Elsevier

Solar Energy

Volume 109, November 2014, Pages 105-110
Solar Energy

Laser etch back process to fabricate highly efficient selective emitter c-Si solar cells

https://doi.org/10.1016/j.solener.2014.08.022Get rights and content

Highlights

  • The selective emitter solar cell was processed using laser doping process combined with etch back process.

  • The cost effective nanosecond pulse with laser was applied to form selective emitter.

  • The laser damage was removed using etch back solution successfully.

  • The fabricated selective emitter solar cell recorded 19.17% with excellent performance uniformity.

Abstract

We developed a novel cost effective process scheme for the fabrication of highly efficient selective emitter solar cells, which uses a laser doping method combined with an etch back process. The laser doping process using a 150 ns pulse width green (532 nm) laser effectively controls the doping profiles to form a selective emitter. However, laser damage was created on the laser-doped surface and eventually the performances and stabilities of laser-doped cells were degraded due to this damage. Using a transmission electron microscope (TEM), the damage was examined and found to have a thickness of 40 nm of amorphous silicon. This thin damage layer was effectively removed in an acid mixture solution. The combined process of laser doping and etch back is called the laser etch back process. After removal of this thin damage layer, the cell efficiencies were significantly improved up to 19.17%.

Introduction

The conventional screen printed c-Si solar cell technology has been transferred to industry. To achieve further efficiency gains for c-Si solar cells, various technologies such as selective emitter solar cells, rear local contact solar cells, heterojunction solar cells and back contact solar cells are actively being investigated (Lee, 2009, Meemongkolkiat, 2008, Oliver Schultz et al., 2008, Kerschaver, 2006, Wen et al., 2013, Wang and Green, 1990, Choi et al., 2014). Among them, selective emitter c-Si solar cell technology is gaining attention due to its high potential for improving efficiency with a competitive production cost (Hilali and Rohatgi, 2012, Rohatgi et al., 2012). The efficiency improvement of the selective emitter solar cell originates from reduced auger recombination at the front side of the cell since the excess doping concentration increases the recombination rate (Jager et al., 2009, Ruby et al., 1997).

To fabricate a selective emitter solar cell, there are several approaches which include doping paste printing, oxide mask process, ion implantation, resist etch back, laser doping, and so on (Tang et al., 2013, Debarge et al., 2002, Poplavskyy et al., 2010, Book et al., 2008, Ruby et al., 1997; Mouhoub et al., 2003; Dastgheib-Shirazi et al., 2008, Meier et al., 2000, Horzel et al., 1997, Volk et al., 2011). However, each of these technologies has its disadvantages. The doping paste method uses an expensive consumable material. Both the oxide mask process and the ion implantation method require multiple process steps and expensive equipments. In the resist etch back method, an additional resist removal process is necessary, where the cleaning of tiny resist particles may degrade the cell performance. The contaminated surface of the wafer traps charges. Among laser doping methods, the laser doped selective emitter (LDSE) is being developed but it requires expensive equipments such as a picosecond pulse width laser and electrode plating systems.

Recently, one step selective emitter solar cell technology was introduced, which utilizes a low cost nanosecond pulse width laser to form selective emitter profiles (Roder et al., 2009). This approach is a cost-effectively designed one step process and demonstrated an absolute 0.4% efficiency gain compared to the conventional screen print c-Si solar cell. However, since the front contact between Ag and selective emitter is relatively more resistive, the laser doping may lead to loss of the efficiency gain from the improved front of the selective emitter. Here, we investigated an approach to stabilize the contact resistance of laser doped selective emitter solar cell by applying solution etch back process. This study demonstrates the solution etch back process effectively removes laser damage and the cell efficiency recorded extra gain due to the stabilized front contact.

Section snippets

Experimental details

The scheme to fabricate a laser doped selective emitter solar cell is introduced in Fig. 1. The selective emitter solar cell is fabricated using 6 inch boron doped p-type Czochralski (CZ) wafers. The wafers are textured using an alkaline solution to form random pyramids. To form a p–n junction, a POCl3 precursor is diffused onto the wafer in a tube thermal furnace and the resulting sheet resistance of base doping is 40 ± 2 ohm/sq. The laser doping was processed using laser of 532 nm wavelength and

Results and discussion

First, the impact of laser irradiation on the pyramids was investigated. The SEM image in Fig. 2b shows the laser-doped morphology at 5.9 W/mm2 and the surface of silicon pyramids is slightly melted. When the sample is irradiated with a laser energy density of more than 9.6 W/mm2, the silicon pyramid morphology is almost melted out and the textured morphology is lost, resulting in a decrease in the light transmittance (Fig. 2d and e). In other aspect, the sheet resistance showed a strong

Conclusion

High efficiency selective emitter solar cells were fabricated by applying a laser doping process combined with etch back. The best LEB cell recorded a power conversion efficiency of 19.17%. The improved efficiency originates from the removal of amorphous silicon laser damage layer, which is created during the laser doping process and removed in the acid solution. After applying LEB process successfully, the fill factors of LEB cells are significantly improved to the level of the conventional

Acknowledgements

This work was supported by the “National Research Foundation of Korea Grant funded by the Korean Government (MSIP)” (2014, University-Institute cooperation program); by World Class 300 Project R&D (No. 10043264) of the Korea Evaluation Institute of Industrial Technology (KEIT) grant funded by the Korea government Ministry of Trade, Industry and Energy.

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