Low cost multicrystalline bifacial PERC solar cells – Fabrication and thermal improvement
Introduction
Multicrystalline silicon solar cell is still the main product in the photovoltaic industry due to its advantage of the low cost. At present, “black silicon” technology (Agarwal et al., 2011, Yoo et al., 2011, Tsujino et al., 2006, Liu et al., 2014) and PERC (Blakers et al., 1989, Zhao et al., 1999) technology are the main technical used to improve multicrystalline silicon cells performance. In consequence, the production cost of multicrystalline silicon cells is also increased since the rear passivation film deposition and the laser contact opening (LCO) processes (Blakers et al., 1989, Zhao et al., 1999) are used. Compared with monocrystalline PERC cells, multicrystalline PERC cells have less efficiency improvement and potential under the same investment, which reduces its cost advantage. In order to maximize the advantages of multicrystalline materials, multicrystalline cells should choose a technical route that reduces cost without reducing the cell efficiency significantly. In 2008, Cesar et al. (2008) proposed to use Al grid lines rather than all-aluminum rear electrode for the bifacial monocrystalline PERC cell. Firing-through aluminum paste was used to directly etch the rear passivation film to form local aluminum-silicon contact without LCO step. However, solar cells with this kind of structural have low fill factor and poor passivation area on the edge of aluminum fingers.
In this paper, a low cost bifacial multicrystalline PERC cell structure was developed through Al fingers structure with direct firing through of rear passivation film. We then conduct the optimization of rear passivation film, aluminum paste matching and co-firing process are presented. This cell structure has the front-side cell efficiency of this structure is similar to that of monofacial multicrystalline PERC cell with additional rear efficiencies. This paper shown that the LCO step was removed and the amount of aluminum paste can be reduced (Dullweber et al., 2016) via using these processes. As a result, this can significantly reduce the process cost. Through the irradiation annealing technology for the finished cells, the efficiency of this structure can be further improved by more than 0.2% absolute.
Section snippets
Experimentals
Diamond-wire sawn p-type multicrystalline silicon wafers of 156.75 mm × 156.75 mm in size, 180–190 μm thick, resistivity of 1–2 Ω·cm were used. The two basic structures of PERC cell are shown in Fig. 1. The bifacial PERC cell has the similar structure as the monofacial PERC cell, but the aluminum grid line is used on the rear instead of the all-aluminum back field. The rear-side passivation film structure of bifacial PERC cells is similar to that of monofacial PERC cells, where layer stack of Al
Bifacial PERC cell fabrication process
Fig. 3 shows the optical picture of the prepared bifacial PERC cell. The cell adopts 5 bus bars design, and the front finger line structure design was consistent with the Ag electrode structure of full-area Al-BSF multicrystalline cells. The rear-side finger electrode was composed of Al, and the main body of the bus bar was Al. Ag was used at the welding point to be compatible with the later module fabrication process. In this experiment, 101 fingers on the front-side and rear-side were used.
Conclusion
The industrialization method for fabricate bifacial multicrystalline PERC cells was explored. By use of firing-through aluminum paste to fabricate aluminum electrodes on the rear of the solar cell enables the PERC cell to generate electricity on both sides. At the same time, compared with the traditional PERC cells, the LCO step is omitted and the consumption of aluminum paste is reduced. The key to the fabrication of this cell structure is to control the passivation in the alloyed area of
Acknowledgements
The authors wish to thank Yan Di and Yimao Wan for the language modification of the article. This work has been supported by Ruxing Technology Development Co., Ltd.
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