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Facile growth of SnS and SnS0.40Se0.60 thin films as an absorber layer in the solar cell structure
Materials Today: Proceedings xxx (xxxx) xxx
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Facile growth of SnS and SnS0.40Se0.60 thin films as an absorber layer in the solar cell structure Arun Banotra a,b, Naresh Padha a,⇑ a b
Department of Physics, University of Jammu, Jammu 180006, India School of Sciences, Cluster University of Jammu, Jammu 180001, India
a r t i c l e
i n f o
Article history: Received 21 August 2019 Received in revised form 2 November 2019 Accepted 12 November 2019 Available online xxxx Keywords: Tin SnS1xSex Solar absorbers Thermal evaporation X-ray diffraction Bandgap
a b s t r a c t SnS (S1) and SnS0.4Se0.6 (S2) thin films were prepared using thermal evaporation on annealing at 523 K of the thermally deposited films. Compositional analysis of the films reveals the formation of SnS and SnS0.40Se0.60 phase. X-ray diffraction results demonstrate the formation of orthorhombic structure corresponding to SnS and SnS0.40Se0.60 phase. Moreover, a decrease in the intensity of SnS0.40Se0.60 peaks as compared to those of SnS was observed. Thus, causes insertion of selenium (Se) atoms in SnS lattice and results in the modification of lattice parameters. These structural modifications leads to variations in crystallite size, lattice strain and dislocation density, thus, confirms the formation of ternary SnS0.40Se0.60 phase. The changes corresponding to optical parameters are also observed in the films in the form of lower transmission, bandgap value and absorption coefficient on Se insertion. Moreover, both the films exhibit absorption coefficient >105 cm1 with bandgap values of 1.44 and 1.23 eV and found suitable for absorption in solar cell. Ó 2019 Elsevier Ltd. All rights reserved. Selection and peer-review under responsibility of the scientific committee of the International Conference on functional materials and simulation techniques.
1. Introduction Efficient selective solar absorbers are required for an effective conversion of solar radiation into electric energy. Literature reports several selective solar absorbers with high absorption and, thus, exhibit capability to reach terawatt energy production with sustainable development [1–5]. To achieve such absorbers, thin-film technologies like CdTe [6] and Cu(In,Ga)Se2 [7] are not suitable because of high cost, supply issues, toxicity and associated disposal management. Moreover, CZTS material seen to have excellent absorber properties, but theoretical investigations reveal several issues, related to multiple valances of Sn with narrow stability range due to the presence of multiple constituents [8–10]. Thus, materials such as SnS, CuS2, SnSe and their ternary derivatives have recently gained interest in photovoltaic (PV) community due to its abundance, low cost and less toxic constituents [11–15]. Recently, these chalcogenide semiconductor materials attracted attention and finds applications as photovoltaic devices, detectors, and
⇑ Corresponding author. E-mail address: [email protected] (N. Padha).
biomedical arena owing to their narrow bandgap energy [16]. These IV-VI compound semiconductors possess high optical conductivity in the visible and ultraviolet region of the EM spectrum [17]. Further, high chemical and environmental stability have been seen in comparison to toxic lead chalcogenides [18–19]. There are large numbers of solution based reports available for the growth of tin chalcogenides which render high electrical resistivity and consume time. These tin chalcogenide thin films were reported to be formed using different physical evaporation methods due to its ease of availability with excellent optical properties. Sulphide and selenide based compound semiconductor materials exhibit high photosensitivity with composition variation and, thus, found favourable as selective solar absorbers and photoelectrochemical (PEC) applications [20–23]. One of the potential photosensitive absorbers available in these tin chalcogenide materials is SnS. It has exhibited high optical absorption 104 cm1, possess optimal direct (1.07 eV) and indirect (1.3 eV) photon energy close to already developed Silicon technology. Further, SnS presents a vulnerable interlayer in its deformed NaCl layer type structure for the penetration of impurities due to weak Van der Waals forces [24,25]. This makes the material more suitable for doping and exhibits growth of facile phases which leads to variations in their structural and optical properties. Tin Selenide another prominent
https://doi.org/10.1016/j.matpr.2019.11.154 2214-7853/Ó 2019 Elsevier Ltd. All rights reserved. Selection and peer-review under responsibility of the scientific committee of the International Conference on functional materials and simulation techniques.
Please cite this article as: A. Banotra and N. Padha, Facile growth of SnS and SnS0.40Se0.60 thin films as an absorber layer in the solar cell structure, Materials Today: Proceedings, https://doi.org/10.1016/j.matpr.2019.11.154
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material in this class presents high absorption coefficient (104 cm1) with low bandgap value of 0.9 eV. Thus, the incorporation of Se atoms in SnS lattice leads to stoichiometric changes and provides bandgap tunability from 1.3 eV to 0.9 eV and results into viable optical solar selective absorber. The other important aspect for the formation of this ternary compound semiconductor is the solar cell efficiency. According to literature, sulphoselenide based ternary solar cells showed better quantum efficiency of 10.1% for S/S + Se ratio of about 40% in comparison to sulphide (8.4%) and selenide (9.15%) based solar cells [26–30]. In the present work, tin and sulphur mixture (powder) have been used as an evaporant material to deposit thin films using multisource thermal evaporation. Further, the as-deposited films were supplemented with additional layer of sulphur and selenium which were undertaken for annealing at 523 K. This results into the transformation of binary SnS to ternary SnS0.4Se0.6 phases with modified structural, optical properties having high photosensitivity. 2. Experimental Highly pure constituents Sn and S powders of the purity 99.99% and 99.50% respectively, procured from sigma Aldrich taken in stoichiometric proportions, were mixed in planetary ball mill at 120 rpm for six hours. The powders Sn + S were deposited on organically cleaned corning glass substrate by multi source thermal evaporation method. Further, the films were supplemented with additional sulphur (S) and selenium (Se) layer of the thickness 200 nm in each case over the as-grown films [30–31]. The resultant films were annealed in a tubular furnace under the vacuum 1 103 mbar at 523 K for one hour. The obtained samples were indexed as S1 (SnS) and S2 (SnS0.40Se0.60) respectively. The deposition has been carried out at a base vacuum pressure 2 106 mbar using vacuum coating unit (Make: HindhiVac, Model: 12A4DM) at room temperature. During deposition, the substrates were rotated continuously at low speed to obtain uniform and homogeneous films. The thickness of the deposited films were monitored using digital thickness monitor (Model: DTM 101, Make HindhiVac) by maintaining 3 to 6 Å/s rate of evaporation. The substrates used for deposition were cleaned in an ultrasonic bath cleaner using trichloroethylene, acetone and methanol (TAM) under constant ambient conditions. The annealed films were characterized for the determination of structural parameters using X-ray diffractometer (Make: PANalytical, Model: X’pert3 powder). The films were scanned in 2h range 10°–70° using filtered CuKa1 radiation (k = 1.5406 Å) equipped with HD Bragg Brentano incident geometry. During data collection, step size of 0.03° and time per step 0.15/ sec (s) were used at a grazing incidence angle of 1.7°. This X-ray diffraction data has been analyzed using highscore software provided by Panalytical for the determination of structural parameters like crystallite size, strain and dislocation density. The study of film surface and composition was carried out using scanning electron microscope (SEM) (Make: JFEI, Model: Nova Nano SEM-450) equipped with energy dispersive x-ray analysis (EDAX). The optical measurements were performed using UV–Vis-NIR spectrophotometer (Make: Shimadzu, Model: UV-3600) in optical transmittance mode in the wavelength range 300 to 1800 nm at room temperature. 3. Results and discussion Thin films of SnS and SnS1xSex were prepared by sequential deposition of SnS and Se layers using multisource thermal evaporation method. The films demonstrate simple preparation technique with low cost, less toxic and highly abundant constituents,
Fig. 1. X-ray diffraction patterns of S1 and S2 films along with JCPDS card of SnS0.5Se0.5 (bars).
yet exhibit homogeneous, uniform and smooth surfaces, with high photosensitive and absorption response.
3.1. X-ray diffraction Fig. 1 shows the X-ray diffraction patterns of S1 and S2 films. The JCPDS data of SnS0.50Se0.50 has been included at the bottom of Fig. 1. It was found that the film S2 possess SnS1xSex phase, corroborated well with corresponding JCPDS pattern of SnS0.50Se0.50. The film S1 demonstrated formation of orthorhombic SnS phase corresponding to 2h values of 31.94 (1)°, 26.005 (1)° and 39.115 (1)° along (0 4 0), (1 2 0) and (1 3 1) planes which matched well with the SnS phase as per the JCPDS card no. 39–0354 with (0 4 0) as most significant peak. The analysis of the film S1 at lower thickness of 200 nm as mixed SnxSy phase at different annealing temperatures has already been reported [31]. However, the film S2 was also found to possess dominant (0 4 0) and (1 0 1) SnS peaks at 2h values of 31.97 (1)° and 30.65 (1)° respectively along with SnS1xSex peaks. Besides this, the film showed an impact of Se atoms diffused into SnS lattice and caused shift in the 2h values along with emergence of various SnS1xSex Bragg peaks due to doping of greater effective ionic radii of Se2 than that of S2 ions. The position of 2h is shifted towards smaller angles after Se doping into SnS phase. Therefore, the formation of SnS1xSex occurs on doping of Se in SnS which also causes strain in the lattice as has also been reported earlier [32]. These results are confirmed on comparison with orthorhombic SnS0.50Se0.50 (JCPDS card no. 48– 1225) corresponding to the peaks observed at 2h value of 31.225 (1)°, 38.095 (1)° and 25.525 (1)° along (1 1 1), (3 1 1) and (2 0 1) planes. Thus, the S2 films exhibit mixed phase character of SnS and SnS1xSex crystallites in the form of a solid solution. Such mixed phase behaviour of sulphoselenide composition has been reported by Thomas Schnabel et al. [33]. The presence of SnS along with SnS1xSex phase might be attributed to one of the following reasons: Either deposited additional layer contains less ‘Se’ atoms which were unable to convert the whole SnS into SnS1xSex phase or the deposited additional Se layer at the top of SnS unable to diffuse the Se through whole thickness of 600 nm to convert the entire SnS lattice into SnS1xSex phase. Thus, might results in the formation of SnS1xSex phase at the top and SnS phase at the bottom. While the presence of other binary phase SnS2 is attributed to the annealing temperature. This emergence corresponding to annealing has already been reported previously [31].
Please cite this article as: A. Banotra and N. Padha, Facile growth of SnS and SnS0.40Se0.60 thin films as an absorber layer in the solar cell structure, Materials Today: Proceedings, https://doi.org/10.1016/j.matpr.2019.11.154
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A decrease in the intensity of S2 films has also been observed as compared to those of S1 films. This decrease in the intensity of SnS peaks in S2 films attributed to the insertion of foreign (Se) atoms in SnS lattice at substitutional sites. Thus, caused strain in the film due to insertion of larger atomic radii of ‘Se’ (198 pm) ions as compared to those of ‘S’ (184 pm) ions. Further, the film exhibit lower crystallinity due to decrease of intensity on the insertion of Se atoms in the SnS lattice and, thus, leads to structural modifications. The observed d-spacing values were used to calculate lattice parameters of films S1 and S2 given by the relation (1) [34]
1 2
d
2
¼
2
2
h k l þ þ a2 b2 c2
ð1Þ
The film S1 possesses lattice parameter a = 4.31 Å; b = 11.2231 Å; c = 3.990 Å which are in good agreement to the reported JCPDS values as presented in Table 1, while the variation in the lattice parameters corresponding to SnS and SnS0.50Se0.50 (JCPDS data) phase has been observed in film S2. The shift in peaks leads to change in unit cell parameters of SnS phase and correspondingly films showed change in their lattice parameter values a = 11.802 Å; b = 4.039 Å; c = 4.322 Å as compared to that of JCPDS values. These changes in the lattice parameters caused changes in the unit cell volume which increased from 192.762 (1) Å3 to 206.022 (1) Å3 with selenisation of the films at 523 K. This increase in the unit cell volume of the S2 films confirms the insertion of larger atomic radii of Se into the existing SnS lattice structure. These changes in the unit cell parameters due to insertion of Se atoms leads to decrease in crystallite size and increase in lattice strain and dislocation density. The axis strain (ezz) values of the unit cells of SnS and SnS1xSex films have been calculated using Eq. (2) [35].
ezz ð%Þ ¼
c c0 100 c0
ð2Þ
where, c is the lattice parameter of strained lattice calculated from the x-ray diffraction data and c0 is the unstrained lattice parameter of bulk SnS and SnS0.50Se0.50. The unit cell parameters of SnS are in close association with JCPDS value while the unit cell parameters of S2 are deviated from the standard JCPDS data value of SnS0.50Se0.50 and SnS. The film S2 shows strain along a- and c-axis having values 4.165 and 0.552 respectively. Thus, the S2 films exhibit diffusion of Se atoms in the SnS lattice which leads to higher strain in the a-axis due to the replacement of higher atomic radii of Se atoms in place of S atoms at the substitution sites. This strained lattice parameter values depicts the formation of SnS0.40Se0.60 phase in analogy to the lattice parameters of SnS0.50Se0.50. The confirmation of SnS0.40Se0.60 phase can also be verified from the lattice strain values calculated corresponding to SnSe phase and found to have values 2.648, 2.745 and 2.657 along a, b and c-axis respectively. The values corresponding to axis strain calculated from the bulk value of SnS0.50Se0.50 are small as compared to the axis strain values obtained from SnSe except the lattice strain corresponding to a-axis. The results of the axis strain, thus, concludes the formation of SnS0.40Se0.60 phase which produces
higher a-axis strain due to the diffusion of Se atoms at the substitution sites. According to Quin and Szpunar model, vacancies and vacancy clusters can produce the strain in the lattice due to grain boundary related to excess volume [36]. These results were verified from the calculation of structural parameters like crystallite size, dislocation density and number of crystallites. The average crystallite size and residual strain of the films S1 and S2 are obtained from W-H plots as presented in Fig. 2. These W-H plots are generated using HighScore software supplied by Panalytical taking highly processed bulk silicon sample as a reference. The crystallite size (D) is then used to find the dislocation density (d) and number of crystallites (N) using the relation given in Eqs. (3) and (4) respectively.
d¼
N¼
1
ð3Þ
D2 t
ð4Þ
D3
where, t is the film thickness. The crystallite size obtained using WH plot is found to be 46.2 nm for SnS phase which decreased to 31.7 nm due to the doping of Se atoms in the already existing SnS lattice. This insertion of Se atoms creates new nucleation centres (60%) which leads to the deformation of already existing SnS lattice. Thus, increases dislocation density from 4.64 1016 cm2 (S1) to 10.05 1016 cm2 (S2) due to its inverse relation with crystallite size. This decrease in the crystallite size and increase in the dislocation density caused due to doping of Se atoms might attributed to lattice distortion due to difference in ionic radii of Se2 and S2. Based on this structural analysis, it is found that the films of S1 and S2 samples exhibit high crystalline character as compared to the recently reported SnS1xSex films [37].
3.2. Morphology Fig. 3 shows SEM images of SnS (S1) and Se (S2) doped nanocrystalline films. The obtained images exhibit different surface morphologies for S1 and S2 films. The film S1exhibit uniform and homogeneous grain growth of spherical shape to those of SnS0.40Se0.60 which showed low surface smoothness with the presence of mixed and facile grains of SnS and SnS0.40Se0.60 phase. However, S2 films exhibit decrease in grain growth as compared to the sulphurized films (S1). The surface study of film S1 reveals the uniform growth of SnS crystallites in comparison to S2 films which presents nonuniform and inhomogeneous growth of the crystallites on the surface and shows lump formation due to the diffusion of Se atoms from the surface. This leads to some vacancies on the surface of the film and made the films rough as compared to SnS films. These results were also supported from the Surface smoothness of the films interpreted on the basis of extinction coefficient calculated from the data obtained from UV–Vis-NIR spectrophotometer.
Table 1 Structural parameters of SnS and Se doped films with their respective standard JCPDS card. Material
SnS JCPDS S1 S2 SnS0.5Se0.5 JCPDS
(1 1 1)SnS*/SnS0.5Se# 0.5
(0 4 0)*SnS/(4 0 0)# SnS0.5Se0.5
(1 2 0)*SnS/(2 0 1)# SnS0.5Se0.5
2h
Intensity
2h
Intensity
2h
Intensity
31.530 — 31.225 31.093
100 — 451 100
31.970 31.94 31.975 31.566
50 1029 196 63
26.008 26.005 25.525 25.719
50 348 153 32
Volume Å3
Cell parameters Å
a = 4.329; b = 11.19; c = 3.983 a = 4.31; b = 11.2231; c = 3.990 (a = 11.802; b = 4.039; c = 4.322) a = 11.33; b = 4.047; c = 4.376
$
193.03 192.762 206.022 200.69
Please cite this article as: A. Banotra and N. Padha, Facile growth of SnS and SnS0.40Se0.60 thin films as an absorber layer in the solar cell structure, Materials Today: Proceedings, https://doi.org/10.1016/j.matpr.2019.11.154
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Fig. 2. W-H Plots of S1 and S2 films obtained from profile fit of the data using Highscore software.
Fig. 3. SEM micrographs of SnS (S1) and Se (S2) doped films.
3.3. Compositional characterization The composition of S1 and S2 films has been obtained using energy dispersive x-ray technique and presented in Table 2. The Table 2 Atomic percentage of the pure SnS and Se doped films. Compound
SnS SnS0.43Se0.57
% Atoms
Sn/S
Sn
S
Se
50.44 56.29
49.56 18.65
0.00 25.08
S/(S + Se)
film S1 showed S/Sn ratio of 1.01 which is nearly stoichiometric while S2 films exhibit decrease in the sulphur content with the presence of selenium. These changes in the atomic percentage values of S2 film has been confirmed from the structural and optical results. Thus, the S2 films demonstrate the interaction of ‘Se’ atoms with SnS lattice and resulted in the formation of SnS0.40Se0.60 compound.
Se/(S + Se)
3.4. Optical characterization 1.01 3.01
– 0.43
– 0.57
Fig. 4 shows the optical transmission of SnS (S1) and Se (S2) doped thin films. The doping of Se atoms into SnS influences the
Please cite this article as: A. Banotra and N. Padha, Facile growth of SnS and SnS0.40Se0.60 thin films as an absorber layer in the solar cell structure, Materials Today: Proceedings, https://doi.org/10.1016/j.matpr.2019.11.154
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optical properties of S1 films and correspondingly decreases transmission values. The S1 films showed a transmission of 30–35% in the wavelength range of 1000 nm to 1800 nm while S2 films demonstrates decrease in transmission % upto 5% to 10% in the wavelength range 950 nm to 1800 nm. This decrease in the transmission % has been attributed to the grain boundaries and lattice defects that existed in the nano-crystalline films due to the insertion of ‘Se’ ions. Thus, causes decrease in the crystallite size. The absorption coefficient (a) for films S1 and S2 were estimated from transmittance using Eq. (5) and presented in Fig. 5 [38].
1 t
100 T%
ð5Þ
a ¼ ln
Fig. 5 shows the decrease in the absorption coefficient (a) value of S2 films in comparison to S1 films. However, the films found to have absorption value 105 cm1 in the visible region of the EM spectrum which is above 2–3 104 cm1 at 1.5 eV (optimum bandgap) as reported [39], thus, demonstrates excellent solar absorbers for use in photovoltaic applications having thickness less than 1 mm. The absorption co-efficient (a) calculated using equation (5) has been used for the determination of bandgap (Eg) as per the Eq. (6) [40].
ahm ¼ B hm Eg
1n
Fig. 5. Absorption co-efficient as a function of for S1 and S2 films. wavelength for SnS (S1) and Se (S2) doped films.
ð6Þ
where n is 2 and ½ for direct and indirect allowed transition respectively, B; proportionality constant and hm; photon energy. Graph of (ahm)2 versus photon energy (hm) for S1 and S2 films has been presented in Fig. 6(a). Both the films demonstrate direct as well as indirect bandgap with sharp direct transitions found in the wavelength range 700 nm to 900 nm. The direct bandgap of the films is determined by extrapolating the linear region of the graph to the energy axis (hm) and found to have 1.44 eV and 1.227 eV for S1 and S2 films respectively. The decrease in the bandgap value has been attributed to the stoichiometric changes in the film composition due to doping of Se atoms. According to literature reports, indirect bandgap semiconductors presents weak phonon assisted absorption and requires a considerable thickness of 200 mm for complete absorption. Moreover, SnS thin-film requires less than 1 mm thickness for absorption in direct allowed optical transitions. Further, the absorption coeffi-
Fig. 4. Transmission % as a function of wavelength.
Fig. 6. a (ahm)2 versus energy (hm) plot for SnS and Se (b): (ahm)1/2 versus energy (hm) plot for doped films SnS and Se doped films.
Please cite this article as: A. Banotra and N. Padha, Facile growth of SnS and SnS0.40Se0.60 thin films as an absorber layer in the solar cell structure, Materials Today: Proceedings, https://doi.org/10.1016/j.matpr.2019.11.154
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ficient of 105 cm1 for thickness less than 1 mm and highly suitable for absorbing sunlight from the visible to NIR region as a photoabsorber layer in the solar cell. Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper
References
Fig. 7. Extinction co-efficient versus wavelength plot for SnS (S1) and Se (S2) doped films.
cient needs to be about 2–3 104 cm1 at photon energy 1.5 eV for complete absorption. Thus, there is an offset of about 0.1 eV between the effective absorption threshold and the band gap, which is considerably larger than in direct-gap absorbers. This offset is undesirable as the energy difference is lost for thermalization of electrons and holes. The indirect bandgap calculation shows the value of 1.03 eV and 0.76 eV for S1 and S2 films respectively. This represents a band offset of about 0.41 eV for S1 and 0.46 eV for S2 films. This increased offset on the addition of Se atoms might be realized as the fact that higher thermalisation is required for S2 films due to absorption arise from defects like strain and dislocation density. The surface smoothness of the films has been calculated from extinction coefficient using the relation (7) [41]
K¼
ak 4p
ð7Þ
Surface smoothness and homogeneity of the films has been studied from extinction coefficient ‘K’. Fig. 7 shows low value of ‘K’ and demonstrates smooth and homogeneous films for pure SnS phase while the extinction coefficient value has increased with the doping of ‘Se’ atoms in SnS. Hence, S2 films demonstrate decrease in surface smoothness with the insertion of ‘Se’ atoms which were corroborated with the surface defect results presented in structural analysis. The optical results demonstrate high absorption coefficient 105 cm1 with optimum bandgap values (1.24– 1.40 eV) having sharp edge transition. Thus, the prepared films exhibits excellent photosensitive response to the bandgap values for selective solar absorbers and PEC applications. 4. Conclusions A detailed analysis is presented confirming the doping of larger atomic radii into SnS lattice. This insertion in the pure SnS films affects the structural, optical and morphology of the films. Both the pure and doped films were polycrystalline in nature in which the crystallinity, bandgap and surface smoothness of the films decreased while lattice strain, dislocation density and band offset increased. These modifications in the structural and optical properties are attributed to the mismatch due to the insertion of higher atomic radii at substitutional sites. The films showed a decrease in the direct and indirect optical bandgap with increase in band offset on Se doping. Moreover, still possesses the absorption coef-
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Please cite this article as: A. Banotra and N. Padha, Facile growth of SnS and SnS0.40Se0.60 thin films as an absorber layer in the solar cell structure, Materials Today: Proceedings, https://doi.org/10.1016/j.matpr.2019.11.154
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Materials Today: Proceedings journal homepage: www.elsevier.com/locate/matpr
Facile growth of SnS and SnS0.40Se0.60 thin films as an absorber layer in the solar cell structure Arun Banotra a,b, Naresh Padha a,⇑ a b
Department of Physics, University of Jammu, Jammu 180006, India School of Sciences, Cluster University of Jammu, Jammu 180001, India
a r t i c l e
i n f o
Article history: Received 21 August 2019 Received in revised form 2 November 2019 Accepted 12 November 2019 Available online xxxx Keywords: Tin SnS1xSex Solar absorbers Thermal evaporation X-ray diffraction Bandgap
a b s t r a c t SnS (S1) and SnS0.4Se0.6 (S2) thin films were prepared using thermal evaporation on annealing at 523 K of the thermally deposited films. Compositional analysis of the films reveals the formation of SnS and SnS0.40Se0.60 phase. X-ray diffraction results demonstrate the formation of orthorhombic structure corresponding to SnS and SnS0.40Se0.60 phase. Moreover, a decrease in the intensity of SnS0.40Se0.60 peaks as compared to those of SnS was observed. Thus, causes insertion of selenium (Se) atoms in SnS lattice and results in the modification of lattice parameters. These structural modifications leads to variations in crystallite size, lattice strain and dislocation density, thus, confirms the formation of ternary SnS0.40Se0.60 phase. The changes corresponding to optical parameters are also observed in the films in the form of lower transmission, bandgap value and absorption coefficient on Se insertion. Moreover, both the films exhibit absorption coefficient >105 cm1 with bandgap values of 1.44 and 1.23 eV and found suitable for absorption in solar cell. Ó 2019 Elsevier Ltd. All rights reserved. Selection and peer-review under responsibility of the scientific committee of the International Conference on functional materials and simulation techniques.
1. Introduction Efficient selective solar absorbers are required for an effective conversion of solar radiation into electric energy. Literature reports several selective solar absorbers with high absorption and, thus, exhibit capability to reach terawatt energy production with sustainable development [1–5]. To achieve such absorbers, thin-film technologies like CdTe [6] and Cu(In,Ga)Se2 [7] are not suitable because of high cost, supply issues, toxicity and associated disposal management. Moreover, CZTS material seen to have excellent absorber properties, but theoretical investigations reveal several issues, related to multiple valances of Sn with narrow stability range due to the presence of multiple constituents [8–10]. Thus, materials such as SnS, CuS2, SnSe and their ternary derivatives have recently gained interest in photovoltaic (PV) community due to its abundance, low cost and less toxic constituents [11–15]. Recently, these chalcogenide semiconductor materials attracted attention and finds applications as photovoltaic devices, detectors, and
⇑ Corresponding author. E-mail address: [email protected] (N. Padha).
biomedical arena owing to their narrow bandgap energy [16]. These IV-VI compound semiconductors possess high optical conductivity in the visible and ultraviolet region of the EM spectrum [17]. Further, high chemical and environmental stability have been seen in comparison to toxic lead chalcogenides [18–19]. There are large numbers of solution based reports available for the growth of tin chalcogenides which render high electrical resistivity and consume time. These tin chalcogenide thin films were reported to be formed using different physical evaporation methods due to its ease of availability with excellent optical properties. Sulphide and selenide based compound semiconductor materials exhibit high photosensitivity with composition variation and, thus, found favourable as selective solar absorbers and photoelectrochemical (PEC) applications [20–23]. One of the potential photosensitive absorbers available in these tin chalcogenide materials is SnS. It has exhibited high optical absorption 104 cm1, possess optimal direct (1.07 eV) and indirect (1.3 eV) photon energy close to already developed Silicon technology. Further, SnS presents a vulnerable interlayer in its deformed NaCl layer type structure for the penetration of impurities due to weak Van der Waals forces [24,25]. This makes the material more suitable for doping and exhibits growth of facile phases which leads to variations in their structural and optical properties. Tin Selenide another prominent
https://doi.org/10.1016/j.matpr.2019.11.154 2214-7853/Ó 2019 Elsevier Ltd. All rights reserved. Selection and peer-review under responsibility of the scientific committee of the International Conference on functional materials and simulation techniques.
Please cite this article as: A. Banotra and N. Padha, Facile growth of SnS and SnS0.40Se0.60 thin films as an absorber layer in the solar cell structure, Materials Today: Proceedings, https://doi.org/10.1016/j.matpr.2019.11.154
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material in this class presents high absorption coefficient (104 cm1) with low bandgap value of 0.9 eV. Thus, the incorporation of Se atoms in SnS lattice leads to stoichiometric changes and provides bandgap tunability from 1.3 eV to 0.9 eV and results into viable optical solar selective absorber. The other important aspect for the formation of this ternary compound semiconductor is the solar cell efficiency. According to literature, sulphoselenide based ternary solar cells showed better quantum efficiency of 10.1% for S/S + Se ratio of about 40% in comparison to sulphide (8.4%) and selenide (9.15%) based solar cells [26–30]. In the present work, tin and sulphur mixture (powder) have been used as an evaporant material to deposit thin films using multisource thermal evaporation. Further, the as-deposited films were supplemented with additional layer of sulphur and selenium which were undertaken for annealing at 523 K. This results into the transformation of binary SnS to ternary SnS0.4Se0.6 phases with modified structural, optical properties having high photosensitivity. 2. Experimental Highly pure constituents Sn and S powders of the purity 99.99% and 99.50% respectively, procured from sigma Aldrich taken in stoichiometric proportions, were mixed in planetary ball mill at 120 rpm for six hours. The powders Sn + S were deposited on organically cleaned corning glass substrate by multi source thermal evaporation method. Further, the films were supplemented with additional sulphur (S) and selenium (Se) layer of the thickness 200 nm in each case over the as-grown films [30–31]. The resultant films were annealed in a tubular furnace under the vacuum 1 103 mbar at 523 K for one hour. The obtained samples were indexed as S1 (SnS) and S2 (SnS0.40Se0.60) respectively. The deposition has been carried out at a base vacuum pressure 2 106 mbar using vacuum coating unit (Make: HindhiVac, Model: 12A4DM) at room temperature. During deposition, the substrates were rotated continuously at low speed to obtain uniform and homogeneous films. The thickness of the deposited films were monitored using digital thickness monitor (Model: DTM 101, Make HindhiVac) by maintaining 3 to 6 Å/s rate of evaporation. The substrates used for deposition were cleaned in an ultrasonic bath cleaner using trichloroethylene, acetone and methanol (TAM) under constant ambient conditions. The annealed films were characterized for the determination of structural parameters using X-ray diffractometer (Make: PANalytical, Model: X’pert3 powder). The films were scanned in 2h range 10°–70° using filtered CuKa1 radiation (k = 1.5406 Å) equipped with HD Bragg Brentano incident geometry. During data collection, step size of 0.03° and time per step 0.15/ sec (s) were used at a grazing incidence angle of 1.7°. This X-ray diffraction data has been analyzed using highscore software provided by Panalytical for the determination of structural parameters like crystallite size, strain and dislocation density. The study of film surface and composition was carried out using scanning electron microscope (SEM) (Make: JFEI, Model: Nova Nano SEM-450) equipped with energy dispersive x-ray analysis (EDAX). The optical measurements were performed using UV–Vis-NIR spectrophotometer (Make: Shimadzu, Model: UV-3600) in optical transmittance mode in the wavelength range 300 to 1800 nm at room temperature. 3. Results and discussion Thin films of SnS and SnS1xSex were prepared by sequential deposition of SnS and Se layers using multisource thermal evaporation method. The films demonstrate simple preparation technique with low cost, less toxic and highly abundant constituents,
Fig. 1. X-ray diffraction patterns of S1 and S2 films along with JCPDS card of SnS0.5Se0.5 (bars).
yet exhibit homogeneous, uniform and smooth surfaces, with high photosensitive and absorption response.
3.1. X-ray diffraction Fig. 1 shows the X-ray diffraction patterns of S1 and S2 films. The JCPDS data of SnS0.50Se0.50 has been included at the bottom of Fig. 1. It was found that the film S2 possess SnS1xSex phase, corroborated well with corresponding JCPDS pattern of SnS0.50Se0.50. The film S1 demonstrated formation of orthorhombic SnS phase corresponding to 2h values of 31.94 (1)°, 26.005 (1)° and 39.115 (1)° along (0 4 0), (1 2 0) and (1 3 1) planes which matched well with the SnS phase as per the JCPDS card no. 39–0354 with (0 4 0) as most significant peak. The analysis of the film S1 at lower thickness of 200 nm as mixed SnxSy phase at different annealing temperatures has already been reported [31]. However, the film S2 was also found to possess dominant (0 4 0) and (1 0 1) SnS peaks at 2h values of 31.97 (1)° and 30.65 (1)° respectively along with SnS1xSex peaks. Besides this, the film showed an impact of Se atoms diffused into SnS lattice and caused shift in the 2h values along with emergence of various SnS1xSex Bragg peaks due to doping of greater effective ionic radii of Se2 than that of S2 ions. The position of 2h is shifted towards smaller angles after Se doping into SnS phase. Therefore, the formation of SnS1xSex occurs on doping of Se in SnS which also causes strain in the lattice as has also been reported earlier [32]. These results are confirmed on comparison with orthorhombic SnS0.50Se0.50 (JCPDS card no. 48– 1225) corresponding to the peaks observed at 2h value of 31.225 (1)°, 38.095 (1)° and 25.525 (1)° along (1 1 1), (3 1 1) and (2 0 1) planes. Thus, the S2 films exhibit mixed phase character of SnS and SnS1xSex crystallites in the form of a solid solution. Such mixed phase behaviour of sulphoselenide composition has been reported by Thomas Schnabel et al. [33]. The presence of SnS along with SnS1xSex phase might be attributed to one of the following reasons: Either deposited additional layer contains less ‘Se’ atoms which were unable to convert the whole SnS into SnS1xSex phase or the deposited additional Se layer at the top of SnS unable to diffuse the Se through whole thickness of 600 nm to convert the entire SnS lattice into SnS1xSex phase. Thus, might results in the formation of SnS1xSex phase at the top and SnS phase at the bottom. While the presence of other binary phase SnS2 is attributed to the annealing temperature. This emergence corresponding to annealing has already been reported previously [31].
Please cite this article as: A. Banotra and N. Padha, Facile growth of SnS and SnS0.40Se0.60 thin films as an absorber layer in the solar cell structure, Materials Today: Proceedings, https://doi.org/10.1016/j.matpr.2019.11.154
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A decrease in the intensity of S2 films has also been observed as compared to those of S1 films. This decrease in the intensity of SnS peaks in S2 films attributed to the insertion of foreign (Se) atoms in SnS lattice at substitutional sites. Thus, caused strain in the film due to insertion of larger atomic radii of ‘Se’ (198 pm) ions as compared to those of ‘S’ (184 pm) ions. Further, the film exhibit lower crystallinity due to decrease of intensity on the insertion of Se atoms in the SnS lattice and, thus, leads to structural modifications. The observed d-spacing values were used to calculate lattice parameters of films S1 and S2 given by the relation (1) [34]
1 2
d
2
¼
2
2
h k l þ þ a2 b2 c2
ð1Þ
The film S1 possesses lattice parameter a = 4.31 Å; b = 11.2231 Å; c = 3.990 Å which are in good agreement to the reported JCPDS values as presented in Table 1, while the variation in the lattice parameters corresponding to SnS and SnS0.50Se0.50 (JCPDS data) phase has been observed in film S2. The shift in peaks leads to change in unit cell parameters of SnS phase and correspondingly films showed change in their lattice parameter values a = 11.802 Å; b = 4.039 Å; c = 4.322 Å as compared to that of JCPDS values. These changes in the lattice parameters caused changes in the unit cell volume which increased from 192.762 (1) Å3 to 206.022 (1) Å3 with selenisation of the films at 523 K. This increase in the unit cell volume of the S2 films confirms the insertion of larger atomic radii of Se into the existing SnS lattice structure. These changes in the unit cell parameters due to insertion of Se atoms leads to decrease in crystallite size and increase in lattice strain and dislocation density. The axis strain (ezz) values of the unit cells of SnS and SnS1xSex films have been calculated using Eq. (2) [35].
ezz ð%Þ ¼
c c0 100 c0
ð2Þ
where, c is the lattice parameter of strained lattice calculated from the x-ray diffraction data and c0 is the unstrained lattice parameter of bulk SnS and SnS0.50Se0.50. The unit cell parameters of SnS are in close association with JCPDS value while the unit cell parameters of S2 are deviated from the standard JCPDS data value of SnS0.50Se0.50 and SnS. The film S2 shows strain along a- and c-axis having values 4.165 and 0.552 respectively. Thus, the S2 films exhibit diffusion of Se atoms in the SnS lattice which leads to higher strain in the a-axis due to the replacement of higher atomic radii of Se atoms in place of S atoms at the substitution sites. This strained lattice parameter values depicts the formation of SnS0.40Se0.60 phase in analogy to the lattice parameters of SnS0.50Se0.50. The confirmation of SnS0.40Se0.60 phase can also be verified from the lattice strain values calculated corresponding to SnSe phase and found to have values 2.648, 2.745 and 2.657 along a, b and c-axis respectively. The values corresponding to axis strain calculated from the bulk value of SnS0.50Se0.50 are small as compared to the axis strain values obtained from SnSe except the lattice strain corresponding to a-axis. The results of the axis strain, thus, concludes the formation of SnS0.40Se0.60 phase which produces
higher a-axis strain due to the diffusion of Se atoms at the substitution sites. According to Quin and Szpunar model, vacancies and vacancy clusters can produce the strain in the lattice due to grain boundary related to excess volume [36]. These results were verified from the calculation of structural parameters like crystallite size, dislocation density and number of crystallites. The average crystallite size and residual strain of the films S1 and S2 are obtained from W-H plots as presented in Fig. 2. These W-H plots are generated using HighScore software supplied by Panalytical taking highly processed bulk silicon sample as a reference. The crystallite size (D) is then used to find the dislocation density (d) and number of crystallites (N) using the relation given in Eqs. (3) and (4) respectively.
d¼
N¼
1
ð3Þ
D2 t
ð4Þ
D3
where, t is the film thickness. The crystallite size obtained using WH plot is found to be 46.2 nm for SnS phase which decreased to 31.7 nm due to the doping of Se atoms in the already existing SnS lattice. This insertion of Se atoms creates new nucleation centres (60%) which leads to the deformation of already existing SnS lattice. Thus, increases dislocation density from 4.64 1016 cm2 (S1) to 10.05 1016 cm2 (S2) due to its inverse relation with crystallite size. This decrease in the crystallite size and increase in the dislocation density caused due to doping of Se atoms might attributed to lattice distortion due to difference in ionic radii of Se2 and S2. Based on this structural analysis, it is found that the films of S1 and S2 samples exhibit high crystalline character as compared to the recently reported SnS1xSex films [37].
3.2. Morphology Fig. 3 shows SEM images of SnS (S1) and Se (S2) doped nanocrystalline films. The obtained images exhibit different surface morphologies for S1 and S2 films. The film S1exhibit uniform and homogeneous grain growth of spherical shape to those of SnS0.40Se0.60 which showed low surface smoothness with the presence of mixed and facile grains of SnS and SnS0.40Se0.60 phase. However, S2 films exhibit decrease in grain growth as compared to the sulphurized films (S1). The surface study of film S1 reveals the uniform growth of SnS crystallites in comparison to S2 films which presents nonuniform and inhomogeneous growth of the crystallites on the surface and shows lump formation due to the diffusion of Se atoms from the surface. This leads to some vacancies on the surface of the film and made the films rough as compared to SnS films. These results were also supported from the Surface smoothness of the films interpreted on the basis of extinction coefficient calculated from the data obtained from UV–Vis-NIR spectrophotometer.
Table 1 Structural parameters of SnS and Se doped films with their respective standard JCPDS card. Material
SnS JCPDS S1 S2 SnS0.5Se0.5 JCPDS
(1 1 1)SnS*/SnS0.5Se# 0.5
(0 4 0)*SnS/(4 0 0)# SnS0.5Se0.5
(1 2 0)*SnS/(2 0 1)# SnS0.5Se0.5
2h
Intensity
2h
Intensity
2h
Intensity
31.530 — 31.225 31.093
100 — 451 100
31.970 31.94 31.975 31.566
50 1029 196 63
26.008 26.005 25.525 25.719
50 348 153 32
Volume Å3
Cell parameters Å
a = 4.329; b = 11.19; c = 3.983 a = 4.31; b = 11.2231; c = 3.990 (a = 11.802; b = 4.039; c = 4.322) a = 11.33; b = 4.047; c = 4.376
$
193.03 192.762 206.022 200.69
Please cite this article as: A. Banotra and N. Padha, Facile growth of SnS and SnS0.40Se0.60 thin films as an absorber layer in the solar cell structure, Materials Today: Proceedings, https://doi.org/10.1016/j.matpr.2019.11.154
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Fig. 2. W-H Plots of S1 and S2 films obtained from profile fit of the data using Highscore software.
Fig. 3. SEM micrographs of SnS (S1) and Se (S2) doped films.
3.3. Compositional characterization The composition of S1 and S2 films has been obtained using energy dispersive x-ray technique and presented in Table 2. The Table 2 Atomic percentage of the pure SnS and Se doped films. Compound
SnS SnS0.43Se0.57
% Atoms
Sn/S
Sn
S
Se
50.44 56.29
49.56 18.65
0.00 25.08
S/(S + Se)
film S1 showed S/Sn ratio of 1.01 which is nearly stoichiometric while S2 films exhibit decrease in the sulphur content with the presence of selenium. These changes in the atomic percentage values of S2 film has been confirmed from the structural and optical results. Thus, the S2 films demonstrate the interaction of ‘Se’ atoms with SnS lattice and resulted in the formation of SnS0.40Se0.60 compound.
Se/(S + Se)
3.4. Optical characterization 1.01 3.01
– 0.43
– 0.57
Fig. 4 shows the optical transmission of SnS (S1) and Se (S2) doped thin films. The doping of Se atoms into SnS influences the
Please cite this article as: A. Banotra and N. Padha, Facile growth of SnS and SnS0.40Se0.60 thin films as an absorber layer in the solar cell structure, Materials Today: Proceedings, https://doi.org/10.1016/j.matpr.2019.11.154
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5
optical properties of S1 films and correspondingly decreases transmission values. The S1 films showed a transmission of 30–35% in the wavelength range of 1000 nm to 1800 nm while S2 films demonstrates decrease in transmission % upto 5% to 10% in the wavelength range 950 nm to 1800 nm. This decrease in the transmission % has been attributed to the grain boundaries and lattice defects that existed in the nano-crystalline films due to the insertion of ‘Se’ ions. Thus, causes decrease in the crystallite size. The absorption coefficient (a) for films S1 and S2 were estimated from transmittance using Eq. (5) and presented in Fig. 5 [38].
1 t
100 T%
ð5Þ
a ¼ ln
Fig. 5 shows the decrease in the absorption coefficient (a) value of S2 films in comparison to S1 films. However, the films found to have absorption value 105 cm1 in the visible region of the EM spectrum which is above 2–3 104 cm1 at 1.5 eV (optimum bandgap) as reported [39], thus, demonstrates excellent solar absorbers for use in photovoltaic applications having thickness less than 1 mm. The absorption co-efficient (a) calculated using equation (5) has been used for the determination of bandgap (Eg) as per the Eq. (6) [40].
ahm ¼ B hm Eg
1n
Fig. 5. Absorption co-efficient as a function of for S1 and S2 films. wavelength for SnS (S1) and Se (S2) doped films.
ð6Þ
where n is 2 and ½ for direct and indirect allowed transition respectively, B; proportionality constant and hm; photon energy. Graph of (ahm)2 versus photon energy (hm) for S1 and S2 films has been presented in Fig. 6(a). Both the films demonstrate direct as well as indirect bandgap with sharp direct transitions found in the wavelength range 700 nm to 900 nm. The direct bandgap of the films is determined by extrapolating the linear region of the graph to the energy axis (hm) and found to have 1.44 eV and 1.227 eV for S1 and S2 films respectively. The decrease in the bandgap value has been attributed to the stoichiometric changes in the film composition due to doping of Se atoms. According to literature reports, indirect bandgap semiconductors presents weak phonon assisted absorption and requires a considerable thickness of 200 mm for complete absorption. Moreover, SnS thin-film requires less than 1 mm thickness for absorption in direct allowed optical transitions. Further, the absorption coeffi-
Fig. 4. Transmission % as a function of wavelength.
Fig. 6. a (ahm)2 versus energy (hm) plot for SnS and Se (b): (ahm)1/2 versus energy (hm) plot for doped films SnS and Se doped films.
Please cite this article as: A. Banotra and N. Padha, Facile growth of SnS and SnS0.40Se0.60 thin films as an absorber layer in the solar cell structure, Materials Today: Proceedings, https://doi.org/10.1016/j.matpr.2019.11.154
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ficient of 105 cm1 for thickness less than 1 mm and highly suitable for absorbing sunlight from the visible to NIR region as a photoabsorber layer in the solar cell. Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper
References
Fig. 7. Extinction co-efficient versus wavelength plot for SnS (S1) and Se (S2) doped films.
cient needs to be about 2–3 104 cm1 at photon energy 1.5 eV for complete absorption. Thus, there is an offset of about 0.1 eV between the effective absorption threshold and the band gap, which is considerably larger than in direct-gap absorbers. This offset is undesirable as the energy difference is lost for thermalization of electrons and holes. The indirect bandgap calculation shows the value of 1.03 eV and 0.76 eV for S1 and S2 films respectively. This represents a band offset of about 0.41 eV for S1 and 0.46 eV for S2 films. This increased offset on the addition of Se atoms might be realized as the fact that higher thermalisation is required for S2 films due to absorption arise from defects like strain and dislocation density. The surface smoothness of the films has been calculated from extinction coefficient using the relation (7) [41]
K¼
ak 4p
ð7Þ
Surface smoothness and homogeneity of the films has been studied from extinction coefficient ‘K’. Fig. 7 shows low value of ‘K’ and demonstrates smooth and homogeneous films for pure SnS phase while the extinction coefficient value has increased with the doping of ‘Se’ atoms in SnS. Hence, S2 films demonstrate decrease in surface smoothness with the insertion of ‘Se’ atoms which were corroborated with the surface defect results presented in structural analysis. The optical results demonstrate high absorption coefficient 105 cm1 with optimum bandgap values (1.24– 1.40 eV) having sharp edge transition. Thus, the prepared films exhibits excellent photosensitive response to the bandgap values for selective solar absorbers and PEC applications. 4. Conclusions A detailed analysis is presented confirming the doping of larger atomic radii into SnS lattice. This insertion in the pure SnS films affects the structural, optical and morphology of the films. Both the pure and doped films were polycrystalline in nature in which the crystallinity, bandgap and surface smoothness of the films decreased while lattice strain, dislocation density and band offset increased. These modifications in the structural and optical properties are attributed to the mismatch due to the insertion of higher atomic radii at substitutional sites. The films showed a decrease in the direct and indirect optical bandgap with increase in band offset on Se doping. Moreover, still possesses the absorption coef-
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Please cite this article as: A. Banotra and N. Padha, Facile growth of SnS and SnS0.40Se0.60 thin films as an absorber layer in the solar cell structure, Materials Today: Proceedings, https://doi.org/10.1016/j.matpr.2019.11.154