Investigation of spray pyrolyzed cubic structured Cu doped SnS films

Abstract Undoped and Cu doped SnS films were deposited onto glass substrates by spray pyrolysis technique in order to investigate the effect of Cu doping on their physical properties. Surface investigations showed that Cu doping reduced the surface roughness of SnS films from 36.5 to 8.8 nm. XRD studies revealed that all films have recently solved large cubic phase of SnS (π-SnS) with a-lattice of 11.53 Å and Cu doping led to a reduction in crystallite size from 229 to 198 Å. Additionally, all deposited films were found to be under compressive strain. Optical band gaps of SnS:Cu varied in the range of 1.83–1.90 eV. Hall-effect measurements exhibited that all films have p-type conductivity with low hole concentration (∼1011–1012 cm−3) and high electrical resistivity (∼104–105 Ωcm). Graphical Abstract


Introduction
Thin films solar cells (TFSCs) have great importance due to their low cost and competitive conversion efficiency compared to c-Si solar cells. Among these TFSCs, Cu (In,Ga)(S,Se) 2 (CIGS) and CdTe solar cells have high conversion efficiency (g) of over 20% and are commercially available. The highest efficiency for CIGS and CdTe-based solar cell have been reported as 23.35 and 21.0%, respectively [1] . However, the toxicity of elemental Cd and scarcity of elemental In, Ga and Te have force researchers to work on new materials which consist of environmentally friendly and abundantly elements, such as Cu 2 ZnSn(S,Se) 4 (CZTS) [2,3] . On the other hand, its structural complexity and undesired secondary phases make it difficult to achieve high efficiency in CZTS solar cells [4] . In order to overcome the above-mentioned drawbacks, thin-film absorbers, which are binary compounds and contain inexpensive earth-abundant elements have emerged as a suitable material for solar cell applications [5] .
In this context, tin mono sulfide (SnS) is a good candidate because it meets the above requirements and also possess a high absorption coefficient (>10 4 cm À1 ) and suitable direct band gap ranging from $1.3 eV (for orthorhombic structure) to $1.7 eV (for cubic structure) [6,7] . Externally doping of thin film semiconductors is most frequently referred, as a way to tailor or improve their physical properties for photovoltaic applications. Recently, some studies have shown that doping of SnS with aluminum (Al), iron (Fe), indium (In), copper (Cu), and silver (Ag) can enhance its photovoltaic properties [8][9][10][11][12] . Doped and undoped SnS films can be deposited by several techniques such as RF magnetron sputtering [8] , vapor transport deposition (VTD) [13] , e-beam evaporation [14] , thermal evaporation [15] , electrodeposition [10] , chemical bath depositions [7] , and spray pyrolysis [12] . However, very few studies that investigate the effect of Cu doping on the physical properties of large cubic phase of SnS by spray pyrolysis exist in the literature. Therefore, variations in physical properties of spray pyrolyzed SnS:Cu films as a function of Cu doping concentration were investigated in this work.

Morphological and elemental analysis
The SEM and AFM images were taken in order to investigate both surface morphologies and determine of film thickness. SEM images, AFM images, and EDS spectra are given in Figure 1. As seen from SEM images given in Figure  1(a-c), the film surfaces are formed homogeneously without any cracks or voids. By means of the cross-sectional images given as inset in Figure 1(a-c), the average film thicknesses were noted as 800, 600, and 795 nm for undoped, Cu doped films SnS film at 4 and 8%, respectively. Variations through the surface morpholgy of all films which were observed by AFM images given in Figure 1(d-f) were consistent with the SEM images. For undoped SnS films, agglomeration-like morphology leading to high surface roughness was noticed in AFM images as well as SEM images. These agglomeration-like structures disappeared after Cu doping and the surface roughness decreased. The average surface roughness for SnS:Cu films are listed in Table 1. Figure 1(g-i) illustrates EDS spectra of SnS:Cu films. All elements, i.e., Sn, Cu, and S were detected in these spectra and atomic percentages of these elements are listed in Table 1.

Structural analysis
XRD patterns utilized for the structural analysis are given in Figure 2. As seen from Figure 2(a), a sharp XRD peak detected at 2h$44.4 along with some weak diffractions peaks at 2h$26.8 , 31.0 , 64.8, and 78.0 . Moreover, it was noticed that other peaks were suppressed due to the high intensity of the peak located at $44.4 , and suppressed peaks appeared after vertical axis was enlarged as seen from Figure 2(b). XRD patterns in Figure 2(b) were compared with both orthorhombic phase of SnS (o-SnS) given by JCPDS card No: 39-0354 and recently resolved large cubic phase of SnS (p-SnS) consisting of 64 atoms in unit cell and lattice parameters of a ¼ 11.5873 Å [16][17][18] . As seen from Figure 2(b), XRD patterns of SnS:Cu films deposited in this study fit well with that of large cubic SnS  phase. Accordingly, a-lattice parameter was calculated for all detected peaks using following equation [19] and mean values listed in Table 2.
By supposing that the crystallite size and strain contributions to line broadening are independent to each other, crystallite size and micro-strain values of the deposited films were evaluated in accordance with Williamson-Hall approach under a uniform deformation model [20] .
where b is FWHM, h is Bragg angle, k is wavelength of xray (1.54056 Å), D is crystallite size, K is shape factor, and e is lattice distribution of micro-strain. Hence, D and e can be extracted easily from the bcosh vs. 4sinh plots given in Figure 2(c). D and e values are also listed in Table 2.
As seen Table 2, the largest average crystallite size was noted as 229 Å for undoped SnS films and it decreased to 198 and 199 Å for Cu doped films SnS film at 4 and 8%, respectively. The ionic radii of Sn 2þ and Cu 2þ are 0.93 Å and 0.73 Å, respectively [21] . This decrease in crystallite sizes may be due to the difference in the ionic radii of the host and doping atoms. On the other hand, calculated a-lattice parameter is lower than the reference value a ¼ 11.5873 Å, and it indicates that compressive strain on these films is expected. Compressive nature of microstrain for Cu doped SnS films was also reported in Patel and Ray. [22] Also as seen from Table 2, variation of micro-strain has an inverse relationship with the change in crystal size.

Optical analysis
Transmittance and absorbance spectra recorded for investigation of optical properties of SnS:Cu films are given in Figure 3(a). As seen from Figure 3(a), optical transmittance is $50%, and it increased for Cu doped films at 4% and then decreased again for Cu doped films at 8%. Transmittance of thin films tightly depends on film thickness, crystalline level, and surface homogeneity of material, so combination of these effects determines optical transmittance level [23] . Although SnS:Cu (4%) films have a worse crystallization level compared to that of undoped SnS films, the high optical transmittance may be due to their low thickness. In addition, as seen from the absorbance spectra of SnS:Cu films given as inset in Figure 3(a) fundamental absorption edge shifted toward lower wavelengths, which is the sign of broadening in optical band gaps. Optical band gaps of SnS:Cu films were determined by using (ah) 2 vs. h plots drawn according to Tauc's relation given in Equation (3) and listed in Table 3.
where E g is the optical band gap, h is the Planck's constant, A is a constant, is frequency and a is the absorption coefficient calculated according to equation of a ¼ 2:303 Absorbance d h i [24,25] . By extrapolating the linear part of the (ah) 2 vs. h plots to a ¼ 0, as given in Figure  3(b), optical band gap values of deposited films were determined as 1.83, 1.90, and 1.84 eV for SnS, SnS:Cu (4%) and SnS:Cu (8%) films, respectively. Some of studies on Cu doped SnS films which exist in the literature reported that Cu doping led to shrinkage in band gap of SnS films until the certain Cu doping concentration and enlarged for further doping concentrations, regardless of their crystal system [8,21,26,27] . Sebastian et al. and Bommireddy et al. stated that decrease in bad gap due to the increase in crystallite/ grain size [21,26] . In this work, a variation in band gap may be related with the variation in crystallite size. Intentional doping of impurity atoms may generate band tails called Urbach tails which indicate the width of the localized states within the forbidden gap. Urbach energies (E u ) were determined from the slope of ln(a) vs. h plots (given in as inset in Figure 3(b)) according to Equation (4) and listed in Table 3.
where a 0 is a constant [25] . As seen from Table 3, Urbach energy values increased from 134 to 237 meV. The increase in Urbach energy implies that tail width of localized states increased due to increase in the degree of electronic disorder by Cu doping [28] .

Electrical analysis
Hall-effect measurement were carried out to investigate the electrical transport properties of SnS:Cu films as a function of Cu concentration using at room temperature. Multiple measurements for each film were taken and results were averaged and then listed in Table 3. It was seen that all films have p-type conductivity with low hole concentration and high electrical resistivity. The highest hole concentration and mobility along with the lowest electrical resistivity were noted for SnS:Cu (8%) films. Such high electrical resistivity values for SnS were also reported by others [29] . However, these electrical properties are not adequate for photovoltaic applications. Especially, low hole concentration limits their utilization in solar cells as an absorber layer. Vidal et al. stated that Sn vacancies are responsible for intrinsic p-type conductivity and S-rich condition enhance hole concentration [30] . Low hole concentration in deposited SnS:Cu films in this work may be result from a sulfur deficiency in the films as seen from EDS spectra. So, loss of sulfur spray pyrolyzed SnS film may be compensated by using an excessive amount of sulfur source in the spray solution as reported [31] or annealing in a sulfur-containing atmosphere (i.e., sulphurization) and thereby hole concentration may be enhanced.

Characterization techniques
To perform structural investigations, X-ray diffraction (XRD) patterns were obtained in the range of 2h ¼ 25 -80 by means of X-ray diffractometer (Bruker AXS, Discovery D8, Billerica, MA) with CuK a radiation of k ¼ 1.54 Å. Surface images and cross-sectional images of SnS:Cu films were captured using scanning electron microscopy (SEM) (Carl Zeiss, Gemini 300, Jena, Germany) equipped with EDS set up in order to analyze surface properties, determine film thickness and perform elemental analysis. Additionally, Park Systems XE 100 model atomic force microscope was used to obtain the AFM images. Optical properties were studied via transmittance and absorbance spectra taken between wavelength range of 350-1400 nm by using a Shimadzu UV-2600 spectrophotometer (Shimadzu Corporation, Kyoto, Japan). Ecopia HMS-3000 Hall Effect measurement system equipped with 0.556 T magnets was used in Van Der Pauw geometry for determining carrier density, mobility, and resistivity of the SnS:Cu films.

Conclusion
In summary, undoped and Cu doped SnS films at 4% and 8% were deposited onto glass substrates by using spray pyrolysis technique in order to investigate Cu doping effect. AFM and SEM images showed that the surface homogeneity of SnS films improved after Cu doping and roughness decreased almost by four times. Additionally, EDS spectra revealed that all films were S-deficient. Based on structural analysis, it was reported that deposited SnS thin films have a large cubic phase with a-lattice $11.53 Å and Cu doping led to reduction in crystallite size. Transmittance and absorbance spectra illustrated that absorption edge was located at between 650 and 700 nm. Calculated optical band gaps were found to be in the range of 1.83-1.90 eV. Hall-effect measurement showed that all films have p-type conductivity with low hole concentration and high electrical resistivity, which may arise due to S-deficiency and limits the utilization of spray pyrolyzed SnS:Cu films in thin film solar cells as an absorber layer. Therefore, it is proposed that excessive sulfur source may be added in spray solution or as-grown SnS:Cu films may be post-annealed in sulfur-containing atmosphere.

Disclosure statement
The author declares that he has no conflicting interests.