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.
Growth mechanism of nanocrystalline silicon at the phase transition and its application in thin film solar cells
Nanocrystalline (or microcrystalline) silicon (nc-Si:H or μc-Si:H) is an absorber material that is crucial for obtaining thin film silicon tandem solar cells with high efficiency. This material is conventionally produced by direct plasma-enhanced chemical vapor deposition (PECVD), which is based on the dissociation of silane (SiH4) and hydrogen (H2) in a radiofrequency (rf) plasma. During the last few decades, the plasma deposition parameter regime has been explored intensively, in particular to improve the quality of nc-Si:H and to increase its deposition rate. The desired formation of nanocrystallites occurs under growth conditions close to the transition regime. A technique that is not plasma based is hot-wire CVD (HWCVD), which is based on catalytic decomposition of SiH4 and dissociation of H2 gases on a hot filament. Because source gases are catalytically decomposed, the method is often referred to as catalytic CVD (Cat-CVD). The HWCVD technique has been shown to be a viable method for the deposition of silicon-based thin films and solar cells. We discuss the progress at our laboratory. Specifically, recent single junction and multijunction n–i–p-type solar cells are highlighted. Improvements have been achieved by addressing, among others, the morphology of the light-scattering rough surfaces, to avoid cavities and shunting paths, and by studying the structural evolution of the nanocrystalline phase during growth. The obtained solar cells are highly stable against light soaking.
Unidirectional hexagonal rare-earth disilicide nanowires on vicinal Si(100)-2×1
Rare-earth disilicide nanowires grown on vicinal Si(100) with a miscut of 2–2.5° toward the [110] azimuth at 600 °C were studied by scanning tunneling microscopy and compared with those grown on flat Si(001). In contrast to rare-earth disilicide nanowires grown on flat Si(100) surfaces, the nanowires grow unidirectionally along the [01̄1] direction of the vicinal Si(100) surface. Rare-earth disilicide nanowires form bundles composed of single nanowire units on both flat and vicinal surfaces. Yet, on the vicinal surface, the bundle width is comparable to the width of the terrace. The average nanowire length on the vicinal substrate is longer than that on the flat substrate. Scanning tunneling spectroscopy shows that the rare-earth disilicide nanowires have metallic properties.