18-PERCENT EFFICIENT SILICON PHOTOVOLTAIC DEVICES BY RAPID THERMAL-DIFFUSION AND OXIDATION

For the first time, cells formed hy rapid thermal processing (RTP) hav e resulted in 18%-efficient 1 and 4 cm(2) single-crystal silicon solar cells. Front surface passivation by rapid thermal oxidation (RTO) sig nificantly enhanced the short wavelength response and decreased the ef fective front surface recombination velocity (including contact effect s) from 7.5 x 10(5) to about 2 x 10(4) cm/s, This improvement resulted in an increase of about 1% (absolute) in energy conversion efficiency , up to 20 mV in V-oc, and about 1 mA/cm(2) in J(sc). These RTO-induce d enhancements are shown to be consistent with model calculations. Sin ce only 3 to 4 min are required to simultaneously form the phosphorus emitter and aluminum back-surface-field (BSF) and,5 to 6 min are requi red for growing the RTO, this RTP/RTO process represents the fastest t echnology for diffusing and oxidizing greater than or equal to 18%-eff icient solar cells. Both cycles incorporate an in situ anneal lasting about 1.5 min to preserve the minority carrier lifetime of lower quali ty materials such as dendritic-web and multicrystalline silicon. These high-efficiency cells confirmed that RTP results in equivalent perfor mance to cells fabricated by conventional furnace processing (CFP), De tailed characterization and modeling reveals that because of RTO passi vation of the front surface (which reduced J(oe),, by nearly a factor of ten), these RTP/RTO cells have become base dominated ( J(ob) much g reater than J(oe)), and further improvement in cell efficiency is poss ible by a reduction in back surface recombination velocity (BSRV), Bas ed upon model calculations, decreasing the BSRV to 200 cm/s is expecte d to give 20%-efficient RTP/RTO cells.