p-Type a-Si:H Doping Using Plasma Immersion Ion Implantation for Silicon Heterojunction Solar Cell Application

International audiencePlasma immersion ion implantation doping of thin a-Si:H layers is proposed as a new and easy-to-process solution for the fabrication of interdigitated back contacted a-Si/c-Si heterojunction solar cells (SHJ). This study is focused on boron implantation (at low acceleration voltages in the range of 500-1500 V) in a-Si:H layers on a c-Si substrate, to create the strongly doped emitter while maintaining very high c-Si surface passivation, required for high efficiency SHJ solar cells. The influence of implantation parameters and post-annealing temperature on the a-Si:H layer conductivity and passivation quality are assessed. The doped layers conductivity is also investigated by direct and indirect electrical characterization of the density of states in a-Si:H after ion implantation, and the post-implantation annealing. Eventually, an interesting passivation/doping trade-off is obtained after annealing at 300 C of samples implanted at 1000 V with implied open circuit voltage values of 710 (AE5) mV and conductivity values of the doped a-Si:H layer of 3.0 (AE1.0) Â 10 À5 V À1 cm À1 , which demonstrates that such approach is promising for processing IBC-SHJ cells. Introduction: The interdigitated back contact amorphous/ crystalline silicon (a-Si:H/c-Si) heterojunction solar cell (IBC-SHJ) is one of the most promising architectures. Indeed, the standard SHJ cell architecture already achieved very high efficiencies (25.1% on large area [1]), and the interdigitated back contact architecture allows to suppress shadowing related to th

Homo-hétérojonction en silicium : une nouvelle architecture pour des cellules à rendement record

National audienc

Solar‐grade boron emitters by BF3_3 plasma doping and role of the co‐implanted fluorine

International audienceWe investigate the electrical properties and dopant profiles of boron emitters performed by plasma immersion ion implan-tation from boron trifluoride (BF$_3$) gas precursor, thermally annealed and passivated by silicon oxide/silicon nitride stacks. High thermal budgets are required for doses compatible with screen-printed metal pastes, to reach very good activation rates. However, if good sheet resistances and saturation current densities may be obtained, we met strong limitations of the implied open-circuit voltage of the n-type Czochralski silicon substrates, which is incompatible with high-efficiency solar cells. Such limitations are not encountered with beamline where pure B$^+$ ions are implanted. Efforts on the passivation quality may improve the implied open-circuit voltage but are not sufficient. We provide experimental comparison between beamline and plasma immersion allowing us to discriminate the causes explaining this observation (implantation technique or ion specie used) and to infer our interpretation: The co-implantation of fluorine seems to indirectly impact the lifetime of the core substrate after thermal annealing

20.5% efficiency on large area N-type PERT cells by ion implantation

4th International Conference on Silicon Photovoltaics, SiliconPV 2014International audienceWe developed a high efficiency N-type PERT (Passivated Rear Totally Diffused) bifacial structure based on B and P ionimplantation doping, SiO2 passivation and conventional screen-printing metallization. Two process flows were compared: a “coanneal” process and a process using separated anneals for B and P activation. We highlight the impact of the variations of the Bemitter and P- BSF profiles on the solar cells performance. The impact of the boron implantation dose was studied allowing tooptimize this parameter. Concerning the BSF, two temperature ranges were studied for the P activation leading to very differentBSF profiles. A shallower profile enables to reach high implied Voc while keeping low contact resistivity. The overalloptimization was integrated into a simplified and industrial process flow on large area Cz-Si solar cells (239cm²). An averageefficiency of 19.7% was reached using the “co-annealing” process. The efficiency in this case was limited by a low PFF. Thislimitation was solved using the “separated anneal” process where an average efficiency of 20.2% was obtained on a 15 cellsbatch with a 20.5% champion cell

High efficiency fully implanted and co-annealed bifacial n-type solar cells

SiliconPV: March 25-27, 2013, Hamelin, GermanyInternational audienceThe aim of the study was to develop a very simple process for the fabrication of large area n-type PERT cells by means of ion implantation. We showed an improvement of the implanted boron activation rate with the annealing temperature by comparing boron SIMS and ECV concentration profiles. A direct positive impact on the boron emitter saturation current density (J 0e) was measured. We also investigated the effect of varying the oxidation conditions during the annealing on the implanted boron emitter and the phosphorus BSF quality. Low emitter saturation current density (J 0e) of 131 fA/cm 2 was measured on textured surfaces, close to the value obtained with diffused B-emitters. A process flow was developed leading to an average efficiency of 19% on 239 cm 2 bifacial solar cells, using only eight processing steps with two implantations and one activation annealing

Capping stability of Mg-implanted GaN layers grown on silicon

International audienceThe morphological stability during activation annealing of Mg‐implanted GaN layers (2 μm thick) grown on Si (111) is studied for several protective layers and fluencies in the 1013–1015 at. cm−2 range. We show that a thin capping, composed of a few nanometer thick AlN and SiNx stacks grown in situ just after GaN deposition, provides a good solution to retain flat morphology and no strain cracking up to 1 h annealing at 1100 °C in N2. These results are compared to thicker protective stackings with AlN layers of Si3N4 or SiO2 deposited after the implantation that withstand a thermal budget of up to 1 h at 1200 °C in N2. The efficiency of these different cap layers to limit GaN damage during high‐temperature annealing is studied as well as the impact of Mg implantation process on the cap resilience. The quality of the GaN sublayer is studied by low‐temperature photoluminescence to analyze structural/optical defects and Mg related complexes. X‐ray diffraction is performed to evaluate residual strains at the different process stages

Homo-heterojunction concept: From simulations to high efficiency solar cell demonstration

International audienceThe novel solar cell architecture called silicon homo-heterojunction (HHJ) cell is investigated combining experimental and simulation approaches. This structure intends to overcome the limitations of the silicon heterojunction technology regarding the amorphous/ crystalline silicon interface ($p$) a-Si:H/$(i)$ a-Si:H /$(n)$ c-Si) by the addition of a (p$^+$) c-Si layer at the hetero-interface. First, the added (p$^+$) c-Si layer is experimentally investigated using boron implantation through the realization and characterization of symmetric solar cell precursors. An adapted process flow taking into account the (p$^+$) c-Si profile optimization, the annealing effects on substrate degradation, and the impact on surface passivation, is deeply explored. Then, large area solar cells are processed and the solar cell performance are discussed in view of the data obtained on precursors and with the help of realistic numerical simulations. Overall, we observe that the HHJ solar cells exhibit a small performance improvement compared to reference heterojunction cells. In particular, a gain in the fill factor is observed, which is shown to be originated from both an improvement in field effect and a decrease of the vertical series resistance from the a-Si:H layers. The experimental data obtained on the processed homo-heterojunction solar cells confirm that this technology can lead to improved conversion efficiencies compared to the high quality reference heterojunction solar cells

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

International audienceIon 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

Thermal Evolution of Implantation Damages in Mg-Implanted GaN Layers Grown on Si

International audienc