X-ray diffraction (XRD) analysis of thin silicon films was carried out using both the symmetric Bragg-Brentano and asymmetric thin-film attachment geometries. The asymmetric configuration allows quantitative phase analysis of the films and reveals that amorphous silicon films deposited from silane diluted with hydrogen have the strongest peak in the XRD patterns located around 27.5 degrees. This peak corresponds to the signal from ordered domains of tetragonal silicon hydride and not from cubic silicon crystallites. The full width at half maximum (FWHM) of this peak narrows from 5.1 to 4.8 degrees as the ratio of hydrogen to silane flow (R) increases to 20 and does not change significantly for higher hydrogen dilutions. The amorphous silicon films fabricated at different hydrogen dilution were applied as absorber layers in single-junction solar cells. Degradation experiments confirm a substantial reduction of the degradation when the dilution ratio is increased from R=0 to R=20. The light induced degradation of solar cells with absorber layers prepared at R > 20 is not further reduced by increasing R.