Argon plasma treatment of silicon nitride (SiN) for improved antireflection coating on c-Si solar cells

Materials Science and Engineering B 215 (2017) 29–36

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Argon plasma treatment of silicon nitride (SiN) for improved antireflection coating on c-Si solar cells Hemanta Ghosh, Suchismita Mitra, Hiranmay Saha, Swapan Kumar Datta, Chandan Banerjee ⇑ Centre of Excellence for Green Energy and Sensor Systems, Indian Institute of Engineering Science & Technology, Shibpur, Howrah 711103, India

a r t i c l e

i n f o

Article history: Received 15 July 2016 Received in revised form 18 September 2016 Accepted 7 November 2016

Keywords: Argon plasma treatment Silicon nitride Silicon oxynitride Double layer anti-reflection coating PECVD Solar cell

a b s t r a c t Antireflection properties of argon plasma treated silicon nitride layer and its effect on crystalline silicon solar cell is presented here. Hydrogenated silicon nitride (a-SiN:H) layer has been deposited on a silicon substrate by Plasma Enhanced Chemical Vapour Deposition (PECVD) using a mixture of silane (SiH4), ammonia (NH3) and hydrogen (H2) gases followed by a argon plasma treatment. Optical analysis reveals a significant reduction in reflectance after argon plasma treatment of silicon nitride layer. While FESEM shows nanostructures on the surface of the silicon nitride film, FTIR reveals a change in SiAN, SiAO and NAH bonds. On the other hand, ellipsometry shows the variation of refractive index and formation of double layer. Finally, a c-Si solar cell has been fabricated with the said anti-reflection coating. External quantum efficiency reveals a relative increase of 2.72% in the short circuit current density and 4.46% in conversion efficiency over a baseline efficiency of 16.58%. Ó 2016 Elsevier B.V. All rights reserved.

1. Introduction According to the Shockley–Queisser limit or detailed balance limit, the maximum conversion efficiency that can be achieved by a solar cell is 33.7% assuming a single p-n junction with a band gap of 1.34 eV (AM 1.5 solar spectrum) taking optical and electrical losses into consideration [1]. Reflection loss from the top surface of a solar cell is one of the most important sources of losses in photovoltaic energy conversion apart from recombination and contact losses. While the integrated reflectance of bare silicon in the wavelength range of 300–1100 nm is
30%, it is reduced to 10–12% in presence of appropriate texturization of silicon surface [2]. Further, the integrated reflectance with respect to the incident photon flux is reduced to
5% in presence of an anti-reflection coating (ARC). Several materials like silicon oxide (SiO), silicon dioxide (SiO2), silicon nitride (Si3N4), titanium dioxide (TiO2), magnesium fluoride (MgF2) and zinc sulphide (ZnS) [3–9] have been used as antireflection coatings. With the view point of physics and technology, each material has inherent advantages and disadvantages. Among these, silicon nitride is the most widely used material for antireflection coating, especially in crystalline silicon solar cell industry.

⇑ Corresponding author. E-mail address: [email protected] (C. Banerjee). http://dx.doi.org/10.1016/j.mseb.2016.11.003 0921-5107/Ó 2016 Elsevier B.V. All rights reserved.

Conventional solar cells use a single antireflection coating for reduction of reflectivity in front surface. However with a single layer antireflection coating designed for one specific wavelength, their performance is limited over the entire solar spectrum. Double layer antireflection (DLAR) coatings have been found to be more effective over the entire visible spectrum [10,11]. For DLAR coatings, materials with suitable refractive index and low absorption coefficient are chosen. For example, SiO2 and TiO2 are often used as materials for DLAR coatings for their appropriate refractive index, good passivation qualities, chemical stability and easy fabrication process [12]. Reflectance can be further tuned with triple layered [13] or multilayered graded index anti-reflection coating [14] with desired value of refractive index. Earlier reports suggest that incorporation of nanostructures of dielectric materials like silica can also be used to reduce reflectance, especially in thin film solar cells, where texturization of thin substrate is difficult [15,16]. It has been demonstrated that refractive index of dielectric nanoparticles can be tuned by varying the structure, area coverage and particle size to enhance photon injection into the solar cell. Refractive index as low as
1.05 can be achieved for materials like porous SiO2 [15]. Such nanostructures of dielectric nanoparticles can be used to fabricate graded index antireflection coating, which result in drastically reduced reflectance over a wide range of wavelength. In order to realize DLAR coatings or multilayer graded index antireflection coatings, especially with nanostructures, different

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techniques are used like chemical vapour deposition [17], sputtering [18], atomic layer deposition [19] and chemical methods like sol-gel [20], chemical spray pyrolysis [21] and hydrolysis [22]. While processes like chemical vapour deposition require high substrate temperatures (
300 °C to 400 °C) during deposition [23] and
1000 °C for long durations during annealing thereafter, chemical processes like sol-gel or spray pyrolysis do not offer precise control over film thickness. Thus, realizing multilayer graded index antireflection coating, especially with nanostructures is not only technologically difficult, but also an expensive method for large scale production. Recently, surface of porous layers of silicon nitride were treated with argon and nitrogen RF plasmas in order to densify and alter their surface, so that abrupt transition can be obtained between porous and dense films [24] to be used as optical filters. Another report suggests that Ar/N2O plasma treatment improves the surface passivation qualities of silicon nitride layer [25]. Studies have also been pursued to observe the effect of plasma treatment on the first SiNx layer of double layer stack to be used as encapsulation barriers for flexible organic photovoltaic cells [26]. But, the effect of argon plasma treatment on the antireflection properties of silicon nitride layer has not been studied before. In this article, a novel process of application of argon plasma treated silicon nitride layer on front surface of crystalline silicon solar cell as an improved anti-reflection coating has been illustrated. Silicon nitride is first deposited on a textured silicon wafer by Plasma Enhanced Chemical Vapour Deposition (PECVD) process. It is then, subjected to argon plasma treatment resulting into nanostructured silicon oxynitride as top layer. This process of development of antireflection coating with nanostructured silicon oxynitride as top layer and bottom silicon nitride layer using PECVD and subsequent argon plasma treatment is a low temperature, controllable and reproducible one, which is also compatible with the present solar cell fabrication technology. This structure has also been simulated using Finite Difference Time Domain (FDTD) solutions from Lumerical Inc. [27] in order to observe the effect of dielectric nanoparticles on top of silicon nitride layer in improving the antireflection properties.

where q is the electron charge. The improvement in JL in presence of an anti reflection coating (JL(with ARC)) can be determined by:

2. Theory

where q is the charge of electron and EQE(k) is the wavelength dependent external quantum efficiency given by the ratio of electron-hole pairs collected to number of incident photons. EQE (k) is measured experimentally.

2.1. Criteria for antireflection coatings design

DJL JL ðidealARCÞ  JL ðwithARCÞ ¼ J L ðidealARCÞ JL

ð4Þ

where, JL(ideal ARC) is the photocurrent that can be achieved when R = 0. 2.2. Simulation model Fig. 1 shows the simulation model which is developed to study the antireflection properties of dielectric nanoparticles on top of silicon nitride layer using FDTD Solutions. Hemispherical nanoparticles have been considered as the structures that were observed after argon plasma treatment were hemispherical in shape. A plane wave source with AM1.5G spectra is incident on the cell structure with the double layer anti reflection coating. The boundary condition of FDTD simulation region in the z direction is considered as perfectly matched layer (PML) so that reflections from the rear surface can be avoided. On the x and y directions, the boundary condition is set as periodic as the structure repeats itself in these directions. Area coverage of 70% is considered as the nanoparticles were closely packed. A power monitor is placed above the plane wave source which gives the reflectance from the front surface of the silicon. Simulations have been performed for different radii of nanoparticles. 2.3. Quantum efficiency Practically, a solar cell suffers from recombination and contact losses other than reflection losses. The photocurrent of a solar cell under short circuit condition, obtained after fabrication of the device is wavelength dependent and is given by Jsc(k). Shortcircuit current density of the device can be calculated from the following relation,

Z J sc ¼ q

k2

k1

EQEðkÞUðkÞdk

ð5Þ

To estimate the reduction in reflectance of an anti reflection coating, the integrated reflectance with respect to the incident photon flux density should be taken into account [28,29]. The integrated reflectance, Rint for a wavelength range k1 to k2, 300–1100 nm in this case, is given by,

R k2 Rint ¼

RðkÞUðkÞdk R k2 UðkÞdk k1

k1

ð1Þ

where R(k) is the wavelength dependent reflectance of the antireflection coating and U(k) is the wavelength dependent photon flux density. Similarly, the normalised fraction of photons injected per unit area per second into the substrate is given by,

R k2 Injected photon fraction ¼

k1

½1  RðkÞUðkÞdk R k2 UðkÞdk k1

ð2Þ

Ignoring the recombination losses and contact losses the ideal photocurrent density is given by the relation,

Z JL ¼ q

k2

k1

½1  RðkÞUðkÞdk

ð3Þ

Fig. 1. Simulation model of silicon solar cell with double layer antireflection coating (DLARC).

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H. Ghosh et al. / Materials Science and Engineering B 215 (2017) 29–36 Table 2 Parameters of argon plasma treatment on SiN coated textured wafers.

3. Experimental details 3.1. Deposition of silicon nitride The hydrogenated amorphous silicon nitride films (a-SiNx:H) were simultaneously deposited on polished silicon as well as textured silicon wafers in each run by PECVD (HHV CT-100)powered by 13.56 MHz radiofrequency power supply. The polished silicon samples were used for Fourier Transform Infra Red (FTIR) spectroscopy measurements and textured silicon samples were used for Finite Element Scanning Electron Microscopy (FESEM), ellipsometry and reflection measurements. Deposition parameters were optimized after standard optical characterizations like reflectance and ellipsometry and investigating chemical compositions by FTIR measurements [29,30]. The deposition parameters used are given in the Table 1. The structural properties were observed by infrared spectroscopy using a Perkin Elmer Spectrum two FTIR spectrometer with a bare (no deposition) c-Si wafer as a reference. Bentham PVE300 spectrophotometer was used to measure the reflectance for the wavelength range of 300–1100 nm. FESEM from Carl Zeiss Sigma was used to observe the images before and after argon plasma treatment in the nanoscale range. Laser ellipsometer SE 400adv from Sentech was used for refractive index measurements. These samples were then, subjected to argon plasma treatment. The different parameters used for argon plasma treatment have been discussed in the following sub-section.

Sample

Pressure (Torr)

Time

SiN (before treatment) Sample 1 Sample 2 Sample 3 Sample 4 Sample 5 Sample 6 Sample 7

1 1 1 1 0.5 0.5 0.5

40 min 1h 1.5 h 2h 1h 1.5 h 2h

HF solution, followed by edge isolation by plasma etching method using SF6 gas. Next, silicon nitride was deposited on the emitter layer, which serves as the antireflection coating. The process parameters have been discussed in the previous sub section. While metallization was carried out for few wafers after deposition of silicon nitride, other wafers were subjected to argon plasma treatment before metallization. For metallization, silver (Ag) paste was used as front contact and aluminium (Al) paste was used for partial rear contact. The external quantum efficiency (EQE) was measured from device spectral responsivity using Bentham PVE 300. I-V characteristics under AM 1.5G was also measured to obtain the basic parameters of the solar cell including open circuit voltage, fill factor and short circuit current in order to estimate the enhancement in efficiency corresponding to the plasma treated anti reflection coating.

3.2. Argon plasma treatment 4. Results and discussion Silicon nitride was deposited on the polished and textured c-Si wafers using PECVD, the parameters of deposition being same for all samples (Table 1). In order to obtain the optimized parameters for argon plasma treatment of silicon nitride layer, the polished wafer samples as well as textured wafer samples were simultaneously subjected to argon plasma treatment in each condition. Different parameters like the working pressure, time duration and power density has been varied keeping a constant gas flow rate of argon (100 sccm). The pressure was varied from 0.5 Torr to 1 Torr, treatment time was varied from 30 min to 120 min and power density was varied from 566 mW/cm2 to 1700 mW/cm2. The IR absorption, refractive index and reflectance have been observed for each case and have been discussed in the successive section. In Table 2, the treatment parameters have been summarized. 3.3. Solar cell fabrication Fabrication of device was started with a 180 lm thick p-type ascut wafer. The area of the wafer was 58 cm2. After proper cleaning, saw damage removal was done with 10% KOH (Potassium Hydroxide) for 5 min. After this, texturization of the wafer was carried out using 3.5% KOH and 5% IPA (Iso Propyl Alcohol) for 40 min. Further, a shallow n+ emitter layer with sheet resistance of 60 X/h on a p-type substrate was formed by the process of diffusion using POCl3 in a closed tube diffusion furnace (SVCS-SVFUR-AH3). During the process of diffusion, an insulating layer of Phospho Silicate Glass (PSG) is generally formed which was removed using 2–4%

4.1. Reflectance at different conditions Fig. 2(a) and (b) show the effect of argon plasma treatment on the reflectance of silicon nitride for different durations of treatment. Fig. 2(a) shows the variation of reflectance when the working gas pressure is 1 Torr and Fig. 2(b) shows the variation for 0.5 Torr. The results have been shown for a power density of 1132 mW/cm2. It can be observed that significant reduction in reflection takes place as compared to that of untreated ARC cells in the wavelength region 300–500 nm and 800–1100 nm in most of the cases. The reduction can be attributed to the formation of double layer antireflection coating (as discussed in Section 4.2) as well as the formation of nanostructures confirmed by FESEM images (Fig. 4(b)). Table 3 shows the integrated reflectance of the samples after silicon nitride is treated with argon plasma at different working pressures and for different time durations. The integrated reflectance after silicon nitride deposition and before argon plasma treatment on textured wafer is 5.7% which might lead to an achievement of maximum current of 40.04 mA/cm2. After argon plasma treatment, integrated reflectance of 4.7% is achieved for sample 2 where the working gas pressure is 1 Torr and the time duration of treatment is 1 h. In this case the maximum current that can be achieved increases by 1.07–40.47 mA/cm2 with a proportionate increase in efficiency. During fabrication of crystalline silicon solar cell, the metallization is done after deposition of antireflection coating. If metalliza-

Table 1 Deposition parameters for a-SiNx:H. Substrate temperature

Time duration (min)

Working pressure (Torr)

Power density (mW/cm2)

Gas flow (sccm) SiH4

NH3

H2

350 °C

2

1

178

5

25

25

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H. Ghosh et al. / Materials Science and Engineering B 215 (2017) 29–36

Fig. 2. Reflectance observed after argon plasma treatment for a gas pressure of (a) 1 Torr (b) 0.5 Torr.

Table 3 Integrated reflectance obtained after argon plasma treatment.

Fig. 3. Reflectance observed after firing of silicon nitride at high temperature.

Sample type

Integrated reflectance (%)

Percentage of incident photons injected (%)

Maximum current that can be obtained (mA/cm2)

SiN (before treatment) Sample 1 Sample 2 Sample 3 Sample 4 Sample 5 Sample 6 Sample 7

5.70 5.40 4.70 4.95 5.02 5.20 5.17 5.39

94.30 94.59 95.30 95.05 94.98 94.80 94.83 94.61

40.04 40.17 40.47 40.37 40.33 40.26 40.27 40.18

To investigate the repeatability of the experiment, ten SiN coated samples were subjected to argon plasma treatment for 1 h at 1 Torr pressure. SiN was deposited on all the samples simultaneously with integrated reflectance of 5.7%. The histogram shown in Fig. 4 gives a statistical variation of reflectance obtained for the samples after argon plasma treatment. It can be observed from Fig. 4 that, the integrated reflectance varies in a very small range of 4.6–4.9% where the maximum number of samples exhibit 4.7–4.8% integrated reflectance.

4.2. Structural characterization

Fig. 4. Histogram depicting reproducibility of integrated reflectance after argon plasma treatment of SiN layer for 1 h at 1 Torr pressure.

tion is done by the method of screen printing, then the cells must go through a very high temperature process of firing where a maximum temperature of
900 °C is attained. In order to examine whether the reflectance of argon treated silicon nitride layer remains unaltered, the sample exhibiting the best reflectance i.e. sample 2, was fired at the standard high temperature profile viz. 350–730–900 °C in the 3 zone infrared fast firing belt furnace (Hengli) used in our laboratory. It was found that, the integrated reflectance changes insignificantly after firing. The reflectance curve is shown in Fig. 3.

FESEM images were of silicon nitride before and after argon plasma treatment are compared and shown in Fig. 5(a) and (b), respectively. The smooth surface of silicon nitride on textured silicon is seen in Fig. 5(a). On the other hand, Fig. 5(b) shows the nanostructures formed on the textured surface after argon plasma treatment. FTIR results are shown in Fig. 6(a) and (b). The primary focus is on the change in the bonding structures which vary due to the plasma treatment. Fig. 6(a) shows the variation in bonding structure when the working pressure is 1 Torr and Fig. 6(b) shows the variation for 0.5 Torr. FTIR spectra of untreated SiN show SiAN stretching (825 cm1), SiAH stretching (2140 cm1), NAH stretching (3300 cm1) and NAH wagging (1185 cm1) modes which is typical of silicon nitride layers [31]. The SiAN stretching vibration mode depicts the NASiAH bonds at a Si site with one N atom, one H atom, and two Si atoms as the nearest neighbors. The SiAH stretching bond appears at 2140 cm1, which is slightly higher than usual case. This is because the nitrogen atom is more electronegative than silicon which results in the increase in the number of nitrogen atoms

H. Ghosh et al. / Materials Science and Engineering B 215 (2017) 29–36

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Fig. 5. FESEM images (a) before argon plasma treatment on textured Si (b) after argon plasma treatment on textured Si (c) after argon plasma treatment on polished Si.

Fig. 6. FTIR spectra of untreated SiN layer and argon plasma treated layer when the working gas pressure is (a) 1 Torr (b) 0.5 Torr (Spectra have been offset for clarity).

which are back bonded to SiAH causing the SiAH stretching mode to shift from a lower value of 2080 cm1 to a higher value of 2140 cm1 due to the induction effect [31]. Presence of NAH wagging and stretching modes denotes the high hydrogen content present in the films as silane is used as working gas. After plasma treatment, it can be seen that NAH bonds disappear and SiAO bonds appear at 1080 cm1, which indicate the occurrence of some oxidation. Since, silicon oxynitride (SiOxNy) is a tetrahedral amorphous alloy, the presence of peaks corresponding to SiAO and SiAN bonds indicate the formation of silicon oxynitride layer [32–34]. The shifting of absorption peak maxima and change in area under the peaks corresponding to SiAO and SiAN bonds depend on substitution of nitrogen atoms by oxygen ones during plasma treatment and the (O/O + N) ratio in the silicon oxynitride layer.

We have analyzed mainly the peaks in the range 700– 1200 cm1, by using the standard deconvolution technique [35] since we can observe significant changes in this region. The main absorbance peak in this wavenumber range has been deconvoluted into four Gaussian peaks. The peak height, area under each of these peaks and FWHM has been found. The area under the curve has been calculated. De-convolution of two FTIR spectra have been shown in Fig. 7 as an example. The four wavenumber components (t1–t4) along with their corresponding integrated absorbance intensities (At1–At4) and FWHM have been presented in the following table. From Table 3 it can be observed that, integrated absorption intensity of SiAN bonds (At1) decrease. The intensity increases

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H. Ghosh et al. / Materials Science and Engineering B 215 (2017) 29–36

Fig. 7. Multiple Gaussian component fits for (a) as deposited silicon nitride (b) argon plasma treated silicon nitride under a pressure of 1 Torr for 60 min.

for SiAO bonds (At4) with duration of plasma treatment. The maximum intensity is observed for working pressure = 0.5 Torr and treatment duration of 90 min. These intensities indicate the formation of silicon oxynitride as a result of argon plasma treatment. This may have occurred due to argon plasma induced damaged SiN layers which usually get easily oxidised. Lower pressure leads to increase of Ar-ion velocity because of longer mean free path and it leads to creation of more damages which easily make bonds with O atoms which are present in the gas ambient within the chamber at a pressure of 0.5–1 Torr. If treatment duration is long, the SiAO bond intensity is higher for the plasma treated film and the SiAN and NAH bond intensity decrease. As the presence of SiAO bonds increase, the reflectance of the plasma treated silicon nitride layer decreases as the refractive index of plasma treated silicon nitride layer tends to be towards that of silicon oxide (RI = 1.45) instead of silicon nitride (RI = 2). 4.3. Simulation results FTIR results reveal the formation of silicon oxynitride (SiOxNy) due to argon treatment of silicon nitride layers. The reduction in reflection may be attributed to the formation of a silicon oxynitride layer on the top surface, thereby leading to the formation of double layer anti reflection coating. The refractive index of silicon oxynitride may vary from 1.45 to 2.3 depending on the volume fraction silicon oxide (RI = 1.45) and silicon nitride (1.9 6 RI 6 2.3). The optical behaviour of homogeneous SiOxNy can be successfully described by using the Bruggeman effective medium approximation (EMA) model considering a simple dielectric physical mixture of two distinct phases, SiO2 and SiNx, using Eq. (6) [36,37]. According to the EMA, the effective refractive index, n, of SiOxNy can be obtained from the following equations:

f SiO2

n2SiO2  n2 n2SiO2

þ

2n2

þ f SiNx

n2SiNx  n2 n2SiNx þ 2n2

¼ 0 and f SiO2 þ f SiNx ¼ 1

remained unaltered after argon treatment. Thus, two sets of simulations structures were designed in order to observe the effect of double layer anti reflection coating. In the first set, only a single anti-reflection layer of silicon nitride with a refractive index of 1.96 and 90 nm thickness was simulated. In the second set, the refractive index of top silicon oxynitride hemispherical nanoparticle was tuned from 1.5 to 2.1 with wavelength and the refractive index of bottom silicon nitride layer was taken as 1.96. Antireflectance properties were observed for different radii of hemispherical nanoparticles. The results are shown in the Fig. 8. It can be observed from the reflectance curves that reduction of reflectance takes place at lower wavelength range 300–550 nm and at higher wavelengths from 850 nm to 1100 nm, similar pattern to the experimental values. Although, the experimental results show much lower reflectance values in the wavelength range 300– 550 nm, the same reduction cannot be observed in theoretical values as textured surface of silicon was not taken into account during software simulations. Observing the FDTD simulations, it can be predicted that the time duration of argon plasma plays an important role as the nanoparticle radius and refractive index of silicon oxynitride affects the performance of double layer anti reflection coating. The hemispherical nanoparticle forward scatters light into the silicon substrate by reradiating the incident light energy depending on the radius of the nanoparticle. The resonance frequency of nanoparticles of silicon oxynitride depends on the size, shape, and refractive index of the surrounding medium of SiN in this case [38].

ð6Þ

where fSiO2, and fSiNx represent the relative volume fractions of SiO2, and SiNx in the mixture, and nSiO2 and nSiO2 are the refractive indices of SiO2 and SiNx, respectively. Furthermore, for a given spectral range, the refractive index of SiOxNy has been found to be a linear function of its constituent volume fraction at each wavelength. Considering all these facts, a double layer antireflection coating on a silicon wafer was simulated using FDTD solutions to observe the antireflection properties. The refractive index of the bottom silicon nitride layer was found to be 1.96 at 600 nm and argon treated silicon nitride layer as 2 at 600 nm by using laser ellipsometer. The thickness of silicon nitride layer was found as 90 nm which

Fig. 8. Simulated reflectance of single layer antireflection coating (SiNx = 90 nm) and double layer antireflection coating (SiNx = 50 nm + hemispherical nanoparticles of different radii).

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H. Ghosh et al. / Materials Science and Engineering B 215 (2017) 29–36

4.4. Device characteristics In this section, we shall discuss the results of the solar cell device which was fabricated with the argon treated silicon nitride layer as antireflection coating. Since, the metallization is done after argon treatment of silicon nitride layer, the effect of the argon plasma treated antireflection coating on the device is observed by comparing it with a solar cell with untreated silicon nitride. Fig. 9 shows the reflectance curve before and after argon plasma treatment. During the plasma treatment working pressure was 1 Torr, argon gas flow rate was 100 sccm and power density was 1132 mW/cm2. The treatment was carried out for 1 h. In this case, the integrated reflectance of silicon nitride layer without treatment

Fig. 11. EQE vs wavelength for the fabricated solar cells.

Fig. 9. Reflectance before and after argon treatment on silicon nitride layer in optimized conditions.

was 5.75%, which was reduced to 4.80% after argon plasma treatment-about 16.5% decrease in integrated reflectance. The current-voltage characteristic of the fabricated solar cells, with untreated SiN (baseline) and with argon plasma treated SiN were measured under AM1.5G illumination. Fig. 10 shows the IV characteristics and Table 4 shows the PV parameters and the calculated efficiency. It can be observed from the IV curves that the short circuit current density has increased by 2.72%. The increase in open circuit voltage is due to the increase in photogenerated current. Overall, the absolute efficiency increases from 16.58% to 17.32%, a relative increase of 4.46% over the baseline efficiency is achieved (Table 5). The external quantum efficiency has been measured for both the devices. As expected from the reflectance curves, the percentage increase in the EQE is maximum in the range of 300–500 nm. An increase in EQE is also observed in the high wavelength ranges of 800–1100 nm as shown in Fig. 11.

5. Conclusions In this article, an improvement in antireflection properties of SiN coating by argon plasma treatment has been demonstrated. Experiments have been carried out to find the different parameters required for argon plasma treatment, in order to achieve the optimum reflectance. FESEM and FTIR spectra reveal the formation of silicon oxynitride nanostructures as a result of argon plasma treatment. The ratio of SiAN and SiAO bonds depends on the working gas pressure as well as the duration of treatment. Further, ellipsometry have been performed to find out the reason behind the reduction in reflectance. The results are used to simulate and observe the reflectance of nanostructured and double layer

Fig. 10. Current voltage characteristics of fabricated solar cells.

Table 4 Peak positions, integrated absorbance intensities and FWHM for the Gaussian components of the main absorbance peaks. Sample

t1

A t1

FWHM (t1)

t2

A t2

FWHM (t2)

t3

A t3

FWHM (t3)

t4

A t4

FWHM (t4)

SiN SiN SiN SiN SiN

825 823 831 820 819

6.48 5.72 2.26 2.74 6.27

98.14 96.17 43.55 61.48 85.31

912 918 914 902 918

4.2 3.7 1.37 1.58 2.55

114.25 92.46 45.42 80.45 72.94

1185 – – – –

0.089 – – – –

29.8 – – – –

– 1080 1083 1073 1074

– 6.67 13.16 15.05 30.84

– 151.56 73.65 81.74 87.06

before Treatment (1 Torr, 60 min) (1 Torr, 90 min) (0.5 Torr, 60 min) (0.5 Torr, 90 min)

Table 5 PV parameters of the fabricated solar cells. Sample No.

Integrated reflection (%)

Jsc (mA/cm2)

Voc (V)

FF

Pm (mW/cm2)

Efficiency (%)

Before argon treatment After argon treatment

5.75 4.80

34.90 35.85

0.621 0.629

76.50% 76.80%

16.58 17.32

16.58 17.32

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anti-reflection coating, which is expected to be formed as a result of argon plasma treatment. Simulation results are in conformity with the experimental results. Finally, a solar cell with argon plasma treated anti-reflection coating has been fabricated. While the photogenerated current increases by 2.72% due to reduction in reflectance, open circuit voltage also increases as a result of increase in photogenerated current. As a result, a relative increase of 4.46% over baseline efficiency of 16.58% is observed due to the argon plasma treatment of SiN antireflection. Acknowledgements This work is supported by Department of Science and Technology, Government of India. The authors are thankful to BHEL-ASSCP for laser ellipsometer measurements. The authors would also like to thank Prof. Subhendu Guha and Prof. Utpal Gangopadhyay for fruitful discussions. References [1] W. Shockley, H.J. Queisser, Detailed balance limit of efficiency of p-n junction solar cells, J. Appl. Phys. 32 (1961) 510–519. [2] U. Gangopadhyay, S.K. Dutta, H. Saha, Texturization and Light Trapping in Silicon Solar Cells, Nova Science Publishers, 2009. [3] M. Asad, M. Kowsari, M. Hossein Sheikhi, Enhancement of nano-/ microtextured crystalline silicon solar cells efficiency using hydrogen plasma surface treatment, Optik 126 (2015) 5762–5766. [4] P. Li, Y. Wei, Z. Zhao, X. Tan, J. Bian, Y. Wang, C. Lu, A. Liu, Highly efficient industrial large-area black silicon solar cells achieved by surface nanostructured modification, Appl. Surf. Sci. 357 (2015) 1830–1835. [5] C. Martinet, V. Paillard, A. Gagnaire, J. Joseph, Deposition of SiO2 and TiO2 thin films by plasma enhanced chemical vapor deposition for antireflection coating, J. Non-Cryst. Solids 216 (1997) 77–82. [6] S.K. Dhungel, J. Yoo, K. Kim, S. Jung, S. Ghosh, J. Yi, Double-layer antireflection coating of MgF2/SiNx for crystalline silicon solar cells, J. Korean Phys. Soc. 49 (2006) 885–889. [7] R. Kishore, S.N. Singh, B.K. Das, Screen printed titanium oxide and PECVD silicon nitride as antireflection coating on silicon solar cells, Renewable Energy 12 (1997) 131–135. [8] U. Gangopadhyay, K. Kim, D. Mangalaraj, Y. Junsin, Low cost CBD ZnS antireflection coating on large area commercial mono-crystalline silicon solar cells, Appl. Surf. Sci. 230 (2004) 364–370. [9] R.R. Bilyalov, L. Stalmans, L. Schirone, Use of porous silicon antireflection coating in multicrystalline silicon solar cell processing, IEEE Trans. Electron Devices 46 (1999) 2035–2040. [10] D.N. Wright, E.S. Marstein, A. Holt, Double layer anti-reflective coatings for silicon solar cells, Conference Record of the Thirty-first IEEE Photovoltaic Specialists Conference, Florida, USA, 2005, pp. 1237–1240. [11] Chunlan Zhou, Tao Li, Yang Song, Zhou Su, Wenjing Wang, Lei Zhao, Hailing Li, Yehua Tang, Hongwei Diao, Gao Zhihua, Duan Ye, Ye Duan, Youzhong Li, SiOx (C)/SiNx dual-layer antireflectance film coating for improved cell efficiency, Sol. Energy 85 (2011) 3057–3063. [12] K. Ali, S.A. Khan, M.Z.M. Jafri, Effect of double layer (SiO2/TiO2) anti-reflective coating on silicon solar cells, Int. J. Electrochem. Sci. 9 (2014) 7865–7874. [13] T.W. Kuo, N.F. Wang, Y.Z. Tsai, P.K. Hung, M.P. Houng, Broadband triple-layer SiOx/SiOxNy/SiNx antireflective coatings in textured crystalline silicon solar cells, Mater. Sci. Semicond. Process. 25 (2014) 211–218. [14] Y. Zhao, F. Chen, Q. Shen, L. Zhang, Optimal design of light trapping in thin film solar cells enhanced with graded SiNx and SiOxNy structure, Opt. Express 20 (2012) 11121–11136. [15] S. Chhajed, M.F. Schubert, J.K. Kim, E.F. Schubert, Nanostructured multilayer graded-index antireflection coating for Si solar cells with broadband and omnidirectional characteristics, Appl. Phys. Lett. 93 (2008) 251108.

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