An industrially viable TOPCon structure with both ultra-thin SiOx and n+-poly-Si processed by PECVD for p-type c-Si solar cells

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We report an industrial compatible tunnel oxide passivated contact (TOPCon) structure on solar-grade p-type c-Si wafer as the rear emitter for high-efficiency solar cells, where the ultrathin silicon oxide (SiOx) is made by plasma-assisted oxidation and the P-doped n-type poly-Si (n+-poly-Si) contact layer by plasma-enhanced chemical vapor deposition (PECVD) of hydrogenated amorphous silicon (a-Si:H) with in-situ doping with PH3 and following the high-temperature annealing. The fabrication processes of SiOx and n+-poly-Si layers are systematically optimized. It is found that the optimized annealing temperature for the crystallization and dopant activation is in the range of 850 °C and 880 °C, which is higher than the similar structure with chemically oxidized SiOx (820 °C). In addition, the samples made with higher PH3/SiH4 doping ratio during the a-Si:H deposition need a lower annealing temperature to reach the similar dopant distribution within the samples made with lower doping ratio but annealed at higher temperature, it means that the optimized annealing temperature decreases with the increase of the PH3/SiH4 doping ratio during the deposition of a-Si:H precursor. The optimized process with post-hydrogenation yields excellent surface passivation on the p-type Si substrate with the implied open-circuit voltage (iVoc) of ∼742 mV, the single-side saturated recombination current density (J0) of ∼3.0 fA/cm2, the contact resistivity (ρc) of ∼2–4 mΩ cm2, and the effective minority carrier lifetime (τeff) of ∼1050 μs (Δn = 1 × 1015 cm-3). With these passivation parameters, a simulation study demonstrates the advantage of rear emitter over the conventional front emitter TOPCon solar cell with ∼22.8% achievable efficiency on solar-grade p-type wafers with current industrial constraints, and ∼24.9% on high quality p-type c-Si wafers with the optimized conditions in R&D laboratories. Our study suggests that the p-type c-Si solar cell with a rear n+-poly-Si TOPCon emitter is a viable structure for high-efficiency solar cell production.