The Studies of Regular Texture Thickness and Finger Pattern of the Front Surface by Using Silvaco TCAD Tools

1. Introduction

The photovoltaic industry has rapidly grown since 2000 and has diversified in technology how the solar cell are manufactured. As of 2010, 70% of these cells were made from mono- and multicrystalline silicon wafer, 20% from thin films and 10% from silicon ribbons. In 1975, screen printing was first applied to solar cells for the formation of the front and rear contacts replacing expensive vacuum metallization [1]. This process and equipment for the screen-printed solar cell has been further optimized and new technologies have been introduced to improve this technology. These include an anti-reflection Silicon nitride coating with excellent surface and bulk passivity properties. Surface texture has reduced reflectivity. [2, 3] Laser edge isolation and single-side etching is used for the electrical separation of the front and rear contacts.

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, Device 3D, and Luminous 3D.

In this article we will study a flat-type solar cell under different diffusion temperatures and concentrations. We will also study a texture-type solar cell with texture thickness and finger pattern at fixed shielding area of the front surface. We will find the maximum solar efficiency under different operation and design parameters.

2. Screen-Printed Silicon Solar Cell

Most screen-printed solar cells are fabricated in the industry today using the following process sequence: (1) Saw damage removal, texture and cleaning silicon wafer; (2) N-type diffusion; (3) Plasma edge isolation; (4) N-type glass removal; (5) Silicon nitride deposition; (6) Ag screen printing of the front contact and drying; (7) Al/Ag screen print of the rear busbars and drying; (8) Al screen printing of the backside contact and drying; (9) Conforming of the front and rear contacts; (10) Measure the IV curve.

3. Simulation of Solar Cell Characteristics Using Silvaco TCAD Tools

The parameters which we defined are listed in Table 1. Additional parameters used in simulation of solar cells, and its defaults, can be found in Silvaco TCAD manuals.

3.1 Flat-type Solar Cell
(1) Diffusion concentration:
The simulation results are shown in Figure 1. We find that the Anode Voltage (Voc) increases along with the diffusion concentration is when the concentration is less than 5e+20. But we can also read from this figure that the Anode current (Isc) is almost the same at around -0.4.

Figure 1. IV curve with diffusion concentration.

(2) Diffusion temperature:
Figure 2 shows the simulation results with the diffusion temperature effect. We find that the Anode Voltage (Voc) increases with decreasing diffusion temperature. But we can read also from this figure that the Anode current (Isc) is almost the same at around -0.44.

Figure 2. IV curve with diffusion temperature.

3.2 Texture-type Solar Cell
(1) Texture Thickness Effect:
The reflectivity and the IV curves of the simulation results are shown in Figures 3 and 4. From Figure 3 we can find that the reflectivity of the texture-type is smaller than that of the flat-type. The optimal condition, minimum reflectivity, for the texture thickness is 10um. Moreover, we also find that there is the maximum Anode Voltage value at texture thickness=10um.

Figure 3. Reflectivity curve with texture thickness.

Figure 4. IV curve with texture thickness.

The summary of the solar cell efficiency is listed in Table 2. We find that the maximum efficiency at texture thickness equals to 10um.

(2) Finger Pattern under the Fixed Shielding Area:
According to the previous studies, we find that the optimal texture thickness is 10um, so we modified the finger pattern of the front surface under the shielding area so that the ratio equals 10%.

The pitch widths are 90um, 45um, 30 um and 15 um. The summary of the solar cell efficiency is listed in Table 3. We find that the maximum efficiency is where pitch width equals to 30um.

4. Conclusion

In conclusion, Silvaco TCAD tool provides a complete solution for solar cell technology. In this article, it helps us to study the regular texture thickness and finger pattern of the front surface. We find the optimal design condition at texture thickness equals to 10um and the finger pitch width at 30um. It also enables researchers to study the all operation and design parameters of the Screen-Print Silicon Solar Cell.

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. D. L. King and M. E. Buck, “Experimental optimization of an anisotropic etching process forrandom texturization of silicon solar cell,” Conference (PVSC ’91), vol. 1, pp. 303-308, Las Vegas, Nev, USA, October 1991.
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, Anaheim, Calif, USA, September-October 1997.
4. “ATLAS User Manual”, Silvaco, Santa Clara, California, USA.

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