Record fast thermal processing of 17.5% efficient silicon solar cells
This paper presents the development and analysis of 17.5% efficient silicon solar cells on 0.9 Ω cm PV-grade Czochralski silicon (Cz–Si), featuring a record small thermal process time of 1 min for diffusion and oxidation. The cells have been processed by rapid thermal processing (RTP) using incoherent light as an energy source. The involved processes consist of a 5 s RTP step at 930 °C for the simultaneous formation of the phosphorus-doped emitter and of the aluminium back surface field (Al–BSF) and a 30 s rapid thermal oxidation (RTO) at 950 °C for the emitter surface passivation. Both processes feature high heating and cooling rates of 100 K s−1. The results show that no lengthy gettering or post diffusion annealing steps are required for the fabrication of high efficiency Cz–Si solar cells and that high heating and cooling rates are applicable. Detailed analysis reveals that the emitter saturation current J0e is reduced by a factor of 3–4 after the RTO passivation in spite of the high carrier surface concentrations of 3 to 4 × 1020 cm−3. Furthermore, one-dimensional device simulations are presented which show that the achieved efficiencies are limited by the effective back surface recombination velocity Sback resulting from the thin Al–BSF applied.