Nuclear Instruments and Methods in Physics Research Section B: Beam Interactions with Materials and Atoms
Homojunction silicon solar cells doping by ion implantation
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.
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