1 Introduction

Antimony trisulfide (Sb2S3) is a low-toxic semiconductor consisting of chemically interconnected ribbons of (Sb4S6)n parallel to the c-axis that are kept together by weak interactions along the b-axis [1]. This semiconductor has a direct energy band gap of 1.7 eV, optical absorption coefficient greater than 104 cm−1 and p-type electrical conductivity [2,3,4]. These properties have made Sb2S3 a promising candidate for use in applications such as photocatalysis [5, 6], photovoltaics [7, 8], thermoelectric devices [9, 10] and sodium ion batteries [11]. Researchers have reported the synthesis of Sb2S3 thin films by techniques such as successive ionic layer adsorption and reaction (SILAR) [12], electrodeposition [13], spray pyrolysis [14], vacuum thermal evaporation [15] and chemical bath deposition (CBD) [16]. Particularly, CBD is a low-cost and low temperature technique that has been suitably used to prepare homogeneous and good quality films on different types of substrates and large-scale deposition areas. For this reason, a considerable number of reports on the synthesis of Sb2S3 thin films with different Sb and S precursors as well as complexing agents are found in the literature [17,18,19,20,21,22,23,24,25,26,27,28]. Usually, this low temperature deposition technique produces amorphous Sb2S3 films, so an annealing process is needed for them to crystallize in the orthorhombic structure. Mandal and Mondal [17], pioneers in the deposition of Sb2S3 films by CBD, reported that an annealing at 300 °C under N2 atmosphere for 1 h caused a decrease in the optical bandgap from 1.85 to 1.74 eV. A similar approach demonstrated that the electrical resistivity of annealed Sb2S3/glass and Sb2S3/SnO2/glass samples was 4.2 × 108 and 5.3 × 106 Ω cm, respectively [18]. This indicates that the type of substrate has a significant influence on the deposition and final properties of the films. Some authors have used the CBD method to prepare Sb2S3 films doped with different metal ions and studied the modification of their electrical and optical properties. For instance, Ag-doped Sb2S3 films were prepared via a CBD route employing a reaction solution with metal salt precursors and sodium thiosulfate at a temperature of 283 K [29]. As an effect of Ag doping, a bandgap decrease from 2.57 to 2.08 eV was observed. Diliegros-Godines et al. [30] used a citrate-mediated CBD process and found that Ag doping promoted an increase in dark conductivity from 1.84 × 10−9 to 1.33 × 10−8 Ω−1 cm−1, but also a decrease in photosensitivity. The insertion of Sn ions in Sb2S3 films was performed by a CBD process that used ethylenediamine tetraacetic acid (EDTA) as complexing agent [31]. Results indicate that, although resulting films (with Sn content from 1.0 to 5.5 at%) had no significant changes in the optical band gap (1.71–1.72 eV), the electrical resistivity decreased from 2.0 × 105 to 1.4 × 104 Ω cm. Cu doping of Sb2S3 films was also attempted with the EDTA-CBD process [32]. The bandgap values of the resulting films (with Cu-doping concentration from 1.3 to 8.7 at%) were reported in the range of 1.70–1.72 eV, while their electrical resistivity decreased from 2.0 × 105 to 0.2 × 104 Ω cm. However, doping of Sb2S3 films with Cu ions by CBD is challenging as the resulting films exhibit the CuSbS2 phase. Although doping with Cu produces a decrease in the conductivity of a Sb2S3 film, the study of electrical conduction due to the incidence of a light field (known as photoconductivity), which is an important parameter to be investigated for the development of optoelectronic devices, is still required.

Herein, we discuss the influence of Cu doping on the crystalline structure, morphology, and optical properties of Sb2S3 films synthesized by a novel CBD route where triethanolamine was used as complexing agent. The resulting films had a deliberate low Cu concentration to avoid the presence of secondary phases that could deteriorate the photoelectrical properties of Sb2S3. Thus, the substitution of Cu ions at the Sb sites promoted the improvement of photoconductivity and photosensitivity. The performance of photovoltaic (PV) devices based on chemically deposited CdS/Sb2S3 and CdS/Cu:Sb2S3 heterojunctions were also investigated.

2 Experimental

2.1 Materials

Antimony (III) chloride (SbCl3), thioacetamide (TAA, C2H5NS), copper (II) chloride dihydrate (CuCl2·2H2O), zinc acetate dihydrate (Zn(CH3COO)2·2H2O), ethylenediaminetetraacetic acid (EDTA, C10H16N2O8) and Triethanolamine (TEA, C6H15NO3) were purchased from Sigma-Aldrich and used without further modification.

2.2 Synthesis of undoped and Cu-doped Sb2S3 thin films

Undoped and Cu doped Sb2S3 thin films were grown on highly resistive ZnS layers previously deposited on soda-lime glass slides. The reaction solution for the deposition of the ZnS layers was prepared in a beaker according to an earlier study [33]. Under constant stirring, 30 ml of 0.1 M Zn(CH3COO)2, 10 ml of 0.1 M EDTA, 30 ml of 0.6 M TAA and 10 ml of deionized water were mixed. Next, the glass substrates were immersed in the beaker with the reaction solution, whose temperature was maintained at 80 °C in a water bath system. After 3 h, optically transparent films were deposited.

The reaction solution for the deposition of Sb2S3 films was prepared in a beaker, under constant stirring, by dissolving 0.5 g of SbCl3 in a mixture of 1 ml of HCl and 2 ml of ethanol. Next, 3 ml of TEA, 6 ml of 1 M TAA and 86 ml of deionized water were sequentially added. Finally, drops of NH4OH were added to adjust the aqueous solution to pH 10. The ZnS/glass substrates were immersed in the beaker with the reaction solution at a temperature of 80 °C, controlled by a water bath system, for 140 min. The resulting films, with red–orange coloration, were strongly adhered to the substrate. The crystallization process was then carried out by annealing the film at 300 °C for 30 min in an Ar atmosphere. The annealed Sb2S3 film, labelled as SSC0, was observed to adopt the characteristic dark brown coloration.

CuCl2/SbCl3 ratios of 1/24, 2/23, 3/22, and 4/21 were included in reaction solutions for the deposition of Cu-doped Sb2S3 films, which were respectively labeled as SSC1, SSC2, SSC3, and SSC4. In all cases the total weight of metal precursors was kept at 0.5 g. The steps to carry out the deposition and annealing process of each film were the same as those used to prepare the undoped Sb2S3 film. In all cases, dark brown films with good adherence to the substrate were obtained.

2.3 Fabrication of PV devices

Sb2S3/CdS and Cu:Sb2S3/CdS heterojunctions were formed on fluorine doped tin oxide (FTO) coated glasses, with surface resistivity of ~ 7 Ω/sq. CdS films with a thickness of approximately 260 nm were prepared by a CBD process consisting of immersing the FTO/glass substrates in a beaker with a reaction solution at a controlled temperature of 80 °C for 120 min. The reaction solution was prepared under constant stirring by adding 37.1 ml of distilled water, 17.5 ml of 0.1 M cadmium nitrate (Cd(NO3)2*4H2O), 10 ml of 1 M trisodium citrate (C6H5Na3O7*2H2O), 1.4 ml of NH4OH and 3.5 ml of 1 M (CH4N2S). The resulting CdS films were annealed at 300 °C for 30 min in Ar atmosphere to improve their conductivity. The yellowish CdS films, used as window layers in the superstrate configuration, were homogeneous and adherent to the substrate. SSC0 and SSC2 films were then deposited as absorber layers on the CdS films. The device structure was then completed by printing circular conductive graphite contacts, with an area of ~ 0.19 cm2, on the SSC0 and SSC2 layers. An additional annealing treatment at 300 ◦C under Ar atmosphere for 30 min was performed to promote the crystallinity of the absorbing films and improve the adhesion of the graphite contact.

2.4 Characterization

The crystalline structure of the films was analyzed with an X-ray diffractometer Bruker D8 Advance (Cu-Kα radiation, λ = 1.54 Å). The system is equipped with an accessory equivalent to the asymmetric Bragg-Brentano geometry to perform measurements in the grazing angle mode. As a complementary technique, Raman spectroscopy measurements were carried out at room temperature by using a Renishaw inVia system equipped with a 514 nm line Ar ion laser. The surface view and cross-section of the films were captured with a field emission scanning electron microscope (FESEM) JEOL (7800 Prime) in order to study their morphology and determine their thickness. Energy-dispersive spectroscopy (EDS) from an EDAX-Pegasus system with accelerating voltage of 25 kV was used to identify the elemental composition of the films. The optical transmittance and reflectance of the films were performed with a UV–vis spectrometer USB2000 + Ocean Optics in the wavelength range from 250 to 800 nm. In order to determine the conductivity of the Sb2S3 films, electrical measurements were accomplished by a programmable source meter Keithley 2400. This system was also used to measure the I–V response of PV devices under the AM1.5G spectrum of an Oriel LCS-100 solar simulator model 94011A that supplies an irradiance of 100 mW/cm2 (1 sun).

3 Results and discussion

3.1 Growth mechanism

Figure 1 shows the evolution of the reaction solution in the preparation of the sample SSC4. Changes in the coloration indicate the different stages in the formation and growth of particles. As known, the CBD process requires the availability of metal-complexes in order to avoid the premature precipitation of oxides. Thus, the film growth is controlled by a complex-decomposition mechanism that is explained as follows:

Fig. 1
figure 1

Reaction solution coloration: a bluish due to Sb-TEA and Cu-TEA complexes, b greenish after adding TAA, c brownish due to the formation of CuS particles and d orange due to the formation of Sb2S3 particles

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The availability of Sb3+ ions in the aqueous media begins with the dissociation of SbCl3 as

$${\text{SbCl}}_{{\text{3}}} \to {\text{Sb}}^{{{\text{3 + }}}} {\text{ + 3Cl}}^{ - }$$
(1)

while the presence of Cu2+ ions in the aqueous medium can be obtained by the dissociation mechanism,

$${\text{Cu}}\left( {{\text{NO}}_{{\text{3}}} } \right)_{{\text{2}}} \to {\text{Cu}}^{{{\text{2 + }}}} {\text{ + 2NO}}_{{\text{3}}}^{ - } {\text{ }}$$
(2)

In presence of TEA molecules at pH 10, complexes with the Sb3+ and Cu2+ ions are formed as follows,

$${\text{ Sb}}^{{{\text{3 + }}}} {\text{ + n}}\left( {{\text{TEA}}} \right) \to {\text{Sb}}\left( {{\text{TEA}}} \right)_{{\text{n}}}^{{{\text{3 + }}}} {\text{ }}$$
(3)
$${\text{Cu}}^{{{\text{2 + }}}} {\text{ + n}}\left( {{\text{TEA}}} \right) \to {\text{Cu}}\left( {{\text{TEA}}} \right)_{{\text{n}}}^{{{\text{2 + }}}}$$
(4)

Since the Sb-TEA complexes in aqueous media are colorless, the reaction solution becomes predominantly bluish due to the presence of Cu-TEA complexes, as evidenced in Fig. 1(a). TAA in alkaline solution decomposes into

$${\text{C}}_{{\text{2}}} {\text{H}}_{{\text{5}}} {\text{NS + 2OH}}^{ - } \to {\text{CH}}_{{\text{3}}} {\text{COO}}^{ - } {\text{ + NH}}_{{\text{3}}} {\text{ + HS}}^{ - } {\text{ }}$$
(5)

A greenish solution is observed upon adding TAA (see Fig. 1b), which turns brown when it starts heating to 80 °C, as shown in Fig. 1c. This suggests that in solution Cu ions are available earlier than Sb ions. The brown coloration of the solution is characteristic of the formation of CuS nuclei

$${\text{Cu}}^{{{\text{2 + }}}} {\text{ + HS}}^{ - } \to {\text{CuS + H}}^{{\text{ + }}}$$
(6)

After 10 min., the reaction solution has an orange-brown color mixture indicating the formation of the first Sb2S3 nuclei

$${\text{2Sb}}^{{{\text{3 + }}}} {\text{ + 3HS}}^{ - } \to {\text{Sb}}_{{\text{2}}} {\text{S}}_{{\text{3}}} {\text{ + 3 H}}^{{\text{ + }}}$$
(7)

and later appears completely orange (see Fig. 1d). However, throughout the reaction time some available Sb3+ and Cu2+ ions can also be complexed by the OH– ions supplied by NH4OH, forming metallic hydroxides that participate in the heterogeneous nucleation process:

$${\text{Cu}}\left( {{\text{TEA}}} \right)_{{\text{n}}}^{{{\text{2 + }}}} {\text{ + 2OH}}^{ - } \leftrightarrow {\text{Cu}}\left( {{\text{OH}}} \right)_{{\text{2}}} {\text{ + nTEA}}$$
(8)
$${\text{Sb}}\left( {{\text{TEA}}} \right)_{{\text{n}}}^{{{\text{3 + }}}} {\text{ + 3OH}}^{ - } \leftrightarrow {\text{Sb}}\left( {{\text{OH}}} \right)_{{\text{3}}} {\text{ + nTEA}}$$
(9)

Therefore, the CuS or Sb2S3 nuclei can be formed through the reactions,

$${\text{Cu}}\left( {{\text{OH}}} \right)_{{\text{2}}} {\text{ + HS}}^{ - } \to {\text{CuS + H}}_{{\text{2}}} {\text{O + OH}}^{ - }$$
(10)
$${\text{2Sb}}\left( {{\text{OH}}} \right)_{{\text{3}}} {\text{ + 3HS}}^{ - } \to {\text{Sb}}_{{\text{2}}} {\text{S}}_{{\text{3}}} {\text{ + 3 H}}_{{\text{2}}} {\text{O + 3OH}}^{ - }$$
(11)

The presence of OH– ions also promote the availability of S2– ions

$${\text{HS}}^{ - } {\text{ + OH}}^{ - } \to {\text{S}}^{{{\text{2 - }}}} {\text{ + H}}_{{\text{2}}} {\text{O}} \cdot$$
(12)

Therefore, as long as Sb3+ or Cu2+ ions are available to react with S2– ions, Sb2S3 or CuS nuclei participate in the formation of primary particles that collide and react to form aggregates in solution or be adsorbed on the substrate to form the film. All the as-deposited films were orange in color and turned dark brown after being annealed.

3.2 Crystalline structure

The XRD patterns shown in Fig. 2 were analyzed to determine the crystalline structure of the Sb2S3 films prepared with different Cu concentrations. As seen, the positions of the diffraction peaks of the sample SSC0 are in good agreement with those of the standard (JCPDS Card No. 42-1393) which corresponds to the orthorhombic phase of Sb2S3. Particularly, it is shown that the deposition process favored the preferential growth of [hk0] grains. This effect can be attributed to the constraints of nucleation and growth of particles imposed by the substrate surface. Besides, compared to the standard, the diffraction peaks appear to be slightly shifted towards larger angles due to internal stress. For instance, Table 1 shows the position of the most intense diffraction peak, assigned to the (130) plane. With the insertion of Cu, the diffraction peaks of samples SSC1, SSC2, SSC3, and SSC4 underwent further changes in position towards larger angles which could occur by the difference in ionic radii of Sb3+ (0.76 Å) and Cu2+ (0.71 Å). Moreover, the XRD patterns do not exhibit peaks associated with other crystalline phases. However, the substitution of Cu ions in Sb sites produced crystalline distortions, due to extrinsic defects, which are confirmed by variations in the lattice parameters. Compared to the sample SSC0, the value of lattice parameter b for the Cu-doped Sb2S3 films tends to be lower. In contrast, the values of the lattice parameter a are slightly higher. Basically, it is inferred that the doping process promoted a cation redistribution in the crystalline structure. The average crystallite sizes (D) of the samples, depicted in Table 1, were calculated using the Scherrer equation,

Fig. 2
figure 2

Diffraction patterns of the undoped and Cu-doped Sb2S3 films

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Table 1 Structural parameters of the samples from XRD patterns
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$$D{\text{ }} = {\text{ }}\frac{{0.9\lambda }}{{\beta \cos \theta }}$$
(13)

where D is the crystal size, λ is the wavelength of X-ray, β is the FWHM, and θ is the diffraction angle. Results reveal that the Cu doped Sb2S3 films are formed by crystals with D values decreasing from 20 to 18 nm, indicating that the insertion of Cu ions induces disruptions to the crystal growth process. Raman spectroscopy was used to confirm the XRD findings but, given its high sensitivity, also to rule out the presence of secondary crystalline phases. Figure 3 shows the Raman spectra of the undoped and Cu-doped Sb2S3 films. In the Raman spectra of the undoped Sb2S3 film are peaks located at positions that have been associated to the orthorhombic Sb2S3 [34,35,36,37,38]. Specifically, peaks at 139, 191 and 266 and 290 cm−1 are related to Ag modes. Peaks at 114 and 172 cm−1 are associated to B2g vibrational modes and the peak at 223 cm–1 is attributed to the B3g mode. The peaks identified above are also present in the spectra of the Cu-doped Sb2S3 samples with some variations in intensity due to the distortions produced by the insertion of Cu ions in Sb sites. However, the appearance of a peak at 345 cm–1 in the spectrum of the sample SSC4 indicates the emerging formation of CuSbS2 [39].

Fig. 3
figure 3

Raman spectra of the undoped and Cu-doped Sb2S3 films

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3.3 Morphology analysis

The surface morphology of the undoped and Cu-doped Sb2S3 thin films were analyzed by SEM. It is known that Sb2S3 particles can grow to adopt different morphologies, including nanotubes and nanorods [40,41,42]. Particularly, the sample SSC0 in Fig. 4(a) is formed by a porous structure of rod-shaped nanoparticles grown parallel to the substrate (see inset) that in most of cases aggregate or coalesce to form flat islands. In good agreement with the XRD results, the presence of planar structures parallel to the substrate correlates with the growth of crystals with preferential orientation (hk0). With the insertion of the Cu precursor, the reaction solution undergoes a supersaturation change that favors the formation of a greater number of nucleation centers. Therefore, there is a greater number of rod-shaped nanoparticles that grow parallel on the lateral face of others, giving rise to the formation of branched planar structures (see Fig. 4b for the sample SSC2). The increase in the concentration of Cu ions in solution leads to the coalescence of many more particles forming ribbon-like structures (see Fig. 4c and d for samples SSC3 and SSC4, respectively). The thickness of the films was measured from cross-sectional images. The samples SSC0, SSC1, SSC2, SSC3, and SSC4 had average thicknesses of 153, 257, 244, 189 and 108 nm, respectively. In congruence with the analysis of surface morphology the nucleation and growth of particles at earlier stages of the reaction promoted the deposition of thicker films. By way of illustration, Fig. 5a and b show cross-sectional images of the samples SSC0 and SSC2, respectively. Unfortunately, the increased nucleation rate due to the increase in the concentrations of Cu ions leads to premature growth of large particles. Some of them precipitate at the bottom of the beaker, while others are deposited on the surface of the substrate. In this way, it is observed that the higher the nucleation speed, the thinner the films. Energy dispersive spectroscopy (EDS) was carried out to identify the presence of the elements in the samples. The EDS spectrum of the sample SSC4 in Fig. 5c shows peaks for Sb and S indicating the presence of Sb2S3. Moreover, there is a weak signal associated with the presence of Cu. Accordingly, the Cu content in samples SSC1, SSC2, SSC3, and SSC4 was found to be 0.04, 0.07, 0.22, and 0.25 at%, respectively. Table 2 depicts the average values semi-quantitative EDS analyses corresponding to Sb, S and Cu content of the samples.

Fig. 4
figure 4

Surface SEM image of the sample a SSC0, b SSC2, c SSC3 and d SSC4

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Fig. 5
figure 5

a and b Cross-section SEM image of samples SSC0 and SSC2, respectively; c EDS spectrum of sample SSC4

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Table 2 Elemental composition of the samples and film thickness
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3.4 Optical properties

The optical transmittance (T) spectra of samples SSC0, SSC2, SSC3 and SSC4 are shown in Fig. 6a–d. Aspects such as homogeneity, distribution of surface grains, compactness and thickness are factors that can affect the optical properties of the films. Particularly, the average transmittance of the samples, evaluated in the range from 700 to 1000 nm, exhibit a slight variation between 39.52 and 41.93%. However, the most noticeable effect of the insertion of Cu ions in Sb2S3 is the shift of the absorption edge towards longer wavelengths, indicating a modification of optical band gap which was determined by using the following equation,

Fig. 6
figure 6

Optical transmittance spectra (inset: plot of (αhv)2 vs. hv) of samples (a)SSC0, (b)SSC2, (c) SSC3 and (d) SSC4

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$$\alpha {\text{hv = A(hv - Eg)}}^{{\frac{{\text{1}}}{{\text{2}}}}}$$
(14)

where α is the absorption coefficient, hν is the photon energy, A is the constant of proportionality, d is the film thickness of the films and Eg is the optical bandgap. Therefore, the Eg value can be approximated by the intercept of the extrapolated linear portion of (αhv)2 versus hv, as shown by the curve plots in the inset of Fig. 6a–d, for which α was calculated as

$$\alpha {\text{ = }}\frac{{\text{1}}}{{\text{d}}}{\text{ ln(T)}}$$
(15)

The Eg values depicted in Table 3 indicate that the insertion of Cu ions promotes a bandgap narrowing from 1.9 to 1.8 eV which can be ascribed to the emerging of a band formed by the overlap of defect states. Such defect states could originate by structural distortions produced by different accommodations of Cu ions (in the 2+ state) and Sb ions (in the 3+ state) bonded to S ions, and hence the presence of cation or anion vacancies.

Table 3 Optical band gap and photoresponse parameters of the samples
Full size table

3.5 Electrical properties

It has been reported that the inclusion of Cu ions in Sb2S3 films prepared by RF magnetron sputtering promoted a sulfur deficiency, so they presented n-type conductivity [43]. On the contrary, the hot probe test measurements performed on our Cu-doped Sb2S3 films exhibited p-type conductivity, which according to the EDS results can be attributed to rich sulfur content. The electrical conductivity of the films was measured by a pair of graphite electrodes of 5 mm length and a separation of 10 mm printed on the surface. Figure 7 shows the photoresponse plots of the samples as result of measuring the electrical conductivity in dark (σd) and illumination (σl). It is important to note that the conductivity in dark of the Sb2S3 films, whose values are depicted in Table 3, increases monotonically with the concentration of Cu from 2.7 × 10–7 to 9.6 × 10–7 Ω−1cm−1. This is explained below: Substitution of Cu at the Sb site promotes an increase in the density of charge carriers by forming offset defect bands that shift up the valence band [44]. Furthermore, the higher compactness of Cu-doped Sb2S3 films results in better charge transport. Under illumination, electron − hole pairs are generated due to photon absorption which in turn produce a photocurrent (Iph) defined as Iph = Ilight − Idark. After Cu doping, the band gap narrows, and a larger charge carrier density is generated. According to Table 3, the photoconductivity tends to increase until reaching a maximum, as observed in the case of sample SSC2. However, for higher Cu concentrations, more and more charge carriers are generated near the surface region, so they recombine quickly through surface states and do not contribute to the photocurrent. Thus, the decrease in photoconductivity of samples SSC3 and SSC4 reveals that the photocurrent is limited by surface recombination. The photosensitivity (S = (σl – σd)/σd) values listed in Table 3 indicate that the highest S value of sample SSC2 is originated by the highest photocurrent response due a good balance between light absorption enhanced by surface defects and defect-induced carrier traps.

Fig. 7
figure 7

Photoresponse of the un-doped and Cu-doped Sb2S3 films

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3.6 Performance of PV devices

A PV device (labeled as DSSC2) was fabricated in a glass/FTO/CdS/SSC2/C/Ag configuration, as illustrated in the inset of Fig. 8, to assess the performance of the Cu-doped Sb2S3 film with the better photoresponse. Figure 8 shows the plot of current density versus voltage (J–V) response of such a device, under illumination, compared with that of a device (labelled as DSSC0) fabricated in a glass/FTO/CdS/SSC0/C/Ag configuration. According to Table 4, the open circuit voltage (VOC) and short-circuit current density (JSC) values exhibited by the device DSSC2 were 205 mV and 0.34 mA/cm2, respectively. These values are 73.72% and 150%, respectively, higher than those displayed by the device DSSC0. It has been reported the use of Sb2S3 films prepared by the CBD method for the fabrication of PV devices in the configuration n-Sb2S3/p-Si [45, 46], while other approaches have used post-sulfurized Sb2S3 films in configurations that include electron and hole transport layers [47,48,49,50,51]. However, to the best of our knowledge, there are still no reports on PV devices based on a CdS/Sb2S3 heterojunction. Although the performance of the device DSSC2 is quite low, compared to PV devices that include Sb2S3 films prepared by thermal evaporation [52,53,54], undoubtedly the insertion of Cu ions was a convenient strategy to promote an increase of VOC and JSC. Moreover, the improvement of the fill factor (FF) indicates a slight reduction of series resistance and increase of shunt resistance. Based on the formulation used to prepare the DSSC2 film, six devices with Cu-doped Sb2S3 films were also fabricated by varying the deposition time and number of layers. The device with the best performance, labelled as 2DSSC2 was that processed with a 160 nm CdS film and a 500 nm Cu-doped Sb2S3 film. As shown in Fig. 8, such a device under illumination showed photovoltaic behavior with VOC and JSC values of 226 mV and 0.94 mA/cm2, respectively. Although the conversion efficiency of the device 2DSSC2 (0.32%) is higher than that of the device DSSC2 (0.12%), the decrease in fill factor (FF) from 34 to 30% suggest that further optimizations should be performed by employing metal back contacts instead graphite and buffer or window layers other than CdS [55]. Another challenging strategy is to achieve the chemical deposition of Sb2S3 films with (hk1) orientations in order to get a further improvement in the performance of PV devices, as has been shown with Sb2S3 films deposited by hydrothermal method [56].

Fig. 8
figure 8

J–V curves of PV devices DSSC0, DSCC2 and 2DSCC2

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Table 4 Electrical parameters of PV devices
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4 Conclusions

Successful chemical deposition of Cu-doped Sb2S3 films was demonstrated from the formation of Cu-TEA and Sb-TEA complexes in aqueous solutions. The crystalline structure analysis of the annealed Cu-doped Sb2S3 films indicated the permanence of the orthorhombic phase of Sb2S3. However, the incipient presence of CuSbS2 was detected by Raman spectroscopy in the sample prepared with the highest concentration of Cu doping. Changes in the deposition mechanism due to the presence of Cu ions in the reaction solution were evidenced by observing that the Sb2S3 film was formed of rod-shaped nanoparticles, while the Cu-doped Sb2S3 films were formed of ribbon-like structures. Moreover, the substitution of Cu ions in Sb sites promoted a narrowing of bandgap energy from 2.05 to 1.8 eV, and the improvement in photoconductivity, where the sample SSC2 reached a maximum value of 30.47 × 10–6 Ω−1cm−1. The enhanced photo-generated current due to the presence of Cu ions was also a beneficial strategy to promote a better performance of the PV devices. In summary, the present results support the prospective of the fabricated Cu-doped films to be used as absorbing layers for the development of photosensors and PV devices.