Optical Properties of Caf2 Thin Film Deposited on Borosilicate Glass and Its Electrical Performance in PV Module Applications

applied sciences

Article Optical Properties of CaF2 Thin Film Deposited on Borosilicate Glass and Its Electrical Performance in PV Module Applications

Muhammad Aleem Zahid 1,2, Shahzada Qamar Hussain 3, Young Hyun Cho 1,* and Junsin Yi 1,* 1 College of Information and Communication Engineering, Sungkyunkwan University (SKKU), Suwon 16419, Korea; [email protected] 2 Electrical Engineering Department, University of Engineering & Technology, Taxila 47050, Pakistan 3 Department of Physics, COMSATS University Islamabad, Lahore Campus, Lahore 54000, Pakistan; [email protected] * Correspondence: [email protected] (Y.H.C.); [email protected] (J.Y.)

Received: 22 July 2020; Accepted: 11 August 2020; Published: 14 August 2020

Featured Application: Minimization of reﬂection losses in conventional and light weight PV modules.

Abstract: Calcium fluoride (CaF2) is deposited via vacuum thermal evaporation on borosilicate glass to produce an anti-reflection coating for use in solar modules. Macleod’s essential simulation is used to optimize the thickness of the CaF coating on the glass. Experimentally, a 120 4 nm-thin CaF film 2 ± 2 on glass shows an average increase of ~4% in transmittance and a decrease of ~3.2% in reflectance, respectively, when compared to that of uncoated glass (Un CG), within the wavelength spectrum of approximately 350 to 1100 nm. The electrical PV performance of CaF2-coated glass (CaF2-CG) was analyzed for conventional and lightweight photovoltaic module applications. An improvement in the 2 short-circuit current (Jsc) from 38.13 to 39.07 mA/cm and an increase of 2.40% in the efficiency (η) was obtained when CaF2-CG glass was used instead of Un CG in a conventional module. Furthermore, 2 Jsc enhancement from 35.63 to 36.44 mA/cm and η improvement of 2.32% was observed when a very thin CaF2-CG was placed between the polymethyl methacrylate (PMMA) and solar cell in a lightweight module.

Keywords: high efficiency; reflection; transmittance; anti-reflection coating; calcium fluoride; conventional PV module; light weight PV module; statistical analysis

1. Introduction A typical photovoltaic (PV) module can be classified as a conventional or light weight module. In a conventional module, glass is used as a top cover and accounts for approximately 70% of the module weight. On the contrary, in lightweight c-Si PV modules, glass is replaced by acrylic (poly methyl methacrylate: PMMA) or other polymers [1–3] to enable such modules to be used in applications where the light weight of the PV module must be limited. Reflection losses are one of the major issues that decrease the effective efficiency of PV modules. A reflection loss of approximately 8% occurs at the front surface of the glass and PMMA because their higher refractive indexes are higher than that of air [4]. This issue can be resolved by using an anti-reflection coating (ARC) material with an ultra-low refractive index (much lower than that of glass) on the front surface of glass. The use of ARC has already been recognized as crucial for various optical and PV panel applications [5,6]. A simple and cost-effective ARC comprises a single layer coating of a material with a refractive index suitable for the desired wavelength spectrum [7]. In this context, optical coatings of fluoride and oxide materials have

Appl. Sci. 2020, 10, 5647; doi:10.3390/app10165647 www.mdpi.com/journal/applsci Appl. Sci. 2020, 10, 5647 2 of 9 been investigated widely owing to their higher band gap. Optical coatings have been widely used in the areas of optoelectronics [8], optics [9], and PV modules [10]. Recently, magnesium fluoride (MgF2) has gained much attention as an ARC suitable for application on the front cover of PV modules, owing to its low refractive index (n = 1.37 at 600 nm); however, it was proven to be less durable in outdoor environments [11]. CaF2 belongs to a similar category of materials and has been widely investigated by the optics community [12], owing to its lower refractive index (1.43 at 600 nm), when compared to that of glass substrates. It also has a broad transmittance spectrum from 130 to 10,000 nm [13]. Furthermore, it exhibits less solubility in water when compared to MgF2 [14], which can be a significant factor in view of the chemical resistance towards environmental conditions. Furthermore, it is also used as an ARC in outdoor conditions, optical applications, IR imaging system, and photolithography systems [15]. Therefore, the application of CaF2 as an ARC for PV modules has been explored in this work. It is important to note that the excellent surface uniformity of ARC is highly dependent on the deposition technique. Numerous techniques have been introduced for depositing CaF2 thin films on substrates for various purposes such as electron beam evaporation [16–19], radio frequency magnetron sputtering [20], molecular beam epitaxial [21], pulsed laser deposition [22,23] and thermionic vacuum arc [24]. Of these techniques, thermal evaporation has been found to be the most suitable because of its advantages of large-area deposition, low-temperature, and simple process, and it has been proven to be a cheap fabrication process [25]. In this work, a CaF2 coating was deposited by vacuum thermal evaporation on borosilicate glass (BSG). In this process, the substrates were tilted and rotated at constant speed, resulting in the uniform deposition of the material. The thickness of the ARC was first optimized using the Essential Macleod simulation tool. The optical properties of the thermally evaporated CaF2 on the glass substrate were investigated using ultraviolet–visible spectroscopy. The electrical performance of the solar cell (SC) was compared by replacing the uncoated glass (Un CG), with CaF2-coated glass (CG) in a conventional PV module and placing a thin CaF2-CG between the PMMA and SC in the light weight PV module.

2. Materials and Methods An SC with an active area of ~2.5 2.8 cm2 was used as a reference SC. The BSG and PMMA × sheets were cut according to the active area of the SC. First, the substrates were cleaned sequentially with acetone, isopropyl alcohol (IPA) and deionized water (DIW) for 10 min each in an ultra-sonic bath. The BSG substrates were dried by blowing N2 gas and then heated on a hot plate at ~100 ◦C for 20 min to remove moisture. The cleaned BSG substrates were loaded immediately into the vacuum thermal evaporation chamber to deposit the thin CaF2 as the ARC. The deposition was performed 6 at ~110 A current and under a pressure of ~10− torr. The thickness of the CaF2 coating on the glass was measured by nano-view Spectroscopic Ellipsometer MF-1000 which was 120 4 nm. The PMMA ± was cleaned using soapy water and dried by blowing N2 gas. Finally, the electrical parameters such as the short circuit current (Jsc) and eﬃciency (η) were characterized by a light-current–voltage (LIV) measurement system under Air Mass 1.5 G 1 sun illumination conditions (100 mW/cm2). The potential was swept from short circuit condition towards open circuit. The step voltage and delay time were 10 mV and 1 ms, respectively.

3. Results and Discussions

3.1. Structure Characterization The morphology of the thin film was observed using field-emission scanning electron microscopy (FE-SEM; JEOL JSM-7600F). The cross-sectional FE-SEM image of the CaF2 film on glass are clearly shown in Figure1a. The image showed the information about the ARC thickness, its uniformity and the interaction between the coating and substrate. X-ray diffraction (XRD) patterns were recorded in Appl. Sci. 2020, 10, 5647 3 of 9 a 2θ range of 20–70 degrees using an X-ray diffractometer (Bruker, D8 Advance, Turbo X-ray source Appl. Sci. 2020, 10, x FOR PEER REVIEW 3 of 9 18 KW). The typical XRD pattern for the CaF2 thin film on glass are presented in Figure1b. It shows that the CaF2 film is polycrystalline and has a strongest peak of 111, which is in good agreement with Figure 1b. It shows that the CaF2 film is polycrystalline and has a strongest peak of 111, which is in the Bannon and Coogan result [26]. The subsequent diffraction lines at 49.6 and 55.6 were identified as good agreement with the Bannon and Coogan result [26]. The subsequent diffraction lines at 49.6 and 220 and 311 reflexes of the CaF2 phase, respectively [27]. 55.6 were identified as 220 and 311 reflexes of the CaF2 phase, respectively [27].

(a) (b)

Figure 1. (a) Cross sectional FE-SEM image of CaF thin film; (b) XRD pattern for the CaF film. Figure 1. (a) Cross sectional FE-SEM image of CaF2 2 thin film; (b) XRD pattern for the CaF2 2 film. 3.2. Optical Characterization 3.2. Optical Characterization The optical characteristics of the CaF2-CG were studied in the wavelength range of ~350 to The optical characteristics of the CaF2-CG were studied in the wavelength range of ~350 to 1100 1100 nm. By varying the thickness of the coating material, the optical properties of the thin film nm. By varying the thickness of the coating material, the optical properties of the thin film deposited deposited on the substrate can be changed [28]. The prototype of the Macleod simulation parameters on the substrate can be changed [28]. The prototype of the Macleod simulation parameters are shown are shown in the Supporting Document. in the Supporting Document. 3.2.1. Reflectance, Transmittance and Absorbance 3.2.1. Reflectance, Transmittance and Absorbance The transmittance and absorbance of the Un CG and CaF2-CG were measured using an ultraviolet-visibleThe transmittance spectrophotometer and absorbance (SCINCO of the S-3100).Un CG Theand reflectanceCaF2-CG were can bemeasured calculated using using an theultraviolet–visible relation: spectrophotometer (SCINCO S-3100). The reflectance can be calculated using the relation: R(λ) = 100 T(λ) A(λ) (1) − − ( ) ( ) where T is the transmittance, and A is theR λ absorbance= 100 − T atλ λ−A(wavelength.λ) (1) whereThe T is spectral the transmittance, behaviors and of transmittanceA is the absorbance and at reflectance λ wavelength. of the Un CG and CaF2-CG, obtained through simulations and experiments, are compared in Figure2. The Un CG shows an average transmittance and reflectance of ~91% and 8.1%, respectively, across the wide spectrum from ~350 to 1100 nm (Figure2a), whereas CaF 2-CG shows improved an transmittance and reflectance of ~95.7% and 4.3%, respectively, in the broad spectrum of ~ 350–1100 nm as shown in Figure2b. The observed results are in good agreement with the results reported by Hahn et al. [29] and exhibit an improvement in the result, as discussed by Cetin et al. in their study on vacuum-evaporated CaF2 [24]. The observed absorbance is the minimum (~0.015%) at 600 nm, which is quite low as expected.

3.2.2. Incidence Angle and Refractive Index The optical characteristics of the PV panel are mainly dependent on the incident angle of light. The influence of different incident angles on the reflectance and transmittance of the CaF2-CG computed using Essential Macleod is clearly shown in Figure 3a. The graph shows that the reflectance increases and the transmittance decreases(a) as we go from a low incidence angle to a(b high) incidence angle, and the results are in good agreement with the analysis presented in [30]. Figure 2. Comparison of the simulated and experimentally obtained optical spectra of: (a) the uncoated and (b) the CaF2-coated borosilicate glass (BSG).

3.2.2. Incidence Angle and Refractive Index Appl. Sci. 2020, 10, x FOR PEER REVIEW 3 of 9

Figure 1b. It shows that the CaF2 film is polycrystalline and has a strongest peak of 111, which is in good agreement with the Bannon and Coogan result [26]. The subsequent diffraction lines at 49.6 and 55.6 were identified as 220 and 311 reflexes of the CaF2 phase, respectively [27].

(a) (b)

Figure 1. (a) Cross sectional FE-SEM image of CaF2 thin film; (b) XRD pattern for the CaF2 film.

3.2. Optical Characterization

The optical characteristics of the CaF2-CG were studied in the wavelength range of ~350 to 1100 nm. By varying the thickness of the coating material, the optical properties of the thin film deposited on the substrate can be changed [28]. The prototype of the Macleod simulation parameters are shown in the Supporting Document.

3.2.1. Reflectance, Transmittance and Absorbance

The transmittance and absorbance of the Un CG and CaF2-CG were measured using an ultraviolet–visible spectrophotometer (SCINCO S-3100). The reflectance can be calculated using the relation: R(λ) = 100 − T(λ) −A(λ) (1) whereAppl. Sci. T2020 is the, 10 ,transmittance, 5647 and A is the absorbance at λ wavelength. 4 of 9

Appl. Sci. 2020, 10, x FOR PEER REVIEW 4 of 9

The optical characteristics of the PV panel are mainly dependent on the incident angle of light. The influence of different incident angles on the reflectance and transmittance of the CaF2-CG computed using Essential Macleod is clearly shown in Figure 3a. The graph shows that the reflectance increases and the transmittance(a) decreases as we go from a low incidence(b) angle to a high incidence angle, and the results are in good agreement with the analysis presented in [30]. FigureFigure 2.2. ComparisonComparison of of the the simulated simulated and and experimentally experimentally obtained obtained optical optical spectra spectra of: (a) the of: uncoated (a) the uncoatedand (b) the and CaF (b2)-coated the CaF borosilicate2-coated borosilicate glass (BSG). glass (BSG).

50 100 2.0 3.2.2. Incidence Angle and Refractive Index

40 90 1.6

30 80 1.2

20 70 0.8 Reflectance (%)Reflectance

10 60 Transmittance (%) 0.4 Refractive Index (n) Index Refractive 0 50 -60 -40 -20 0 20 40 60 0.0 Incidence Angle 400 500 600 700 800 900 1000 1100 Wavelength (nm)

(a) (b)

Figure 3. (a()a )Influence Influence of ofthe the different different incident incident angles angles on onthe thereflectance reflectance and and transmittance; transmittance; (b) (refractiveb) refractive index. index.

n The refractiverefractive indexindex ((n)) providesprovides knowledgeknowledge regardingregarding thethe locallocal fields,fields, electronicelectronic polarizationpolarization and the phase velocity velocity of of light light spreading spreading inside inside a amaterial material [25]. [25 ].The The refractive refractive index index depends depends on the on thedeposition deposition technique technique of coating of coating because because the the mass mass density density and and crystal crystal orientation orientation of of the materialmaterial n n influencesinfluences n.. TheThe Lorentz–Lorenz Lorentz–Lorenz equation equation provides provides information information on on the the increase increase in the in valuethe value of withof n the increase in the film density [31]. The dependence of n for the CaF2-coated film on BSG over a broad with the increase in the film density [31]. The dependence of n for the CaF2-coated film on BSG over rangea broad of range wavelength of wavelength is shown is in shown Figure in3b. Figure We used 3b. theWe Cauchyused the model Cauchy for model fitting for the fitting data from the data the VB-250 VASE ellipsometer for determining the n value of the produced CaF2 thin film. The measured from the VB-250 VASE ellipsometer for determining the n value of the produced CaF2 thin film. The refractive index of ~1.39 at 600 nm indicated the excellent transparency of ~95% of CaF2-CG. There was measured refractive index of ~1.39 at 600 nm indicated the excellent transparency of ~95% of CaF2- goodCG. There improvement was good in theimprovement refractive index,in the as refractive a result ofindex, the vacuum as a result thermal of the evaporation vacuum process,thermal whenevaporation compared process, to that when presented compared in the to literaturethat presented and that in the of literature bulk material and that [24]. of bulk material [24]. 3.3. Electrical Characterization 3.3. Electrical Characterization As a result of the improvement in the optical characteristics of CaF2-CG, a similar improvement As a result of the improvement in the optical characteristics of CaF2-CG, a similar improvement was expected in the electrical parameters when the CaF2-CG was placed on a pristine SC. was expected in the electrical parameters when the CaF2-CG was placed on a pristine SC. 3.3.1. Conventional Module 3.3.1. Conventional Module The performance of the ARC was analyzed by placing the CaF2-CG on an SC, as shown in FigureThe4. Theperformance structure of represents the ARC thewas conventional analyzed by moduleplacing modelthe CaF in2-CG which on an the SC, Un as CG shown is replaced in Figure by 4. The structure represents the conventional module model in which the Un CG is replaced by the the CaF2-CG. CaF2-CG. Appl. Sci. 2020, 10, 5647 5 of 9 Appl. Sci. 2020, 10, x FOR PEER REVIEW 5 of 9

Figure 4. CaF2-coated BSG on solar cell to represent the conventional photovoltaic (PV) module model. Figure 4. CaF2-coated BSG on solar cell to represent the conventional photovoltaic (PV) module Tablemodel.1 shows a comparison of the di fferent PV parameters of the Un CG and CG samples on SC, having average and corresponding standard deviation values. The data were analyzed after recording the performanceTable 1 shows data a ofcomparison five different of the samples. different PV parameters of the Un CG and CG samples on SC, having average and corresponding standard deviation values. The data were analyzed after recordingTable 1.theComparison performance of the data different of five electrical different parameters samples. of a solar cell (SC), CaF2-coated glass (CG)/SC and uncoated glass (Un CG)/SC measured under 1 sun illumination (100 mW/cm2). Table 1. Comparison of the different electrical parameters of a solar cell (SC), CaF2-coated glass Measurement Conditions V (V) J (mA/cm2) FF η (%) ∆η (%) (CG)/SC and uncoated glass (Unoc CG)/SC meassc ured under 1 sun illumination (100 mW/cm2). SC 0.7072 0.0008 40.58 0.28 0.7406 0.0017 21.25 0.16 0.00 ± ± ± ± CG/SC 0.7057 0.0007 39.03 0.45 0.7402 0.0020 20.39 0.24 4.05 Measurement Voc ± Jsc ± ± ±η − Δη Un CG/SC 0.7045 0.0011 38.10 0.43 0.7406FF0.0030 19.88 0.24 6.45 Conditions (V) ± (mA/cm± 2) ± ±(%) −(%) SC 0.7072 ± 0.0008 40.58 ± 0.28 0.7406 ± 0.0017 21.25 ± 0.16 0.00 StatisticalCG/SC Analysis 0.7057 ± 0.0007 39.03 ± 0.45 0.7402 ± 0.0020 20.39 ± 0.24 −4.05

TheUn analysis CG/SC showed 0.7045 that there± 0.0011 is a reasonable 38.10 ± 0.43 improvement 0.7406 ± being0.0030 observed 19.88 in ± J0.24sc and η −as6.45 a result of applying ARC on glass. The CG on SC have an average Jsc and standard deviation of 39.03 0.45 ± withStatistical an eff ectiveAnalysisη of 20.39 0.24, respectively, as compared to SC having Un CG with Jsc and standard ± deviation of 38.10 0.43, with an effective η of 19.88 0.24, respectively, as shown in Figure5a. The analysis showed± that there is a reasonable improvement± being observed in Jsc and η as a Theresult CG of couples, applying lighter ARC on in glass. the device, The CG enhance on SC have the J scanand averageη of J thesc and structure standard [32 deviation]. The increase of 39.03 in ± e0.45ffective withη ofan 2.40%effective is observed η of 20.39 by ± increasing 0.24, respectively, the transmittance as compared of ~4% to afterSC having applying Un ARCCG with on glass Jsc and as comparedstandard deviation to Un CG. of The 38.10 relation ± 0.43, between with an theeffective current–voltage η of 19.88 ±(J–V) 0.24, curverespectively, for the Unas shown CG and in CGFigure on 2 SC,5a. measuredThe CG couples, under 1lighter sun illumination in the device, (100 enhance mW/cm the) is Jsc shown and η inof Figurethe structure5b. [32]. The increase in effectiveSupporting η of 2.40% Documents is observed show by the increasingJ–V curve the for transmittance five different of PV ~4% module after structures.applying ARC on glass as compared to Un CG. The relation between the current–voltage (J–V) curve for the Un CG and CG 3.3.2. Light Weight Module on SC, measured under 1 sun illumination (100 mW/cm2) is shown in Figure 5b. The addition of thin glass between the PMMA and SC can help the surface mechanical integrity and the reduce moisture in a light weight PV module. It improves the reliability of module. The performance of the coating in the light weight module model was analyzed by placing the CaF2-CG between the PMMA and SC. Table2 shows a comparison of the di fferent PV parameters by placing the Un CG and CG between the PMMA and SC having average and corresponding standard deviation values. The data were analyzed after recording the performance data of five different samples.

Table 2. Comparison of the electrical parameters of a the polymethyl methacrylate (PMMA)/SC, PMMA/CG/SC and PMMA/Un CG/SC measured under 1 sun illumination (100 mW/cm2).

2 Measurement Conditions Voc (V) Jsc (mA/cm ) FF η (%) ∆η (%) PMMA/SC 0.7089 0.0010 37.80 0.45 0.7603 0.0015 20.37 0.25 0.00 ± ± ± ± PMMA/CG/SC 0.7082 0.0010 36.43 0.43 0.7612 0.0012 19.64 0.24 3.58 ± ± ± ± − PMMA/Un CG/SC 0.7074 0.0008 35.65 0.36 0.7601 0.0018 19.17 0.20 5.90 ± ± ± ± − Appl. Sci. 2020, 10, x FOR PEER REVIEW 6 of 9 Appl. Sci. 2020, 10, 5647 6 of 9

21.66 45 ) 2 21.09 40 20.52 35

19.95 mA/cm Efficiency Efficiency (%)

( 30

19.38 25 SC CG/SC Un CG/SC

41 20 SC

) CG/SC 2 40 15 Un CG/SC 39 10 (mA/cm sc

J 5 38 Current Density 0 37 SC CG/SC Un CG/SC 0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 Voltage (V)

(a) (b)

Figure 5.Figure(a) Statistical 5. (a) Statistical analysis analysis of Jsc ofand Jsc andη on η on the the SC, SC, CG CG/SC/SC and the the Un Un CG/SC. CG/SC. The The graph graph shows shows the the datadata from from ﬁve five diﬀ erentdifferent cells, cells, which which were were analyzed analyzed with with standard standard deviationdeviation values; (b (b) )the the J–VJ–V characteristicscharacteristics of a SC, of CG a SC,/SC CG/SC and Un and CG Un/SC. CG/SC.

Statistical AnalysisSupporting Documents show the J–V curve for five different PV module structures.

The3.3.2. analysis Light showed Weight thatModule there is a reasonable improvement being observed in Jsc and η as a result of putting CG between PMMA and SC. The PMMA/CG/SC have an average Jsc and standard deviation The addition of thin glass between the PMMA and SC can help the surface mechanical integrity of 36.43 0.43 with an eﬀective η of 19.64 0.24 as compared to PMMA/Un CG/SC, which have Jsc and± the reduce moisture in a light weight± PV module. It improves the reliability of module. The and a standard deviation of 35.65 0.36 with an eﬀective η of 19.17 0.20, respectively, as shown performance of the coating in± the light weight module model was analyzed± by placing the CaF2-CG in FigurebetweenAppl.6a. Sci. The 2020 the relation, 10 PMMA, x FOR betweenPEER and REVIEWSC. Table the current 2 shows density–voltagea comparison of the(J–V) differentcurve PV for parameters the Un CGby placing and7 of 9 CG 2 betweenthe the Un PMMA CG and and CG SC, between measured the under PMMA 1 sun and illumination SC having average (100 mW and/cm corresponding), is shown in standard Figure6b. deviation values. The data were analyzed after recording the performance data of five different samples. 45

21.00 ) 2 40 Table20.25 2. Comparison of the electrical parameters of a the polymethyl methacrylate (PMMA)/SC, 35 2 PMMA/CG/SC19.50 and PMMA/Un CG/SC measured under 1 sun illumination (100 mW/cm ). 30 Efficiency (%) 18.75 mA/Cm

Measurement Voc ( Jsc η Δη 25 18.00 2 FF ConditionsPMMA/SC PMMA/CG/SC PMMA/Un CG/SC (V) (mA/cm ) (%) (%)

39 20 PMMA/SC 0.7089 ± 0.0010 37.80 ± 0.45 0.7603 ± 0.0015 20.37 ± 0.25 0.00 ) 2 38 15 PMMA/SC PMMA/CG/SC 0.7082 ± 0.0010 36.43 ± 0.43PMMA/CG/SC 0.7612 ± 0.0012 19.64 ± 0.24 −3.58 37 10 (mA/cm

sc PMMA/Un CG/SC PMMA/UnJ CG/SC 0.7074 ± 0.0008 35.65 ± 0.36 0.7601 ± 0.0018 19.17 ± 0.20 −5.90 36 5

35 PMMA/SC PMMA/CG/SC PMMA/Un CG/SC 0 Statistical Analysis Current Density 0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 The analysis showed that there is a reasonable improvement being observed in Jsc and η as a Voltage(V) result of putting CG between PMMA and SC. The PMMA/CG/SC have an average Jsc and standard deviation of 36.43 ± 0.43 with an effective η of 19.64 ± 0.24 as compared to PMMA/Un CG/SC, which (a) (b) have Jsc and a standard deviation of 35.65 ± 0.36 with an effective η of 19.17 ± 0.20, respectively, as

Figureshown 6.Figure(a in) StatisticalFigure 6. (a) Statistical 6a. analysis The relationanalysis of Jsc ofbetweenand Jsc andη on η theon the the current PMMA PMMA/SC, density–voltage/SC, PMMAPMMA/CG/SC/CG /(J–V)SC and and curve PMMA/Un PMMA for the/Un CG/SC Un CG CG./ SC. and TheCG graph betweenThe shows graph the shows dataPMMA the from data and ﬁve from SC, diﬀ fiveerentmeasured different SCs, whichunder SCs, which were1 sunanalyzed were illumination analyzed with standardwith(100 standard mW/cm deviation deviation2), is values;shown in (b) theFigureJ–values;V 6b.characteristics (b) the J–V characteristics of a PMMA /ofSC, a PMMA/SC, PMMA/CG PMMA/CG/SC/SC and PMMA and/ PMMA/UnUn CG/SC. CG/SC.

SupportingSupporting Documents Documents show show the J–Vthe J–Vcurve curve for for ﬁve five di differentﬀerent lightlight weight weight PV PV module module structures. structures.

3.4. Weather-Resitance3.4. Weather-Resitance Test Test The brine pollution as a weather resistant test was performed to check the adhesive property of The brine pollution as a weather resistant test was performed to check the adhesive property of ARC ARC on glass [33]. The coated glass was immersed in a 0.5 M NaCl solution for 30 min. The on glasstransmittance [33]. The coated of CG glass before was performing immersed the in test a 0.5 wa Ms 95.12% NaCl solutionwhile it became for 30 94.95% min. The after transmittance the test. The of decrease of only 0.18% was seen in transmittance. The results suggested that this ARC has weather- resistant ability.

4. Conclusions

In summary, the optical properties of a CaF2 as ARC using vacuum thermal evaporation on borosilicate glass substrates are investigated successfully. The optical characteristics of uncoated and CaF2-CG were simulated using Macleod simulations and compared experimentally. In addition, the electrical performance of the CaF2-CG was analyzed for conventional and light weight module applications. The results showed reasonable improvement in the short circuit current density and module efficiency. The findings suggest that, because of the lower solubility of CaF2 when compared to that of MgF2, the CaF2-ARC on glass can be beneficial for improving the electrical performance of solar cell modules, and can be utilized effectively in both types of PV modules (conventional and light weight). The CaF2-ARC showed a good weather-resistant ability. Moreover, various treatments like annealing and hexamethyldisilazane on coated surface could be an interesting topic in the future to further improve the optical properties and environmental sustainability of this material on glass.

Funding: This research was funded by the Korea Institute of Energy Technology Evaluation and Planning, Korea Government. Appl. Sci. 2020, 10, 5647 7 of 9

CG before performing the test was 95.12% while it became 94.95% after the test. The decrease of only 0.18% was seen in transmittance. The results suggested that this ARC has weather-resistant ability.

4. Conclusions

In summary, the optical properties of a CaF2 as ARC using vacuum thermal evaporation on borosilicate glass substrates are investigated successfully. The optical characteristics of uncoated and CaF2-CG were simulated using Macleod simulations and compared experimentally. In addition, the electrical performance of the CaF2-CG was analyzed for conventional and light weight module applications. The results showed reasonable improvement in the short circuit current density and module efficiency. The findings suggest that, because of the lower solubility of CaF2 when compared to that of MgF2, the CaF2-ARC on glass can be beneficial for improving the electrical performance of solar cell modules, and can be utilized effectively in both types of PV modules (conventional and light weight). The CaF2-ARC showed a good weather-resistant ability. Moreover, various treatments like annealing and hexamethyldisilazane on coated surface could be an interesting topic in the future to further improve the optical properties and environmental sustainability of this material on glass.

Author Contributions: Conceptualization, Y.H.C., and M.A.Z.; methodology, Y.H.C.; software, M.A.Z.; validation, S.Q.H., and Y.H.C.; formal analysis, M.A.Z., and S.Q.H.; investigation, M.A.Z.; data curation, M.A.Z., and S.Q.H.; writing—original draft preparation, M.A.Z.; writing—review and editing, M.A.Z., and S.Q.H.; supervision, Y.H.C., and J.Y.; project administration, Y.H.C., and J.Y.; funding acquisition, Y.H.C., and J.Y. All authors have read and agreed to the published version of the manuscript. Funding: This research was funded by the Korea Institute of Energy Technology Evaluation and Planning, Korea Government. Acknowledgments: This research was supported by the Korea Institute of Energy Technology Evaluation and Planning (KETEP) 20193010014850. Conﬂicts of Interest: The authors declare no conﬂict of interest.

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