High efficiency a-Si:H based solar cells employing novel p-nc-SiC:H buffer layers prepared by the mercury-sensitized photo-CVD technique

Abstract

Highly conductive boron-doped nanocrystalline silicon-carbide (p-nc-SiC:H) films were successfully prepared by the mercury-sensitized photo-CVD using ethylene gas as a carbon source. A dark conductivity as high as $5×10^{-1} S/cm^{-1}$, together with an $E_{04}$ bandgap of 2.1 eV, was obtained. From TEM image and Raman spectrum, it is considered that this nanocrystalline film consists of ~ 10 nm-sized crystalline silicon grains embedded in amorphous silicon-carbide matrix. Considering the low $H_{2}$ dilution ratio of 15 ~ 30, this high conductivity shows the possibility of producing higher quality p-nc-SiC:H thin films with this method. Each effect of the film thickness, hydrogen dilution, ethylene addition, boron doping, substrate temperature variation, and mercury bath temperature variation on the formation of nc-SiC:H films was investigated by inspecting the electrical, optical, and structural properties. By virtue of these efforts, we found that the hydrogen, carbon, and boron atoms introduced to the growing surface play important roles on the film properties, respectively. Hydrogen atoms introduced to the growing surface improve the crystallinity of the SiC:H films. High carbon and/or boron content suppress the nucleation of nanocrystallites. The electronic study on the nc-SiC:H films was performed for the first time: The paramagnetic spin density is directly proportion to the crystallinity due to surface-related DB states under the constant carbon content. However, the effect of C-DB states surpass that of surface-related DB states with the carbon content variation. Also, it was found that two temperature parameters, the substrate temperature and mercury bath temperature, influence strongly the characteristics of the films. It was found that the surface diffusion of precursors, which contributes to the nanocrystalline growth, depends considerably upon the substrate temperature. A considerable high quality nanocrystalline growth at 120℃ suggests a low...