# Triple Layer Antireflection Design Concept for the Front Side of c-Si Heterojunction Solar Cell Based on the Antireflective Effect of nc-3C-SiC:H Emitter Layer

Authors: Erick Omondi Ateto; Makoto Konagai; Shinsuke Miyajima
Journal: International Journal of Photoenergy
 (2016)

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## Abstract

We investigated the antireflective (AR) effect of hydrogenated nanocrystalline cubic silicon carbide (nc-3C-SiC:H) emitter and its application in the triple layer AR design for the front side of silicon heterojunction (SHJ) solar cell. We found that the nc-3C-SiC:H emitter can serve both as an emitter and antireflective coating for SHJ solar cell, which enables us to realize the triple AR design by adding one additional dielectric layer to normally used SHJ structure with a transparent conductive oxide (TCO) and an emitter layer. The optimized SHJ structure with the triple layer AR coating (LiF/ITO/nc-3C-SiC:H) exhibit a short circuit current density (Jsc) of 38.65 mA/cm2 and lower reflectivity of about 3.42% at wavelength range of 300 nm–1000 nm.


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## Body

## 1. Introduction

Crystalline silicon (c-Si) wafer based solar cell is currently the dominant technology in the market with a market share of more than 80% and is expected to remain the most prominent PV technology for the next decade. The recent research focus in c-Si wafer based solar cell is to increase the energy conversion efficiency and reduce the cost to make it more competitive with the conventionally used fossil fuel. In this context, SHJ solar cell technology has been demonstrated to be a promising candidate for high conversion efficiency solar cells beyond 25%.A typical example is the SHJ solar cell with the back contact configuration which has achieved up to 25.6% efficiency [1]. A high Voc of 0.750 V has also been achieved on n-type c-Si wafer using amorphous silicon (a-Si:H) as the emitter and back surface field [2]. This higher Voc is due to good passivation quality of intrinsic (i) a-Si:H used for passivation. However, the loss analysis of these SHJ solar cells indicates that there is nonnegligible Jsc loss attributed to the front a-Si:H emitter layer [3]. This is because charge carrier generated from a considerable amount of shorter wavelength photon of the solar spectrum absorbed in a-Si:H emitter layer instantaneously recombine due to high defect density in the layer. Another disadvantage of a-Si:H is its high refractive index, which is similar to that of c-Si. This leads to high reflection at the interface between a-Si:H and transparent conductive oxide. To solve this problem, relatively wider band-gap material with lower refractive index and low absorption coefficient should be used. For this end, hydrogenated nanocrystalline cubic silicon carbide (nc-3C-SiC:H) has been shown to be a promising material as an emitter layer for the heterojunction solar cell [4–8].One way to reduce the cost of c-Si solar cell is to reduce the wafer thickness used to fabricate the solar cells. It is estimated that the wafer processing accounts for approximately 50% of the solar cell cost. Thinner wafers can therefore play a fundamental role in the further reduction of the cost of solar cell [9]. SHJ solar cell structure is suitable for thinner c-Si wafer application [10]. The main disadvantage of thin c-Si wafer based SHJ solar cell is the poor absorption performance in the red and NIR part of the solar spectrum, which leads to lower Jsc. Thus excellent antireflection and light trapping is a requirement for such cells. In the state-of-the-art SHJ solar cell, antireflection and light trapping is realized by pyramidal texturing that can be achieved through alkali etching [11–13]. This combined with the transparent conductive oxide (TCO) leads to a very low reflectance and also increases the optical path length for all photons in the solar spectrum [11, 12, 14–17]. However such pyramidal texturing requires micron-size features that are inapplicable to novel thin wafers based SHJ solar cell since they become very fragile and delicate to handle. Moreover greater surface area increases carrier recombination in the surface between thin film and c-Si wafer, which has a detrimental effect on the Voc. Therefore other approaches for highly efficient antireflection and light trapping schemes have to be considered for thinner planar c-Si wafers based solar cell.For light trapping, advanced methods have been proposed to enhance light absorption in thinner wafers based solar cell [18–22]. Regrettably, all these light trapping schemes encounter some inherent limitation for use especially in the front side of the SHJ solar cell. Use of nanoparticles for the front emitter still has to be embedded in dielectrics layer to improve their light trapping effect. The nanoparticles at the front side also suppress the absorption of blue/green region light into the c-Si due to interference effect thereby leading to a decrease in Jsc for SHJ solar [23]. This suppression can only be avoided by locating nanoparticles array at the rear side of the SHJ solar cell to scatter and trap poorly absorbed longer wavelength light of the solar spectrum [24]. Other advanced light trapping schemes such as grating produced via nanoimprint lithography or self-assembled hexagonal sphere gratings that have been proposed are industrially incompatible and also can only be applied at the rear side of the structure. Therefore for front side of SHJ solar cell, multi-AR coating that can be used in conjunction with these advanced light trapping schemes need to be considered.In this paper, we investigated a triple layer AR concept for the front side of SHJ solar cell that capitalizes on the antireflective effect of nc-3C-SiC:H emitter layer. Use of emitter layer for AR effect has not been investigated before. This paper, therefore, provides a foundation for the design of the triple layer AR using transparent emitter layer and only one additional dielectric layer to the normally used SHJ structure. The optimization is conducted using device simulator, AFORS-HET [25]. Wide band-gap nc-3C-SiC:H with low absorption coefficients and refractive indices between c-Si and ITO therefore can serve both as an emitter and AR coating. In this paper lithium fluoride (LiF) is used as the additional layer; however, MgF2, Al2O3, and SiO2 also show very promising results [26, 27]. This work focuses only on the reflection and parasitic reduction at the front side of SHJ solar cell. To validate our analysis we compared our simulation result for our structure with the conventionally used a-Si:H emitter.


## 2. Cell Structure and Optical Model

### 2.1. SHJ Solar Cell Structure

The SHJ solar cell structure used in the simulation is shown in Figures1(a) and 1(b). The difference of these two is the front side structure. Figure 1(a) is the conventional structure consisting of an intrinsic (i) type buffer layer, emitter layer, and indium tin oxide (ITO) TCO layer at the front side. Figure 1(b) is the structure with additional layer of LiF dielectric layer, which henceforth will be called a triple AR coating structure. For the emitter layer, n-type a-Si:H and n-type nc-3C-SiC:H emitter layers are used. Intrinsic hydrogenated amorphous silicon (i-type a-Si:H) and intrinsic hydrogenated amorphous silicon carbide (i-type a-SiC:H) buffer layer are used as the i-type layer for a-Si:H and nc-3C-SiC:H emitter SHJ structure, respectively. For the rear side, intrinsic amorphous silicon oxide (i-type a-Si1-xOx:H) and p-type hydrogenated microcrystalline silicon oxide (p-type μc-Si1-xOx:H) were used as the back surface field (BSF). Aluminum (Al) was used as the front and rear electrode. The triple layer AR coating has been optimized on 200 μm thick silicon wafer. However, we varied the thickness of the silicon from 200 μm to 20 μm to ascertain the effect of c-Si thickness on the reflection especially at longer wavelength.Figure 1
(a) The schematic structure of the c-Si heterojunction structure with single AR coating. (b) Heterojunction structure with additional layer of LiF.
(a)
(b)


### 2.2. Optical Parameters of the Materials

The optical refractive indices and absorption coefficients of the materials used are shown in Figure2. The complex refractive indices of n-type nc-3C-SiC:H, i-type a-SiC:H, i-type a-Si1-xOx:H, p-type μc-Si1-xOx:H, ITO, and LiF were obtained from Woollam spectroscopic ellipsometry measurements of the individual layers deposited on c-Si wafers.Figure 2
(a) The refractive indices of c-Si, emitter layers, ITO, and LiF used in the simulation. (b) The absorption coefficient of c-Si, emitter layers, ITO, and LiF used in the simulation.
(a)
(b)The n-type nc-3C-SiC:H emitter was deposited by very high frequency plasma enhanced chemical vapor deposition (VHF-PECVD) method with an excitation frequency of 60 MHz. The p-typeμc-Si1-xOx:H and i-type a-Si1-xOx:H were deposited by RF-PECVD (13.56 MHz) and VHF-PECVD, respectively. The LiF layer was deposited by thermal evaporation. ITO was deposited by radio frequency (RF) magnetron sputtering. The details of the nc-3C-SiC:H emitter deposition, ITO deposition, and the overall solar cell fabrication process are reported in the previous publications [5, 7, 28, 29]. The complex refractive index included in the AFORS-HET package for n- and i-type a-Si:H, p-type c-Si, and Al electrode were used.


### 2.3. Optical Analysis Method

One of the useful figures of merit for the optimization of the ARC design is the weighted reflectanceRav shown as follows [30]:(1)Rav%=∫3001200∅λRλdλ∫3001200∅λdλ×100%,where Rλ is the simulated reflectance of the solar cell and ∅(λ) is the photo flux density of the AM 1.5 solar spectrum, which is adopted as the illuminating source in the simulation, with a wavelength range 300 nm–1200 nm in 1 nm step.Conventionally, weighted reflectance has been used as the only merit for optimization of the AR coating. However, due to the parasitic absorption losses in ITO layer, buffer layer, emitter layer, and rear metal electrode layer, it is very crucial to also consider absorbance in c-Si, to achieve more accurate design of AR coating as will be shown later. In the case of start-of-the art c-Si solar cells with well-passivated front and rear surface, the absorbance in c-Si is what contributes to the photo current density. The absorbance in c-Si can be evaluated using the weighted average absorbance (Aav) as shown in(2)Aav%=∫3001200∅λAc-Siλdλ∫3001200∅λdλ×100%,where Ac-Si (λ) is the absorbance in c-Si in each wavelength. This equation can also be used to calculate absorption in other layers of the SHJ structure. The denominator for (1) and (2) calculates a total photon flux of 2.9 × 1021 (photons s−1m−2), which corresponds to photo current density of 46.46 mA/cm2, at an internal quantum efficiency of 1 and reflectance of 0. Using Ac-Si, the photo current density due to absorbance in c-Si can therefore be calculated as shown in(3)Jc-SimAcm2=46.46×Ac-Si.This equation can also be used to calculate the parasitic current losses incurred in TCO, emitter, front i-type and rear metal electrode layers.


## 2.1. SHJ Solar Cell Structure

The SHJ solar cell structure used in the simulation is shown in Figures1(a) and 1(b). The difference of these two is the front side structure. Figure 1(a) is the conventional structure consisting of an intrinsic (i) type buffer layer, emitter layer, and indium tin oxide (ITO) TCO layer at the front side. Figure 1(b) is the structure with additional layer of LiF dielectric layer, which henceforth will be called a triple AR coating structure. For the emitter layer, n-type a-Si:H and n-type nc-3C-SiC:H emitter layers are used. Intrinsic hydrogenated amorphous silicon (i-type a-Si:H) and intrinsic hydrogenated amorphous silicon carbide (i-type a-SiC:H) buffer layer are used as the i-type layer for a-Si:H and nc-3C-SiC:H emitter SHJ structure, respectively. For the rear side, intrinsic amorphous silicon oxide (i-type a-Si1-xOx:H) and p-type hydrogenated microcrystalline silicon oxide (p-type μc-Si1-xOx:H) were used as the back surface field (BSF). Aluminum (Al) was used as the front and rear electrode. The triple layer AR coating has been optimized on 200 μm thick silicon wafer. However, we varied the thickness of the silicon from 200 μm to 20 μm to ascertain the effect of c-Si thickness on the reflection especially at longer wavelength.Figure 1
(a) The schematic structure of the c-Si heterojunction structure with single AR coating. (b) Heterojunction structure with additional layer of LiF.
(a)
(b)


## 2.2. Optical Parameters of the Materials

The optical refractive indices and absorption coefficients of the materials used are shown in Figure2. The complex refractive indices of n-type nc-3C-SiC:H, i-type a-SiC:H, i-type a-Si1-xOx:H, p-type μc-Si1-xOx:H, ITO, and LiF were obtained from Woollam spectroscopic ellipsometry measurements of the individual layers deposited on c-Si wafers.Figure 2
(a) The refractive indices of c-Si, emitter layers, ITO, and LiF used in the simulation. (b) The absorption coefficient of c-Si, emitter layers, ITO, and LiF used in the simulation.
(a)
(b)The n-type nc-3C-SiC:H emitter was deposited by very high frequency plasma enhanced chemical vapor deposition (VHF-PECVD) method with an excitation frequency of 60 MHz. The p-typeμc-Si1-xOx:H and i-type a-Si1-xOx:H were deposited by RF-PECVD (13.56 MHz) and VHF-PECVD, respectively. The LiF layer was deposited by thermal evaporation. ITO was deposited by radio frequency (RF) magnetron sputtering. The details of the nc-3C-SiC:H emitter deposition, ITO deposition, and the overall solar cell fabrication process are reported in the previous publications [5, 7, 28, 29]. The complex refractive index included in the AFORS-HET package for n- and i-type a-Si:H, p-type c-Si, and Al electrode were used.


## 2.3. Optical Analysis Method

One of the useful figures of merit for the optimization of the ARC design is the weighted reflectanceRav shown as follows [30]:(1)Rav%=∫3001200∅λRλdλ∫3001200∅λdλ×100%,where Rλ is the simulated reflectance of the solar cell and ∅(λ) is the photo flux density of the AM 1.5 solar spectrum, which is adopted as the illuminating source in the simulation, with a wavelength range 300 nm–1200 nm in 1 nm step.Conventionally, weighted reflectance has been used as the only merit for optimization of the AR coating. However, due to the parasitic absorption losses in ITO layer, buffer layer, emitter layer, and rear metal electrode layer, it is very crucial to also consider absorbance in c-Si, to achieve more accurate design of AR coating as will be shown later. In the case of start-of-the art c-Si solar cells with well-passivated front and rear surface, the absorbance in c-Si is what contributes to the photo current density. The absorbance in c-Si can be evaluated using the weighted average absorbance (Aav) as shown in(2)Aav%=∫3001200∅λAc-Siλdλ∫3001200∅λdλ×100%,where Ac-Si (λ) is the absorbance in c-Si in each wavelength. This equation can also be used to calculate absorption in other layers of the SHJ structure. The denominator for (1) and (2) calculates a total photon flux of 2.9 × 1021 (photons s−1m−2), which corresponds to photo current density of 46.46 mA/cm2, at an internal quantum efficiency of 1 and reflectance of 0. Using Ac-Si, the photo current density due to absorbance in c-Si can therefore be calculated as shown in(3)Jc-SimAcm2=46.46×Ac-Si.This equation can also be used to calculate the parasitic current losses incurred in TCO, emitter, front i-type and rear metal electrode layers.


## 3. Result and Discussion

For improving conversion efficiency, it is important for a c-Si solar cell to have minimal reflectance over the entire visible and near infrared spectrum. This can easily be achieved on SHJ solar cell by using textured wafer combined with single AR coating of TCO; the TCO also serves as a lateral transport medium for photo generated carriers to metal finger contact. In contrast to the textured wafers, a single AR coating is not enough for planar wafers. In the case of the single AR coating, there is the wavelength, which gives the minimum reflectivity, and low reflectivity is only obtained near the wavelength. On the other hand, the local minimum of the reflectivity is obtained at multiple wavelengths for multiple AR coating, which can widen the wavelength range for good antireflective performance.


### 3.1. Optical Simulation of the SHJ with Only ITO AR Coating

Figure3 shows the simulated reflectance and absorbance spectra of the SHJ solar cells with optimum single AR coating of ITO. We calculated the spectra for SHJ solar cells with nc-3C-SiC:H emitter and a-Si:H emitter. The optimal thicknesses of a-Si:H and nc-3C-SiC:H emitter were 10 nm and 25 nm, respectively. In the AR coating design, the thickness of the coating material is important to minimize the reflection. In our simulations, the optimal thicknesses of ITO obtained for a-Si:H and nc-3C-SiC:H SHJ structures were 80 nm and 60 nm, respectively.Figure 3
(a) The reflectance spectra for SHJ solar cells with nc-3C-SiC:H emitter and a-Si:H emitter with a single AR coating (b). The corresponding absorbance. The absorbance spectra are the absorbance in c-Si layer in the solar cells.
(a)
(b)The reflectance spectra of the SHJ solar cell with nc-3C-SiC:H emitter closely resemble the a-Si:H emitter counterpart with a slight variation at wavelength range of 300 nm–600 nm. This variation is due to the difference in parasitic absorption and the thicknesses of the emitter layer and ITO layer of the structures. For both structures, minimum reflectance is observed at one certain wavelengthλmin. The SHJ solar cell with nc-3C-SiC:H emitter shows the minimum reflectance is obtained at λmin=653 nm while λmin is found to be 635 nm for the SHJ solar cell with a-Si:H emitter.For a single AR coating, based on the reflectance spectra of the SHJ solar cell with nc-3C-SiC:H emitter shown in Figure3(a), it cannot be explicitly deduced whether nc-3C-SiC:H demonstrates an antireflective effect. However, it is worth noting that there is a minimum reflectance observed at wavelength lower than 300 nm which contributes to relatively lower reflectance in the range of 300 nm to 400 nm as compared to the SHJ solar cell with a-Si:H emitter.The total reflectance for the SHJ solar cell with a-Si:H emitter was found to be 17.2% corresponding to a reflectedJsc loss of 7.98 mA/cm2. While for the SHJ solar cell with nc-3C-SiC:H emitter was 17.6% corresponding to a reflected Jsc loss of 8.17 mA/cm2. Although the reflectance spectra for the two structures are comparable, with only slight variation at wavelength range 300 nm–600 nm, the absorbance in c-Si exhibited a significant difference. This difference is attributed to the absorption loss in the emitter layer. The nc-3C-SiC:H emitter significantly boosts light absorption in c-Si at wavelength range 300 nm–600 nm due to its higher transmittance compared to that of the a-Si:H emitter. In case of the a-Si:H emitter, the parasitic absorption in the a-Si:H emitter reduces the short wavelength (300 nm–600 nm) light that reaches the c-Si. This has a detriment effect on the overall Jsc that can be obtained from the SHJ solar cell.It should be also noted that even though the optimum thickness of nc-3C-SiC:H emitter (25 nm) is more than twice the optimum thickness of a-Si:H emitter (10 nm), the absorbance in a-Si:H emitter is evidently high. The absorbance in the a-Si:H emitter and nc-3C-SiC:H emitter is 4% and 0.42%, respectively. This absorbance in a-Si:H emitter contributes 1.86 mA/cm2  Jsc loss as compared to only a Jsc loss of 0.2 mA/cm2 for nc-3C-SiC:H emitter. In addition, other parasitic absorption incurred in i-type buffer layer and ITO layers of the two structures also is attributed to lower Jsc. The absorbance in ITO was 1.3% and 0.84% for SHJ solar cells with a-Si:H and nc-3C-SiC:H emitter, respectively. These correspond to a Jsc loss of 0.39 mA/cm2 and 0.60 mA/cm2 in ITO for SHJ with nc-3C-SiC:H and a-Si:H emitter, respectively. The Jsc loss in the i-type buffer layer of a-Si:H and a-SiC:H was found to be 0.48 mA/cm2 and 0.2 mA/cm2, respectively. However, as Holman et al. duly noted, 30% of the aforementioned i-layer Jsc losses can still contribute to Jsc [3]. The Jsc loss due to rear electrode was about 0.80 mA/cm2 for both structures. From this finding, it should be therefore emphasized that only minimizing reflectance does not guarantee maximization of the absorbance in c-Si. The parasitic absorption especially at the emitter and TCO layer also need to be taken into account when designing AR coating. Overall, Jsc calculated from absorbance in c-Si shown in Table 1 for SHJ solar cells with a-Si:H emitter with single AR coating was 34.74 mA/cm2 while that of SHJ solar cell with nc-3C-SiC:H emitter was 36.72 mA/m2.Table 1
Comparative result of the total reflectance and the total absorbance in each layer with different AR coating schemes and emitter layer for flat SHJ solar cell. The thicknesses of AR and emitter are optimized values.
AR coating
Weighted reflectance (%)
Absorbance (%)
ITO
Emitter
i-layer
c-Si
Metal
a-Si:H emitter
ITO
17.18
1.30
4.00
1.03
74.77
1.72
ITO & LiF
14.95
1.28
4.57
1.15
76.27
1.78
nc-3C-SiC:H emitter
ITO
17.55
0.85
0.42
0.42
79.04
1.72
ITO & LiF
13.18
0.73
0.63
0.50
83.18
1.78


### 3.2. Optical Simulations for SHJ Solar Cells with an Additional LiF Dielectric Layer

As previously mentioned, a single layer AR coating only has a good antireflection effect for a single wavelength while when multiple AR coating is used, good antireflection effect can be achieved within a wide spectrum. A set of optimized and well-designed multilayer AR coating can therefore be an effective way to improve the optical absorption of the solar cell in thin planar wafers. The common method employed for the design of multiple AR coatings on silicon is transfer matrix method [31]. Bouhafs et al. designed a hypothetical triple layer AR coating for a silicon substrate [32]. Their calculation yielded hypothetical materials with optimum refractive indices 1.4, 1.97, and 2.78 for outer layer, medium layer, and inner layer. The optimum thicknesses they found were thicknesses 104 nm, 55 nm, and 51 nm for outer layer, medium layer, and inner layer, respectively. Bett et al. also calculated front side antireflection concept for silicon solar cell using TiO2, SiNxOy, and MgF2 where TiO2 is the inner layer with thickness of 52 nm [33]. In both cases, they found that the inner layer has to be over 40 nm thick to achieve a good antireflective effect.Our approach is to use emitter layer as the inner layer of the triple antireflection design concept. Conventionally, in silicon heterojunction solar cells, the emitter thickness tends to be in the range of 5 nm to 10 nm especially in the case of a-Si:H where parasitic absorption has an adverse effect onJsc. This does not give enough room for the design of the AR coating. On the contrary, due to its transparency at shorter wavelength, nc-3C-SiC:H emitter thickness can be increased up to 40 nm without having any parasitic absorption effect. Moreover, as shown in Figure 2(a), nc-3C-SiC:H has a refractive index of about 2.72 at 300 nm to 2.37 at 1200 nm, therefore making it a close to ideal material for the inner layer of the AR coating as designed by Bouhafs et al.To investigate the antireflective effect of the emitter layer, we optimized the thicknesses of LiF and ITO and the emitter (nc-3C-SiC:H and a-Si:H) layers that yield minimum total reflectance and maximum absorbance in c-Si calculated using (1) and (2). The optimum thicknesses of ITO and LiF for 10 nm a-Si:H emitter SHJ structure were found to be 70 nm and 90 nm, respectively, while those of 40 nm nc-3C-SiC:H emitter SHJ structure were 60 nm and 100 nm, respectively. With optimum LiF and ITO thicknesses, emitter layer is varied and Jsc loss due to total reflection and emitter parasitic absorption is calculated. Figures 4(a) and 4(b) show the emitter Jsc loss due to parasitic absorption and reflection as a function of the emitter thickness. For a-Si:H emitter, Jsc loss due to parasitic absorption increases linearly from 1.76 mA/cm2 for 8 nm thick a-Si:H to 4.89 mA/cm2 for 30-nm thick a-Si:H emitter. This increase in parasitic absorption in a-Si:H emitter concurs with the results reported by Holman et al. [3]. For nc-3C-SiC:H emitter, Jsc loss due to parasitic absorption also increases linearly, although extremely small, from 0.12 mA/cm2 to 0.36 mA/cm2. Moreover, in Figure 4, while reflectance Jsc loss in SHJ solar cell with a-Si:H emitter increases with emitter thickness, SHJ solar cell with nc-3C-SiC:H emitter exhibits an opposite behavior as anticipated for an antireflective emitter. This explicitly demonstrates the antireflective effect of nc-3C-SiC:H emitter layer. This also revealed that nc-3C-SiC:H emitter can be used to design a triple layer AR coating concept for the front side of SHJ solar cell. On the other hand, a-Si:H emitter cannot be used in the AR coating design; therefore two more layers of dielectrics have to be deposited to achieve a triple layer AR coating.Figure 4
(a) The emitter current density loss as a function of the emitter thickness of the SHJ solar cell (b). The reflected current density loss as a function of emitter thickness for SHJ solar cell.
(a)
(b)Figures5(a) and 5(b) show the reflectance spectra and absorbance in c-Si comparing SHJ solar cell with nc-3C-SiC:H emitter with only ITO and with ITO and an additional LiF layer. For these structures we used an optimum thickness of 60 nm for ITO. It is observed that SHJ solar cell with nc-3C-SiC:H emitter with LiF and ITO exhibits a local minimum reflectance at three distinct wavelengths λmin = 320 nm, λmin = 520 nm, and λmin = 900 nm (zero reflectance). Therefore low reflection can be achieved over a broad wavelength range of the visible light. As shown in Figure 5(a) the Jsc loss due to reflection is remarkably reduced from 8.17 mA/cm2 to 6.12 mA/cm2 for nc-3C-SiC:H emitter when LiF is used. This reduction in Jsc loss due to triple layer AR coating leads to high Jsc. The corresponding total weighted reflectance is shown in Table 1. On the contrary, Jsc loss is reduced by 1.04 mA/cm2 for the case of a-Si:H emitter. The Jsc for SHJ with nc-3C-SiC:H emitter therefore had an appreciable increase by 1.92 mA/cm2 when LiF and ITO are used. While for a-Si:H emitter it is only increased by 0.7 mA/cm2 when LiF and ITO are used. Overall, our triple layer AR coating structure achieved appreciable Jsc of 38.65 mA/cm2 for optimum thickness of 40 nm nc-3C-SiC:H, 50 nm ITO, and 100 nm LiF. This result does not take into account the shading loss due to the front electrode.Figure 5
(a) The reflectance spectra on three SHJ solar cells with nc-3C-SiC:H emitter with only ITO, with ITO and LiF dielectric, and SHJ solar cell with a-Si:H emitter, ITO, and LiF dielectric. (b) The corresponding absorbance in c-Si of those SHJ structures.
(a)
(b)As shown in Table1, the total reflectance for nc-3C-SiC:H emitter can be reduced from 17.5% to 13.2%. It should be noted here that the triple layer antireflection coating only reduces reflectance at the front side of the SHJ solar cell. As can be seen in the reflectance spectra shown in Figure 5(a), higher reflection is observable at longer wavelength (λ>1000nm). To precisely ascertain the effect of triple layer antireflection coating, we calculated the weighted reflectance for spectrum with a wavelength range (300 nm–1000 nm). The total reflectance for nc-3C-SiC:H emitter with only ITO AR coating was found to be 6.81% corresponding to Jsc loss of 3.16 mA/cm2 and this was further reduced to 3.42% corresponding to Jsc loss of 1.58 mA/cm2 when additional layer of LiF was used. This is comparable to the values already obtained on textured wafers.Figure6 shows the total absorbance (1-reflectance) simulated for the optimized triple layer AR coating structure for our SHJ solar cell with nc-3C-SiC:H emitter. It is divided into several regions depending on the loss mechanism exhibited in each layer. This is to elucidate the dire importance of optimization of each layer in the overall short circuit current density that can be obtained from the SHJ solar cell.Figure 6
The absorbance at each layer of SHJ solar cell with nc-3C-SiC:H emitter, ITO, and LiF dielectric AR coating.When the thickness of c-Si is varied from 200μm to 20 μm, there is an increase in reflection at longer wavelength (λ> 800 nm). The reflection Jsc loss increases from 6.31 mA/cm2 for 200 μm thick wafer to 10.63 mA/cm2 for 20 μm thick wafer. This is because our triple AR coating concept on planar wafers fulfills only one function: optical enhancement by antireflection effect, which plays a fundamental role in the visible range of the spectrum. For textured wafers, a low reflection can be achieved over a broad wavelength range because front sides can fulfill two functions concerning the optical properties: first an antireflective effect and second a light path length enhancement by scattering or diffracting light into higher propagation angles. The light trapping effect cannot be realized by the front antireflective effect. However, it can be used in conjunction with other advanced light trapping schemes at the rear side, to enhance optical absorption in c-Si for near infrared wavelength, where penetration depth exceeds the cell thickness. We believe therefore that our triple layer AR coating designed here for the front side of the SHJ solar cell should be used in conjunction with plasmonic nanoparticles at the rear side or other rear side light trapping schemes to achieve a low reflectance over a broad wavelength of the visible light.


### 3.3. Triple Layer AR Coating on an Encapsulated Flat SHJ Solar Cell

For PV manufacturing processes, to provide protection for mechanical and electrical stability, the solar cell is encapsulated using ethyl vinyl acetate (EVA) and glass. This greatly influences the optical performance of the solar cell. Therefore to reflect real industrial PV application, the effect of triple layer AR coating in an encapsulated flat SHJ solar cell is investigated. Since the absorption in EVA and glass is not taken into account, we assume the glass and EVA structure has a thickness of 3 mm with refractive index of 1.5. As can be seen from Table2, both the double layer and triple layer ARC structures lead to a reduction in Jsc under the encapsulation condition. When the solar cell is encapsulated, LiF needs to be further optimized. The optimized structure with a LiF thickness of 10 nm leads to Jsc of 36.78 mA/cm2. This is still slightly higher than the double structure with only ITO and encapsulated double structure.Table 2
Comparative result of the total reflectance and the total absorbance in each layer with different AR coating schemes and emitter layer for flat SHJ solar cell. The thicknesses of AR coating and emitter layers are optimized values.
Reflected flux (%)
Current loss due to reflected flux (mA/cm2)
Absorbance (%) in c-Si
J
s
c
(mA/cm2)
ITO/nc-3C-SiC:H
17.58
8.17
79.07
36.73
Glass/ITO/nc-3C-SiC:H
18.34
8.52
78.13
36.30
LiF (100 nm)/ITO/nc-3C-SiC:H
13.58
6.31
82.63
38.65
Glass/LiF (10 nm)/ITO/nc-3C-SiC:H
17.15
7.97
79.17
36.78
Glass/Al2O3/ITO/nc-3C-SiC:H
16.75
7.78
79.64
37.00To achieve an effective optical interface, a selective material with a refractive index between 1.5 (EVA) and 2.0 (ITO) should be considered. Therefore use of dielectric material such as Al2O3 is highly recommended. As shown in Table 2, structure with Al2O3 shows Jsc of 37.08 mA/cm2.


## 3.1. Optical Simulation of the SHJ with Only ITO AR Coating

Figure3 shows the simulated reflectance and absorbance spectra of the SHJ solar cells with optimum single AR coating of ITO. We calculated the spectra for SHJ solar cells with nc-3C-SiC:H emitter and a-Si:H emitter. The optimal thicknesses of a-Si:H and nc-3C-SiC:H emitter were 10 nm and 25 nm, respectively. In the AR coating design, the thickness of the coating material is important to minimize the reflection. In our simulations, the optimal thicknesses of ITO obtained for a-Si:H and nc-3C-SiC:H SHJ structures were 80 nm and 60 nm, respectively.Figure 3
(a) The reflectance spectra for SHJ solar cells with nc-3C-SiC:H emitter and a-Si:H emitter with a single AR coating (b). The corresponding absorbance. The absorbance spectra are the absorbance in c-Si layer in the solar cells.
(a)
(b)The reflectance spectra of the SHJ solar cell with nc-3C-SiC:H emitter closely resemble the a-Si:H emitter counterpart with a slight variation at wavelength range of 300 nm–600 nm. This variation is due to the difference in parasitic absorption and the thicknesses of the emitter layer and ITO layer of the structures. For both structures, minimum reflectance is observed at one certain wavelengthλmin. The SHJ solar cell with nc-3C-SiC:H emitter shows the minimum reflectance is obtained at λmin=653 nm while λmin is found to be 635 nm for the SHJ solar cell with a-Si:H emitter.For a single AR coating, based on the reflectance spectra of the SHJ solar cell with nc-3C-SiC:H emitter shown in Figure3(a), it cannot be explicitly deduced whether nc-3C-SiC:H demonstrates an antireflective effect. However, it is worth noting that there is a minimum reflectance observed at wavelength lower than 300 nm which contributes to relatively lower reflectance in the range of 300 nm to 400 nm as compared to the SHJ solar cell with a-Si:H emitter.The total reflectance for the SHJ solar cell with a-Si:H emitter was found to be 17.2% corresponding to a reflectedJsc loss of 7.98 mA/cm2. While for the SHJ solar cell with nc-3C-SiC:H emitter was 17.6% corresponding to a reflected Jsc loss of 8.17 mA/cm2. Although the reflectance spectra for the two structures are comparable, with only slight variation at wavelength range 300 nm–600 nm, the absorbance in c-Si exhibited a significant difference. This difference is attributed to the absorption loss in the emitter layer. The nc-3C-SiC:H emitter significantly boosts light absorption in c-Si at wavelength range 300 nm–600 nm due to its higher transmittance compared to that of the a-Si:H emitter. In case of the a-Si:H emitter, the parasitic absorption in the a-Si:H emitter reduces the short wavelength (300 nm–600 nm) light that reaches the c-Si. This has a detriment effect on the overall Jsc that can be obtained from the SHJ solar cell.It should be also noted that even though the optimum thickness of nc-3C-SiC:H emitter (25 nm) is more than twice the optimum thickness of a-Si:H emitter (10 nm), the absorbance in a-Si:H emitter is evidently high. The absorbance in the a-Si:H emitter and nc-3C-SiC:H emitter is 4% and 0.42%, respectively. This absorbance in a-Si:H emitter contributes 1.86 mA/cm2  Jsc loss as compared to only a Jsc loss of 0.2 mA/cm2 for nc-3C-SiC:H emitter. In addition, other parasitic absorption incurred in i-type buffer layer and ITO layers of the two structures also is attributed to lower Jsc. The absorbance in ITO was 1.3% and 0.84% for SHJ solar cells with a-Si:H and nc-3C-SiC:H emitter, respectively. These correspond to a Jsc loss of 0.39 mA/cm2 and 0.60 mA/cm2 in ITO for SHJ with nc-3C-SiC:H and a-Si:H emitter, respectively. The Jsc loss in the i-type buffer layer of a-Si:H and a-SiC:H was found to be 0.48 mA/cm2 and 0.2 mA/cm2, respectively. However, as Holman et al. duly noted, 30% of the aforementioned i-layer Jsc losses can still contribute to Jsc [3]. The Jsc loss due to rear electrode was about 0.80 mA/cm2 for both structures. From this finding, it should be therefore emphasized that only minimizing reflectance does not guarantee maximization of the absorbance in c-Si. The parasitic absorption especially at the emitter and TCO layer also need to be taken into account when designing AR coating. Overall, Jsc calculated from absorbance in c-Si shown in Table 1 for SHJ solar cells with a-Si:H emitter with single AR coating was 34.74 mA/cm2 while that of SHJ solar cell with nc-3C-SiC:H emitter was 36.72 mA/m2.Table 1
Comparative result of the total reflectance and the total absorbance in each layer with different AR coating schemes and emitter layer for flat SHJ solar cell. The thicknesses of AR and emitter are optimized values.
AR coating
Weighted reflectance (%)
Absorbance (%)
ITO
Emitter
i-layer
c-Si
Metal
a-Si:H emitter
ITO
17.18
1.30
4.00
1.03
74.77
1.72
ITO & LiF
14.95
1.28
4.57
1.15
76.27
1.78
nc-3C-SiC:H emitter
ITO
17.55
0.85
0.42
0.42
79.04
1.72
ITO & LiF
13.18
0.73
0.63
0.50
83.18
1.78


## 3.2. Optical Simulations for SHJ Solar Cells with an Additional LiF Dielectric Layer

As previously mentioned, a single layer AR coating only has a good antireflection effect for a single wavelength while when multiple AR coating is used, good antireflection effect can be achieved within a wide spectrum. A set of optimized and well-designed multilayer AR coating can therefore be an effective way to improve the optical absorption of the solar cell in thin planar wafers. The common method employed for the design of multiple AR coatings on silicon is transfer matrix method [31]. Bouhafs et al. designed a hypothetical triple layer AR coating for a silicon substrate [32]. Their calculation yielded hypothetical materials with optimum refractive indices 1.4, 1.97, and 2.78 for outer layer, medium layer, and inner layer. The optimum thicknesses they found were thicknesses 104 nm, 55 nm, and 51 nm for outer layer, medium layer, and inner layer, respectively. Bett et al. also calculated front side antireflection concept for silicon solar cell using TiO2, SiNxOy, and MgF2 where TiO2 is the inner layer with thickness of 52 nm [33]. In both cases, they found that the inner layer has to be over 40 nm thick to achieve a good antireflective effect.Our approach is to use emitter layer as the inner layer of the triple antireflection design concept. Conventionally, in silicon heterojunction solar cells, the emitter thickness tends to be in the range of 5 nm to 10 nm especially in the case of a-Si:H where parasitic absorption has an adverse effect onJsc. This does not give enough room for the design of the AR coating. On the contrary, due to its transparency at shorter wavelength, nc-3C-SiC:H emitter thickness can be increased up to 40 nm without having any parasitic absorption effect. Moreover, as shown in Figure 2(a), nc-3C-SiC:H has a refractive index of about 2.72 at 300 nm to 2.37 at 1200 nm, therefore making it a close to ideal material for the inner layer of the AR coating as designed by Bouhafs et al.To investigate the antireflective effect of the emitter layer, we optimized the thicknesses of LiF and ITO and the emitter (nc-3C-SiC:H and a-Si:H) layers that yield minimum total reflectance and maximum absorbance in c-Si calculated using (1) and (2). The optimum thicknesses of ITO and LiF for 10 nm a-Si:H emitter SHJ structure were found to be 70 nm and 90 nm, respectively, while those of 40 nm nc-3C-SiC:H emitter SHJ structure were 60 nm and 100 nm, respectively. With optimum LiF and ITO thicknesses, emitter layer is varied and Jsc loss due to total reflection and emitter parasitic absorption is calculated. Figures 4(a) and 4(b) show the emitter Jsc loss due to parasitic absorption and reflection as a function of the emitter thickness. For a-Si:H emitter, Jsc loss due to parasitic absorption increases linearly from 1.76 mA/cm2 for 8 nm thick a-Si:H to 4.89 mA/cm2 for 30-nm thick a-Si:H emitter. This increase in parasitic absorption in a-Si:H emitter concurs with the results reported by Holman et al. [3]. For nc-3C-SiC:H emitter, Jsc loss due to parasitic absorption also increases linearly, although extremely small, from 0.12 mA/cm2 to 0.36 mA/cm2. Moreover, in Figure 4, while reflectance Jsc loss in SHJ solar cell with a-Si:H emitter increases with emitter thickness, SHJ solar cell with nc-3C-SiC:H emitter exhibits an opposite behavior as anticipated for an antireflective emitter. This explicitly demonstrates the antireflective effect of nc-3C-SiC:H emitter layer. This also revealed that nc-3C-SiC:H emitter can be used to design a triple layer AR coating concept for the front side of SHJ solar cell. On the other hand, a-Si:H emitter cannot be used in the AR coating design; therefore two more layers of dielectrics have to be deposited to achieve a triple layer AR coating.Figure 4
(a) The emitter current density loss as a function of the emitter thickness of the SHJ solar cell (b). The reflected current density loss as a function of emitter thickness for SHJ solar cell.
(a)
(b)Figures5(a) and 5(b) show the reflectance spectra and absorbance in c-Si comparing SHJ solar cell with nc-3C-SiC:H emitter with only ITO and with ITO and an additional LiF layer. For these structures we used an optimum thickness of 60 nm for ITO. It is observed that SHJ solar cell with nc-3C-SiC:H emitter with LiF and ITO exhibits a local minimum reflectance at three distinct wavelengths λmin = 320 nm, λmin = 520 nm, and λmin = 900 nm (zero reflectance). Therefore low reflection can be achieved over a broad wavelength range of the visible light. As shown in Figure 5(a) the Jsc loss due to reflection is remarkably reduced from 8.17 mA/cm2 to 6.12 mA/cm2 for nc-3C-SiC:H emitter when LiF is used. This reduction in Jsc loss due to triple layer AR coating leads to high Jsc. The corresponding total weighted reflectance is shown in Table 1. On the contrary, Jsc loss is reduced by 1.04 mA/cm2 for the case of a-Si:H emitter. The Jsc for SHJ with nc-3C-SiC:H emitter therefore had an appreciable increase by 1.92 mA/cm2 when LiF and ITO are used. While for a-Si:H emitter it is only increased by 0.7 mA/cm2 when LiF and ITO are used. Overall, our triple layer AR coating structure achieved appreciable Jsc of 38.65 mA/cm2 for optimum thickness of 40 nm nc-3C-SiC:H, 50 nm ITO, and 100 nm LiF. This result does not take into account the shading loss due to the front electrode.Figure 5
(a) The reflectance spectra on three SHJ solar cells with nc-3C-SiC:H emitter with only ITO, with ITO and LiF dielectric, and SHJ solar cell with a-Si:H emitter, ITO, and LiF dielectric. (b) The corresponding absorbance in c-Si of those SHJ structures.
(a)
(b)As shown in Table1, the total reflectance for nc-3C-SiC:H emitter can be reduced from 17.5% to 13.2%. It should be noted here that the triple layer antireflection coating only reduces reflectance at the front side of the SHJ solar cell. As can be seen in the reflectance spectra shown in Figure 5(a), higher reflection is observable at longer wavelength (λ>1000nm). To precisely ascertain the effect of triple layer antireflection coating, we calculated the weighted reflectance for spectrum with a wavelength range (300 nm–1000 nm). The total reflectance for nc-3C-SiC:H emitter with only ITO AR coating was found to be 6.81% corresponding to Jsc loss of 3.16 mA/cm2 and this was further reduced to 3.42% corresponding to Jsc loss of 1.58 mA/cm2 when additional layer of LiF was used. This is comparable to the values already obtained on textured wafers.Figure6 shows the total absorbance (1-reflectance) simulated for the optimized triple layer AR coating structure for our SHJ solar cell with nc-3C-SiC:H emitter. It is divided into several regions depending on the loss mechanism exhibited in each layer. This is to elucidate the dire importance of optimization of each layer in the overall short circuit current density that can be obtained from the SHJ solar cell.Figure 6
The absorbance at each layer of SHJ solar cell with nc-3C-SiC:H emitter, ITO, and LiF dielectric AR coating.When the thickness of c-Si is varied from 200μm to 20 μm, there is an increase in reflection at longer wavelength (λ> 800 nm). The reflection Jsc loss increases from 6.31 mA/cm2 for 200 μm thick wafer to 10.63 mA/cm2 for 20 μm thick wafer. This is because our triple AR coating concept on planar wafers fulfills only one function: optical enhancement by antireflection effect, which plays a fundamental role in the visible range of the spectrum. For textured wafers, a low reflection can be achieved over a broad wavelength range because front sides can fulfill two functions concerning the optical properties: first an antireflective effect and second a light path length enhancement by scattering or diffracting light into higher propagation angles. The light trapping effect cannot be realized by the front antireflective effect. However, it can be used in conjunction with other advanced light trapping schemes at the rear side, to enhance optical absorption in c-Si for near infrared wavelength, where penetration depth exceeds the cell thickness. We believe therefore that our triple layer AR coating designed here for the front side of the SHJ solar cell should be used in conjunction with plasmonic nanoparticles at the rear side or other rear side light trapping schemes to achieve a low reflectance over a broad wavelength of the visible light.


## 3.3. Triple Layer AR Coating on an Encapsulated Flat SHJ Solar Cell

For PV manufacturing processes, to provide protection for mechanical and electrical stability, the solar cell is encapsulated using ethyl vinyl acetate (EVA) and glass. This greatly influences the optical performance of the solar cell. Therefore to reflect real industrial PV application, the effect of triple layer AR coating in an encapsulated flat SHJ solar cell is investigated. Since the absorption in EVA and glass is not taken into account, we assume the glass and EVA structure has a thickness of 3 mm with refractive index of 1.5. As can be seen from Table2, both the double layer and triple layer ARC structures lead to a reduction in Jsc under the encapsulation condition. When the solar cell is encapsulated, LiF needs to be further optimized. The optimized structure with a LiF thickness of 10 nm leads to Jsc of 36.78 mA/cm2. This is still slightly higher than the double structure with only ITO and encapsulated double structure.Table 2
Comparative result of the total reflectance and the total absorbance in each layer with different AR coating schemes and emitter layer for flat SHJ solar cell. The thicknesses of AR coating and emitter layers are optimized values.
Reflected flux (%)
Current loss due to reflected flux (mA/cm2)
Absorbance (%) in c-Si
J
s
c
(mA/cm2)
ITO/nc-3C-SiC:H
17.58
8.17
79.07
36.73
Glass/ITO/nc-3C-SiC:H
18.34
8.52
78.13
36.30
LiF (100 nm)/ITO/nc-3C-SiC:H
13.58
6.31
82.63
38.65
Glass/LiF (10 nm)/ITO/nc-3C-SiC:H
17.15
7.97
79.17
36.78
Glass/Al2O3/ITO/nc-3C-SiC:H
16.75
7.78
79.64
37.00To achieve an effective optical interface, a selective material with a refractive index between 1.5 (EVA) and 2.0 (ITO) should be considered. Therefore use of dielectric material such as Al2O3 is highly recommended. As shown in Table 2, structure with Al2O3 shows Jsc of 37.08 mA/cm2.


## 4. Conclusion

We have demonstrated a triple layer antireflection design concept for planar SHJ solar cell that capitalizes on the transparent quality of nc-3C-SiC:H emitter. The antireflective effect of nc-3C-SiC:H is observed when one additional layer of dielectric is added to the SHJ structure with ITO to form a triple layer AR coating where nc-3C-SiC:H serves both as an emitter and AR coating. This leads to an appreciable reduction in reflection, with reflectance as low as 3.42% for optimized thicknesses, at wavelength range of 300 nm to 1000 nm. This, as confirmed with our studies, is not possible for the conventional SHJ based on a-Si:H emitter. The triple antireflective concept for the front side of SHJ solar cell investigated here can be used in conjunction with rear side light trapping plasmonic nanostructure schemes to achieve lower reflectance in thinner planar SHJ solar cells where texturing cannot be done. The parasitic absorption for the case of the optimized triple layer AR coating can be minimized when nc-3C-SiC:H emitter is used as compared to when a-Si:H emitter is applied. We conclude therefore that nc-3C-SiC:H is ideally suitable material for the emitter layer and design of triple layer AR coating for the front side of the planar thinner SHJ solar cell.



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*Source: 5282851-2016-02-25.xml*