Homojunction silicon solar cells doping by ion implantation

https://doi.org/10.1016/j.nimb.2017.06.020Get rights and content

Abstract

Production costs and energy efficiency are the main priorities for the photovoltaic (PV) industry (COP21 conclusions). To lower costs and increase efficiency, we are proposing to reduce the number of processing steps involved in the manufacture of N-type Passivated Rear Totally Diffused (PERT) silicon solar cells. Replacing the conventional thermal diffusion doping steps by ion implantation followed by thermal annealing allows reducing the number of steps from 7 to 3 while maintaining similar efficiency.

This alternative approach was investigated in the present work. Beamline and plasma immersion ion implantation (BLII and PIII) methods were used to insert n-(phosphorus) and p-type (boron) dopants into the Si substrate. With higher throughput and lower costs, PIII is a better candidate for the photovoltaic industry, compared to BL. However, the optimization of the plasma conditions is demanding and more complex than the beamline approach.

Subsequent annealing was performed on selected samples to activate the dopants on both sides of the solar cell. Two annealing methods were investigated: soak and spike thermal annealing. Best performing solar cells, showing a PV efficiency of about 20%, was obtained using spike annealing with adapted ion implantation conditions.

Introduction

In the PV industry, the main challenge is to increase the efficiency of the solar cells while reducing the manufacturing costs. For this, the process flow must be optimized with a simplification, or a decrease of the number of the process steps.

In this study, we propose to change the traditional gas diffusion doping process by an ion implantation doping process. The ion implantation allows to better control the doping profile inside the material with a better uniformity and reproducibility [1]. Also it allows to reduce significantly the number of process steps. In future, this doping process will reach the new cell technologies such as Interdigitated Back Contact (IBC) or selective emitter cells. The first tests on silicon solar cells using ion implantation date from 1980, where encouraging yields were reached [2]. Nevertheless, this doping technology is not very used in the PV industry, besides in microelectronic industry, because up to now, the ion implantation tools were relatively expensive and the throughput was not adapted to the PV industry. These last years, the development of the ion implantation technologies, such as Beam Line, Plasma Immersion or Shower, has consented to reduce the price of the ion implantation tools and to increase the throughput to become competitive for the PV industry.

In this paper we investigated the n-type and p-type doping for the fabrication of N-type homojunction silicon solar cells using ion implantation followed by an activation annealing. Our study consisted on comparing Beam Line Ion Implantation (BLII) versus plasma immersion ion implantation (PIII) techniques and also Soak annealing versus Spike annealing techniques. Firstly, we have separately optimized and electrically characterized the doping process of each type. Thus, these optimized doping processes were integrated on the total process flow for the fabrication of entire solar cells.

Section snippets

Experimental

In the PV industry, there are two cell technologies: Homojunction & Heterojunction, with different cell architectures. In our study, we used homojunction N-type implanted Passivated Rear Totally Diffused (PERT) silicon solar cells. N-type silicon has many advantages like the absence of light induced degradation (LID) [3], [4], a low sensitivity to metallic impurities and a high lifetime potential [5]; and PERT architecture conciliates high efficiency and cost effective ($/W) processes [6].

A

Diffusion

Since the beginning of the PV industry, the standard doping process is the diffusion on the both sides of the cell. It is a mature process where it is difficult to control the concentration and depth of the profile. Moreover, it is a technology that is not adapted to all cell architectures. The fact of tuning only diffusion parameters like temperature, time and gas flow, does not allow reaching cell efficiencies beyond 20% in production.

Fig. 3 shows SIMS profiles, which are the chemical

Conclusion

In this work we studied the doping of n-type silicon solar cells using two ion implantation techniques: BLII and PIII, allowing to reduce the number of doping process steps for the n-type PERT silicon solar cells, from 7 to 3. Compared to the diffusion techniques, we obtained a record cell of 20.33% efficiency using BLII doping and separated Soak annealing.

In PIII, we developed boron emitter using B2H6 as precursor gas. Hybrid cells combining PIII for emitter doping and BLII for BSF reached

Acknowledgments

This work was done under the LETI/INES collaboration on the process development of solar cell doping by ion implantation.

References (20)

There are more references available in the full text version of this article.

Cited by (8)

  • Effect of diffusion parameters on emitter formation in silicon solar cells by proximity rapid thermal diffusion

    2018, Materials Science in Semiconductor Processing
    Citation Excerpt :

    Formation of the p-n junction is a key stage in the cell fabrication process and is traditionally carried out in conventional thermal furnaces using solid or gaseous dopant sources. Ion implantation has also recently made inroads in PV manufacturing [5]. These techniques can have limitations in terms of control of junction depth and/or uniformity when applied to 3 dimensional structures such as micro and nanowire solar cells which require shallow junctions.

  • Characterization of Si<inf>x</inf>O<inf>y</inf>N<inf>z</inf> coating on CF/PPS composites for space applications

    2018, Surface and Coatings Technology
    Citation Excerpt :

    Beloto et al. [27] treated the surface of porous silicon by PIII, with 12 kV, and the modified depth was approximately 90 nm. Another study of implantation profile was made by Milési et al. [28], that treated solar cell devices with elevated temperatures (1000 °C), and observed a modified depth of 150 nm. Thus, in this work, the modified depth of the silicon film should be inferior than 80 nm.

  • Borided Materials

    2019, Engineering Materials
View all citing articles on Scopus
View full text