Iron sulfide, FeS₂ (iron(II) disulfide), is a mineral called pyrite, or iron pyrite, and also called fool's gold for its metallic luster and pale brass-yellow hue, that makes it looks like gold. Due to its abundance in nature and its low toxicity, FeS₂ is a possible PV material. Its extraction cost is so low that a pyrite-based solar cell with only 4% efficiency could be as economical as a monocrystalline silicon solar cell with 20% efficiency (Wadia et al., 2009). Pyrite has a very high absorption coefficient (~5 × 10⁵ cm⁻¹) and an energy bandgap (E_g ~ 0.95 eV) suitable for converting PV energy. XRD pattern and Raman spectrum are reported in Figures 2A,B, respectively¹. Various methods for the synthesis of thin films of pyrite have been adopted, such as Fe₂O₃ sulphuration, sputtering, spray pyrolysis (SP), vacuum evaporation (VE), chemical bath deposition (CBD), molecular beam epitaxy (MBE), electrochemical deposition, and colloidal NC synthesis (Bi et al., 2011; Puthussery et al., 2011; Morrish et al., 2012; Bai et al., 2013; Prabukanthan et al., 2017).

FeS₂ has a high potential in the large-scale production of PV modules, but for a long time, it was hard to obtain well workable thin-film devices that use pyrite as an absorber (Bi et al., 2011). Indeed, a significant PV efficiency value (8.39%) (Huang et al., 2015) was obtained by using the FeS₂ not as absorber material, but as a counter electrode in a DSC solar cell (Kilic and Turkdogan, 2017). The limiting factors are the high dark current, due to phase impurities, and a high density of acceptor surface states (Cabán-Acevedo et al., 2014). Moreover, FeS₂ is affected by thermal instability (Yu et al., 2011). Lately, Prabukanthan et al. (2017) synthesized FeS₂ thin films by electrochemical deposition at 70°C and reported efficiency of 1.98% for a solar cell with architecture ITO/FeS₂/ZnSe/Au (PV parameters in Table 1). With the aim of enhancing the photoresponse and stabilize the structure, they have introduced 3 mole% Co²⁺ as a dopant, reaching a remarkable efficiency of 5.42%. They suggest that substituting Co²⁺ for Fe²⁺ makes the bandgap decrease, improving the charge separations at the interface with ZnSe. X-ray diffraction and Raman spectra confirmed that doped and undoped FeS₂ thin films have a cubic pyrite structure; the atomic force microscopy revealed that the 3 mole% of Co²⁺ doped FeS₂ thin film exhibits a smoother surface and lower roughness values (Prabukanthan et al., 2017).

Iron selenides can be found in two stoichiometric phases, FeSe and FeSe₂. This last one, also called ferroselite, has been investigated as an electrode material in tandem PV; it is a p-type semiconductor material with a 1.0 eV bandgap (Qurashi, 2014).

Cuprous chalcogenide films, CuS, Cu₂S, Cu₂Se, are p-type semiconductors. They can be considered ideal materials for use in low-cost and non-toxic solar cells. Cu₂S, for example, is an indirect gap semiconductor with a bandgap of 1.21 eV, which is optimal for use as a light absorber (Liu et al., 2003). Since the early 80s, thin-film solar cells based on a CdS/Cu₂S junction were produced, reaching efficiencies of 9.15%, with open circuit voltage (V_oc) of 516 mV, short circuit current density (J_sc) equals to 19.3 mA/cm² and fill factor (FF) of 71.4% under a sunlight intensity of 87.9 mW/cm² (Bragagnolo et al., 1980). Subsequently, the research on these devices was abandoned due to the diffusion of Cu⁺ in CdS, to the high electron-hole recombination, and to the possibile coexistence of mixed phases ranging from CuS₂, that has a metallic conduction (Munson et al., 1967), to Cu₂S, which could give to the material a quasi-metallic behavior (Moitra and Deb, 1983; Niemegeers and Burgelman, 1986; Page et al., 2009). Nowadays, Cu₂S is successfully employed as sensitizer in DSCs: Mousavi-Kamazani et al. (2016) employed Cu₂S quantum dots as a barrier layer in DSCs, showing a considerable improvement in the efficiency of about 37%. In thin film architecture, the last efforts are devoted to stabilizing the solar cells: Wu et al. (2008) presented the synthesis of colloidal Cu₂S nanocrystals and realized a solution-processed solar cell, coupled with CdS nanorods, with 1.6% efficiency and 4 months stability. Cu_xS shows at room temperature five stable phases, ranging from the more stable structure chalcocite (x = 2), to the defective structure djurleite (x = 1.94), digenite (x = 1.80), anilite (x = 1.75), and covellite (x = 1.00); mixed phases has been observed in the intermediate compositions. Congiu et al. (Congiu et al., 2016), have indeed proposed an ink-based method for realize Cu_2−xS counter electrode for DCS, showing high stability using ferrocene as redox shuttle. Various grown methods are reported: CBD (Loferski et al., 1979), successive ionic layer adsorption and reaction (SILAR) (Mani et al., 2014), MBE (Gautier et al., 1998), VE (Nair et al., 1998), solid state reaction (Sanchez Ranjel et al., 2015), SP (Sabah et al., 2015), hydrothermal method (Patil et al., 2018), atomic layer deposition (ALD), and chemical vapor deposition (CVD) (Ye et al., 2015). CuS is 2D metal chalcogenide (Karthick Kannan et al., 2015) mostly employed as a hole transport material in solar cells, rather than as a light absorber (Lei et al., 2014; Rao et al., 2016), and thanks to the high catalytic activity, proved to guarantee high performing DSCs (Li et al., 2014; Zhang et al., 2016a,b).

Copper selenides exist in many phases and structural forms, such as CuSe (klockmannite), Cu₂Se_x, CuSe₂ (marcasite), α-Cu₂Se (bellidoite), Cu₃Se₂ (umagnite), Cu₅Se₄ (athabaskite), Cu₇Se₄, and in isometric form as Cu_2−xSe (berzelianite). They may also have different crystallographic forms (monoclinic, cubic, tetragonal, hexagonal, etc.). Copper selenides are p-type semiconductors, with both direct and indirect bandgap (Petrović et al., 2017), which may be potentially employed as PV absorbers. CuSe at room temperature has a hexagonal structure and undergoes an orthorhombic transition at 48°C and a hexagonal transition at 120°C. At higher temperatures, CuSe decomposes into Cu_2−xSe and selenium. Cu_2−xSe, at room temperature, has a cubic structure with faces centered with 0.15 ≤ × ≤ 0.2 and; when x = 0.2, it shows a direct bandgap of 2. 2 eV and an indirect bandwidth of 1.4 eV (Qurashi, 2014).

Tin can form sulfides with different Sn/S ratio, such as: SnS₂, Sn₂S₃, Sn₃S₄, Sn₄S₅, SnS (Jiang and Ozin, 1998). The most investigated are tin(II) sulfide (SnS), with a distorted GeS structure and tin(IV) sulfide (SnS₂) with a PbI₂ structure (Rimmington et al., 1972). Tin(IV) sulfide can be found in 70 polytype structures, with a hexagonal close-packed structure and different c parameters (Pałosz et al., 1990). Tin monosulphide (SnS) is a mineral called herzenbergite, and the synthesized SnS is a p-type semiconductor material, as the tin vacancies generate acceptor levels (Thangaraju and Kaliannan, 2000). Figure 3 shows the XRD pattern (a) and the Raman spectrum (b) for single-crystal SnS (Raadik et al., 2013).

When then tin to sulfur ratio is SnS_0.8, it has a bandgap of 1.16 eV (Price et al., 1999), which is like that of silicon, but with a higher optical absorption coefficient. Although the maximum theoretical efficiency of SnS thin-film solar cells is 32%, the maximum efficiency is 4.4%, reported by Sinsermsuksakul et al. (2014). They deposited, by ALD, SnS films (400 nm), using alternating doses of tin precursor vapor and a gas mixture of 4% H₂S in N₂, to enlarge the grains and reduce the recombination at the boundaries. Additionally, they engineered the n-type Zn(O,S) buffer layer (30 nm), reducing the sulfur and adding nitrogen as a dopant, to improve the rectifying quality of the p-n junction, and introduced a 1 nm thick SnO₂, to enhance the quality of the SnS/Zn(O,S) interface (Sinsermsuksakul et al., 2014). The low solar cells performance can be due also to phase impurity and off-stoichiometry (Sinsermsuksakul et al., 2014). Kim and coworkers (Kim et al., 2018) investigated the presence of Sn-S polytypes (SnS, SnS₂, Sn₂S₃) and their effect on the solar cells performance. SnS requires high sulfurization temperature, 500°C, meanwhile, the transition to SnS₂ and Sn₂S₃ happens between 150 and 300°C (Banu et al., 2017). Unfortunately, SnS₂ shows n-type conductivity and its presence affects the open circuit voltage. Sn₂S₃ has an energy gap of 1.09 eV, like the of SnS one, but a phase mixture can influence the carrier transport, generating a type II junction between Sn₂S₃ and SnS (Burton et al., 2013). Kim et al. (2018) did not verify the presence of Sn₂S₃, but they found out that the SnS₂ phase can be predominant on the thin film surface and in the shallow bulk area. Therefore, another key point is the selection of an adequate buffer layer for the SnS absorber: lately, many researchers are focused on the realization of SnS/SnS₂ p-n junction, with the aim to improve the SnS/buffer interface. Firstly, Sanchez-Juarez et al. (2005) fabricated an SnS/SnS₂ thin film hetero-junction by plasma-enhanced chemical vapor deposition, verifying the PV effect. More recently, Gedi et al. (2016) fabricated SnS/SnS₂ solar cells by CBD, the efficiency of 0.51%, proving that the junction is a type-II heterostructure. Degrauw et al. (2017) grown nanowire arrays of SnS/SnS₂ heterojunction, by chemical vapor transport (CVT) catalyzed by Cu particles, and fabricated devices with the efficiency of 1.4 %.

Tin and selenium form tin(II) selenide (SnSe), also called stannous selenide, which is a p-type semiconductor, with both an indirect bandgap at 0.90 eV and a direct bandgap at 1.30 eV (Lefebvre et al., 1998). It is receiving increasing attention for potential applications in low-cost PV, as it is an easy processable two-dimensional layered material (Wang et al., 2016; Jeong et al., 2017; Razykov et al., 2018; Ul Haq et al., 2018). fabricated SnSe thin films by chemical molecular beam deposition, using synthesized polycrystalline SnSe as precursors: XRD analysis proves that SnSe films can be grown in the orthorhombic crystalline structure. The realized films displays a direct optical bandgap of 1.1–1.2 eV and a remarkable high absorption coefficient of about 10⁵ cm⁻¹. The authors have proven that the physical properties of SnSe thin films are strongly affected by the growth conditions, and that a proper develop of the deposition methodology can lead to a material suitable for thin film solar cells. Lately Minnam Reddy et al. (2018), have fabricated a SnSe solar cells a power conversation efficiency of 1.42%.

As mentioned above, iron shows top abundance to annual production ratio, therefore p-type Cu₂FeSnS₄ (CFTS) is a very good candidate as cheap chalcogenide PV absorber layer. In addition, CFTS shows bandgap (1.28–1.50 eV) and absorption coefficient (>10⁴ cm⁻¹) suitable for PV applications.

The crystal structure of CFTS, which is a lustrous dark gray mineral, is basically stannite, however early X-ray diffraction (XRD) studies demonstrated that the tetragonal stannite phase I (−42 m) is stable in the temperature range between 420 and 500°C (Quintero et al., 1999). At higher temperature, cubic polymorphous modifications with disordered sphalerite-like structure I (−43 m) were also observed (Evstigneeva and Kabalov, 2001). XRD pattern and Raman spectrum (widely used for the identification of possible secondary phases in the fabricated films) of typical stannite CFTS² are depicted in Figures 4A,B, respectively.

So far, various methods have been reported for the preparation of CFTS absorber layers (Guan et al., 2014; Kevin et al., 2015; Khadka and Kim, 2015; Meng et al., 2015a, 2016; Chatterjee and Pal, 2017; Chen et al., 2017; Miao et al., 2017). Most of them are based on non-vacuum techniques, since they are simple, low-cost, often efficient and do not require sophisticated deposition set-up. Very few efficiency values were reported up to now for CFTS based PV devices, the best one being recently obtained by Chatterjee and Pal (2017) through SILAR method. SILAR is a low temperature non-vacuum technique, which offers a good balance between fabrication cost and phase purity, while being easily scalable for large area depositions. The as-formed CFTS thin films prepared in Chatterjee and Pal (2017) showed stannite structure with high crystallinity and a lower limit of crystallite size around 10 nm, as confirmed by XRD, Scanning Electron Microscopy (SEM), and cross-sectional SEM. The optical bandgap calculated through the Tauc's plot obtained from transmittance measurements was 1.5 eV, in agreement with the literature. Several n-type semiconductors were considered to complete the heterojunction, all of them obtained by SILAR as well. The best conversion efficiency of 2.95% was achieved with promising reproducibility in CFTS/Bi₂S₃ heterojunctions (Chatterjee and Pal, 2017). Meng et al. (2015a, 2016) also reported PV device efficiencies, obtained by solar cells based on CFTS prepared by sulfurization of the sputtered metal precursors. They showed that CFTS thin films sulfurized under fast heating rate around 40°C/min were S-poor and showed a bilayer structure with many micro-grains at the bottom of the CFTS thin film. By reducing the heating rate to 20°C/min, CFTS isolated grains were formed and the in general the grain size increases, most probably since a lower heating rate gives enough time and energy to favor the precursor crystallization and alloying. The best sputtering based CFTS solar cell prepared in Meng et al. (2016) showed a 0.11% efficiency (V_oc = 129 mV, J_sc = 3.25 mA/cm², FF = 26.6%). Guan et al. (2014) reported for the first time on the preparation of CFTS thin films by the SILAR method, obtaining CFTS layers with large agglomeration of rod-shaped grains, a bandgap of 1.22 eV and an absorption coefficient higher than 10⁴ cm⁻¹. Khadka and Kim (Khadka and Kim, 2015) employed electrostatic field assisted spray pyrolysis followed by sulfurization to produce stannite structured CFTS and CFTSe. Both the structural quality (i.e., crystalline texture and grain size) and the carrier mobility of CFTS/CFTSe thin films improved after the sulfur addition. Kevin et al. (2015) reported on CFTS, CFTSe, and CFTSSe thin films deposited by Aerosol Assisted Chemical Vapor Deposition (AACVD) at 350°C using mixtures of molecular precursors. More recently, CFTS thin films were prepared by doctor blade deposition of oxides (CuO, Fe₂O₃, and SnO₂) on glass followed by sulfurization (Chen et al., 2017) and by electrochemical deposition followed by annealing at 500–550°C (Miao et al., 2017).

Incidentally, beside its role as low-cost photo-absorber layer in thin film solar cells, CFTS was recently considered both as counter electrodes and as cheaper alternative to platinum (Pt) in dye sensitized solar cells. An extensive review on these topics is reported in Vanalakara et al. (2018).

As mentioned above, Mn provides relevant abundance to annual production ratio, therefore p-type Cu₂MnSnS₄ (CMTS) is also a good candidate as cheap chalcogenide PV absorber layer. CMTS was investigated above all as single crystal (Podsiadlo et al., 2015) or nanocrystal (Cui et al., 2012; Liang et al., 2012) for the diluted magnetic semiconductor characteristic, while, in the last few years, some papers reported on CMTS thin films for PV applications (Chen et al., 2015a,b, 2016a; Wang et al., 2015; Prabhakar et al., 2016; Le Donne et al., 2017; Marchionna et al., 2017; Yu et al., 2017). Wang et al. (2015) obtained stannite CMTS thin films by sulfurization of different precursor (Cu,Sn)S/MnS and (Cu,Sn,Mn)S films, all deposited on glass by chemical methods. The surface morphologies basically consisted of small and irregular crystalline grains (size around 18–22 nm) with a few voids. The optical bandgap values ranged between 1.15 and 1.26 eV, while the occurrence of a p-type conductivity was confirmed. Chen et al. (2015a) reported on the synthesis and the properties of stannite CMTS thin films grown by direct liquid coating and by annealing in nitrogen atmosphere, which were employed in Chen et al. (2015b) as photo-absorbers in solar cells achieving 0.49% of maximum efficiency. Some of the same authors showed also that CMTS thin films can be prepared by direct liquid coating and combining the annealing in nitrogen atmosphere and the post-sulfurization in sulfur vapors, reporting a 0.38% efficiency (Chen et al., 2016a). The same group recently investigated CMTS thin films grown by sulfurization of electrodeposited Cu-Sn/Mn metal precursors (Yu et al., 2017), which are a promising starting point for the preparation of CMTS layers by a simple, environmentally friendly and low-cost method. Prabhakar et al. (2016) presented a study on thin film solar cells based on stannite CMTS and Cu₂MnSn(S,Se)₄ (CMTSSe) layers, fabricated by spray pyrolysis using water as solvent. Proof-of-concept PV devices with structure Mo/CMTS/CdS/TCO provided the best performance (i.e., 0.73% efficiency) when CMTS layers were doped with Na in the form of NaCl during the spray process. Na in fact is widely known to improve grain growth and reduce non-radiative recombination both in CIGS and CZTS thin films. Through a careful electrical analysis (Hall measurements on exfoliated films and J-V curves under illumination of the PV devices), a very high carrier density in the CMTS/CMTSSe layers was identified as responsible for the low V_oc and FF values.

Some of the present authors reported on CMTS PV absorber synthesized by a two-step process: firstly the metal precursor stacks have been deposited by thermal evaporation, and then they have been annealed in nontoxic sulfur vapors (Le Donne et al., 2017; Marchionna et al., 2017). Of the many possible stoichiometries, Cu-poor/Mn-rich CMTS films with Mn/Sn ratio around 1 were chosen to avoid the arise of both highly conductive (e.g., Cu_2−xS) and insulating (e.g., MnS) secondary phases. In (Marchionna et al., 2017), an extensive characterization of both CMTS and any possible secondary phases by SEM, Energy Dispersive Spectroscopy, Raman, XRD, and Photoluminescence (PL) was presented. Large grain size (see Figure 5), direct band suitable for PV applications and high absorption coefficient have been reported for Cu-poor/Mn-rich samples synthesized by sulfurization with temperatures between 500 and 585°C, starting the annealing process at 115°C to trigger the alloy formation. Limited electrical performance were reached (efficiency 0.33%, V_oc = 226 mV, J_sc = 4 mA/cm², FF 36.3%) (Marchionna et al., 2017), mainly due to the presence of Cu_7.38(11)Mn₄Sn₁₂S₃₂, a spinel-type insulating secondary phase.

Cu-poor/Mn-rich CMTS layers with better homogeneity of the metal ratios were presented in Le Donne et al. (2017), reached through an enhanced control of the precursor evaporation rate; PV devices showed improved performance compared to (Marchionna et al., 2017): efficiency 0.5 vs, 0.33%, V_oc = 302 vs, 226 mV, J_sc = 4.6 vs. 4 mA/cm², FF 36 vs. 36.3%. Considering the beneficial consequences of low temperature post-deposition thermal treatments on kesterite based solar cells (Neuschitzer et al., 2015; Jiang et al., 2016), the effect of annealing temperature ranging from 200 to 275°C on the same CMTS devices was investigated, too. The best annealing in air at 225°C for 40 min led to a significant reduction of recombination losses without a strong increase of CdS absorption, allowing for a remarkable improvement of CMTS solar cell performance (J_sc = 5.8 mA/cm², FF 40%, V_oc = 354 mV, efficiency 0.83%) (Le Donne et al., 2017).

As it is known in the literature, V_oc in CZTS/CZTSSe is mainly limited by the band tailing related to cation-cation antisite disorder and by the following potential fluctuations (Gokmen et al., 2013). The main antisite defects, responsible of the antisite disorder, are copper-on-zinc (Cu_Zn) and zinc-on-copper (Zn_Cu) (Chen et al., 2013). Furthermore, Sn multivalency could induce deep levels in the bandgap, which in turn could cause non-radiative recombination, when Sn²⁺ occupies a Zn²⁺ site (Chen et al., 2013). Thanks to the distinct electronic properties exhibited by Cu, Sn and Ba, the formation of cation–cation antisite defects in Cu₂BaSnS₄ (CBTS)/Cu₂BaSn(S,Se)₄ (CBTSSe) has been proven to be difficult (Xiao et al., 2017). Therefore, in addition to the good abundance to annual production ratio provided by Ba, CBTS/CBTSSe could potentially lead to better optoelectronic properties than CZTS/CZTSSe. Up to now, very few works reported on CBTS/CBTSSe, which mainly deal with vacuum deposition methods (Ge et al., 2016; Shin et al., 2016, 2017). Shin et al. investigated Cu₂BaSnSe_xS_4−x films with different Se contents prepared by co-sputtering followed by sulfurization/selenization (Shin et al., 2016). Of the different examined stoichiometries, combined experimental and theoretical analyses showed that Cu₂BaSnSe_xS_4−x thin films with 0 < x ≤ 3 compositions are isostructural to CBTS with space group P3₁ and display a tunable bandgap in the 1.6–2 eV range, which well fits the optimal values for being employed both in single junction (i.e., 1–1.6 eV) and top solar cell in multijunction (i.e., 1.7–2.0 eV) PV devices. Furthermore, optical absorption, External Quantum Efficiency and PL data confirmed the expected absence of band tailing in CBTS. Last but not least, prototype CBTS-based thin-film solar cells with 1.54% average efficiency have been prepared and tested (V_oc = 699 mV, J_sc = 4.1 mA/cm², FF = 53.5%) (Shin et al., 2016). Some of the same authors obtained further improvements by combining the bandgap tuning related to the introduction of Se with a post-deposition annealing in air, producing a CBTSSe-based PV device with 5.2% efficiency (Shin et al., 2017). Ge et al. (2016) studied polycrystalline Cu₂BaSn(Se_0.83S_0.17)₄ (CBTSSe) thin films grown on fluorine tin oxide (FTO) by co-sputtering of a sulfide precursor followed by selenization. In contrast to the XRD results reported in (Shin et al., 2016), these layers (Ge et al., 2016) are isostructural to CBTSe with space group Ama2. Conversely, they exhibit, as expected, an optical bandgap around 1.85 eV, absorption coefficient higher than 10⁴ cm⁻¹, and p-type conductivity. A proof-of-concept PV device with FTO/BCTSSe/CdS/ZnO/AZO structure showed 1.57% efficiency (V_oc = 613 mV, J_sc = 6.78 mA/cm², FF = 37.7%) (Ge et al., 2016).

Theoretical calculations claim that the substitution of Zn with Ni in CZTS could reduce the optical bandgap and potentially enhance electrical conductivity (Ghosh et al., 2014). These attractive features along with the sufficient abundance to annual production ratio provided by Ni make Cu₂NiSnS₄ (CNTS) worth to be considered for TW range PV applications. Currently, few works on CNTS are present in the literature, most of them reporting on thin films prepared by non-vacuum techniques, namely electrodeposition followed by sulfurization (Chen et al., 2016b; Yang et al., 2016), spray sandwich method (Dridi et al., 2017; Bitri et al., 2018) and direct solution coating followed by sulfurization (Mokurala et al., 2017). As expected, the layer morphology was generally found to be strongly dependent on the substrate, the larger grain size being obtained on SLG due to the higher diffusion of Na from the substrate. Hall measurements showed that after sulfurization an increase in mobility and a decrease in resistivity occur (Mokurala et al., 2017), which were attributed to the passivation of defects (mainly V_S) and to the enhanced crystallization and in the CNTS thin films. The high absorption coefficient (>10⁴ cm⁻¹) and the suitable optical bandgaps (i.e., 1.29–1.50 eV) generally obtained, along with some preliminary low resistivity values [≅ 0.14 Ωcm (Mokurala et al., 2017)], suggest that cubic p-type CNTS thin films are promising for PV applications.

Among the chalcogenide thin film absorbers based on less abundant elements (i.e., Co, Cd, and Ag), most of the studies reported in the literature deals with Cu₂CdSnS₄ (CCdTS)/Cu₂CdSn(S,Se)₄ (CCdTSSe). In fact, despite the low abundance to annual production ratio provided by cadmium, the replacement of Zn by the bigger Cd atom leads to a reduced presence of detrimental Zn_Cu antisite defects, which could enhance the conductivity (Hussain et al., 2016). Furthermore, the CCdTS bandgap is nearer than the CZTS one to the optimal value calculated by Shockley and Queisser (i.e., 1.34 eV) (Smets et al., 2016), which could potentially lead to more efficient PV devices. CCdTS, which more stable crystalline form is stannite, shows p-type conductivity related to the presence of Cu vacancies (V_Cu) and/or Cu_Cd antisites (Meng et al., 2015b). So far, CCdTS/CCdTSSe was mainly investigated in the form of nanocrystal (Fan et al., 2011; Ramasamy et al., 2013), while few works reported on the deposition of CCdTS/CCdTSSe thin films (Timmo et al., 2013; Nie et al., 2015; Zhao et al., 2015; Henry et al., 2016; Xu et al., 2016; Rouchdi et al., 2017) and in particular on CCdTS/CCdTSSe based PV devices (Timmo et al., 2013; Zhao et al., 2015). Among the latter, Timmo et al. prepared CCdTS monograin powders with 1.4 eV bandgap from molten CdI₂ or KI as flux materials sealed in quartz ampoules at 610°C, reaching a 2.7% efficiency (Timmo et al., 2013). Zhao et al. (2015) reported on stannite CCdTSSe thin films prepared by a chemical solution approach, which showed large densely packed grains and suitable bandgap around 1 eV. Solar cell prototypes based on them showed a 3.1% efficiency. However, it should be remarked that Cd as well as many Cd based compound are toxic, making CCdTS/CCdTSSe not fully compatible with a green energy supply.

Finally, few literature works explored Cu₂CoSnS₄ and Ag related quaternary alloys as thin film PV absorbers, namely (Krishnaiah et al., 2015; Gershon et al., 2016a,b; Ghosh et al., 2016). Particularly attractive results were obtained for Ag₂ZnSnSe₄ (AZTSe) (Gershon et al., 2016a) and for (Ag_xCu_1−x)₂ZnSnSe₄ (Gershon et al., 2016b), which both showed efficiencies much higher than those reported in most of the previous paragraphs (see Table 1). Most probably, the reason is that the introduction of Ag in CZTSe was demonstrated to strongly reduce the band-tailing effect responsible for the V_oc loss in kesterite thin film solar cells, which was discussed above in the CBTS section. Theoretical calculations (Chagarov et al., 2016) showed in fact that the formation energy of I-II antisite in AZTSe is significantly higher than in CZTSSe, which reduces the I-II antisite density by at least one order of magnitude. The band-tailing reduction is particularly relevant for pure-Ag materials (i.e., AZTSe), however Gershon et al. (2016b) showed that introducing only 10% Ag in CZTSe allows to obtain a remarkable 10.2% efficiency. Despite a further optimization could potentially improve the performance of (Ag_xCu_1−x)₂ZnSnSe₄ based solar cells (Gershon et al., 2016b), silver is a further element under serious threat of extinction in the next 100 years (Harland et al., 2013), so most probably they will have a marginal role in the PV TW challenge.