Profiled poly-silicon films by Hot-Wire Chemical Vapour Deposition for solar cells on cheap metal substrate

Films grown by Hot-wire CVD at high hydrogen dilution of SiH4 (Polyl) showed a polycrystalline initial growth, though with high defect density and randomly oriented grains. The crystal columns consist of many small grains. Oxygen penetrates deep into the film through large voids between the columns perpendicular to the substrate. Films grown with low hydrogen dilution (Poly2) showed device quality (purely intrinsic nature, only (220) oriented growth) but with an incubation phase during which amorphous initial growth takes place. The presence of only 2000 cm(-1) IR vibration and a high temperature hydrogen evolution (similar to 600 OC) proves the compact structure which inhibits oxygen incorporation ([O]=5x10(18) cm(-3) from SIMS) into the film. A new approach was used to integrate these two growth regimes to make profiled poly-Si layers in which a Polyl layer of fixed thickness acts as a seed layer, on top of which the Poly2 is deposited. The top layer selectively grows along the (220) direction even though the grains in the seed layer are randomly oriented, The top layer is compact with V shaped columns consisting of twinned grains with a height approaching the layer thickness. A sharp reduction in void content is observed at the transition from seed layer to top layer. The profiled films show complete absence of amorphous incubation phase. Cells incorporating the profiled poly-Si:H films were made on stainless steel substrates in the configuration SS/n-mu c-Si:H(PECVD)/i-poly-Si:H(HWCVD)/p-mu c-Si:H(PECVD)/ITO. An efficiency of 4.41% and a FF of 0.607 have been achieved. A current density of 19.95 mA/cm(2) is generated with an only similar to 1.22 mu m thick i-layer which was deposited at a rate of more than 5 Angstrom/s.