Ion implantation has the advantage of being a unidirectional doping technique. Unlike gaseous diffusion, this characteristic highlights strong possibilities to simplify solar cell process flows. The use of ion implantation doping for n-type PERT bifacial solar cells is a promising process, but mainly if it goes with a unique co-annealing step to activate both dopants and to grow a SiO 2 passivation layer. To develop this process and our SONIA cells, we studied the impact of the annealing temperature and that of the passivation layers on the electrical quality of the implanted B-emitter and P-BSF. A high annealing temperature (above 1000°C) was necessary to fully activate the boron atoms and to anneal the implantation damages. Low J 0BSF (BSF contribution to the saturation current density) of 180 fA/cm 2 was reached at this high temperature with the best SiO 2 passivation layer. An average efficiency of 19.7% was reached using this simplified process flow ("co-anneal process") on large area (239 cm 2 ) Cz solar cells. The efficiency was limited by a low FF, probably due to contaminations by metallization pastes. Improved performances were achieved in the case of a "separated anneals" process where the P-BSF is activated at a lower temperature range. An average efficiency of 20.2% was obtained in this case, with a 20.3% certified cell.
The use of ion implantation doping instead of the standard gaseous diffusion is a promising way to simplify the fabrication process of silicon solar cells. However, difficulties to form high-quality boron (B) implanted emitters are encountered when implantation doses suitable for the emitter formation are used. This is due to a more or less complete activation of Boron after thermal annealing. To have a better insight into the actual state of the B distributions, we analyze three different B emitters prepared on textured Si wafers: (1) a BCl 3 diffused emitter and two B implanted emitters (fixed dose) annealed at (2) 950°C and at (3) 1050°C (less than an hour). Our investigations are in particular based on atom probe tomography, a technique able to explore 3D atomic distribution inside a material at nanometer scale. Atom probe tomography is employed here to characterize B atomic distribution inside textured Si solar cell emitters and to quantify clustering of B atoms. Here, we show that implanted emitters annealed at 950°C present maximum clusters due to poor solubility at lower temperature and also highest emitter saturation current density (J 0e = 1000 fA/cm 2 ). Increasing the annealing temperature results in greatly improved J 0e (131 fA/cm 2 ) due to higher solubility and a consequently lower number of clusters. BCl 3 diffused emitters do not contain any B clusters and presented the best emitter quality. From our results, we conclude that clustering of B atoms is the main reason behind higher J 0e in the implanted boron emitters and hence degraded emitter quality.
We investigate the electrical properties and dopant profiles of boron emitters performed by plasma immersion ion implantation 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.
Ion Beam Services (IBS) has developed processes dedicated to silicon-based solar cell manufacturing using a plasma-immersion ion implantation equipment. It enables the realization of various doping profiles for phosphorus-doped emitters which fit the requirements of high-efficiency solar cells. PH 3 plasma-implanted emitters are chemically, physically and electrically characterized to demonstrate their excellent quality. Those emitters are then integrated into a low cost p-type monocrystalline silicon solar cell manufacturing line from the National Solar Energy Institute (INES) in order to be compared with usual POCl 3 diffusion. Starting from a basic process flow with blanket emitter and conventional full-area aluminum back-surface field, plasma-immersion implanted emitters enable to raise conversion efficiencies above 19.1%. Thanks to an optimized double layer anti-reflective coating, a 19.4% champion cell has been achieved. Depending on different plasma process parameters, lightly doped emitters are then engineered aiming to study doping modulation using a dedicated laser.
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