Insights to the production of SnS-cubic thin films by vacuum thermal evaporation for photovoltaics

Thin films of SnS-CUB with a lattice constant of 11.6 Å, 32 units of SnS per cell and an optical bandgap (E g) of 1.7 eV (direct), are mostly produced by chemical techniques. This cubic polymorph is distinct from its orthorhombic polymorph (SnS-ORT) with an E g of 1.1 eV. This work is on the deposition of SnS-CUB thin films of 100–300 nm in thickness by thermal evaporation at substrate temperatures of 400 °C–475 °C on glass or on a chemically deposited SnS-CUB thin film (100 nm). Under a slow deposition rate (3 nm min−1) from a SnS powder source at 900 °C, the thin film formed on a SnS-CUB film or glass substrate at 450 °C is SnS-CUB. At a substrate temperatures of 200 °C–350 °C, the thin film is of SnS-ORT. A low atomic flux and a higher substrate temperature favor the growth of SnS-CUB thin film. The E g of the SnS-CUB film is nearly 1.7 eV (direct gap), and that of the SnS-CUB film is 1.2 eV (indirect gap). The electrical conductivity (σ) of SnS-CUB and SnS-ORT films are 10–7 and 0.01 Ω–1 cm−1, respectively. A proof-of-concept solar cell of the SnS-CUB thin film showed an open circuit voltage of 0.478 V, compared with 0.283 V for the SnS-ORT solar cell. The insights to the deposition of SnS-CUB and SnS0.45Se0.55-CUB (E g, 1.57 eV; σ, 0.02 Ω−1 cm−1) thin films by vacuum thermal evaporation offer new outlook for their applications.


Introduction
Tin sulfide (SnS) is known to form in the orthorhombic (ORT) and cubic (CUB) crystalline phases.The mineral form of SnS-ORT is herzenbergite, named after Roberto Herzenberg (1885-1955) with its x-ray powder diffraction files (PDF) 39-0354, 75-1803, etc.The unit cell axes of SnS-ORT are: * Author to whom any correspondence should be addressed.
Original content from this work may be used under the terms of the Creative Commons Attribution 4.0 licence.Any further distribution of this work must maintain attribution to the author(s) and the title of the work, journal citation and DOI. a = 4.3291, b = 11.1923, and c = 3.9838 Å, with four SnS formula units in it and with a mass density of 5.187 g cm −3 [1].This material has a direct bandgap (E g ) of 1.3 eV and an indirect E g of 1.1 eV [2].Its enthalpy of formation is −100 kJ mol −1 , melting point is of 880 • C and boiling point, 1230 • C [3].This is the most common crystalline phase in which SnS is found in laboratory preparations and as a mineral.In the year 1967, SnS of cubic crystalline rock salt structure (SnS-RS) was assigned to thin film produced by thermal evaporation of SnS powder source on NaCl crystal, freshly cleaved with its (100) or (110) planes [4].This thin film of SnS-RS was of 100 nm in thickness and was produced at a deposition rate of 60 nm min −1 at a substrate temperature of 150 • C. When the substrate was at room temperature (23 • C), the film was of mixed SnS-ORT/SnS-RS.This observation suggests that the increased atomic mobility of Sn and S on the film surface at a higher temperature promoted the formation of the SnS-RS polymorph.The lattice constant of SnS-RS is 5.80 ± 0.02 Å, as cited in PDF 77-3356 [5].
In 2006, a new crystalline structure in the cubic system was detected in SnS nanocrystals prepared by a reaction of SnCl 2 and elemental sulfur powder in oleylamine at 170 • C [6].Based on the x-ray diffraction peaks of the material, a zinc blende structure (SnS-ZB) with a lattice constant a = 5.845 Å was assigned to it.The prominent peaks in the diffraction pattern were assigned to the diffraction from the (111) and (200) crystalline planes of a material with a ZB structure.Thin films of SnS produced by chemical deposition at 26 • C [7] and by successive ion-layer adsorption and reaction on chemically deposited seed layers [8] reported during 2008-2011 appeared to belong to the same crystalline structure.Hence, they were also considered as SnS-ZB.However, a theoretical study reported in 2012 [9] found that the initial assignment of a zinc blende (ZB) structure to this material is unlikely to hold true because of its thermodynamic instability: a SnS-ZB structure will not be stable under the reported experimental conditions.
The correct structural assignment for the material reported as SnS-ZB came in 2015 from new results on SnS nanocrystals [10].The proposed structure was a large cubic cell with a lattice constant of 11.6 Å-double that of SnS-RS (5.80 Å) [5].This unit cell held 32 SnS formula units.Lacking in a mirror plane symmetry, this is a chiral material.Thin films of SnS produced by chemical deposition [11] were assigned the same structure, with a = 11.600± 0.025 Å [12].This material was designated as π-SnS, or as SnS-CUB (large-cubic).Based on further study on nanocrystals prepared in a similar way as in [6,10], PDF 86-9477 for π-SnS (SnS-CUB) became available in 2017 [13].The cited lattice constant of the material was of 11.5680 Å and its calculated mass density was of 5.175 g cm −3 , essentially the same as that of SnS-ORT (5.187 g cm −3 ) [1].The diffraction peaks previously assigned to (111) and (200) crystalline planes of SnS-ZB correctly belonged to the diffraction from the (222) and (400) crystalline planes of SnS-CUB.The ratio of the intensity of XRD peaks [I (222) /I (400) ] from these planes was nearly unity, shown previously as improbable with [I (111) /I (200) ] for RS or ZB structures of SnS [9].The XRD pattern of SnS-CUB has a 'signature triple peak' [11] with intensities, 100:30:15 belonging to the diffraction from the (400), ( 410) and (411) crystalline planes.Henceforth, this signature became the distinguishing feature of the SnS-CUB polymorph.This material has an E g of 1.74 eV [11], notably larger than that of SnS-ORT (1.1-1.3 eV) [1,2].
Theoretical studies in 2017 found that the enthalpy of formation of π-SnS (SnS-CUB) was −92.46 kJ mol -1 , compared with −95 kJ mol −1 for the 'ground state structure,' SnS-ORT [14,15].The lattice constant resulting from the model for the cubic phase was 11.506 Å, which is nearly 0.1 or 0.07 Å, respectively, lower than that reported for SnS-CUB thin films [11,12] or for nanocrystals [6,13].The E g values from the theoretical model for the π-SnS (SnS-CUB) was 1.72 eV (indirect) and 1.74 (direct), in agreement with the values reported for SnS-CUB thin films [11].Both SnS-ORT and SnS-CUB polymorphs would be 'phonon-stable' at temperatures even above 600 • C [14,15], which suggests that they may coexist at such temperatures.Thin films or precipitates of SnS-CUB were indeed stable upon heating at temperatures of up to 500 • C, under controlled ambient, as reported in 2018 [16].The high thermal stability of the SnS-CUB polymorph is consistent with the results reported on its nanocrystals and for thin films [7] when these were being considered as SnS-ZB.
For developing device applications for SnS-CUB thin films, vacuum thermal evaporation from powder sources or by sputtering techniques becomes desirable to ensure large area capability and reproducibility on a production line, as in CdTe and Cu-In-Ga-S-Se (CIGS) photovoltaic module technologies [22,23].The advantage of having a SnS-CUB (1.74 eV)/SnS-ORT (1.1 eV) absorber to secure a higher V oc (0.488 V) while maintaining a short circuit current density (J sc ) of 6.93 mA cm −2 compared with a SnS-CUB-only absorber produced by chemical deposition was discussed in [18].Thus, it is desirable to adapt this result [18] for vacuum techniques and move toward a photovoltaic technology using these materials.A need for a higher process temperature to produce 'device quality SnS-CUB' to achieve the technological prospects for SnS-CUB was expressed in [14].Vacuum thermal evaporation method is reliable and suitable for scale-up.However, the reported work to date showed much difficulties in obtaining SnS-CUB by this route; the thin films produced turned out to be of SnS-ORT or of mixed polymorphs of SnS-ORT/CUB.
The present work provides insight on how to ensure the deposition of SnS-CUB films by vacuum thermal evaporation.A proof-of-concept solar cell with a SnS-CUB absorber prepared this way with a relatively high V oc is presented.Preliminary results on SnS-Se-CUB thin films produced by thermal evaporation, which may serve to improve these photovoltaic structures, are also presented.An outlook on the technological prospects of SnS-CUB, SnS-Se-CUB and Sn-Ge-S-CUB thin films as photovoltaic and optical materials is included to promote further research in this theme.

Setting up the conditions to deposit SnS-CUB thin films by vacuum thermal evaporation
In order to seek guidelines to prepare SnS-CUB thin films by vacuum thermal evaporation, one may consider past work.In a 1994 report on n-CdS/p-SnS solar cell, the SnS thin film was deposited at substrate temperature of 250 • C-285 • C from SnS powder source [24].The film formed was of SnS-ORT; and the solar cell gave a V oc of 0.12 V with η, 0.29%.In 2014, a solar cell of SnS-ORT absorber film with η of 3.88% was produced by thermal evaporation at a deposition rate of 6 nm min −1 from powder source on Mo-substrate at a temperature of 240 • C [20].In a solar cell reported, also in 2014, with η of 4.36% the SnS film was deposited from vapor phase at a slow rate inherent in ALD, of 0.24 nm min −1 , at a substrate temperature of 200 • C [19].Results on SnS thin films by thermal evaporation in 2017 [25] were published after SnS-CUB was already recognized as a polymorph of SnS [10][11][12][13].In that work, thin films deposited at a substrate temperature of 250 • C showed the presence of SnS-CUB as well as of SnS-ORT.At higher substrate temperatures of 300 • C-350 • C, the thin films were of SnS-ORT.The deposition rate (not specified) might have been 10 nm min −1 .In thin films produced by ALD reported in 2021 [26], a dominant presence of SnS-CUB along with SnS-ORT was observed when the substrate temperature was below 140 • C. At higher temperatures (200 • C) as used for the solar cells [19], the thin film was of only SnS-ORT.
In aerosol-assisted chemical vapor deposition (AA-CVD) of thin films prepared via the decomposition of dimethylamido(N-phenyl-N',N'-dimethyl thioureate)Sn(II) dimer reported in 2020 [27], SnS-ORT was the material of the film produced at a substrate temperature of 300 • C, but at 375 • C, the film was of SnS-CUB.This followed an earlier work in 2015 [28], but the observation reported at the time was that SnS-CUB film was formed at 300 • C; but at higher temperatures, 350 • C-450 • C, the thin film deposited was of SnS-ORT.These might have been the first instances when a SnS-CUB thin film (100% phase-pure) was formed from vapor phase on substrates at 300 • C-375 • C. The lattice constant of the material of this SnS-CUB film was 11.62658 Å [27], within the value reported for chemically deposited SnS-CUB thin film (11.600 ± 0.025 Å) in [12].These values are larger than 11.5680 Å stated in PDF 86-9477 based on XRD results on SnS-CUB nanocrystals [13], where such disparity between the values was recognized.Nevertheless, in the chemical vapor transport (CVT) deposition from SnS powder held at 600 • C on Mo-substrates at 490 • C under a high rate of growth of 100 nm min −1 reported in 2018 [29], the film formed was of SnS-ORT with no evidence for SnS-CUB.A solar cell using this material showed a V oc of = 0.29 V with η of 2.94%.
In 2022, the coexistence of SnS-ORT and SnS-CUB platelets was reported in the condensate formed by CVT at a temperature of 520 • C under an argon gas flow downstream from the SnS powder source at 550 • C [30].From the results on scanning tunneling microscopy/spectroscopy (STM-S) on the SnS-CUB platelets, a bandgap of 1.7 eV was obtained, which was previously determined for the SnS-CUB thin films deposited at 17 • C [11,12] and also predicted from theoretical models [14,15].The results implied that a higher vapor flux at a higher substrate temperature is not conducive toward the formation of SnS-CUB, but at a lower flux, SnS-CUB polymorph would compete with or dominate over SnS-ORT.As reported in 2018 [31], thin films (720 nm in thickness) produced at a rate of 20 nm min −1 from SnS powder source at 930 • C on substrates at 400 • C by vacuum thermal evaporation was SnS-ORT with orientation of (111) planes parallel to the substrate.Thin films of tin sulfide deposited from a SnS target by radio frequency sputtering at a rate of 350 nm min −1 on substrates at a temperature of 200 • C reported in 2021 were also of SnS-ORT [32].These films were of crystalline grains oriented with (040) planes parallel to the substrate.Solar cells of these films showed a V oc of 0.087 V.

Rate of deposition-a vital parameter to assure the formation of SnS-CUB
A notable aspect seen among the results on SnS-CUB polymorph presented above is that it can be the 'preferred material' deposited at temperatures of 200 • C-520 • C under suitable conditions.The results in [29][30][31][32] showed that at a high deposition rate of 20-350 nm min −1 , SnS-ORT polymorph prevails in the material at substrate temperatures of 200 • C-550 • C. Unlike in vacuum deposition techniques (ALD, CVD, CVT, thermal evaporation or sputtering), in chemical deposition the source and substrate are at the same temperature.In a 2014 work on chemically deposited SnS thin films [33], it was found that at a deposition temperature of 20 • C where the rate of the film growth was 0.16 nm min −1 , the film deposited was notably compact with an E g close to 1.7 eV.From its distinct XRD pattern, different from that of SnS-ORT, a 'cubic structure' was assigned to it.At a deposition temperature of 40 • C, the film growth rate increased to 0.4 nm min −1 .This film had a flaky surface morphology, typical of SnS-ORT; its XRD pattern was in agreement with PDF 39-0354.The E g of this film was 1.1 eV (indirect), distinct from that of the film deposited at 20 • C-26 • C. One may note that by the year 2016, π-SnSe as nanocrystals [34] or as SnSe-CUB thin films [35] were also reported, produced via chemical routes with a = 11.9702 or 11.9632 Å, respectively, and 32 SnSe units per unit cell.This opened up the prospects for the formation of SnS-Se-CUB materials as well [36], in this case as thermoelectric materials.
From the results on SnS-CUB portrayed above, guidelines emerge to set up the conditions for the deposition of SnS-CUB films by vacuum thermal evaporation (TE) and to avoid the deposition of SnS-ORT or of mixed polymorph thin films of SnS-CUB-ORT: (i) These films would form at temperatures 375 • C [27] to 525 • C [30] if the growth rate (vapor flux on the growing film) remains below 5 nm min −1 , which is unusually low for typical vacuum-deposition of thin films.(ii) The preference for the growth of the SnS-CUB compared with SnS-ORT at a higher temperature under low vapor flux [27,30] is analogous to the preferred formation of SnS-RS in the epitaxial growth on NaCl (100) and (110) planes at 150 • C, whereas at 23 • C mixed phase of SnS-RS and SnS-ORT resulted [4].(iii) The coexistence of SnS-ORT and SnS-CUB observed in thin films produced by TE at substrate temperature of 275 • C, and the disappearance of the SnS-CUB phase at a higher temperature are caused by a relatively higher growth rate (10 nm min −1 ) [25], which is analogous to the absence of the SnS-CUB platelets in the material prepared by CVT [30] on the substrate at 550 • C subjected to a higher vapor flux.(iv) It is unlikely that SnS-CUB thin films may be deposited in a high growth rate condition of several 100 nm min −1 as in sputtering [32], irrespective of the substrate temperature.In these cases SnS-ORT polymorph would dominate, caused by low ad-atom mobility on the growing surface, which is not conducive for the formation of SnS-CUB.Hence, close-space vapor deposition or electron beam evaporation used for thin film solar cell technology with high growth rate at substrate temperature 400 • C-550 • C [22,23] also may not be applicable in the production of SnS-CUB to substitute as an absorber film in such technologies.
In the present work, in SnS thin films produced by TE on glass substrates at 450 • C under a growth rate of 3 nm min −1 from SnS powder source, SnS-CUB polymorph dominated.When the film was deposited on a layer of chemically deposited (CD) SnS-CUB thin film (80-100 nm in thickness), the material of the thin film produced was of 200-350 nm in thickness; it was entirely of SnS-CUB.Corning soda lime microscope glass slides (product 0215) of 25 mm x 75 mm area and of 1 mm in thickness were used.Mechanical stability of these substrates is appreciably good at temperatures of up to 510 • C-545 • C. Thus, these were adequate for depositing the thin films at substrate temperatures of up to 475 • C planned for this work.These substrates were cleaned in neutral soap solution, rinsed in deionized water, and dried prior to the deposition of the thin films.

SnS-CUB thin films on glass substrates by chemical
deposition.The following procedure, based on past work, leads to the deposition of compact SnS-CUB films, which were well-adhered to the Corning glass substrates-(i) Pretreatment of glass substrates for improved adhesion of the thin films-Cleaned Corning microscope glass slides were immersed for 30 min in a 0.02 M (mol dm −3 ) aqueous solution of SnCl 2 • 2H 2 O (J T Baker laboratory reagent, 96.4% assay) at room temperature.These substrates were rinsed in water and dried.This is an alternative pretreatment for the chemical deposition of SnS-CUB thin films reported in a 2020 work [36], which is distinct from that of a Na 2 S pretreatment mentioned in 2016 for the chemical deposition of SnS-CUB thin films [11].For the Na 2 S pretreatment, an aqueous solution of Na 2 S.9H 2 O (Fermont, laboratory reagent, formula mass 240 g mol −1 ), of 0.03 M in concentration was prepared by dissolving 0.72 g of the material in 100 ml of deionized water.The cleaned glass substrates were immersed for 16 h (overnight) in this aqueous solution at room temperature.The treated substrates were rinsed in deionized water and dried prior to depositing the thin films.In both these cases, the underlying mechanism for improved adhesion is not well understood [37].this by adding deionized water to it.This solution should be used within three days of its preparation and storage at room temperature to assure the deposition of good quality SnS-CUB thin films.After three days, the solution slowly turns turbid.Henceforth this stock solution would not be suitable for the deposition of the SnS-CUB films-they would lack in adhesion to substrate and would lack homogeneity.(iii) Chemical deposition (CD) of a SnS-CUB film of 80-100 nm in thickness on glass substrates-These would serve as a 'substrate film' for the deposition of SnS-CUB thin films on it by TE.For the deposition of these films, 10 ml of the above stock solution was taken in a 100 ml beaker.The following reagents (J T Baker, laboratory grade) were added to it with stirring: 30 ml of 3.7 M triethanolamine (TEA, diluted to 50% of the as-supplied reagent), 16 ml of 30% NH 3 (aq., as supplied, 16 M), and 10 ml of 0.1 M solution of thioacetamide.Deionized water was added to this mixture to take the volume to 100 ml.Initial pH of this solution was 11.Up to six pre-treated glass substrates were supported on the wall of the beaker containing this solution, which was placed in a circulation bath at 8 • C for 16 h (overnight).The films were taken out, rinsed and dried under ambient and stored.These were specularly reflecting compact SnS-CUB films, which appear orange-red in transmitted daylight.(iv) SnS powder source for the thermal evaporation-The solution in the beaker left after the SnS-CUB thin film deposition, mentioned as in (iii) was kept at room temperature (25 • C) for a further 10 h.The precipitate settling to the bottom of the beaker was filtered, rinsed in deionized water, dried at 80 • C in a drying oven, and stored in a sample bottle.In the present work we established that this precipitate would serve as a thermal evaporation source instead of the commercially acquired powder source.In a similar way, powder source of SnS-ORT may be collected as precipitate produced at 40 • C-50 • C, as in the work reported on SnS-ORT thin films [33].The work presented here was done with commercially available SnS powder source of SnS-ORT (Sigma Aldrich, 98% assay) supplied in 10 g sample bottles, discussed in a previous work [16].
Typically 650 mg of this powder will be used in a molybdenum boat to produce by vacuum thermal evaporation SnS-ORT or SnS-CUB thin films, depending on the substrate temperature, vapor flux at the substrate, and the nature of the substrate-glass or a chemically deposited SnS-CUB substrate film (iii).

Vacuum thermal evaporation (TE) of SnS powder sources
The vacuum system-Details of the TE system used for this work (Torr International TP2.5), with substrate heating and source-to-substrate distance of 40 cm may be found in a 2021 report [38].Two cleaned glass slides along with 2 glass slide/SnS-CUB substrate films of 80-100 nm in thickness were clamped on a stainless steel substrate holder disc of 15 cm in diameter.It was then attached to a motor which rotates the sample holder at 5 revolutions per min.Tungsten-halogen lamps in quartz envelopes with a reflector placed behind the rotating sample holder maintained the temperature of the substrates at 200 • C-475 • C by controlling the on-off duration of the lamps.The stated temperature may vary within ±12 • C at the surface of the glass substrates of 1 mm in thickness.The powder source, typically of 650 mg, was placed in Mo-boats (Kurt J Lesker).A few drops of propylene glycol were added to it so as to mix and smear it well inside the boat.It was then dried at 80 • C in air for 180 min in a drying oven so that the solvent evaporated and the SnS powder source maintained a good thermal contact with the Mo-boat.This boat was clamped across Cu-posts.The pumping system consisting of a mechanical pump and a turbo-molecular pump brought the chamber pressure down to 10 -6 torr in about 30 min.A direct current applied across the boat takes the source temperature to typically 800 • C-900 • C in 5-10 min.The source temperature was controlled in such a way that a quartz crystal thickness monitor gave a deposition rate of 0.5-0.8Å s −1 or 3-5 nm min −1 for the SnS thin film, based on the input data on its mass density (5.15 g cm −3 ).The system lacked a feedback control circuit between the thickness monitor and the source heater; thus, the control was done manually.
The deposition rate for the film was maintained near 3-5 nm min −1 for this work, based on the insights acquired from the past work, presented in section 2. The duration of the deposition was about 120 min to produce a thin film of 250 nm in thickness under this slow growth rate.For the sake of comparison, we also present the polymorphic composition of the films deposited under a faster growth rate (13 nm min −1 ) to illustrate its effect on the relative composition of SnS-CUB/ORT phases in the film.Example of a thicker SnS film (450 nm) deposited from SnS source of 1000 mg at a substrate temperature of 450 • C is also presented to illustrate the uniformity of the phase-composition of the film across its thickness.Preliminary result on the formation of SnS-Se-CUB thin films through the evaporation of SnS (300 mg) + SnSe (300 mg) as the source is also presented.

Characterization
The thickness of the films deposited on the glass slides or on glass-SnS-CUB thin films was measured using an Ambios Technology XP200 step-measurement unit.In some cases, the value for the thickness was confirmed from a cross sectional view of the film recorded in a Hitachi 5500 field emission scanning electron microscope (FESEM).This FESEM carried a Bruker energy dispersive x-ray emission spectrum (EDS) analyzer, which provided an assay of the chemical composition of the films.The electron beam was at 7 keV so as to inhibit the emission from the elements of the glass substrate.In all cases of SnS films reported here, the S:Sn atomic ratio in the films was at nearly 1:1.The XRD data were obtained in a Rigaku ULTIMA IV diffractometer using Cu-K α1 radiation of wavelength (λ) of 1.5406 Å in the usual θ-2θ mode (θ is the Bragg angle) or in the grazing incidence (GIXRD) mode.The crystalline structure/chemical composition in the film was studied along its thickness, from the surface toward the film/glass interface.The penetration depth (PD) of the xrays into the SnS film (PD = 8.81 µm) was calculated from the mass density of 5.15 g cm −3 for SnS-CUB or SnS-ORT and its mass absorption coefficient (221 cm 2 g -1 ) for these x-rays as described in [39].The sampling depth (SD) = PD (sin δ) into SnS (CUB or ORT) films in GIXRD was calculated.For the Cu-K α1 x-ray beam making an angle (δ = 0.5 • , 1.5 • and 2.5 • ) with the sample plane while entering the film, the SD's are 77, 231, and 384 nm, approximated to 80, 240 and 400 nm, respectively.Thus, the presence of SnS-CUB with the characteristic triple peaks for the diffraction from the (400), ( 410) and (411) planes with relative intensities of nearly 100%, 30% and 15% in the 2θ of 30-33 • interval from SnS-CUB may be distinguished from the (111) and (040) diffraction peaks with relative intensities 100% and 50% from the SnS-ORT polymorph appearing within the same interval.The presence of any SnS-ORT component in the material of the film dominated by SnS-CUB, would manifest itself in an increase in the intensity to >35% of the (410) SnS-CUB peak at 2θ of 31.82 • , because the (111) and (040) diffraction peaks of SnS-ORT are at 2θ of 31.53 • and 31.57• , causing an overlap of intensities at this position.
The optical transmittance (T) and specular reflectance (R) of the films were recorded using a JASCO 670 spectrophotometer in the spectral range 250-2500 nm for 'film-side incidence' with air as the reference for T and a front-aluminized standard mirror (supplied) for R. In the case of solar cells, the T and R spectra were recorded for the glass-FTO side incidence to represent the solar cell operation.The optical absorption coefficient (α) of the film for any wavelength was calculated by considering multiple reflections within the film [40].From the plots of αhν raised to a power 2 /3 or 1 /2 versus photon energy (hν), a value for E g of nearly 1.7 eV (direct gap) for SnS-CUB film or of nearly 1.1 eV (indirect gap) for a SnS-ORT film may be determined.For the electrical measurement, pairs of silver paint lines of 5 mm in length at 5 mm separation were applied on the film surface and were allowed to dry at 80 • C for 30 min.The sample was placed in a darkened box and it was allowed to stabilize under the dark.A computerized measurement system with a Keithley 619 multimeter and a 230 programmable voltage source allowed the measurement of the current under a bias voltage of 5 V applied across the electrodes.The current through the sample was measured at 0.05 s interval during 2 s in the dark, 2 s under an illumination and 2 s in the dark.The illumination at the sample plane was of 850 W m −2 in intensity, provided by a tungsten halogen lamp (3400 K, 100 W).From the sample dimensions, these data were converted to the electrical conductivity in the dark (σ d ) and under the illumination (photo, σ p ) by assuming a uniform illumination across the thickness of the film (which is an approximation).

Results and discussion
Figures 1 and 2 show the XRD patterns recorded on thin films of 200-300 nm in thickness deposited at T s of 200 • C-475 • C on glass substrates (figure 1) and on glass/SnS-CUB(100 nm) (figure 2).The temperature of the surface of the glass substrate (1 mm in thickness) may be lower by about 6 • C-10 • C, depending on the set temperature, T s .While the temperature of the boat was raised toward 900 • C with temperature of the substrates already set at the T s , the evaporation rate settled at 3 nm min −1 , briefly passing through 4 nm min −1 , and finally dropping to below 2 nm min −1 in about 120 min when the thermal evaporation was terminated.Some powder residue (20-30 mg) remained in the Mo-boat.In setting up the condition for the deposition, a lack of repeatability was observed on the polymorphic composition of the film at T s at or above 400 • C, which was soon linked to the dependence of the rate of deposition on the morphology of the powder.In section 4.2 we shall illustrate that at a rate of deposition exceeding 7 nm min −1 , the SnS-ORT polymorph would dominate the content of the film.We identify such higher deposition rate as the principal reason why SnS-CUB thin films by thermal evaporation were not commonly reported so far.
In figure 1, the XRD pattern of the thin film deposited at a T s of 200 • C shows that it is of SnS-ORT polymorph.The combined intensity of (111) diffraction peak (100%) observed at 2θ of 31.64 • and of the (040) diffraction peak (50%) at 2θ of 31.97 • and any preferred orientation of these crystalline planes within the crystallites with respect to the glass substrate surface explain this diffraction pattern.This feature is commonly observed in SnS-ORT thin films formed from the vapor phase [31,32].The SnS-ORT composition of the film is accompanied by its E g of 1.2 eV and its p-type electrical conductivity of 0.01 Ω −1 cm −1 (following sections).An increase in the T s to 350 • C increases the mobility of the Sn and S atoms condensing on the film surface from the vapor phase.
Many of the diffraction peaks of SnS-ORT are identified in the material of this film, but one also identifies the presence of the diffraction from the (222) planes of the SnS-CUB polymorph near 2θ of 26.8 • , situated between the diffraction peaks from the (120) and (021) planes of SnS-ORT.Raising the T s to 400 • C at the deposition rate of 3 nm min −1 provides the necessary condition to produce SnS-CUB thin films on the glass substrate.One can identify all of the SnS-CUB diffraction peaks of the PDF 86-9477 [13] in the diffraction pattern.Contribution from the SnS-ORT content in the film may be identified from the presence of its (110) peak in the pattern.The XRD patterns for the films prepared at T s of 450 • C and 475 • C on glass substrate at a deposition rate of 3 nm min −1 , carry all of the diffraction peaks identified with those of the SnS-CUB polymorph.Figure 2 shows that the thin films deposited on glass/SnS-CUB at T s of 400 • C, 450 • C and 475 • C by TE at the slow deposition rate conserve SnS-CUB polymorph in the films without notable deviations in the relative intensities of the diffraction peaks compared with those in the standard pattern.
These films are of E g 1.66-1.74eV and of much lower electrical conductivity than of SnS-ORT films deposited at T s of 200 • C or 350 • C, as discussed in the following sections.The presence of a SnS-CUB film prepared by chemical deposition (CD) on the glass substrate does not promote the growth of SnS-CUB if the substrate temperature is 350 • C or below, even if the rate of growth of the film is low, 3 nm min −1 .
Many aspects of the crystalline structure of the films seen in the XRD patterns of figures 1 and 2 are summarized in table 1.
Since there exists differences in the values of a for SnS-CUB: 11.568 Å reported for the SnS-CUB nanocrystals [13, PDF 84-9477], those evidenced in thin films (11.600 ± 0.025 Å) [11,12], and that obtained from theoretical models [14,15] of 11.506 Å, the applicability of WFP software to estimate the abundance of SnS-CUB/SnS-ORT in the films may require caution.We set a guideline that the value for d (411) obtained from the SnS-CUB (411) peak helps determine the value of a.The relative intensities of nearly 100:30:15 for the (400), ( 410) and (411) for the diffraction peaks attest to the near-100% dominance of the SnS-CUB polymorph in the material of the films.The presence of SnS-ORT component in the film would increase the relative intensity of the (410) peak above 35% because of its overlap with the diffraction from the (111) and (040) planes of the SnS-ORT polymorph component.It is observed that by carefully controlling the deposition rate to nearly 3 nm min −1 , the SnS-ORT formation may be inhibited at T s of 400 • C-475 • C. The diameter of the crystallites of the material in the film (column 9, table 1) is estimated from the (411) peak using the Scherrer formula by the XRD software after correcting for the equipment broadening (input data).This value is understood as the edge-length of cube-shaped crystallites stacked perpendicular to the diffracting (hkl) planes, approximated to a 'diameter' [41].
A T s of 400 • C meets only a minimum condition to have a dominant presence of SnS-CUB in the film on glass substrate, but on a SnS-CUB (CD) layer, SnS-CUB growth by TE is assured at such a temperature.Similar results (available from the authors) were obtained if the precipitates collected from chemical deposition bath are used as the thermal evaporation source.

On the homogeneity of SnS-CUB composition along the thickness of the film
Figure 3 shows the GIXRD result on a film of 550 nm in thickness deposited at a T s of 450 • C and at a deposition rate of 3 nm min −1 on glass/SnS-CUB (CD-100 nm).For this, commercial SnS powder of 1000 mg was placed in the Mo-boat.The inset figure shows the GIXRD scheme.With the sampling depth of nearly 80 nm, 240 nm and 400 nm for the GIXRD recorded at gracing incidence angle (δ) of 0.5 • , 1.5 • and 2.5 • for SnS-CUB (or SnS-ORT) [39], the layer thickness of the film analyzed is seen to be entirely of that produced by thermal evaporation on the base film of SnS-CUB (100 nm) prepared by chemical deposition.The GIXRD patterns in all the cases carry the relative intensities of 100:30:15 for the 'triple peaks,' typical of the SnS-CUB polymorph.With the difficulty which still exists in correctly assigning the composition of the film by Rietveld refinement [13], one may consider that the film is constituted entirely of the SnS-CUB polymorph along its thickness.

Low deposition rate-the decisive parameter in the formation of SnS-CUB polymorph
In a 2015 study on the deposition of SnS thin films by AA-CVD process through the dissociation of a tin thioureide complex, a low deposition rate possible at a substrate temperature of 300 • C was found to be essential for the deposition of the cubic polymorph of SnS [28].At a deposition rate of Analyses of the relative content of SnS-CUB and SnS-ORT polymorphs in the thin films deposited at the substrate temperatures (Ts) on glass and glass/SnS-CUB (chem.dep, 100 nm) at a deposition rate 3 nm min −1 on an average and SnS-powder source temperature at 800 • C-900 • C in a molybdenum boat.The source-to-substrate distance was of 40 cm.The relative intensities of the XRD peaks produced by SnS-CUB crystalline planes (222), (400), ( 410) and (411) of the films seen in figures 1 and 2 are listed.Data Row 1 is for nanocrystals of SnS-CUB prepared at 80 oC on which the powder diffaction file is based.Data Row 2 is for SnS-CUB thin films prepared by chemical deposition.Bold type letters are used to imply that the materials belong predominantly to either SnS-ORT or SnS-CUB.Thus, a high T s of 450 • C, or the presence of a SnS-CUB substrate layer helps to secure the deposition of SnS-CUB on it only if the deposition rate is relatively low, at <7 nm.For a lower T s of 400 • C or in the absence of a SnS-CUB film, a deposition rate of <5 nm min −1 may be a necessary condition to obtain the SnS-CUB film by TE.At T s < 400 • C, the thin films formed even at the low deposition rate of 3 nm min −1 is predominantly constituted by SnS-ORT polymorph, and at a T s of 200 • C or below, the film is entirely of this polymorph.Overall, a higher substrate temperature together with a low deposition rate facilitates the growth of the SnS-CUB polymorph because of the improved mobility of the arriving atomic species over the film surface, in agreement with the conclusions made in [30].
Other than the distinct XRD patterns of the ORT and CUB polymorphs of SnS discussed above, their films also carry distinct surface and cross-sectional morphologies, as shown in the FESEM images of figure 5.
The SnS-ORT film carries a discontinuous surface with some voids along its thickness (i); the presence of a chemically deposited SnS-CUB base film does not remedy this situation.In SnS solar cell produced by thermal evaporation followed by a reaction in H 2 S at an elevated temperature to improve the crystalline grains benefits from a second layer of SnS produced by thermal evaporation at a lower T s to fill-up the voids and thus improve the parameters of the solar cell [42].The micrographs of the films in figure (iii) and (iv) show a compact surface morphology and a compact cross sectional feature of the film, which may provide a large parallel resistance for the cell, and hence would lead to a higher V oc .

Compatibility of thermal evaporation to develop SnS-CUB solar cell structures
The GIXRD patterns in figure 6 show that it is possible to build a glass/SnO 2 :F(FTO)-250 nm/CdS-100 nm/SnS-CUB(CD)-SnS-CUB(TE)-450 nm solar cell structure.The SnS-CUB(TE) film was produced by thermal evaporation of the commercial powder sample on SnS-CUB (CD-100 nm) at a T s of 450 • C with a deposition rate of 3 nm min −1 .The CdS thin film of thickness 100 nm was produced by chemical deposition at 80 • C.This film has a hexagonal crystalline structure [43], with a preferred orientation of the c-axis perpendicular to the SnO 2 :F substrate film.The diffraction from the CdS-(002) planes overlaps with that from the (222) planes of SnS-CUB with 2θ near 26.8 • .Thus, the intensity of this peak is significantly more than that from the (400) planes of SnS-CUB for a 2θ near 31.3• for the sampling depth of 400 nm (at the gracing incidence angle 2.5 • ), thereby reaching the SnS-CUB/CdS interface.
The SnS-CUB diffraction peaks fully account for the GIXRD pattern recorded for δ = 0.5 • (depth, 80 nm) and they dominate the pattern recorded at δ = 2.5 • .There is no evidence for the SnS-ORT polymorph throughout its thickness in the material deposited by thermal evaporation.Thus, SnS-CUB layers may be integrated into solar cells or other device structures by thermal evaporation under the deposition condition established as above.
The optical and electrical characteristics of the materials produced under different conditions are given in section 4.4 for SnS-ORT (200 • C), SnS-CUB (450 • C) or for a mixed phase of SnS-CUB-SnS-ORT (350 • C).Solar cells produced as in figure 6 are discussed in section 4.6.

Optical and electrical characteristics of the SnS-CUB and SnS-ORT films
In figure 7, the top plots (i) and (ii) of T and R spectra correspond to the thin films deposited at 200 • C and 450 • C, of thickness 250 nm prepared on glass substrates (a), (b) and glass-SnS-CUB substrates (a') and (b').
According to the XRD data on these films given in figures 1 and 2, these are of crystalline structure SnS-ORT (a), (a') and SnS-CUB (b), (b'), respectively.The films deposited at 200 • C carried some voids, as illustrated in the FESEM images in figure 5.A clear difference exists among these thin films in T (in accordance with previous reports), on the wavelength (λ g ) at which the onset of the optical absorption occurs: it is near 1000 nm (1.2 eV) for SnS-ORT and near 750 nm (1.65 eV) for SnS-CUB film, shown by the vertical dashed lines in the T, R Figures.The optical absorption coefficient (α) for each film was calculated from the T and R values by taking into account multiple reflections and absorption within the thin film, discussed in [40].The plots of (αhν) 1 /2 versus photon energy hν (eV) give straight-line fits with the indirect bandgap of 1.2-1.34eV for SnS-ORT.
For SnS-CUB film (100 nm) obtained by chemical deposition at 8 • C, the plot of (αhν) 2 /3 versus hν (eV) gives a straightline fit with direct bandgap (forbidden transitions) of 1.76 eV.For the SnS-CUB films deposited by thermal evaporation, the values are of 1.66 and 1.73 eV for the films prepared on glass slide at substrate temperatures of 450 • C and 475 • C, respectively.That the E g of 1.76 eV for the SnS-CUB film (100 nm in thickness) prepared by chemical deposition at 8 • C during 18 h (rate, 0.09 nm min −1 ) and that of the film (1.73 eV) of 250 nm in thickness prepared at 475 • C (rate, 3 nm min −1 ) by thermal evaporation are nearly the same is a significant finding here.This result is in accordance with the phase stability of SnS-CUB reported in [14,15].
The above result agrees with that reported for the 'spiral platelets' of SnS-CUB condensed over a substrate at 520 • C with (electronic) bandgap of 1.5-1.8eV, centered near 1.7 eV [30].Under a slow deposition rate, SnS-CUB is formed at elevated temperature (450 • C-475 • C), but at a lower temperature (<350 • C) SnS-ORT is the preferred polymorph.
Once formed at a low temperature (8 • C-17 • C) by chemical deposition, SnS-CUB thin films or the precipitate collected from the bath remain stable upon heating for short periods, 5-15 min at 500 • C in a nitrogen ambient [16].However heating SnS-CUB [44] or SnS-ORT [45] thin films at 450 • C in sulfur-vapor for 30 min converts these films into p-type, or to n-type Sn 2 S 3 .That the SnS-CUB polymorph became the preferred material to condense from the vapor phase of SnS at such temperature was a result consistently observed during many experimental runs in the present work using commercial powder sample or precipitate.The result presented above showed a good level of repeatability, if the rate of deposition was kept at 3-5 nm min −1 at substrate temperatures 400 Photoconductivity response of the films given in figure 8 shows that all of the SnS thin films discussed above show a notable increase from the dark to photo-level by a factor of two from 0.008 to 0.016 Ω −1 cm −1 for SnS-ORT (200 • C) and 3-6 x 10 -5 Ω −1 cm −1 for SnS-CUB deposited on glass substrates in (a), left panel.The electrical conductivity is two orders of magnitude higher in SnS-ORT films than in SnS-CUB films.In the case of mixed SnS-CUB + SnS-ORT thin films deposited under relatively higher deposition rate (7-13 nm min −1 ) at 430 • C or 450 • C, the electrical conductivity is at an intermediate level between these.For the films deposited at 200 • C and 350 • C on glass/SnS-CUB substrate in (c), right panel, the electrical conductivity remains nearly the same as of those deposited on glass substrates.However, the presence of a chemically deposited SnS-CUB substrate film reduces the dark and photoconductivity for the films deposited at 400 • C and 450 • C, maintaining these near the same level as for the SnS-CUB film prepared by chemical deposition at 8 • C given in (b), mid panel.From the XRD pattern in figure 2, and from the optical properties (figure 7, E g 1.7 eV), these films were found almost entirely of SnS-CUB polymorph.For the SnS-CUB film deposited at 475 • C, the crystalline grain diameter was 29-39 nm, leading to a higher photoconductivity of 2 x 10 -4 Ω −1 cm −1 .
The increase in the photoconductivity seen in all cases is related to a higher mobility-lifetime product of the photogenerated carriers [17], desirable for solar cell application.

On the type of the electrical conductivity.
For the SnS-ORT films deposited at 200 • C or 350 • C on glass or glass/SnS-CUB substrates, the electrical conductivity in the dark (σ d ) is 0.01 Ω −1 cm −1 .In the 'hot-probe test,' the hot probe acquires a negative potential with respect to the cold probe held on the film surface.Hence this material is ptype.If the drift mobility for holes (µ p ) is assumed to be 5 cm 2 V −1 s −1 , typical for thin films [17], the hole concentration, p p = (σ d /µ p q) is 10 16 cm −3 .This value is six orders of magnitude higher than its intrinsic carrier concentration (10 10 cm −3 ) at 300 K for a crystalline semiconductor with an E g of 1.12 eV (Si) or of 1.14 eV (SnS-ORT).For single crystal SnS-ORT, µ p of 90 cm 2 V −1 s −1 and p p of 10 18 cm −3 have been cited [1].The p-type electrical conductivity in SnS-ORT arises from Sn-vacancies (activation energy, E v-Sn 0.68 eV), which act as acceptors contrasted with S vacancies (E v-S 2.17 eV), which are donors [46].Thus, holeconcentration may be as high as 10 19 cm −3 in SnS-ORT.This takes its σ d above 10 Ω -1 cm −1 .In that case, thermoelectric generation offers itself as an application for these thin films [36].
In SnS-CUB thin films, σ d is typically 10 -6 −10 -5 Ω −1 cm −1 .For a carrier drift mobility (µ p or µ n ) of 5 cm 2 V −1 s −1 for these thin films assumed as above, the carrier concentration, n n or p p = (σ d /µq), is 10 12 -10 13 cm −3 .For an E g of 1.7 eV for SnS-CUB, seen in figure 7, the intrinsic carrier concentration is 10 5 cm −3 , which is seven orders of magnitude lower than that inferred from figure 8. Hot probe test is unreliable to determine the electrical conductivity type in such resistive thin films.From photo-electrochemical characterization under pulsed illumination, the electrical conductivity in SnS-CUB (known as SnS-ZB at the time of the report) prepared at 300 • C was described as 'ambipolar' [28].The electrical conductivity in the 'spiral platelets' of SnS-CUB prepared at 520 • C was predominantly n-type [30].The depletion layer width for an one-sided p-n junction for an extrinsic carrier concentration of 10 13 cm −3 is nearly 10 000 nm, compared with 350 nm for 10 16 cm −3 [47] for typical cases.Hence, the SnS-CUB film of 250-450 nm in thickness formed by the thermal evaporation, described above would require a p + material with a thickness of 60 nm with a p p of nearly 10 18 cm -3 , to provide a 'back surface field (BSF)' in a configuration, n + window/SnS-CUB/p + (BSF).Alternatively, suitable dopants are required for SnS-CUB thin films to increase the carrier concentration p p toward 10 16 cm -3 .The following section describes a proofof-concept solar cell of SnS-CUB film produced by thermal evaporation described above.

Proof-of-concept solar cells of SnS-CUB and SnS-ORT thin films produced by thermal evaporation
The results presented above suggest that it is possible to deposit polymorph-pure SnS-ORT or SnS-CUB thin films for solar cells by thermal evaporation from SnS powder sources by controlling the substrate temperature and the rate of deposition.It may also be possible to deposit sequentially SnS-ORT and SnS-CUB or their mixed phases by varying the deposition parameters.The result presented in this section pertains only to the polymorph-pure thin films.Figure 9 shows the GIXRD pattern recorded at grazing incidence angle of 1.5 • (sampling depth of nearly 240 nm in SnS) of solar cell structures of A: FTO (TEC-7)/CdS/SnS-ORT (250 nm); and of B: FTO (TEC-7)/CdS/SnS-CUB(CD-80 nm)-SnS-CUB (250 nm), each made by thermal evaporation of commercial SnS powder, on substrates at 200 • C for SnS-ORT and at 450 • C for SnS-CUB.The deposition rate was of 3 nm min −1 in each case.The positions of the diffraction peaks from the CdS and FTO films in the cell structure are indicated, as in figure 6.The XRD patterns confirms that in A, the SnS film is of SnS-ORT polymorph with all of the major peaks clearly identified.In B, the SnS is of SnS-CUB polymorph, also with all of its characteristic peaks identified.

Light-generated current density (J L ) in the SnS-CUB and SnS-ORT cell structures.
Figure 10 illustrates the method of evaluating the light generated current density for the solar cell structures A and B shown in the insets in figure 9, discussed in detail in a previous work [38].In the top panel is the photon flux density distribution across the wavelength interval 250-2500 nm for air mass 1.5-global tilt (AM1.5G)solar radiation obtained from standard textbook data [48].
In the mid-panel (b) is the transmittance (T) and reflectance (R) spectra recorded for these cell structures for the FTO-side incidence, given along with T plot for the FTO.The onset of the optical absorption in the SnS film near 950 nm for SnS-ORT and near 750 nm for SnS-CUB are indicated here.The average reflectance of the cell structure in the 250-950 nm interval is <0.1.
The free-carrier optical absorption due to the electrically conductive transparent FTO film initiates at a wavelength near 950 nm and it increases toward higher wavelength, 2500 nm [49].Such optical absorption brings the transmittance of the cell structures to near-zero at such wavelength.The freecarriers in FTO, which produces its electrical conductivity of 5 × 10 3 Ω −1 cm −1 lead to a conductor-like optical reflectance at longer wavelength [49] with R > 0.6 at 2500 nm.
The spectral distribution of the light-generated current density for the cell structure A or B given in (c) is obtained by taking into account the transmittance and reflectance losses for the FTO-side incidence and the N phλ values in the spectral range 350 nm to wavelength λ g (nm) = 1240/E g (eV).The area under the curves of 0.1q N phλ (T FTO -T cell -R cell ) in the bottom panel (c) gives 24 and 17 mA cm -2 for the cells A and B, respectively.These are the light generated current density (J L ) values for the cells.These represent the maximum values for the short circuit current density (J sc ) of the respective solar cells if the collection losses of the photo-generated carriers across the cell structure are negligible.
In figure 11 are given the J-V characteristics of the solar cells A and B measured under simulated AM1.5G (1000 W m -2 ) solar radiation, verified by measurements made under solar radiation at our site.The notable feature here is that the V oc of cell B (SnS-CUB) is 0.478 V compared with that of cell A (SnS-ORT), 0.283 V.The value of J sc for the solar cell B is 1.5 mA cm −2 which is only 10% of the value of J L seen in figure 10 due to carrier collection losses; but it is 15.4 mA cm −2 , which is 64% of the J L predicted for the cell A.
The need for improving the electrical conductivity of SnS-CUB thin films produced by thermal evaporation for improved solar cells or the use of BSF material to improve the collection of photo-generated carriers is obvious for solar cell B. The objective here was to demonstrate that the SnS-CUB absorber in cell B, with an E g above 1.65 eV compared with SnS-ORT (E g , 1.3 eV) leads to a notably higher V oc .The present work offers also the possibility of stacking SnS-CUB/SnSe-ORT layers by lowering the substrate temperature or by increasing the deposition rate during the deposition, as suggested from the results in figures 1, 2 and 4. In the case of chemically deposited SnS-CUB/SnS-ORT absorber stack, the performance of such a cell improved [18] compared with a SnS-CUB solar cell, even though the conversion efficiency of the solar cell still remained below 1.5%.
Given below is the preliminary results to consider an alternate approach, where a SnS-CUB/SnS-Se-CUB stack may be made by thermal evaporation from two crucibles, which may serve as a BSF layer.

SnS-Se-CUB thin films by thermal evaporation-preliminary results
The SnS-CUB polymorph with an E g of 1.65-1.73eV obtained for the thin films prepared by thermal evaporation, brings-up the possibility to create SnS x Se 1−x -CUB thin films of E g between 1.4 eV (SnSe-CUB) [35] and 1.65-1.76eV (SnS-CUB) [36].For a proportional variation of the bandgap with composition x, SnS 0.5 Se 0.5 -CUB offers a material with an E g of approximately, 1.55 eV.
In this section we illustrate that it is possible to produce such films by using a mixture of SnS and SnSe powder as the evaporation source.Since SnSe powder is commercially scarce, we used the precipitate collected from the work on chemically deposited SnSe thin films [35][36][37].First, 2 g of Se-powder and 12.3 g Na 2 SO 3 were refluxed at 100 • C in 100 ml of deionized water, which produced a 0.2 M solution of Na 2 SeSO 3 .A solution of 100 ml in volume for the precipitation/chemical deposition of SnSe was prepared by dissolving 0.7 g of SnCl 2 • 2H 2 O (Aldrich) in 5 ml of acetone, to which were sequentially added by stirring 35 ml of 3.5 M triethanolamine, CH 2 CH 2 OH) 3 N, 18 ml of 2.0 M solution of sodium hydroxide (NaOH), 0.25 ml of 0.5% polyvinyl pyrrolidone (PVP), 4.0 ml of 0.2 M of the Na 2 SeSO 3 solution, and 16 ml of water.Initial pH of this deposition bath is 14.The reagents were 'Baker Analyzed.'The reaction is rather fast at room temperature, with a SnSe-ORT film of thickness 150-200 nm deposited on a glass substrate within 2 h [37].Alternatively, a SnSe-CUB film of 200 nm in thickness is deposited in 2-3 h on a SnS-CUB (50-80 nm) base film on glass substrate [35,36].The SnSe-ORT precipitate settling down in the beaker was filtered after 24 h, flushed with distilled water, dried and stored for use in thermal evaporation.For this initial work a powder mixture of 300 mg of this SnSe powder and 300 mg of commercial SnS powder were used.The procedure for thermal evaporation was the same as for depositing SnS-CUB films (section 3).The substrate temperature was 450 • C, and the film growth rate was kept low, 3 nm min −1 , established in this work for SnS-CUB film.The results presented below are for the films deposited on glass/SnS-CUB (80−100 nm) substrate layer.It was seen in figure 2 that the presence of a SnS-CUB layer promotes the deposition of the 'cubic-only' polymorph at a substrate temperature in the interval of 400 • C-475 • C.
The GIXRD pattern recorded for the film at 0.5 • with a sampling depth of nearly 80 nm is shown in figure 12.Its comparison with the XRD pattern for SnS-CUB (PDF 86-9477) and for SnSe-CUB given in [35] permits one to assign the XRD peaks to the (hkl) planes of SnS x Se 1−x -CUB.The logintensity plots for the patterns for SnS-CUB b' in the bottom panel and (c) for the SnSe-CUB in the top panel illustrate that all of the XRD peaks in the GIXRD pattern may be assigned to SnS x Se 1−x -CUB.
All the peaks in the GIXRD pattern are located near midway between the corresponding peaks of SnS-CUB and SnSe-CUB.The lattice constant a = d (hkl) x(h 2 + k 2 + l 2 ) 1 /2 has an average value of 11.7944 Å for the material, given in the inset table.By assuming a linear variation of the lattice constant for materials of intermediate composition between 11.60 Å for SnS-CUB [12] and 11.9632 Å for SnSe-CUB [35], the average value for a = 11.7944Å for the material in a) suggests a chemical composition, SnS 0.45 Se 0.55 .The melting point of both SnS-ORT and SnSe-ORT is 880 • C [1].However, their different vapor pressures may not permit a constant composition of the vapor on the growing film.
While the (400) diffraction peak appears to be sharp, with a FWHM in 2θ of 0.4 • and a crystalline grain diameter of 22 nm, the (222), ( 410) and (411) peaks may be resolved for contributions arising from the underlying SnS-CUB base film, and an overlying SnSe-CUB component.These features may be addressed in future work.The preliminary work presented here shows the feasibility of producing materials intermediate to SnS-CUB and SnSe-CUB with E g in the 1.4-1.74eV interval by thermal evaporation.The substrate temperature of 450 • C maintained for the deposition of this film is close to that employed in CdTe or CIGS polycrystalline solar cell technology described in [22,23].
In figure 13 is shown the optical and electrical properties of the SnS 0.45 Se 0.55 -CUB film prepared as above.In (a), the average R for the long wavelength region of the film is 0.39, which gives a refractive index for the material [40], n = (1 + R 1 /2 )/(1 − R 1 /2 ) of 4.33 and its high frequency relative permittivity ε r = n 2 , of 18.7.This value is higher than 16, cited for SnS-ORT or SnSe-ORT [1].The analysis of E g from the plot of (αhν) 2/3 versus hν in (c) gives a direct gap of 1.57 eV for this material, involving 'forbidden transitions' as in SnS-CUB ( [11,33] and present work) or SnSe-CUB [35] thin films.The same value of 1.57 eV for E g is suggested from the plot (b) of α versus hν, from the value of hν on the horizontal axis at which α → 0.
The value for E g (1.57eV) obtained for the SnS 0.45 Se 0.55 -CUB film of an intermediate composition is in agreement with a proportional variation of E g with x = 0.45 in SnS x Se 1−x -CUB: 0.45 × 1.74 eV (SnS-CUB) + 0.55x1.4eV (SnSe-CUB) = 1.55 eV.In this analysis, it is assumed that SnS 0.45 Se 0.55 -CUB is the dominant absorber in the film.This is an approximation as evidenced from the details of the XRD peaks from the (222), (410 and (411) diffracting planes of the SnSe-CUB, SnS 0.45 Se 0.55 -CUB, and SnS-CUB components in the film (figure 12).The photoconductivity response for this film in figure 13(d) shows a dark-level electrical conductivity and photoconductivity of 0.015 and 0.03 Ω −1 cm −1 , respectively.This is nearly four orders of magnitude higher than that of SnS-CUB thin films (devoid of SnS-ORT component) shown in figure 9.The observed p-type electrical conductivity is close to that of SnS-ORT films prepared by thermal evaporation at a T s of 200 • C-350 • C. The p-type conductivity originates from tin vacancies in both cases.The presence of Se in the vapor phase, and its incorporation into the SnS x Se 1−x -CUB film produced by thermal evaporation on the substrate at 450 • C is seen to inhibit the chalcogenide vacancies, which would otherwise compensate for the Sn-vacancies and thereby reduce the majority carrier concentration, holes (p p ).
If one assumes a carrier drift mobility (µ) of 5 cm 2 V −1 s −1 , the electrical conductivity in the dark (σ d ) for the SnS 0.45 Se 0.55 -CUB component gives the carrier concentration, [p p = σ d /(q µ)] of nearly 10 16 cm −3 .Thus, SnS x Se 1−x -CUB films produced by thermal evaporation can combine an E g of the material intermediate to 1.4 eV (SnSe-CUB)-1.74eV (SnS-CUB).Its increased majority carrier concentration is desired for solar cell technology for use of this thin film as a BSF layer with SnS-CUB absorber or by itself as the principal absorber.Further work is required for the realization of such solar cell structures.The close up of the photoconductivity rise and decay curves for the film given in figures 13(e) and (f) shows that that the response time to duplicate the electrical conductivity or to reduce it to half its steady state value (and hence of the carrier concentration) in response to the illumination is, 0.1 s.Thus, the primary reason for the increase in the electrical conductivity upon illumination seen in figure 13(e), is photo-generation of carriers; it is unlikely to be caused by heating of the sample.The carrier generation of 10 16 cm -3, which occurs upon the illumination could contribute toward an acceptable level of J sc in a solar cell, reduce the reverse saturation current density (J o ) due to a reduction in the minority carrier concentration (holes, n p ) monority carrier concentration (electrons, n p ) and improve the V oc of such cells; V oc (300 K) being approximately 0.026 V ln(J L /J o ], [47,48].

SnS-CUB and SnS x Se 1−x -CUB thin films produced by thermal evaporation-an outlook
The correct assignment of a π-SnS or 'large cubic' SnS-CUB polymorphic identity within the 'earth-abundant tin sulfide' with a bandgap of 1.74 eV in nanocrystals and in thin films during 2015-'16 [10][11][12][13][14][15][16], in addition to SnS-ORT [1-3], consisted of experimental findings and their theoretical validation.An exceptional range of applications for this material was suggested-from solar cells to optical devices for a semiconductor material, which lacks a mirror plane symmetry (chiral material) [12,14].However, the expectation that this material may overcome the limitations of SnS-ORT as a solar cell material was not met in the subsequent years, nor were the optical devices explored.A concern was raised in 2017 on the lack of reports on SnS-CUB film produced from high temperature processing usual with solar cell production [14].
There were also apparent contradictions among the results in materials produced by TE, in which the substrate temperature alone was considered to explain the conditions under which SnS-ORT or SnS-CUB polymorph may be formed.By using 'AA-CVD' for the dissociation of Sn(II)-thioureide precursor, an initial report in 2015 found that a possible lowdissociation/deposition rate at 300 • C produced SnS-CUB (known at the time as SnS-ZB), whereas at temperatures 350 • C-450 • C, SnS-ORT resulted [28].In a 2019-'20 work, with the same process and the same precursor but with a distinct furnace, the formation of SnS-ORT was observed on an FTO substrate at 300 • C and that of SnS-CUB on the substrate at 375 • C [27].The role of the rate of deposition as a decisive parameter on the formation of the particular SnS-polymorph appears to have been overlooked.
In a 2022 work on the condensation of SnS-vapor on substrate subjected to a temperature gradient (550 • C-520 • C downstream over 1.2 cm in length), the 'supersaturation' of vapor on the substrate surface at 550 • C produced SnS-ORT.Such a formation was explained on the reduced ad-atom mobility on the substrate under a high-flux condensation of SnS into a solid phase [30].The 'cooler zone' at 520 • C situated downstream of the vapor flux carried by the argon flow received less vapor flux, and thereby allowed for an increased ad-atom mobility during its condensation on the substrate.Along with the prevailing temperature, this condition facilitated the growth of the SnS-CUB polymorph with a bandgap of 1.7 eV.Thus, when the temperature is sufficiently high (400 • C-520 • C) it is the low vapor flux, rather than the substrate temperature itself, which is decisive to secure the growth of the SnS-CUB polymorph.
In the present work on SnS thin films produced by vacuum thermal evaporation also, the finding is that the low rate of deposition (3 nm min −1 ) as well as the temperature of the substrate (400 • C-475 • C) are equally relevant for the formation of the SnS-CUB polymorph.The presence of a SnS-CUB substrate film promotes the condensation of SnS vapor into SnS-CUB polymorph, even when the low-flux and high temperature conditions are not optimally met.In chemical deposition or precipitation techniques, the rate of deposition is controlled by the thermally activated dissociation of the Sn-complexes.The substrate/growth surface is at the same temperature, and hence, independent control of the complex dissociation and ad-atom mobility on the surface is difficult to achieve.The transition from the growth of SnS-CUB to SnS-ORT occurred for a seemingly small increase in temperature, from 30 • C-40 • C due to an increase in the temperature activated atomic/ionic flux on the growing film, which caused a reduced ad-ion mobility and promoted the growth of the SnS-ORT polymorph [33].
The understanding on the growth conditions for the deposition of thin films of SnS-ORT-only or SnS-CUB-only polymorphs and the preliminary results on SnS x Se 1−x -CUB thin films produced from SnS and SnSe powder mix at substrate temperature 450 • C opens-up technological possibilities.For a range of composition x, E g may be varied in the 1.4-1.74eV interval in SnS x Se 1−x -CUB, and the electrical conductivity may also be varied by orders of magnitude.It will be interesting to see whether a variation in the electrical conductivity in SnS-CUB also occurs through the introduction of Se at parts per million level.In such a case, E g of SnS:Se-CUB would remain close to 1.7 eV, whereas its p-type electrical conductivity would increase like in typical semiconductors subjected to an intake of dopants.The role of Se in this case would be to inhibit S-vacancies [46], which would otherwise compensate for some of the Sn-vacancies, responsible for its ptype electrical conductivity, thereby adversely affecting solar cell performance of the material.
Instead of the substitution of S-sites with Se in SnS-CUB, illustrated in section 4.7, it may be interesting to see the substitution of Sn sites with Ge.The possibility of observing a stable π-GeS with a cubic lattice constant of 11.257 Å and a direct E g of 1.36 eV has been predicted from theoretical studies reported in 2017 [50], though not yet experimentally confirmed as of 2019 [51].If a linear variation with the composition exists in this case, E g of 1.36-1.74eV may be obtained in Sn y Ge 1−y S-CUB.Similar to that illustrated in section 4.7, such materials may be produced at a substrate temperature of 400 • C-475 • C by thermal evaporation of SnS-GeS powder mixtures with a possibility to introduce SnSe to this mixture to modify the electrical conductivity of the thin films.Powder form of GeS is commercially available; its melting point is 615 • C, which is compatible with that of SnS or SnSe (880 • C) to serve as a powder source in thermal evaporation.
The success in photovoltaic technology using these novel materials would require attention to materials and interfaces as discussed in 2019 specifically for CdTe solar cell technology [52], and expressed in 'PV Roadmap 2020' [23].In the case of SnS polymorphs, solar cell studies have so far engaged in SnS-ORT absorbers, thereby leaving much to explore in the cubic polymorphs.During the past fifty six years since 1967 [4], the understanding on tin sulfide (SnS) has constantly evolved, because chemical deposition [7,11,33,53] and chemically based techniques [8,54] offered easy access to methodologies to deposit their thin films pertaining to ORT or CUB structures.Production of the SnS-ORT or SnS-CUB or the related cubic or orthorhombic materials with certainty by controlling the deposition rate, temperature, and the substrate type in vacuum thermal evaporation is attractive for the industrial production of these materials for device applications.

Conclusions
The present work helped to resolve some disparities that existed among the results reported during 2016 to date on the formation of the polymorphs of tin sulfide with respect to the substrate temperature in vapor phase deposition by bringingin section 2 the rate of deposition as well to gain insight to the deposition process.It was thus possible to arrive at conditions conducive for the formation of the SnS-CUBonly or SnS-ORT-only polymorph.The experimental results showed that the rate of deposition is an important parameter, along with the substrate temperature to infer whether it is the SnS-ORT or SnS-CUB polymorph that would result from a particular experiment.The formation of SnS-CUB in thermal evaporation or by sputtering in a vacuum ambient was seldom observed because of a high deposition rate inherent in these techniques, irrespective of the substrate temperature.Under a low deposition rate, of 3 nm min −1 , SnS-CUB is the preferred polymorph deposited at a substrate temperature, 400 • C-475 • C.This result is in accordance with a finding reported in 2022 on the formation of SnS-CUB from vapor phase of SnS under low vapor flux at a temperature of 520 • C. Thus, the production of SnS-CUB thin films by thermal evaporation has become available through this work.A proof-of-concept SnS-CUB solar cell produced a V oc of >0.475 V compared with a V oc of <0.3 V for a SnS-ORT solar cell.Improvement of the electrical conductivity of SnS-CUB and/or the use of a BSF layer may improve the SnS-CUB solar cell.Addition of a BSF layer of SnS 0.45 Se 0.55 -CUB produced at 450 • C by thermal evaporation requires further work.This material with a bandgap of 1.57 eV and a majority (hole) carrier concentration of 10 16 cm −3 , appears to be suitable for the BSF role of this material.
In section 5 on 'an outlook', possibilities of developing Sn-Ge-S-Se thin films of cubic structure by thermal evaporation was presented.This may bring to reality the prospects of these materials envisioned during 2015-'16, when the π-CUB polymorph was correctly identified as a tin sulfide polymorph, but so far remains mostly as an 'enigmatic material'.

Figure 1 .
Figure 1.(b)-(f) GIXRD patterns recorded at 1.5 • with Cu-Kα 1 radiation for tin sulfide thin films prepared by thermal evaporation at a deposition rate of 3 nm min −1 on glass substrates maintained at temperatures Ts of 200 • C-475 • C. The sampling depth into the film is approximately, 250 nm at this grazing incidence.The observed peaks are compared with those of: (a) SnS-ORT (PDF 39-0354) in the top panel and of (g) SnS-CUB (PDF 86-9477) in the bottom panel.

Figure 2 .
Figure 2. (b)-(f) GIXRD patterns recorded with Cu-Kα 1 radiation at 1.5 • for tin sulfide thin films prepared by thermal evaporation at a deposition rate of 3 nm min −1 on glass substrates (1 mm in thickness) carrying (g) a chemically deposited SnS-CUB thin film of 100 nm in thickness (CD), bottom plot.This substrate was maintained at temperatures Ts of 200 • C-475 • C. The sampling depth into the film is approximately, 250 nm.The observed peaks are compared with those of: (a) SnS-ORT (PDF 39-0354) in the top panel and (h) of SnS-CUB (PDF 86-9477) in the bottom panel.

Figure 3 .
Figure 3. GIXRD patterns of a thin film of nearly 450 nm in thickness produced by thermal evaporation on chemically deposited SnS-CUB film 100 nm (CD) in thickness recorded at gracing incidence angle (δ) of 0.5 • , 1.5 • and 2.5 • with the sampling depth of nearly 80, 240 and 400 nm, respectively, which illustrates the homogeneity of the SnS-CUB polymorphic composition of the film.Bottom plot is for the powder diffraction file (PDF 86-9477) for SnS-CUB.

Figure 4 .
Figure 4. XRD patterns of: (c) SnS thin films produced on glass/SnS-CUB (CD) at Ts of 450 • C at a deposition rate of 7 nm min −1 closely resembling that of SnS-CUB and (b) at 13 nm min −1 , showing a pronounced presence of SnS-ORT; in (a) and (d) are the standard PDF for SnS-ORT and SnS-CUB.

Figure 5 .
Figure 5. Field effect scanning micrograph images of the surface (left panel) and of the cross section of the films formed by thermal evaporation at a deposition rate of 3 nm min −1 : (i) 200 • C, glass substrate, (ii) 200 • C on a SnS-CUB substrate film (100 nm) in both cases constituted by SnS-ORT; (iii) 450 • C, glass substrate and (iv) glass/SnS-CUB of SnS-CUB in both cases (figures 1 and 2).The position of the chemically deposited SnS-CUB film on glass is indicated by a red band in the right side panel of (ii) and (iv).

Figure 7 .
Figure 7. (i) Optical transmittance (T) and reflectance (R) plots of SnS-ORT films deposited at a rate of 3 nm min −1 on glass substrates at 200 • C on glass slide (a) and on glass-SnS-CUB (a´) by thermal evaporation of commercial powder source of SnS; and (ii) of SnS-CUB thin films deposited in the same way on glass (b) and glass-SnS-CUB (b ′ ) substrates at 450 • C. The vertical dashed lines indicate the onset of the optical absorption for the two cases.Bottom plots (iii)-(vii) help determine the type of the bandgap and of the electronic transition: 'indirect-allowed' for SnS-ORT and direct-forbidden for SnS-CUB films deposited on glass slides at different substrate temperatures (Ts).

Figure 8 .
Figure 8. Photoconductivity response (2 s dark, 2 s light, and 2 s dark) recorded using pairs of coplanar silver-paint electrodes 5 mm each at 5 mm separation printed on the SnS films of nearly 250 nm in thickness produced by thermal evaporation at different substrate temperatures: (a) on glass or (c) on glass/SnS-CUB (CD) (right panel).The mid-panel (b) is of a SnS-CUB film (100 nm) produced by chemical deposition.

Figure 9 .
Figure 9. GIXRD patterns recorded at grazing incidence angle 1.5 • on solar cell structures of SnS prepared by thermal evaporation at substrate temperature of: (b) 200 • C for A with SnS-ORT, matching its standard pattern (a) at the top panel and (c) of 450 • C for SnS-CUB, matching its standard pattern (d) in the bottom panel.Contributions to the pattern from the crystalline planes of the underlying CdS and SnO 2 :F (FTO) thin films in the cell structure are as indicated in figure 6.

Figure 10 .
Figure 10.(a) Top panel, spectral distribution of photon flux density N phλ for AM1.5G (1000 W m −2 ) solar radiation; (b) mid-panel, optical transmittance (T) and reflectance (R) for the cell structures A and B for SnO 2 :F(FTO)-side incidence as in a working solar cell and T of FTO for glass (2.2 mm)-side incidence.(c) Bottom panel-the spectral distribution of light-generated current density in mA cm -2 nm −1 for the cell structures.The area under the respective curves for wavelength 250 nm to λg gives the light generated current density (J L ) for the cell, which sets the maximum for the short-circuit current density (Jsc), 24 and 17 mA cm −2 , respectively, for the solar cells A (SnS-ORT) and B (SnS-CUB).

Figure 11 .
Figure 11.Current density (J) versus voltage (V) plots for solar cell A of SnS-ORT and solar cell B of SnS-CUB, with parameters listed in the inset table.While cell B has a high Voc, 0.478 V, its high specific series resistance of 2330 Ω cm 2 adversely affects other cell parameters.

Figure 12 .
Figure 12.(a) GIXRD pattern recorded at 0.5 • for a thin film deposited on glass/SnS-CUB substrate at a temperature of 450 • C by thermal evaporation of a SnS (300 mg) + SnSe (300 mg) powder source.The inset table shows the positions of the major peaks, the d-spacing and the 'large cubic' lattice constant (a) evaluated from it with average value of 11.7944 Å and a chemical composition SnS 0.45 Se 0.55 for the material with the cubic structure.The diffraction pattern is compared with the standard PDF data in the linear scale (b) and log scale (b ′ ) for SnS-CUB in the bottom panels and (c) and (c ′ ) for SnS-ORT in the top panels.

Figure 13 .
Figure 13.(a) Optical transmittance (T) and reflectance (R) of the SnS 0.45 Se 0.55 film; (b) optical absorption coefficient (α) of this material as a function of photon energy (hν); (c) the (αhν) 2/3 versus hν plot suggesting a direct gap (forbidden transitions) of 1.57 eV; (d) the photoconductivity response of the film.The increase in the photoconductivity in (e) for the film upon illumination and in (f) its decrease when the illumination is shut-off occur within 0.1 s.