Abstract
This research work focuses on augmenting the power conversion efficiency of the polycrystalline silicon solar cell with the aid of antireflection coating (ARC) of synthesized molybdenum disulphide (MoS2). The sol-gel technique and electrospraying method were preferred for synthesizing and depositing MoS2 as transparent thin films on the surface of the solar cells. The optical, electrical, structural, and thermal properties of the coated solar cells were analyzed for understanding the influence of the MoS2 coating. Five different samples (A-II, A-III, A-IV, A-V, and A-VI) were coated with varying coating time. Among them, 120 min coated sample experienced a maximum power conversion efficiency (PCE) of 17.96% and 18.82% under direct sunlight and neodymium light with resistivity as low as . The investigation of optical properties of the coated solar cells revealed a maximum transmittance of 93.6% and minimum reflectance of 6.3%, achieved for A-IV sample in the visible UV spectrum. Sample A-IV showed prominent results in the temperature analysis with temperatures as low as 38.9°C in uncontrolled and 43.2°C in controlled source environments. The results from various analyses proved that MoS2 was an appropriate material for an antireflection coating to enhance the performance of polycrystalline solar cell.
1. Introduction
The rapid consumption of fossil fuels has increased the demand for renewable energy [1]. Solar energy, one of the major contributors to the renewable energy industry, can be harnessed through photovoltaic cells (PV cells) [2]. The average power conversion efficiency (PCE) of these cells was usually around 12-15% due to various losses. Inorganic PV cells were highly stable in nature, hence delivering consistent power for longer period of time [3]. Moreover, organic PVs have also grasped the attention due to their low cost, tunable spectral response, and flexible nature. It can be installed over any curvature owing to considerable magnitude of power conversion efficiency [4, 5].The major loss which occurs in these solar cells was the reflective loss, also including charge recombination and energy loss [6]. 20-30% of the incident light was reflected from the surface of polycrystalline silicon solar cell which leads to a decrease in rated power conversion efficiency (PSSC) [7]. This drawback can be rectified through antireflective surface coatings and thereby increase the PCE of the solar cell [8, 9]. Antireflection coating (ARC) increases the transmittance of the solar cell by increasing its light trapping capacity resulting in enhanced solar cell performance [10]. The metal oxide semiconductors like Al2O3 [11], SiO2 [12, 13], ZnO [14, 15], and TiO2 [16] have been extensively employed as ARC materials. In general, the transition metal dichalcogenides exhibit better optical and electrical properties [17]. Among the transition metal dichalcogenides, molybdenum disulphide (MoS2) has gained greater attraction in recent times. It was extensively utilized in photodetectors and supercapacitor electrode applications [18, 19].
The reduction in reflective losses depends on the quality of the coating which is influenced by parameters such as number of layers, coating thickness, and coating methodology [20, 21]. The methods employed for depositing the ARC materials as nano-thin films include thermal evaporation [22], spray pyrolysis [23], RF sputtering [10, 24, 25], plasma-enhanced chemical vapor deposition (PECVD) [26], atomic layer deposition [27], and electrospinning [28]. Sol-gel technique is a low-cost room temperature synthesis of nanomaterials after undergoing series of process such as aggregation, gelation, drying, and tempering [29].
This method provides an advantage of uniform deposition of droplets with sizes varying from micrometers to nanometers. Through electrospraying technique, the dispersed solution was ejected at higher velocity forms a thin film. The thin film was dried only under the influence of higher input voltage (not due to the effect of temperature). The basic characteristics of thin film formed from electrospraying technique can be altered easily by changing the input operating conditions such as mass flow rate, substrate to target distance, type of collector, supply voltage, and viscosity of material. The heterostructured MoS2 was employed as effective photodetector for X-ray imaging applications [30]. Transition metal chalcogenide MoS2 with polycrystalline Si was used as heterojunction solar cells which have the tendency to absorb the incoming photons in minimal quantity and deliver equivalent photocurrent. Electrospraying is one of the methods in which the deposition material solution is dispersed towards the target material charged with high electric potential [13]. This method provides an advantage of uniform deposition of droplets with sizes varying from micrometers to nanometers. The electrospraying method enables us to control the coating parameters like time and flow rate of the deposition material [31, 32]. The MoS2 was synthesized using the sol-gel method which enables us to produce high-quality thin films with low complexity [33]. Synthesized MoS2 holds energy band gap of 1.35 eV and hence finds out the applications in detection of light and gas, energy storage systems [34], energy harvester, perovskite solar cells [35], optoelectronics, etc. [36]. The lab-scale thin film coating shows better uniformity than large-scale coating due to lesser working area. Almost uniform coating can be achieved in large-scale coating through proper optimization of operating parameters [37]. In this work, neodymium light was used in place of sunlight for analyzing the AR-coated Si solar cells. The color temperature of neodymium was 2800 K and emits light bluer in nature much similar to the sunlight. With the solar power meter and AC regulator, the intensity of neodymium light was matched with the solar radiation. The artificial solar light (neodymium) was used for evaluating the coated solar cell performance which possesses very minimal deviation in incident radiation.
This work is aimed at utilizing sol-gel-derived MoS2 as the ARC material to increase the efficiency of PSSC through electrospraying technique. The PCE of the samples was analyzed under controlled source environment using a neodymium lamp as the source of illumination and uncontrolled source environment (under direct sunlight). The obtained I-V values were used to interpret the influence of the coated films at varying operating conditions. Moreover, the structural, thermal, optical, and electrical properties of various MoS2-coated samples were evaluated to determine the performance of the polycrystalline silicon solar cells.
2. Experimental Techniques
The solar cells used in this research are polycrystalline solar Si solar cells bought from Vikram Solar, India.
2.1. Methodology
The work flow of the process which is depicted in Figure 1 includes the synthesis of MoS2 and deposition of synthesized MoS2 on solar cells, analyzing the electrical properties of the samples to understand the effects and inspecting the structural properties.
2.2. Synthesis of MoS2
Materials involved in the synthesis of MoS2 were ammonium molybdate tetrahydrate, thioacetamide and diethylenetriaminepentaacetic acid which act as the sources of molybdenum and sulphur and chelating agent. The synthesis of MoS2 dichalcogenide involves dissolving () of (NH4)6Mo7O24.4H2O (ammonium molybdate tetrahydrate) in litres of deionized water. The obtained solution was stirred constantly, and then 400 mg () of CH3CSNH2 (thioacetamide) was added to the solution. With the obtained final solution, 0.05 grams of diethylenetriaminepentaacetic acid was added and stirred for 2 hours to attain the brown sol [30]. Then, sol was kept at 60°C in an oven for 13 hours which results in bronze gel formation. This xerogel was dried for 2 days and further placed within a tubular furnace under argon gas atmosphere flowing at 180 sccm to form the powdered MoS2 [38, 39].
2.3. Electrospray Deposition
The calculated volume of synthesized MoS2 was dissolved in ethanol and then stirred for 2 hours at room temperature to form appropriate coating solution. Ethanol was further used to clean the surface of the solar cells before coating. Then, the cells were placed in electrospinning setup, and the prepared solution was deposited as charged droplets through strong electrostatic force of attraction for varying time period. The electrospraying method enables the deposition material to be charged and dispersed as small droplets scaling from micrometers to nanometers.
The sample A-I indicates the bare uncoated cell. The samples A-II to A-VI indicate that MoS2-coated solar cells varied by coating time. During the analysis, a significant decline in the efficiency for sample A-V was found. It was speculated that this was due to the excessive coating time period. To confirm the optimal coating time as 120 min and the decline of electrical properties in samples coated for greater than 120 min, another sample was coated with a coating time slightly longer than A-IV (135 min). The deposition parameters for electrospraying of MoS2 over solar cell surface are tabulated in Table 1.
2.4. Characterization Techniques
The surface roughness and topography of the coated MoS2 on the solar cells were examined through atomic force microscopy (AFM). X-ray diffraction (XRD) was used for analyzing the structural properties and crystallinity of calcinated MoS2 sample. The layer thickness of the coated MoS2 and surface morphology were interpreted from the results of field emission scanning electron microscopy (FE-SEM). The Raman analysis was performed to study the vibrational modes of the synthesized samples using Micro Raman spectroscopy from LabRAM HR Evolution, HORIBA France. The current-voltage relationships and resistivity of all the samples were determined through I-V analysis and the four-probe method, respectively. The transmittance and reflectance of the MoS2-coated samples and uncoated samples were inspected through optical analysis. The infrared thermal imaging technique was used to capture thermal images, and the temperature changes on the cells due to the coating were studied.
3. Result and Discussion
The XRD result of calcinated transition metal dichalcogenide MoS2 at 900°C was compared with the standard MoS2 diffraction data (JCPDS card No. 00-037-1492). The comparison of diffraction peaks with the standard file confirmed its cubic crystal arrangement as the diffraction peaks match precisely. The sharp diffraction peaks indicate the high crystalline nature of the synthesized MoS2 powder. The interpreted miller indices of the ARC material were (100), (101), (102), (103), (106), (105), (106), (110), (008), and (108). Figure 2 depicts the observed diffraction peaks of sol-gel synthesized MoS2.
The crystallite size was calculated using the basic equation of the Scherrer formula:
Microstrain is defined as the RMS value of variation in lattice parameters across individual crystallites. It is also known as the ratio of peak width to peak position of obtained diffraction peaks. Dislocation density was the measure of number of dislocations within the unit volume of a crystalline material, i.e., number of lines of dislocation per unit volume. The dislocation density and microstrain of coating materials were determined using following equations and in Table 2. where is the average size of crystallite (nm), is the Scherrer constant, is the X-ray wavelength (Ǻ), is the dislocation density (nm-2), is the microstrain (radians), is the line broadening at FWHM (radians), and is the Bragg’s angle (degree).
Further, a confirmative analysis was performed using the facile Raman spectroscopic measurement to analyze the vibrational modes obtained via sol-gel synthesized MoS2 as shown in Figure 3. The sample revealed two bands at approximately 350 cm-1 and 408 cm-1 which are related to the (in-plane optical vibration of the Mo-S bond) and (out-of-plane optical vibrational mode of S atom), respectively, thereby confirming the formation of MoS2 which is well supported by XRD. These two vibrational modes occurred due to the vibrations in S-Mo-S layers.
Through AFM analysis, surface topography of coated samples was analyzed. The surface roughness of the coating was a viable parameter in the aspect of reflective losses, as rough surface results in higher transmittance values. The AFM result was used to find out the root mean square (RMS) roughness value of the coated samples. The roughness value of the samples increases with respect to coating time [40]. The obtained surface roughness of A-I–A-VI samples were 46 nm, 79 nm, 97 nm, 124 nm, 153 nm, and 176 nm. The surface topography of the coated samples A-II to A-V in three-dimensional view is shown in Figure 4. From experimentation, it was evident that increased coating time increases the surface roughness and coating thickness of deposited thin film. The calculated error values of measured surface roughness of samples A-I to A-VI were ±1.47, ±2.27, ±1.41, ±1.09, ±3.03, and ±2.26. Up to optimal coating thickness, the light transmittance increases which allows more photons to get into the depletion region resulting in generation of more electrons in response with the incident photons.
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Extinction coefficient was used to represent the magnitude of total amount of photons attenuated whenever the electromagnetic waves travel into the target material, as indicated in Figure 5. The higher absorption spectra were determined for the A-IV sample. The extinction coefficient was found to be maximum for the A-IV sample, resulting in lesser scattering of light than other coated and uncoated samples. From the observed results, the various MoS2-coated solar cells facilitate more photons to enter into the depletion region than uncoated solar cells [41].
Within the UV visible spectra of wavelength 300 nm to 800 nm, the coating was highly transparent with the highest transmittance of 93.6% and lowest reflectance of 6.3%. Figures 6 and 7 illustrate the highest transmittance and minimum reflectance of the sample A-IV due to its high light trapping capacity. The increase in thickness of the coating layer until achieving optimal thickness results in high transmittance and low reflection for samples A-I to A-IV. Despite the increasing coating layer thickness for the sample A-V, a drop in transmittance and an increase in the reflection due to constructive interference were attained by coated particle agglomeration. Apart from the coating thickness, surface roughness also contributes for increasing the PCE of the solar cells. The samples with greater thickness than the optimal thickness offers a resistance to the light to pass into the depletion region resulting in decrement in efficiency of solar cell. Hence, the experiment was repeated for the time interval of 2.15 hrs (A-V) and 2.30 hrs (A-VI) resulting in declined power output because the lesser number of photons was facilitating the power conversion process.
The relationship between the optical properties of the sample and its structural properties is consolidated in Table 3. The enhancement of electrical properties of the solar cells through antireflection coating was evaluated through performance analysis of coated and uncoated samples under a controlled and uncontrolled/open-source environment.
The generated power output for various MoS2-coated and uncoated solar cells under direct sunlight is tabulated in Table 4. In an uncontrolled environment, the solar cells were analyzed under maximum solar radiation measured using solar radiation monitoring system (at 12.30 p.m.), for achieving maximum power conversion efficiency. From the results, it was found that power conversion efficiency increases with the increase in short circuit photocurrent density and open circuit voltage for coated solar cells. Sample A-IV exhibited maximum PCE of 17.96% (, , and ). Due to the increase in exciton generation, the I-V graph shifts gradually away from the origin. The fill factor is the maximum area of a quadrilateral plane accommodated under the I-V curve. Shifting of I-V curve away from the origin alo increases the area under the curve leading to increase in fill factor. Photons of energy greater than the energy band gap of semiconducting material have the tendency to break the depletion region and expel as charge carrier. These charge carriers were responsible for the power generation process in solar cells. All these analyses were performed with the aid of kick start interfacing software and Keithley 2450 I-V source meter along with a pyranometer. The decrease in transmittance of sample A-V led to a decline in electric output, which was clearly indicated in Figure 8.
The same steps were followed for both uncoated and MoS2 solar cells in the controlled environment, and the values are tabulated in Table 5. In a controlled source environment, an artificial solar setup regulated by alternating current was fabricated. To provide a steady source of radiation, neodymium lamp was used as the source of light with the radiation intensity prefixed as 1000 W/m2. The output radiation of fabricated artificial solar setup was prefixed as 1000 W/m2. The sample A-IV had shown prominent PCE of 18.82% compared to 15.36% PCE (, , and ) of the bare cell. Due to excessive heat flux incurred by light scattering, a decline in the efficiency of sample A-V was observed (as in Figure 9). The antireflective MoS2-coated solar cells exert increasing efficiency until the coating thickness reaches optimal coating thickness. Further coating of MoS2 leads to poor transmittance of incident photons leading to the decreased power generation. This is also due to more scattering of incident light.
The cross-sectional thickness and surface morphology of the thin film coatings were studied using FESEM. The morphology of the layers and coating thickness depends on various coating constraints such as coating time, flow rate, distance of nozzle from the substrate, supply voltage, type of substrate, and substrate temperature [42]. Figure 10 displays the FESEM image of coating structure and its cross-sectional thickness. The cross-sectional thickness of the coated MoS2 was observed to be 320 nm, 567 nm, 746 nm, 938 nm, and 986 nm, especially for samples A-II to A-VI, respectively. From the obtained results, it was evident that the increase in coating time increases the coating thickness. The optimal coating thickness for achieving maximum performance was 746 nm (sample A-IV). This leads to phenomenal electrical properties and high PCE in the I-V analysis in both controlled and uncontrolled environments. The calculated error values of measured coating thickness of samples A-II to A-VI were ±1.31, ±1.09, ±2.08, ±2.58, and ±2.20. In addition to this, EDX analysis of best operating solar cell is indicated in Figure 11. EDX analysis shows the elements present in the inspected sample at various proportions.
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The resistivity of the samples was obtained through four-probe method. The coated cells exhibited less resistivity than the bare uncoated cell. It can be associated with the existence of the Mo and S elements present in the surface of solar cell. The decline in the values of resistivity can be correlated with increase of conductivity resulting in high photocurrent density generation. The sample A-IV holds superior electrical properties with resistivity of . As compared to the resistance value of uncoated solar cell (), sample A-IV had significantly lower resistivity values. The measure of movement of holes or electrons within the semiconductor or conductor material was termed as hall mobility [43]. The variations in the carrier concentration, hall mobility, and resistivity are cordially illustrated in Figure 12 [44, 45]. Coating of antireflective materials with increased grain size and reduced grain boundary results in increased exciton separation and recombination [9]. It was also found that resistivity drops with the increase in light transmittance for the electrosprayed MoS2-coated solar cells. The A-IV sample possesses the ability to generate more photons because more number of excitons lie within the localized state of valence band [9, 46].
The temperature analysis of uncoated and MoS2-coated solar cells was executed using IR thermal imaging technique under both controlled and uncontrolled environment as indicated in Figures 13 and 14. This method was employed widely in fields like welding, medicine, and manufacturing industries. The temperature of the solar cell affects the electrical performance of solar cell. Likewise, the efficiency of the solar cell seemed to decrease with the increase in the temperature for both controlled and uncontrolled environments [45, 47–49].
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The sample A-IV possessed lower surface temperature than other solar cells in both controlled and uncontrolled environments. The observed reduction in transmittance of sample A-V and A-VI was due to the increased light scattering which leads to the increase in heat flux. The increase in scattering of electron-phonon leads to a decrease in PCE of the solar cells. Due to the reduction in mobility and charge carrier concentration, there is a declined PCE of solar cell [34, 35]. Hence, MoS2 acts as an excellent antireflective material for augmenting the PCE of the solar cell.
4. Conclusion
MoS2 was synthesized using the sol-gel method and employed as an antireflection coating for a polycrystalline silicon solar cell using electrospraying method. The samples were coated with varying deposition time under constant voltage of 17 kV. From XRD analysis, (100), (101), (102), (103), (106), (105), (106), (110), (008), and (108) were the obtained miller index values of synthesized crystalline MoS2. The obtained diffraction peaks precisely matched with the standard data of the MoS2 confirming the crystalline nature of synthesized material. The dislocation density and microstrain were calculated for the synthesized material identified using the observed XRD results. The thickness of the coatings was observed as 320 nm, 567 nm, 746 nm, 938 nm, and 859 nm through the results of FESEM. With the aid of I-V and optical analysis, the influential factors such as photogenerated current and voltage, surface roughness, and thickness of the coating were identified. A-IV solar cell experiences highest PCE of 17.96% under direct sunlight and 18.82% in a controlled source environment. Comparatively, there was a phenomenal increase in the current density of 13% higher than the uncoated solar cell especially generated by 120 min coated solar cell. The resistivity values were reduced to minimum of than the other solar cells. The coated cells hold greater optical properties with highest transmittance and lowest reflectance of 93.6% and 6.3%, respectively. The sample A-IV facilitates more photons to pass through the coating reducing the scattering of light. Hence, synthesized MoS2 holds the antireflective property and can be employed for attaining maximum power conversion efficiency of polycrystalline Si solar cells.
Data Availability
All data generated or analyzed during this study are included in this published manuscript.
Additional Points
Highlights. (i) The power conversion efficiency of polycrystalline solar cell is augmented with ARC. (ii) An antireflection coating is made by molybdenum disulphide through sol-gel method. (iii) Electrospraying method is employed for fine distribution of ARC. (iv) The surface roughness and coating thickness were correlated to the increase in PCE of solar cells
Conflicts of Interest
The authors declare that they have no conflicts of interest.
Acknowledgments
Author V. K. Gobinath acknowledges the Department of Science & Technology (DST), Government of India, for the successful execution of this research work through the Teachers Associateship for Research Excellence (Ref No. TAR/2021/000173).