# Effect of Cadmium Concentration on Structural, Optical, and Electrical Properties of Cu2Zn1−xCdxSnS4 Quinternary Alloy Nanofibres, Synthesized by Electrospinning Technique

Authors: A. S. Ibraheam; Y. Al-Douri; Nabeel Z. Al-Hazeem; U. Hashim; Deo Prakash; K. D. Verma
Journal: Journal of Nanomaterials
 (2016)

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## Abstract

The Cu2Zn1−xCdxSnS4 quinternary alloy nanofibres with different Cd concentrations were grown on glass substrate using the electrospinning technique. The structural properties of Cu2Zn1−xCdxSnS4 quinternary alloy nanofibres were investigated by X-ray diffraction (XRD), field emission-scanning electron microscopy (FE-SEM), and atomic force microscopy (AFM). Optical properties were analysed through UV-visible spectrophotometry (UV-Vis) and photoluminescence (PL) spectroscopy, which revealed that there is a decrease in band gap from 1.75 eV to 1.61 eV, with the increasing Cd concentration from x = 0 to x = 1. The current-voltage measurements exhibited a power conversion efficiency of 3% under the solar illumination with intensity of 100 mW/cm2. Electrical properties supported that the Cu2Zn1−xCdxSnS4 quinternary alloy can be used as an absorber in solar cells. The bulk modulus, refractive index, and dielectric constant were also investigated.


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## Body

## 1. Introduction

The quaternary CuIn1−xGaxSe2 (CIGS) thin film is a promising candidate for low cost absorber layer in thin film solar cell, due to its excellent properties and higher efficiency [1–3]. In recent years, enormous efforts have been dedicated to find out the low cost, earth-abundant elements materials for solar cell applications. Cu2ZnSnS4 (CZTS) is a potential material to substitute the CuIn1−xGaxSe2 (CIGS) and has attracted great interest due to its direct band gap of Eg = 1.45–1.5 eV, optical absorption coefficient higher than 104 cm−1 [4], good photostability [5], low toxicity [6], high photovoltaic efficiency [7], and presence of earth-abundant elements [8–13]. Various technologies have been developed for deposition of CZTS, which include thermal evaporation [14–16], sputtering [17], electrodeposition [18, 19], spray pyrolysis [20], electrochemical deposition [21], and pulsed laser deposition [22–24]. Third-generation approaches to photovoltaics (PVs) aim to decrease costs and significantly increasing efficiencies but maintain the economic and environmental cost advantages of thin-film deposition techniques. PV technologies have shown significant progress lately in terms of annual production capacity and life cycle environmental performances, which necessitates the assessment of environmental impacts of such technologies. The different PV technologies show slight variations in the emissions when comparing the emissions from conventional energy technologies replaced by the latest PV technologies.Lately, Spinelli et al. [25] have proposed dielectric nanoparticle arrays as antireflection coatings and light-trapping schemes for thin-film c-Si solar cells. However, practical aspects about the integration of these nanostructures with solar modules have yet to be investigated. They have studied the effect of ethylene vinyl acetate (EVA) encapsulation on the optical properties of dielectric nanoparticles placed on top of c-Si substrates and showed that Si nanoparticle encapsulated in EVA layer yields ultralow reflectivity over the entire 300–1000 nm spectral range. Furthermore, they have studied the case of plasmonic nanoparticle coatings for c-Si solar cells. Breitenstein [26] has investigated novel one-diode model for illuminated solar cells, which contains an additional variable resistance describing minority carrier diffusion from the bulk to the p-n junction. This model naturally describes the differences between photo- and electroluminescence imaging, as well as a well-known departure from the superposition principle. Oishi et al. [27] have grown Cu2ZnSnS4 thin films on Si (100) substrate by vacuum evaporation technique, which exhibited the broad emissions at around 1.45 eV and 1.31 eV in the photoluminescence spectrum measured at 13 K.Electrospinning technique is one of the most promising methods for the fabrication of 1D-nanostructure materials [28]. Researches on the synthesis of various metal-oxide nanowires, via electrospinning, have recently increased [29, 30]. The electrospinning route is a simple, versatile, and relatively inexpensive technique for the synthesis of a broad range of hybrid nanocomposites. Unlike other methods, it can produce short nanorods or nanotubes, along with the continuous nanowires and nanotubes [31]. For the first time, electrospinning technique was successfully used for the synthesis of CZTS fibres [32].In this work, electrospinning technique has been described as an alternative synthesis route for the preparation of Cu2Zn1−xCdxSnS4 quinternary alloy nanofibres, which were further studied as the photovoltaic (PV) absorber layers without sulfurization. The obtained Cu2Zn1−xCdxSnS4 quinternary alloy nanofibres were analysed and characterized, and the optimum Cd concentration (x) for PVs applications was determined. In addition, the bulk modulus, refractive index, and optical dielectric constant were also investigated.


## 2. Experimental Process

Cu2Zn1−xCdxSnS4 quinternary alloy nanofibres were deposited onto glass substrate using the electrospinning method. Firstly, the precursor solutions were prepared from copper (II) chloride dihydrate, zinc (II) chloride dihydrate, tin (II) chloride dihydrate, cadmium (II) chloride, thiourea, 2-methoxyethanol (2-metho), and monoethanolamine (MEA). The molar ratio of Cu, (Zn + Cd), Sn, and S in the solution was 2 : 1 : 1 : 4. To obtain the solutions with different Cd concentration (x), the molar ratio of Cd to (Zn + Cd) was varied to give x values of 0, 0.2, 0.4, 0.6, 0.8, and 1. The precursor solutions were mixed on the basis of required ratio of the formula, that is, Cu2+2(Zn1−xCdx)+2Sn+4S4-8. The 2-metho and MEA were used as solvent and stabilizer, respectively. The molar ratio of MEA to precursor solution of Cu2Zn1−xCdxSnS4 was maintained at 0.5. The solution was stirred at 50°C for 3 h to completely dissolve the metal salts, followed by addition of thiourea, which results in the yellow and transparent solution. After that, polyvinyl alcohol (PVA) was weighted precisely (concentration ratios of 25 wt.%) and mixed with solution of Cu2Zn1−xCdxSnS4 (x = 0, 0.2, 0.4, 0.6, 0.8, 1), under vigorous magnetic stirring for 20 h, leading to a clear white solution. During a typical electrospinning process, 2 mL of the above solution was added into the syringe which was pumped with a flow rate of 0.6 mL/h, at a voltage of 12 kV and the electrode distance of 12 cm. Precursor fibres collected on glass substrate were then annealed at 300°C under N2 atmosphere for 40 min.To study the performance of solar cells based on Cu2Zn0.4Cd0.6SnS4 quinternary alloy nanofibres, prototype of CZCTS solar cells with Cd concentration equals 0.6 was prepared. The CdS and ZnO:Al were successively deposited on Cu2Zn0.4Cd0.6SnS4 quinternary alloy to obtain ZnO:Al/CdS/Cu2Zn0.4Cd0.6SnS4/Al/glass structure, including Al doped ZnO which equals 5% as shown in Figure 1.Figure 1
Schematic structure of typical Cu2Zn0.4Cd0.6SnS4 solar cell.The crystal structure of the Cu2Zn1−xCdxSnS4 quinternary alloy nanofibres was examined by X-ray diffraction (XRD; PW 1710 X-ray diffractometer, Phillips, USA) with Cu Kα radiation (λ = 1.54 Å). The optical properties were measured at room temperature using UV-visible spectrophotometry (UV-vis; Lambda 950, PerkinElmer, USA) and photoluminescence spectroscopy (PL; Jobin Yvon model HR 800 UV system, Jobin Yvon, USA) was carried out at room temperature using a He-Cd laser (λ = 325 nm). Surface morphology and grain size were investigated by field emission-scanning electron microscopy (FE-SEM; NOVA NANO SEM 450, USA). The topography of the films was characterized by atomic force microscopy (AFM; SII SPI 3800N Probe, Seiko Instruments Inc., USA). For the power conversion efficiency measurement of Cu2Zn0.4Cd0.6SnS4 based solar cell, the current-voltage characteristics were measured under simulated AM 1.5 G solar illumination with intensity of 100 mW/cm2.


## 3. Results and Discussion

### 3.1. Structural Properties

Figure2 shows the XRD patterns of CZCTS quinternary alloy nanofibres obtained from 15 wt.% PVA/CZTS nanofibres at x = 0 and the increasing concentration ratios of 25 wt.% (PVA) at x = 0.2, 0.4, 0.6, 0.8, and 1. The XRD patterns of Cu2Zn1−xCdxSnS4 quinternary alloy nanofibres show five peaks at 2θ = 16.147°, 28.436°, 32.986°, 34.549°, and 47.875° that attributed to (002), (112), (200), (004), and (220) planes of the tetragonal kesterite phase of Cu2ZnSnS4 (ICDD PDF2008, 01-075-4122) and stannite phase of Cu2CdSnS4 (ICDD (PDF2008), 00-029-0537). The estimated errors as given in Table 1 are accomplished based on three prepared samples for each concentration. As seen, the peak intensity of (112) was higher than (002) and (200), revealing that (112) is the preferred growth plane, which is comparable to the standard XRD pattern of CZTS [33]. The peaks were shifted to the lower angle side with increasing Cd concentration in the Cu2Zn1−xCdxSnS4 quinternary alloy nanofibres, due to the increasing lattice constant. This is due to radius of Cd ion (1.53 Å), which is larger than Zn (1.33 Å), as supported by previous work [34]. It is important to note that XRD patterns of Cu2Zn1−xCdxSnS4 nanofibres showed no additional peaks corresponding to other impurities, such as CdS, ZnS, CuS, or Cu2S, over the entire composition range. The lattice constants a and c were calculated for the CZTS nanocrystals based on the (112) peak, which are given in Table 1: (1)1d2=h2+k2a2+l2c2.The crystallite sizes (D) of CZCTS nanofibre were calculated using the Scherrer formula for the peak (112) [35]: (2)D=kλβcos⁡θ,where k is a constant, taken to be 0.94, and β is the full width at half maximum (FWHM).Table 1
The structural properties of Cu2Zn1-xCdxSnS4 quinternary alloy nanofibres using XRD for different Cd concentrations, x = 0–1.
x
Peak (θ)
Crystallite size (nm)
d
h
k
l (112) (Å)
Lattice constants (Å)
Bulk modulus (B0) (GPa)
Estimated error (%)
0
28.436
30.90
3.387
a
=
5.339
82.30
1
a
=
5.428
∗
c
=
10.507
c
=
10.842
∗
0.2
28.428
39.79
3.545
a
=
5.434
84.20
1.5
c
=
10.808
0.4
28.398
49.66
3.623
a
=
5.438
84.25
1.5
c
=
10.820
0.6
28.388
56.34
3.695
a
=
5.583
85.15
1.5
c
=
10.831
0.8
28.383
59.86
3.734
a
=
5.567
85.05
1.5
c
=
11.168
1
28.379
64.28
3.804
a
=
5.668
87.07
1
a
=
5.487
#
c
=
11.349
c
=
11.389
∗
∗Ref. [34, 47] exp., #ref. [47] exp.Figure 2
XRD patterns of Cu2Zn1−xCdxSnS4 quinternary alloy nanofibres at different Cd concentrations (x = 0–1).Material stiffness is important in different industries, which is reflected in bulk modulus [36–41]. Many attempts have been made to explore the thermodynamic properties of solids. In the present study, thermodynamic properties, including interatomic separation and bulk modulus of solids, have been studied with different approximations and best-fit relations [38–42]. Additionally, more approaches that are empirical have been developed to accurately compute the structural and electronic properties of the solids, despiteab initiocalculations, which are very complex and require numerous efforts [43]. Although the empirical methods give applicability advantage to a broad class of materials and illustrate the trends in many cases, they might not be extremely accurate for each specific material.Cohen [44] had established an empirical formula for the calculation of bulk modulus B0. This formula was based on the nearest-neighbour distance and presented good consistency of results with the experimental data. At the same time, an analytical expression for the calculation of bulk modulus from the total energy has been derived by Lam et al. [45], which was structurally different from the empirical formula and yet offered the same numerical results. In addition, they have obtained an analytical expression of the bulk modulus. Another concept based on lattice constant has been established [46], where the obtained results are consistent with the experimental data and other calculations. The two interesting features of this formula are as follows: only the lattice constant is required as input and the computation of B0 is trivial.To determine the possible relation of bulk modulus with the lattice constant (despite being a qualitative concept) is the main objective. It was advocated that the dominant effect is the degree of covalence, which was characterized by the Phillips homopolar gapEh [42]. The reason to include these data in present work is the validity of calculations which is not restricted in computed space. Thus, it will not be wrong to consider that the data might be proven valuable for the future work in this field. Here the study of B0 is important, as the differences between the lattice constants of Cu2Zn1−xCdxSnS4 quinternary alloy nanofibres can be clearly seen in Table 1. Here, B0 was calculated using the following empirical formula [46]:(3)B0=3000-100λa2-3.5,where a is the lattice constant (in Å); λ is an empirical parameter for the effect of ionicity (λ=0, 1, 2 for groups IV, III–V, and II–VI semiconductors, resp.). The calculated values of B0 are indicated in Table 1. Here it can be concluded that the present bulk modulus, calculated in a different way from the definition of Cohen, exhibit the same trends. It was observed that Cd concentration was directly proportional to the stiffness of Cu2Zn1−xCdxSnS4 quinternary alloy nanofibres.


### 3.2. Morphological Study

The morphology of the Cu2Zn1−xCdxSnS4 quinternary alloy nanofibres was studied using FE-SEM. Figure 3 shows a series of FE-SEM images of PVA/Cu2Zn1−xCdxSnS4 quinternary alloy nanofibres obtained from precursor solutions, with concentration ratios of 15 and 25 wt.% (PVA) and different x values (x=0, 0.2, 0.4, 0.6, 0.8, and 1). The formation of fibres can be easily controlled by optimizing the PVA concentration. At low concentration, formation of defects such as beads along the fibres was observed (Figure 3 (x=0)), which is due to the capillary breakup of the spinning jet because of surface tension [47]. Also, increasing the concentration of polymer (PVA) in solution with increasing Cd concentration, the number of defects was found to decrease, resulting in the formation of uniform fibres with increasing diameters from 58.615 for x=0, 125.582 for x=0.2, 134.21 for x=0.4, 185.33 for x=0.6, 290.45 for x=0.8, and 424.85 for x=1. This suppression of defects during the fibre formation can perhaps be understood as follows: increasing concentration of solution helps to raise the viscoelastic forces, which might have resulted in reducing the influential forces due to surface tension. The obtained results illustrate that the morphology of the composite fibres was optimized with minimal defects when the PVA concentration was 25 wt.% (x=1). It can be seen from the FE-SEM images that a random mesh of nanofibres was obtained. This randomness of the fibre mesh was the direct consequence of bending instability associated with the electrospinning jet.Figure 3
FE-SEM images of Cu2Zn1−xCdxSnS4 quinternary alloy nanofibre at different Cd concentrations (x = 0–1).AFM was used to confirm the cylindrical morphology of the electrospinned fibres. Figure4 displays the AFM images of PVA/Cu2Zn1−xCdxSnS4 nanofibres obtained from precursor solutions with 15 wt.% of PVA at x=0 and 25 wt.% of PVA at x=0.2, 0.4, 0.6, 0.8, and 1. From Figure 4, it is obvious that the PVA/Cu2Zn1−xCdxSnS4 has the cylindrical morphology. The fibres were uniformly distributed over the collector screen. The average roughness of the surface was observed to be 25.6, 46.3, 96.6, 124, 188.50, and 236.23 nm at x = 0, 0.2, 0.4, 0.6, 0.8, and 1, respectively. These results indicate that the quality of Cu2Zn1−xCdxSnS4 quinternary alloy nanostructure is improved as the Cd concentration increases.Figure 4
AFM images of Cu2Zn1−xCdxSnS4 quinternary alloy nanofibres at different Cd concentrations (x = 0–1).


### 3.3. Optical Properties

PL spectra of CZCTS quinternary alloy nanofibres with different Cd concentrations measured at room temperature are shown in Figure5. PL spectra consist of one broad asymmetric band at 1.75 eV for Cu2ZnSnS4 and at 1.61 eV for Cu2CdSnS4 as given in Table 2. A shift of the PL band of Cu2Zn1−xCdxSnS4 quinternary alloy nanofibres towards higher wavelengths with increasing Cd concentration was observed in the region 0 ≤ x ≤ 1. This shifting is due to the substitution of Zn atoms with Cd atoms to produce a lower energy gap [48, 49].Table 2
The energy band gaps corresponding to refractive index and optical dielectric constant using Ravindra et al. [51], Hervé and Vandamme [52], and Ghosh et al. [53] of Cu2Zn1-xCdxSnS4 quinternary alloy nanofibres.
x
Band gap using UV (eV)∗
Band gap using PL (eV)∗∗
n
ε
∞
0
1.81
1.75
2.95a∗  2.82b∗  2.70c∗
8.70a∗  7.95b∗  7.29c∗
2.83a∗∗  2.70b∗∗  2.58c∗∗
8.00a∗∗  7.29b∗∗  6.65c∗∗
0.2
1.80
1.72
2.97a∗  2.84b∗  2.72c∗
8.82a∗  8.06b∗  7.39c∗
2.85a∗∗  2.73b∗∗  2.61c∗∗
8.12a∗∗  7.45b∗∗  6.81c∗∗
0.4
1.79
1.70
2.99a∗  2.86b∗  2.74c∗
8.94a∗  8.17b∗  7.50c∗
2.87a∗∗  2.75b∗∗  2.63c∗∗
8.23a∗∗  7.56b∗∗  6.91c∗∗
0.6
1.69
1.67
3.01a∗  2.88b∗  2.76c∗
9.06a∗  8.29b∗  7.61c∗
2.89a∗∗  2.77b∗∗  2.65c∗∗
8.35a∗∗  7.67b∗∗  7.02c∗∗
0.8
1.68
1.65
3.04a∗  3.00b∗  2.78c∗
9.24a∗  9.00b∗  7.72c∗
2.91a∗∗  2.79b∗∗  2.68c∗∗
8.46a∗∗  7.78b∗∗  7.18c∗∗
1
1.63
1.61
3.07a∗  3.02b∗  2.81c∗
9.42a∗  9.12b∗  7.89c∗
2.93a∗∗  2.82b∗∗  2.71c∗∗
8.58a∗∗  7.95b∗∗  7.34c∗∗
aRef. [51]; bref. [52]; cref. [53].
∗refers to Band gap using UV.
∗
∗refers to Band gap using PL.Figure 5
PL of Cu2Zn1−xCdxSnS4 quinternary alloy nanofibres at different Cd concentrations (x = 0–1).The absorption coefficient associated with the high absorption region was calculated from absorbance (A) and the film thickness (t) using the following relation:(4)a=2.303At.Figure 6 shows the absorption coefficient (α) of Cu2Zn1−xCdxSnS4 quinternary alloy nanofibres deposited at different Cd concentration (x = 0 to 1). It can be observed that the absorption coefficient was greater than 104 cm−1. Thus, it would be better to infer that the grown nanostructures have good absorbance in the visible region of electromagnetic spectrum. The absorption spectra are shifted towards longer wavelengths as Cd concentration increases. The fundamental absorption corresponds to the transition from valence to conduction band that can be used to determine the band gap of the material. The energy band gap (Eg) was estimated using(5)hναγ=βhν-Eg,where hν is the incident photon energy, β is parameter, and γ is index which characterizes the optical absorption process (γ=2 and 1/2 for indirect and direct transitions, resp.). The best linear fit was obtained for γ=1/2 for direct transition. Eg was determined by extrapolating the straight line portion hναγ = 0 value and showed a decrease from 1.81 eV for x=0 to 1.80 eV for x=0.2, 1.79 eV for x=0.4, 1.69 eV for x=0.6, 1.68 for x=0.8, and 1.63 for x=1, as illustrated in Figure 7 and given in Table 2. It was found that the energy band gap of Cu2Zn1−xCdxSnS4 quinternary alloy nanofibres drifts towards lower energy with an increase in Cd concentration. Thus, the optical properties of CZCTS absorber layers can be improved by substituting the Zn atoms by Cd atoms to produce a lower energy gap [34].Figure 6
Absorption coefficient of Cu2Zn1−xCdxSnS4 quinternary alloy nanofibres at different Cd concentrations (x = 0–1).Figure 7
Optical band gap of Cu2Zn1−xCdxSnS4 quinternary alloy nanofibres at different Cd concentrations (x = 0–1).The refractive indexn is a significant physical parameter in microscopic atomic interactions. Theoretically, the refractive index is related to the density and the local polarizability of these entities [50]. Many simple relationships between refractive index n and the energy gap Eg have been attempted [51–58]. Here, various relationships between n and Eg have been reviewed in order to validate the current work. As suggested by Ravindra et al. [51], the band gap and the high frequency refractive index presented a linear relationship: (6)n=α+βEg,where α = 4.048 and β = −0.62 eV−1Inspired by simple physics of light refraction and dispersion, Hervé and Vandamme [52] have proposed an empirical relation as(7)n=1+AEg+B2,where A=13.6 eV and B=3.4 eV.Ghosh et al. [53] had taken a different approach by considering the band structural and quantum-dielectric formulations of Penn [59] and Van Vechten [60]. Introducing A (contribution from the valence electrons) and B (constant additive to the lowest band gap Eg), the expression was written as(8)n2-1=AEg+B2,where A=25Eg+212, B=0.21Eg+4.25, and (Eg+B) refers to an appropriate average energy gap of the material.Thus, these three models for variation ofn with energy gap have been tried. In addition, the calculated values of the optical dielectric constant (ε∞) were obtained using the relation ε∞=n2 [61]. The calculated refractive index and optical dielectric constant are given in Table 2. This is showing that the Ghosh et al. model is an appropriate model for solar cells applications.Figure8 shows the transmittance spectra of the Cu2Zn1−xCdxSnS4 quinternary alloy nanofibres at different Cd concentrations. The transmittance of the nanofibres decreases as Cd concentration increases, which is attributed to the increasing absorbance. The highest value of transmittance was 78% at x=0 and the lowest was 33% at x=1, with a shift in transmittance towards high wavelengths as Cd concentration increases.Figure 8
Transmittance spectra of Cu2Zn1−xCdxSnS4 quinternary alloy nanofibres at different Cd concentrations (x = 0–1).


### 3.4.I-V Characteristics

To study the performance of CZCTS-based solar cells, the optimal prototype device with glass/ZnO:Al/CdS/Cu2Zn0.8Cd0.2SnS4/Al structure was prepared (Figure 1). I-V curve was studied under 100 mW/cm2 via simulated AM 1.5 G solar illuminations (Figure 9). Our results suggest that Cu2Zn1−xCdxSnS4 at x equals 0.6, has an optimal band gap, and can be a suitable material as absorber layer in single junction solar cells. Although the power conversion efficiency (3%) is low without elaborate optimization (Figure 9), our preliminary results suggest that the performance of CZCTS-based solar cells is quite tunable at varied Cd concentration [34]. It is of prime importance to consider the trade-off between various parameters and the design of tandem solar cells with the optimal performance.Figure 9
I-V characteristics for the glass/ZnO:Al/CdS/Cu2Zn0.4Cd0.6SnS4/Al solar cell at Cd concentration x=0.6.


## 3.1. Structural Properties

Figure2 shows the XRD patterns of CZCTS quinternary alloy nanofibres obtained from 15 wt.% PVA/CZTS nanofibres at x = 0 and the increasing concentration ratios of 25 wt.% (PVA) at x = 0.2, 0.4, 0.6, 0.8, and 1. The XRD patterns of Cu2Zn1−xCdxSnS4 quinternary alloy nanofibres show five peaks at 2θ = 16.147°, 28.436°, 32.986°, 34.549°, and 47.875° that attributed to (002), (112), (200), (004), and (220) planes of the tetragonal kesterite phase of Cu2ZnSnS4 (ICDD PDF2008, 01-075-4122) and stannite phase of Cu2CdSnS4 (ICDD (PDF2008), 00-029-0537). The estimated errors as given in Table 1 are accomplished based on three prepared samples for each concentration. As seen, the peak intensity of (112) was higher than (002) and (200), revealing that (112) is the preferred growth plane, which is comparable to the standard XRD pattern of CZTS [33]. The peaks were shifted to the lower angle side with increasing Cd concentration in the Cu2Zn1−xCdxSnS4 quinternary alloy nanofibres, due to the increasing lattice constant. This is due to radius of Cd ion (1.53 Å), which is larger than Zn (1.33 Å), as supported by previous work [34]. It is important to note that XRD patterns of Cu2Zn1−xCdxSnS4 nanofibres showed no additional peaks corresponding to other impurities, such as CdS, ZnS, CuS, or Cu2S, over the entire composition range. The lattice constants a and c were calculated for the CZTS nanocrystals based on the (112) peak, which are given in Table 1: (1)1d2=h2+k2a2+l2c2.The crystallite sizes (D) of CZCTS nanofibre were calculated using the Scherrer formula for the peak (112) [35]: (2)D=kλβcos⁡θ,where k is a constant, taken to be 0.94, and β is the full width at half maximum (FWHM).Table 1
The structural properties of Cu2Zn1-xCdxSnS4 quinternary alloy nanofibres using XRD for different Cd concentrations, x = 0–1.
x
Peak (θ)
Crystallite size (nm)
d
h
k
l (112) (Å)
Lattice constants (Å)
Bulk modulus (B0) (GPa)
Estimated error (%)
0
28.436
30.90
3.387
a
=
5.339
82.30
1
a
=
5.428
∗
c
=
10.507
c
=
10.842
∗
0.2
28.428
39.79
3.545
a
=
5.434
84.20
1.5
c
=
10.808
0.4
28.398
49.66
3.623
a
=
5.438
84.25
1.5
c
=
10.820
0.6
28.388
56.34
3.695
a
=
5.583
85.15
1.5
c
=
10.831
0.8
28.383
59.86
3.734
a
=
5.567
85.05
1.5
c
=
11.168
1
28.379
64.28
3.804
a
=
5.668
87.07
1
a
=
5.487
#
c
=
11.349
c
=
11.389
∗
∗Ref. [34, 47] exp., #ref. [47] exp.Figure 2
XRD patterns of Cu2Zn1−xCdxSnS4 quinternary alloy nanofibres at different Cd concentrations (x = 0–1).Material stiffness is important in different industries, which is reflected in bulk modulus [36–41]. Many attempts have been made to explore the thermodynamic properties of solids. In the present study, thermodynamic properties, including interatomic separation and bulk modulus of solids, have been studied with different approximations and best-fit relations [38–42]. Additionally, more approaches that are empirical have been developed to accurately compute the structural and electronic properties of the solids, despiteab initiocalculations, which are very complex and require numerous efforts [43]. Although the empirical methods give applicability advantage to a broad class of materials and illustrate the trends in many cases, they might not be extremely accurate for each specific material.Cohen [44] had established an empirical formula for the calculation of bulk modulus B0. This formula was based on the nearest-neighbour distance and presented good consistency of results with the experimental data. At the same time, an analytical expression for the calculation of bulk modulus from the total energy has been derived by Lam et al. [45], which was structurally different from the empirical formula and yet offered the same numerical results. In addition, they have obtained an analytical expression of the bulk modulus. Another concept based on lattice constant has been established [46], where the obtained results are consistent with the experimental data and other calculations. The two interesting features of this formula are as follows: only the lattice constant is required as input and the computation of B0 is trivial.To determine the possible relation of bulk modulus with the lattice constant (despite being a qualitative concept) is the main objective. It was advocated that the dominant effect is the degree of covalence, which was characterized by the Phillips homopolar gapEh [42]. The reason to include these data in present work is the validity of calculations which is not restricted in computed space. Thus, it will not be wrong to consider that the data might be proven valuable for the future work in this field. Here the study of B0 is important, as the differences between the lattice constants of Cu2Zn1−xCdxSnS4 quinternary alloy nanofibres can be clearly seen in Table 1. Here, B0 was calculated using the following empirical formula [46]:(3)B0=3000-100λa2-3.5,where a is the lattice constant (in Å); λ is an empirical parameter for the effect of ionicity (λ=0, 1, 2 for groups IV, III–V, and II–VI semiconductors, resp.). The calculated values of B0 are indicated in Table 1. Here it can be concluded that the present bulk modulus, calculated in a different way from the definition of Cohen, exhibit the same trends. It was observed that Cd concentration was directly proportional to the stiffness of Cu2Zn1−xCdxSnS4 quinternary alloy nanofibres.


## 3.2. Morphological Study

The morphology of the Cu2Zn1−xCdxSnS4 quinternary alloy nanofibres was studied using FE-SEM. Figure 3 shows a series of FE-SEM images of PVA/Cu2Zn1−xCdxSnS4 quinternary alloy nanofibres obtained from precursor solutions, with concentration ratios of 15 and 25 wt.% (PVA) and different x values (x=0, 0.2, 0.4, 0.6, 0.8, and 1). The formation of fibres can be easily controlled by optimizing the PVA concentration. At low concentration, formation of defects such as beads along the fibres was observed (Figure 3 (x=0)), which is due to the capillary breakup of the spinning jet because of surface tension [47]. Also, increasing the concentration of polymer (PVA) in solution with increasing Cd concentration, the number of defects was found to decrease, resulting in the formation of uniform fibres with increasing diameters from 58.615 for x=0, 125.582 for x=0.2, 134.21 for x=0.4, 185.33 for x=0.6, 290.45 for x=0.8, and 424.85 for x=1. This suppression of defects during the fibre formation can perhaps be understood as follows: increasing concentration of solution helps to raise the viscoelastic forces, which might have resulted in reducing the influential forces due to surface tension. The obtained results illustrate that the morphology of the composite fibres was optimized with minimal defects when the PVA concentration was 25 wt.% (x=1). It can be seen from the FE-SEM images that a random mesh of nanofibres was obtained. This randomness of the fibre mesh was the direct consequence of bending instability associated with the electrospinning jet.Figure 3
FE-SEM images of Cu2Zn1−xCdxSnS4 quinternary alloy nanofibre at different Cd concentrations (x = 0–1).AFM was used to confirm the cylindrical morphology of the electrospinned fibres. Figure4 displays the AFM images of PVA/Cu2Zn1−xCdxSnS4 nanofibres obtained from precursor solutions with 15 wt.% of PVA at x=0 and 25 wt.% of PVA at x=0.2, 0.4, 0.6, 0.8, and 1. From Figure 4, it is obvious that the PVA/Cu2Zn1−xCdxSnS4 has the cylindrical morphology. The fibres were uniformly distributed over the collector screen. The average roughness of the surface was observed to be 25.6, 46.3, 96.6, 124, 188.50, and 236.23 nm at x = 0, 0.2, 0.4, 0.6, 0.8, and 1, respectively. These results indicate that the quality of Cu2Zn1−xCdxSnS4 quinternary alloy nanostructure is improved as the Cd concentration increases.Figure 4
AFM images of Cu2Zn1−xCdxSnS4 quinternary alloy nanofibres at different Cd concentrations (x = 0–1).


## 3.3. Optical Properties

PL spectra of CZCTS quinternary alloy nanofibres with different Cd concentrations measured at room temperature are shown in Figure5. PL spectra consist of one broad asymmetric band at 1.75 eV for Cu2ZnSnS4 and at 1.61 eV for Cu2CdSnS4 as given in Table 2. A shift of the PL band of Cu2Zn1−xCdxSnS4 quinternary alloy nanofibres towards higher wavelengths with increasing Cd concentration was observed in the region 0 ≤ x ≤ 1. This shifting is due to the substitution of Zn atoms with Cd atoms to produce a lower energy gap [48, 49].Table 2
The energy band gaps corresponding to refractive index and optical dielectric constant using Ravindra et al. [51], Hervé and Vandamme [52], and Ghosh et al. [53] of Cu2Zn1-xCdxSnS4 quinternary alloy nanofibres.
x
Band gap using UV (eV)∗
Band gap using PL (eV)∗∗
n
ε
∞
0
1.81
1.75
2.95a∗  2.82b∗  2.70c∗
8.70a∗  7.95b∗  7.29c∗
2.83a∗∗  2.70b∗∗  2.58c∗∗
8.00a∗∗  7.29b∗∗  6.65c∗∗
0.2
1.80
1.72
2.97a∗  2.84b∗  2.72c∗
8.82a∗  8.06b∗  7.39c∗
2.85a∗∗  2.73b∗∗  2.61c∗∗
8.12a∗∗  7.45b∗∗  6.81c∗∗
0.4
1.79
1.70
2.99a∗  2.86b∗  2.74c∗
8.94a∗  8.17b∗  7.50c∗
2.87a∗∗  2.75b∗∗  2.63c∗∗
8.23a∗∗  7.56b∗∗  6.91c∗∗
0.6
1.69
1.67
3.01a∗  2.88b∗  2.76c∗
9.06a∗  8.29b∗  7.61c∗
2.89a∗∗  2.77b∗∗  2.65c∗∗
8.35a∗∗  7.67b∗∗  7.02c∗∗
0.8
1.68
1.65
3.04a∗  3.00b∗  2.78c∗
9.24a∗  9.00b∗  7.72c∗
2.91a∗∗  2.79b∗∗  2.68c∗∗
8.46a∗∗  7.78b∗∗  7.18c∗∗
1
1.63
1.61
3.07a∗  3.02b∗  2.81c∗
9.42a∗  9.12b∗  7.89c∗
2.93a∗∗  2.82b∗∗  2.71c∗∗
8.58a∗∗  7.95b∗∗  7.34c∗∗
aRef. [51]; bref. [52]; cref. [53].
∗refers to Band gap using UV.
∗
∗refers to Band gap using PL.Figure 5
PL of Cu2Zn1−xCdxSnS4 quinternary alloy nanofibres at different Cd concentrations (x = 0–1).The absorption coefficient associated with the high absorption region was calculated from absorbance (A) and the film thickness (t) using the following relation:(4)a=2.303At.Figure 6 shows the absorption coefficient (α) of Cu2Zn1−xCdxSnS4 quinternary alloy nanofibres deposited at different Cd concentration (x = 0 to 1). It can be observed that the absorption coefficient was greater than 104 cm−1. Thus, it would be better to infer that the grown nanostructures have good absorbance in the visible region of electromagnetic spectrum. The absorption spectra are shifted towards longer wavelengths as Cd concentration increases. The fundamental absorption corresponds to the transition from valence to conduction band that can be used to determine the band gap of the material. The energy band gap (Eg) was estimated using(5)hναγ=βhν-Eg,where hν is the incident photon energy, β is parameter, and γ is index which characterizes the optical absorption process (γ=2 and 1/2 for indirect and direct transitions, resp.). The best linear fit was obtained for γ=1/2 for direct transition. Eg was determined by extrapolating the straight line portion hναγ = 0 value and showed a decrease from 1.81 eV for x=0 to 1.80 eV for x=0.2, 1.79 eV for x=0.4, 1.69 eV for x=0.6, 1.68 for x=0.8, and 1.63 for x=1, as illustrated in Figure 7 and given in Table 2. It was found that the energy band gap of Cu2Zn1−xCdxSnS4 quinternary alloy nanofibres drifts towards lower energy with an increase in Cd concentration. Thus, the optical properties of CZCTS absorber layers can be improved by substituting the Zn atoms by Cd atoms to produce a lower energy gap [34].Figure 6
Absorption coefficient of Cu2Zn1−xCdxSnS4 quinternary alloy nanofibres at different Cd concentrations (x = 0–1).Figure 7
Optical band gap of Cu2Zn1−xCdxSnS4 quinternary alloy nanofibres at different Cd concentrations (x = 0–1).The refractive indexn is a significant physical parameter in microscopic atomic interactions. Theoretically, the refractive index is related to the density and the local polarizability of these entities [50]. Many simple relationships between refractive index n and the energy gap Eg have been attempted [51–58]. Here, various relationships between n and Eg have been reviewed in order to validate the current work. As suggested by Ravindra et al. [51], the band gap and the high frequency refractive index presented a linear relationship: (6)n=α+βEg,where α = 4.048 and β = −0.62 eV−1Inspired by simple physics of light refraction and dispersion, Hervé and Vandamme [52] have proposed an empirical relation as(7)n=1+AEg+B2,where A=13.6 eV and B=3.4 eV.Ghosh et al. [53] had taken a different approach by considering the band structural and quantum-dielectric formulations of Penn [59] and Van Vechten [60]. Introducing A (contribution from the valence electrons) and B (constant additive to the lowest band gap Eg), the expression was written as(8)n2-1=AEg+B2,where A=25Eg+212, B=0.21Eg+4.25, and (Eg+B) refers to an appropriate average energy gap of the material.Thus, these three models for variation ofn with energy gap have been tried. In addition, the calculated values of the optical dielectric constant (ε∞) were obtained using the relation ε∞=n2 [61]. The calculated refractive index and optical dielectric constant are given in Table 2. This is showing that the Ghosh et al. model is an appropriate model for solar cells applications.Figure8 shows the transmittance spectra of the Cu2Zn1−xCdxSnS4 quinternary alloy nanofibres at different Cd concentrations. The transmittance of the nanofibres decreases as Cd concentration increases, which is attributed to the increasing absorbance. The highest value of transmittance was 78% at x=0 and the lowest was 33% at x=1, with a shift in transmittance towards high wavelengths as Cd concentration increases.Figure 8
Transmittance spectra of Cu2Zn1−xCdxSnS4 quinternary alloy nanofibres at different Cd concentrations (x = 0–1).


## 3.4.I-V Characteristics

To study the performance of CZCTS-based solar cells, the optimal prototype device with glass/ZnO:Al/CdS/Cu2Zn0.8Cd0.2SnS4/Al structure was prepared (Figure 1). I-V curve was studied under 100 mW/cm2 via simulated AM 1.5 G solar illuminations (Figure 9). Our results suggest that Cu2Zn1−xCdxSnS4 at x equals 0.6, has an optimal band gap, and can be a suitable material as absorber layer in single junction solar cells. Although the power conversion efficiency (3%) is low without elaborate optimization (Figure 9), our preliminary results suggest that the performance of CZCTS-based solar cells is quite tunable at varied Cd concentration [34]. It is of prime importance to consider the trade-off between various parameters and the design of tandem solar cells with the optimal performance.Figure 9
I-V characteristics for the glass/ZnO:Al/CdS/Cu2Zn0.4Cd0.6SnS4/Al solar cell at Cd concentration x=0.6.


## 4. Conclusion

In this work, Cu2Zn1−xCdxSnS4 quinternary alloys nanofibres were successfully fabricated via electrospinning technique. Our findings suggest that the Cd concentrations correlate directly with the lattice constant and bulk modulus that reflect the stiffness of Cu2Zn1−xCdxSnS4 quinternary alloys nanofibres. The diameters of nanofibres were found to be in between 58.615 and 442.85 nm at x=0 and 1, respectively. The band gap is found to decrease with increasing Cd concentrations. The power conversion efficiency of the Cu2Zn1−xCdxSnS4-based solar cells with Cd concentration, x = 0.6, is 3%. Ghosh et al. model is found to be more appropriate for solar cells application.



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*Source: 7314714-2016-03-22.xml*