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Record fast thermal processing of 17.5 % efficient silicon solar cells

: Peters, S.; Ballif, C.; Borchert, D.; Schindler, R.; Warta, W.; Willeke, G.


Semiconductor Science and Technology 17 (2002), Nr.7, S.677-681
ISSN: 0268-1242
ISSN: 1361-6641
Fraunhofer ISE ()
silicon solar cell; record fast thermal processing; PV-grade Czochralski silicon; thermal process time; diffusion; oxidation; rapid thermal processing; incoherent light energy source; phosphorus-doped emitter; aluminium back surface field; rapid thermal oxidation; emitter surface passivation; high heating rate; high cooling rate; fabrication; emitter saturation current; high carrier surface concentration; one-dimensional device simulation; efficiency

Presents the development and analysis of 17.5% efficient silicon solar cells on 0.9 Omega 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 degrees 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 degrees C for the emitter surface passivation. Both processes feature high heating and cooling rates of 100 K s/sup -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 J/sub 0e/ is reduced by a factor of 3-4 after the RTO passivation in spite of the high carrier surface concentrations of 3 to 4 x 10/sup 20/ cm/sup -3/. Furthermore, one-dimensional device simulations are presented which show that the achieved efficiencies are limited by the effective back surface recombination velocity S/sub back/ resulting from the thin Al-BSF applied.