Exploring the potential of Cu₂FeSnS₄: a comprehensive review on structural properties, optoelectronic features, and future prospects in earth-abundant thin film solar cells

Abstract

Cu-based earth-abundant ternary and quaternary chalcogenides are gaining intense interest in environmentally benign thin film solar cells. Recently, Cu₂FeSnS₄ (CFTS), has pulled prime attention in research and development due to their exceptional optoelectronic properties in addition to the nontoxic nature and composition consisting of earth abundant constituent elements. CFTS has a high optical absorption coefficient (>10⁴ cm⁻¹) and a suitable bandgap range (1.2-1.5 eV) to be used as an absorber layer in thin film solar cells. Despite several preliminary works in the recent past, the potential of CFTS in solar cell applications has not yet been systematically explored. This mini review article begins with a brief insight into the importance and background of this potential material. Various synthesis methods employed for the deposition, characteristics of such deposition along with a few important results obtained are presented. Moreover, recent advancements in the CFTS thin film based photovoltaic device performance studies, challenges, strategies to realize for such device applications and outlook for further developments are discussed. Additionally, this review gives a comprehensive insight into the various possible defects and their impact on the performance and feasible solutions to mitigate these issues to enhance the optoelectronic properties of CFTS thin films.

Keywords:

• Cu2FeSnS4
• thin films
• solar photovoltaics
• device applications

Review editor:

• Ian Jones, University of Birmingham, United Kingdom

Subjects:

• Electronic Devices & Materials; Optics
• Optoelectronic Effects; Devices & Systems; Semiconductors; Power & Energy; Materials Science

1. Introduction

It is a well-accepted fact that the sun produces sufficient energy to sustain the power needs of all humanity. The total solar flux reaching the Earth’s surface is estimated to be 1.08 × 10⁸ GW, which is 7000–8000 times the total global energy consumption (Wray, 2008). This opens a huge potential for utilizing solar energy to power all our energy needs at little expense to the environment. Traditional solar cells have been successfully deployed for commercial energy production. By 2016, every continent had the capacity to produce at least 1 GW of solar energy, and at least 24 countries had installed over 1 GW of solar PV capacities (Ashok Kumar et al., 2020; Wray, 2008). The evolution of solar cell technology spans several generations, each marked by significant advancements that have propelled the efficiency and practicality of harnessing solar energy. The first generation, represented by traditional crystalline silicon solar cells, laid the foundation for photovoltaic technology and remains prevalent in the market. Second-generation solar cells, including thin-film technologies like amorphous silicon and cadmium telluride, introduced cost-effective alternatives with improved manufacturing processes. The third generation focused on enhancing efficiency through advanced materials such as organic photovoltaics and dye-sensitized solar cells (Agyei-Tuffour et al., 2022). Emerging fourth-generation technologies aim to overcome limitations of previous generations by employing novel concepts like perovskite solar cells, offering higher efficiency, flexibility, and lower production costs. The dynamic progression across these generations underscores the relentless pursuit of innovation within the solar industry, as researchers and engineers continue to push the boundaries of what is possible in sustainable energy generation.

The massive material requirements remain a major challenge to be solved when implementing solar energy. The large area requirement leads to large requirements for materials and manufacturing, which can make solar panels unsustainable for large-scale production. Thin-film photovoltaics have been explored as one of the alternatives to combat the energy crisis. Thin-film solar photovoltaics have grown in the field of study owing to their increasing efficiency, compactness, and low material requirements (Siddharth et al., 2022).

Cadmium Telluride (CdTe) and Copper Indium Gallium Sulfide (CIGS)-based thin-film solar cells have increased in popularity over the last few years, owing to their high efficiency and ease of manufacturing (Lee & Ebong, 2017). However, due to the high toxicity of Cadmium and scarcity of Indium, they pose a severe risk to both consumers and the environment. Another alternative is perovskite-based PV cells, which have been extensively studied owing to their high efficiency, reaching efficiencies of up to 24.4% (Li et al., 2022). However, the stability of perovskite-based PV technologies is a major obstacle to their commercialization. This has sparked the need for safer, more environmentally friendly, and stable materials (Lee & Ebong, 2017; Rahi et al., 2022).

This has led to an increase in the study of copper-based quaternary chalcogenide materials such as Copper Zinc Tin Sulfide (CZTS) (Le Donne et al., 2019) and Copper Iron Tin Sulfide (CFTS) (Kevin et al., 2015). Owing to the low toxicity and abundance of the constituent materials, CZTS was found to be an attractive option for current Thin Film Solar Cells (TFSCs), which could also be attributed to its high theoretical Power Conversion Efficiency (PCE) of approximately 32% (Baid et al., 2021). CZTS devices usually exhibit a bandgap of 1.0 – 1.5 eV (Kumar, 2021) which is ideal due to the high intensity of solar radiation (AM 1.5 G) around the same range; therefore, it is optimal for high conversion efficiency under sunlight, along with a high absorption coefficient of ∼10⁴ cm⁻¹. CZTS is a highly viable material alternative for TFSCs, while competing with other major players such as CIGS and CdTe thin-film technologies. Structurally, CZTS forms in kesterite, stannite and wurzite form depending on the lattice parameter. The wurzite CZTS is the naturally occuring structure which is similar to the CFTS structure, stannite CZTS is where the Cu and Zn interchange their atomic position in the CZTS lattice and kesterite CZTS is the most stable form of CZTS which is suitable for the PV applications but has a very narrow region of structural stability owing to the dependence on the chemical composition (Baid et al., 2021). However, CZTS faces challenges arising from defects caused by the intermingling of Cu and Zn atoms due to their closely matched atomic sizes. Band grading presents another obstacle hindering the advancement of thin-film solar cells based on CZTS. Moreover, the fluctuation in electrostatic potential of Cu and Zn within the Cu_Zn+Zn_Cu defect adversely affects the performance of CZTS solar cells. Consequently, there is an imperative need to explore alternatives to kesterite CZTS and investigate approaches for substituting other elements in place of Cu or Zn (Gokmen et al., 2013; Zhong et al., 2016). CFTS comprises solely of elements that are abundant in the earth, relatively non-toxic, and cost-effective. Furthermore, the substitution of Fe for Zn in CZTS thin films results in a reduction of the optical band gap energy. Given that Fe is more soluble in the lattice, it enhances conductivity, thereby increasing both the solar-to-electricity conversion efficiency. CFTS has also been demonstrated to have a high optical absorption coefficient (>10⁴ cm⁻¹) and a suitable bandgap range (1.2-1.5 eV) (Guan et al., 2014). Despite the potential this material, there are no review articles available other than one published long back in 2018 (Vanalakar et al., 2018).

This mini review article unveils the important properties of CFTS and its viability of utilizing as a material suitable for manufacturing TFSCs. Various synthesis methods used for deposition of CFTS are also outlined along with a few important results obtained using the respective methods. Moreover, recent advancements in the device performance studies, challenges, strategies to realize for device applications and outlook for further developments on CFTS based solar cells are discussed. Additionally, a comprehensive insight into the various possible defects and their impact on the performance and feasible solutions to mitigate these issues to enhance optoelectronic properties are provided.

2. Synthesis of CFTS thin films

Thin films are often coated using various physical and chemical techniques to achieve uniform and consistent coatings. The following are the coating methods commonly employed to coat CFTS thin films: SILAR (Madhusudanan et al., 2019, 2022; Sankapal et al., 2000), Sol-gel (Meng et al., 2024), Spin coating (Madhusudanan et al., 2021), spray pyrolysis (Nefzi et al., 2020; 2020; Nilange et al., 2019), Chemical bath (El Radaf et al., 2020), solvothermal (Mukurala et al., 2021), electrochemical deposition (Miao et al., 2017; Mokurala & Mallick, 2017), chemical vapor deposition (Oueslati et al., 2018), and physical vapor deposition (Meng et al., 2015; Rahmani & Ghanaatshoar, 2022; Tripathi et al., 2021), Chemical techniques encompass spray pyrolysis, SILAR (successive ionic layer adsorption and reaction), spin coating, and chemical bath deposition. Physical techniques, such as sputtering and pulsed laser deposition (PLD), are utilized for the fabrication of CFTS thin films. While both methods have their own advantages and limitations, the quality of the thin film is significantly influenced by the chosen fabrication method. Notably, post-annealing is a crucial step in the fabrication process, regardless of the use of physical or chemical techniques. This step proves essential for addressing defects, non-homogeneity, non-compactness, and amorphous structure in the CFTS thin films. Furthermore, post-annealing contributes to enhancing crystal quality and grain growth, ultimately leading to improved power conversion efficiency (PCE) (see Figure 1) (Qu et al., 2016; Vanalakar et al., 2014). The post-annealing process for CFTS involves exposure to inert or sulfur-rich environments at temperatures exceeding 500 °C, using conventional or rapid thermal furnaces.

Figure 1. Performance distribution of solar cells fabricated using CZTSSe thin films annealed at different temperatures [Reproduced with permission from (Qu et al., 2016)].

Spin coating is one of the most straightforward techniques for producing thin films on a fairly even surface. In this process, a substrate is affixed to a rotating fixture, and a solution containing the coating material is applied onto the surface. Rapid rotation of the fixture then ensures even distribution of the solution across the substrate. A couple of reports are available on the deposition of compact CFTS thin films via the spin coating method. These films exhibited a notably high absorption coefficient and a band gap of approximately 1.53 eV (Dong et al., 2018). Moreover, the deposition of CFTS thin films are carried out using the spin coating method, incorporating cobalt substitution into the CFTS thin films. The observed band gap fell within the range of 1.1-1.5 eV, with an absorption coefficient exceeding 10⁴ cm⁻¹. These findings support the potential utilization of these thin films as absorbers in solar cells, showcasing an adjustable optical band gap (Drissi et al., 2023).

Spray pyrolysis is an economical and viable method involving the thermal decomposition of a liquid sample sprayed as aerosol. This technique is characterized by its low-cost nature and its potential for large area deposition. Investigations into the impact of thiourea concentration in the precursor solution on the characteristics of spray deposited CFTS thin films revealed that the crystallinity of these films improves with increasing thiourea content in the precursor solution (Nilange et al., 2019). Using the spray pyrolysis technique, thin films of CFTS were produced and then sulfurized at different temperatures. These films exhibited p-type conductivity, showcasing carrier mobilities within the range of approximately 2 to 11 cm²/V·s. Notably, they achieved an impressive power conversion efficiency of 8.03% (Prabhakar et al., 2014). Using the spray pyrolysis technique, CFTS thin films were deposited at various substrate temperatures ranging from 175 °C to 325 °C. These films exhibited a consistent band gap of approximately 1.54 eV and demonstrated stable electrical properties, suggesting their potential suitability for solar cell applications (Nilange et al., 2019). Likewise, CFTS thin films were deposited at various substrate temperatures (T_s = 160, 200, 240, and 280 °C). The results indicate that at T_s = 240 °C, the energy band gap and absorption coefficient values were approximately 1.46 eV and 9.8 x 10⁴ cm⁻¹, respectively (Nefzi et al., 2018). Utilizing non-toxic aqueous precursors, CFTS thin films are deposited via spray pyrolysis followed by sulfurization. It has been shown that Dye-Sensitized Solar Cells (DSSCs) employ CFTS thin film as a photocathode in an iodine/iodide electrolyte demonstrate a favorable power conversion efficiency of 8.03%. This suggests that CFTS holds promise as a cost-effective alternative to platinum (Pt) for use as a counter electrode in DSSCs (Prabhakar et al., 2014).

Sol-gel coating is a method wherein a thin film or coating is created by converting a solution (sol) into a gel, which then solidifies into the desired material. CFTS thin films were fabricated using a combination of the sol-gel method and spin-coating technique, employing different stabilizers, and taking through a sulfurization process. These films displayed diverse band gap values, while electrical bistability was evident in the as-prepared CFTS films with stabilizers. However, bistability was absent in the sulfurized films, which instead exhibited favorable photocurrent responses and photostability. Both the as-prepared and sulfurized CFTS films showcased characteristics well-suited for thin-film solar cell production (Madhusudanan et al., 2021). Using the same technique, Cu₂Fe_xCo_1-XSnS₄ thin films were produced without sulfurization. These films exhibited a tetragonal stannite structure characterized by varying lattice parameters and crystallite sizes. The introduction of higher cobalt content led to reduced crystallite size, heightened dislocation density, and modified optical properties, resulting in band gaps spanning from 1.1 to 1.53 eV. These observations indicate the feasibility of regulating optical properties in spin-coated Cu₂Fe_xCo_1-XSnS₄ thin films, offering promising prospects for TFSC applications (Drissi et al., 2023). Moreover, CFTSSe thin films were prepared via sol-gel deposition followed by a post-annealing step. The percentage of Se affected the XRD diffraction peaks, lattice constant, and Raman peaks of the films. XPS analysis unveiled the existence of different states within the CFTSSe thin film. SEM images depicted smooth, densely packed surfaces, with the inclusion of Se leading to a decrease in grain size within the thin films (Meng et al., 2024).

Physical Vapor Deposition (PVD) is a technique employed in vacuum environments to create thin films of exceptional quality. PVD serves as a broad term covering multiple methods utilized for thin-film deposition, with two primary processes being prominent: 1) Sputtering, and 2) Thermal evaporation (Ohring, 2002) A two-step approach involving magnetron sputtering and subsequent post-sulfurization processing was employed by the researchers for the deposition of CFTS films. No impurity phases were detected, revealing Cu-poor and Fe-rich states in the samples. The structural phase transition observed was from a rhodostannite structure to a stannite structure (Meng et al., 2015). CFTS films were also fabricated using the thermal evaporation technique and then subjected to sulfur atmosphere annealing. The annealed films exhibited enhanced crystallinity, while the presence of interference fringes in the film’s transmittance indicated a high level of layer homogeneity (Oueslati et al., 2019).

3. Structural, morphological, and optoelectronic properties

SILAR deposition technique was utilized to deposit CFTS films along with various n-type materials to construct an optimal solar cell. Upon performing an XRD analysis, the peaks obtained were all in accordance with the stannite phase, while the diffraction peaks showed a narrow Full Width at Half Maxima (FWHM) which indicates a high degree of crystallinity of the material in thin films. The XRD analysis indicated the presence of secondary phases, that is, CuS and SnS. Raman spectroscopy was necessary to confirm the presence of binary and ternary phases, such as Fe-S and Cu-Sn-S. The strong peak at 320.9 cm⁻¹ indicates the main vibrational mode of the CFTS thin film, and the existence of weak peaks at 150 cm⁻¹ and 453 cm⁻¹ may have arisen from the glass substrate. The method followed by the authors lead to the formation of CFTS thin film without the high temperature sulfurization process which could help in cut down the cost of the solar cell. However, due to the single step process of CFTS, few traces of CuS and SnS have been found in the final thin film (Chatterjee & Pal, 2017). CFTS thin films were deposited utilizing the spray pyrolysis technique, with a thorough investigation into the influence of substrate temperature. Figure 2 illustrates the significant impact of substrate temperature on the elemental composition of the deposited thin films (Nefzi et al., 2018).

Figure 2. Impact of substrate temperature on atomic composition of CFTS films [Reproduced with permission from (Nefzi et al., 2018)].

The films, deposited at a substrate temperature of 240 °C, exhibited atomic ratios of Cu:Fe:n:S at approximately 2:1:1:4, aligning with earlier research findings. X-ray diffraction (XRD) analysis revealed that films deposited at 160 °C exhibit a predominant peak at 2θ = 28.61°, which corresponds to the (112) plane and is indicative of the stannite CFTS structure. X-ray diffraction (XRD) analysis indicates that films deposited at 160 °C feature a prominent peak at 2θ = 28.61°, corresponding to the (112) plane and indicative of the stannite CFTS structure. Additionally, a secondary phase of SnS is observed at 26.5°. Rietveld analysis was conducted to enhance the precision of the X-ray diffraction (XRD) findings. It demonstrated that the most effective substrate temperature was 240 °C. The grain size was determined to be 43.8 nm, and no impurity phases were detected. During Raman analysis, vibration bands were observed at 252 cm⁻¹ and 319 cm⁻¹, suggesting the presence of the CFTS stannite phase. The morphology of the film which is deposited with substrate temperature of 240 °C displays large grains accompanied by micro-aggregates, which exhibit a spherical form on the surface. The study suggested that the substrate temperature of 240 °C helps in the secondary phase free growth of CFTS with larger grains and better composition with optimum band gap. This can be attributed to the completion of the binary and ternary reactions to form the quaternary CFTS with less loss of volatile sulfur to maintain the required stoichiometry which will greatly impact the film’s properties. The CFTS thin films are fabricated via the sol-gel method and subsequently annealed in a nitrogen atmosphere at temperatures ranging from 300 °C to 450 °C for 1 hour. Their X-ray diffraction (XRD) analysis revealed major peaks at 28.45°, 47.50°, and 56.59°, corresponding to the (112), (204), and (312) planes of the CFTS stannite structure. Films annealed at 400 °C exhibit no secondary phases and undergo complete transformation into the stannite structure. Raman analysis indicated vibration bands at 286 cm⁻¹ and 320 cm⁻¹, representing the CFTS stannite phase, while a vibration band at 475 cm⁻¹ suggested the presence of Cu_(2-x)S. Raman analysis confirmed a well-defined crystalline structure with the absence of secondary phases in the film annealed at 400 °C. The deposited films were homogeneous, granular, and crack-free morphology, with a composition that is Cu-poor, Sn-rich, and nearly stoichiometric. Single step synthesis of CFTS thin films was attempted in this study. Although, the temperature to attain the single phase CFTS thin films was high (400 °C), the single phase CFTS thin films were obtained without the additional sulfurization step (Ait Elhaj et al., 2021).

CFTS thin films were synthesized using electrodeposition method and the XRD patterns of the prepared samples mostly followed the peaks assigned to the stannite structure of CFTS which are inferred from the intense diffraction lines at 28.50°, 32.85°, 47.50°, and 56.66°, corresponding to the (112), (200/004), (220/204), and (312/116) reflections, respectively, as shown in Figure 3. Raman spectroscopy using an excitation wavelength of 633 nm revealed significant peaks at 210 cm⁻¹, 255 cm⁻¹, 289 cm⁻¹, 330 and 346 cm⁻¹ which can be attributed to the CFTS stannite structure. Raman spectroscopy also showed low peaks at 314-315 cm⁻¹ which correspond to the hexagonal phase SnS₂ due to excess Sn and possibly its diffusion to the surface, which is not visible in the XRD spectrum, further confirming the possibility of the phase being present only on the surface. Some samples displayed other smaller peaks, which may be due to unwanted phases, thereby lowering the overall quality of the film. The two-stage synthesis including CFTS deposition at an applied potential of -1.25 V and sulfurization step at 500 °C allowed the formation of single phase CFTS thin films with required stoichiometry (El Khouja et al., 2021).

Figure 3. XRD patterns of CFTS films produced at different potentials [Reproduced with permission from (El Khouja et al., 2021)].

CFTS as an electron-acceptor nanocrystal in hybrid solar cells were studied. The synthesised nanocrystals fitted well with the stannite phase of CFTS. The Raman spectra showed peaks at 320 cm⁻¹, a characteristic peak of stannite CFTS, while the peaks at 214 cm⁻¹ and 282 cm⁻¹ corresponding to FeS and at 267 cm⁻¹, 303 cm⁻¹ and 356 cm⁻¹ corresponding to Cu₂SnS₃, therefore depicting the phase purity of the crystals. The HRTEM images show spherical nanocrystals with diameters ranging from 3 to 7 nm. The lattice fringes are separated by 0.31 nm, matching the d-spacing of the (111) planes in the stannite form. Wang et al. (Dong et al., 2017) synthesized a CFTS/Se film using a blade coating process. The structural parameters of the resulting film were found very good with a higher crystallite size (D) and are summarised in Table 1.

Table 1. Structural parameters of CFTS/Se films obtained using blade coating (47).

Sample	2θ	FWHM (degree)	D (nm)	a (Å)	c (Å)
Post-sulfurized film	28.54	0.144	54.5	5.11	5.58
Post-selenized film	27.43	0.129	60.9	5.63	11.21

The Raman spectroscopy results showed peaks at 318 cm⁻¹, 283 cm⁻¹ and 370 cm⁻¹ that is attributed to CFTS, and 186 cm⁻¹ and 234 cm⁻¹ which is ascribed to CFTSe. The shift in the dominant characteristics of the spectrum was an effect of the partial substitution of Se. The SEM images showed a rough surface finish with a multitude of holes due to the evaporation of the organic solvent from the CFTS films. The removal of the organic solvent before annealing to prevent the degradation of the photovoltaic performance was suggested, as the defects caused blockages in the charge carrier transport.

A melt-quench technique was employed for synthesizing CFTSSe materials, followed by deposition using a thermal evaporation method. X-ray diffraction (XRD) analysis revealed tetragonal CFTSSe phase. To identify secondary phase impurities, Raman spectroscopy was employed, while Fourier-transform infrared (FTIR) spectroscopy was utilized to investigate the presence of organic and inorganic ligands. The study intends to employ method to employ the synthesis of single-phase source material as a precursor for the thermal evaporation to obtain the final CFTSSe thin films. But the formation has lesser Fe and more Sn in the compound and has resulted in the needle like structure observed in the SEM images (Tripathi et al., 2021) A facile spray pyrolysis techniqueutilized a to deposit CFTS films. They attempted to optimize the synthesis conditions of films with the help of a combination of spray durations (of 20, 40 and 60 minutes) and thermal annealing (sulfur atmosphere at a temperature of 450 °C for 30 minutes). After annealing, the measured absorptance was greater than 10⁵ cm⁻¹. Additionally, the band gap was measured to be 1.55 eV, which is similar to the range reported in most theoretical and experimental studies (Dridi et al., 2020). A quaternary chalcogenide CFTS thin layers was grown with the help of spray pyrolysis technique on glass substrates using various substrate temperatures (T_s = 160 °C, 200 °C, 240 °C and 280 °C). Absorbance values were measured for a wavelength range of 300 – 2000 nm, and a maximum value of 78% was observed for T_s = 240 °C, which was attributed to an increase in the crystallite size. The measured band gap values were 1.62 eV, 1.05 eV, 1.46 eV and 1.58 eV for the temperatures 160 °C, 200, 240, and 280 °C, respectively. An optimal value of 1.46 eV was obtained at 240 °C. The refractive index increased with an increase in temperature, before peaking at T = 240 °C, and falling drastically at T = 280 °C. This can also be attributed to the variation in crystallite size of the coated films (Nefzi et al., 2018).

As we can see in the overall discussion, there is a variation in the characteristics of CFTS recorded by various researchers. This can be attributed as follows. Structural and hence other characteristics heavily depend on the synthesis and processing conditions. Precursor quality, deposition temperature, annealing, substrate interaction, pressure and gas environment, growth rate and kinetics are some of the important causes for the observed variation in the structural characteristics of CFTS thin films.

4. Device performance of Cu₂FeSnS₄ thin films

CFTS thin films typically exhibit an optical band gap range of 1.2 eV - 1.5 eV and have been shown to form a type II band alignment in the p-n junction. All the literature on CFTS aims at improving the performance of CFTS thin film devices by changing the deposition techniques to improve the structural and morphological properties for photovoltaic application.

The effect of heating rate on the electrical properties of the CFTS prepared using RF magnetron sputtering process was studied (Meng et al., 2016). The PV cell was fabricated using an Al: ZnO/ZnO/CdS/CFTS/Mo/glass structure. The best electrical properties among the samples, V_oc, J_sc and FF were 129 mV, 3.25 mA/cm², and 27.9% respectively and the corresponding power conversion efficiency was 0.11%. They have obtained optical band gap close to the ideal value of ∼1.4 eV. As per the report, a heating rate of 20 °C/min during sulfurization provided the optimum results. This study with a focus on the heating rate suggests that due to the volatile nature of sulfur, the corresponding binary and ternary reactions during the high temperature processing and the necessity to let the reaction of binary and ternary compounds to complete to form the final CFTS thin films. This temperature variations can help in obtaining the single phase CFTS with the better control over the composition which plays a vital role in the defect formations and subsequent device performance. Similarly, another study reported the following properties: resistivity of 14 W-cm, Hall mobility of 0.09 cm² V s⁻¹, with J_sc, V_oc, FF, and ECE of, 3.6 mA/cm², 200 mV, 31% and 0.4 ± 0.04% respectively (Ananthoju et al., 2016).

The CFTS is used as p-type and Bi₂S₃ used as n-type layer in photovoltaic cells (Chatterjee & Pal, 2017). It exhibited the best electrical properties with an ECE of 2.95% with V_oc, J_sc, and FF of 0.61 (0.58 ± 0.02) V, 9.3 (8.8 ± 0.37) mA/cm², and 52 (48 ± 0.03) %, respectively, compared to pure CFTS, which exhibited V_oc, J_sc, and FF of 0.30 (0.30 ± 0.04) V, 2.0 (1.6 ± 0.30) mA/cm², and 33 (30 ± 0.03) %, respectively. The CFTS/Bi₂S₃ junction exhibited a clear type-II band alignment. The fabrication of the CFTS/Bi₂S₃ followed a low temperature and low vacuum method which helps in considerable reduction in the cost of the final solar cell. The photocurrent density of CFTSSe, ranging from 14 mA/cm² to 16 mA/cm², surpasses that of standard CFTS thin film, which falls between 11 mA/cm² to 13 mA/cm² (Wang et al., 2017). This highlights the significance of the selenization process in enhancing photoresponse, rendering it suitable for photovoltaic applications. The optical bandgap of CFTS powder measures at 1.40 eV, while for CFTS and CFTSSe films, it is recorded at 1.32 eV and 1.25 eV, respectively.

A numerical study (Khattak et al., 2018) claimed that an optimized CFTS thin film theoretically achieved PCE, V_oc, Jsc, and FF of 19.97%, 0.995 V, 23.37 mA/cm² and 85.94%, respectively. They indicated an optimal band gap value of ≥1.3 eV for maximum power conversion efficiency. Study concentrated on the effects of CFTS film thickness, CFTS acceptor concentration density, electron transport layer, back contact work function and working temperature of PV cell. This study covered some significant aspects of the solar cell layers which has a significant impact on the final device performance. A numerical study using SCAPS 1D (Konan et al., 2019) suggested that an optimum thickness of 3 µm with an acceptor concentration of 3 × 10¹⁸ cm⁻³, a back metal work function of 4.9 eV at an operating temperature of 300 K yields a PCE of 22.27%, J_sc of 24.92 mA/cm², V_oc of 1.033 V, and FF of 86.54%. The bandgap was measured to be 1.36 eV with an absorption coefficient of 5 × 10⁴ cm⁻¹. The work function of the back metal contact has a significant impact on the performance of the device too, with a major increase in all parameters of the cell (efficiency, fill factor, open circuit voltage and current density) between 4.7 eV and 4.5 eV, with the highest efficiency of 22.35%.

5. Other applications of CFTS

Owing to its inherent similarities to CZTS/Se, CFTS/Se has applications in various areas, such as photovoltaic cells, photocatalysis, water splitting, memory switching applications, and DSSCs. Like CZTS, CFTS also exhibits p-type semiconductor properties; hence, when used in tandem with a compatible n-type semiconductor to form a p-n junction. CFTS shows promising results, with a theoretical power conversion above 19% (Khattak et al., 2018; Konan et al., 2019). The CFTS also performs better with the inclusion of selenium (Wang et al., 2017). A few studies have explored the photocatalytic activity of CFTS (Mokurala et al., 2016; Nefzi et al., 2018; 2020; Zaman et al., 2019). CFTS demonstrates promising results with respect to photocatalytic properties, with a photodegradation rate greater than 80% for Methylene Blue dye. CFTS nanoparticles also demonstrate excellent photocatalysis and may, therefore, be used in environmental pollution control (Mukurala et al., 2021) and memory switching properties of CFTS due to its similarities to CZTS (Madhusudanan et al., 2019). The fabricated device exhibited electrical bistability in the p-type CFTS layer, which is usually associated with memory phenomena. CFTS is also used in water-splitting applications. Ni-TiO₂ (nickel-doped TiO₂) coated with CFTS using the wet chemical approach showed an increase in the photoelectrochemical performance of the device when compared to Ni-TiO₂ alone. As the holes are separated from the valence band of TiO₂, they transfer to the valence band of CFTS, resulting in the accumulation of holes at the surface of the heterostructure, where the water reacts with the holes to generate O₂. Owing to the higher charge carrier density and lower electron-hole pair recombination, CFTS-coated Ni-TiO₂ exhibited higher PEC performance than its counterparts (Mukurala et al., 2021). CFTS shows excellent photoconversion efficiency in DSSCs over TFSCs at 7% when compared to the standard DSSC with Pt at 8.2%. Therefore, it is a viable alternative to platinum (Mokurala et al., 2016; Mokurala & Mallick, 2017; Prabhakar et al., 2014). Hence, CFTS can be used in the application like photo catalysis, memory switching, thin film solar cells, thermoelectric and similar energy device applications where the cost can be brought down with the usage of low cost and abundant materials.

6. Summary and outlook

Current research on CFTS is highly limited, and its full potential is yet to be explored in depth. One method for increasing its efficiency is to use a tandem construction approach. Tandems have been successfully used to improve the efficiency of CZTS solar cells (Adewoyin et al., 2019; Amiri & Dehghani, 2020). CFTS has been explored for its potential use in memory-switching applications. CFTS was used as an alternative material, with the primary application being thin-film solar cells. However, it is observed that there is a huge gap between the efficiencies proposed by simulation studies and those of experimental studies, that is, 19% vs. 2.9%, highlighting the limited research done on the material. One reason for such a shortcoming could be the lack of in-depth research on the nature of the defects in the material. As per our understanding, the following are the kind of film growths caused due to the process parameters which gives rise to deep and shallow defects causing the limited performance of CFTS thin films. Non-uniform grain size, presence of secondary phases, lack of crystallinity occurring during the synthesis affects optical properties, mechanical strength, and film stability. Porosity and voids can cause leakage paths, decrease film density, and impact thermal stress levels. Surface roughness increases scattering losses and reduces film smoothness, affecting optical performance. Another important reason is compositional inhomogeneity. Also, intrinsic defects such as point defects (Missing or extra atoms within the crystal lattice can create energy traps and affect conductivity), Dislocations and grain boundaries (These defects can act as pathways for diffusion and recombination, impacting device performance). These defects can have various consequences on CFTS thin films such as reduced optical clarity and transmission, increased light scattering, decreased electrical conductivity. Film instability and degradation is another reason for the device failure and decreased efficiency. Mitigation strategies for reducing the defect can be a) Optimizing deposition parameters: Temperature, pressure, precursor purity, and deposition rate can significantly impact film quality. b) Substrate pre-treatment: Cleaning and surface activation can improve adhesion and reduce interfacial defects. c)Doping and compositional control: Tailoring the film composition can enhance desired properties and minimize unwanted phases and d) Post-deposition annealing: Thermal or laser treatment can improve crystallinity and reduce stress.

In addition, the values of the open-circuit voltage and fill factor were rather small, which in turn led to poor device performance and efficiency. More research needs to be given in order conducted to fully exploit CFTS as a p-type absorber material. Another area lacking research in CFTS is the optimization of the design of solar panels. Only a handful of simulation studies have aimed at optimizing the design of PV cells, and almost no experimental work exists in this area. It was also observed that CFTS is an effective material for water treatment and pollution control. CFTS nanocrystals exhibit good molecular splitting properties, making them effective for photocatalysis, water splitting, and breaking down polluting materials. A major scope for further research on CFTS as a suitable acceptor material is to study the nature of its nanocrystals in greater detail. Several applications of CFTS nanocrystals have also been emphasized in the article, and more research in these fields can greatly improve their usability as a material. As stated earlier, CFTS lacks defect studies and a significant understanding of improving the structural and morphological properties such as specific area, stoichiometric ratio of the elements, sulfurization processes, and surface characteristics, which are crucial for improving the electrical and optical properties. The subpar efficiency observed in CFTS-based thin-film solar cells (TFSCs) is linked to inherent bulk defects in the absorber, stemming from compositional irregularities, grain size variations, interfacial recombination at the buffer/absorber or back electrode/absorber interfaces, fluctuations in bandgap, and other contributing factors (Mukurala et al., 2021).

Furthermore, CFTS can find potential applications in fields such as sensors and memory switching applications, which have rarely been explored. The efficiency of thin-film CFTS PV cells can be further improved by producing tandem solar cells, a technique commonly used to improve their efficiency of solar cells. Such research currently does not exist for CFTS and can help potentially improve the efficiency of CFTS PV cells to the point of commercialization.

To sum it up, the exploration of Cu₂FeSnS₄ (CFTS) in thin film solar cells has encountered several challenges that merit consideration for the advancement of this promising material. One primary challenge is the mitigation of structural defects within the CFTS crystal lattice, which significantly impacts its optoelectronic performance. Achieving defect-free films remains a hurdle that demands innovative approaches and strategic material engineering. Additionally, the scalability and reproducibility of fabrication processes for CFTS thin films pose challenges for large-scale production and commercial viability. Moreover, despite its potential, CFTS-based solar cells have yet to attain their full efficiency, and understanding and optimizing the material’s electronic properties are imperative for enhanced device performance. Future prospects for CFTS thin film solar cells lie in addressing these challenges through advanced characterization techniques, novel fabrication methods, and tailored defect engineering strategies. Continued research efforts should focus on unravelling the intricate interplay between the structural aspects and optoelectronic characteristics of CFTS, fostering breakthroughs that can pave the way for environmentally friendly and efficient thin film solar cell technologies.

Authorship contributions

Shrenik Kalambur: Investigation; Data curation; Formal analysis; Writing - original draft. Mouli Rajesh: Validation; Writing - original draft; Writing-review & editing. Nagabhushan Jnaneshwar Choudhari and Kaya D M: Data curation; Formal analysis; Validation; Writing-review & editing. Dr Raviprakash Y: Resources; Conceptualization; Validation; Writing-review & editing; Supervision.

Disclosure statement

No potential conflict of interest was reported by the author(s).

Data availability statement

Data can be made available on a reasonable request to the corresponding author.