Purely Intrinsic Poly-silicon Films for n-i-p Solar Cells

Abstract

Polycrystalline silicon films have been prepared by hot wire chemical vapor deposition (HWCVD) at a relatively low substrate temperature of 430° C at a high growth rate (>5 Å/s) by optimizing the hydrogen dilution of the silane feedstock gas, the gas pressure and the wire temperature. The optimized material has 95% crystalline volume fraction with complete coalescence of grains. The grains with an average size of 70 nm have a preferential orientation along the (220) direction. Large structures up to 0.5 µ m could be observed by atomic force microscopy (AFM). An activation energy of 0.54 eV for the electrical transport and a low carrier concentration (11 cm^-3) confirmed the intrinsic nature of the films. A white light photoconductivity of 1.9×10^-5 Ω^-1 cm^-1, a high minority carrier diffusion length of 334 nm and a low (17 cm^-3) defect density ensure that the poly-Si:H films possess device quality. A very small temperature dependence of the Hall mobility (0.012 eV) indicates negligible barrier to carrier transport at the grain boundaries. A single junction n-i-p cell incorporating HWCVD poly-Si:H in the configuration n⁺-c-Si/i-poly-Si:H/p-µc-Si:H/ITO yielded 3.15% efficiency under 100 mW/cm² AM1.5 illumination and a current density of 18.2 mA/cm² was achieved for only 1.5 µ m thick i-layer.