The Studies of Anti-Reflective (AR) Coatings Effects for Solar Cell by Using Silvaco TCAD Tools

 

1. Introduction

In 1975, screen printing was first applied to solar cells for the formation of the front and rear contacts replacing expensive vacuum metallization [1]. The encapsulate used in a photovoltaic (PV) module has many requirements. It must be optically transparent, electrically insulating, mechanically compliant, adherent to both glass and cells, and sufficiently robust to withstand 20–30 years in the field. Since the early 1980s, the encapsulate in all PV modules has been ethylene vinyl acetate (EVA) [2, 3].

It’s a popular strategy to place Anti-Reflective (AR) coatings on light detecting devices to improve device quantum efficiency. Such coatings rely on destructive interference of reflected waves to reduce overall reflection coefficient of light incident on the detecting device.

Silvaco TCAD [4] provides complete and well integrated simulation software for all solar cell technology. The TCAD modules required for solar cell simulation included: S-Pisces, Blaze, Luminous, Device3D, and Luminous3D.

This article will present the results of ray-trace simulations of conventional silicon PV modules with different AR coating material, EVA and nitride components. We can find the optimal thickness of these materials under the different design and operation parameters.

 

2. The Accurate Optical Assessment

An accurate optical assessment of a PV module is not trivial. As illustrated by Figure 1, incident rays reflect from the air–glass (1), glass–encapsulate (3), encapsulate–cell (6) and encapsulate–backsheet (8) interfaces; and in the latter two cases, the reflection is often diffuse, leading to some of the reflected light being total-internally reflected at the glass–air interface and then returned to the cells. Furthermore, incident rays are absorbed in the glass (2), the encapsulate (4), the cell’s anti-reflection coating or metal fingers (5), and the backsheet (7). These eight interactions depend on the light’s incident wavelength and incident angle. Therefore, a comprehensive optical assessment is aided by the application of ray tracing.

Cross-sectional diagram of a conventional PV module (to scale), and the optical loss mechanisms as listed in the text.

 

3. The Design and Operation Parameter Parameters of the Flat-type Solar Cell

The parameters of the solar cell which we defined are listed in Table 1. Other parameters are using by defaults in Silvaco TCAD tools.

Parameter
Value
Unit
Cell Width
100
μm
Cell Depth
200
μm
Shielding area ratio
0.1
-
Pitch Size
90
μm
Substrate Con.N-type
1e17
/cm3
Diffusion Con. P-type
5e20
/cm3
Diffusion Time
75
min
Diffusion Temp.
790
oC
Backside Conc. N-type
1e21
/cm3
Backside Conc. Depth
9.25
μm
Lifetime
10
μs
Light Source AM15 -
AM15
-

Workfunction

For Al
For Ag

 

4.4
5.0

 

eV

Table 1. Design Parameters for Solar Cell.

 

3.1 The Simulation Results of EVA
The optical properties are summarized by reference [5]. The simulation results are shown in Figures 2 and 3. We find the optimal EVA thickness is 0.1um in the Figure 2. Besides, we also find that the minimum reflectivity value will be shifted with increasing the ARC thickness. In the Figure 3, we can find that the maximum value, -0.33, of the Anode Current (Isc) under the ARC thickness is 0.1um. But we can also find from this figure that the Anode Voltage (Voc) are almost the same and the value of is around 0.6.

Figure 2. The reflectivity curve with different EVA thickness.

 

Figure 3. The IV curve with different EVA thickness.

 

3.2 The simulation Results of Nitride
The optical properties are referenced form the material vendor. Figures 4 and 5 are shown the simulation results. We find the optimal Nitride thickness is 0.7um in the Figure 4. Besides, we also find that the minimum reflectivity value will be shifted with increasing the ARC thickness. In the Figure 4, we can find that the maximum value, -0.32, of the Anode Current (Isc) under the ARC thickness is 0.07um. But we can also read from this figure that the Anode Voltage (Voc) are almost the same and the value of is around 0.6.

Figure 4. The reflectivity curve with different Nitride thickness.

 

Figure 5. The IV curve with different Nitride thickness.

 

 

The summary of the solar cell efficiency are listed in Table 2. We can find the efficiencies with the ARC materials are larger than that without the ARC material. We also find the solar cell efficiency with EVA is larger than that with nitride.

 

Design condition
Efficiency (%)
Without ARC
5.45
The optimal condition with EVA, thickness=0.1um
8.27
The optimal condition with Nitride, thickness=0.07um
8.01
Table 2. The Solar Efficiency without or with ARC.

 

4. Conclusion

In conclusion, Silvaco TCAD tool provides a complete solution for solar cell technology. In this article, it helps us to study the ARC thickness and material. We can find the optimal design condition for ARC thickness, 0.1um for EVA and 0.07um for nitride. It also enables researchers to study the all operation and design parameters of the silicon solar cells.

 

Reference

  1. E. L. Ralph, “Recent advancements in low cost solar cell processing,” in Proceedings of the 11th IEEE Photovoltaic Specialists Conference (PVSC ’75), pp. 315–316, Scottsdale, Ariz, USA, May 1975.
  2. J. Zhao, A. Wang, P. Altermatt and M. A. Green, “24% efficiency silicon solar cells with double layers antireflection coatings and reduced resistance loss,” pp3636-3638, Appl. Phys. Lett, 66, 1995
  3. R. Einhaus, E. Vazsony, J. Szlufcik, J. Nijs, and R. Mertens, “Isotropic texturing of multicrystalline silicon wafers with acidic texturing solutions,” in Proceedings of the 26th IEEE Photovoltaic Specialists Conference (PVSC ’97), pp. 167-170,
  4. “ATLAS User Manual”, Silvaco, Santa Clara, California, USA.
  5. K. R. McIntosh, J. N. Cotsell, J. S. Cumpston, A. W. Norris, N. E. Powell, and B. M. Ketola, “An optical comparison of silicone and eva encapsulants for conventional silicon pv modules: a ray-tracing study,” in Proceedings of the 34th IEEE Photovoltaic Specialists Conference (PVSC ’09) pp.544-579

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