. With CBD, a large number of substrates can be coated in a single run with a proper jig design. For the synthesis of Cu₃BiS₃ thin films on glass substrate, copper (II) acetate, bismuth (III) nitrate pentahydrate, and thiourea were employed by Deshmukh et al. The as-deposited Cu₃BiS₃ thin films had a direct band gap between 1.56–1.58 eV and absorption coefficient ~10⁵ cm⁻¹. SEM images showed the formation of nanoparticles having diameter 70–80 nm. The atomic ratio of Cu:Bi:S was reported 41:13:45 which is closed to the stoichiometry of Cu₃BiS₃. The optical study directed that the Cu₃BiS₃ films could be applied in thin-film solar cells as an absorber layer.

Spin-coating deposition technique

Spin-coating technique is very simple and low cost for the deposition of various semiconductor thin films. It involves the three steps in the deposition of Cu₃BiS₃ thin films: (1) Preparation of the chemical precursor solution, which contains the ions of interest; (2) spin coating the precursor solution on a substrate to obtain the desired thin film; and (3) annealing the thin film at suitable atmosphere to form Cu₃BiS₃ material. Recently, Zhang et al. [12] in 2016 for the first time reported the synthesis of Cu₃BiS₃ thin films by sulfurizing the mixed metal oxide precursor film deposited by spin-coating technique. The copper nitrate trihydrate, bismuth nitrate pentahydrate, and 2-methoxyethanol were used as precursor to obtain Cu₃BiS₃ thin films. The XRD pattern confirmed the orthorhombic phase of Cu₃BiS₃ above the sulfurization temperature 420 °C. The band gap energy of the Cu₃BiS₃ thin films varied between 1.15 to 1.10 eV due to the larger grain size at higher annealing temperature. The measurement of Hall effect showed that Cu₃BiS₃ had a p-type conductivity with carrier concentration of 5.1 × 10¹⁶/cm³ and Hall mobility of 3.73 cm²/VS.

Non-vacuum-based other deposition techniques

Apart from these standard techniques, there are several non-vacuum or innovative techniques employed by a researcher for the deposition of Cu₃BiS₃. Microwaving is a one process used for the preparation of nanostructured materials. It resolves the problems of temperature and concentration gradients and provides uniform development. By concentrating microwave radiation into the solution, the vibrating electric field applies a force on dissolved species to induce vibrations with dissimilar modes. To avoid over-boiling, cyclic microwave radiation (CMR) was used instead of continuous. In 2011, Aup-Ngoen et al. [56] reported CMR synthesis of Cu₃BiS₃ dendrites using l-cysteine as a sulfur source and a complexing agent. For the synthesis of Cu₃BiS₃ dendrites, 3 mM CuCl and 1 mM BiCl₃ were dissolved in 30 mL EG with 3 mM l-cystein (C₃H₇NO₂S) to form a solution. Then this solution was irradiated using 300–700 W CMR for 40 cycles. The XRD revealed the formation of orthorhombic phase with strongest intensity peak at 31.3° corresponds to (131) plane of Cu₃BiS₃ and photoluminescence (PL) emission of Cu₃BiS₃ dendrites was blue emission at 367 nm.

Zhong et al. [57] reported a facile biomolecule-assisted solvothermal method for the preparation of flower-like Cu₃BiS₃ nanorods using l-cysteine as sulfur source and a complexing agent. Primary, CuCl₂·2H₂O (3 mmol), Bi(NO₃)₃·5H₂O (1 mmol), and l-cysteine (3 mmol) were dissolved in 40 mL N₁N-dimethylformamide. This mixed solution was filled into auto clave and heated at 200 °C for 16 h. The SEM showed the formation of flower-like Cu₃BiS₃ structure with nanorods having an average diameter of 150 nm. The Cu₃BiS₃ exhibited four Raman vibrational peaks at ~116, 153, 355, 459 cm⁻¹, and a strong band at 356 nm in PL spectra.

Hot injection solution chemical technique is a well-known approach for the preparation of materials with well-defined shape and size. This technique usually used in the synthesis of semiconductor nanocrystals. In 2013, Yan et al. [58] synthesized wittichenite Cu₃BiS₃ nanocrystals by utilizing a hot-injection method for the first time. For this, the recipe was 2.25 mM copper (II) acetylacetonate, 0.75 mM bismuth (III) nitrate pentahydrate (atom ratio Cu:Bi = 3), and 12 ml oleylamine which were added into a three-neck flask and heated at 140 °C for 1 h. When the temperature was raised up to 220 °C, 2.5 ml OLA containing 1.2 mol·l⁻¹ sulfur was quickly injected into the three-neck flask. The diameter of the obtained Cu₃BiS₃ nanocrystals ranges from 8 to 11 nm with a band gap of 1.56 eV. Raman spectra at 486 and 122 cm⁻¹ confirmed the existence of Cu₃BiS₃. Also, Cu₃BiS₃ showed a good photoresponse in I–V experiments.

Hu et al. [59] reported a simple and low-cost screen-printing approach for the preparation of the Cu₃BiS₃ absorber layer. For the formation of the Cu₃BiS₃ composite coating, Bi₂S₃ and CuS powders obtained by CBD were used. The as-deposited CuS and Bi₂S₃ powders were mixed with polyacrylic acid, acting as a binder, and resulting mixture was used as a paste to form Cu₃BiS₃ composite coating. When the annealing temperature was equal or higher than 250 °C, there was an interfacial diffusion of atoms at the CuS–Bi₂S₃ interface leads to the formation of ternary compound Cu₃BiS₃. The sheet resistance and electrical conductivity of Cu₃BiS₃ annealed in nitrogen was reported 10³ Ω/□ and 1 S cm⁻¹, respectively. The proposed reaction mechanism for the formation of Cu₃BiS₃ is as follows:

$$ 6{\text{CuS}} + {\text{Bi}}_{2} {\text{S}}_{3} \to 2{\text{Cu}}_{3} {\text{BiS}}_{3} + 3{\text{S}} \uparrow $$

(7)

Recently, low-temperature solution methods such as solvothermal or hydrothermal have been employed for the synthesis of Cu₃BiS₃ nanocrystallites, but this requires a long reaction time to ensure the well crystallinity of the Cu₃BiS₃. Shen et al. [60] in 2003 described the rapid synthesis of Cu₃BiS₃ at low temperature (195 °C) via a rapid polyol process from single source precursors. For this, a stoichiometric mixture of Bi(S₂CNEt₂)₃, Cu(S₂CNEt₂)₂, and sodium diethyldithiocarbonate was placed into a three-neck flask which contained 50 ml EG. This system was heated and maintained at 195 °C for 60 min under magnetic stirring. The SEM confirmed the formation of coral shaped nanocrystallites of Cu₃BiS₃ and XRD pattern confirmed the orthorhombic phase.

In 1997, Nair et al. [61] reported the formation of ternary Cu₃BiS₃ during annealing of chemically prepared CuS (0.3 μm) films on Bi₂S₃ (0.1 μm) films. The deposition of Bi₂S₃ was carried out at RT for 2.5 h. containing 10 ml Bi³⁺ (0.5 M), 8 ml TEA (3.7 M), and 8 ml thioacetamide (0.5 M). The CuS was prepared from 10 ml copper (II) chloride (0.5 M), 8 ml TEA (3.7 M), 8 ml 30% aqueous ammonia, 10 ml NaOH (1 M) solution at RT for duration of 3, 5, 8, and 11 h on the coating of Bi₂S₃/glass followed by air annealing in an oven at 100–350 °C for 60 min. The XRD confirmed the formation of wittichenite Cu₃BiS₃ phase. The obtained films were smooth, continuous, and phase-pure. These films were optically absorbing in the visible region (absorption coeff. 4 × 10⁴ cm⁻¹ at 2.48 eV) and of the p-type with electrical conductivity of 10²–10³ Ω⁻¹ cm⁻¹.

It is seen that in the above synthesis methods of Cu₃BiS₃, a wide variety of Cu₃BiS₃ crystal morphologies had been described. This included flower-like structure (Fig. 7a, b), nanosheets (Fig. 7c), nanorods (Fig. 7d, e), dendrites (Fig. 7f), flower-like hierarchical structure (Fig. 7g), coral-like nanostructure (Fig. 7h), bent-like nanorods (Fig. 7i), spherical nanoparticles (Fig. 7j), nanoneedle-like (Fig. 7k), and 3D flower-like ball (Fig. 7l), which are presented in Fig. 7. Compared with regular thin films [22], a flower-like, nanosheet, 3D flower-like ball, etc., pattern indicates a high surface area, which has potential applications in the area of energy conversions and sensors [42]. In addition to this, Liang et al. [62] reported the efficiency of solar cells depends on the surface morphology. As per the evidences from SEM micrographs (Fig. 7) of Cu₃BiS₃ thin films, it reveals that such morphologies may be found the potential application as an absorber layer in solar PV cells.

Fig. 7

Flower-like (a, b) [43, 57]; nanosheets (c) [42]; nanorods (d, e) [37, 47]; dendrites (f) [56]; flower-like hierarchical (g) [38]; coral-like (h) [60]; bent-like nanorods (i) [39]; spherical nanoparticles (j) [55]; nanoneedle-like (k) [29] and 3D flower-like ball (l) [45] morphologies of Cu₃BiS₃

Computational and simulation aspects of Cu₃BiS₃

Till now, it has been reported that Cu₃BiS₃ has a direct band gap in the range of 1.10–1.86 eV [12, 13] and a p-type conductivity with a carrier concentration of 2 × 10¹⁶ cm⁻³, which are the appropriate properties for an absorber material in photovoltaics. However, thin-film PVs based on Cu₃BiS₃ are not yet extended to the device level. This is mainly due to the fundamental physical properties of Cu₃BiS₃ are not yet well understood and the quality of the thin film is not optimized. Henceforth, it is necessary to understand the details in the optical and electronic properties for the improvement of PV solar cells based on Cu₃BiS₃.

Yu et al. [63] presented a fundamental analysis of the factors for Cu–V–VI (V=P, As, Bi, Sb; VI–S, Se) system that control absorption strength using the density function theory (DFT) approach. It was reported that the high-valence Cu–V⁵⁺–VI compounds have stronger absorption than high-valence CuIn³⁺Se₂. The Cu₃–V–VI semiconductor absorbers contain many members exhibiting spectroscopic limited maximum efficiency (SLME) > 23% at a film thickness of 200 nm. This inherent efficiency, coupled with tunable band gaps, useful electrical properties, makes Cu₃–V–VI family materials attractive as potential candidates for investigation and development of new single-junction solar cell. Later on, Tablero [8] studied the electronic properties of Cu₃BiS₃ by using first-principles density functional method. He had reported maximum efficiency obtained for Cu₃BiS₃:O with the results of generalized gradient approximation (GGA) and local density approximation (LDA) first-principles calculations was 50%. This obtained supreme efficiency is larger than the efficiency of a Cu₃BiS₃ single-junction solar cell with equivalent solar concentration.

Further, Kumar and Persson [64, 65] analyzed the structural, optical, and electronic properties of Cu₃BiS₃ using a first-principles approach within the DFT. It was found that the compound Cu₃BiS₃ has an indirect band gap of Eg = 1.5–1.7 eV. The analysis revealed that Cu₃BiS₃ has a stronger absorption coefficient than other Cu–S based materials like Cu₂ZnSnS₄ and CuInS₂. The stronger absorption in Cu₃BiS₃ was explained by the localized Bi 6p states in the lowest conduction band, forming a flat energy band that increases the absorption coefficient in the lower energy region. Hence, Cu₃BiS₃ can be regarded as a potential absorber material in thin-film PV technologies.

Recently, Mesa et al. [66] first time used the finite elemental method to simulate the nucleation of dislocations of Cu₃BiS₃ thin films. They reported the critical thickness of the thin films of Cu₃BiS₃ through the finite element method was 6b. Today, the wxAMPS tool is an important application for simulating solar cells with high reliability. In wxAMPS, the obtained values are, V _OC = 0.712 V, J _SC = 36.25 mA/cm², FF = 79.54%, and an efficiency of 19.86%, which allows Cu₃BiS₃ is an outstanding alternative new material for the designing of photovoltaic devices. This is the first report on the efficiency of Cu₃BiS₃ thin-film-based solar cells using simulation.

Conclusions

A comprehensive review of synthesis, characterizations, processing, and device applications of Cu₃BiS₃ is presented here. The recent significant progress in Cu₃BiS₃ has shown the feasibility of developing photovoltaic solar cells with low-cost, abundant, and/or readily available elements. A broad range of methods, including vacuum as well as non-vacuum-based approaches, have been explored to deposit the Cu₃BiS₃ thin films. This progressive improvement in orthorhombic based pure Cu₃BiS₃ thin film solar cell leads to a highest efficiency of 1.281%. However, this efficiency is very low as compared to its counterpart CIGS, CZTS, CdTe, and CIS thin-film-based solar cells, which are already at the commercialization stage exhibiting the conversion efficiency greater than 14%.

In order to improve the efficiency of Cu₃BiS₃ based thin film solar cells, a more detailed understanding of the fundamental properties of Cu₃BiS₃, particularly the nature of the defects as well as their impact on the properties of Cu₃BiS₃ material is important. It is also essential to detect the secondary phases and their effects in order to optimize the synthesis process to make Cu₃BiS₃ thin films with preferred properties. To develop the successful technology, the detailed understanding of Cu₃BiS₃ material synthesis is necessary. This can be achieved by studying material experimentally as well as theoretically. Also for the long-term durability of wittichenite-based Cu₃BiS₃, thin-film solar cell device under light, moisture, and heat should be studied to form a low-cost, environment-friendly, and high-output approach for the deposition of Cu₃BiS₃ thin films.

This review presents the results on Cu₃BiS₃ thin-film properties as well as its applications in solar cell devices observed at an early stage, which subsequently suggests that it is new and efficient material for low-cost PV solar cells. However, the progress of chemical methods suitable for the synthesis of a ternary Cu₃BiS₃ still remains a major challenge. The wide range of techniques that have been employed to prepare Cu₃BiS₃ semiconductor material had been reviewed with the hope of distinguishing new paths for productive research based on the present author’s work.