Magnetron sputtered Cu doped SnS thin films for improved photoelectrochemical and heterojunction solar cells

Abstract

Tin(II) sulfide (SnS) is a promising low cost photovoltaic material due to its favorable direct optical band gap (∼1.3 eV) and high absorption coefficient (>10⁴ cm⁻¹). However, SnS solar cells are reported to have low efficiency due to band misalignment that can be reduced by the proper optimization of acceptor concentration in p-SnS. This work describes the effect of extrinsic Cu doping in sprayed SnS thin films on SnO₂:F glass for a possible enhancement in the photocurrent in photoelectrochemical cells and the open circuit voltage in heterojunction solar cells. The structural, morphological, optical and photoelectrochemical properties of the Cu:SnS films are studied in detail. A process temperature of 325 °C was found to be optimum for Cu doping at the Sn vacancies in the host lattice. An improvement in the photocurrent density from 1.1 mA cm⁻² to 1.8 mA cm⁻² was observed in the photoelectrochemical cell prepared by this doping process. A further enhancement in photocurrent of up to 3.2 mA cm⁻² was shown when the residual surface Cu was removed by HCl etching. The developed Cu:SnS heterojunction solar cell showed a record open circuit voltage of 462 mV with In₂S₃ as a buffer layer.

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1 Introduction

Tin(II) sulfide (SnS) is a low cost alternative for photovoltaic absorbers¹ as well as photoelectrodes for water splitting.² It is a p-type semiconductor with a direct optical band gap of ∼1.3 eV and a high absorption coefficient of ∼10⁴ cm⁻¹.^1,3 As an n-type heterojunction partner in solar cells, various buffer materials are possible, such as CdS, In₂S₃, ZnS etc. However, their performance is limited by the band offset between the heterojunction candidates.⁴ A negative (type-II) conduction band offset (CBO) is common in most cases. Extrinsic doping of SnS can modify the band positions and gap of SnS, leading to either a reduction in the negative CBO or creation of a small positive CBO (type-I configuration).⁵ Doping at the Sn-vacancy sites can also change the acceptor concentration in SnS. The band gap narrowing of p-SnS in a type-II heterojunction has a severe effect on the valence band offset (VBO) compared to type-I. When the acceptor concentration approaches 10¹⁸ cm⁻³, it becomes a ‘broken-gap’ (type-III) arrangement.⁶ An optimization of the doping is therefore required to manipulate the CBO as well as the VBO in SnS heterojunction solar cells.

Intrinsically doped SnS shows p-type conductivity. However, it can be modified by changing the Sn/S ratio. Ab initio studies have shown that the Sn vacancies act as shallow acceptors, giving rise to a p-type conductivity, whereas the Sn on S antisites act as donor defects. Hence, a S rich growth condition should be avoided.⁷ A p-type conductivity is required for an ideal absorber layer application in thin film solar cells. While growing the SnS material, the scope of controlling the hole concentration by changing the Sn/S ratio is restricted.⁷ The desired hole concentration of the SnS material can be achieved by ex situ doping with pure metals such as Cu,^5,8 Ag,^9–11 In,^12,13 Al (ref. 14) and Pb.¹⁵ SnS can be n-type by doping with Bi (ref. 16) and Sb.^17,18 By diffusion of Bi and Sb in the p-type SnS, homojunction solar cells can be feasible. Most studies evidence the resistive nature of the SnS layers. Extrinsic doping can adjust this acceptor concentration (N_A) to give the right band gap for a type-II configuration. It has been reported that an optimum acceptor concentration in SnS of between 1.5 × 10¹⁵ and 8.6 × 10¹⁹ cm⁻³ is possible by Cu-diffusion in thermally evaporated SnS film on a glass substrate.^5,8

A potential of 1.23 V is needed for water splitting under standard conditions from a thermodynamic standpoint, therefore a semiconductor with a minimum band-gap of E_g = 1.23 eV (an absorption wavelength cut-off of 1008 nm) could be effective in such an application. Based on the standard AM1.5G solar spectrum (1000 Wm⁻²), a semiconductor with such a band-gap would operate at a maximum overall solar-to-hydrogen conversion efficiency of η_STH = 47.4%, assuming there exists 100% quantum conversion efficiency and no other losses in the system.^19,20 Therefore, SnS can offer the possibility of deployment in water splitting applications as well. Recently, Sun et al. have demonstrated that an all-surface atomic SnS sheet based photoanode has a photon-to-current conversion efficiency of 67.1% at 490 nm with an overall photocurrent density of 5.27 mA cm⁻². However, the photocurrent was less than 0.2 mA cm⁻² for bulk SnS (thickness more than 200 nm).² The earth abundant SnS semiconductor is composed of weakly interacting layers held together by van der Walls interactions, where in each monolayer, Sn and S atoms are tightly bound by chemical bonds with 100% exposed surface atoms. Moreover, no surplus charge is present on the chemically stable surfaces of SnS monolayers that are devoid of dangling bonds or a surface density of states.^2,5,21 Therefore, achieving a higher photocurrent in bulk SnS (thickness more than 200 nm) remains a challenge, and may be addressed by a proper doping scheme.

In the present study, a thin film of SnS produced by compressed air assisted chemical spray pyrolysis (CSP) was doped with Cu by pulsed DC-magnetron sputtering. The Cu:SnS thin films were characterized for their crystal structure, morphology, elemental composition, and electrical and optical properties. Copper was doped ex situ into SnS by varying the substrate temperature. Our results show an improvement in the photocurrent density in the photoelectrochemical cell, and an exceptional rise in open circuit voltage in the doped SnS thin film solar cell. This doping scheme will be useful in developing solution processed SnS solar cells with higher efficiency over the existing efficiency of 1–2%.^1,22,23

2 Results and discussion

2.1 Structural and phase analysis

The XRD patterns of the as-sprayed and doped (at different temperatures from 275 °C to 350 °C for fixed sputtering parameters) films are shown in Fig. 1b. The observed XRD spectra are used to determine the phases, lattice parameters, space groups, and dimensions of the unit cell. The dominant peak broadening of the characteristic phase is used to estimate the strain and crystallite size. The SnS material was considered to have orthorhombic crystal symmetry with different space groups of Pbnm (62) and Cmcm (63) according to COD-AMSCD data. The unit cell of the SnS material with a space group of Pbnm (62) is shown in Fig. 1a. The XRD data were processed as described elsewhere.²⁴ The background-corrected XRD patterns of all the sprayed films, after performing the phase analysis, reveal peaks corresponding to the (110), (120), (021), (101), (111), (040), (131) and (152) planes of reflection, which are characteristic of the SnS phase having Pbnm orthorhombic symmetry. The ideal planes of reflection of COD-AMCSD 900-8785 are shown in Fig. 1a. The as-sprayed SnS film appears as a mixed phase of Pbnm (62) and Cmcm (63) (COD-AMCSD-900-8295) symmetries. The (021) plane of reflection corresponds to the Cmcm (63) space group. The XRD profiles of the SnS crystal structures with space groups of Pbnm (62) and Cmcm (63) were generated by Pseudo-Voigt functions from the crystallographic information files (COD: 600-8285 and 600-8295), and they are shown in Fig. S3 and S4.† The detail of samples A to F is provided in Table 1, where the substrate temperature of Cu diffusion is a parameter. It is interesting to note that the mixed phase appearance of the SnS films was removed for samples E and F. In all cases, the (111) plane remains the preferentially oriented one. It was revealed from peak analysis that the SnS phase is not affected by the doping process. It is important to check whether Cu was doped by substitution or at interstitial sites. The Cu peak (2θ = 43.3°; COD-AMCSD-710-1264) is anticipated if Cu goes to the interstitial sites of the SnS planes, however no peak corresponding to Cu was found, which may indicate a substitutional doping of Cu at the Sn vacancies of SnS. Other impurity phases such as Cu₂SnS₃, Sn₂S₃ and SnS₂ were not detected in the deposited films during the Cu diffusion by sputtering.

Fig. 1 Effect of Cu doping on the structural properties of the sprayed SnS film. (a) Isometric view of the orthorhombic unit cell of the SnS material (COD: 900-8785) and (b) effect of Cu diffusion at various annealing temperatures (see Table 1) on the XRD patterns.

Table 1 Summary of structural properties (2θ, FWHM, d-spacing, lattice parameters, crystallite size and micro-strain) of the as-deposited and Cu doped SnS thin films

Sample code	Substrate temperature (°C)	2θ (°)	FWHM (°)	d-spacing (Å)	Lattice constants (Å)			Grain size t (nm)	Micro-strain Δ
					a	b	c
900–8765	—	31.538	—	2.834	4.33	11.18	3.98	—	—
A	As-sprayed	31.387	0.931	2.847	4.35	11.23	4	9.84	3.910
B	35	31.517	0.81	2.836	4.33	11.19	3.98	11.32	3.401
C	275	31.53	0.71	2.835	4.33	11.18	3.98	12.92	2.981
D	300	31.527	0.717	2.835	4.33	11.18	3.98	12.79	3.010
E	325	31.661	0.401	2.823	4.31	11.14	3.97	22.88	1.683
F	350	31.696	0.405	2.821	4.31	11.13	3.96	22.65	1.700

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The finite size of the crystallite causes a broadening of the diffraction lines which can be related to its size by the Debye–Scherrer formula,²⁵ where the micro-strain of the films was obtained using this relation.²⁶ In thin films, the residual strain may occur at the scale of the microstructure and crystal structure, which is, by necessity, balanced by stresses in other locations or crystal planes within the material for an equilibrium configuration.²⁷ XRD data can determine the residual strain only at the level of the crystal structure. The intergranual micro stress can be determined by optical interferometric methods. However, both types of strain measurement on the same sample are rarely found.²⁸ The estimated d-spacing, crystalline size (t) and residual strain (Δ) for all the films are listed in Table 1. From Fig. 1 and Table 1, the shift of the (111) peak position to a higher θ value, from 31.387° to 31.696°, indicates the existence of crystal lattice compression after Cu-doping. This confirms the compressive nature of the micro-strain. As a result, the lattice volume was decreased from 192.67 ų to 189.87 ų as Cu diffused in the SnS lattice. Therefore, the unit cell volume reduction in the temperature range 275–350 °C during the sputtering was probably due to the substitution of smaller Cu²⁺ in the larger Sn²⁺ site. From the XRD spectra, the substrate temperature of 325 °C was found to be optimum for Cu doping in SnS.

2.2 Microstructure and elemental analysis

Fig. 2 shows topographical FESEM images of the as-deposited and Cu doped SnS films at various diffusion temperatures. The obtained microstructure is in agreement with the reported SnS thin films synthesized using vacuum techniques such as atomic layer deposition,²¹ thermal evaporation²² and sputtering.²⁹ Fig. 2(a) shows the morphology of an as-sprayed SnS film as represented by the vertically oriented petal-like polycrystalline grains propagating almost vertically to the surface. The average length of the petal-like microstructure was measured to be 500 nm in the horizontal direction. The sputtered Cu thin layer (∼100 nm) on the SnS can be observed from the topography image in Fig. 2(b). It was identical to that of the Cu thin layer on the glass substrate (please refer the ESI, Fig. S7,†where FESEM topography of the sputtered Cu layer on the glass substrate with different magnifications is shown). While sputtering, Cu is observed to be diffused into the SnS layer and its transition from surface to bulk can be observed in Fig. 2(c)–(f). Cu doping at a substrate temperature of 325 °C while sputtering was found to be optimum for denser SnS films, and a visible Cu fraction could not be observed from the topography. The thickness of each sample was estimated by cross-sectional FESEM image analysis as shown in Fig. 3. Non-uniformity is a common issue with thin films processed by most non-vacuum techniques. The ex situ doping process has a special advantage with solution processed thin films, which assists with easy diffusion into the structure. Our FESEM images reveal that the morphology of the SnS films was modified due to Cu diffusion into the SnS material. However, the Cu doped films remained pinhole free (please refer to Fig. 2). The cross-section FESEM images (Fig. 3) reveal a similar observation for Cu doped SnS on glass substrates. A summary of the elemental (EDS) analysis is provided in Table 2, where the atomic percentages of Sn, S and Cu are given. The Sn/S ratio of the as-prepared and Cu deposited films at room temperature is in agreement, and was near to ∼1. The as-deposited SnS material was Sn rich intrinsically. The EDS analysis revealed that the SnS material was doped by sputtered Cu at 325 °C and Cu took the Sn site in the lattice. The ratio of (Sn + Cu)/S is in agreement with the Sn/S of sample A. EDS in the cross-section of Cu:SnS was performed to investigate the diffusion profile. The result is shown in Fig. 4. It shows a consistent distribution of Cu in the SnS layer because of a diffusion process taking place at an optimum temperature of 325 °C. Here, it is shown for the Cu:SnS/FTO processed at 325 °C for 30 minutes.

Fig. 2 Surface topographical FESEM images of 500 nm thick (a) undoped, and magnetron sputtered Cu on sprayed SnS at different temperatures; (b) 35 °C, (c) 275 °C, (d) 300 °C, (e) 325 °C and (f) 350 °C, on a glass substrate.

Fig. 3 Cross-sectional FESEM images of samples A–F (a) undoped, and magnetron sputtered Cu on sprayed SnS at different temperatures; (b) 35 °C, (c) 275 °C, (d) 300 °C, (e) 325 °C and (f) 350 °C, on a glass substrate.

Table 2 Summary of the elemental analysis of as-deposited and Cu doped SnS thin films

Sample code	Atomic percentage (%)			Sn/S	(Cu + Sn)/S
	Sn	S	Cu
A	51.57	48.43	—	1.065	—
B	31.38	30.57	38.05	1.026	2.271
C	16.27	38.3	45.43	0.425	1.611
D	25.42	42.16	32.42	0.603	1.372
E	32.3	48.41	19.41	0.667	1.068
F	34.75	52.24	13.01	0.665	0.914

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Fig. 4 The depth profile of Cu:SnS on the FTO coated glass substrate, showing the concentration distribution of the elemental percentages of Sn, S and Cu.

2.3 Optical properties

The absorption co-efficient (α), Tauc plot and derivative of ln(hνα) with respect to hν for samples A–F are shown in Fig. 5(a)–(c), respectively. The thickness of the SnS layer, absorption co-efficient, optical band gap (E_g) and absorption length (L = 1/α) of each sample are summarized in Table 3. The transmittance and absorption spectra of the SnS films are provided in Fig. S6(a) and (b) of the ESI,† respectively. The method of estimation of α and E_g is provided elsewhere.^24,30 The band gap was estimated by the Tauc as well as by derivative, d(ln(hνα))/d(hν), methods. The latter provided a more accurate value of E_g and is therefore considered in our study. The results are consistent with published data. From the XRD and FESEM results, it was revealed that Cu was doped into SnS by a diffusion process in the temperature range of 325–350 °C. Therefore, we analyzed the optical properties of the samples processed in this temperature range only. The estimated absorption coefficient (α) of the Cu doped film was found to be enhanced by a factor of 5.4 in the annealed film, from that of 1.4 × 10⁴ cm⁻¹ in the case of the as-prepared film. This reveals that Cu:SnS only 140 nm thick can absorb most of the incident photons having an energy greater than 1.6 eV. The estimated α for sample E was maximum. The E_g of the as-prepared SnS film was 1.44 eV. By the Cu doping into the SnS material, the band gap was increased and reached up to 1.66 eV. Therefore, a blue shift of the optical band gap is observed as an effect of Cu doping, confirming that there are no deep defect states created due to Cu. A similar observation was made by Gremenok et al. for Pb doped SnS, where E_g was increased from 1.22 eV to 1.32 eV by Pb doping.¹⁵ Moreover, a similar shift in E_g was observed from 1.34 eV to 1.43 eV by Ag doping in an SnS material.¹¹ The ab initio study, to the contrary, suggests that the indirect E_g should decrease from 0.982 eV to 0.864 eV when replacing a Sn atom by Cu.⁵ Such a band gap reduction was reported only for Al as a dopant in SnS, where E_g was reduced from 1.5 eV to 1.29 eV.¹⁴

Fig. 5 Optical properties of undoped and Cu doped SnS thin films on a glass substrate (a) absorption coefficient, (b) Tauc plot and (c) first derivative plot of the normalized value of d(ln(hνα))/d(hν). For substrate temperatures of samples A–F see Table 1.

Table 3 Summary of the optical properties of un-doped and Cu doped SnS thin films on a glass substrate. The thickness of each sample was determined by FESEM analysis. Please refer to Fig. 3, where the thicknesses of the Cu doped SnS films on glass substrates are shown

Sample code	Thickness (nm)	Eg (eV)	α ( × 10^4 cm^−1)	L = 1/α (nm)
A	550	1.44	1.4	715
B	520	1.84	7.8	128
C	400	1.42	2.9	345
D	370	1.5	3.4	295
E	350	1.66	7.2	140
F	330	1.48	4.1	244

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2.4 Photo-response (photoelectrochemical cell and heterojunction solar cell)

The photoelectrochemical (PEC) cell response of the undoped and Cu doped SnS films on the FTO glass substrate with a K₄Fe(CN)₆ + K₃Fe(CN)₆ electrolyte was recorded using a linear sweep photo-voltammogram technique. The topography and cross-section morphology of the SnS/FTO photocathode is shown in Fig. 6. The thickness of the fabricated Cu doped SnS working electrode was 600 nm (Fig. 6(c)). The steady state light and dark J–V envelopes of the constructed PEC cell with the Cu doped SnS electrode is presented in Fig. 7. The PEC cell was illuminated by a pulsed high power white LED light from the front side, where a forward bias voltage was applied. The photo-responses of the PEC cells consisting of working electrodes of as-deposited SnS, Cu:SnS, and after HCl treatment are shown in Fig. 8. In this case the photogenerated carriers are collected due to the electric field present at the semiconductor-electrolyte interface. The flat band potential (V_FB), photocurrent density at zero bias and maximum photocurrent density at a given bias voltage are summarized in Table 4. A flat band potential of 0.38–0.48 V was identified from the PEC response. A significant improvement in PEC response was found for the Cu doped SnS film compared to the pristine film. According to the observed cathodic photocurrent of PEC, all the films were photoactive and p-type in nature. The dark and light envelope of J–V (vs. Ag/AgCl) of the Cu doped SnS electrode is presented in Fig. 7, where the photogenerated current density of 1 mA cm⁻² at zero bias and 1.8 mA cm⁻² at a bias of −0.15 V vs. Ag/AgCl is highlighted. The achieved photogenerated current density was 0.65 mA cm⁻² higher than that in the reported SnS electrode grown by electrodeposition. The improvement in dark current and photogenerated current density can be attributed to the enhanced build in potential because of the improved carrier concentration of the SnS material on the FTO substrate. Summarized electrochemical cell parameters are provided in Table 4. The atomic layer thin intrinsic SnS (less than 200 nm) electrode employed for water splitting applications offered a maximum current density of 5.27 mA cm⁻² for an applied potential of 0.6 V vs. a Ag/AgCl reference electrode.² This large photocurrent is mainly attributed to the n-type conductivity of the atomic layer thin SnS material, however, the bulk thin film of SnS shows a photocurrent density of 0.05 mA cm⁻². The dark current for the Cu doped SnS films shows a large magnitude at an applied bias more than the flat band potential. From the EDS analysis, it was revealed that a trace of elemental Cu was found on the surface of the SnS material, which was yet to be diffused. This residual Cu could be removed by HCl treatment. This is evident from the distinct shaped grains of Cu-doped SnS in Fig. 6(b), as compared to that of the as-prepared SnS in Fig. 6(a). The dark current was found to be much reduced after the HCl treatment. The removal of Cu from the surface provided a better semiconductor/electrolyte interface. As a result, a larger photocurrent density of 3.2 mA cm⁻² was obtained for an applied potential of −0.42 V vs. a Ag/AgCl reference electrode. The obtained larger photocurrent is therefore attributed to the improved N_A. EDS analysis reveals that the Cu was present in SnS even after the HCl treatment, therefore the HCl treatment did not affect the Cu at the doping sites in the SnS material.

Fig. 6 The FE-SEM topographic image of (a) undoped SnS and (b) Cu-doped SnS. The cross sectional FE-SEM images of (c) Cu-doped SnS and (d) heterojunction solar cell on a FTO substrate.

Fig. 7 Dark and light envelope of the constructed photoelectrochemical cell with a Cu doped SnS electrode on a FTO substrate.

Fig. 8 The linear sweep photo-voltammogram under pulsed illumination of the undoped and Cu doped SnS–K₄Fe(CN)₆ + K₃Fe(CN)₆ electrolyte interface in the forward and reverse bias regimes. (a) As deposited SnS film, (b) Cu doped SnS at 325 °C and (c) after HCl treatment. The scan direction of the potential was from positive to negative.

Table 4 The estimated parameters from the PEC response of undoped and Cu doped SnS thin films over the FTO coated glass substrates. The photo-generated current density J_photo = J_light − J_dark. Current density is in mA cm⁻²

Sample	VFB	Jdark	Jphoto at V = 0	Jphoto (max)
As deposited	0.49	0.2	1	1.1
Cu:SnS	0.5	0.1	1.2	1.7
Cu:SnS (etched)	0.38	0.01	1.5	3.2

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The J–V characteristics of grown SnS and Cu:SnS solar cells having In₂S₃ as a buffer layer are shown in Fig. 9. The cross-section of the grown SnS solar cell is shown in Fig. 6(d). All of the layers are distinguished. The estimated thickness of the SnS layer is 700 nm. The V_OC of 210 mV is enhanced to 462 mV over the as-grown SnS solar cell. The achieved V_OC of 462 mV is the highest in its class of solar cell. This improvement is because of the enhanced N_A of the SnS layer. The enhanced V_OC may be attributed to the local degenerate nature of SnS close to the surface causing a back surface field by the Cu doping. However, the EDS analysis revealed the equal diffusion of Cu into the SnS layer at 325 °C. The diffusion temperature of Cu had an influence on the performance of the solar cell. It was found that a diffusion temperature of more than 350 °C could reduce the shunt resistance and hence the solar cell performance. The efficiency of the developed Cu doped SnS solar cell is limited by the poor fill factor. Further study to improve the fill factor would improve the efficiency of this low cost solar cell to a substantial level.

Fig. 9 The J–V characteristics of the undoped and doped SnS solar cells under AM1.5G.

3 Experimental

3.1 Thin film deposition by spray pyrolysis

A thin film of tin(II) sulfide was prepared by the spray pyrolysis technique using an aqueous solution of as-received SnCl₂·2H₂O (SC) (>99%) and thiourea (TU) (>99%). The optimized molar concentration ratio of 1 : 1.25 of SC/TU was maintained. We added 0.55–0.65 ml of HCl (35–38% LR, S D Fine Chem. Ltd.) to enhance the solubility of 0.05 M SnCl₂·2H₂O. The F:SnO₂ (FTO) coated glass substrates (of sheet resistance 15 Ω □⁻¹, area 0.5 × 1.5 cm² and thickness 2.2 mm) were thoroughly cleaned as described elsewhere.²⁴ All chemicals and substrates were procured from Sigma-Aldrich and used without any further purification. The prepared aqueous chemical solution was transported to the spray nozzle from a syringe pusher and sprayed on the glass substrate under ambient conditions. The process parameters and details of the CSP are described elsewhere,²⁴ with the temperature of the glass substrates maintained at 375 ± 5 °C during the spray. After the spray, the substrate was naturally cooled down to 50 °C and then removed from the spray station. The films appeared to be dark red in daylight transmission.

3.2 Cu doping by pulsed DC magnetron sputtering

The sprayed SnS thin films on FTO and glass substrates were loaded inside a pulsed DC magnetron sputtering deposition system procured from Milman Inc. for Cu deposition and ex situ diffusion. The Cu target (purity > 99.999%) was DC sputtered under argon plasma at 10 W of power with 50% of duty with a pulsed frequency of 10 kHz. The ultimate pressure of 3 × 10⁻⁵ mbar was achieved by running a turbo molecular pump backed with a rotary pump. The sputtering pressure of 6.5 × 10⁻³ mbar was maintained by flowing Ar (purity > 99.999%) gas at the flow rate of 20 SCCM. 100 nm of Cu at the deposition rate of 60 Å m⁻¹ was deposited at a temperature of 35 °C on the 500 nm thick SnS layer and then Cu diffusion into the SnS layer was allowed; the substrate temperature was varied from 275 °C to 350 °C to find an optimum temperature of diffusion. The thickness of Cu was monitored through a thickness/rate monitor from Inficon (model, SQM-160).

3.3 Material characterization

The structural characterization of the SnS thin film was carried out using an X-ray diffractometer from PANalytical (model, X'Pert Powder) with Cu-Kα radiation, λ_Kα = 1.540598 Å, step size = 0.05, and time/step size = 0.5 s per step in ‘Gonio mode’. The thickness and average surface roughness of the SnS films were characterized using a surface profiler from Vecco (model, Dektak 150). The planar and cross-sectional morphologies were analyzed using a field emission scanning electron microscope (FESEM) from Zeiss (model, Ultra 55) with 5 kV of field voltage, using an SE2 detector. The elemental composition of the as-prepared film was determined by an energy dispersive spectroscopy (EDS) attachment to the SEM with a field voltage of 20 kV. Optical characterization was carried out by a UV-vis spectrophotometer from Shimadzu (model, UV-2600) by recording the transmission spectra of the thin films in the range 320–1400 nm. The extrinsic nature of the conductivity of the films was determined using the hot point probe method.

3.4 Photoelectrochemical and heterojunction solar cell

The SnS-based photoelectrochemical (PEC) cells were fabricated with spray deposited SnS over a FTO coated glass substrate using the same technique as described above, where 50 ml of 0.1 M (K₄Fe(CN)₆) + 0.01 M K₃Fe(CN)₆ aqueous solution was used as an electrolyte. A PEC solar cell having configuration Pt (2 cm²)|0.1 M (K₄Fe(CN)₆) + 0.01 M K₃Fe(CN)₆|SnS (0.25 cm²)—FTO was constructed. The current–voltage profile under a chopped light illumination condition was recorded by an Autolab potentiostat/galvanostat using Ag/AgCl as a reference electrode. The film surface inside the PEC cell was illuminated by a white high power LED from Wenrun (product, WR-EC150150UW-1000C-9P40), excited (600 W m⁻²) at 12 V DC with manual chopper of frequency 2 Hz. The active area of the SnS electrode was 5 mm × 5 mm, and was kept fixed for all experiments. The heterojunction of the undoped and doped SnS solar cells was fabricated with an In₂S₃ buffer layer as described elsewhere (the device fabrication scheme is illustrated in Fig. S5,†; this process was explored for the development of a superstrate configured SnS solar cell).^31,32 Please refer to the ESI, Fig. S1 and S2,† where structural and optical characterizations of the In₂S₃ buffer layer have been shown, respectively. The J–V characteristics were recorded using a source measure unit from Agilent (model, U2722A), combined with a AAA class solar simulator from Photo Emmision Tech Inc. (model, SS80) under AM1.5G illumination conditions. The light intensity was calibrated by the certified standard silicon reference cell.

4 Conclusions

In conclusion, we have deposited high quality SnS films on glass and FTO coated glass substrates by the affordable chemical spray pyrolysis method using the ambient air assisted transport of aqueous solutions of Sn²⁺ and S²⁻. The SnS films are doped by pulsed DC magnetron sputtering of elemental Cu at different temperatures. A substrate temperature of 325 °C is optimum for Cu diffusion in the SnS layer, where Cu substitutes sites of Sn vacancies. Our results demonstrate good control over the structural, morphological, optical and optoelectronic properties of Cu doped SnS films. A significant improvement in the FWHM of 0.4° is obtained for Cu doped SnS thin film at 325 °C. The observed morphological properties of the SnS films are identical to those films grown by CVD, thermal evaporation and sputtering processes. A photogenerated current density of 1.8 mA cm⁻² is achieved for a constructed electrochemical cell having an active area of 0.25 cm², which shows further improvement up to 3.2 mA cm⁻² after HCl etching. The enhanced acceptor carrier concentration due to Cu doping causes a high V_OC of 462 mV in a Cu:SnS/In₂S₃ heterojunction device, which is more than double that of the un-doped case.

Acknowledgements

Authors acknowledge the internal research grant from Pandit Deendayal Petroleum University (PDPU) and the Solar Research & Development Centre, PDPU for providing XRD and FESEM facilities. Prof. Indrajit Mukhopadhyay is gratefully acknowledged for his advice on photoelectrochemical measurements.

References

1. P. Sinsermsuksakul, K. Hartman, S. B. Kim, J. Heo, L. Sun, H. H. Park, R. Chakraborty, T. Buonassisi and R. G. Gordon, Appl. Phys. Lett., 2013, 102, 053901.
2. Y. Sun, Z. Sun, S. Gao, H. Cheng, Q. Liu, F. Lei, S. Wei and Y. Xie, Adv. Energy Mater., 2014, 4, 1–11.
3. M. Patel, I. Mukhopadhyay and A. Ray, Opt. Mater., 2013, 35, 1693–1699.
4. M. Sugiyama, K. Reddy, N. Revathi, Y. Shimamoto and Y. Murata, Thin Solid Films, 2011, 519, 7429–7431.
5. W. Wang, K. Leung, W. Fong, S. Wang, Y. Hui, S. Lau, Z. Chen, L. Shi, C. Cao and C. Surya, J. Appl. Phys., 2012, 111, 093520.
6. S. M. Sze and K. K. Ng, Physics of semiconductor devices, John Wiley & Sons, 2006.
7. J. Vidal, S. Lany, M. dAvezac, A. Zunger, A. Zakutayev, J. Francis and J. Tate, Appl. Phys. Lett., 2012, 100, 032104.
8. S. Zhang and S. Cheng, Micro Nano Lett. IET, 2011, 6, 559–562.
9. W. Albers, C. Haas, H. Vink and J. Wasscher, J. Appl. Phys., 1961, 32, 2220–2225.
10. M. Devika, N. K. Reddy, K. Ramesh, K. Gunasekhar, E. Gopal and K. R. Reddy, J. Electrochem. Soc., 2006, 153, G727–G733.
11. H. J. Jia, S. Y. Cheng and P. M. Lu, Adv. Mater. Res., 2011, 152, 752–755.
12. M. Reghima, A. Akkari, C. Guasch and N. Kamoun-Turki, J. Renewable Sustainable Energy, 2012, 4, 011602.
13. W. Huang, S. Cheng and H. Zhou, ECS Trans., 2012, 44, 1295–1301.
14. S. Zhang, S. Y. Cheng, H. J. Jia and H. F. Zhou, Adv. Mater. Res., 2012, 418, 712–716.
15. V. Gremenok, V. Y. Rud, Y. V. Rud, S. Bashkirov and V. Ivanov, Semiconductors, 2011, 45, 1053–1058.
16. A. Dussan, F. Mesa and G. Gordillo, J. Mater. Sci., 2010, 45, 2403–2407.
17. P. Sinsermsuksakul, R. Chakraborty, S. B. Kim, S. M. Heald, T. Buonassisi and R. G. Gordon, Chem. Mater., 2012, 24, 4556–4562.
18. K. Santhosh Kumar, C. Manoharan, S. Dhanapandian and A. Gowri Manohari, Spectrochim. Acta, Part A, 2013, 115, 840–844.
19. M. F. Weber and M. J. Dignam, J. Electrochem. Soc., 1984, 131, 1258–1265.
20. K. Sivula and M. Gratzel, Photoelectrochemical Water Splitting: Materials, Processes and Architectures, 2013, p. 83.
21. P. Sinsermsuksakul, J. Heo, W. Noh, A. S. Hock and R. G. Gordon, Adv. Energy Mater., 2011, 1, 1116–1125.
22. A. Schneikart, H. Schimper, A. Klein and W. Jaegermann, J. Phys. D: Appl. Phys., 2013, 46, 305109.
23. K. Ramakrishna Reddy, N. Koteswara Reddy and R. Miles, Sol. Energy Mater. Sol. Cells, 2006, 90, 3041–3046.
24. M. Patel, I. Mukhopadhyay and A. Ray, J. Phys. D: Appl. Phys., 2012, 45, 445103.
25. A. Bradley and A. Jay, Proc. Phys. Soc., 1932, 44, 563.
26. S. Prabahar and M. Dhanam, J. Cryst. Growth, 2005, 285, 41–48.
27. D. Dye, H. Stone and R. Reed, Curr. Opin. Solid State Mater. Sci., 2001, 5, 31–37.
28. P. Tyagi and A. Vedeshwar, Phys. Rev. B: Condens. Matter, 2002, 66, 075422.
29. S. Zimin, E. Gorlachev, I. Amirov, V. Naumov, G. Dubov, V. Gremenok and S. Bashkirov, Semicond. Sci. Technol., 2014, 29, 015009.
30. J. Sharma, G. Saint, N. Goyal and S. Tripathi, J. Optoelectron. Adv. Mater., 2007, 9, 3194–3199.
31. M. Patel, I. Mukhopadhyay and A. Ray, Semicond. Sci. Technol., 2013, 28, 055001.
32. M. Patel and A. Ray, ACS Appl. Mater. Interfaces, 2014, 6, 10099–10106.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c4ra06219a

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