PURELY INTRINSIC POLYSILICON FILMS FOR N-I-P SOLAR-CELLS

Polycrystalline silicon films have been prepared by hot wire chemical vapor deposition (HWCVD) at a relatively low substrate temperature of 430 degrees C at a high growth rate (> 5 Angstrom/s) by optimizing the hydrogen dilution of the silane feedstock gas, the gas pressure and t he wire temperature. The optimized material has 95% crystalline volume fraction with complete coalescence of grains. The grains with an aver age size of 70 nm have a preferential orientation along the (220) dire ction. Large structures up to 0.5 mu m could be observed by atomic for ce microscopy (AFM). An activation energy of 0.54 eV for the electrica l transport and a low carrier concentration (< 10(11)cm(-3)) confirmed the intrinsic nature of the films. A white light photoconductivity of 1.9 x 10(-5)Ohm(-1)cm(-1), a high minority carrier diffusion length o f 334 nm and a low (< 10(17)cm(-3)) defect density ensure that the pol y-Si:H films possess device quality. A very small temperature dependen ce of the Hall mobility (0.012 eV) indicates negligible barrier to car rier transport at the grain boundaries. A single junction n-i-p cell i ncorporating HWCVD polySi:H in the configuration n(+)-c-Si/i-poly-Si:H /p-mu c-Si:H/ITO yielded 3.15% efficiency under 100 mW/cm(2) AM1.5 ill umination and a current density of 18.2 mA/cm(2) was achieved for only 1.5 mu m thick i-layer.