Investigations of the structural, optoelectronic and band alignment properties of Cu2ZnSnS4 prepared by hot-injection method towards low-cost photovoltaic applications

Abstract Cu2ZnSnS4 is a promising, versatile and inexpensive quaternary semiconductor with suitable optoelectronic properties for solar energy conversion. In this work, we report the synthesis of CZTS nanocrystals (NCs) using low-cost homemade hot-injection method. Oleylamine was used as both the binder and stabilizer for the CZTS NCs during the growth process. Detailed investigation of the influence of sulphur concentration and reaction temperature on the structural, stoichiometric, morphological, and optoelectronic attributes of CZTS NCs was carried out. The XRD, Raman, and TEM measurements confirm the formation of phase-pure tetragonal kesterite CZTS NCs. The synthesized CZTS NCs exhibit particle sizes in the range of 15–30 nm and display strong optical absorption in the visible region. The nearly optimal chemical composition of the CZTS NCs was confirmed by energy dispersive X-ray spectroscopy. UV–Visible spectroscopy and electrochemical measurements predicts the band gap of the CZTS NCs in the range of 1.3–1.6 eV, which is very close to the optimum values for the fabrication of single junction solar cells. The estimated conduction band offset (CBO) and valence band offset (VBO) of the CZTS-3M/CdS heterostructure are predicted as 0.11 and 0.98 eV, respectively, whereas for CZTS-225 °C/CdS heterostructure, CBO and VBO are 0.10 and 1.0 eV, respectively. The small conduction band offset measured at the CZTS/CdS interface are encouraging characteristics for the carrier transport and the deeper understating of band alignment and interface properties provides a hopeful approach for designing higher efficiency and more efficient carrier separation in CZTS solar cells.


Introduction
Photovoltaic (PV) technology, which makes use of the superabundant and freely available Sun's energy to generate electricity, has obvious economic, environmental and societal benefits. However, for PV technology to provide a significant fraction of the world's energy demands, devices must be composed of cheap and readily available materials [1]. In thin film technologies, CIGS, CdTe and thin film silicon solar cells have exhibited 10e25% efficiencies [2e5], but the high cost of solar cell fabrication, particularly that of silicon, and the degradation of the performance over time remain major issues, which prevent their wide-spread deployment into the power grid scale applications [6]. Emerging next generation solar cells based on chalcogenide thin films like CIGS (CuInGaSe 2 ) and CdTe, show terrestrial cell efficiencies of 17.5e21.7% [7,8], but the scarcity, cost and toxicity associated with the In, Ga, and Cd elements present in these cells limit their sustainability in the future. Among the alternative absorber materials to replace CIGS and CdTe, kesterite-copper-zinc-tin chalcogenides Cu 2 ZnSnS 4 (CZTS) and Cu 2 ZnSnSe 4 (CZTSe) have recently emerged as promising candidates to have the capacity to meet or exceed annual worldwide electricity consumption with a significant cost-reduction [9e12]. Kesterite-CZTS compounds are ideally positioned as a next generation PV materials because they combine: (i) near optimum direct bandgaps (1.5 eV for CZTS and 1.13 eV for CZTSSe), (ii) high optical absorption coefficient of~10 4 cm À1 in the visible light region, with predicted theoretical power conversion efficiency (PCE) more than 30% [9,13], and (iii) component elements that are earth-abundant, cheap, and non-toxic. These characteristics make CZTS absorbers promising candidates for scalable production of cost-effective and environment-friendly thin-film photovoltaics [14e16]. In recent years, with increasing research efforts that have focused on improving the stability, interface properties and charge carrier dynamics of CZTS materials, promising power conversion efficiencies exceeding 10% have been achieved [10,11,17].
The development of simple and cost-effective synthesis approaches to fabricate CZTS thin films have also received significant attention. Compared to vacuum based techniques, non-vacuum based chemical techniques for the synthesis of CZTS thin films, are characterized by low-cost and high throughput [18e20]. Yang, C. et al., reported more than 10% efficiency for a CZTS solar cell. With heat treatment, they controlled the charge carrier recombination at the p-n junction and achieved 11% certified efficiency for CZTS solar cell with 730 mV open-circuit voltage [21]. Thin films prepared by solution based non-vacuum methods have demonstrated the highest power conversion efficiency of 12.6% for CZTSe devices [11,22]. The commonly employed physical and chemical techniques used for the synthesis of CZTS material includes RF magnetron sputtering [23], thermal evaporation [24], pulsed laser deposition [25], e-beam evaporation [26], spray pyrolysis [27], sol-gel method [28], electro-deposition [29], etc. Among the above mentioned synthesis techniques, the hot injection method (HIM) has received a great interest for the synthesis of high-quality CZTS nanocrystals. The simplicity, safety, and low-cost associated with the hot injection method makes it suitable for large-scale production of CZTS NCs. Riha et al. reported the synthesis of mono-disperse CZTS NCs using stoichiometric initial precursor concentration at the synthesis temperature of 300 C by hot injection method [30]. The effect of process temperatures on particle size in the HIM was investigated by Chernomordik et al. [31]. Wang et al. achieve power conversion efficiency of 12.6% for kesterite CZTSSe fabricated using the hydrazine pure-solution approach [11]. The challenges in kesterite photovoltaics has been summarized in recent reports and mentioned very important issues, such as massive deep defects, uncontrolled grain growth, band trailing, unoptimized interfaces, which need to be addressed for better efficiency [32,33]. There are extensive literature reports on the use of CdS as a buffer layer (n-type material) for CZTS solar cell, and all reports provides strong evidence that CdS has all essential properties to form desirable junction with CZTS to achieve efficient carrier transport [34e38]. A deep understating of band alignment and interface properties (band offset) is, however, needed to achieve proper optimization of the CZTS/CdS heterojunction towards improved charge carrier dynamics and device performance. Rondiya et al. reported conduction and valence band offset of 0.10 and 1.21 eV, respectively, from CV measurements and 0.28 and 1.15 eV from the density functional theory calculations [39].
The structural, morphological, and opto-electrical properties of CZTS NCs synthesized by the hot injection method can be controlled by optimizing synthesis parameters such as process temperature, injection temperature, reaction time, precursor concentrations etc. In this communication, we report the successful synthesis of high-quality CZTS NCs in oleylamine (OLA) via the hot injection method. We have primarily focused on optimizing the stoichiometry of the CZTS by varying sulphur concentration during synthesis. Also, the influence of reaction temperature on the structural, morphological, and opto-electrical properties of CZTS NCs was comprehensively investigated and discussed. From our predicted energy band alignment between the CZTS absorber and commonly used CdS buffer material, we provided insights into the transport mechanism of photogenerated charge carriers across the CZTS/CdS interface. The influence of reaction temperature on the crystal structure, which further affects other physicochemical and optoelectronic properties of the CZTS NCs is unraveled. The study provides further understanding of the versatility of CZTS, not just as a traditional absorber layer, but as a charge transport layer.

Synthesis of high-quality CZTS nanocrystals
The colloidal CZTS NCs were synthesized by the hot injection method. Oleylamine solution was used as the solvent, surfactant, and capping ligand. Cupric sulphate (CuSO 4 $5H 2 O), zinc sulphate (ZnSO 4 $7H 2 O), and stannous sulphate (SnSO 4 ) were used as sources for Cu, Zn, and Sn, respectively. 0.15 M (CuSo 4 .5H 2 O), 0.075 M (ZnSO 4 $7H 2 O) 0.075 M (SnSO 4 ), and 10 mL of oleylamine were mixed in a three-neck flask at room temperature. The first neck of flask is connected to vacuum and argon gas via a two-way connector, the second neck was used for injection of elemental sulphur solution and the third neck of flask was used for insertion of a thermocouple to monitor reaction temperature. The resulting solution was heated to 130 C for 1 h with constant stirring in alternatingly vacuum and argon gas environment, which ensured the complete elimination of air and moisture from the reaction zone. Once the CueZneSn-oleylamine complex turned into a light brownish solution, the reaction temperature was raised to 225 C. Meanwhile, another solution was prepared, for which 1 M sulphur powder was mixed with 5 mL oleylamine in a glass beaker. This sulphur solution was stirred and heated at constant temperature of 80 C until all the sulphur powder was completely dissolved, and the solution turned to red color. The resulting heated sulphur precursor solution was rapidly injected into the preheated CueZneSn-oleylamine complex solution, which immediately change the color from dark brown to black. The schematic of the experimental setup is shown in Scheme 1. The resulting mixture was maintained at the temperature of 225 C for half an hour, forming a homogeneous black solution. After the reaction was complete, the three-neck flask was taken out from the heating bath and put in a cold-water bath for cooling. The same experiments were repeated in this set for 2, 3, 4 and 5 M sulphur concentrations. To get rid of impurities and unreacted precursors from the CZTS NCs, it was precipitated using 1:8 toluene to propanol mixture. The colloidal CZTS NCs were recovered from the solution by centrifugation at 4000 rpm. The recovered NCs were annealed at 350 C for 1 h and finally taken for characterization. In an another set of experiments, the reaction temperature was systematically varied from 200 C to 300 C, while keeping all other parameters constant as listed in Table 1. The experimental procedure was same for this second set of the experiments in which 4 M sulphur solution were injected into CueZneSn-oleylamine complex solution.

Characterization techniques
The synthesized Cu 2 ZnSnS 4 NCs were systematically investigated using different characterization techniques namely, X-ray diffraction (XRD), Raman spectroscopy, UVeVisible spectroscopy, Energy dispersive X-ray spectroscopy (EDS), field emission scanning electron microscopy (FESEM) and scanning electron microscopy (SEM). The average crystallite size, lattice parameter and phase identification of the Cu 2 ZnSnS 4 NCs were studied using x-ray diffraction (Bruker D8 Advance, Germany make) with Cu Ka radiation (l ¼ 1.54 Å). The phase characterization of the as-synthesized samples was obtained by the Raman spectroscopy with "Renishaw Raman Microscope" using argon laser wavelength of 532 nm, in the back-scattering geometry". The optical bandgap of Cu 2 ZnSnS 4 NCs were estimated from absorbance spectra measured using a JASCO, V-670 UVevisible spectrophotometer in the range of 200e1400 nm. The Scanning electron microscope (SEM) characterization was performed using the JEOL JSM-6360-LA instrument. The electrochemical measurements for the estimation of band edges and electrochemical band gaps were performed using Metrohm Galvanostat/Potentiostat (Auto lab 100 PGSTAT). The standard 3 electrode system having Au disk, Ag wire, and Pt wire loop as working, reference and counter electrode, respectively. The working electrode is cleaned/polished over the 0.5 mm Al 2 O 3 powder and rinsed with copious amount of DI water, followed by potentiodynamically cycling in 0.5 M H 2 SO 4 , in the potential window (À0.8Ve1.5V) to ensure the clean surface of gold. In a typical experiments, the predried 0.341 g of TBAP typically 100 mM in 15 mL pre-dried ACN was transferred to electrochemical cell through silicone septa. The blank cyclic voltammogram were recorded for the reference in TBAP-ACN mixture to ensure there were no peaks corresponding to contamination on the Au electrode. Subsequently the Au electrode was loaded with 100 mL CZTS sample dispersion in ACN (1.0 mg/mL) and vacuum dried. The scan rate was kept constant (100 mV/s) for all measurements. After completion of each set of experiments, the potentials were calibrated using ferrocene as an internal standard with respect to the normal hydrogen electrode (NHE). The band gaps are estimated directly from the cathodic and anodic peak difference. The band edges are extracted from the oxidation and reduction peak potential vs. NHE. These positions are then converted with respect to local vacuum by the standard difference of À4.5 vs NHE [13,40].

Structural properties
The XRD patterns of the CZTS NCs synthesized at different sulphur concentrations (1e5 M) and reaction temperatures (200 Ce300 C) respectively, are shown in Fig. 1 (a) and 1 (b). The major diffraction peaks located at 2q ¼ 28.   [41,42]. It is also evident from Fig. 1 (b) that the intensity and sharpness of the (112) plane increases with reaction temperature (up to 250 C), suggesting that the crystallinity of the CZTS NCs improved gradually with increasing reaction temperature. The observed high intensity and broad shoulder peak located at 2q ¼ 28.9 in all samples reflects the preferential growth of CZTS along the (112) plane. The variation in crystallite size (d x-ray ) of the synthesized CZTS NCs are calculated by measuring the full-width half maxima (FWHM) corresponding to (112) planes using the Debye-Scherrer formula [43]. The calculated crystallite size of the CZTS NCs for set-I and set-II was found to be in the range of 12 nme28 nm. The maximum crystalline size is observed at 28.5 nm corresponding to the 3 M sulphur concentration in set-1. The observed variations in crystallite size in both sets, can be attributed to the non-uniform lattice strain or different molar concentration of the reaction. Besides, the nanocrystal size distribution in the hot injection method depends on the process temperature, injection temperature of the sulphur source, reactivity and concentrations of the CueZneSn and S precursors, and the type of solvent used [44]. The inter-planar distance of the CZTS NCs for the first-order diffraction is estimated at 3.132 Å, which is very well matched with earlier reported by value of 3.12 Å [45e47]. The lattice constants of CZTS NC for tetragonal structure was calculated using equation (1): where, d is interplanar distance, a and c are a lattice constants and h, k, l are the planes. Shown in Table 2 are the structural parameters of the CZTS NCs prepared with varying sulphur concentration and reaction temperature. The estimated lattice constants for the CZTS kesterite tetragonal structure show good agreement with reported values in the literature [48]. It is clear from the predicted structural parameters coupled with the recorded XRD pattern that all the synthesized CZTS NCs have good crystallinity. Raman spectroscopy is a unique and powerful characterization technique for the identification secondary phases in a wide range of Raman active materials. Fig. 1 (c) and 1 (d) show the Raman spectra of the CZTS NCs synthesized at different sulphur concentration and reaction temperatures, respectively. CZTS kesterite structures are typically characterized by three main Raman peaks located at 286 cm À1 , 336 cm À1 , and 371 cm À1 [49]. Among these peaks, the strongest peak of CZTS appeared at 336 cm À1 , which can be assigned to the A 1 symmetry mode sulphur-metal vibrations in the kesterite lattice [50]. Consistent with previous assignments, we observed two major peaks at 283.7 cm À1 and 334.9 cm À1 (Fig. 3c) for varying sulphur concentrations, which can be assigned to Raman active mode of the tetragonal kesterite CZTS structure [51]. The high-intensity Raman peak located at 334.9 cm À1 , corresponds to the A 1 symmetry mode of a single-phase CZTS NCs [50,52,53]. Similar to the varying sulphur concentrations, the Raman spectra of the CZTS NCs synthesized at varying reaction temperatures (200 Ce300 C) show Raman peaks at 283.7 cm À1 and 333 cm À1 . However, the peak intensity decreases and the most intense peak of CZTS NCs shift from 335 cm À1 to 332 cm À1 with increasing reaction temperature as shown in Fig. 1 (d). The absence of the vibrational modes of cubic ZnS (352 and 275 cm À1 ) and orthorhombic Cu 3 SnS 3 (318 cm À1 ) confirm the formation of single-phase CZTS by the hot injection method [54,55]. Moreover, energy dispersive X-ray spectroscopy (EDS) composition analysis (Table 3) confirmed that both sets of CZTS NCs have nearly optimal stoichiometric Cu:Zn:Sn:S ratios in all samples. Fig. 1 (e) shows the lower magnification TEM image of the CZTS NCs synthesized with 4 M sulphur concentration, which reveals that the particles are slightly polydispersed. The selected area electron diffraction (SAED) pattern of a CZTS NCs shown in Fig. 1 (f) reveal bright spot patterns that are very well matched to the planes observed from the X-ray diffraction spectra. The average size of the CZTS NCs is found to be 21 nm, also in good agreement with the X-ray diffraction data.

Optical properties
The performance of solar cell depends sensitively on the optical properties such as optimum band gap and high absorption coefficient. The optical absorption coefficients of the CZTS NCs were studied in the range of 200e1400 nm. The UVeVisible spectra are recorded on the CZTS NCs dispersed in dimethyl sulfoxide (DMSO). Shown in Figure (2a and 2b) are the absorption spectra of the CZTS NCs synthesized at different sulphur concentration (1 Me5 M) and different reaction temperatures (200 Ce300 C). All samples show strong absorption in the visible region, indicating that CZTS is a good absorber material in agreement with previous reports [56e58]. The reflectance spectra for the CZTS nanocrystal samples with varying sulphur concentration and reaction temperatures are shown in Fig. 2c and d, respectively. For varying sulphur concentration (Fig. 2c), the lowest reflectance is observed for the samples prepared with 5 M concentration whereas for the temperature variation, the lowest reflectance is observed for 250 C in the wavelength range of 200e1400 nm. CZTS is a direct band gap material, based on the allowed direct inter band transition. We have estimated the band gap of the synthesized CZTS material using Tauc's relation as reported previously [43] and shown in Fig. 2e and f. The overall band gap for the different sulphur  concentration synthesized CZTS materials was found to be in the range of 1.25e1.55 eV as shown in Table 4. As can be seen from  Table 4), which is well matched with previous experimental reports [57,59e64] and theoretical predictions [65]. The estimated band gaps fall in the optimum band gap range for solar energy conversion in a single-junction device.
The refractive index (n) and extinction coefficient (K 0 ), which are important properties for optical device and solar cell applications were also determined for the as-synthesized CZTS NCs. n is related to the electric polarizability of ions and local field into the material [66,67] whereas the K 0 gives a measures of the fraction of light loss due to scattering and absorption per unit distance in materials [68]. The refractive index (n) of the CZTS NCs was calculated from the reflectance (R) and extinction coefficient (K 0 ) using equations (2) and (3).
where, R is the reflectance, K 0 is the extinction coefficient, a is the absorption coefficient and l is the wavelength. The graph of refractive index (n) and photon energy (հʋ) is shown in Fig. 3 (a) and (b) synthesized CZTS samples with varying sulphur concentration and reaction temperatures, respectively. For sulphur concentration variation (Fig. 3a), the refractive index changed from 10.5 to 3 when the photon energy is varied from 0.6 to 2.5 eV and remains steady for higher photon energies.
In the reaction temperature variation set, the refractive index increases with increasing reaction temperature of CZTS NCs, especially 200 C, 225 C, 250 C and 300 C. The refractive index varies from 9.0 to 2.5, when photon energy varies from 0.6 to 2.0 eV. The higher refractive index is attributed to the crystallinity in the CZTS NCs, which is evident for the 3 M sulphur concentration in first set and 250 C reaction temperature in second set. By increasing the sulphur concentration the material becomes denser. So, increasing densification of material causes a decrease in the velocity of the light into the material and leads to increase in the refractive index of the materials [69]. These finding agrees with the X-ray diffraction analysis and further applauded by SEM observation of CZTS NCs. The SEM morphology shows a homogeneous and highly dense surface. Similar observations are also noted in the literature data [70,71]. Fig. 3(c and d) shows the extinction coefficient of CZTS NCs. The extinction coefficient of CZTS NCs increases from 1.0 eV to 2.5 eV photon energy range and decrease in the range of 2.5e4.0 eV photon energy. In the sulphur concentration variation set, the extinction coefficient increases with increasing sulphur concentration, especially 1 M, 2 M, 4 M and 5 M and in the reaction temperature set, the extinction coefficient increases with increasing reaction temperature for 225 C and 275 C. The values of K 0 are ranging from 0.0002 to 0.0009. These values of extinction coefficient are low in comparison with reported results, the lower  values are probably due to the weak absorption in the CZTS NCs. The high and low value of the extinction coefficient have direct relation to the absorption of light. The dielectric constant is one of the important intrinsic parameters of the semiconductor to describe the optical behavior of the material for application in solar cell. The dielectric constant has two parts i.e. real part of dielectric constant (ε r ) and imaginary part of the dielectric constant (ε i ). The real part of dielectric constant signifies deceleration of the incident light velocity and imaginary part of dielectric constant signifies interaction of matter from an electric field due to the dipole moment. The real part of dielectric constant (ε r ) and imaginary part of the dielectric constant (ε i ) was calculated by using the values of the refractive index (n) and extinction coefficient (K 0 ) by using following equations (4) and (5) [72e74]: Fig. 4aed shows the variation of ε r and ε i for the sulphur concentration and process temperature variation as a function of photon energy (հʋ). The real part dielectric constant increases from 10 to 130 for both sets of samples. It is interesting to note that the values of the real part of dielectric constant are higher than that of imaginary part of dielectric constant and both decreases exponentially with increasing photon energy. The optical response of the material is explicitly studied in term of the optical conductivity. The optical conductivity (s) of the synthesized CZTS NCs was calculated using the relation: s ¼ anc 4p , where, a is absorption coefficient, n is refractive index, and c is speed of light. Fig. 4(e and f) shows the plot of optical conductivity (s) against photon energy (eV) for the different sulphur concentration and different reaction temperature. It is evident from Fig. 4 (e, f) that the optical conductivity of both sets of samples have very large optical conductivity in the lower wavelength range. These high optical conductivity values can be attributed to the high refractive index and high absorption coefficient of the CZTS NCs. The nature of the plot of "s opt " is attributed to the photo-excited electrons [75].

Morphological properties
The low-resolution and high-resolution Field Emission Scanning Electron Microscope (FESEM) images of the CZTS NCs synthesized at varying sulphur concentration (1e5 M) and reaction temperatures (200e300 C) are presented in Fig. 5a and b, respectively, where we observe significant morphological differences in the CZTS NCs. With increasing sulphur concentration and reaction temperatures, we observed large agglomeration of grains, which has potential implications for photovoltaic application, as larger grains help to reduce recombination rate charge carriers [64,65,76,77]. It is clear from the recorded morphological images of the CZTS powders that the surface morphology is strongly influenced by both the reaction temperature and sulphur molar concentration.

Cyclic voltammetric measurements
Cyclic voltammetry measurements have been used to estimate the band structure parameters of the prepared CZTS NCs. An argon atmosphere is maintained throughout the electrochemical measurements. In the electrochemical cell, 15 mL of dry acetonitrile (ACN) is used as solvent, 100 mM TBAP as supporting electrolyte and 1 mg/mL solution of CZTS sample in ACN is used to drop-cast on the electrode and used as the working electrode. The blank CV measurement was taken on a bare Au disk electrode and the CV measurement for the CZTS was recorded by drop casting 100 mL CZTS solution on the working electrode, which is dried in vacuum desiccator. Fig. 6 (a) shows the typical CV pattern for the CZTS 225 C -blue line and CZTS 3 M À red line, drop-casted on the gold electrode. The scan-rate was 100 mV/s. For comparison, the blank CV recorded on the bare Au electrode (without CZTS loading) is shown as black line. For the CZTS prepared at 225 C, a prominent anodic peak at 0.92 V (labelled as A 1 ) and cathodic peak À0.60 V (labelled as C 1 ), over and above the background trace is noted in the repeated cycles. Similarly, for the CZTS prepared using the sulphur precursor concentration of 3 M, we have noticed prominent cathodic and anodic peaks at À0.61 V and 0.94 V, respectively. Interestingly, in the 3 M CZTS sample we have observed one additional complementary peak at 0.80 V in the reverse cycle, which corresponds to anodic peak at 0.94 V. The observed complementary peak at 0.80 V in the cyclic voltammogram shows that the CZTS NCs gains electron after oxidation at 0.94 V in the reverse cycle i.e., the CZTS NCs donate electron to the electrode (0.80 V) and get it back in the reverse cycle (0.94 V). These results suggest that the CZTS NCs are more stable in charge transfer condition [78,79]. The oxidation and reduction peaks in the CV are nothing but the electron transfer (donation and acceptance) from valence band and conduction band, which happens because of the applied bias/potential. From the anodic and cathodic peaks, we have deduced the values of valence and conduction band edges (Table 5), which are in very good agreement with previous spectroscopic data [40]. The electrochemical (EC) band gap calculated as the potential difference between the anodic peak A 1 and cathodic peak C 1 is estimated at be 1.55 V for CZTS-225 C (blue line) and 1.52 V for CZTS-3M (red line). The close agreement between the estimated band gaps obtained from the CV and UVeVisible NIR measurements confirmed the formation of high-quality CZTS NCs by the hot injection method.

Band alignment investigation at CZTS/CdS interface
Cadmium sulfide (CdS) is a commonly used buffer layer materials for CZTS-based solar devices, hence we have determined the energy band alignment at the CZTS/CdS interface. The magnitude of band offsets controls transport phenomena at the interfaces and characteristics of devices hence the need for their accurate determination. Using the electrochemical band gap and electron affinities for the CZTS (3 M) and CZTS (225 C) as summarized in Table 5, the band offsets can be estimated. The band gap and electron affinity for the n-type CdS are 2.42 and 4.0 eV, respectively [80]. Shown in Fig. 6 (b) is the schematic energy band alignment at the CZTS-3M/CdS and CZTS-225 C/CdS interface after heterojunction formation, revealing a type-II band alignment. The conduction band offset (CBO) and valence band offset (VBO) predicted at 0.11 and 0.98 eV, respectively, at the CZTS (3 M)/CdS interface. For the CZTS-225 C/CdS heterojunction, the CBO and VBO are predicted at 0.10 and 1.0 eV, respectively. The estimated small CBO at the both heterojunctions suggests lesser resistance to electron transport across the CZTS/CdS interface, and hence expected to favor efficient charge carrier separation towards achieving efficient CZTS-based solar devices.

Conclusion
Tetragonal kesterite Cu 2 ZnSnS 4 (CZTS) NCs were successfully synthesized by the hot injection method. The as-synthesized CZTS NCs were comprehensively characterized using a range of techniques such as XRD, Raman, UVeVis Spectroscopy, FESEM and EDS all of which confirmed the formation of phase-pure tetragonal kesterite CZTS. The nearly optimal chemical composition of the CZTS NCs was confirmed by our energy dispersive X-ray spectroscopy (EDS) composition analysis. The estimated band gaps from UVeVisible NIR and electrochemical CV measurements are in the range of 1.3e1.6 eV, which is very close to the optimum value for the fabrication of single junction thin film solar cells. Besides, the CZTS samples show strong absorption in the visible region, indicating their suitability as active absorber layer material. We determined a type-II staggered band alignment at the CZTS/CdS heterojunction with smaller conduction band offset (CBO)~0.1 eV, which is suitable for efficient separation of photo-excited electrons to the CdS and holes to holes to CZTS. We believe that the controlled synthesis parameters for the hot injection method and the generated data presented here will have immediate applications in the synthesis of high-quality CZTS-based materials for device fabrication.

Funding
This research was funded by the UK's Engineering and Physical Sciences Research Council (EPSRC), grant number EP/S001395/1.

CRediT authorship contribution statement
Bharat R. Bade: completed all the samples synthesis, All authors discussed the results, contributed to manuscript write up, and reviewed the manuscript. Sachin R. Rondiya: Conceptualization, Writing -original draft, All authors discussed the results, contributed to manuscript write up, and reviewed the manuscript. Yogesh A. Jadhav: completed all the samples synthesis, All authors discussed the results, contributed to manuscript write up, and reviewed the manuscript. Mahesh M. Kamble: completed all the samples synthesis, All authors discussed the results, contributed to manuscript write up, and reviewed the manuscript. Sunil V. Barma: completed all the samples synthesis, All authors discussed the results, contributed to manuscript write up, and reviewed the manuscript. Sagar B. Jathar: completed all the samples synthesis,