1 Introduction

Currently, the market of semiconductor materials is characterized by an active search for effective non-silicon individual materials and multilayer heterostructures, as well as the development of accessible methods for the formation of functional layers using non-epitaxial processes. Tin sulfide, SnS, is one of the most important semiconductors of the AIVBVI class, which has attracted considerable attention in recent years [1, 2]. This material is characterized by an increased band gap value, which is in the range of 1.1–1.8 eV depending on the structural-chemical state [3, 4]. Typically it is a p-type semiconductor with a high optical absorption coefficient in the visible and near-infrared regions of the spectrum (104–105 cm–1) [3, 5]. In addition, SnS is characterized by a layered orthorhombic structure, the atomic layers of which are connected only by van der Waals forces. In this regard, there are no unfilled electronic levels on the surface of SnS crystals, and the Fermi level pinning is not observed, making the surface of the material highly stable [6]. These features of the crystalline and, accordingly, the electronic structure of this semiconductor promote the use of SnS to create bulk and thin heterojunctions with a low content of intermediate electronic levels on the contact surface of functional layers that are in a polycrystalline or a nanocrystalline state. In addition, significant progress has recently been achieved in obtaining n-type SnS both in the form of single large crystals [7, 8], and thin films [9,10,11,12].

In the last decade, various aspects of the use of SnS-based film structures in electronic devices have been actively studied [13]. SnS-based materials have been implemented in solar cells [14,15,16], gas sensors [17, 18], lithium-ion and sodium-ion batteries [19, 20], supercapacitors [21], and field-effect transistors [22]. Unique properties of this material also explain great interest in the field of actively developed solar energy, in which a special type of high-yield SnS-based cells has been developed [16, 23]. Thin films of this semiconductor can be used as base layers of solar cells (SCs) with theoretical prerequisites for creating photoconverters with an efficiency of up to 24% [23], which exceeds the characteristics of modern traditional SCs based on single-crystal silicon. It is expected that upcoming SnS-based SCs will be able to replace their analogs containing toxic Se and Cd, as well as expensive In and Te, for example, to replace SCs with base layers of cadmium telluride (CdTe) and copper-, indium-, and gallium selenide (CuInxGa1–xSe2).

To date, significant progress has been achieved in the deposition of SnS films by reproducible but yet expensive vacuum processes [14, 24, 25]. Currently, there is a tendency to develop simpler liquid-phase methods for obtaining SnS-based materials, including methods of chemical deposition [16, 26,27,28], electrodeposition [4, 15, 29, 30], spray pyrolysis [31], thermal evaporation [32, 33], layer-by-layer synthesis [34, 35], and sol–gel method [36]. One of the key problems in the synthesis of SnS films is the achievement of the reproducible minimal deviation of their composition from the monosulfide stoichiometry. In the case of vacuum methods, the resulting SnSx films often lack sulfur [37], while chemical deposition from aqueous media leads to the incorporation of oxygen into films. In the case of electrochemical deposition, an excess of sulfur is usually observed even in the form of the individual impurity phase due to incomplete reduction of sulfur-containing ions from the electrolyte [29]. Therefore, it is reasonable to create functional heterostructures based on the SnS layer by different liquid-phase deposition techniques and further optimize the technology and output parameters of the created photovoltaic cells.

Several semiconductors have been proposed for use in photovoltaic structures with a functional heterojunction based on the absorbing SnS layer. These semiconductors perform the function of a wide-gap window. Among them are sulfides CdS [38], ZnS [39], CdxZn1−xS [40], oxides ZnO [41], TiO2 [42], SnO2 [43], combined multilayer structures such as ZnO/CdS/SnS [44], and other materials [45,46,47]. In general, CdS/SnS structures have a solar energy conversion efficiency level of about 4% [48]. However, this result was surpassed (η ≈ 5%) when TiO2 oxide was used as a wide gap window [42].

Unfortunately, the electrical parameters of the reported heterojunction SnS-based structures are still insufficient comparing to the theoretical results [49]. The use of the oxide wide-gap layer seems to be preferable due to the increased resistance and manufacturability of oxide layers in comparison with sulfide films. In addition, in some experiments [50], ZnO/SnS structures showed a higher efficiency compared to sulfide ZnS/SnS and SnS2/SnS heterostructures. ZnO is a very promising wide-bandgap material that could be used as a replacement for CdS, which is toxic and has a narrow band gap (2.42 eV) [51]. In turn, ZnO has a fairly large band gap (about 3.5 eV), good chemical and thermal stability in the air, and is nontoxic. It is characterized by high transparency in the visible range of the electromagnetic spectrum and electrical resistance varying in a wide range [52]. ZnO can be easily deposited at low temperatures with good film quality. Moreover, thin layers of ZnS can be employed to minimize series resistance and facilitate charge carrier transport in such devices [53, 54].

The aim of this study was to optimize the properties of single layers and investigate the formation and properties of photovoltaic ZnO/SnSx heterostructures. In this contribution, we propose to use a combined cycle for creating a ZnO/SnS heterostructure, including the stage of electrochemical deposition of a light-absorbing SnS layer of increased thickness with the following stage of chemical (ionic) layer deposition of a thin light-transmitting layer of wide-gap ZnO.

2 Experimental

2.1 Deposition of SnSx and ZnO layers

The deposition of functional layers was performed on indium tin oxide (ITO) coated glass substrates with varying surface electrical resistivity. Samples with a typical surface area of 1–2 cm2 were cut from commercially available ITO/glass substrates obtained from JSC INTEGRAL (Minsk, Belarus). An indium-tin target containing 10% of tin and oxygen plasma was used. The specific feature examined in this contribution was the use of the ITO substrates with relatively high resistivity (not less than 30 Ω/□ with a thickness not exceeding 150 nm), which complicates the electrodeposition of uniform semiconductor films.

At the first stage, individual SnS and ZnO layers were obtained on the ITO/glass surface and further examined. The electrochemical deposition of SnSx films was carried out in a complex electrolyte of the following composition, mol/L: 0.03 SnCl2 × 2H2O, 0.1 Na2S2O3 × 5H2O, and 0.06 CH3COONa × 3H2O. The pH of the electrolyte was maintained at ca. 2.0 and monitored by a pH meter with 0.1 accuracy. The electrodeposition process was performed in a standard three-electrode glass cell using an Autolab 302N potentiostat/galvanostat in a pulsed potentiostatic mode with a relatively low deposition rate (no more than 20 nm/min) to ensure the formation of high-quality and smooth coatings. Based on this, the duration of impulses was 15 s (polarization) and 5 s (pause). The deposition potential was set depending on the surface ohmic resistance of the substrate and varied in a range of −(0.8 − 1.0) V vs. sat. Ag/AgCl. Platinum mesh was used as the counter electrode. Based on the results of the preliminary experiment, the number of cycles was set to 120. After the deposition, the samples were rinsed with deionized water and annealed for 30 min at 200 °C in air. The proposed process has a lower annealing temperature compared to previously examined SnSx films obtained by the SILAR approach [55] to minimize sublimation of sulfur and oxidation of tin in air.

Thin ZnO films were deposited by the chemical successive ionic layer adsorption and reaction (SILAR) method, which was carried out by cyclic treatment of substrates in cationic and anionic solutions with intermediate rinsing to remove excess precursor from the surface. As a cationic precursor, an ammonia-based 0.01 M ZnSO4 (pH 11) solution was used, and the deposition was performed at room temperature. As an anionic precursor, a 1% H2O2 solution at a temperature of 65–75 °C was used. The duration of treatment in the cationic treatment bath was 20 s and in the anionic treatment and washing bath—30 s. The final annealing was performed in air at 350 °C for 30 min. During the formation of ZnO films, the number of SILAR cycles varied from 5 to 20.

The formation of functional ZnO/SnSx heterostructures was carried out in the cycle “electrodeposition of SnSx—chemical layer deposition of ZnO”. The deposition parameters during each step were selected based on the examination of individual coatings and are discussed below in the text of this contribution. The formed heterostructure was annealed at 350° for 30 min.

2.2 Characterization of obtained films

The phase composition of the formed films was determined by X-ray phase analysis (XRD) using a D8 Advance AXS X-ray diffractometer (Bruker, Germany) in the Bragg–Brentano configuration. XRD patterns were recorded using a KαCu = 0.15418 nm source in the 2θ angle range of 11°–80° with a scanning step of 0.01°. Scanning electron microscopy (SEM) images of the surface (top) and cross-cut (side) appearance of obtained layers, as well as energy-dispersive X-ray spectroscopy (EDS) data, were obtained using an S-4800 (Hitachi, Japan) scanning electron microscope and a Quantex EDS analyzer, operating at an accelerating voltage of 12–15 kV. In the cross-cut analysis, the tilt angle of the measurement was 70°–90°.

The X-ray Photoelectron Spectroscopy (XPS) measurements were performed using an RS 40B1 unmonochromatized MgKα (1253.6 eV) X-ray source (PREVAC, Poland) with a power of 180 W. The operating parameters of an anode were 12 kV and 15 mA. A Gammadata Scienta hemispherical analyzer SES R4000 was used for signal collection. The base pressure in the analysis chamber was about 1 × 10–9 mbar. The high-resolution spectra were collected at a pass energy of 100 eV with a step of 25 meV. The data analysis was performed in the CasaXPS 2.3.23 software by fitting the experimental curve with a combination of Gaussian and Lorentzian lines of variable proportions (70:30). All binding energy values were charge corrected to the carbon C 1 s excitation, which was set at 285.0 eV.

Optical transmission spectra for samples on transparent ITO-coated glass substrates were obtained in the wavelength range of 300–700 nm on an SP8001 (Metertech, Taiwan, R.O.C.) spectrophotometers.

The surface electrical resistivity of the formed structures was measured in a thermostated cell by the two-probe method at room temperature (ca. 20 °C) and in heating–cooling cycles in the temperature range from 20 to 150 °C. The measurements were at least triplicated. To evaluate the photosensitive properties of the obtained semiconductor films and heterostructures, the values of photoelectromotive force (photoEMF) were measured depending on the intensity of the activating optical mixed radiation in the visible range. In these measurements, an RN8-20-1 incandescent lamp with adjustable radiation power from 0 to 10 W was used as a light source. The used multielectrode contact system (made of Ag or graphite) made it possible to measure the photoEMF arising both in the SnS/ITO contact and in the ZnO/SnS/ITO heterojunction by varying the position of the electrodes. The current–voltage characteristic of the formed structures was measured using an HP4156B Semiconductor Parameter Analyzer (Agilent, USA) with a measurement accuracy of ± 0.01 mV and ± 1 nA.

3 Results and discussion

To form SnS layers of photosensitive heterostructures with an increased optical absorption coefficient, the electrochemical deposition method, which is highly productive and adapted for industrial use, was used. Several studies [29, 56] have shown that in the case of a DC electrodeposition of sulfide semiconductor films it is possible to use dilute electrolytes based on soluble salts of a metal component of the film and sodium thiosulfate as a source of sulfur. The disadvantage of this technology is the formation and accumulation of colloidal sulfur in the volume of the electrolyte and on the surface of the substrate due to the chemical decomposition of thiosulfate ions in the acidic environment (Eq. 1), which, in turn, is necessary to suppress the undesirable hydrolysis of Sn2+ cations [15]:

$${\text{S}}_{{2}} {\text{O}}_{{3}}^{2 - } + 2{\text{H}}^{ + } \to {\text{S}} + {\text{SO}}_{2} + {\text{H}}_{{2}} {\text{O}}{.}$$
(1)

As a result, significant deviations from the equimolar composition are observed during the electrodeposition of SnS films due to the inclusion of elemental sulfur in the composition of the coatings. One of the ways to overcome this problem is to use a pulsed deposition mode, in which pauses at an open circuit potential help to reduce the inclusion of sulfur in the formed coating. Cathodic deposition of the SnS film can be expressed as:

$${\text{Sn}}^{2 + } + {\text{S + 2}}e^{ - } \to {\text{SnS}}{.}$$
(2)

Deposition of ZnO films in the SILAR process can occur through the stage of the chemisorption of \({\text{Zn}}[{\text{NH}}_{3} ]_{4}^{2 + }\) complex ions with the following sorption of H2O2 and their reaction according to the scheme [57]:

$${\text{Zn}}[{\text{NH}}_{3} ]_{4}^{2 + } + 4{\text{H}}_{2} {\text{O}}_{2} \to {\text{ZnO}} + 4{\text{NH}}_{3} \times {\text{H}}_{2} {\text{O}} + 3/2{\text{O}}_{2} .$$
(3)

3.1 Microstructure and chemical composition of electrodeposited SnSx films

The formation of visually detectable gray-black SnS coatings on the ITO surface was observed after 120 cycles of pulsed electrodeposition (40 min). Examination of the cross-cut SEM images of the obtained films (Fig. 1) revealed that the dense and relatively uniform globular-shaped coatings were formed on the ITO surface. The size of the formed globules varied from ca. 30 nm (Fig. 1b) to ca. 250 nm (Fig. 1a). The thickness of the deposited SnSx films on the ITO substrates with the surface resistivity of 37–83 Ω/□ (the thickness of the ITO layer 90–140 nm) was in the range from 190 to 433 nm (Table 1). Examination of the top surface SEM images (Fig. 2) showed that deposits of smaller thicknesses (up to 300 nm) have lower inhomogeneity, which is better for the further formation of heterogeneous structures.

Fig. 1
figure 1

SEM cross-cut images of SnSx/ITO coatings deposited onto substrates with ITO thickness of a 140 nm and b 115 nm

Full size image
Table 1 Properties of ITO substrates and electrodeposited SnSx films
Full size table
Fig. 2
figure 2

SEM images of SnSx films with a thickness of a 433 nm and b 190 nm

Full size image

The results of the SEM and EDS analysis showed a sufficient difference from the theoretically expected dependences of the thickness and stoichiometry of the sulfide layer on the properties of the ITO layer (Table 1). In particular, the thickness of the deposited SnSx films near linearly depends on the resistivity of the ITO substrate and monotonously decreased as the latter increased (Fig. 3). This indicates that in the used range of cathodic potentials, the electrodeposition rate of semiconducting SnSx films on the ITO surface (with a surface resistance not less than 30 Ω/□) is limited by the electrophysical process of electron transfer at the ITO/SnSx interface. In turn, for the chemical composition of the formed SnSx films, an increasing supralinear dependence of the S content on the resistivity of the ITO layer was obtained (Fig. 3). This dependence is characterized by a rapid increase in the sulfur content as the surface resistivity reaches 50 Ω/□. Such a trend may be due to a negative effect of the chemical formation of colloidal sulfur (Eq. 1) and its incorporation into the coating at lower cathodic polarizations due to sufficient hindering of the electron transfer in the ITO substrates of higher resistivity.

Fig. 3
figure 3

Dependence of SnSx film thickness and S/Sn ratio on the electrical resistivity of ITO substrate

Full size image

Therefore, characteristic gray-black colored films with the composition SnS0.9–0.95 closest to the expected SnS stoichiometry with a thickness of more than 400 nm were formed on the ITO substrates with a thickness of 130 nm and the electrical conductivity less than 40 Ω/□. The maximum electrodeposition rate of the SnS0.9–0.95 layers on the ITO substrate with the surface resistivity of 35–45 Ω/□ exceeded 10 nm/min, which is better than in the previous reports where galvanostatic [29] deposition in a non-pulse mode in the same electrolyte was used.

The phase composition of the formed films was determined by the XRD analysis. The results showed that SnSx films electrodeposited on ITO substrates (Fig. 4) have a polycrystalline structure. The exact phase identification in a thin-film SnSx/ITO system is somewhat difficult due to the overlapping of the main XRD reflexes of the SnS and ITO phases. In the obtained XRD patterns, the strongest relatively wide peak located at 2θ 30.4°–31.6°, most probably originates from the superimposition of the reflexes of the SnS phase (peaks at 2θ 30.47° (101) and 31.53° (111) with the parameters a = 0.4329 nm, b = 1.1190 nm, and c = 0.3980 nm according to JCPDS card no. 39-0354) and the ITO (peak at 2θ 30.54° (222) for cubic In1.88Snx0.12O3 according to JCPDS card no. 01-089-4598). Similarly, superimposed peaks in the 2θ range of 21.4–22.1° (peak at 2θ 22.01° (110) of SnS and at 2θ 21.47° (211) of ITO) and 50.5–51.2° (peak at 2θ 51.09° (112) of SnS and 50.95° (440) of ITO), were detected. Individual peaks of the SnS phase were identified at 2θ 26.0° (120), 42.4° (210), and 44.7° (141). Individual peaks of the ITO phase were identified at 2θ 35.4° (400), 60.6° (600). These data together with the results of the EDS analysis confirm the electrodeposition of the orthorhombic SnS (herzenbergite, syn) phase, which is in agreement with the results of other studies [15, 29, 58].

Fig. 4
figure 4

XRD patterns of a SnSx film electrochemically deposited onto ITO and b ZnO film chemically deposited onto glass

Full size image

The deposition of ZnO thin films is a well-developed technology and several approaches exist. In this study, the SILAR technique was used, which allows the obtaining of thin films of various materials on conductive and non-conductive surfaces [57, 59]. At the preliminary stage, the number of the SILAR deposition cycles varied from 5 to 20 to optimize the process parameters and obtain thin films with a thickness of ca. 200 nm. The cross-cut SEM images of the obtained films (Fig. 5b) showed that fine films with a thickness of ca. 200 nm were obtained after 10 SILAR cycles.

Fig. 5
figure 5

Cross-cut SEM images of ZnO coatings deposited onto glass substrates during a 5, b 10, and c 20 SILAR cycles

Full size image

The analysis of the XRD pattern of the ZnO film obtained on the glass substrate (Fig. 4b), revealed peaks of hexagonal ZnO wurtzite phase at 2θ 31.8 (100), 36.5 (101), and 62.9° (103) with the parameters a = 0.325 nm and c = 0.521 nm (JCPDS card no. 36-1451). A low crystallinity of the obtained ZnO films is due to a relatively small annealing temperature (350 °C), which was used to prevent oxidation and sublimation during the formation of ZnO/SnSx/ITO heterostructures.

Summarizing, XRD and SEM measurements of the SnSx and ZnO films, obtained by electrochemical and SILAR techniques, respectively, confirmed the formation of the orthorhombic SnSx films with a thickness of about 200–400 nm and hexagonal ZnO films with a thickness of 100–200 nm.

3.2 Optical and electrical properties of SnSx and ZnO films

The development of an effective photovoltaic device based on the ZnO/SnSx heterostructure requires maximum sunlight absorption in the SnSx layer and high enough transmittance of the ZnO layer. Spectrophotometric measurements of electrodeposited SnSx films showed that the values of optical transmission of the samples significantly depended on their thickness (Fig. 6). The films with a thickness of more than 400 nm were opaque in the visible range of the optical spectrum. For them, the absorption coefficient was about (1−2) × 104 cm−1. The optical transmission of films with a thickness of up to 200 nm did not differ significantly from that of the transparent ITO/glass substrate. The optical transmission in the visible range of ZnO films obtained in ten SILAR cycles was at the level of 60%.

Fig. 6
figure 6

Optical transmittance spectra of SnSx/ITO/glass samples depending on the thickness of SnSx film

Full size image

Band gap is an intrinsic property of a semiconductor material and has a direct influence on the photovoltaic properties of the device [60,61,62]. Therefore, spectrophotometric measurements were also used to determine the bandgap energy, Eg, of the formed SnSx and ZnO layers. This allowed estimating the difference between Eg (SnSx) and Eg (ZnO) and evaluating the possibility of the usage of the formed heterostructures in photovoltaic applications. Using the Tauc relation [63], in the approximation of direct interband transitions at n = 2, the values of the Eg were determined for individual SnSx and ZnO films (Fig. 7). The found values were 1.4 and 3.3 eV for SnSx and ZnO, respectively. It can be noted that the Eg value for the SnSx films obtained in the present contribution is larger than the data reported in [15, 58] and corresponds to the theoretical maximum of the efficiency of a solar panel depending on the band gap energy of the semiconducting absorber layer, which is ca. 1.4 eV and is observed in GaAs [64]. The obtained results indicate the perspectives of using SnSx films as an effective light-absorbing layer of a photovoltaic heterostructure and ZnO films as a wide-gap window [51, 54].

Fig. 7
figure 7

Tauc plots of SnSx and ZnO films

Full size image

The requirements for the ohmic parameters of photovoltaic structures follow from the current–voltage characteristics of a solar cell, which can be described using a standard approximation equation [44]:

$$I = I_{{{\text{ph}}}} - I_{0} \left[ {\exp \frac{{e(U + IR_{{\text{s}}} )}}{nkT} - 1} \right] - \frac{{U + IR_{{\text{s}}} }}{{R_{{{\text{sh}}}} }}$$
(4)

where Iph is light-generated current, mA; I0 is the saturation current of the contact, mA; n is the diode ideality factor; Rs is series resistance, Ω; Rsh is shunting resistance, Ω; e is the elementary charge, C; k is the Boltzmann's constant, J/K; and T is the absolute temperature, K.

This dependence contains two resistances, Rs and Rsh. The value of Rs directly depends on the resistance of functional layers and contacts in a photovoltaic system. To increase the current generated in the solar panel, Rs should be decreased. At the same time, effective separation of photocarriers, which determines the magnitude of the generated photoEMF is only possible at sufficiently high levels of electrical resistance of semiconductor regions and a high potential barrier at the heterojunction interface.

The surface electrical resistance of the obtained SnSx films measured at room temperature was about several MΩ/□ or 34–68 Ω cm (Table 2), which corresponds to a relatively high conductivity [23]. A supralinear decreasing dependence of the surface electrical resistance of the SnSx films on their thickness was observed. This dependence is in good agreement with an inverse linear relationship of the surface electrical resistance R on a film thickness h according to Eq. (5):

$$R = \rho /h = \, 1/(\sigma h),$$
(5)

where ρ and σ are electrical resistivity (Ω cm) and electrical conductivity (S/cm), respectively. On the other hand, a stronger supralinear increase in the surface electrical resistance with a decrease in the thickness of the SnSx film can be also caused by the chemical state of the obtained thin films, namely by excessive stochiometric amounts of sulfur. The inclusion of sulfur agglomerates in the coating and their predominance in grain boundaries may cause local destruction of percolation clusters in a polycrystalline semiconductor, in which the electrical conductivity occurs by stoichiometric crystallites and/or defect grain boundaries.

Table 2 Electrical parameters of electrodeposited SnSx films
Full size table

During exploitation, solar panels absorb sunlight and are heated. Therefore, the dependence of the electrical resistance of the functional layers on temperature is an important parameter. Thin SnSx films with high electrical resistance (Table 2, samples 3 and 4) were characterized with semiconductor-type dependences of the electrical resistance on temperature with large hysteresis. For this reason, more attention was paid to the more promising SnSx films with thicknesses higher than 400 nm (Tables 1 and 2, samples 1 and 2). Figure 8 shows the dependences of the electrical resistance on the temperature in the range of 20–150 °C for sample 1 with a thickness of 433 nm. The obtained dependencies are of the semiconductor-like type with a small hysteresis. These dependencies reproduced well in three heating–cooling cyclic experiments, which is important for the stable exploitation of a thin-film solar panel. At the same time, the average temperature coefficient of electrical resistance was almost equal for all obtained SnSx films and is in a range of (7.1–7.3) × 10−3 K–1. This suggests that there are no major differences in the mechanism of charge transfer in films with different thicknesses.

Fig. 8
figure 8

Dependences of electrical resistance of SnSx/ITO sample (SnSx layer thickness 433 nm) on temperature in a R = f(T) and b lnσ = f(1/T) for cooling dependence

Full size image

Transformation of the dependences of the electrical resistance on temperature for the sample with the largest thickness (Table 2, sample 1) in the semi-logarithmic coordinates with recalculation of the surface resistance R into electroconductivity σ is shown in Fig. 8b. The obtained dependence is almost linear in the examined temperature range and was fitted with linear dependence. The obtained value of the activation energy of conduction was estimated to be 0.253 eV, which is in good agreement with the data of Sun et al. [65]. This corresponds to the presence of activation centers of charge carriers with the activation energy larger than the average thermal energy of particles (around 0.025 eV at ambient temperature). Regardless of the deposition technique, holes are usually reported to be charge carriers in polycrystalline undoped orthorhombic SnS films [15, 22, 31, 37, 39, 50, 56, 66]. Oppositely, cubic monocrystals [7, 8] or dopped films [9,10,11,12] can also provide n-type conductivity.

In turn, ZnO films had higher surface electrical resistance (5–18 MΩ/□ or 141–223 Ω/cm) compared to SnSx films. These values are slightly higher compared to those previously reported for ZnO films deposited by the SILAR method (105 Ω/cm) [51]. Obtained ZnO films were characterized by a decreasing semiconductor dependence of the surface electrical resistance on temperature with a reduced temperature coefficient of not more than 10−3K–1.

As the obtained films are intended to be used in photovoltaic devices, their photoconductivity is an important parameter, which describes the possibility of a semiconductor to increase its electric conductivity due to the generation of nonequilibrium charge carriers in the case of interband or impurity absorption of electromagnetic radiation. Separation of such charge carriers results in the occurrence of valve photoEMF, providing the possibility of the conversion of the light energy into electric energy. The dependence of the surface electrical resistance of SnSx films on the intensity of irradiation with mixed optical radiation (white light) indicated rather high photosensitivity of SnSx films (photoresistance decreased by 2.2 times) due to rather intense own light absorption, also noted by the Alagarasan et al. [67], Fig. 9. This result additionally indicates that the obtained SnSx layer is promising as an efficient photoresistive absorber for photovoltaic heterostructures.

Fig. 9
figure 9

Dependence of photoresistance of SnSx film on the intensity of irradiation

Full size image

Examination of the physical properties of individual films showed that photosensitive light-absorbing SnSx films with a thickness of about 400 nm with Eg 1.4 eV and an electrical resistance of about 40 Ω cm, obtained at 120 cycles of pulsed cathodic polarization, as well as light-transmitting ZnO films with a thickness of about 200 nm with Eg 3.3 eV and the electrical resistance of about 150 Ω cm, obtained in 10 SILAR cycles show the most promising properties and were selected for further studies.

3.3 Formation and characterization of photosensitive ZnO/SnSx heterostructures

Several ways to create ZnO/SnSx heterostructures have been reported in the literature. The most common approach is to use the same technique to form different layers of the heterostructure, for example, vacuum deposition [39, 44], electrodeposition [29, 68], atomic layer deposition [48, 65], SILAR [55], or spray pyrolysis [50]. In this work, to obtain photosensitive ZnO/SnSx heterostructures, a two-stage approach “electrodeposition of SnSx—SILAR deposition of ZnO” was used. This approach was selected because it allowed, on the one hand, deposition of a sufficiently thick (more than 300 nm) SnSx absorbing layer with stoichiometry close to tin monosulfide, and, on the other hand, to carry out the layer-by-layer synthesis of a light-transmitting wide-gap ZnO layer (100–200 nm). For the formation of the heterostructure, firstly ca. 430 nm-thick SnSx film was deposited on the conductive ITO/glass substrate during 120 cathodic pulses. Then, ZnO film was obtained in 10 SILAR cycles. This resulted in the formation of a uniform multilayer structure (Fig. 10a). The crosscut SEM image (Fig. 10b) showed that the obtained ZnO/SnSx heterostructures have a well-defined interface between microcrystalline SnSx and nanocrystalline ZnO layers. After the SILAR process, the thickness of the pre-formed SnSx layers decreased from 430 nm to around 340 nm, which is ca. 20%. This can be explained by the chemical dissolution of the SnSx layer in the SILAR solutions (pH ≈ 11), most probably through the formation of stannite ions following the equation:

$${\text{SnS}} + 3{\text{OH}}^{ - } \to {\text{Sn(OH)}}_{3}^{ - } + {\text{S}}^{2 - } .$$
(6)
Fig. 10
figure 10

SEM images of a surface and b cross-cut of 10ZnO/120SnSx/ITO

Full size image

This process results in smoothing the surface of the SnSx layer, which increases the quality of the interface in the functional contact SnSx/ZnO.

The results of the EDS analysis of the ZnO/SnSx heterostructure formed on ITO/glass substrates (Table 3) indicate that the S:Sn ratio in the sulfide layer of the heterostructure, estimated taking into account the 10% tin content in the ITO film composition, is 0.94 and is in good agreement with the stoichiometry of individual electrodeposited films (Table 1). The oxygen content in the heterostructure slightly exceeds the stoichiometry of the corresponding amount of ZnO and In1.8Sn0.2O3 (ITO), which is due to the presence of adsorbed oxygen-containing particles on the surface of the samples. Note that the ratio between the main metals (Zn, Sn, and In) of the heterostructure correlates well with the thickness of the corresponding films, taking into account the lower lattice density of SnS in comparison with ZnO and ITO.

Table 3 Results of EDS analysis of main elements of formed 10ZnO/120SnSx/ITO heterostructures
Full size table

To investigate the chemical states of the obtained 10ZnO/120SnSx/ITO heterostructures, XPS measurements were performed. XPS spectra were recorded from samples in the area of crosscut to elucidate the chemistry of SnSx and ZnO layers in the formed heterostructure. The survey spectrum (Fig. 11) indicated that the surface of the studied system is composed of C (65.1 at.%), O (25.8 at.%), Zn (3.1 at.%), Sn (2.3 at.%), S (2.5 at.%), and In 1.3 at.%). The presence of carbon results from the surface contamination and annealing of the samples. The presence of indium originates from the substrate. The Sn/S ratio confirms that SnS is the main form of tin presented at the surface.

Fig. 11
figure 11

Survey XPS spectrum of 10ZnO/120SnSx/ITO

Full size image

High-resolution XPS spectra in the binding energy range of Sn 3d, S 2p, and Zn 2p are shown in Fig. 12. The high-resolution spectrum in the Sn 3d region (Fig. 12a) was deconvoluted into three peak doublets. The main component Sn(1) with Sn 3d5/2 peak at 485.6 eV was assigned to Sn2+ compounds in the form of sulfide, SnS, which is in good agreement with the previous reports [69,70,71]. The second component Sn(2) located at 488.6 eV was assigned to Sn4+ compounds. Their presence most probably originates from a thin surface oxide film, which can be formed on the surface during annealing [72]. In the case of the conditions applied, Sn2+ preserves annealing being only slightly oxidized at the surface. Moreover, an additional third component Sn(3) at 483.0 eV was deconvoluted. The origin of this peak is somehow speculative. Most probably this component originates from the ligand-type bonding of Sn atoms with remnant acetate anions from the deposition bath [73, 74]. However, it should be noted that the XPS measurement is only surface-sensitive (the measurement depth in the case of the SnS layer did not exceed ca. 5 nm) and the surface fraction of the Sn(3) component is very small (ca. 10 at.%). Therefore, the presence of the residual complex ions on the surface did not significantly affect the properties of the obtained SnSx layers. The high-resolution spectrum of S 2p (Fig. 12b) showed two peak doublets. In this case, the first component S(1) with the S 2p3/2 peak located at 161.5 eV verifies the presence of S2– in the form of SnSx sulfide [69, 75, 76]. The second component S(2) with the S 2p3/2 peak located at 168.8 eV most probably originates from the species in which sulfur has a positive oxidation state. In the case of the electrochemical formation of SnSx layers, these species are remnant S2O32− ions from the deposition bath, or \({\text{SO}}_{3}^{2 - }\) ions, which could be formed through the partial dissolution in water of gaseous SO2 formed by Eq. (1). In the case of the ZnO film, the high-resolution spectrum of Zn 2p (Fig. 12c) was deconvoluted into two peak doublets. The main component Zn(1) with the Zn 2p3/2 peak located at 1022.0 eV corresponds to ZnO oxide, while the second component Zn(2) with the Zn 2p3/2 peak at 1023.6 eV corresponds to Zn–OH compounds or remnant Zn2+ ions from the SILAR bath [77].

Fig. 12
figure 12

High-resolution XPS spectra in binding energy ranges of a Sn 3d, b S 2p, and c Zn 2p

Full size image

Examination of the electrophysical properties of functional heterostructures was performed based on the current–voltage (I–V) curves, which were obtained for individual SnSx films and the functional heterostructure. For individual SnSx films, symmetric I–V curves were obtained (Fig. 13). When interpreting the I–V curves of contact structures, the absolute value of the direct and reverse currents were analyzed, as well as the asymmetry of the I–V curve as the ratio of direct and reverse currents flowing in the examined sample at a voltage of 2 V. A decrease in current on the I–V curve of the ZnO/SnSx heterostructure comparing to the individual SnSx film is due to the increased electrical resistance of the ZnO film. At the same time, the observed decrease in current was insignificant compared to the base SnSx film, which may indicate a low contact electrical resistance at the ZnO/SnSx interface. The asymmetry coefficient of the I–V characteristic for the ZnO/SnSx heterostructure exceeded 2 (Fig. 13). This result indicates the formation in the resulting heterostructure of a nonohmic functional contact with rectifying properties.

Fig. 13
figure 13

I–V curves of SnSx film and ZnO/SnSx/ITO heterostructure

Full size image

As stated above, photoEMF in photocatalytically-active materials is generated due to the spatial separation of photocarriers. This effect occurs due to significant differences in the mobility of diffusing photocarriers of different signs (Dember photoEMF), or potential barrier on the path of diffusing photocarriers (gate photoEMF), or other factors [78]. In turn, photoEMF results in the current flow of in the corresponding system. In this contribution, the photovoltaic activity of the obtained structures was evaluated by measuring the photoEMF values depending on the intensity of light source radiation. The measurements were performed by applying a special system of electrodes. The data obtained for the ITO layer show very low values of photoEMF, which did not exceed 15 mV and are typical for Dember photo EMF (Fig. 14), which gives grounds to confirm the expediency of using the ITO layer as an electrode in the formation of photovoltaic heterostructures.

Fig. 14
figure 14

Dependencies of photoEMF of ITO, SnSx/ITO, and ZnO/SnSx/ITO on the power of the light source

Full size image

The measured photoEMF in the SnSx/ITO structure indicates the generation of a potential difference of about 80 mV, which is likely not connected to Dember photoEMF but is the result of the potential barrier at the boundary of the light-absorbing SnSx layer and the transparent wide-gap ITO layer. A comparison of the photoEMF values obtained for the SnSx/ITO and ZnO/SnSx/ITO heterostructures shows that the use of the ZnO/SnSx/ITO heterostructure increases the photoEMF value from 81 to 198 mV, or 2.4 times. Such a significant increase in the photoEMF value due to a layer of wide-gap ZnO oxide indicates the dominant effect of the ZnO/SnSx contact in the generation of the total photoEMF in the ZnO/SnSx/ITO photovoltaic structure. The obtained absolute values of the photoEMF exceed those obtained by other authors, for example, for electrodeposited ZnO/SnS heterostructures (135 mV) [68], ZnO/CdS/SnS heterostructures (magnetron-sputtered ZnO and SnS layers and chemically deposited CdS layer) (132 mV) [44]. At the same time, photoEMF values obtained in this contribution are smaller than the results reported, for example, by the authors of [50] for spray pyrolyzed ZnO/SnS heterostructures (275 mV). The photoEMF values obtained in this contribution for ZnO/SnSx heterostructures correspond well to the values reported in our previous study for ZnS/SnSx heterostructures [55], obtained by the SILAR method using a higher annealing temperature.

4 Conclusions

This work focused on the development of ZnO/SnS functional heterostructures for implementation in solar cell applications. Characteristic gray-black colored SnS0.9–0.95 films with the composition closest to the expected SnS stoichiometry were formed on the ITO substrates with a thickness of about 100 nm and an electrical conductivity of less than 40 Ω/□. The formation of the orthorhombic SnS phase with the preferable (111) orientation was confirmed by EDS, XRD, and XPS methods. Chemical deposition of ZnO thin films by the SILAR method allowed to deposit of hexagonal films with a thickness of about 200 nm and (100) preferred crystal orientation.

The band gap (1.4 eV for SnSx and 3.3 eV for ZnO), as well as the high light absorption coefficient of SnSx films (1–2) × 104 cm–1 were determined. The SnSx films were characterized by a reduced electrical resistance of about 50 Ω cm, while the ZnO films had higher resistance (about 200 Ω cm). Light-absorbing SnSx films showed a stable temperature dependence of electrical resistance with an activation energy of conduction of 0.253 eV. The dependence of the surface electrical resistance of SnSx films on the intensity of irradiation indicated rather high photosensitivity of SnSx films (photoresistance decreased by 2.2 times) due to intense own light absorption.

The obtained ZnO/SnSx heterostructures formed by the electrodeposition-SILAR cycle had a well-defined interface between microcrystalline SnSx and nanocrystalline ZnO layers. A comparison of the photoEMF values obtained for the SnSx/ITO and ZnO/SnSx/ITO heterostructures showed that the use of the ZnO/SnSx/ITO heterostructure increases the photoEMF value from 81 to 198 mV, or 2.4 times, which is a competing value for the two-layer ZnO/SnS heterostructures.