Solution-processed CZTS thin films and its simulation study for solar cell applications with ZnTe as the buffer layer

Using zinc tellurium (ZnTe) as the buffer layer in the Cu2ZnSnS4 (CZTS)-based solar cells showed an improvement in overall efficiency. ZnTe is investigated as an alternative to replace the conventional toxic Cd-contained buffer layers. It may also reduce the overall cost of these cells as both layers (ZnTe and CZTS) have eco-friendly and earth-abundant constituents. The sol–gel spin coating method is used for the deposition of CZTS thin films on the corning glass substrates. The X-ray diffraction studies showed the peaks corresponding to (112), (200), (220), and (312) planes which confirmed the formation of the essential kesterite phase. The optical band gap of the deposited films was found at around 1.45 eV by the UV–visible-NIR spectrophotometer. The optimum thickness of the absorber layer (CZTS) and buffer layer (ZnTe) was investigated based on the performance of the ZnO:Al/ZnO/ZnTe/CZTS/Mo cell structure by using the AMPS-1D simulation tool. In contrast, the tool was molded by the experimentally investigated data for the constituent materials of the cell structure. The solar cells’ efficiency was increased by 23.47% at 2500 nm and 50 nm thickness of the CZTS and ZnTe layers, respectively. In addition, it was analyzed and found that the current density value showed an improvement with operating temperature as it is one of the requirements in the high solar radiation areas where the temperature even rises more than 50 °C in the summer.


Introduction
Photovoltaic (PV) technology is an alternative approach to fossil fuels for providing the next generation the safe, secure, sustainable, and affordable energy (Butler 2008).The challenges involve reducing costs and improving power conversion efficiency (PCE).Silicon-based solar cells with more than 26.7% efficiency have dominated the terrestrial solar panel market.Still, due to high parasitic absorption losses, PCE for silicon solar cell is limited (Yoshikawa et al. 2017).Organic solar cells, which are less expensive and flexible, have a reported efficiency of around 11.2% (Mori et al. 2014).However, longterm stability and durability are severe problems for these cells.Furthermore, perovskite structure-based solar cells with PCE of 22.1% (Yang et al. 2015) and perovskite/silicon tandem solar cells with PCE of 23.1% also have been developed (Bush et al. 2017).Nevertheless, the large-scale production of these cells is a big challenge for researchers.In this connection, thin-film chalcopyrite PV technology offers large-scale production with high efficiency (22.6%) (Kato et al. 2017).These technologies are primarily based on CuInGaSe 2 (CIGS) and CuInSe 2-x S x and are highly stable, but the high cost of rareearth elements, i.e., indium and gallium, is the main obstacle to the mass production of CIGS PV.
Recently, Cu 2 ZnSnS 4 (CZTS) has emerged as an attractive alternative to CIGS due to its similar crystal structure, nontoxic, cost-effective, and earth-abundant elemental composition (Rawat and Shishodia 2016).The CZTS has wide acceptability in the photovoltaic industry due to its mentioned magnificent properties with the optimum band gap of around 1.4 eV (Jiahui et al. 2017;Khalate et al. 2018, Jirage andBhuse 2019).
Several physical and chemical routes for the deposition of CZTS thin films have been reported (Yan et al. 2017;Schelhas et al. 2017, Seto andAraki 2017).Vacuum-based deposition techniques are popular methods for preparing chalcogenide absorber layers and have resulted in highly efficient CZTS PV devices.Yan et al. (2017) achieved 11.5% efficiency of cells by using the RF magnetron sputtering technique of CZTS film deposition (Yan et al. 2017).Miyazaki et al. (2017) extensively analyzed the surface of CZTS films achieved by RF magnetron sputtering (Miyazaki et al. 2017).Here, the major challenge in commercializing CZTS solar cells is the involvement of high capital costs in vacuum technologies.Therefore, recent research on CZTS solar cells has focused on developing solution-based approaches in ambient conditions.Cho et al. (2013) prepared CZTS thin films from precursor paste using the spin-coating technique on a Mo-covered glass substrate.The deposited films were annealed in the air ambient and showed densely packaged morphology.Here, CdS was used as an essential buffer layer.The efficiency was achieved at around 3%, and the open circuit voltage (V OC ), short circuit current (J SC ), and fill factor (FF) were measured to be 556 mV, 13.5 mA/ cm 2 , and 40.3% respectively (Cho et al. 2013).Chen et al. (2015) deposited CZTS film using a solution-based process while ethanol was the primary solvent.They used it as a counter electrode for dye-sensitized solar cells (Chen et al. 2015).Larramona et al. (2015) deposited CZTS films by ultrasonic spray, and PCE was reported at around 10.8% with 510 mV, 32.5 mA/cm 2 , and 65% values of V OC , J SC , and FF respectively.Non-hydrazine solvents were used while the toxic CdS buffer layer was still present (Larramona et al. 2015).Su et al. (2015) spin-coated full sulfur CZTS films by sol-gel route using 2-methoxy-ethanol as an essential solvent and found the efficiency around 9.24% while V OC , J SC , and FF were measured at 581 mV, 24.1 mA/cm 2 , and 66% respectively (Su et al. 2015).Guchhait et al. (2016) studied Ag doping in CZTS films spin-coated by the sol-gel route, and it was found that the incorporation of silver enhanced grain growth and improved carrier lifetime.The PCE for these structures was found at 7.2% (Guchhait et al. 2016).Thus, solution-based fabrication techniques are preferred in the efforts to reduce the overall cost and manufacturing on a large commercial scale (Xiong et al. 2020;Lin et al. 2020).However, commonly investigated CZTS solar structure, i.e., transparent conducting oxide (TCO)/i-ZnO/CdS/CZTS/Mo/ glass, includes CdS as a buffer layer (Mahajan et al. 2017, Jhuma andRashid 2020).Moreover, CdZnS is also investigated as the buffer layer (Mahajan et al. 2017;Zhang et al. 2022); these all have Cd as an essential element.Cd is considered a highly toxic material for health and the environment; it is expected to avoid using cadmium.Thus, there is an urgent need to find an alternative to Cd-contained buffer layers.Some metal sulfides such as ZnS and SnS 2 are also explored to replace the Cd-limited buffer layer (Mahajan et al. 2017, Jhuma and Rashid 2020, Liu et al. 2021, Tripathi et al. 2021).These metal sulfides are promising materials for the buffer layer and have demonstrated high efficiency for the CZTS-based solar cells.However, these metal sulfide buffer layers have experienced other kinds of problems, such as ZnS having a poor conductivity, not being easy to deposit, and precursors used in the deposition process typically are materials highly volatile, toxic, and harmful to the environment and health (Rodríguez et al. 2014).Moreover, due to its wider band gap of 3.6 eV, it becomes hard to select an appropriate TCO for solar cell fabrication.Meanwhile, the deposition of tin sulfide (SnS, SnS 2 , Sn 2 S 3 ) has been occurring in many phases, and few of these have a very narrow band gap of around 1.3 eV, even less than the band gap of the absorber layer of CZTS (1.4 eV) (Norton et al. 2021).These demerits of such metal sulfides somewhat limited their claim for CZTS-based solar cells.
ZnTe is explored as a new possible material for the buffer layer for CZTS-based solar cells.ZnTe is a cost-effective, earth-abundant, and eco-friendly material.Furthermore, the band gap of ZnTe is the same as that of the conventional CdS, it is also found with a good lattice match with the CZTS, and it has low electron affinity compared to commonly used Cdcontained and metal sulfide materials (Suthar et al. 2020).
In the present work, a typical structure of Al:ZnO/i-ZnO/ZnTe/CZTS/Mo/glass (Mo-coated glass substrate) is investigated by the AMPS-1D simulation method.The cell structure is shown in Fig. 1.All the constituent layers of the proposed structure are selected with nontoxic and earthabundant elements.The materials chosen for the essential constituent layers of the cell structure can be easily deposited by the cost-effective solution-processed techniques.The data sheet of AMPS-1D simulation codes was molded with the experimentally obtained optical and microstructural parameters of the constituent layers, as mentioned in Table 1.Meanwhile, the CZTS, which is considered the absorber layer, is deposited here by a simple sol-gel spin coating route using environment-friendly non-toxic This molded AMPS-1D tool was used to examine the performance of the modeled solar cell to optimize the thickness of the buffer (ZnTe) and the absorber (CZTS) layer.The performance of the modeled solar cell structure (Al:ZnO/i-ZnO/ZnTe/CZTS/Mo) was analyzed via means of the cell parameters such as η, J SC , V OC , FF, and quantum efficiency.
Moreover, the effect of operating temperature on these performance parameters is also investigated.

Deposition of CZTS powder and thin films
The CZTS thin films were prepared by using the sol-gel spin coating method.Furthermore, this sol was also used to prepare the slurry for the differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA) study.The precursor sol was prepared  by mixing zinc chloride, stannic chloride, cupric chloride, and thiourea in the molar ratio of 1:1:2:16.This mixture was dissolved in 2-methoxy-ethanol in the molar ratio of 40:1 to the copper chloride.This solution was continuously stirred at 70 °C for 2 h.A homogenous, clear, and green-yellowish transparent sol was obtained.This sol was divided into two parts.The first part was kept safe for 48 h for the hydrolysis process, and the second one was continuously stirred.With the addition of a few drops of mono-ethanolamine during the continuous stirring, it converted into the green-blackish slurry, which was further dried using a vacuum oven (~ 10 −3 Torr) at 80 °C for 24 h.This dried dark black paste was crushed with the help of a mortar and pestle to convert it into powder for the DSC study.To deposit the CZTS thin films, the hydrolyzed sol was spin-coated at 1500 rpm on the cleaned corning glass substrates for 1 min.To obtain the multi-layered structure with enough thickness for following up on various characterizations, five layers of CZTS were coated, and each layer was successively dried at 110 °C with the help of a hot plate.The films were annealed in the mixture of nitrogen and hydrogen sulfide (1:1) ambient by using a tubular furnace at 350 °C for 15 min.A blackish coating was obtained on the glass substrate, as shown in the inset of Fig. 3.

Characterizations of the deposited CZTS
The crystallization dynamic was investigated using TGA and DSC from HITACHI STA7300, as shown in Fig. 2. The temperature scanning program was performed from 35 to 900 °C at a scanning rate of 10 °C/min under a nitrogen purge of 50 ml/min.The DSC curve shows significant endothermic peaks at 244.6 °C, 330.6 °C, and 784.8 °C due to the oxidation of materials.The TGA curve represents the change onset temperature of the material is 230 °C, at which material loss was 5 wt%.Furthermore, at 371 °C, its loss was found to be around 33 wt%, and at 900 °C, its loss was about 60 wt%.The micro-structural properties of the deposited CZTS films were studied using X-ray diffraction (XRD) by using PANalytical X'pert Pro with Cu-Kα radiation (λ = 1.54056Å).The peaks in the XRD pattern confirm the formation of the kesterite structure corresponding to the planes ( 112), ( 200), (220), and (312), as shown in Fig. 2. The low background and sharp peaks in the XRD spectra show the poly-crystalline nature of the deposited films (Fig. 3).The average crystallite size (D) is calculated by using Scherrer's formula (Rakhshani 2020): where "λ" is the X-ray wavelength, "β" is the full width at half maximum, and "θ" is the Bragg diffraction angle.The average crystallite size was found to be around 170 nm.
The surface morphology of the deposited CZTS films was investigated using field effect scanning electron microscope D = 0.9 cos (FE-SEM) from Nova Nano FE-SEM 450 (FEI) equipped with the energy dispersive spectrometer.The microstructural image from FESEM is shown in Fig. 4. The granular structures can be observed with large grain sizes in the deposited films.The larger grain size results in a higher diffusion length of charge carriers (Katagiri et al. 2001).
The EDS (energy dispersive spectroscopy) analysis was carried out to determine the composition of CZTS thin films (shown in Fig. 5).The EDS data demonstrated that the films The optical energy band gap of the deposited thin films was calculated from the transmission spectra (shown in Fig. 6) observed by using an Agilent UV-vis-NIR spectrophotometer within the wavelength range of 800 to 1100 nm.The band gap of the deposited CZTS is calculated by employing Tauc's plots using the relation for the direct band gap materials (Singh et al. 2013) as follows: where α is the absorption coefficient, β is a constant, and E g is the band gap of sample material under the investigation.It was found to be around 1.45 eV by extrapolating the linear portion of the graph (inset of Fig. 6).

Material parameters: designing and optimization of cell structure
A prototype structure of the solar cell (Al:ZnO/i-ZnO/CdS/ CZTS/Mo/glass) is shown in Fig. 1.The design of the cell is selected from the conventionally fabricated solar cell structure (Al:ZnO/i-ZnO/CdS/CZTS/Mo/glass) (Jhuma et al. 2019;Cantas et al. 2018); here, CdS buffer layer is replaced with ZnTe.The molybdenum-coated corning glass substrate, with a thickness of around 1 mm, is overlaid by the CZTS layer.It is further followed by the buffer layer of ZnTe, then the window layer of n-type intrinsic ZnO (i-ZnO) with a thickness of around 50 nm.A final aluminum-doped ZnO (Al:ZnO) layer with a thickness of about 300 nm as the TCO is considered.The illumination over the cell structure is done by AM 1.5 solar radiation, and the sunlight is assumed to be passed from the TCO layer toward the absorber layer (CZTS).A detailed description of the parameters used for simulation is shown in Table 1.The performance parameters of modeled solar cell were calculated by AMPS-1D simulation code by changing the thickness of the absorption layer and buffer layer.
The energy band diagrams of the proposed solar cell structure (Fig. 1) under the equilibrium conditions are shown in Fig. 7.The spike in the band gap diagram is found at the interface of ZnTe and i-ZnO layers.These spikes are caused at the interface of buffer and window layers due to lattice mismatch of these layers.These spikes create extra barriers for the electrons to overcome or tunnel through and may work as potential wells with discrete energy states.Dangling bonds appearing due to difference in affinity at the interface become intermediate energy states and acts as recombination centers at the interface.These centers are responsible for lowering the efficiency of the cells.ZnTe layer varied from 50 to 400 nm with an interval of 50 nm, whereas the thickness of the CZTS layer was kept at a fixed value of 2500 nm.Due to the changes in the thickness of the ZnTe layer, a significant variation in the overall efficiency of the CZTS solar cell has been observed and it is shown in the inset of Fig. 8.The highest efficiency is found for the lower thickness of the ZnTe layer, i.e., for 50 nm.The efficiency of the cell was found to be 23.47%; and the J SC , FF, and V OC were observed at 30.367 mA/cm 2 , 0.808, and 0.957 V, respectively.These cell parameters are far better than conventional CZTS solar cell structures (Al:ZnO/i-ZnO/CdS/CZTS/Mo) having CdS as a buffer layer simulated with the overall efficiency of around 12 to 14% (Jhuma et al. 2019;Cantas et al. 2018).At lower thicknesses, more charge carriers can pass the junction to reach the electrodes, leading to less recombination.Thus, higher efficiency and current density are found at the lower thickness of the ZnTe layer.

Variation in thickness of buffer layer
The quantum efficiency of Al:ZnO/ZnO/ZnTe/CZTS/Mo/ glass solar cell is influenced by variation in thickness of the buffer layer, which can be seen clearly in Fig. 9.The quantum efficiency of solar cells is measured in the wavelength range from 350 to 1000 nm.In the lower wavelength region, the higher quantum efficiency is found for the lower thickness of the ZnTe layer.As quantum efficiency is a measure of how effectively charge carriers are being separated near the depletion region (Singh et al. 2016), less recombination is found for the lower thickness of the buffer layer leading to higher quantum efficiency.

Variation in thickness of absorber layer
J-V curves for CZTS solar cells with variation in thickness of CZTS are obtained (Fig. 10).The thickness of the CZTS layer varied from 2000 to 4000 nm with a step size of 500 nm.Meanwhile, the thickness of the ZnTe layer was kept at a fixed value of 50 nm, and the performance parameters of the solar cell were analyzed (Fig. 11).It is found that the current density and efficiency of the cell are increased if the thickness of the CZTS layer is increased up to 2500 nm; beyond that, these parameters start to decrease as shown in the inset of Fig. 10 and Fig. 11a.Although it is observed, the overall efficiency increased minutely from 23.473 to 23.475% as the thickness of the absorber layer increased from 2500 to 3000 nm.2500 nm is more feasible and is the optimum thickness of the CZTS layer for such solar cell structures.At this thickness, maximum charge separation and low recombination are found.Moreover, the fill factor, which reflects how much maximum power can be achieved, is decreased from 0.814 to 0.801 with an increment in the thickness of the CZTS layer, as shown in Fig. 11b.At lower thicknesses, charge separation is easier, whereas the recombination rate is higher at the higher thicknesses of the absorber layer.This fact leads to higher values of the V OC (Fig. 11c) and the fill factor reduces at the higher thickness of the absorption layer.

Effect of operating temperature
Operating temperature of a solar cell is another important parameter for determining its efficiency.To investigate the effect of the operating temperature on cell performance, its temperature varied from 27 to 77 °C.It is found that with the increase in the operating temperature, the current density increases, but overall efficiency decreases (as shown in Fig. 12).The performance parameters at 300 K (27 °C) are found to be J SC = 30.367mA/ cm 2 , η = 23.475%,V OC = 957 mV, and FF = 80.8% for 50 nm thickness of the buffer layer, and 2500 nm thickness of absorption layer.Whereas for 350 K (77 °C), cell performance parameters are observed with values around to be J SC = 30.755mA/ cm 2 , η = 22.424%, V OC = 887 mV, and FF = 82.2%.The band gap of semiconductor materials slightly decreases with the increment in the temperature; also, less energy is required to break the bonds (Kumari and Verma 2014).So, the energy of charge carriers increases with the temperature and the conduction is enhanced (Kumari and Verma 2014).It may be the possible reason for the observed improvement in the current density.Still, as the V OC is reduced, the cell's overall efficiency decreases with the operating temperature.
Moreover, the quantum efficiency increases with an increase in the operating temperature for the longer wavelength region (Fig. 13).At higher temperatures, the light absorption for wide band gap materials increases in the longer wavelength regions (Seraphin 1979).This improvement in light absorption is responsible for the higher rate of charge carrier generation and the photocurrent; hence, there is an increase in the quantum efficiency.

Conclusions
The CZTS thin films were successfully deposited by a simple sol-gel spin coating method where earth-abundant and cost-effective raw materials were used.The deposited films were characterized by SEM with EDX, XRD, and UV-vis absorption spectra.The band gap of the material was found to be 1.45 eV as the samples were annealed in the mixture of nitrogen and hydrogen sulfide ambient.Furthermore, the simulation study of Al:ZnO/ZnO/ZnTe/CZTS/Mo heterojunction solar cells was done using the found experimental parameters for the CZTS layer.Effects of the thickness of the buffer layer, absorption layer, and operating temperature are also studied over performance parameters.It is found that by using an optimum value of the thickness of the CZTS and ZnTe layer, the efficiency of the cell may increase.Thus, the results confirm that the developed CZTS thin film layer can be applied to the development of low-cost prototype solar cells.This also provides ZnTe as a possible replacement for the widely used toxic Cd-contained CdS buffer layer.
The submitted work is original and not have been published elsewhere in any form or language (partially or in full), unless the new work concerns an expansion of previous work.

Fig. 1
Fig. 1 Schematic of simulated CZTS based solar cell structure

Fig. 2
Fig. 2 DSC study and in the inset TGA analysis of CZTS powder.In the left inset, synthesized CZTS powder

Fig. 3
Fig. 3 XRD of CZTS thin films and in the inset, the picture of the deposited CZTS thin films

Fig. 5 Fig. 6
Fig. 5 Elemental analysis of deposited CZTS thin film by EDS

Figure 8
Figure 8 shows J-V curves for CZTS solar cell with variation in the thickness of the ZnTe layer.The thickness of the

Fig. 7
Fig. 7 The schematic energy band diagram of a typical Al:ZnO/ZnO/ ZnTe/CZTS/Mo solar cell under equilibrium conditions

Fig. 11
Fig. 11 Effect of thickness of absorption layer over performance parameters Al:ZnO/ZnO/ZnTe/CZTS/Mo solar cell.a Efficiency.b Fill factor.c Open circuit voltage