Investigation of a-Si:H solar cells with a protocrystalline-like amorphous silicon intrinsic layer
We investigate the effects of the hydrogen dilution in a-Si:H film on the crystallization and the absorption property. Crystallization of the a-Si:H films from amorphous to several tens nanometer-sized crystal state is observed by increase of hydrogen dilution ratio using PECVD. The phase transition of the a-Si:H films from amorphous to micro-crystal occurred very rapidly in a very narrow range of the silane concentration(SC). Protocrystalline-like properties of silicon (pc-Si:H) is obtained fractionally within the a-Si:H matrix. It could be assumed that the volume fraction of the protocrystalline silicon is the highest in the range of SC=3.7∼4.0%, whereas amorphous or micro-crystalline phases are dominant in the other range of SC<4.0% and SC>3.0%, respectively. The μc-Si:H films with SC=1.0%~1.4% have the micro-crystalline volume fraction 75%∼80%. But the μc-Si:H films still contain the amorphous, proto- or nano-crystalline silicon in the rest volume so that they show some different characteristics with other micro-crystalline silicon. The a-Si:H films with SC=3.7∼4.0% doesn't contain the micro-crystalline volume fraction but different characteristics in absorption from the other a-Si:H films with higher SC>4.0%. Such films could be assumed to have the properties of the protocrystalline silicon. They, consequently, showed the low degradation property from the light soaking and the rapid stability.
Protocrystalline Silicon at High Rate from Undiluted Silane
Hot Wire Chemical Vapor Deposition (HWCVD) is shown to be a fast method for the deposition of protocrystalline silicon films from undiluted silane. Intrinsic silicon-hydrogen films (2 µm thick) have been deposited by HWCVD on plain stainless steel as well as on stainless steel precoated with a n-type doped microcrystalline silicon layer. In X-ray diffraction experiments, the linewidths of the first sharp peak (FSP) were 5.59 ± 0.09 degrees and 5.29 ± 0.11 degrees, respectively, indicating improved medium-range order and a template effect due to the µc-Si:H n-layer. For thinner layers (0.7 µm thick), the linewidths of the FSP were 5.29 ± 0.09 degrees and 5.10 ± 0.09 degrees. These FSPs are as narrow as for optimized i-layers made by H2-diluted plasma deposition, however, at a much higher deposition rate (1 nm/s), at moderate temperature (250 °C), and without the use of H 2 dilution. In accompanying transmission electron micrographs, the layers show a significant concentration of elongated small voids in the growth direction that are not interconnected. Small Angle X-ray Scattering (SAXS) results are consistent with these observations. We suspect that the void nature allows the bulk of the film to be more ordered. The utilization of such layers in n-i-p solar cells on plain stainless steel leads to cells with a remarkably good stability, showing a decrease of the fill factor of less than 10 % during 1500 h of light soaking.
Nuclear spin-lattice relaxation in transition metal compounds with local tetragonal symmetry.
We report on the various contributions to the total spin-lattice relaxation rate in metallic materials with local tetragonal symmetry. The analytical formulae are given in the tight-binding approximation. The calculations show the relation between various partial electron densities of states and corresponding contributions to the relaxation rates. The presented formulae can be used to compare theoretically calculated electron band structure parameters with those obtained from NMR spin-lattice relaxation time measurements.