# Influence of Silicon Dioxide-Titanium Dioxide Antireflective Electrosprayed Coatings on Multicrystalline Silicon Cells

Authors: Rajasekar Rathanasamy; Gobinath Velu Kaliyannan; Santhosh Sivaraj; Abishek Saminathan; Bharathikannan Krishnan; Dhayananth Palanichamy; Md. Elias Uddin
Journal: Advances in Materials Science and Engineering
 (2022)

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## Abstract

This research work primarily focuses on enhancing the power conversion efficiency (PCE) of polycrystalline silicon solar cells by using a single-layer and a double-layered antireflection coating deposited through the electrospraying technique. The usage of titanium dioxide and silicon dioxide as antireflection coating materials has shown a significant increase in the optical and electrical properties of solar cells under open and controlled light sources. The sample with TiO2 as a base layer and SiO2 as the top layer (sample B-IV) exhibited a maximum PCE of 18.90% in direct sunlight and 21.19% in a neodymium setup with cell temperatures of 40°C and 52.1°C, respectively. Sample B-IV has also shown the lowest resistivity of 3.1 × 10−3 Ω·cm among the coated samples. Also, an increase of 11.6% light transmittance and a reduction of 9.6% light reflectance were exerted by sample B-IV. The results obtained from different analysis proves that TiO2/SiO2 was an appropriate antireflection coating material for enhancing the PCE of the polycrystalline silicon solar cell.


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## Body

## 1. Introduction

Renewable energy is harnessed from natural sources or processes which continually replenish at a faster rate than consumption. This makes renewable energy a more efficient solution to the world's power problem [1]. The need for cleaner energy in the past decade has paved way for a promising feature in the field of solar power generation [2]. Power generation through solar energy is widely encouraged as it is cheap [3], clean [4] and nonpolluting. A polycrystalline solar cell is a type of solar cell made from various crystals of silicon fused into a single photovoltaic cell. This type of solar cell is widely used in the field of commercial power generation through solar energy. The practical efficiency of these solar cells is usually around 12–15%. The factors like humidity, dust, and reflection of sunlight play an important role in altering the efficiency of solar cells. In general, 20–30% of sunlight gets reflected from the surface of the solar cell [5]. The reflection losses in the polycrystalline solar cells can be reduced by increasing the power conversion efficiency (PCE) with the help of an antireflection coating [6]. For antireflection coating, a variety of materials can be employed like ZnO [7], SiO2 [8], TiO2 [9], Al2O3 [10], and ZnS [11]. In this research work, SiO2 and TiO2 have been chosen as antireflection coating (ARC) materials for minimising the incident light reflection and thereby increasing the power conversion efficiency of solar cells. SiO2 was chemically stable at high temperatures and has scratch resistance properties [12]. TiO2 exhibits mechanical hardness, chemical stability, and less moisture absorption and has a suitable refractive index and minimum absorption throughout the visible spectra region of solar cells. The refractive indices of SiO2 and TiO2 were 1.44 and 2.20, respectively [13]. Four different coated samples were prepared, B-II (SiO2), B-III (TiO2), B-IV (TiO2/SiO2), B-V (SiO2/TiO2), and B-I as uncoated solar cells [14]. The antireflection coating had been deposited by various methods such as spin coating [15], slot die coating [16], electrospraying [17], doctor blading [18], dip coating [19], and sputter deposition [5]. The electrospraying method is preferred for this research work as it has already been extensively employed in various applications including film coating, drug delivery [20], chocolate processing [21], and encapsulation of nutraceuticals [22]. In electrospraying, both size and generation of the droplet can be controlled through the supplied voltage at the capillary nozzle and the flow rate of the liquid which enables the desired output of deposition of the ARC material.This research work aims in utilising TiO2 and SiO2 as the ARC material to increase the PCE of polycrystalline silicon solar cells by electrospraying technique. The necessary analysis was carried out to determine the effects of TiO2 and SiO2 coating on the optical, structural, electrical, and thermal properties of the coated samples. The coated polycrystalline Si solar cells were inspected under sunlight source and neodymium light sources.


## 2. Materials Used

Precursors—titanium dioxide (TiO2) and silicon dioxide (SiO2) with 99.9% purity were procured from Sigma–Aldrich. Ethanol (C2H5OH) with 99.9% purity was bought from Changshu Hongsheng Fine Chemicals, China. Polycrystalline silicon solar cells were bought from Eco Worthy, China.


### 2.1. Methodology

The components involved in electrospraying are a liquid feeder (syringe), a blunt hypodermic needle, and a high voltage source [23]. The overall coating processes were represented in Figure 1. The voltage applied to the droplets for electrospraying is typically between 5 kV and 30 kV of positive polarity. The antireflection coating material was completely dissolved in ethanol using a magnetic stirrer. The target solar cell was mounted over a flat plate covered with aluminium foil. The mass flow rate and supplied voltage were configured in the electrospraying machine as 2 ml/h and 17 kV, respectively. The overall operating parameters for achieving electrosprayed antireflective thin surface films were indicated in Table 1. The ARC solution in the syringe was charged and dispersed as fine droplets towards the target solar cell. The droplets formed a coating on the surface of the solar cell as a result of the strong electrostatic force. As indicated in Table 1, the coated samples were specified as B-II, B-III, B-IV, and B-V representing the single layer TiO2, single layer SiO2 coating, double layer of TiO2/SiO2, and double layer of SiO2/TiO2 samples, respectively, whereas, B-I indicates the uncoated polycrystalline silicon solar cell.Figure 1
Pictorial representation of electrospraying method.Table 1
Sample description and maintained coating parameters.
SampleCoating materialFlow rate (ml/h)Voltage (kV)Substrate target distance (cm)B-IISiO22173B-IIITiO2B-IVTiO2/SiO2B-VSiO2/TiO2


### 2.2. Characterisation Techniques

FESEM (Field Emission Scanning Electron Microscopy) analysis was a strong investigative tool used to study the surface morphology and cross-sectional of the coated and uncoated samples [24], Chemical composition of the coated solar cells was examined through the energy dispersive X-ray analysis (EDAX). The film thickness and surface topography of the coated and uncoated polycrystalline samples were recorded using AFM (Atomic Force Microscopy) analysis. The current-voltage characteristics of the solar cells were measured by using I–V analysis with the help of Keithley 2450 source metres in both controlled and uncontrolled source environments. The influence of the coating over the solar cells and the optical properties of the cells were observed through the optical transmittance and reflectance analysis.


## 2.1. Methodology

The components involved in electrospraying are a liquid feeder (syringe), a blunt hypodermic needle, and a high voltage source [23]. The overall coating processes were represented in Figure 1. The voltage applied to the droplets for electrospraying is typically between 5 kV and 30 kV of positive polarity. The antireflection coating material was completely dissolved in ethanol using a magnetic stirrer. The target solar cell was mounted over a flat plate covered with aluminium foil. The mass flow rate and supplied voltage were configured in the electrospraying machine as 2 ml/h and 17 kV, respectively. The overall operating parameters for achieving electrosprayed antireflective thin surface films were indicated in Table 1. The ARC solution in the syringe was charged and dispersed as fine droplets towards the target solar cell. The droplets formed a coating on the surface of the solar cell as a result of the strong electrostatic force. As indicated in Table 1, the coated samples were specified as B-II, B-III, B-IV, and B-V representing the single layer TiO2, single layer SiO2 coating, double layer of TiO2/SiO2, and double layer of SiO2/TiO2 samples, respectively, whereas, B-I indicates the uncoated polycrystalline silicon solar cell.Figure 1
Pictorial representation of electrospraying method.Table 1
Sample description and maintained coating parameters.
SampleCoating materialFlow rate (ml/h)Voltage (kV)Substrate target distance (cm)B-IISiO22173B-IIITiO2B-IVTiO2/SiO2B-VSiO2/TiO2


## 2.2. Characterisation Techniques

FESEM (Field Emission Scanning Electron Microscopy) analysis was a strong investigative tool used to study the surface morphology and cross-sectional of the coated and uncoated samples [24], Chemical composition of the coated solar cells was examined through the energy dispersive X-ray analysis (EDAX). The film thickness and surface topography of the coated and uncoated polycrystalline samples were recorded using AFM (Atomic Force Microscopy) analysis. The current-voltage characteristics of the solar cells were measured by using I–V analysis with the help of Keithley 2450 source metres in both controlled and uncontrolled source environments. The influence of the coating over the solar cells and the optical properties of the cells were observed through the optical transmittance and reflectance analysis.


## 3. Result and Discussion

The surface morphology and cross-section thickness of the coated thin films were analysed using FESEM. The coating structure and the coating thickness of sample B-IV are displayed in Figure2. The cross-sectional thickness of the coated layers in samples B-II, B-III, B-IV, and B-V were observed to be 247 nm, 360 nm, 610 nm, and 762 nm, respectively. Various parameters impact the quality of the coating such as flow rate, supplied voltage, and distance between the substrate and nozzle. The optical coating thickness was determined to be 610 nm for sample B-IV with titanium dioxide as a base layer and silicon dioxide as the top layer. The sample B-IV has performed exceptionally by exhibiting maximum PCE in the I–V analysis.Figure 2
(a) Surface morphology of B-IV (TiO2/SiO2). (b) Cross-section of B-IV (TiO2/SiO2) solar cells.
(a)(b)Through EDAX, the chemical composition of the coated material on the surface of the solar cell was determined. The EDAX graph of sample B-IV is displayed in Figure3, which confirms the presence of both silicon and titanium elements on the surface of solar cells. Surface roughness plays a significant role in trapping solar light as rough surfaces lead to more light trapping capacity. The surface topography of the coatings was analysed through AFM analysis. The AFM result (Figure 4) was used to determine the surface roughness values of coated samples B-II, B-III, B-IV, and B-V to be 93 nm, 108 nm, 121 nm, and 129 nm, respectively.Figure 3
Energy dispersive X-ray analysis (EDAX) of B-IV sample coated solar cell.Figure 4
3D AFM image of coated solar cell samples (a) B-II, (b) B-III, (c) B-IV, (d) B-V.
(a)(b)(c)(d)Table 2
Transmittance and reflectance of coated and uncoated solar cells.
SampleCoating materialCoating thickness (nm)Transmittance (%)Reflectance (%)B-IUncoated083.913.6B-IISiO224787.610.4B-IIITiO236090.38.2B-IVTiO2/SiO261095.54B-VSiO2/TiO276293.15.9Under the influence of antireflective surface coatings, more incident light gets trapped by minimising the light reflectance. The incident light rays were found to be constructive or destructive which was purely dependent on the thickness of the antireflective layer [25]. Until reaching the optimal coating thickness, the incident light gets coupled together and then reaches the depletion region. Beyond the optimal coating thickness, the incoming light was found to exert destructive interference [26]. Hence, a lesser number of photons reaches the depletion region resulting in a decrement in the power generation ability of solar cells. For obtaining the effective antireflective property, amplitudes of reflected beam at a coated thin film—air interface and coated thin film—substrate interface should be equal and 180° out of phase [27]. This results in destructive interference of reflected beams. There are two important conditions with which the optimal thickness of the antireflective coating can be evaluated (as indicated in equations (1) and (2)).(i)
The product of refractive indices of the coated substrate (n0) and working medium (n2) must be equal to the square of the refractive index of the deposited antireflective film (n1).(1)n1n0×n2.(ii)
The optimal thickness of antireflective film must be equal to one-fourth of the wavelength at which minimal reflectance is obtained.(2)n×t=λmin4.The thickness at which zero reflectance was achieved at≈ 610 nm for the B-IV sample with a refractive index of 2.16. Constructive interference of reflected beams tends to arise beyond the optimal coating thickness of the antireflective surface.The sample B-IV (TiO2/SiO2) exhibited higher transmittance of 95.5% in the UV visible region compared to other samples in the visible UV spectrum. This was due to the minimised reflective losses in the sample. The rough surface and optimal thickness of the coating reduced the scattering of light from the surface and resulted in higher transmittance. The presence of Ti and Si enabled more photons from the incident light to pass through the coated samples. This phenomenon was evident from obtained optical studies of coated samples (as shown in Figures 5 and 6), whereas, the sample B-V (SiO2 as a base layer) revealed comparatively low transmittance than B-IV (TiO2 as a base layer which can be correlated with the excessive thickness of the coated layer. The consolidated transmittance and reflectance values observed in the optical analysis are tabulated in Table 2.Figure 5
Optical transmittance spectra of various solar samples.Figure 6
Optical reflectance spectra of various solar samples.I–V analysis for the coated solar cells and the bare solar cell was conducted in both controlled and uncontrolled open environments to learn the influence of the coating on the electrical properties of the solar cells. In an open-source environment, the analysis was done under direct sunlight using a Keithley source metre, power source metre, and kickstart interfacing software. The power output generated by the samples' direct sunlight is tabulated in Table3. The results of the I–V analysis imply that the efficiency of the solar cells increases with the increase in short circuit photocurrent density and open circuit voltage. Sample B-IV has shown maximum current density and open circuit voltage of 36.46 mA/cm2 and 0.656 V leading to a maximum efficiency of 18.90% (Figure 7).Table 3
Measured values of samples under direct sunlight.
SamplesOpen circuit voltage (V)Short circuit current density (mA/cm2)Fill factor (%)Power conversion efficiency (%)B-I (uncoated)0.62831.030.74614.54B-II (SiO2)0.63133.40.7515.81B-III(TiO2)0.63534.70.7817.19B-IV (TiO2/SiO2)0.65636.460.7918.90B-V (SiO2/TiO2)0.64935.50.7717.74Figure 7
Measured values of the samples under direct sunlight.A neodymium lamp was used as a source of illumination in the controlled source environment which can emit radiation similar to the sunlight. Measured I–V values of the coated samples under closed source were tabulated in Table4. The sample B-IV experiences superior results in a closed environment with 21.19% PCE (Figure 8). As expected, a decline in the power output of sample B-V was observed. The sample B-V is confirmed to have exceeded coating thickness and roughness values than the optimal level. The solar cell samples performed better in the fabricated neodymium setup than in direct sunlight. The source of illumination in the controlled setup emits constant radiation for the solar cell without any fluctuation. However, in direct sunlight, the radiation is subjected to variation with time due to several factors.Table 4
Measured values of samples under a controlled source of environment.
SamplesOpen circuit voltage (V)Short circuit current density (mA/cm2)Fill factor (%)Power conversion efficiency (%)B-I (uncoated)0.63232.160.7314.84B-II (SiO2)0.63436.10.7517.17B-III (TiO2)0.6438.970.7618.96B-IV (TiO2/SiO2)0.65840.760.7921.19B-V (SiO2/TiO2)0.65240.020.7720.09Figure 8
Measured values of the samples under a controlled environment.The simulation results for various coated and uncoated solar cells under consistent light incidence. The simulation was carried out for antireflective coated multicrystalline silicon solar cells using SCAPS software. The best operating solar cells that exert maximum cell performance were represented in Figure9. However, the experimental results were found to be lesser compared to simulation results, due to certain errors such as calibration errors, uncontrolled temperature change, deposition of light inhibitor layers (dust particles), minor internal cracks, and resistive losses [28]. Among all other coated solar cells, B-IV solar cells exert maximum photocurrent generation. For B-IV solar cells, the simulation results were obtained as maximum Voc, Isc, and PCE as0.664 V, 41.85 mA/cm2, and 22.06% (simulated under constant light illumination). Increment in electron-hole pair generation leads to an increase in the power conversion efficiency of photovoltaic cells. As a counteraction, electron-hole pair recombination does not improve power conversion efficiency. Instead, the photon gets expelled while the electron falls from conduction to the valence band. Such a recombination process was known as the radiative-recombination process [29].Figure 9
Simulation results of various coated and uncoated solar cells: (a) B-I; (b) B-II; (c) B-III; (d) B-IV; (e) B-V.
(a)(b)(c)(d)(e)The resistivity of the coated samples was inspected through the four-probe technique. Double coated solar cells had considerably low resistivity and sample B-IV held the lowest resistivity of 3.1 × 10−3 Ω·cm. The SiO2coated sample experienced the highest resistivity among the coated solar cells, but it was still lower than the resistivity of the uncoated solar cell. This reduction in the resistivity was due to the increase in the carrier concentration and Hall mobility which was illustrated in Figure 9. The conductivity and photo-generated current get increased for the multilayer antireflective coated solar cell (B-IV) than in other solar cells. With the decrement in measured resistivity of coated solar cells, Hall mobility and carrier concentration gets increased as compared to the bare solar cell. The mobility of electrons was improved by the larger-sized grains with lesser grain boundary leads to enhanced photocurrent generation [30]. The Hall mobility and carrier concentration were determined using the following equations (3) and (4) The measured electrical characteristics of various solar cells were represented in Figure 10.(3)nH=1e×RH,(4)µH=RH×σ,where nH = Carrier concentration (cm−3), µH = Hall mobility (cm2/VS), e = Charge of an electron, RH = Hall coefficicent (cm3/coulomb), σ = electrical conductivity of coated thin films.Figure 10
Electrical characteristics of uncoated and coated samples.The bare cell and coated samples were analysed in accordance with the heat flux measurement using a Fluke—thermal imager in both controlled and uncontrolled environments as indicated in Figures11 and 12. The thermal analysis revealed that the efficiency of solar cells depends on the temperature of the solar cell. Also, the increase in heat flux increases the resistivity in solar cells and affects the power generation of the solar cell [31, 32]. Sample B-IV exhibited lower temperatures of 40.0°C and 39.6°C compared to other samples in both controlled and uncontrolled source environments, respectively. The temperature of samples decreases from B-I to B-IV, whereas sample B-V possessed an elevated temperature than expected. The sample with SiO2 as a base layer could not provide better thermal properties than the sample with TiO2 as a base layer. From the analysis of thermal, electrical, and structural properties of the coated solar cells, the double-layered coating of TiO2/SiO2 acts as excellent antireflective coating elements for increasing the PCE of polycrystalline silicon solar cells.Figure 11
Thermal analysis under controlled source environment: (a) B-I (uncoated), (b) B-II (SiO2), (c) B-III (TiO2), (d) B-IV (TiO2/SiO2), and (e) B-V (SiO2/TiO2).
(a)(b)(c)(d)(e)Figure 12
Thermal analysis under direct sunlight: (a) B-I (uncoated), (b) B-II (SiO2), (c) B-III (TiO2), (d) B-IV (TiO2/SiO2), and (e) B-V (SiO2/TiO2).
(a)(b)(c)(d)(e)


## 4. Conclusion

Antireflection coating materials were prepared using titanium dioxide and silicon dioxide and employed on the solar cells through the electrospraying method. The samples were prepared with two single-layered coatings and two double-layered coatings. The cross-sectional thickness of the coated layer in samples B-II, B-III, B-IV, and B-V was observed to be 247 nm, 360 nm, 610 nm, and 762 nm, respectively. The results from EDAX confirm the proper deposition of TiO2& SiO2 elements on the samples. The surface roughness values of the samples B-II, B-III, B-IV, and B-V were reckoned to be 93 nm, 108 nm, 121 nm, and 129 nm. Sample B-IV has shown a significant increase in the PCE of the solar cell with 18.90% of efficiency under direct sunlight and 21.19% in a controlled source environment with the highest transmittance of 95.5% and reflectance as low as 4%. The resistivity was remarkably reduced in the double-layered coated samples especially the B-IV sample had 3.1 × 10−3 Ω - cm resistivity. The temperature of the coated solar cells and the heat flux variations were observed from temperature analysis, which confirms the lowest surface temperature in both controlled and uncontrolled setup by the sample B-IV (40.0°C and 39.6°C). Hence, the antireflective property of sample B-IV (TiO2/SiO2) was proved to be effective for enhancing the PCE of polycrystalline silicon solar cells.



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