Optoelectronic properties of p-type layers for amorphous-silicon solar cells prepared using helium dilution

This paper is focused on the investigation of the properties of p-doped fil ms to be used as emitters in hydrogenated-amorphous-silicon thin-him solar cells. In order to obtain highly conductive and transparent materials, plas ma-enhanced chemical vapour deposition of helium-diluted silane (SiH4) has been used. Small amounts of diborane (B2H6) have been added to the gas mixt ures for doping purposes. The influence of preparation conditions on the op tical and electrical properties of the films has been systematically studie d. It has been found that high radio frequency power densities and suitably tuned concentrations of silane in helium favour doping efficiency and film transparency simultaneously. A substrate temperature of 250 degrees C has proven to be adequate. Remarkably, the films of this study were between 25 and 35 nm thick, that is, about 10 times thinner than those reported in mos t papers on this subject and roughly only two to three times as thick as th ose actually grown in solar-cell preparation. The best samples obtained hav e 2.1 eV, 10(-2) S cm(-1) conductivity and 0.1 eV extended-state- conductiv ity activation energy. (C) 1999 Elsevier Science B.V. All rights reserved.