Micro and nano-structuration of silicon by femtosecond laser: Application to silicon photovoltaic cells fabrication
Introduction
In order to increase the efficiency of actual solar cells many different ways are currently being developed by researchers. They include the nano and micro-structuration of the surface [1], [2], [3], [4], [5], [6], [7], [8], the use of antireflection (AR) coatings [9], rear totally diffused structures [10], or the use of absorbing nanoparticles [11].
It has been recently shown by Mazur et al. [1], [2], [3], [4], [5], [6], [7] that a simple way to improve the silicon-based solar cell efficiency is to irradiate the silicon surface with a series of femtosecond laser pulses, in the presence of a sulfur containing gas [1], [2], [3], [4], [5], [6], [7]. This produces micro-spikes on the silicon surface that strongly reduces the incident solar light reflection (Black Silicon). We have in this study created a photovoltaic structure in a silicon wafer, which the illuminated surface was locally nanostructured (squares of 1 mm2) using a femtosecond laser, before the formation of p-n junction. Various parameters, like polarization, spot size, energy density, number of shots, scanning parameters were chosen to make an appropriate nanotexturization without SF6, i.e. the laser treatment was performed under vacuum (10− 5 mbar).
The p-n junction was obtained by counterdoping the wafer surface by means of the Plasma Immersion Technique (PULSION tool, developed by IBS [11]) followed by Rapid Thermal Annealing (RTA).
It will be shown that the photocurrent increases by 25 to 30% in the texturized areas.
Section snippets
Processing steps
Samples were n-type silicon doped phosphorus to 1015 cm− 3 (5–20 Ω.cm), cleaned by means of the conventional RCA treatment.
The surfaces were first phosphorus diffused from a POCl3 source in order to create a n+ layer which helps the formation of a back ohmic contact, while the n+ front layer was chemically removed (CP4 etch) before the laser treatment.
After the laser structuring of the surface, the samples have been boron implanted by Plasma Immersion (PULSION, BF3, 2 kV, 900 °C, 30 mn) and RTA
Conclusions
We have prepared laser-microstexturized Si structures which reduce the reflection of a silicon surface. 1 mm2 areas were structured on n-type silicon with a femtosecond laser, under vacuum. The boron doped p+ regions were obtained using a Plasma Ion Immersion technique. An increase in the photocurrent was detected by LBIC and reaches more than 30% in the treated zones.
Notice that this study has been done in a framework of renewable energy and sustainable development promotion: therefore it was
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