# Plasma‐immersion ion implantation: A path to lower the annealing temperature of implanted boron emitters and simplify PERT solar cell processing

Abstract : Ion implantation is a suitable and promising solution for the massive industrialization of boron doping, which is a crucial process step for most next-generation solar cells based on crystalline silicon (c-Si). However, the use of ion implantation for boron doping is limited by the high temperature (in the 1050°C range) of the subsequent activation anneal, which is essential to dissolve the boron clusters and reach a high-emitter quality. In this work, we propose the use of plasma-immersion ion implantation (PIII) from B$_2$ H$_6$ gas precursor instead of the standard beamline ion implantation (BLII) technique to decrease this temperature down to 950°C. PIII and BLII boron emitters were compared with annealing temperatures ranging from 950°C to 1050°C. Contrary to BLII, no degradation of the emitter quality was observed with PIII implants annealed at 950°C along with a full activation of the dopants in the emitter. At 1000°C, emitter saturation current densities (J$_{0e}$) below 21 fA/cm$^2$ were obtained using the PIII technique regardless of the tested implanta-tion doses for sheet resistances between 110 and 160 $\Omega$/sq. After metallization steps, the metal/emitter contact resistances were assessed, indicating that these emitters were compatible with a conventional metallization by screen-printing/firing. The PIII boron emitters' performances were further tested with their integration in n-type passivated emitter rear totally diffused (PERT) solar cells fully doped by PIII. Promising results already show a conversion efficiency of 20.8% using a lower annealing temperature than with BLII and a reduced production cost. KEYWORDS annealing temperature, B$_2$ H$_6$ plasma, boron doping, n-type PERT solar cells, plasma-immersion ion implantation, silicon solar cells We report a new way to activate implanted boron emitter at low temperature that is the use of plasma-immersion ion implantation (PIII) from B$_2$H$_6$ plasma. A full activation of the emitter at 950°C was observed even for a high implantation dose corresponding to a sheet resistance of 112 $\Omega$/sq. Promising performances while being integrated in n-PERT solar cells fully doped by PIII were demonstrated with efficiency of 20.8% % using a lower annealing temperature than with BLII and a reduced production cost.
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https://hal-cea.archives-ouvertes.fr/cea-02570680
Submitted on : Tuesday, May 12, 2020 - 11:45:53 AM
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Adeline Lanterne, Thibaut Desrues, Coralie Lorfeuvre, Marianne Coig, Frank Torregrosa, et al.. Plasma‐immersion ion implantation: A path to lower the annealing temperature of implanted boron emitters and simplify PERT solar cell processing. Progress in Photovoltaics, Wiley, 2019, 27 (12), pp.1081-1091. ⟨10.1002/pip.3186⟩. ⟨cea-02570680⟩

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