1 Introduction

In recent times, the ability of nanostructured solar cell with dye sensitized metal oxide as photoanode has received attention of researchers owing to their low cost, simple fabrication technique and feasibility.[1] Although the dye-sensitized solar cells (DSSCs) have considerable conversion efficiency, the complex structure of dyes, their availability and stability put certain constrains on their industrial applications.[2] Therefore, nowadays semiconductor materials with narrow band gap are attractive alternative to the light-harvesting dye molecules because of their tunable electrical, optical and structural properties.[3,4]The semiconductor with unique properties like tunable band gap over a wide range to match the solar spectrum, good photostability, broad excitation spectra, high extinction coefficient and multiple exciton generation capability makes them potentially suitable for photovoltaic’s applications.[3]Some previous investigations in this concern have been carried out using CdS,[4–6]CdSe,[7–10]Ag 2S,[11]In 2S3,[12] PbS,[13]Sb 2S3[14] semiconductor nanoparticles. Among these Sb 2S3 has attracted attention of the researchers owing to its narrow band gap of about 1.7 eV which can allow extension of the absorption band toward the near infrared (NIR) part of the solar spectrum.[14] Sensitization with Sb 2S3 nanoparticles also helps in multiple exciton generation and enhancement in the charge separation process.[15] Many reports are available on the deposition of Sb 2S3 thin films by chemical route,[16–18] while synthesis of crystalline Sb 2S3 at room temperature is seldom reported.[19] Although Sb 2S3 has more compatible properties for photovoltaic applications, very few reports are available on TiO 2/Sb 2S3 configuration.[20–22] However, so far SnO 2/Sb 2S3 system has not been studied for energy harvesting purpose.

With this inspiration, in the present study, we report for the first time, the preparation of crystalline Sb 2S3 by chemical bath deposition at room temperature. Deposited Sb 2S3 is used for the fabrication of Sb 2S3 sensitized SnO 2 photoelectrode via the doctor blade method to explore the system for nanostructured solar cell application. The working mechanism and energy level diagram of Sb 2S3/SnO 2 system have been discussed.

2 Experimental

2.1 Synthesis of Sb2S3, preparation of SnO2 photoelectrode and fabrication of solar cell

Chemical bath deposition (CBD) has been followed for the synthesis of nanospheres of Sb 2S3. Depositions were conducted at room temperature for 1 h using a solution mixture of antimony chloride and sodium thiosulfate (Na 2S2 O 3) as a precursor source of Sb 3+ and S 2− ions, respectively.[19] Ethylene diamine tetraacetic acid (EDTA) was used as complexing agent.

To make SnO 2 paste, 0.5 g of SnO 2 powder was mixed with ethanol, acetic acid, ethylene glycol and α-terpineol in mortar and pestle for 40 min, then SnO 2 film was prepared on fluorine-doped tin oxide (FTO) glass by the doctor blade method.[23] After drying, all samples were annealed at 450 ∘C for 1 h. Further the SnO 2 films were immersed into Sb 2S3 colloidal solution for 1 h to adsorb Sb 2S3 nanoparticles onto the SnO 2 photoelectrode surface.

The solar cell was assembled with Sb2S3 sensitized SnO 2 photoanode, polysulphide as electrolyte and carbon coated FTO as a counter electrode. For fabrication of solar cell, few drops of polysulphide electrolyte solution were added between Sb 2S3/SnO 2 photoelectrode and counter electrode (carbon-coated FTO). Finally, they were clamped together facing conducting surfaces inwards.

The structural analyses were carried out by RigakuDmax-2400 (Cu K α=0.154 nm) X-ray diffractometer. The optical absorbance was recorded on JASCO V-670 spectrophotometer and the emission spectrum was recorded by using Perkin Elmex LS55 photoluminescence spectroscopy technique with He–Ne LASER line (325 nm) as excitation source at room temperature. The morphology of the deposited films was studied by JEOL-JSM 6360 scanning electron microscope (SEM). J–V measurements were conducted in the dark and under an illumination of 50 mW cm −2 to study the performance of fabricated solar cell.

3 Results and discussion

3.1 Structural analysis

Phase and purity of prepared Sb 2S3 nanospheres were confirmed from X-ray diffraction (XRD) analysis as shown in figure 1a. The XRD pattern shows defined peaks around 2 𝜃=28.85∘, 32.13 ∘ and 45.75 ∘, indexed to the diffraction from the (321), (221) and (441) planes, respectively. Almost single larger intensity peak with smaller full-width at half-maxima (FWHM) was observed, indicating highly oriented and crystalline nature of the prepared Sb 2S3 nanospheres. Defined peaks match with the standard peaks from JCPDS file no. 42-1393, readily confirm the orthorhombic phase for Sb 2S3. Many reports are available on the synthesis of polycrystalline Sb 2S3 deposited at various bath temperatures (low as well as high).[19,24] The elemental analysis of prepared Sb 2S3 was carried out by energy-dispersive spectroscopy (EDS). Figure 1b gives the average atomic percentage of Sb:S as 53 .46, revealing the typical stochiometric formation of the antimony sulphide at room temperature.

Figure 1
figure 1

(a) XRD pattern of Sb 2S3 deposited at room temperature and (b) EDS spectrum of Sb 2S3 deposited at room temperature.

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The average crystallite size of the as-prepared Sb 2Se3 was predicted by the Scherer formula,[25]

$$ D=\frac{0.9\lambda} {\beta \;\cos \;\theta} , $$
(1)

where λ (= 1.54 Å) is the wavelength of the X-rays, β the full-width in radians at half-maximum of diffraction peaks and 𝜃 Bragg’s angle of the X-ray pattern at maximum intensity. The average crystallite size estimated is in the range of 30–60 nm in crystalline orientations of (321), (221) and (441) planes.

3.2 Contact angle measurement

The water contact angle measurement was performed for the films of antimony sulphide, in order to get information about the surface wetting capacity of the deposited material. The water droplet was poured with the help of syringe onto the surface of as-deposited films of Sb 2S3 and the images of the drop were captured as shown in figure 2. It has been observed that the angle of contact for films of antimony sulphide with water around 50 ∘. The acute angle of contact reveals the hydrophilic nature of the prepared Sb 2S3 nanospheres.

Figure 2
figure 2

Schematic presentation of contact angle measurement for Sb 2S3 films.

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3.3 Morphological analysis of Sb2S3, SnO2 and SnO2/Sb2S3 thin films

The typical SEM images of as-deposited Sb 2S3 in figure 3a show the well-distributed spherical morphology. The study carried out by Lokhande et al[19] also showed the spherical morphology for antimony sulphide, deposited at very low temperature (6 ± 2 ∘C). While, Han et al[26] observed olivary microcrystallines of Sb 2S3 synthesized via a hydrothermal procedure at high temperature about 180 ∘C. An observation from figure 3b and c reveals that after Sb 2S3 sensitization, the morphology of SnO 2 photoelectrode changes from porous nanoparticles network to the well-developed flakes like structure. The observed changes in the surface morphology is attributed to the growth of Sb 2S3 on SnO 2.

Figure 3
figure 3

(a) Band gap calculation and (b) optical absorption spectra of SnO 2, SnO 2/Sb 2S3 and Sb 2S3.

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3.4 Optical analysis of Sb2S3, SnO2 and SnO2/Sb2S3 thin films

The wavelength dependence of optical absorption spectra of as-prepared Sb 2S3 nanospheres was analysed in the wavelength range of 300–850 nm and the band gap was calculated. Figure 4a shows the plot of (absorbance) 2 vs. h υ for Sb 2S3 nanospheres. By extrapolating the lines obtained to (absorbance) 2=0 on energy axis direct band gap of Sb 2S3 was observed as 2.17 eV for the deposited Sb 2S3 film. Such a value agrees well with the value 2.26 eV for crystalline Sb 2S3 reported in Rajpure et al,[16] but it is somewhat higher than band gap energy of 1.62 eV reported by Desai and Lokhande.[17]

Figure 4
figure 4

SEM images of (a) Sb 2S3, (b) SnO 2 and (c) SnO 2/Sb 2S3.

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Figure 4b shows wavelength-dependent absorption spectra for Sb 2S3, SnO 2 and SnO 2/Sb 2S3. The SnO 2 film exhibits absorption around wavelength less than 340 nm, quite suitable as window material, seldom studied for semiconductor sensitized-based solar cell,[27] as compared with TiO 2.[20–22]Sb 2S3 shows prominent peak of absorption at wavelength about 350 nm and has very promising absorbance extended into far visible region of electromagnetic waves. This makes it very appropriate absorber material for photovoltaic cells.[28]

Optical absorption spectra shown in figure 4b suggest that deposition of a Sb 2S3 helps to enhance the photoresponse of the SnO 2 film by extending absorption up to 613 nm for SnO 2/Sb 2S3 system. The enhancement in the absorbance of the SnO 2/Sb 2S3 system in the visible region from UV-region confirms the growth of the Sb 2S3 on the SnO 2 particles.

3.5 Photoluminescence (PL) study of Sb2S3

Figure 5 shows emission spectra for chemically deposited Sb 2S3 measured at room temperature with excitation wavelength at 350 nm gives four distinct peaks around 472, 532, 610 and 716 nm. The first peak centred around 472, this blue emission can be attributed to deep trap states or defects in material resulted during experimental conditions.[29] Second peak about of 532 nm, corresponds to the green emission, which is in agreement with the previous study by Kulyk et al.[30] Third peak at 610 nm is the PL peak for deposited Sb 2S3 and agrees well with the observations made by Xu et al[31] for Sb 2S3. Fourth emission peak observed approximately at 717 nm, which is at higher wavelength than that of absorption peak approximately at wavelength of 430 nm. This observed red shift in position of emission peak can be attributed to Stoke’s shift, which may be due to the localized centres in band structure.[32]

Figure 5
figure 5

Room temperature PL spectrum of Sb 2S3.

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3.6 Photovoltaic analysis

Figure 6a presents the schematic of charge generation-transfer process for Sb 2S3-sensitized SnO 2 photoelectrode-based solar cell. Illumination of the conducting electrode causes excitation of Sb 2S3, excited electron will enter in the conduction band (E c) of Sb 2S3 and hole will remain in the valence band (E v). The possible charge generation-transfer processes on the basis of band positions of SnO 2, Sb 2S3 and electrolyte has been shown in figure 6b. From the energy level alignment of SnO 2 and Sb 2S3, it is observed that the E c of Sb 2S3 is negative enough to inject electron into SnO 2, a wide band semiconductor, which has the conduction band edge minimum more positive as compare to TiO 2 (at −4.5 eV for SnO 2; −4.2 eV for TiO 2).[33,34]Hence the excited Sb 2S3 can easily inject electrons from its E c quickly into the E c of SnO 2, as compare to TiO 2. The flakes like morphology of SnO 2 helps the received electrons to percolate through SnO 2 to the conducting FTO. Finally, it travels through the external load and complete the circuit by entering the system via counter electrode. The function of redox species is to quickly reduce back the excited Sb 2S3 to its ground state and to set the system for next excitation. Thereafter, the redox species takes electrons from counter electrode for further action and the electron cycle repeats.

Figure 6
figure 6

(a) Schematic presentation of nanostructured solar cell fabricated with SnO 2/Sb 2S3 photoanode and (b) energy band diagram and schematic representation of charge generation and transfer process in Sb 2S3-sensitized SnO 2-based solar cell.

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3.7 Cell performance analysis

Figure 7 represents J–V characteristics for solar cell fabricated using Sb 2S3-sensitized SnO 2 photoelectrode having effective area of 0.25 cm 2 in the dark and under an illumination of 50 mW cm −2 showed V oc∼240 mV and J sc∼0.640 mA cm −2 and FF ∼35%.

Figure 7
figure 7

Photocurrent density–voltage (J–V) characteristics of Sb 2S3/SnO 2 photoanode-based nanostructure solar cell.

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Study of J–V characteristics reveals that, Sb 2S3 a good absorber due to its optimum energy band gap, helps photoelectrochemical active SnO 2 so that it can accept electrons from the adjacent semiconductor layer of Sb 2S3 nanoparticles followed by efficient charge separation. This helps in the enhancement of photocurrent density by 20% for SnO 2/Sb 2S3 system in comparison with SnO 2/CdSe system which shows J sc ∼ 25–30 μA cm−2 as reported by Nasr et al.[28] The low value of fill factor (FF ∼ 35%) attributes to the poor electron transfer at electrolyte–counter electrode interface. In order to improve FF it is essential to optimize electrolyte and counter electrode system for SnO 2/Sb 2S3 photoanode-based solar cells.

4 Conclusion

In this paper, it has been reported that a template free, simple chemical route for the preparation of crystalline Sb 2S3 nanospheres at room temperature can be a substitute for controlled and high temperature synthesis techniques. Prepared Sb 2S3 has orthorhombic phase. Optical absorbance observed in the visible far IR-region, makes it suitable as absorber material in solar cell applications. Noticeable enhancement in absorption of visible light was observed after deposition of Sb 2S3 over SnO 2, confirming the deposition of Sb 2S3 onto SnO 2. The charge generation and transfer processes discussed in detail for SnO 2/Sb 2S3-based solar cell suggest SnO 2 as an alternative to TiO 2 as well as ZnO. The photovoltaic performance for solar cell comprising SnO 2/Sb 2S3 nanostructure shows V oc∼240 mV and J sc ∼0.640 mA cm −2 and FF ∼ 35%.