Potential of TiO2 as a Capping Layer for Industrial c-Si PERC Solar Cells

doi:10.21203/rs.3.rs-4065052/v1

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Potential of TiO2 as a Capping Layer for Industrial c-Si PERC Solar Cells

https://doi.org/10.21203/rs.3.rs-4065052/v1

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published 12 Jun, 2024

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Titanium dioxide (TiO₂) has gained popularity specially in photovoltaic applications, owing to its transparency in the visible region, and scratch resistance. In this work, the potential of TiO₂ as a capping layer for c-Si p-type SiN_x passivated emitter and rear contact (PERC) solar cells is studied through extensive optical and device simulations. The bifacial PERC solar cell model used in this study is calibrated with an experimental device having an efficiency of 22.19%. Device simulation results show that TiO₂ deposited by the mesoporous technique outperforms atmospheric pressure chemical vapor deposition (APCVD) and atomic layer deposition (ALD) based TiO₂ layers when capped over SiN_x (n = 2.1) passivated solar cells. Furthermore, it is shown that the efficiency of SiN_x(n = 2.1)/TiO₂ based solar cells is maintained, even when the TiO₂ layer thickness varies from 75 to 95 nm. To enhance the efficiency further, the type of SiN_x layer (characterized by the n value), and the thicknesses of SiN_x and TiO₂ layers are optimized simultaneously to find the best combination of these parameters. The best front side solar cell efficiency of 22.43%, is obtained when a stack of SiN_x(n = 1.99)/TiO₂ (t = 58/76 nm) is used. Similarly, a rear side efficiency of 16.59% is achieved when the rear side Al₂O₃/SiN_x stack is capped with mesoporous TiO₂. These efficiencies are 0.24 and 1.25% higher, respectively, when compared to the original SiN_x passivated PERC solar cell, demonstrating the prospective of using TiO₂ in commercial photovoltaic applications.

Efficiency

bifacial PERC solar cell

ARC

SiNx

capping layer

mesoporous

TiO2

Over the recent years, titanium dioxide (TiO₂) thin films have gained popularity in photovoltaic applications due to their superior chemical, mechanical, and optical properties [1]. With a bandgap of ~ 3.25 eV, and refractive index of ~ 2.2, TiO₂ is one of the few materials that is environmentally friendly, transparent to UV light, and can be manufactured easily. The bandgap and hence the transmission capability of the material can be tuned easily by doping it with metallic ions, making it an attractive choice for solar cell applications. Apart from the bandgap tunability, TiO₂ is scratch resistant, mechanically strong, and also has self-cleaning properties. The hydrophilic nature of TiO₂ helps rain water to spread out on the film surface, washing away dust and dirt [2]. Moreover, it decomposes any organic compounds on its surface and it is highly efficient in photocatalysing dirt in sunlight [3], which makes TiO₂ one of the ideal materials to be used as an efficient capping layer in solar cells.

As a result, TiO₂ is widely investigated for its use in various photovoltaic applications such as thin-film solar cells, perovskite cells, and dye-sensitized solar cells [4, 5]. However, the potential of using TiO₂ in monocrystalline silicon solar cells is still not fully explored. Monocrystalline silicon solar cells, in particular, the PERC/PERT technology makes up most of the market share for solar cells today and is commonly used in commercial and industrial applications. The anti-reflective coatings currently used in the PERC/PERT solar cells include SiO₂, SiN_x and Al₂O₃ [6, 7]. Despite having superior optical properties, there are very few studies that discuss the impact of using TiO₂ on monocrystalline silicon based solar cells [8–10].

With the PERC/PERT technology dominating the solar cell market, it is imperative to study the effect of using TiO₂ on this commercial solar cell technology. Hence, in the present work, the potential of using TiO₂ as a capping layer for bifacial PERC silicon solar cell is explored. TiO₂ can be deposited by different physical and chemical methods such as Atmospheric Pressure Chemical Vapor Deposition (APCVD), Atomic Layer Deposition (ALD), sputtering, sol-gel, spray, and mesoporous techniques [11–13]. In this work, the impact of APCVD, ALD and mesoporous TiO₂ on the bifacial PERC solar cell efficiency is investigated, through optical and TCAD based device simulations. The solar cell model used in this work is already fully calibrated [14] using Silvaco device and process simulator ATLAS/ATHENA [15], and is based on an actual solar cell manufactured and reported by C. Zhang et al. [16]. Simulation results show that mesoporous TiO₂ outperforms the ALD and APCVD based films due to its relatively low reflectance in visible and IR regions. Moreover, the efficiency of the solar cell is less sensitive to variation in the mesoporous TiO₂ film thickness when compared to other deposition techniques. An overall improvement of 0.14 and 0.23% in the front and rear side efficiency, respectively, is possible when the front and rear surface of the cell is coated with an additional mesoporous TiO₂, as a capping layer.

APCVD, ALD and mesoporous TiO film properties

The APCVD TiO₂ film used in the simulations was manufactured and characterized by K. Davis et al. [17]. The TiO₂ layer was deposited at 200°C using tetraisopropyl titanate and water as precursors, resulting in an amorphous film with a refractive index of 1.6 at 632 nm. Similarly, the ALD deposited TiO₂ layers investigated in [12] were deposited at 75°C using TiCl₄ & H₂O as precursors, which resulted in an amorphous layer with n = 2.23 (632 nm). It is important to consider that while TiO₂ films deposited at temperatures below 350°C are typically amorphous, temperatures above 350°C result in the formation of the metastable crystalline phase of anatase, which transitions to the high temperature stable crystalline phase rutile at temperatures above 800°C [18]. The phase has a significant impact on a number of film parameters, including density, refractive index, chemical resistance, and thermal stability [19].

The mesoporous TiO₂ layers were prepared at 130°C via a hydrothermal process using the precursor titanium isopropoxide [13]. Figure 1 shows the SEM image of a typical mesoporous TiO₂ film [20]. Moreover, the refractive index value (n) versus wavelength for all the three TiO₂ films is shown in Fig. 1.

Simulation procedure and framework

The solar cell design used to investigate the impact of TiO₂ capping layers is a p-type bifacial silicon PERC solar cell fabricated and reported by C. Zhang et al. [16] with an alkaline etch textured front side and planar rear side. The p-type base has a resistivity of 1.6 Ω‧cm² with a 130 Ω/□ phosphorus emitter formed by diffusion process. The front side is passivated with 80 nm SiN_x (n = 2.1 at 632 nm) and the rear side is passivated with 5 nm Al₂O₃/95 nm SiN_x. The photogeneration data for the front and rear stack is obtained by simulating the stack in the OPAL2 tool provided by PVlighthouse [21]. However, the 1D photogeneration data from the tool is not equally spaced with respect to depth, hence the data was interpolated in MATLAB using the spline method before importing it into the device simulator, ATLAS. All relevant models such as Fermi-Dirac statistics, Schenk bandgap narrowing, Auger and SRH recombination, required to achieve a realistic simulation model were also included. The front and rear side metallization fraction used is ~ 5% and ~ 15.8%, respectively. Finally, the series and shunt resistances measured for the cell were incorporated as lump resistors. A detailed set of material and model parameters for the solar cell is previously reported in [14] as Table 2. The resulting simulated/experimental front side J_sc, V_oc, FF and efficiency, are: 41.12/41.12 mA/cm², 0.675/0.676 V, 80/80.13%, and 22.19/22.17%, respectively. Moreover, a similar agreement for the rear side is also obtained, with the simulated/experimental values of J_sc (28.67/28.82 mA/cm²), V_oc (0.665/0.663 V), FF (80.48/80.5%), and efficiency (15.34/15.39%). This bifacial PERC solar cell model is used as a base for the present work, where the effect of adding APCVD, ALD, and mesoporous TiO₂ capping layers on the front and rear side is investigated.

Impact of adding TiO₂ on the front and rear surfaces of the PERC cell

In order to gauge the true merits of using TiO₂ as a capping layer in PERC solar cells, the first set of simulations entail adding a TiO₂ layer on the front and rear side of the solar cell separately, while leaving the original stack (80 nm SiN_x (n = 2.1) on the front and 5 nm Al₂O₃/95 nm SiN_x (n = 2.1) on the rear side) intact. Figure 2 (a) and (b) demonstrate the effect of adding ALD, APCVD and mesoporous TiO₂ layers for a range of thicknesses, on the front and rear sides of the cell respectively, in terms of the photocurrent absorbed in the silicon substrate (J_sub). J_sub can also be defined as the maximum short circuit current that the cell can deliver if there are no shading and electrical losses (such as loss of current due to recombination).

As observed from Figs. 3 (a) and (b), mesoporous TiO₂ improves J_sub of the original PERC cell both on the front and rear side. A maximum J_sub of 43.19 mA/cm² is obtained for 80 nm mesoporous TiO₂ deposited on the front side, which is 0.65% higher than the J_sub of the original PERC cell (42.91 mA/cm²). A 1.65% increase in J_sub, for 90 nm mesoporous TiO₂ is observed on the rear side as well. It is important to point out that the J_sub values for the front side are generally higher because it is a textured surface whereas the rear side of the cell is planar. Despite being the same material, the APCVD and ALD based TiO₂ capping layers do not perform well, as the mesoporous ones, and the J_sub values reduce even further when the APCVD or ALD based layers are added to the original stack. This is because, the optical properties of these films are heavily dependent on their deposition methods, due to the phase of the resulting film. As observed in the refractive index plots in Fig. 1, mesoporous TiO₂ has the lowest refractive index of 1.6 at the design wavelength (λ₀ = 600 nm), whereas it is much higher (2.3) for ALD TiO₂.

For solar cells with a double antireflective coating system (shown in Fig. 4), the refractive index value for the top layer (n_top) that minimizes the reflection at the design wavelength, is given by [22]:

\({n}_{\text{t}\text{o}\text{p}}^{3}={n}_{Si}{n}_{0}^{2}\) (Eq. 1)

Using n_Si (λ₀) = 3.94 [23] and n₀ = 1, the optimal n_top value is calculated to be 1.579. Among the three TiO₂ films investigated in this work, mesoporous TiO₂ exhibits a n value very close to the optimal refractive index, hence it performs better than APCVD and ALD based films. However, it is also important to explore why the APCVD and ALD TiO₂ based films do not lead to any improvement when compared to the original PERC cell with (n = 2.1) SiN_x. To understand this, Fig. 5 compares the reflection, absorption, and transmission spectra for (a) uncapped PERC solar cell and after capping the front surface with 80 nm of (b) mesoporous (c) APCVD and, (d) ALD based TiO₂ layers.

As evident in Fig. 5(b), the absorption in the 300–500 nm range is lower for the mesoporous SiN_x/TiO₂ stack when compared to the ‘SiN_x only’ case in Fig. 5(a). Even though the reflection in Fig. 5(b) for the mesoporous SiN_x/TiO₂ stack is higher in the 300–350 nm range, it still leads to a higher J_sub because the spectral intensity in this wavelength is quite low. Therefore, no significant photocurrent is lost due to the higher reflection in this wavelength range. Moving on to Figs. 5(c) and (d), the stacks based on ALD and APCVD TiO₂ films lead to a peak reflection of ~ 8–10% which spreads across the visible and early IR range (500–800 nm), causing significant loss in J_sub, as this is a region of high solar spectral intensity.

The trends in J_sub observed for different types of TiO₂ capping layers can also be seen in the efficiencies of the PERC solar cells with these layers. Figure 6 (a) and (b) shows the front and rear side illuminated J-V curves, respectively, for the original PERC solar cell (with 80 nm of (n = 2.1) SiN_x on the front and 5 nm/95 nm of Al₂O₃/SiN_x (n = 2.1) on the rear side) and the same cell when it is capped with 80 nm of mesoporous, ALD or APCVD TiO₂ layers. The corresponding conversion efficiencies and solar cell parameters for each case is summarized in Table 1.

Table 1

Front and rear side solar cell parameters for the original PERC solar cell and the same cell when it is capped with 80 nm of mesoporous, ALD or APCVD TiO₂ layers.
	Stack definition	J_sc (mA/cm²)	V_oc (V)	FF (%)	Efficiency (%)
Front (textured)	SiN_x (original PERC)	41.12	0.675	80.00	22.19
	SiN_x/mesoporous TiO₂	41.37	0.675	79.99	22.33
	SiN_x/APCVD TiO₂	40.63	0.674	80.03	21.92
	SiN_x/ALD TiO₂	38.78	0.673	80.13	20.91
Rear (planar)	Al₂O₃/SiN_x (original PERC)	28.67	0.665	80.48	15.34
	Al₂O₃/SiN_x/mesoporous TiO₂	29.06	0.666	80.47	15.57
	Al₂O₃/SiN_x/APCVD	27.00	0.664	80.55	14.44
	Al₂O₃/SiN_x/ALD TiO₂	23.51	0.660	80.56	12.49

The table suggests that a 0.14/0.23% increase in the front/rear side efficiency is possible when 80 nm mesoporous TiO₂ layer is added on the SiN_x passivated PERC cell. Moreover, as illustrated in Figs. 3 (a) and (b), the optical properties, and hence the J_sub is not very sensitive to the changes in thickness of the mesoporous TiO₂ layer which is a desirable property since this TiO₂ layer is prepared using chemical methods where precise control of thickness is often difficult.

Front side optimization of SiN and TiO mesoporous together

Considering the advantages of using mesoporous TiO₂ as a capping layer for the PERC solar cells, and the mature surface passivation provided by the underlying SiN_x layer, the next step in harnessing the full potential of this stack is to optimize the thicknesses of both these layers and the refractive index of the SiN_x layer, simultaneously. This is because, the optimal design point (i.e. refractive index and thickness) of SiN_x when it is used as a standalone ARC will be different when it is used in combination with the TiO₂ layer. In order to find this optimal design point, the SiN_x refractive index value is fixed, and then the thickness of the SiN_x and TiO₂ layers is varied to maximize J_sub. This process is then repeated for SiN_x layers with different refractive indices (in the range of 1.91–2.37). As an example, Fig. 7 demonstrates the impact of varying the thicknesses of mesoporous TiO₂ and SiN_x (n = 1.91) on the front J_sub value.

The J_sub value does not vary significantly for SiN_x films > 50 nm, irrespective of the TiO₂ film thickness, as evident in the figure, and a maximum value of 43.13 mA/cm² is achieved when the SiN_x/TiO₂ film thickness is 55/60 nm, respectively. This J_sub value is 0.22 mA/cm² higher than that of the original SiN_x (n = 2.1) PERC cell. To improve the performance of the SiN_x/TiO₂ stack further, contour plots similar to Fig. 5 are generated for SiN_x layers with different refractive indices, to identify the optimal thicknesses of the SiN_x and TiO₂ layers in each case. These results are summarized in Fig. 8.

As illustrated in the figure, the highest photocurrent possible for the SiN_x/TiO₂ is 43.39 mA/cm², where the thickness of the stack is 58/76 nm, respectively, and the refractive index of the SiN_x layer is 1.99. It is worth noting that ideally, the refractive index (at λ₀) of bottom layer which minimizes reflection in a DLARC system is given by Eq. 2 [22], and found to be 2.49.

\({n}_{\text{b}\text{o}\text{t}}^{3}={n}_{0}{n}_{Si}^{2}\) (Eq. 2)

The optimal n value of 1.99 for SiN_x in the SiN_x/TiO₂ stack found through optical simulations deviates significantly from the value found using Eq. 2. This is because, the goal of Eq. 2 is to minimize the reflection at λ₀, however, it does not account for the photons that fail to transmit because they are absorbed by the films. Hence this deviation could be attributed to the relatively high absorption of the n = 2.37 film (Note: The SiN_x layer with n = 2.37 is chosen as it is closest to the optimal n of 2.49, available in the OPAL2 database). Figures 9(a) and (b) show the reflection and absorption, respectively, for the front surface SiN_x/meso TiO₂ stack with SiN_x n = 1.99 and n = 2.37.

As observed in Fig. 9(a), the reflection around λ₀ is expectedly lesser for SiN_x with the higher refractive index value. Moreover, the total reflection loss for the stack with SiN_x n = 1.99 and 2.37 is 1.2 and 0.4%, respectively, corroborating well with the results found using Eq. 2. Whereas, the absorption for the stack with SiN_x n = 2.37 is higher, as shown in Fig. 9(b) and remains non-zero, which falls into the visible region where the solar intensity is high. The total losses due to absorption in the n = 1.99 and n = 2.37 based SiN_x stack are 0.2 and 3.2%, making the cumulative losses 1.4% and 3.6%, respectively. This makes SiN_x, with n = 1.99, more suitable as a bottom layer in the SiN_x/TiO₂ stack for PERC solar cells with textured surfaces. It is also noteworthy that the reflection for both stacks in Fig. 9(a) remains < 10% throughout the wavelength range indicating that the loss in J_sub for solar cells with textured surfaces is primarily dominated by absorption in the films.

Though, one might argue that the J_sub value obtained via optimizing the SiN_x/TiO₂ stack, can also be achieved if the refractive index and thickness of the standalone SiN_x layer is optimized, without having to deposit the additional mesoporous TiO₂ film. Hence, to investigate the full potential of the SiN_x layer, the refractive index and thickness of the layer in each case is optimized and the results are displayed in Fig. 10.

As shown in the figure, the highest photocurrent possible for a single SiN_x layer (n = 1.92) is 43.2 mA/cm², which is still lower than the photocurrent possible for most of the SiN_x/TiO₂ stack in Fig. 6. Therefore, it can be concluded that capping the SiN_x based PERC cells with mesoporous TiO₂ can enhance the performance of the solar cells even further. In terms of the solar cell performance, Table 2, and Fig. 11 demonstrate the effect of SiN_x and TiO₂ layers optimization on the cell parameters and front side illuminated J-V curves, respectively.

Table 2

Front side solar cell parameters of original SiN_x (n = 2.1), optimized SiN_x (n = 1.92) and optimized SiN_x/meso TiO₂ based PERC solar cell.
Stack definition	J_sc (mA/cm²)	V_oc (V)	FF (%)	Efficiency (%)
SiN_x (original PERC: n = 2.1, t = 80 nm)	41.12	0.675	80.00	22.19
SiN_x (optimized: n = 1.92, t = 79 nm)	41.38	0.675	79.99	22.33
(n = 1.99) SiN_x/mesoporous TiO₂ (t = 58/76 nm)	41.57	0.675	79.98	22.43

Optimizing SiN and TiO mesoporous together for rear side

The process for optimizing the SiN_x and TiO₂ layers on the rear side is very similar to that for the front side presented in the previous section. Though it is necessary to repeat the optimization process as the rear side of the PERC cell used in this work is planar whereas the front side is textured. Figure 12 shows the rear side J_sub values obtained for the Al₂O₃/SiN_x/mesoporous TiO₂ stack with different SiN_x refractive indexes. The Al₂O₃ layer thickness is kept intact (at 5 nm) to ensure that the rear surface recombination and fixed charge values used in the TCAD simulations are still valid.

Unlike the front surface, the refractive index value for SiN_x that gave the best results for the rear side is 2.37, which corroborates well with the value obtained using Eq. 2. For the sake of comparison with the front textured surface, Figs. 13(a) and (b) show the reflection and absorption, respectively for the stacks based on SiN_x with n = 1.99 and n = 2.37 on the rear side.

While the absorption for the stack with SiN_x n = 2.37 is still higher (absorption loss = 2.8%), when compared to n = 1.99 (0.2%), it is the loss due to reflection which dominates for the planar surface, as the light rays do not get a chance to enter the substrate the second time after it is reflected once, especially in the visible region. The total loss due to reflection for the stack with SiN_x n = 1.99 and n = 2.37 is 8.2 and 3.8%, respectively, which results in the highest J_sub value of 41.08 mA/cm² for the Al₂O₃/n = 2.37 SiN_x/TiO₂ stack on the rear surface. In terms of the solar cell parameters, the rear side efficiency for the PERC solar cell improves from 15.34 (for the original Al₂O₃/(n = 2.1) SiN_x stack) to 16.59% when the refractive index value for SiN_x and thicknesses of both SiN_x and TiO₂ are optimized together, as shown in Table 3, and Fig. 14.

Table 3

Rear side solar cell parameters of original Al₂O₃/SiN_x (n = 2.1), optimized Al₂O₃/SiN_x (n = 1.92) and optimized Al₂O₃/ SiN_x/meso TiO₂ based PERC solar cell.
Stack definition	J_sc (mA/cm²)	V_oc (V)	FF (%)	Efficiency (%)
Al₂O₃/SiN_x n = 2.1 (original PERC: t = 5/95 nm)	28.67	0.665	80.48	15.34
Al₂O₃/SiN_x (optimized: n = 1.92, t = 77 nm)	29.94	0.666	80.42	16.03
Al₂O₃/n = 2.37 SiN_x/TiO₂ (t = 5/47/85 nm)	30.59	0.667	80.39	16.59

The potential of TiO₂ as a capping layer has been studies with the help of optical and TCAD based device simulations on the basis of experimental data. Initially the simulation model is calibrated with an experimental solar cells data and then using extensive simulations the potential of TiO₂ is explored. A comparison of TiO₂ deposited through APCVD, ALD, and mesoporous on the surface of bifacial PERC solar cells has been presented, and it is found that the mesoporous TiO₂ improves the efficiency of the solar cells. Our studies have quantitatively provided the positive impact of the capping layer on the efficiency of bifacial PERC solar cells, in addition to the intrinsic benefits of TiO₂, such as hydrophilic nature and hardness. It is concurred that an overall improvement of 0.14 and 0.23% in the front and rear side efficiency is possible when these surfaces are coated with an additional mesoporous TiO₂ at much lower temperature than needed for processing of solar cells.

Author statement

Aamenah Siddiqui: Draft writing, data collection, simulation, Muhammad Usman: Conceptualization, validation, resources, reviewing, supervision, Anders Hallen: Resources

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Data availability

Data will be made available on request.

Acknowledgments

The authors would like to acknowledge Swedish Research Council (VR) to support the research through Swedish Research Links (SRL) project no. 2019–03874.

Author Contribution

A.S.: Draft writing, data collection, simulation, M. U.: Conceptualization, validation, resources, reviewing, supervision, A. H.: Resources, reviewing

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published 12 Jun, 2024

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16 Mar, 2024
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15 Mar, 2024
Reviewers agreed at journal
15 Mar, 2024
Reviewers invited by journal
11 Mar, 2024
Editor assigned by journal
11 Mar, 2024
Submission checks completed at journal
11 Mar, 2024
First submitted to journal
10 Mar, 2024

You are reading this latest preprint version

Potential of TiO2 as a Capping Layer for Industrial c-Si PERC Solar Cells

Status:

Journal Publication

Version 1

Abstract

Figures

Introduction

APCVD, ALD and mesoporous TiO film properties

Simulation procedure and framework

Results and discussion

Impact of adding TiO₂ on the front and rear surfaces of the PERC cell

Front side optimization of SiN and TiO mesoporous together

Optimizing SiN and TiO mesoporous together for rear side

Conclusion

Declarations

Author Contribution

References

Additional Declarations

Status:

Journal Publication

Version 1

Potential of TiO2 as a Capping Layer for Industrial c-Si PERC Solar Cells

Status:

Journal Publication

Version 1

Abstract

Figures

Introduction

APCVD, ALD and mesoporous TiO film properties

Simulation procedure and framework

Results and discussion

Impact of adding TiO2 on the front and rear surfaces of the PERC cell

Front side optimization of SiN and TiO mesoporous together

Optimizing SiN and TiO mesoporous together for rear side

Conclusion

Declarations

Author Contribution

References

Additional Declarations

Status:

Journal Publication

Version 1

Impact of adding TiO₂ on the front and rear surfaces of the PERC cell