Application of Hot Wire Deposited Intrinsic Poly-Silicon Films in N-I-P cells and TFTS

Abstract

ABSTRACTPoly-silicon films have been prepared by hot-wire chemical vapour deposition (HWCVD) from hydrogen diluted silane gas at a low temperature (430 °C). The crystalline volume fraction is 95%. The grains have an average size of 70 nm and coalesce completely. The activation energy (0.54 eV) and the low carrier concentration (6.8 × 1010 cm−3) indicate the fully intrinsic nature of the films. The low (<1017 cm−3) defect density, the absence of 2100 cm−1 mode in infrared spectrum, the low activation energy of Hall mobility (0.012 eV) and the ambipolar diffusion length (LD) of 568 nm all indicate that the grain boundaries in the poly-Si:H films are indeed very thin. Preliminary n-i-p cells incorporating a poly-silicon i-layer yielded 3.15% efficiency and a current density of 18.2 mW/cm2 for only a 1.5 μm i-layer. In the solar cell, the conducting path is along the columnar grains ( (220) preferential orientation from XRD) and the carrier transport bypasses the grain boundary defects. This has been confirmed from the c-Si like optical absorption spectrum (measured by Dual Beam Photoconductivity in the cell configuration) at the low energy region. Inverted staggered thin-film transistors incorporating the poly-silicon layers showed transfer and output characteristics similar to those of state of the art a-Si:H TFTs : the saturation mobility is 0.4 cm2/Vs and the off current is approximately 10−11 A. This leads to the conclusion that the silicon near the SiO2 interface (the channel region) is still amorphous. This is illustrated by XTEM.