Effective Utilization of Synthesized FeS 2 for Improving Output Performance of Polycrystalline Silicon Solar Cell

Solar cells are capable of converting light energy into electrical energy and can completely replace the utilization of fossil fuel energy resources. The current research work majorly concentrates on the development and coating of antireflection materials over the front contact of silicon solar cells. Ferrous disulphide was one of the wide band gap semiconductors employed as a catalytic electrode (in DSSCs), counter electrode (in QDSSs), and solar energy harvester. FeS 2 was synthesized through the hydrothermal method. The antireflective coating was performed over solar cells through the electrospraying technique. It was found that the antireflective material was distributed evenly over the coating substrate at 2 ml/h for 30 (F1), 60 (F2), 90 (F3), and 120 (F4) min. The coated solar cells were examined under neodymium light illumination mimicking sunlight. The effect of electrosprayed FeS 2 films adhered over the front contact of solar cells was evaluated using various characterization techniques. The maximum efficiency attained by coated solar cells under indirect light was 19.6%. With the aid of electrospraying, hydrothermally synthesized FeS 2 assists incoming photons with energy greater than the bandgap of a procured Si solar cell to take part in the photogeneration process. The maximum Isc and Voc of 38.08mA/cm 2 and 0.655V were achieved for the F3 solar cell under neodymium irradiation.


Introduction
In the present situation, the global population and energy consumption are increasing rapidly. is leads to excessive consumption of limited fossil fuel resources, which delivers toxic gases and pollutes the global atmosphere. Researchers were on the verge of identifying and adopting an alternate energy resource with long-term sustainability, the absence of toxic gas emissions during photocurrent generation, and eco-friendly energy generation [1][2][3]. Solar energy was one of the promising candidates because it is copious, consistent, and delivers sustainable energy to the present and future world. It can be e ectively converted and utilized as heat and electrical energy. With the adaption of solar photovoltaic technology, global electrical energy needs can be easily achieved and do not impose a greenhouse e ect on the global atmosphere as compared to conventional energy sources. According to the NREL e ciency chart, a polycrystalline silicon solar cell delivers a higher power output of 23.2%. In India, polycrystalline Si solar cells were preferred mostly for both industrial and domestic purposes because of their low cost, higher power conversion efficiency, and longer working life [4].
Even though polycrystalline Si solar cells deliver higher power output, some of the notable performance inhibitors were reflection loss, thermal loss, recombination loss, and resistive loss. Among the mentioned losses, reflection loss hinders the electron-hole generation process to a greater extent. Antireflective surface coatings over front contact surfaces have the ability to increase short-circuit current along with a sequential decrease in reflectivity of incident light [5]. Antireflective materials should be transparent, have wide bandgap semiconductors, and have a low cost [6][7][8].
e optimal refractive index of the anti-reflective material was essential to trap the reflected photons. Materials with similar anti-reflective properties utilized for improving light transmittance were MoSe 2 , ZnAl 2 O 4 , MnFe 2 O 4 , CaTiO 3 , MoS 2 , SiO 2 , TiO 2 , SiO 2 /TiO 2 , ZnAl 2 S 4 , MoS 2, Ta 2 O 5, etc. [9][10][11][12]. e transmittance of incident light increases with multilayered coatings, until it achieves optimal thickness. Further coating of material leads to incremental scattering of incident light. e major reason for the drop in cell performance was the cluster formation of coating material [2,9].
Transition metal sulfides hold a wider energy band gap and better electrical and optical properties. ey are semiconducting materials that possess explicit stability and find application in the photovoltaic sector, such as lithium-ion batteries, photo sensible capacitors, and photodetectors [13]. As a performance promoter, fewer sulphur-based materials are involved in the collection of reflected photons such as, Al 2 S 3 , MoS 2, and ZnS [14][15][16]. Some of the coating techniques for achieving thin surface films were screen printing, dip coating, sputter deposition, doctor blading, spin coating, laser beam epitaxy, spray pyrolysis, and spray coating [17]. e electrospraying method with optimal running conditions was taken into account for coating the synthesized antireflective FeS 2 particles [18,19]. Due to the abundant nature of FeS 2 , it is utilized as an effective photoabsorber material in place of silicon. e energy band gap of FeS 2 was equivalent to 0.95 eV, which was quite nearer to the Shockley-Queisser limit for photoenergy conversion. It also holds higher carrier diffusion lengths and an elevated coefficient of absorption owing to the thickness of the films.
is material was also available at a low cost [20]. FeS 2 was extensively resistant to a corrosive environment, and hence was used as an alternative for the Pt counter electrode in DSSCs [21,22].
is research work majorly concentrates on the development and coating of ferrous disulphide thin films as a perfect light harvesting material for minimizing the light reflection at front contact of crystalline silicon solar cell. However, the synthesized material was confirmed through the X-ray diffraction method and compared with standard JCPDS files. Initially, an adequate quantity of synthesized FeS 2 was dispersed with ethanol. en, the dispersed liquid was electrosprayed over the front contact surface of the solar cell. e cross-sectional thickness and material distribution were identified using the FESEM analysis. e coated samples were evaluated using electrical, structural, optical, and thermal studies. e photocurrent generation was measured using a Keithley IV source meter.

Materials Used
High-performance polycrystalline silicon solar cells were purchased from Pearlescent Green, Noida. e dimensions of the procured solar cell considered for the performance analysis were 70 mm × 70 mm. Powdered polyvinylpyrrolidone was purchased from Sigma Aldrich and with thiocarbamide and FeSO 4 . 7H 2 O was obtained from Lakshita Chemicals, Mumbai. iourea, C 2 H 5 OH, and sodium hydroxide were brought from Mahadev Chemicals in Chennai.
e pure form of sulphur was purchased from Labogens, India.

Preferred Methodology.
e layer-by-layer assembly of ethanol-dispersed FeS 2 over the front contact surface of solar cells was performed using the electrospray technique. Initially, the surface of solar cells sensible to solar light was precleaned using ethanol. At preset parameters, the coating of synthesized FeS 2 was performed. After each coating, the photovoltaic cell was heat treated at 70°C using a hot air oven.
is process was repeated in cyclic manner for attaining a multilayer antireflective coating.

Synthesis of Metalsulphide-FeS 2 .
One of the abundant transition metal sulfides with explicit optical and electrical properties was FeS 2 , and it was synthesized using the hydrothermal technique (as indicated in Figure 1). Initiator materials such as iron sulphate heptahydrate and polyvinylpyrrolidone were taken in adequate quantity and then mixed with 0.02 L of distilled water. e mixture was heat treated at 95°C for 0.5 h. Following this, 0.01 mol of thiourea was poured into the mixture and heated at 100°C for 0.5 h. e final mixture was taken into an autoclave. Further, the addition of 0.02 mol of sulphur (in powder form) into the mixture was performed. e autoclave was subjected to a constant heating process for 1 day at 210°C, then allowed to cool at room temperature. Now, 0.01 L of sodium hydroxide solution was added to the final solution. e end product was obtained as a result of centrifugation together with multiple washings of ethyl alcohol and CS 2 . e resultant powder was dried in an oven at 70°C. e obtained FeS 2 powder was dispersed with ethanol and then loaded into the 5 ml surgical syringe. e loaded syringe was placed in the spinneret holder. e surface of the solar cell was wiped thoroughly using ethanol and then placed over the flat plate collector. A high supply voltage of 17 kV was given to the syringe. Under the influence of high electrostatic force, the fine droplets of dispersed liquid get ejected from the syringe and deposited over the silicon solar cell surface. e operating parameters for electrospray deposition are specified in Table 1.
e cross-sectional thickness and distribution of coating materials were observed through FESEM analysis (MIRA 3). By matching the standard JCPDS file, the synthesized FeS 2 was confirmed through the XRD technique (XPertPro analyzer). e analysis was performed between the diffraction angles of 20 and 80 , along with the step size and scanning rate, which seemed to be 0.5⁰ and 0.02⁰/min. rough standard software, the roughness of the coated surface was determined using the atomic force microscopy technique (Tosca 400). In this method, the coated solar cells were scanned at 1 × 10 −10 m 2 area for evaluating the surface topography over the front contact surface. e photogenerated energy was calculated with the help of a Keithley I-V source meter at each instant for different solar cells under a neodymium lamp. e input AC regulator and solar power meter help us to tune the neodymium radiation equivalent to solar radiation. e transmittance of various glass samples which are coated under the same operating conditions of different solar cells through UV-Vis-NiR spectroscopy (Cary 5000). e variation in surface temperature of different coated solar cell samples was encountered using an IR fluke thermal imager. e solar simulator setup for determining the power conversion efficiency of solar cells is represented in Figure 2.

Result and Discussion
Solar cells coated with the FeS 2 powder were assessed through FESEM analysis. From the I-V characteristic curve, the F3 sample holds denser and more uniform coatings of materials. e coating thickness and material distribution were clearly identified using the FESEM technique [23][24][25].
e cross-sectional thickness of F1, F2, F3, and F4 cells was 0.95, 1.46, 1.78, and 2.31 μm,a respectively. From the results, the coating thickness goes on increasing with a gradual increment in coating time.  e crystallite size of synthesized material is found to vary from one particle to another. By means of Debye Scherer equation (1), the average crystallite size of hydrothermally synthesized FeS 2 particles was determined and is tabulated in Table 2. e average crystallite size of FeS 2 particles was about 17.5590 nm. e number of crystallites is addedtogether to form a particle. Hence, the measured particle size is found to be less than the crystallite size of synthesized FeS 2 [26].
Debye Scherer equation, where K � Scherer constant (0.9). λ � 0.15406 nm (wavelength of incident X-ray source). β � Line broadening at the     Advances in Materials Science and Engineering FWHM (radians). θ � Position of peaks (radians). D � Average crystallite size (nm). e synthesized powder was examined through HRTEM to obtain the size of the particles [27]. FeS 2 powdered particles were added with ethyl alcohol. A small amount of the dispersed mixture was taken with the help of a glass dropper. en, the glass dropper was pressed against the thin carbon film, which was placed over a copper grid arrangement. An accelerating voltage of 0.2 MV was supplied to the copper grid. e examined particles were of different sizes, varying from 67.43 nm to 310.75 nm. From Figure 5, HRTEM studies of particles appeared to have irregular shapes.
e surface topography at near atomic resolution for coated solar cells was analyzed using the AFM technique [28]. Figure 6 represents the 3D morphology of coated samples evaluated through AFM studies. e RMS values of surface roughness for coated solar cells were found to be 55, 89, 103, and 133 nm, respectively. Various FeS 2 electrosprayed samples were examined under a fixed scanning area in a tapping mode. e coating material firmly gets adhered to the front contact surface, with slight variations in the magnitude of surface roughness for different solar cells. It was found that an increase in coating time increases the surface roughness and thickness of the coated film. e roughened surface of coated cells facilitates the incoming photons to impregnate into the junction of the p-n region.
is should enhance the power conversion efficiency of coated solar cells. e transmittance of coated samples at varying coating times was analyzed through UV-Vis spectroscopy [17,29]. e obtained transmittance studies were graphically represented in Figure 7. e coated samples were analyzed with respect to reference glass slides. Among all coated glass slides, the triple-layer electrosprayed specimen exhibits maximum transmittance than other samples. e size of the glass sample for optical analysis was 2.54 cm × 7.62 cm. e maximum transmittances obtained for one, two, three, and four-layered glass slides were 84.6, 86.2, 92.7, and 82.5%, respectively. e refractive index of the sample was found to be varying and was measured at different wavelengths. As the refractive index decreases, light transmittance tends to rise for coated samples. e refractive indexes of FeS 2 -coated samples were determined as 4.5, 4.1, 3.7, and 3.5 using Eq.
(2). Here, n 2 and n 0 represent the refractive index of thermally evaporated ZnSe and atmospheric air. e antireflective material should be selected by considering its refractive index and transparent nature. For the coated thin films with enhanced light transmittance, hall mobility reaches a maximum of 14.97 cm 2 V −1 s −1 (F3). is indicates the traption of the maximum possible photons reflected from the solar cell surface to achieve the maximum photocurrent generation.
e generated electrons flow into the closed circuit and were measured using an I-V source meter. e source meter was interfaced with the display device. e measured electrical characteristics of coated and uncoated samples were tabulated in Table 3. With the experimental data, the characteristic curve was traced with open-circuit voltage against short-circuit current. e analysis of solar cells was performed under constant light irradiation (neodymium light illumination). From Figure 8, the obtained power conversion efficiencies of uncoated and F3 solar cell samples were found to be 14.88% and 19.6%, respectively. e triplelayer electrosprayed FeS 2 sample showed almost a 5% increase in power conversion efficiency as compared to an uncoated solar cell. e solar cell performance gradually increases until reaching optimal coating thickness. At optimal coating thickness, this promoted electron mobility reduced photon reflection with the front contact surface and  Advances in Materials Science and Engineering elevated the built-in voltage. After 1.78 μm of thickness, there is a drop in solar cell efficiency due to increased reflectivity at the coated surface for the F4 solar cell as compared to the F3 solar cell. Only a certain quantity of photons were involved in the photogeneration process, which was the major reason for the drop in cell performance. e remaining photons were dissipated as heat energy, which was responsible for increasing solar cell temperature.
us, increased cell temperature reduces the electrical conductivity of a solar cell [8,30,31]. e resistivity of coated and uncoated solar cells was assessed through four-probe methods (as represented in Figure 9). e best-performing solar cell (F3) experiences hall mobility and resistivity at 14.97 cm 2 V −1 s −1 and 2.35 × 10 −3 Ω m, respectively. e parameters related to the electrical properties such as hall mobility, carrier concentration, and electrical resistivity were evaluated and tabulated in Table 4. e presence of Fe was responsible for the increment in the overall electrical conductivity of coated solar cells. e spin-coated surface films experience decremented electrical resistivity until reaching the optimal coating thickness of 1.78 μm; beyond that, resistivity starts increasing. is is because clustered coated materials inhibit the penetration of photons into the front contact of the solar cell [12,17]. e rate at which excitons were generated and recombined was dependent on hall mobility. Hall mobility is the measure of representing the generated electron mobility in photovoltaic cells. Certain noticeable changes occur in the size of the crystal without altering the orientation of the crystal, which was the major reason for incremental electrical resistivity (for the F4 sample).   Advances in Materials Science and Engineering e thermal imaging technique was one of the nondestructive examination techniques employed in the fields of energy audit, ecology, foundry and manufacturing industries, etc. e coated and uncoated solar cells get heated under continual light irradiation. An increase in surface temperature reduces photoelectron generation [32]. is results in a drop in the power conversion efficiency of solar cells. Until now, temperature studies on coated solar cells were not well explored. e measured solar cell temperatures for uncoated and coated solar cells were 60.7, 55.1, 53.3, 49.9, and 52.1°C, which were clearly evident from Figure 10. In general, the performance of semiconductors decreases with an increase in operating temperature. For solar cells, the operating temperature directly affects the magnitude of Voc    and Isc. Increasing cell temperature slightly increases the short circuit current but decreases the open circuit voltage. is phenomenon results in shifting the I-V curve toward the origin, leading to a decrease in the measured fill factor. Due to the decreased energy band gap, minimal energy was required for the electron to jump from the valence to the conduction band. is facilitates the mobility of generated electron and hole pairs. Hence, Isc increases in a very minimal quantity with reference to temperature [33].
In most cases, the photogeneration ability decreases with an increase in cell temperatures, owing to enhanced electrical resistivity. Due to a higher number of excitons generation and recombination, maximum hall mobility was achieved for the F3 sample. e charge carrier mobility decreases with an increase in photon reflection at the solar cell surface, leading to an increase in the solar cell temperature. From the observed results, photon scattering at the solar cell surface promotes heat flux and hinders photocurrent generation.

Conclusion
With the aid of certain precursor materials such as polyvinylpyrrolidone, thiocarbamide, iron sulphate heptahydrate, and NaOH, FeS 2 was synthesized through the hydrothermal method. From the results of XRD, the miller  indices approximately coincide with the standard JCPDS file. Also, the average crystallite size of hydrothermally synthesized samples was determined to be approximately 17.5590 nm. From FESEM, the coating thickness for the best operating solar cell (F3) was 0.97, 1.25, 1.78, and 2.19 μm.
Various layers of FeS 2 over silicon substrates were confirmed through a continual increase in coating thickness. e maximum number of photons get diffused in the F3 sample, with a corresponding transmittance of 92.7%. A pyritestructured antireflective material with three coating layers experiences a maximum power conversion efficiency of 19.6%. rough IV studies, the F3 solar cell exhibits a higher Voc, which increases from 14.88% (bare cell) to 19.6%. With a further increase in coating thickness, the Isc tends to drop, resulting in minimal output photocurrent generation. From the observed results, solar cell performance decreases (for the F4 solar cell) with increases in surface temperature and electrical resistivity. As a result, synthesized FeS 2 was found to be transparent, antireflective, and minimize photon scattering at the solar cell surface.

Data Availability
No data were used to support the study.

Conflicts of Interest
e authors declare that they have no conflicts of interest.